A

United States 

Department of 

Agriculture

Forest Service

Pacific Southwest 

Research Station

General Technical Report

PSW-GTR-226

May 2010

An Integrated Science 

Plan for the Lake Tahoe 

Basin: Conceptual 

Framework and 

Research Strategies DEPAR TMENT OF AGRICULT URE
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Editors

Zachary P. Hymanson is Executive Director, Tahoe Science Consortium, 

291 Country Club Dr., Incline Village, NV 89452, and Michael W. Collopy

is Executive Director, Academy for the Environment, 108 Mackay Science 

Building, University of Nevada, Reno, NV 89557. 

Front cover: Lake Tahoe view from Mount Rose Highway scenic pullout, looking 

northwest toward Tahoe City, California. Back cover: Lake Tahoe view from Mount 

Rose Highway scenic pullout, looking south. Both photographs by Peter Goin.
An Integrated Science Plan for the 

Lake Tahoe Basin: Conceptual 

Framework and Research Strategies 

Zachary P. Hymanson and Michael W. Collopy, Editors

U.S. Department of Agriculture, Forest Service

Pacific Southwest Research Station

Albany, CA

General Technical Report PSW-GTR-226

May 2010

Published in cooperation with:

U.S. Environmental Protection Agency

Region 9 

iii

Abstract

Hymanson, Zachary P.; Collopy, Michael W., eds. 2010. An integrated science 

plan for the Lake Tahoe basin: conceptual framework and research strategies. 

Gen. Tech. Rep. PSW-GTR-226. Albany, CA: U.S. Department of Agriculture, 

Forest Service, Pacific Southwest Research Station. 368 p.

An integrated science plan was developed to identify and refine contemporary 

science information needs for the Lake Tahoe basin ecosystem. The main 

objectives were to describe a conceptual framework for an integrated science 

program, and to develop research strategies addressing key uncertainties and 

information gaps that challenge government agencies in the theme areas of 

(1) air quality, (2) water quality, (3) soil conservation, (4) ecology and biodiversity, and (5) integrating the social sciences in research planning. Each 

strategy concludes with a presentation of near-term research priorities. Several 

factors (e.g., changing agency priorities, funding levels, and the emergence of 

new issues, new information, or new technologies) can affect the applicability 

of near-term research priorities. Thus, this science plan is considered a living 

document. The research priorities are best reviewed and revised regularly to 

ensure they reflect the changing information needs and evolving priorities of 

agencies charged with the welfare of the Lake Tahoe basin. 

Keywords: Lake Tahoe basin, air quality, water quality, soil conservation, 

ecology and biodiversity, social sciences.
iv

Summary

The known effects of past actions and the unique character of the Lake Tahoe basin 

have led to broad-based support for substantive conservation and restoration efforts 

over the last two decades. Increased attention and funding over the past decade, in 

particular, have resulted in remarkable progress toward restoration goals, along with 

considerable information on the strengths and weaknesses of different approaches 

to addressing the substantial restoration challenges. Restoration has focused not 

only on Lake Tahoe, but also on the entire watershed. Special attention has been 

given to the highly interdependent nature of terrestrial and aquatic habitats and the 

multifaceted socioeconomic conditions that influence the Tahoe basin ecosystem. 

The Lake Tahoe basin is recognized as a highly complex physical, biological, and 

social environment, and the challenges posed by its restoration and continued 

management for multiple benefits are paralleled by few other locations. 

Conservation and restoration of the Lake Tahoe basin ecosystem have required 

the sustained engagement of federal, state, and local governments, as well as the 

private sector. These entities have worked together to develop and implement a 

variety of programs and activities aimed at achieving common environmental and 

social goals. Yet determining how to proceed with conservation and restoration 

efforts in the face of limited information remains a central challenge to these 

efforts. Science (e.g., monitoring, research, and modeling), particularly applied 

science to inform adaptive management, provides a promising set of tools to 

address information limitations that affect our ability to select and implement 

effective management strategies. However, effort is required to organize and 

describe the science activities needed to inform adaptive management focusing on 

the conservation and restoration of a complex system. This document presents the 

results of science community efforts to organize and describe the initial elements 

of an integrated science plan for the Lake Tahoe basin: a conceptual framework for 

completing science to inform adaptive management, and focused research strategies 

covering topic areas of relevance to Tahoe basin management and conservation. 

Separate, agency-led efforts are underway to develop other essential elements of an 

integrated science plan including programs for status and trends and effectiveness 

monitoring, new data applications aimed at converting data into information and 

knowledge, and the integration of monitoring and applied research efforts.

This science plan was developed to identify and refine science information 

needs for the Lake Tahoe basin. The main purpose of this effort was to develop 

a set of research strategies addressing key uncertainties and information gaps 

that challenge resource management and regulatory agencies. The research needs 
v

identified in these strategies are based on assessments of the issues and information 

needs that currently confront government agencies and stakeholders working in 

the basin. The resulting strategies are intended to guide future research efforts and 

to help maximize the information gained from future science investments. Three 

common needs drive the recommendations presented in each research strategy:

• Increasing our understanding of the factors and processes driving change.

• Developing the tools and knowledge to predict future conditions in 

the Lake Tahoe basin and permit comparisons among alternative futures.

• Providing information for future management decisions aimed at 

conserving and restoring the natural and human environments of the 

Lake Tahoe basin.

This science plan comprises seven chapters. Chapter 1 examines the need and 

approach for developing the science plan, and also provides important contextual 

information. A brief review of past science planning efforts presented in chapter 1 

shows that merely producing a science plan is not enough to ensure the establishment of a sustained science program that can deliver useful information covering 

a diversity of issues. Clear policy direction to obtain and use scientific information 

is essential, but a deeper level of commitment among all relevant parties will guide 

progress from planning to implementation. An explicit assumption of this effort 

is that the agencies charged with responsibility for the welfare of the Lake Tahoe 

basin will collaborate to establish the funding, resources, and infrastructure necessary for sustained implementation of an applied science program. Brief summaries 

and recommended priority science needs for each of the six remaining chapters are 

presented below.

Conceptual Framework for an Integrated Science Plan 

Conceptually, the efforts and activities of an integrated science program can be 

divided among three basic elements: (1) monitoring, (2) empirical research, and 

(3) data application. To be effective, however, it is recommended that efforts be 

integrated across all three elements and applied to land and resource management efforts through adaptive management. This is the basis for the conceptual 

framework presented in chapter 2. The overview conceptual model presented in 

this chapter will orient readers to the important issues covered in this science plan, 

the relationships among those issues, and the format of more specific conceptual 

models presented in each of the research strategies. Near-term priorities for each 

science activity are as follows:
vi

Monitoring—

Effective monitoring schemes are essential elements in integrated science programs. Monitoring results are often the main source of scientific information in 

support of adaptive management systems. Monitoring is used to establish baseline 

conditions, track management activities, and record outcomes. Several monitoring 

needs were identified in discussions between scientists and managers during the 

course of developing this science plan. What emerged from those discussions is not 

a comprehensive list of the monitoring needs for the Lake Tahoe basin; rather, the 

list represents the recommended highest priority monitoring information needs to 

be addressed within the next five years.

• Meteorology and climate change:

▪ Long-term acquisition of spatially relevant meteorological data on air 

temperature; wind-speed and direction; humidity; precipitation timing, 

amount, and type; streamflow; snowpack; and snowmelt are recommended to support environmental studies in all issue areas. The technology to capture these kinds of data has advanced, which allows them to 

be collected more efficiently at more locations than in the past. These 

data are fundamental to informing questions about air quality, water 

quality, soil conservation, ecology and biodiversity, and climate change.

• Air quality:

▪ Improve air quality monitoring in the Tahoe basin. The first step is to 

develop a comprehensive monitoring plan for the basin that addresses the 

criteria air pollutants (covered under the National Ambient Air Quality 

Standards) and species affecting human and ecosystem health (including water clarity), along with the required spatial and temporal distribution of the measurements. Once this plan is prepared, sampling locations 

can be chosen and appropriate air quality sensors can be deployed. Air 

quality and meteorological monitoring are best integrated to maximize 

efficiencies and information gain.

• Water quality:

▪ Initiate long-term status and trend monitoring of watershed hydrology 

and pollutant loads entering Lake Tahoe to (1) inform Lake Tahoe total 

maximum daily load (TMDL) land use and lake clarity models, and 

other water quality-related management models; (2) evaluate progress in 

meeting TMDL allocation requirements and other regulatory obligations; 

and (3) evaluate snowpack and snowmelt trends as they pertain to lake 

clarity. The Lake Tahoe Interagency Monitoring Program (LTIMP) 
vii

partially meets these monitoring needs, but this program has eroded 

over the last decade owing to funding restrictions. The LTIMP does 

not include some key pollutant sources (i.e., urban stormwater and road 

runoff), and it does not include some key water quality constituents 

that directly affect lake clarity (i.e., particle number and particle size 

distribution).

▪ Develop a regional stormwater quality/best management practices retrofit monitoring program to assess the effects and effectiveness of capital 

investment projects at the project, watershed, and basinwide scales.

• Soil conservation:

▪ Implement a consistent and effective monitoring protocol for key soil 

properties and conditions that is process-specific, uniform across agencies and contractors, and relies on current technology. Such a monitoring 

program is considered vital to informing the performance effectiveness 

of projects affecting soil properties and conditions throughout the Tahoe 

basin.

• Ecology and biodiversity:

▪ Develop a forest fuel reduction monitoring program to assess quantitatively the effects and effectiveness of fuel reduction projects at 

the project, watershed, and basinwide scales.

▪ Establish a long-term, basinwide status and trend monitoring program 

for terrestrial and aquatic biotic communities. It is recommended that 

biodiversity monitoring focus on the composition of biotic communities 

and the distribution, frequency of occurrence, and abundance of associated native and nonnative species using direct measures and indicators 

to inform environmental thresholds and desired conditions. Monitoring 

data also could provide a foundation for assessing the effects of climate 

change on terrestrial and aquatic biota of the Lake Tahoe basin.

▪ Initiate status and trends monitoring for select focal species identified 

in regulations and basin management plans. Integration of focal species 

monitoring with biodiversity monitoring is recommended to the degree 

efficiencies can be gained, and to provide reliable estimates of the population parameters targeted in those regulations and plans. 

• Social sciences:

▪ Establish regular and systematic surveys to document the use of existing 

recreation resources and to help inform priorities for future facilities. 
viii

▪ Develop and implement infrastructure and common methods to collect basic social and economic data throughout the Lake Tahoe basin. 

Common sampling methods are recommended to ensure that data can be 

aggregated and analyzed to assess socioeconomic conditions within the 

various municipalities, within geographic regions, and throughout the 

entire basin.

▪ Develop infrastructure to support the centralized collection, inventory, 

and distribution of transportation data at regional and basinwide levels.

▪ Establish a spatially and temporally appropriate monitoring program 

to allow for the assessment of noise conditions relative to established 

threshold indicators.

Empirical research—

Research can be defined as a structured process of inquiry that aspires to discover, 

interpret, and revise our knowledge of facts. Research aims to produce an evergreater knowledge of events, behaviors, processes, theories, and laws. Research may 

include laboratory or field experiments, or the development of models. Research is 

one of the fundamental ways in which science contributes to reducing uncertainties. Research and monitoring operate most effectively together. The single highest 

priority for research is to establish a stable funding stream to ensure that the level 

of scientific understanding of the Lake Tahoe basin continues to develop in order to 

support timely and effective science delivery and inform policy choices and management strategies. Specific, near-term research priorities are presented below for each 

research strategy included in this science plan.

Data application—

Data application includes analysis, reporting, management, and assessment to 

accomplish the following objectives (1) manage data and information in ways that 

ensure their quality and availability; (2) complete analyses that convert data into 

information that can directly guide management; and (3) share that information with 

others, such that information promotes knowledge. From a management standpoint, 

data application to real-world problems may be the most important element of an 

integrated science program, because it is this activity that provides research and 

monitoring results in forms that managers can apply directly to management actions, 

decisions, and policy choices. A dedicated source of funds and resources to support 

data management, analysis, and reporting together with the identification of responsible entities are recommended to ensure that this element of an integrated science 

program is accomplished and maintained, and that redundant efforts are eliminated. 

In the course of developing this science plan, a number of high-priority investments 

in data applications were identified for completion over the next 5 years. 
ix

• Accomplishments report: Completed annually, an accomplishments report 

from the science community would synthesize the results of research and 

other science projects completed during the previous year; this report can 

then be combined with updated running totals of administrative outcomes 

(e.g., dollars spent, number of projects completed) and program outcomes 

(e.g., acres restored, volume of storm water treated) from the management 

agencies. The accomplishments report would summarize scientific findings 

and relate the importance of those findings to ongoing and future management activities and policies. This report also could identify research needs 

that have emerged as a result of monitoring results, new environmental 

conditions, or changes in policy or regulation. Information in the accomplishments report will help agencies and scientists track projects completed 

or in progress, and develop or adjust near-term priorities for capital projects, 

research, monitoring, and analysis. Information in the report also could provide a snapshot of project outcomes.

• Central monitoring database: The value provided by a centralized information depository is manifold; however, a primary goal is to promote the management of both research and monitoring data in a manner that ensures their 

quality and accessibility. It is recommended that infrastructure be developed 

and maintained so that basic data and summary information are stored, 

integrated, and accessible to a diversity of users. The Tahoe Integrated 

Information Management System (TIIMS) has begun efforts to address the 

needs associated with this activity, but more work remains.

• Environmental management knowledge base: Create an environmental 

management knowledge base that documents our current understanding of 

conditions, interactions, threats, and desired outcomes. Use the outputs of 

this knowledge base to inform science and management priorities and activities. Integrated information systems can meet the array of information needs 

across institutions and disciplines in an efficient and transparent manner. 

Knowledge bases with their incumbent conceptual models provide valuable 

tools for integrated information storage and retrieval. A knowledge base system of environmental quality for Lake Tahoe that serves both management 

and research would:

▪ Provide a transparent basis for evaluating desired conditions

▪ Enhance the ability to communicate ideas and outcomes among stakeholders regarding management options at site and landscape scales

▪ Facilitate rapid access by decisionmakers to needed information
x

▪ Help prioritize information gaps to be filled, and illustrate the basis 

of priorities

▪ Compare outcomes associated with various management options to 

help determine priority objectives for specific locations and how to 

balance objectives across landscapes

▪ Serve as a tool to interpret changes in desired conditions over time, 

based on monitoring data. 

• State of the Lake Tahoe basin report: Completed every fifth year, a stateof-the-basin report would include a comprehensive synthesis of the research 

and monitoring that has been completed over the previous 5 years. Results 

could be framed in terms of an environmental report card for the Lake Tahoe 

basin, which would evaluate conditions relative to environmental goals 

and thresholds. This information would be available for use in evaluating 

and (if necessary) modifying management strategies and implementation 

programs (i.e., informing the adaptive management process). This report also 

could alert high-level officials to emerging issues that may require new or 

alternative policies. The state of the Lake Tahoe Basin report would include 

the following subject areas: 

▪ Lake Tahoe

▪ Basin watershed condition 

▪ Basin airshed condition

▪ Living resources condition

▪ Human environment.

Air Quality 

Chapter 3 addresses the fact that air quality in the Lake Tahoe basin is known to 

affect lake water quality, forest health, and human health. To address these issues 

and develop a sound scientific approach for mitigating the impacts of atmospheric 

pollutants, this research strategy provides an update and builds on recent work to 

understand how changes in air quality affect various aspects of the natural and 

human environment in the Lake Tahoe basin. This chapter also delineates current 

knowledge gaps and defines the research needs and strategies to close those gaps. 

Near-term air quality research priorities are as follows:

• Many of the key chemical species and physical parameters leading to 

secondary pollutant formation and/or deposition to the lake are not currently measured. Air quality measurements of key species under a range of 

meteorological conditions and averaging times are recommended following 
xi

the development of a monitoring plan (see air quality monitoring description above). Parameters to measure include nitrous oxides, ammonia, nitric 

acid; size-segregated aerosol mass; particle number and size distribution, 

and aerosol chemical composition. Once these data are obtained, it will be 

possible to assess atmospheric impacts and air pollutant trends. This information also will provide the necessary input for developing an understanding of the processes controlling air quality and atmospheric deposition in 

the basin.

• The only model adapted for use in the Tahoe basin is the Lake Tahoe 

Atmospheric Model, a heuristic model that is based on statistical input. It 

is recommended that an appropriate air quality model incorporating physical processes be developed that can utilize the full suite of meteorological, 

chemical, and particulate data. This will enable managers and scientists to 

better assess air pollutant trends, estimate impacts, and support the development of effective regulations that will assist in meeting air quality and 

other environmental goals. 

• Atmospheric deposition to the lake is the major source of nitrogen and 

a substantial source of phosphorous and particulate matter. There is, 

however, significant uncertainty in deposition flux estimates. To reduce 

this uncertainty, it is recommended that focused studies (i.e., gradient 

or eddy-correlation studies, along with measurements of key species) be 

conducted of the sources and pathways of particle deposition to better 

inform models and restoration efforts. 

• Mobile source emissions are a major source of pollutants in the basin. To 

improve the emissions inventory, it is recommended that Tahoe-specific 

vehicle model year distributions, emission factors, and activity data be conducted for use in mobile source emission factor models. These results will 

reduce the uncertainty in the emissions inventory and enable regulators to 

develop more effective strategies to reduce pollution in the basin.

Water Quality 

Lake Tahoe is the most-studied feature of the Tahoe basin ecosystem with regard 

to water quality. Although a substantial amount of research and monitoring has 

been accomplished, knowledge gaps and uncertainties still exist, particularly in 

understanding how watershed restoration efforts influence the long-term water 

quality of Lake Tahoe. Because water quality restoration efforts in the Tahoe basin 

are expected to exceed $1 billion, it is critical that we continue to collect and deliver 

information in an organized fashion. The water quality research strategy in chapter 
xii

4 is intended to serve as a road map for discussions with resource managers. The 

chapter identifies those science projects necessary to help guide water quality 

restoration efforts and understand related ecosystem processes. Near-term water 

quality research priorities include:

• Pollutant loading and treatment within the urban landscape:

▪ Develop a process-based understanding of sources, transport and 

loading of fine sediment particles (<20 μm) from different urbanized 

land uses in the Tahoe basin. Although this includes all features of 

the urban landscape, roadways appear to be particularly important 

and deserve focused attention.

▪ Quantify the effectiveness of best management practices (BMPs) and 

other watershed restoration activities on the control of fine sediment 

particle and nutrient loading to Lake Tahoe. Major load reduction 

approaches include hydrologic source control (HSC), pollutant source 

control (PSC) and stormwater treatment (SWT). Although some data 

have been collected on BMP and restoration effectiveness in removing 

nutrients and fine sediment, these efforts have been for specific 

projects and have not provided basinwide process-based evaluations. 

A comprehensive basinwide watershed-scale evaluation of BMP and 

erosion control project effectiveness is needed, especially for the Lake 

Tahoe TMDL program.

▪ Conduct focused studies to understand the influence altered urban 

hydrology has on pollutant pathways and determine how alternative 

hydrologic designs can enhance load reduction.

▪ Investigate longer-term impacts from infiltration of stormwater runoff 

around the Tahoe basin, particularly as it relates to different soils, land 

uses, and groundwater quality. 

▪ Continue efforts to establish a Regional Storm Water Monitoring 

Program. Key elements of this program include (1) pollutant source 

monitoring; (2) pollutant reduction monitoring; (3) BMP design, 

operation, and maintenance monitoring; and (4) data management, 

analysis, and dissemination. Although this is not research per se, data 

collected under this program will be used to support research on BMPs 

and pollutant load reduction. 

▪ Validate pollutant reduction crediting tools that are currently being 

developed to track progress in implementing the Lake Tahoe TMDL. 

At the same time, develop a science-based adaptive management program to guide pollutant load reduction activities.
xiii

• Near-shore water quality and aquatic ecology:

▪ Additional research is recommended to determine near-shore processes 

at various temporal and spatial scales. This research will contribute to 

an integrated database that can be used to determine trends and patterns 

for integrated, process-driven models. From this information, construct a 

predictive model to help guide ongoing and future management strategies. Ideally, this model would include features such as nutrient loading, 

turbidity, localized and lakewide circulation patterns, wave resuspension, 

periphyton and macrophyte populations, introduced and native species, 

and recreational uses and activities within the near shore. 

▪ Develop an aquatic invasive species research program with direct ties 

to water quality (e.g., threat of invasive species impacts on: [1] native 

species composition and aquatic food webs, [2] in-lake sources of 

drinking water, or [3] water quality and stimulation of benthic algal 

growth in the near shore). 

▪ Develop analytical approaches for establishing quantitative and realistic 

water quality standards and environmental thresholds for the near-shore 

region.

• Erosion and pollutant transport/reduction within the vegetated landscape: 

▪ Collaboration between researchers and agency representatives is recommended to evaluate fine sediment and nutrient loads resulting from 

forest fuels reduction activities. A major effort would include quantifying BMP effectiveness for controlling fine sediment and nutrient releases 

from wildfire, as well as from forest biomass management practices, 

such as prescribed fire and mechanical treatment.

▪ Fully evaluate the benefits and risks from using large areas of the natural 

landscape (e.g., forests, meadows, flood plains, wetlands) for treatment 

of urban runoff.

• Water quality modeling:

▪ Water quality management in the Tahoe basin has embarked on a pathway that will use science-based models to help guide management into 

the future. Continued support for the development, calibration, and 

validation of these models is recommended.

▪ Develop appropriate linkages between the landscape, climate, and atmospheric and water quality models to provide more comprehensive assessment of primary and secondary drivers whose effects propagate through 

the ecosystem.
xiv

▪ Build decision-support modules for the linked ecosystem models that 

will support evaluation of effects from larger spatial scales.

• Climate change:

▪ Continue to document the effects of climate change on existing and 

future water quality conditions.

▪ Apply predictive scenario testing for evaluating potential effects from 

climate change within the new and developing management models 

used for water quality in the Tahoe basin. In particular, models could 

be used to evaluate basinwide BMP effectiveness and load reduction 

strategies based on the expected changes to temperature, precipitation, 

and hydrology.

▪ Limnological processes in Lake Tahoe such as stratification, depth 

of mixing, particle distribution and aggregation, species succession, 

aquatic habitat based on water temperature, and meteorology are all 

recommended for reevaluation in light of climate change and possible 

management response to the impacts of climate change.

Soil Conservation 

Today soil conservation conveys a concept much broader than preventing soil 

erosion and reclaiming eroded lands. Soil conservation strategies go well beyond 

simply protecting soil from processes of physical erosion to now encompassing 

the protection and enhancement of overall soil quality and ecology. The intent of 

chapter 5 is to identify the most pressing management questions, uncertainties, 

and pertinent research needed to address a wide spectrum of plant-, soil-, and 

water-related issues in the Lake Tahoe basin. Near-term soil conservation research 

priorities include:

• Key soil properties and conditions:

▪ Further quantify the distribution of various watershed properties such as 

soil water repellency, biologic and inorganic nutrient pools, infiltrability, and water balance parameters on a larger spatial scale. The impact 

of natural and anthropogenic activities such as development (impervious 

vs. pervious), forest management (fire suppression vs. mechanical or prescribed fire biomass reduction), vegetation (native vs. nonnative species), 

restoration (physical and chemical amendments vs. reduced fertilization), 

and features that have disrupted natural littoral and eolian processes 

on soil health at the watershed scale (including the intervening zones) 

remains poorly understood. 
xv

▪ Quantitatively assess which restoration methods are most effective in 

controlling event-based runoff and the transport and equilibrium chemistry of fine particles most associated with nutrient and sediment loading. 

Project success and longevity would be evaluated relative to the sustainability of hydrologic function, productivity, and erosion control over 

time. 

▪ Characterize to the extent feasible and quantify where possible historical vs. current and natural vs. anthropogenic induced declines in soil 

status and resulting soil loss at the watershed scale. Research on soils in 

natural as well as disturbed settings is recommended whenever possible, 

with sites established to measure soil conservation parameters including 

inputs such as plant-soil nutrient fluxes through litter-fall, crown-wash, 

and root turnover as well as losses from erosion, leaching, runoff, wind, 

or fire. Research would focus on sites where a suitable control portion is 

available, especially if event (e.g., prewildfire or pretreatment) data are 

available. 

Knowledge advancement potential—The fate of Sierran ecosystems in a changing 

environment will have a direct impact on soil health, fire hazard, biomass mitigation strategies, erosion, and water quality. Manipulative research projects that 

include random assignment of treatments and replication are challenging to perform 

in the Tahoe basin. And yet a crucial research need is to identify and quantify key 

indicator parameters in a variety of historical and current ecological settings, under 

various manipulations, and over time. In such cases where robust experiments 

are possible, restoration methods that are most effective in controlling runoff and 

transport of fine particles, as well as those most effective in the reduction of nutrient discharge loading and its direct effect on water clarity can be better assessed. 

This would allow a more complete understanding of the environmental factors (i.e., 

temperature, moisture, vegetation, and litter) that determine the formation, persistence, and dissipation of seasonal and long-term effects on runoff water quality 

and erosion. In this context, similar slope stabilization, infiltration, revegetation, or 

sedimentation techniques could be tested against each other in similar and divergent environments as a means of ascertaining why some work better than others in 

one locale vs. another. Research focused on the identification, monitoring, evaluation, tracking and adaptive management of individual and collective BMP systems, 

and linking them to geographic information system layers at the watershed and 

basin scale would go a long way toward addressing the concerns of stakeholders 

and agencies alike. Being able to track and revisit BMP strategies would further 

facilitate true adaptive management.
xvi

• Development and application of predictive models as related to soil 

conservation: 

▪ Successful model application dictates the need for site-specific 

parameterization and model calibration. Model use and application 

can then become more consistent, and interpretive assessment more 

uniform among agencies basinwide. Research and monitoring protocols 

are recommended to provide relevant information for predictive model 

development, improvement, calibration, and field validation specific 

to the Lake Tahoe basin. Prioritization is recommended to determine 

which soil, vegetation, and hydrologic parameters are important; what 

should be measured; and what information is needed to parameterize and 

calibrate the models.

▪ Develop a spatially explicit water balance, nutrient cycling, and erosion 

potential model to better understand current sediment and nutrient 

transport at the watershed scale and under conditions of potential 

changes in hydrologic and soil parameters. Further studies of the role of 

hydrophobic soils are recommended to determine the spatial distribution 

of recharge areas vs. those that are overland flow generating, and their 

influence on soil erosion model output estimates. A prioritization of 

which parameters are important, what should be measured, and what 

information is needed to parameterize and calibrate the models is 

recommended.

▪ Develop a better understanding of how various factors or stressors 

change soil status in Tahoe basin watersheds to assist forest managers 

in preparing management plans and make predictions about ecosystem 

response to natural (e.g., fire, insect attack, drought, or erosion) and 

anthropogenic (e.g., air pollution, harvesting, development, or climate 

change) perturbations. For example, a comprehensive assessment of the 

effects of both wildfire and prescribed fire and postfire vegetation on 

long-term response in biological and physicochemical soil parameters is 

needed to better understand fire and its role in restoration ecology.

Knowledge advancement potential—Because regulatory policy is often based 

on the subjective judgment of “risk potential” rather than on a sound quantitative 

decision support system, the application of predictive models can provide important 

tools to understanding and estimating the potential outcome of management 

strategies and programs. Successful model application, however, is accomplished 

through site-specific parameterization and model calibration. Establishing the 

means for prioritization of which ecosystem parameters are important, what should 
xvii

be measured, and what information is needed to parameterize and calibrate the 

models is recommended. In the event that the current models are not adequate 

predictors, appropriate modifications or adjustments are needed to make the 

existing models more functional. If this is not an option, starting from scratch and 

developing a new model that is simple, accurate, and appropriate for the Tahoe 

basin may be necessary. Model use and predictive application is recommended to 

enable consistency among agencies basinwide wherein the acquisition of a more 

robust quantitative database could provide the foundation for policies of future 

management strategies.

• Effects of climate change as related to soil conservation:

▪ There is concern that anthropogenic activities over the last century have 

resulted in nontypical ecosystem structure throughout the basin of which 

the distribution, character, variability, and potential response to climate 

change have not been evaluated. Consequently, strategic efforts directed 

toward long-term site restoration in response to a quasi-natural state will 

be the more likely scenario. Quantitative assessment of what we can and 

cannot hope to accomplish on a long-term basis is recommended. 

▪ More comprehensive localized point source precipitation, surface runoff, 

erosion, and nutrient transport data is recommended to quantify potential 

discharge loads as a function of amount, type (snow vs. rainfall), frequency, and precipitation intensity. It is recommended that research and 

monitoring projects be designed to address potential changes in hydrologic parameters as a result of climate change.

▪ Investigate the implications of climate change on slope stability parameters. It is recommended that surface soil stability, compaction, soil 

structure and aggregate stability, infiltrability and runoff, and potential 

for mass wasting all be tested under scenarios of different temperature 

and moisture regimes to estimate the potential effects of climate change 

on soil erosion in the Lake Tahoe basin.

Knowledge advancement potential—The implications associated with climate 

change cannot be ignored. Predictions for changes in precipitation quantity and 

intensity are quite variable for the Sierra Nevada. One scenario is that precipitation 

will increase in intensity leading to large-scale flooding. Another is that the Lake 

Tahoe area will be subject to overall warmer temperatures and more evapotranspiration, while increased precipitation will be more common farther to the north. 

There is general agreement, however, that with warmer temperatures snow elevation levels will be higher and accumulation likely lower, which will lead to longer 
xviii

fire seasons. Examination of management strategies involving biomass reduction, 

drainage control, and practices to diminish sediment and nutrient transport to 

Lake Tahoe is recommended in the context of shifting the quality and amount of 

hydrologic input. Such an examination will likely necessitate new approaches. It is 

recommended that future planning for the production of resilient, spatially heterogeneous and diverse forest structure be designed to account for potential changes in 

hydrologic function in response to different moisture regimes to determine what the 

ultimate effects of climate change could be on management protocols for sensitive 

areas at the watershed and local scales.

• Policy implications and adaptive management strategies as related to 

soil conservation:

▪ Monitor and study established environmental thresholds for attainment and performance effectiveness. It is recommended that regulatory agencies and land managers develop a protocol for periodic review, 

verification, and update of processes, quantitative thresholds, and policy 

relevance. More research and monitoring of existing regulatory programs 

is recommended, such that their overall effectiveness and applicability 

can be more quantitatively assessed relative to their actual reduction of 

nutrient and sediment loading to the lake, and the subsequent enhancement of water clarity. Develop a basinwide protocol for “standard methods of ecological measurement and monitoring in the Tahoe basin.”

▪ Continued research that addresses critical natural resource issues and 

critical management questions relevant to soil conservation in the Tahoe 

Basin is essential. This exercise can begin by identifying a list of agencyspecific management questions pertinent to soil conservation relative 

to key soil properties and conditions of interest. For example, what 

are the appropriate management strategies that maximize defensible 

space (measures taken to protect homes from wildfire, e.g., removing 

vegetation), but at the same time function to minimize erosion and 

the degradation of runoff water quality? It is then important to take 

advantage of unique opportunities and small-scale experimental 

field trials to quantitatively evaluate potential impacts. To ensure 

credibility and applicability, such research would make every effort 

to be scientifically defensible, applicable to the Tahoe basin or similar 

ecological settings, and publishable in peer-reviewed journals. 

▪ Restoration and BMP strategies are generally implemented to mitigate 

known adverse impacts from either natural events or anthropogenic 
xix

activities. Choosing the most effective strategy therefore necessitates a 

thorough knowledge of the mitigation objective, process mechanics, both 

short- and long-term functionality, and whether or not these components 

will differ depending on location within a given watershed or the Tahoe 

basin in general. Performance evaluation is commonly assessed on a collective (e.g., projectwide) rather than individual (e.g., specific management activity) process basis. Complete evaluation of which strategies are 

truly the most effective in meeting specific restoration objectives would 

require testing each management activity against one another as well as 

assessing their cumulative effects. Management strategies implemented 

for one purpose (e.g., defensible space) may or may not have an effect on 

other issues of concern and ascertaining why some work better than others in one locale vs. another (or not at all) is a critical issue.

Knowledge advancement potential—If greater confidence in performance effectiveness can be developed, consistency will likely follow. With new technology 

comes the opportunity for innovative soil conservation strategies that could alter 

or refine historical threshold values. In the past, technological advancement and 

expansion of the knowledge base was much slower. Today, it is not unusual for 

substantial new advancements to take place on a 5-year rather than a 25-year cycle. 

Key to the success of any such approach, however, is the development of a consistent and effective monitoring protocol for key soil properties and conditions that 

is current, process-specific, and uniform across agencies and contractors. Hence, 

research is recommended to develop a standard protocol for ecological measurement and monitoring in the Lake Tahoe basin, which includes variable levels of 

intensity that can be applied to different types and scale of projects. In its absence, 

implementers and agencies frequently employ different techniques in attempting 

to evaluate the performance effectiveness of similar soil conservation activities. 

Comparative interpretive assessment is then difficult to impossible. Furthermore, 

evaluating which conservation and/or restoration methods are most effective is 

recommended in the context of a more comprehensive framework wherein each 

on-the-ground management strategy could be tested against one another in similar 

and divergent environments. Therein lay key opportunities where new and unique 

soil conservation strategies could be explored. Finally, agency representatives can 

clearly identify agency-specific areas of concern, and then work with scientists 

and implementers to articulate the respective critical soil conservation issues. This 

would greatly assist in the development and design of successful monitoring, opportunistic, and/or experimental research programs that generate data and information 

directly applicable to agency needs.
xx

Ecology and Biodiversity 

The integrity of animal and plant communities serves as a critical measure of the 

effectiveness of implementation policies designed to protect and restore ecosystem 

processes in the Lake Tahoe basin. The conservation of plants and animals in the 

Tahoe basin is wholly dependent on the conservation of its terrestrial and aquatic 

ecosystems. Thus, the research agenda for biological diversity and ecological function is based on investigations that integrate data collection efforts across scientific 

disciplines to maximize the recovery and persistence of biological diversity. The 

Ecology and Biodiversity research strategy in chapter 6 highlights the interactions 

between native species and communities, and natural and human-caused stressors 

that present the greatest ecological and social risk, and for which research can 

reduce uncertainties presenting barriers to more effective management. Near-term 

ecology and biodiversity research priorities include:

• Old-growth and landscape resilience: The ultimate objective of forest 

management is to restore and maintain forest health and resilience such that 

forests and their associated biota are able to maintain the full range of functions, their native biological diversity, and continues to perform the ecosystem services upon which human communities in the basin depend. The 

primary uncertainty limiting management’s ability to meet this objective 

is a clear understanding of what environmental conditions to create, and 

when and where to create them. Specific questions pertain to the historical 

amount and distribution of forest structural conditions, associated plant and 

animal species composition, and how to translate historical conditions into 

target conditions for the future that will enable forested ecosystems to adapt 

to future environmental stressors without the loss of function or biological 

diversity. Old forests are of particular concern and interest, since despite 

the maturity of existing forests, it is apparent that extant forests have lost 

ecological complexity associated with old forests, and therefore species and 

functions restricted to old forests are rare and most at risk from uninformed 

management. Finally, robust measures of forest biological diversity and 

resilience are recommended to enable simple and effective tracking of management progress and success. 

• Fire regime: One of the greatest ecological risks associated with fire is the 

uncertainty associated with the effects of fuel reduction treatments—both 

their effectiveness in changing fire behavior, and the consequences of treatment effects on other ecological conditions, such as biological diversity 

and forest ecosystem resilience. The near-term research priorities pertain 
xxi

to addressing risks and uncertainties posed by current management activities, which target fuel reduction treatments on thousands of acres without 

an indepth understanding of the ecological consequences. Therefore, nearterm research priorities include improving the understanding of the effects 

of various types and intensities of treatments on the spectrum of ecosystem 

management objectives, including but not restricted to fire behavior. Of 

primary concern is the fate of terrestrial species and processes, as they are 

directly affected by forest conditions. It would be most efficient to develop 

and test silvicultural prescriptions in the course of addressing near-term 

research priorities, as opposed to after ecological risks are more clearly 

understood. Once the primary ecological objectives at risk as a function of 

reducing the threat of catastrophic wildfire are understood, it is important 

to develop simple and informative measures of their status for long-term 

monitoring. 

• Special communities: The conservation and restoration of special communities in the basin all rely on similar information: (1) maps of current location and condition throughout the basin, (2) reference conditions based on 

historical data and other relevant data sources, (3) evaluation of the effectiveness of restoration approaches, and (4) the development of performance 

measures to assess status and restoration effects. A few unique information needs are associated with individual special communities. In aspen 

communities, techniques for converting conifer-encroached stands back to 

aspen-dominated habitats is a primary information need. Fens and meadows are under an unknown level of threat from various human activities. 

The current status of marshes is basic information that is lacking. Finally, 

detailed information on genetic and environmental sensitivities of Tahoe 

yellow cress are needed to aid population restoration efforts of this endemic 

species. 

• Aquatic ecosystems: The emphasis of aquatic ecosystem research is on 

conservation and restoration of vertebrate biota in Lake Tahoe and the conservation of species in the rich array of other aquatic ecosystems around the 

basin. In Lake Tahoe, the uncertainties with the greatest potential impact on 

management are those associated with the interactions between nonnative 

and native plant and animal species. These interactions have potential consequences for biodiversity, lake clarity, and near-shore aesthetics. Research 

on measures to control established nonnative species is recommended. This 

research is best pursued through an adaptive management approach, where 
xxii

information from scientific assessments of pilot projects is used to guide 

longer-term management strategies. Restoration of native fishes in Lake 

Tahoe presents a steep challenge, and information on the ecological interactions is key to making progress. The other aquatic ecosystems are in need 

of more basic information, namely the status of vertebrate populations and 

communities, and factors limiting the ability of sites to support native species. Once these things are better understood, the development of efficient 

measures that can be used to track conditions over time is recommended. 

• Urban ecosystems: Research recently conducted in the Lake Tahoe basin 

identified substantial and unexpected effects of urban development and 

human activities on various elements of biological diversity. The patterns 

observed varied by taxonomic group (i.e., birds, small mammals, mammalian carnivores, ants, and plants), among species (some were sensitive 

while others were not), and by type of human disturbance (e.g., habitat loss, 

habitat alteration, habitat enrichment, or different types of human activities). The results of that work suggest the need to understand mechanisms 

of key responses such that development and management can be conducted 

in a manner that minimizes and/or mitigates negative effects on biological 

diversity. Questions of particular priority pertain to better understanding 

thresholds of change in habitat loss, habitat alteration, or habitat use indicated by past research, specifically changes observed at 30 to 40 percent 

development. Above this threshold, it is unclear what happens, but sites 

likely become sinks, traps, or abandoned by a wide array of species. In 

addition to site-scale mechanisms, landscape-wide modeling is recommended to understand the implications of existing results and facilitate the 

rapid evaluation of the implications of new information on landscape design 

and management priorities. Finally, as in other subthemes, as vulnerable 

species and target ecological objectives in more urban areas are clarified, 

development and testing of effective measures are recommended for use in 

monitoring and assessment. 

• Recreation: Recreational uses have been identified to have substantial 

effects on the occupancy and abundance of many and diverse species based 

on multiple research projects. Alternatively, species thought to be impacted 

by certain recreational activities (e.g., effects of off-highway vehicle use 

on occurrence of American marten) did not exhibit negative responses. 

Specific uncertainties in the Tahoe basin pertain to the effects of dogs on 

retaining biological diversity in more urban environments, and the effects 
xxiii

of ski areas on montane obligates, namely American marten. Although population sizes of northern goshawk and bald eagle are limited, their sensitivity to the presence of people has important implications for the management 

of events and on-going recreational uses in the vicinity of known use sites. 

Effective measures of use and effects are lacking and their development 

will be important for monitoring. 

• Climate change: Climate change is perhaps the ultimate source of uncertainty, and arguably poses a high environmental and economic risk. Under 

such circumstances, information needs start at the basics. In the case of 

ecological elements and processes, this translates to applying existing and 

new information to modeling potential responses—plant and animal ranges 

and associated effects on population sizes, species interactions, and ecological services—to predicted or potential climate change and associated broadscale environmental responses. It is important for modeling to be conducted 

in the basin, as opposed to relying on modeling outside the basin or at larger 

scales because detailed information will be needed to inform how management can respond to potential threats. As with all other subtheme areas, as 

information is accrued, effective and reliable measures of key population and 

community metrics are recommended to be developed for monitoring. 

Integrating the Social Sciences in Research Planning 

The evaluation of social science research needs was included in this science plan 

to explicitly recognize that the processes underlying environmental degradation, 

as well as their conservation and recovery, are fundamentally based on human 

decisionmaking. Effective conservation and restoration policies rely on a wellgrounded understanding of human behavior, the interaction of social and natural 

processes, and linking information from the social sciences to decisionmaking. 

The goals of the research strategy in chapter 7 are to (1) identify social science 

data and research needs for management objectives that are not directly environmental, (2) describe the research needs necessary to improve policy design 

and implementation for managing environmental conditions, and (3) develop a 

framework for prioritizing social science research needs in areas that focus on 

the interaction between the human and natural environments. Near-term 

recommended social sciences research priorities include:

• Recreation:

▪ Develop a consistent, basinwide 3-year recreational survey cycle to 

track seasonal changes, crowding behavior, and long-term patterns 

of recreational activity change.
xxiv

▪ Statistical analysis of the factors associated with recreational 

quality in order to determine visitor preference and direct future 

infrastructure investment.

▪ Analysis of recreational capacity in terms of ecological impact 

and infrastructure at both the specific site level and basinwide.

• Transportation:

▪ Collection and analysis of current bicycle use data in order to determine 

road and trail use, and to determine adequacy of current bicycle routes 

and the potential for use as an alternative transportation mode.

▪ Collection and analysis of public transit ridership data to and within 

the basin.

▪ Centralized collection, inventory and distribution of transportation 

data at a basin level.

▪ Develop a centralized Web-based information source about all 

basin-level public transportation systems.

▪ Analysis of effective policy options for addressing parking in 

high-impact areas.

▪ Predictive modeling on vehicle use, transit ridership, demographics, 

and linkages between economic health and emergency preparedness.

• Economics:

▪ Increased data collection and analysis of housing trends at the 

community and basin level.

▪ Increased data collection and analysis of employment trends at 

the community and basin level.

▪ Increased data collection and analysis of tourism trends at the 

community and basin level.

▪ Estimate affordable housing availability for year-round residents, 

public sector employees and workers in service and hospitality 

industry.

▪ Evaluation, prioritization and coordination of Tahoe basin 

environmental improvement projects.

▪ Development of community sustainability and well-being indicators.

▪ Program evaluation of the effectiveness and cost-efficiency of 

existing policies.

▪ Analysis of effective policy options for increasing affordable 

housing options.
xxv

• Scenic resources:

▪ Examination of public perceptions of the scenic quality of natural 

and built environments.

▪ Development of new TRPA Scenic Quality indicators.

▪ Analysis of effective policy options for increasing building design 

diversity.

▪ Analysis of effective policy options for improving overall scenic quality.

• Noise:

▪ Increasing spatial and temporal monitoring to capture single-event 

noise levels.

▪ Research on public acceptability and perception of current noise levels.

• Collaborative information management: 

▪ Information synthesis and knowledge-sharing about effective approaches 

for recreation infrastructure, and about mitigating environmental impacts 

in other alpine resort communities.

• Tahoe basin community management: 

▪ Consolidation of existing information, design of new collection 

instruments, and examination of methods for upscaling data to the 

Tahoe basin level and/or downscaling to the community level.

• Program design, policy evaluation, and policy process evaluation: 

▪ Develop policy conflict resolution mechanisms for programs working 

at cross purposes. Target research on policy evaluation and program 

design of costly programs.

• Fire and natural hazards: 

▪ Develop a basinwide emergency communications network, implement 

reforms to fuels management on private parcels, and begin extensive 

inter-agency review of basinwide coordination of fuels management.

• Climate change: 

▪ Develop predictive models of climate change impacts on the Tahoe 

environment. Focus on outputs that can communicate potential outcomes 

to local government, the general public, and agency personnel.

Those reading multiple chapters of this science plan will soon realize the 

research strategies differ in scope and breadth. These differences are due to the 

diversity of management issues that exist among the theme areas and to variations 

in the state of knowledge. Past efforts to obtain knowledge in each of the theme 

areas have not been equal. This means different levels of investment are needed 
xxvi

to progress from this point forward. For example, we now have an operating Lake 

Tahoe clarity model that can be used to predict future conditions and analyze the 

effects of alternative management strategies aimed at improving Lake Tahoe water 

clarity. Thus, some research needs identified in the “Water Quality” research 

strategy will include recommendations to improve the validity and predictive 

capabilities of this model. In contrast, we are struggling to obtain and aggregate 

basic regional socioeconomic data for the Lake Tahoe basin that can inform us 

about trends in the human environment. These differences in knowledge base 

compromise our ability to understand and quantify interactions among resources, 

habitats, processes, and stressors. Continued commitment of future resources and 

funding across all five theme areas is recommended as the best strategy to even out 

the disparity in our knowledge base.

The near-term research priorities presented in the research strategies and above 

are based on the input of agency and stakeholder representatives received during 

subtheme identification, as well as the best professional judgment of the authors. 

Several factors (e.g., changing agency priorities, funding levels, the emergence of 

new issues or new information, and the availability of new technologies) can simultaneously affect the applicability of chosen research priorities. Thus, the selected 

priorities are best reviewed and revised regularly to ensure the current science 

needs and priorities reflect the changing information needs and evolving priorities 

of agencies charged with the welfare of the Lake Tahoe basin. For this reason, this 

science plan is considered a living document. The agency, stakeholder, and science 

community representatives active in the Lake Tahoe basin all share the continuing 

responsibility to revisit and update this document in the future. 

Each research strategy is meant to serve as a stand-alone document. We think 

this organizational approach is most useful because government agency representatives and stakeholders often seek issue-specific information. This organizational 

approach also should aid those agencies dealing with multiple theme areas, because 

they are organized in distinct programs that generally coincide with the theme 

areas. However, this stand-alone approach affected the way cross-cutting issues 

are treated. In preparing this science plan, several issues that cut across multiple 

theme areas were identified:

• Quantification of key environmental indicators 

• Model application and development 

• Adaptive management functionality and effectiveness 

• Research and policy implementation 

• Effects of climate change

• Effects of fire
xxvii

The stand-alone organization of the research strategies means information on 

cross-cutting issues is presented in multiple chapters. For example, those wanting 

to learn about the research recommended to improve our understanding of climate 

change effects will need to review the appropriate section in several research strategies. Although this organizational approach means the reader will have to do more 

work to synthesize information on cross-cutting issues, this approach does allow for 

better integration of cross-cutting issues within each research strategy.

The target audience for information presented in this document includes those 

individuals within government agencies and the stakeholder community that have a 

role in the protection and management of the Lake Tahoe basin ecosystem. We hope 

this document is of particular use to those individuals who find themselves responsible for deciding if and how new funding for science should be allocated.

Acknowledgments

Numerous individuals contributed to the development of this document. Most of 

these people are acknowledged in the individual chapters and their names are not 

repeated here. In addition, however, the editors and authors would like to acknowledge the following individuals and funding sources. Jane Freeman and Jack Landy 

were the U.S. Environmental Protection Agency contract managers overseeing the 

grant that funded the development and production of this document. Jack Landy 

also managed the independent review of this document. Wendy Trowbridge took 

on the arduous task of formatting this document for general technical report review 

and production with vigor and professionalism. We also acknowledge and appreciate the work of Peter Goin, who provided many of the photographs included in this 

document. Core members of the Tahoe Science Agency Coordination Committee 

provided intellectual and moral support throughout the process to produce this science plan. These individuals include Charlie Donohue, Joey Keely, Lauri Kemper, 

Jason Kuchnicki, Sue Norman, Shane Romsos, Hannah Schembri, and Tricia York. 

Their continued support for this science plan will be valuable when the document 

becomes available for general use, and when it comes time to update the research 

strategies. Southern Nevada Public Lands Management Act funding was provided 

to prepare and produce this document through U.S. Environmental Protection 

Agency Cooperative Agreement X7-96963901-0. 
xxviii

Contents

1 Chapter 1: Overview

Zachary Hymanson

1 Introduction

2 Approach for Developing This Science Plan

5 A Review of Science Planning and Support in the Lake Tahoe Basin

10 Advancing an Applied Science Program in the Tahoe Basin

11 Geographic Scope and Environmental Setting

13 Using This Document and Target Audience

15 English Equivalents

15 References

19 Chapter 2: Conceptual Framework for an Integrated Science Program

Patricia N. Manley, Dennis D. Murphy, and Zachary P. Hymanson

19 Introduction

20 Monitoring

24 Research

25 Data Application

28 Adaptive Management

30 Science Plan Conceptual Models

35 References

37 Chapter 3: Air Quality

A.W. Gertler, T.A. Cahill, T.M. Cahill, S.S. Cliff, C. Emmett, J. Koracin, 

H. Kuhns, J.V. Molenar, J. Quashnick, and J.E. Reuter

37 Introduction

38 Conceptual Model

41 Tahoe Basin Meteorology

44 Atmospheric Deposition of Nitrogen, Phosphorus, and Particles

48 Local Versus Regional Transport of Air Pollutants

52 Tahoe Basin Air Quality: The Criteria Pollutants

56 Visibility in Lake Tahoe Basin

61 Air Pollutant Emission Inventories

65 Atmospheric Modeling of the Lake Tahoe Basin

69 Impact of Fire on Air Quality

75 Near-Term Research Priorities
xxix

76 English Equivalents

76 References

83 Chapter 4: Water Quality

John E. Reuter, James M. Thomas, and Alan C. Heyvaert

83 Introduction

85 Review of Important Background Elements

86 Anticipated Water Quality Topics Requiring Additional Data, 

Research, and Modeling

87 Water Quality Conceptual Model

89 Lake Tahoe Water Clarity

100 Lake Tahoe Near-Shore Water Quality

106 Pollutant Loading From Urban Sources

119 Stream Channel Erosion

122 Water Quality Treatment

131 Upland Watershed Function—Hydrology and Water Quality

137 Water Quality and Forest Biomass Management Practices

145 Drinking Water Protection

148 Water Quality Modeling

162 Water Quality Research Priorities

165 English Equivalents

166 References

183 Chapter 5: Soil Conservation

W.W. Miller, E.M. Carroll-Moore, H. Tretten, and D.W. Johnson

183 Introduction

184 Conceptual Model

185 Key Soil Properties and Conditions

196 Development and Application of Predictive Models as Related to 

Soil Conservation

203 Effects of Climate Change as Related to Soil Conservation

207 Policy Implementation and Adaptive Management Strategies as 

Related to Soil Conservation

222 Near-Term Soil Conservation Research Priorities

227 English Equivalents

227 Acknowledgments

228 References
xxx

237 Chapter 6: Ecology and Biodiversity

Patricia N. Manley, Dennis D. Murphy, Seth Bigelow, Sudeep Chandra, 

and Lisa Crampton

237 Introduction

242 Old-Growth and Landscape Management

247 Fire and Fuels Management

252 Special Communities Management

262 Aquatic Ecosystem Restoration

274 Urbanization

280 Recreation

283 Climate Change

285 Summary of Near-Term Research Priorities

288 Acknowledgments

289 References

303 Chapter 7: Integrating the Social Sciences in Research Planning

Derek Kauneckis and David Halsing

303 Introduction

319 Conceptual Model

320 Recreation

325 Transportation

331 Economics

339 Scenic Resources

343 Noise

345 Metathemes and Emerging Areas of Interest

359 Near-Term Social Science Research Priorities

362 Acknowledgments

362 References
1

Zachary P. Hymanson2

Introduction

A complex suite of interrelated changes has occurred in and around the Lake Tahoe 

basin over the last 150 years. These changes have substantially affected the atmospheric, aquatic, and terrestrial environments, as well as socioeconomic conditions 

in the basin (Elliott-Fisk et al. 1996, Lindström et al. 2000). Human responses to 

these changes have taken many forms; however, the overall trend shows a transition from policies favoring unrestricted use of habitats and living resources for 

development and economic benefit, to policies favoring limitations on development 

and increased habitat conservation and restoration (see Elliott-Fisk et al. 1996 

for a review). This transition ultimately led to the policy declaration establishing 

the Tahoe Region Planning Compact (Public Law 96-551), which aims to ensure 

equilibrium between the region’s natural endowment and its human-developed 

environment, and to the subsequent state of California designation of Lake Tahoe 

as an Outstanding National Resource Water under the Federal Clean Water Act 

(LRWQCB 1995).

The known effects of past actions and the unique character of the Lake Tahoe 

basin have led to broad-based support for substantive conservation and restoration 

efforts over the last two decades (CTC 2006, Elliott-Fisk et al. 1996, Murphy and 

Knopp 2000, U.S. Public Law 106-506 2000, TRPA 2001). Increased attention and 

funding over the past decade, in particular, have resulted in remarkable progress 

toward restoration goals, along with considerable information on the strengths 

and weaknesses of different approaches to addressing the substantial restoration 

challenges (Elliott-Fisk et al. 1996; Murphy and Knopp 2000; TRPA 2002, 2007). 

Restoration has focused not only on Lake Tahoe, but also on the entire watershed. 

Special attention has been given to the highly interdependent nature of terrestrial 

and aquatic habitats, and the multifaceted socioeconomic conditions that influence 

the Tahoe basin ecosystem (Elliott-Fisk et al. 1996, Murphy et al. 2000). The 

Lake Tahoe basin is recognized as a highly complex physical, biological and 

social environment, and the challenges posed by its restoration and continued 

management for multiple benefits are paralleled by few other locations. 

Chapter 1: Overview1

1 Citation for this chapter: Hymanson, Z.P. 2009. Overview. In: Hymanson, Z.P.; Collopy, 

M.W., eds. An integrated science plan for the Lake Tahoe basin: conceptual framework 

and research strategies. Gen. Tech. Rep. PSW-GTR-226. Albany, CA: U.S. Department of 

Agriculture, Forest Service, Pacific Southwest Research Station: 1–17. Chapter 1.

2 Tahoe Science Consortium, Incline Village, NV 89451.
2

GENERAL TECHNICAL REPORT PSW-GTR-226

Conservation and restoration of the Lake Tahoe basin ecosystem have required 

the sustained engagement of federal, state and local governments, as well as the 

private sector. These entities have worked together to develop and implement a 

variety of programs and activities aimed at achieving common environmental and 

social goals (TRPA 2001, 2007). Yet determining how to proceed with conservation 

and restoration efforts in the face of limited information remains a central challenge 

to these efforts. Science (e.g., monitoring, research, and modeling), particularly 

applied science completed to inform adaptive management, provides a promising 

set of tools to address information limitations that affect our ability to select and 

implement effective management strategies. The coordination of scientific activities with management actions is at the core of an effective adaptive management 

approach (Manley et al. 2000). However, effort is required to organize and describe 

the science activities needed to inform an adaptive management system focusing on 

the conservation and restoration of a complex system. This document presents the 

results of science community efforts to organize and describe the initial elements 

of an integrated science plan for the Lake Tahoe basin: a conceptual framework for 

science operating in an adaptive management system, and focused research strategies covering topic areas of relevance to Tahoe basin management and conservation. Separate, agency-led efforts are underway to develop other essential elements 

of an integrated science plan, including programs for status and trends and effectiveness monitoring, new data applications aimed at converting data into information and knowledge, and the integration of monitoring and applied research efforts.

Approach for Developing This Science Plan

This science plan was developed through a collaborative effort among agency, 

science community, and stakeholder representatives to identify and refine science 

information needs for the Lake Tahoe basin. The main purpose of this effort was 

to develop a set of research strategies to address key uncertainties and information 

gaps that challenge resource management and regulatory agencies. The research 

needs identified in these strategies are based on assessments of the issues and 

information needs that currently confront government agencies and stakeholders 

working in the basin. The resulting strategies are intended to guide future research 

efforts and to help maximize the information gained from future science investments. 

The plan begins by presenting an overview of a conceptual model and framework that identify issues of concern in the Tahoe basin and describe how science 

can work to inform polices, management strategies, and actions within the context 

of a Lake Tahoe basin adaptive management process. Next, the plan presents 
3

An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

five chapters describing focused research strategies for the theme areas of (1) air 

quality, (2) water quality, (3) soil conservation, (4) ecology and biodiversity, and 

(5) integrating the social sciences in research planning. Each research strategy 

describes the knowledge gaps and research needs for relevant issues of concern (i.e., 

subthemes). Scientists worked with resource management agency representatives 

and stakeholders to identify relevant subthemes (e.g., fire and fuels management 

is one subtheme under the Ecology and Biodiversity theme area), and to identify 

the management issues and information needs associated with each subtheme (e.g., 

minimizing adverse impacts to wildlife is one management issue under the fire and 

fuels management subtheme). For each subtheme, the authors summarize the current state of knowledge, identify remaining uncertainties and knowledge gaps, and 

list science activities that address the uncertainties and knowledge gaps. 

The research strategy theme areas were chosen based on an examination of 

resource areas considered in four management plans or planning processes: (1) the 

1987 Lake Tahoe Basin Regional Plan, (2) the 2000 Lake Tahoe Watershed Assessment, (3) the Pathway planning process,3

 and (4) update of the Environmental 

Improvement Program for the Lake Tahoe basin (table 1.1). 

Three common needs drive the recommendations presented in each research 

strategy:

• Increasing our understanding of the factors and processes driving change.

• Developing the tools and understanding to predict future conditions in the 

Lake Tahoe basin and permit comparisons among alternative futures.

• Providing information to inform future management decisions aimed at 

conserving and restoring the natural and human environments of the Lake 

Tahoe basin.

Each research strategy concludes with a presentation of near-term research 

priorities. Near-term research priorities are based on input received from agency 

and stakeholder representatives during subtheme identification, as well as the 

best professional judgment of the authors. Several factors (e.g., changing agency 

priorities, funding levels, the emergence of new issues or new information, and 

3 The Pathway planning process (formerly known as Pathway 2007) is a collaborative planning effort among four partner agencies, including the Tahoe Regional Planning Agency, 

USDA Forest Service, the California Lahontan Regional Water Quality Control Board, and 

the Nevada Division of Environmental Protection. These agencies are working together to 

update important resource management plans for the Lake Tahoe basin, which will guide 

land management, resource management, and environmental regulations over the next 

20 years. This planning process is ongoing and is referred to as “Pathway” or “Pathway 

planning process” in this document. More information about the Pathway planning process 

is available at http://www.pathway2007.org/.
4

GENERAL TECHNICAL REPORT PSW-GTR-226

Table 1.1—A comparison among theme areas considered in four Lake Tahoe basin planning documents or 

processes and the Lake Tahoe Science Plan 

TRPA 1987 

Regional Plan: 

threshold categories

2000 Lake Tahoe 

watershed assessment

Pathway planning 

process

Environmental 

Improvement 

Program—update

Lake Tahoe science 

plan theme areas

Air qualitya Air quality Air quality Improving air quality 

 and transportation

Air quality

Soil conservation/ 

 stream environment 

 zone (SEZ)

Upland water quality/ 

 sediment and nutrient 

 discharge

Soil conservation 

 and SEZ habitats

Habitat and vegetation Soil conservation

Water qualitya Water quality Water quality Storm water management Water quality

Vegetationa

Biological integrity and 

 aquatic resources

Vegetation and 

 forest fuels

(1) Forest health and 

 fuels management

(2) Habitat and vegetation Ecology and 

Wildlife biodiversity a

Wildlife and fisheries

(1) Habitat and vegetation

(2) Watershed management

(3) Threatened, endangered, 

 and sensitive species Fisheries

—

Recreation

Socioeconomics

Socioeconomics

Recreation

—

Enhancing recreation 

 and scenic resources

Integrating the social 

 sciences in research 

 planning — — Transportation Improving air quality 

 and transportation

Scenic resources Scenic quality and 

 resources

Enhancing recreation 

 and scenic resources

Noise — Noise —

— Adaptive management 

 strategy

Lake Tahoe adaptive 

 management system

Program support and 

 applied science program

Science plan framework 

 and overview 

 conceptual model

a Theme areas considered in the June 6, 2001, key management questions. See “A Review of Science Planning and Support 

in the Lake Tahoe Basin” below for more information about development of the key management questions. 

— = not applicable, TRPA = Tahoe Regional Planning Agency. 

the availability of new technologies) can simultaneously affect the applicability 

of chosen research priorities. Thus, the selected priorities are best reviewed and 

revised regularly to ensure the current science needs and priorities reflect the 

changing information needs and evolving priorities of agencies charged with the 

welfare of the Lake Tahoe basin. For this reason, this science plan is considered a 

living document. The agency, stakeholder, and science community representatives 

active in the Lake Tahoe basin all share the continuing responsibility to revisit and 

update this document in the future. 
5

An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

A Review of Science Planning and Support in the 

Lake Tahoe Basin

Over the last 30 years, there have been several efforts to organize and describe 

the science needed to improve our understanding of the Lake Tahoe basin ecosystem and inform management actions. The most substantial efforts include 

the following:4

• Research Needs for the Lake Tahoe Basin (LTARCB 1974). A National 

Science Foundation funded project, which aimed to “encourage research 

needed to achieve the planning and management objectives of public and 

private entities” and to “provide scientific expertise and data to support 

effective planning and management programs.” Information shortfalls that 

compromised management of the Tahoe basin’s air, water, vegetation, fish 

and wildlife, social sciences, and resource systems were identified. More 

than 80 separate research needs were proposed. 

• Lake Tahoe Environmental Assessment (WFRC IRTF1979). The compilation and analysis of information prepared in support of this assessment 

evaluated data for a number of resources, habitats, and socioeconomic factors. These were pivotal evaluations formally introducing the concepts of 

carrying capacity and environmental thresholds, which were central to the 

scientific underpinnings of the 1987 Lake Tahoe Basin Regional Plan.

• Lake Tahoe Case Study (Elliott-Fisk et al. 1996). This document took a 

science-based approach to provide an ecosystem and policy assessment of 

the Lake Tahoe basin. The case study synthesized information from these 

assessments to inform the Sierra Nevada Ecosystem Project, and identified 

future science-based management needs for the Lake Tahoe basin.

• Environmental Improvement Program (EIP) (TRPA 2001). The EIP 

described a series of program areas and projects, which if implemented, 

would advance the Lake Tahoe basin toward attainment of the environmental thresholds identified in the 1987 Lake Tahoe Basin Regional Plan. 

The science and research portion of the EIP (updated and expanded in 

2001) identified threshold and EIP-related research and monitoring projects 

designed to (1) advance scientific understanding of ecosystem processes 

and threshold attainment, (2) refine planning and restoration strategies, and 

(3) improve and quantify the effectiveness of capital improvement projects.

4 Information on science planning is taken in part from Murphy (2000).
6

GENERAL TECHNICAL REPORT PSW-GTR-226

• Lake Tahoe Watershed Assessment (Murphy and Knopp 2000). This document provided a synthesis of 20 years of research publications and reports 

dealing with the atmospheric, aquatic, and terrestrial environments; the living 

resources associated with these environments; and socioeconomic conditions. 

Like the documents that precede it, this assessment included recommendations for research and monitoring. The assessment also presented an adaptive 

management strategy, describing a means of organizing current information, 

and linking management planning with essential science activities.

• Key Management Questions (SAG 2001). Scientists and agency representatives worked together to develop a list of Key Management Questions (KMQ) 

to direct new research and monitoring efforts in the Lake Tahoe basin. Some 

of the KMQs were periodically revised and updated (2002–04) to reflect 

the most important questions that land managers, project implementers, and 

regulators had about land use decisions and methods to improve ecosystem 

health in the Lake Tahoe basin. Tahoe basin agency executives prioritized 

some of the KMQs. This information was used by federal and state agencies 

to develop budgets for future science funding.

The documents described above range from lists of research needs and 

questions to indepth reviews and issue-specific analyses. In some cases, the 

documents also describe processes and approaches for obtaining new scientific 

information. To varying degrees, all of these efforts provided recommendations 

for the kinds of science activities, and in several cases, the specific studies 

needed to address existing uncertainties and information gaps. 

In several cases, it was implicitly assumed that providing a description of the 

science needs for the Lake Tahoe basin would lead to establishment of a sustained 

program for addressing those needs. However, such a program has never been 

established and, with the exception of water quality, information gains for many 

critical issue areas generally lag behind the information needs of managers and 

policymakers. In some cases, these information gains are lacking because the 

necessary studies have not been initiated or completed. In other cases, these 

information gains have not been realized because of a lack of synthesis and 

analysis of existing data. 

With the exception of the KMQs, none of the science plans prepared for the 

Lake Tahoe basin have been supported by sustained science planning processes that 

provide a means for objective prioritization and regular revision. Unfortunately, the 

KMQ planning process eventually fragmented with different issue areas receiving 

varying degrees of attention. Functional science planning processes are critical to 
7

An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

ensuring that a science program remains relevant and responsive to management 

information needs. Support for science planning also is needed to ensure that any 

funds available for new science address the highest priority information needs. 

In addition to efforts to organize and describe science needs, other efforts 

have improved the organization of science entities and implementation of science 

activities in the Tahoe basin. A persistent challenge has provided the motivation to 

improve science organization and implementation:5

For years the academic and management communities could not agree on 

a scientific agenda that would answer both key management and research 

questions. Many times, scientific work was not deemed pertinent to what 

the agencies wanted, and the management questions were not articulated in 

ways the science community could use.

Explicit efforts to address this challenge were formalized in 1999 with the 

signing of a memorandum of understanding (MOU) with a primary focus on 

priority research, monitoring, evaluation, and outreach supporting Tahoe basin 

management goals. The entities signing this MOU (Tahoe Regional Planning 

Agency [TRPA], University of Nevada, Reno [UNR], Desert Research Institute 

[DRI], University of California, Davis [UCD], U.S. Department of the Interior, 

Geological Survey [USGS], and the U.S. Department of Agriculture, Forest 

Service [USFS]) committed to work collaboratively to:

• Establish a joint steering committee to evaluate the EIP to determine 

what environmental issues may benefit from broader research inquiry.

• Further develop and improve the communication and coordination among 

existing research groups working in the Tahoe basin to prevent duplication 

of efforts and provide the maximum interdisciplinary teamwork necessary 

to resolve the most important environmental issues.

• Contribute to TRPA’s development of a research master plan and set of 

guiding principles for research inquiry in the Lake Tahoe Region related 

to priority preservation, restoration, and enhancement needs. 

• Encourage the development of competitive research proposals with 

peer review to achieve the highest caliber of scientific assessment of 

problems facing the Lake Tahoe region.

• Identify monitoring tasks and evaluations that would be assured 

continuance to fully inform the research community evaluating the 

Lake Tahoe region.

5 York, T. 2008. Personal communication. Environmental scientist. California Tahoe 

Conservancy, 1061 Third Street, South Lake Tahoe, CA 96150.
8

GENERAL TECHNICAL REPORT PSW-GTR-226

• Develop outreach plans to convey research results and options to the public 

in general and specifically to the communities within the Lake Tahoe region.

In 2000, the TRPA requested the MOU participants convene a Lake Tahoe 

Science Advisory Group (SAG). The SAG also included representatives from 

numerous state, federal, and local agencies. Key objectives of the SAG were to (1) 

develop a scientific work plan that would dovetail with restoration efforts and management objectives and (2) coordinate/facilitate the operating principles of a Tahoe 

Environmental Science System identified in a second MOU signed in February 

2000. The SAG, in conjunction with a number of work groups active in the Tahoe 

basin, identified critical information needs requiring attention for effective management decisions. This was intended to be an ongoing process that included two key 

components: (1) development of a series of KMQs, and (2) development of a science 

plan, produced by the research institutions, which addressed the KMQs. Although 

the SAG did prepare KMQs for a number of theme areas (table 1.1 and summarized 

above), funding and resource commitments to produce a science plan did not occur.

Over time, support and commitment to the SAG and its goals declined. Select 

agencies did pursue new funding initiatives to support scientific investigations 

related to specific agency needs (e.g., technical studies for the Lake Tahoe total 

maximum daily load, and the Lake Tahoe Air Deposition Study). However, with the 

exception of water quality studies, efforts to organize collaborative science efforts 

within and among issue areas continued to struggle. 

In 2003, the federal government began providing funding to support 

implementation of the Lake Tahoe Restoration Act (U.S. PL 106-506 2000). Some 

of this funding was reserved for new science; however, the persistent challenge 

identified above continued to compromise the processes for identifying science 

needs and the objective selection of projects for funding. In addition, all parties 

recognized that the SAG had neither the charge nor the resources to develop 

a vision, structure, and program capable of supporting science and research 

as it applies to Lake Tahoe basin restoration (TRPA and USCOE 2005). Thus, 

discussions were initiated in December 2003 among members of the science 

community and staff from the TRPA, the U.S. Environmental Protection Agency, 

and the U.S. Army Corps of Engineers to evaluate and possibly restructure SAG 

operations to better support allocation of federal funding under the Lake Tahoe 

Restoration Act. These discussions led to a proposal for an enhanced science 

community group known as the Tahoe Science Consortium (TSC). In 2005, 
9

An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

science community and agency representatives signed an MOU forming the TSC,6

and establishing its primary objective: “To provide environmental managers and 

decision makers with comprehensive and well-synthesized scientific findings 

drawn from research, monitoring, and modeling.” The TSC is a partnership among 

five research organizations: (1) UNR; (2) UCD; (3) DRI; (4) the USFS, Pacific 

Southwest Research Station, and (5) the USGS, Carson Science Center. The TSC 

operates independently of the management and regulatory agencies working in the 

Tahoe basin, and TSC efforts have focused on: 

• Science planning: Working with Lake Tahoe basin agency representatives to develop regional monitoring approaches for specific issue areas, 

and developing a science plan for the Lake Tahoe basin that identifies and 

prioritizes research needs. These efforts are intended to contribute to the 

primary objective of the TSC. 

• Peer review: Administering or conducting the scientific peer review of 

research proposals, science products, or technical programs related to 

Lake Tahoe basin management and restoration. Independent peer review is 

intended to ensure that science activities conducted in the basin are scientifically sound, and that the results are technically credible. 

• Technical assistance: Providing scientific input and technical advice to 

resource management and regulatory agencies that addresses management 

issues and concerns as they arise. 

After nearly a decade of effort, agency and science community representatives 

have made progress in addressing the persistent challenge affecting science organization and implementation. Formation of the TSC concurrently with increased 

investments and commitments by the TSC partners represents major efforts of the 

science community to better address Lake Tahoe basin science needs on a sustained 

basis. In addition, the establishment of two new working groups (the Tahoe Science 

Agency Coordination Committee, and the Science and Management Integration 

Team) has provided greater organizational capacity for agencies to communicate 

management issues and information needs to the science community, and to provide 

the ongoing support for science planning processes. Efforts now focus on sustaining 

progress.

6 More information about the TSC and its member organizations is available at 

http://www.tahoescience.org/.
10

GENERAL TECHNICAL REPORT PSW-GTR-226

Advancing an Applied Science Program 

in the Tahoe Basin

Clear policy direction and agency commitments are essential to advancing an effective science program. The Lake Tahoe Restoration Act (PL 106-506) sets forth the 

primary policy directive for providing and sustaining an effective science program 

in the Tahoe basin:

The Secretary shall provide for continuous scientific research on and monitoring of the implementation of projects on the [EIP] priority list, including 

the status of the achievement and maintenance of environmental threshold 

carrying capacities.

This overarching policy directive is supported by several policies and goals 

within the TRPA (TRPA 1986) and through implementation of the Federal Vision 

for the EIP (Lake Tahoe Basin Executives 2006). 

Fundamentally, science comprises several practices and principles, which are 

applied in an integrated fashion to provide objective and verifiable approaches to 

acquiring new information that addresses uncertainties and knowledge gaps. To 

effectively support the ongoing information needs of resource management and 

regulatory agencies, these practices and principles are best organized and implemented as an integrated science program that includes the means to provide timely 

information in formats useful to agency representatives and decisionmakers. 

Conceptually, the efforts and activities of an integrated science program can 

be divided among three basic elements: (1) monitoring, (2) empirical research, and 

(3) data application. To be effective, however, efforts need to be integrated across 

all three elements, and the allocation of resources among all elements is essential. 

This is the basis for the conceptual framework presented in chapter 2.

The brief review of past science planning efforts presented previously shows 

that merely producing a science plan is not enough to ensure the establishment of a 

sustained science program that can deliver useful information covering a diversity 

of issues. Clear policy direction is essential, but a deeper level of commitment 

among all relevant parties would enhance progress from planning to implementation. An explicit assumption of this effort is that the agencies charged with responsibility for the welfare of the Lake Tahoe basin will work to establish the funding, 

resources, and infrastructure necessary for sustained implementation of an applied 

science program. The TSC is prepared to work with agency representatives to make 

a sustained science program for the Lake Tahoe basin a reality.
11

An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

Geographic Scope and Environmental Setting7

Lake Tahoe and its tributary watersheds together make up the Lake Tahoe basin 

(fig. 1.1). In most cases, the Lake Tahoe basin encompasses the entire geographic 

scope of this science plan. Where appropriate, however, the scope is broadened to 

consider external factors (e.g., regional meteorology or climate change) that can 

substantially influence conditions or future management actions within the basin. 

The Lake Tahoe basin lies in the east-central portion of the Sierra Nevada 

mountain range and on the western boundary of the Great Basin. It is a montane, 

lacusterine-dominated ecosystem with several physical characteristics that make it 

a unique feature of the Sierra Nevada mountain ecosystem (table 1.2).

The broad elevation range of the basin (1900 to 3050 m [6,200 to 10,000 ft] 

above sea level) and a topography that strongly controls precipitation and temperature combine to yield a wide diversity of montane vegetation types, ranging from 

coniferous forests and woodlands, riparian forests, subalpine to alpine meadows, 

various wetland communities, and Great Basin shrublands. Soils are thought to 

act as a secondary control (after climate) of vegetation patterns. Geology within 

the basin is dominated by granitic rocks and soils in the southern portions, with an 

overlay of volcanic rocks and soils in the northern portions. The diversity of plant 

communities and vegetation types creates a broad spectrum of wildlife habitats. 

Numerous fish and invertebrate species occupy the stream and lake habitats, but in 

many cases, introduced species dominate (Chandra 2003). 

Human activities have had and continue to have a dominant influence on the 

natural resources and environment of the Lake Tahoe basin. These activities include 

numerous past and present habitat and species modifications (e.g., logging; urban, 

commercial, roads, and infrastructure development; recreation; fire suppression; 

water diversion; species extirpations and introductions; habitat enrichment; and 

habitat restoration). Most anthropogenic activities are considered stressors to the 

natural environment, so restoration projects generally aim to remove or reduce the 

effects of these stressors. Humans and their activities will remain dominant components in the Lake Tahoe basin ecosystem, so ensuring the equilibrium between 

the basin’s natural endowment and its human-provided environment remains the 

primary directive.

7 Information on geographic scope and key physical characteristics is taken in part from 

Elliott-Fisk et al. (1996).
12

GENERAL TECHNICAL REPORT PSW-GTR-226

Figure 1.1—Plan view of the Lake Tahoe basin illustrating select hydrologic and bathymetric features, major roadways, and political 

boundaries. The area of shaded relief indicates the watershed boundary. Courtesy of the U.S. Geological Survey.
13

An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

Using This Document and Target Audience

This science plan includes six chapters that present a conceptual framework and 

focused research strategies for five topics of importance to the Lake Tahoe basin 

ecosystem. Chapter two presents the conceptual framework and overview conceptual model for this science plan. Further, the chapter describes the foundational 

elements and approach for establishing an integrated science program as part of a 

Lake Tahoe basin adaptive management system. The overview conceptual model 

will orient readers to the important issues covered in this plan and the relationships among those issues. Five theme-specific research strategies make up the core 

of this science plan. These chapters cover the topics of air quality, water quality, 

soil conservation, ecology and biodiversity, and integrating the social sciences in 

research planning.

Table 1.2—Key physical characteristics of the Lake Tahoe basin

Characteristic Size Comment

Basin surface area 1300 km2

Land surface area 800 km2

Developed land surface area 83 km2 The developed land area occupies about 10.5 percent of 

the total land area and includes residential, commercial, 

institutional, utilities, and transportation development.

Undeveloped land surface area ~717 km2 The undeveloped land area occupies about 89.5 percent 

of the total land area. This area is dominated by 

undeveloped montane forest habitat.

Lake surface area 500 km2 The lake surface area comprises about 38 percent of

the basin surface area, yielding a watershed area to 

lake area ratio of ~1.6:1.

Maximum lake depth; 502 m; 313 m Lake Tahoe is the 11th deepest lake in the world.

mean lake depth

Lake width and length 19 km × 35 km

Lake volume ~156 km3 The top 6 ft of Lake Tahoe is operated as a draw-down

reservoir with a nominal yield of 903 million m3.

Average water residence time 650 years Average residence time of most pollutants of concern is 

on a decadal time scale or less.

Number of watersheds draining 63 Tributary inflow annually delivers about 430 million m3

into Lake Tahoe to Lake Tahoe.

Number of drainages out of 1 The Truckee River flows northeast from Lake Tahoe to 

Lake Tahoe Pyramid Lake, Nevada.
14

GENERAL TECHNICAL REPORT PSW-GTR-226

The research strategies in this science plan differ in scope and breadth because 

of the diversity of management issues that exist among the theme areas and variations in the state of knowledge. Past efforts to obtain knowledge in each of the 

theme areas have not been equal (TRPA 2007), so different levels of investment 

are needed to progress. For example, we now have a Lake Tahoe clarity model that 

can be used to predict conditions and analyze the effects of alternative management 

strategies aimed at improving Lake Tahoe water clarity. Thus, some research needs 

identified in the “Water Quality” research strategy will include options for improving the validity and predictive capabilities of this model. In contrast, we are struggling to obtain and aggregate basic regional socioeconomic data for the Lake Tahoe 

basin that can inform us about trends in the human environment. These differences 

in knowledge base compromise our ability to understand and quantify interactions 

among resources, habitats, processes, and stressors. Continued commitment of 

future resources and funding across all five theme areas is believed to be the best 

strategy to even out the disparity in our knowledge base.

Each research strategy is meant to serve as a stand-alone document. We think 

this approach is most useful because government agency representatives and 

stakeholders often seek issue-specific information. This approach also should 

aid those agencies dealing with multiple theme areas, because they are internally 

organized across distinct programs that generally coincide with the different theme 

areas. However, this stand-alone approach affected the way cross-cutting issues are 

treated. Through the course of preparing this science plan, several issues that cut 

across multiple theme areas were identified:

• Quantification of key environmental indicators 

• Model application and development 

• Adaptive management functionality and effectiveness 

• Research and policy implementation 

• Effects of climate change 

• Effects of fire

The stand-alone organization of the research strategies means information 

on cross-cutting issues is presented under multiple theme areas. For example, 

those wanting to learn about the research needed to improve our understanding 

of climate change effects will need to review the appropriate section in several 

chapters. Although this organizational approach means the reader will have to do 

more work to synthesize information on cross-cutting issues, this approach does 

allow for better integration of cross-cutting issues within each theme area.
15

An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

The target audience for this document includes those individuals within government agencies and the stakeholder community that have a role in the protection 

and management of the Lake Tahoe basin ecosystem. We hope this document is of 

particular use to those individuals who find themselves responsible for deciding if 

and how new funding for science should be allocated.

English Equivalents

When you know: Multiply by: To get:

Meters (m) 3.28 Feet

Kilometers (km) .621 Miles

Square meters (m2

) 10.76 Square feet

Square kilometers (km2

) .386 Square miles

Cubic meters (m3

) 35.3 Cubic feet

References

California Tahoe Conservancy [CTC]. 2006. California Tahoe Conservancy 

20th anniversary report. South Lake Tahoe, CA. 60 p.

Chandra, S. 2003. The impact of nonnative species introductions and cultural 

eutrophication on the Lake Tahoe food web over time. Davis, CA: University 

of California, Davis. 92 p. Ph.D. dissertation.

Elliott-Fisk, D.L.; Cahill, T.C.; Davis, O.K.; Duan, L.; Goldman, C.R.; Gruell, 

G.E.; Harris, R.; Kattelmann, R.; Lacey, R.; Leisz, D.; Lindstrom, S.; 

Machida, D.; Rowntree, R.; Rucks, P.; Sharkey, D.; Stephens, S.; Ziegler, 

D. 1996. Lake Tahoe case study. Addendum to the Sierra Nevada Ecosystem 

Project; final report to Congress. University of California, Centers for Water 

and Wildland Resources. Wildland Resources Center Report No. 36. http:// 

ceres.ca.gov/snep/pubs/web/PDF/~A_C07.PDF. (October 15, 2008).

Lahontan Regional Water Quality Control Board [LRWQCB]. 1995. 

Water quality standards and control measures for the Lake Tahoe basin. In: 

Water quality control plan for the Lahontan region, north and south basins. 

Chapter 5. http://www.waterboards.ca.gov/lahontan/water_issues/programs/

basin_plan/references.shtml. (October 15, 2008).

Lake Tahoe Area Research Coordination Board [LTARCB]. 1974. Research 

needs for the Tahoe basin. Prepared with the support of the National Science 

Foundation, Office of Intergovernmental Science and Research Utilization, 

Grant No. GI-39556. 29 p.
16

GENERAL TECHNICAL REPORT PSW-GTR-226

Lake Tahoe Basin Executives. 2006. A federal vision for the environmental 

improvement program at Lake Tahoe. http://www.fs.fed.us/r5/ltbmu/local/ 

ltbec/index.shtml. (October 15, 2008).

Lindstöm, S.; Rucks, P.; Wigand, P. 2000. A contextual overview of human land 

use and environmental conditions. In: Murphy, D.D.; Knopp, C.M., eds. Lake 

Tahoe watershed assessment. Volume I. Gen. Tech. Rep. PSW-GTR-175. Albany, 

CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research 

Station. 736 p. Chapter 2.

Manley, P.N.; Tracy, J.C.; Murphy, D.D.; Noon, B.R.; Nechodom, M.A.; 

Knopp, C.M. 2000. Elements of an adaptive management strategy for the 

Lake Tahoe basin. In: Murphy, D.D.; Knopp, C.M., eds. Lake Tahoe watershed 

assessment. Volume I. Gen. Tech. Rep. PSW-GTR-175. Albany, CA: U.S. 

Department of Agriculture, Forest Service, Pacific Southwest Research 

Station. 736 p. Chapter 7.

Murphy, D.D. 2000. Introduction and key findings. In: Murphy, D.D.; Knopp, 

C.M., eds. Lake Tahoe watershed assessment. Volume I. Gen. Tech. Rep. 

PSW-GTR-175. Albany, CA: U.S. Department of Agriculture, Forest Service, 

Pacific Southwest Research Station. 736 p. Chapter 1.

Murphy, D.D.; Knopp, C.M. 2000. Lake Tahoe watershed assessment. Volume I. 

Gen. Tech. Rep. PSW-GTR-175. Albany, CA: U.S. Department of Agriculture, 

Forest Service, Pacific Southwest Research Station. 736 p.

Science Advisory Group [SAG]. 2001. Lake Tahoe basin key management 

questions. South Lake Tahoe, CA. 9 p.

Smith, J.L.; Stone, J.C.; Rowe, T.G.; Gardner, J.V. 1999. Selected hydrologic 

features of Lake Tahoe Basin and surrounding area, California and Nevada, 

1998. Water Resources Investigations Report 99-4031. Reston, VA: U.S. 

Department of the Interior, Geological Survey. 1 sheet.

Tahoe Regional Planning Agency [TRPA]. 1986. Monitoring and evaluation. 

Regional Plan for the Lake Tahoe basin: goals and policies. Stateline, Nevada. 

http://www.trpa.org/documents/docdwnlds/goals.pdf. (October 15, 2008).

Tahoe Regional Planning Agency [TRPA]. 2001. Environmental Improvement 

Program: The cooperative effort to preserve, restore, and enhance the unique 

natural and human environment of the Lake Tahoe region. Stateline, Nevada. 

http://www.trpa.org/default.aspx?tabindex=10&tabid=227. (October 15, 2008).
17

An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

Tahoe Regional Planning Agency [TRPA]. 2002. 2001 Threshold 

Evaluation Report. Stateline, Nevada. http://www.trpa.org/default.

aspx?tabindex=1&tabid=174. (October 15, 2008).

Tahoe Regional Planning Agency [TRPA]. 2007. Lake Tahoe Environmental 

Improvement Program accomplishments report 1997–2006. Stateline, Nevada. 

http://www.tiims.org/Content/Projects/EIP/default.asp. (October 15, 2008).

Tahoe Regional Planning Agency; U.S. Army Corps of Engineers, Sacramento 

District [TRPA and US COE]. 2005. Final letter report. Workgroup to Develop 

the Tahoe Science Consortium Lake Tahoe Basin Nevada and California. On 

file with: U.S. Army Corps of Engineers, Sacramento District, 1325 J Street, 

Sacramento, CA 95814.

U.S. Public Law 96-551. 1980. Tahoe Regional Planning Agency Compact. 

In: HR 8235, 96 United States Congress. (December 19, 1980).

U.S. Public Law 106-506. 2000. Lake Tahoe Restoration Act. In: HR 3388, 

106 United States Congress. (November 13, 2000).

Western Federal Regional Council Interagency Regional Task Force [WFRC 

IRTF]. 1979. The Lake Tahoe Environmental Assessment. [Place of publication 

unknown]. 240 p.
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GENERAL TECHNICAL REPORT PSW-GTR-226
19

An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

Patricia N. Manley,2 Dennis D. Murphy,3 and Zachary P. Hymanson4

Introduction

Uncertainties always accompany the best efforts of environmental and resource 

managers. They are inescapable. Accordingly, many managers seek guidance from 

science to enhance the likelihood that their management actions and restoration 

efforts will meet desired objectives. Science to support land and resource management applies principles and practices in an integrated fashion to acquire objective 

and verifiable information to fill knowledge gaps, thereby reducing uncertainties. 

Scientific principles and practices employed to generate new information include 

the formulation of explicit hypotheses, application of the scientific method (the 

approach for designing experiments to test hypotheses), selection of methods 

to avoid bias (applying techniques that ensure independence and randomness), 

utilization of statistical analyses to transform data into information, and development of tools to transform information into knowledge. To support the information 

needs of land and resource managers and regulatory agencies, scientific activities 

are best organized and implemented through an integrated science program.

Conceptually, the activities carried out under an integrated science program 

that can serve to inform the management, conservation, and restoration of natural 

resources include three basic components—monitoring, research, and data application. Monitoring is used to establish baseline conditions, track management 

activities, and record outcomes. Empirical research seeks to elucidate cause-effect 

relationships, and in so doing enhances the understanding of ecosystem interactions and the effects of management actions in the context of natural environmental variation and anthropogenic influences. Data application includes a broad array 

of activities associated with handling data, including analysis and interpretation; 

modeling; data management; information dissemination; and knowledge application for assessment, evaluation, and decision support. This chapter provides more 

detail—including near-term priorities—for each component of an integrated 

Chapter 2: Conceptual Framework for 

an Integrated Science Program1

1 Citation for this chapter: Manley, P.N.; Murphy, D.D.; Hymanson, Z.P. 2009. Conceptual 

framework for an integrated science program. In: Hymanson, Z.P.; Collopy, M.W., eds. 

An integrated science plan for the Lake Tahoe basin: conceptual framework and research 

strategies. Gen. Tech. Rep. PSW-GTR-226. Albany, CA: U.S. Department of Agriculture, 

Forest Service, Pacific Southwest Research Station: 19–35. Chapter 2. 

2 Pacific Southwest Research Station, USDA Forest Service, Davis, CA 95618.

3 University of Nevada, Reno, Department of Biology, Reno, NV 89557.

4 Tahoe Science Consortium, Incline Village, NV 89451.
20

GENERAL TECHNICAL REPORT PSW-GTR-226

science program. These descriptions are followed by a discussion of adaptive 

management, which describes how an integrated science program can help to 

inform and guide the management of complex systems. The chapter concludes 

with a section that introduces the reader to conceptual models: a tool used 

throughout this science plan. 

Monitoring

Effective monitoring schemes are essential elements in integrated science programs. Monitoring results are often the main source of scientific information 

in support of adaptive management systems. Monitoring is a form of research 

and, like research, is best implemented in an experimental framework that differentiates between alternative explanations of how managed resources respond 

to environmental change. Monitoring has been defined as the “measurement 

of environmental characteristics over an extended period of time to determine 

status or trends in some aspect of environmental quality” (Suter 1993). In the 

Lake Tahoe Watershed Assessment, Manley et al. (2000) provided an expanded 

definition of monitoring, describing three forms of monitoring: 

• Implementation monitoring—The monitoring of management actions 

in relation to planned activities. This form of monitoring catalogues the 

completion of projects or activities. It also documents compliance with 

environmental regulations and mitigation obligations in project implementation. 

• Effectiveness monitoring—The monitoring of the effectiveness of management practices and actions in achieving desired conditions. This type 

of monitoring is recommended as an integral part of capital improvement, 

regulatory, and incentive programs (fig. 2.1), such that the outcomes of 

individual or combined effects of actions taken under various programs 

are known. Effectiveness monitoring does not address the uncertainties 

that are associated with techniques or designs, nor does it attempt to compare the relative effectiveness of different techniques or designs—these 

questions reside in the realm of empirical research. Integration of effectiveness monitoring and research to address priority uncertainties is the 

efficient and desired approach. 
21

An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

Figure 2.1—Illustration of the dynamic interface between an integrated science program (tri-shaded 

box) and the three types of implementation programs used to achieve and sustain the environmental 

thresholds or goals for the Lake Tahoe basin. The arrows indicate the pathways of information 

transfer and communication. 

Monitoring

Empirical research

Data application

Capital

improvements 

Regulatory

requirements 

Incentive

strategies

Environmental

goals 

• Status and trend monitoring—The monitoring of the status and trends of 

resources, habitats, and their agents of change. This is the principal form of 

data gathering that informs management about overall environmental and 

resource conditions relative to established environmental objectives and 

thresholds. Typically, this type of monitoring serves to track the condition 

of indicators that have been selected to represent a set of conditions pertinent to environmental objectives. 

Each of these types of monitoring is necessary to provide information of 

relevance to the management and conservation of habitats and resources in the 

Lake Tahoe basin (fig. 2.2). Integrated and coordinated monitoring programs are 

recommended to provide to agency managers and decisionmakers information 

about how implementation programs are affecting conditions in the Lake Tahoe 

basin. A multifaceted approach involves a dedicated and sustained effort to collect 

and analyze monitoring data on environmental and resource conditions at multiple 

spatial scales (project, watershed, and basin), in order to assess the effects of 

implementation programs, and progress toward achieving and sustaining environmental thresholds. Monitoring programs are best established in a coordinated and 

integrated framework so that information gains are maximized, while monitoring 

costs are minimized. 
22

GENERAL TECHNICAL REPORT PSW-GTR-226

Monitoring Priorities

Several monitoring needs were identified in discussions between scientists and 

managers during the course of developing this science plan. What emerged from 

those discussions is not a comprehensive list of the monitoring needs for the Lake 

Tahoe basin; rather, the list represents priority monitoring information needs 

within the next 5 years. A full array of important monitoring questions still awaits 

articulation by managers, ideally in collaboration with scientists. The allocation 

of resources to address monitoring information needs is outside the scope of the 

science plan, but will be essential to program success.

• Meteorology and climate change. Long-term, spatially relevant meteorological data on air temperature; wind speed and direction; humidity; timing; 

amount, and type of precipitation; annual hydrograph; snowpack; and 

snowmelt are recommended to support environmental studies in all issue 

areas. The technology to capture these kinds of data has advanced, which 

allows them to be collected more efficiently at more locations than in the 

past. These data are fundamental to informing questions about air quality, 

soil conservation, water quality, ecology, biodiversity, and climate change.

Figure 2.2—A conceptual diagram showing the relationships among various types of monitoring and 

the ways in which those types of monitoring can provide information to managers and policymakers. 

The questions also indicate how various types of monitoring provide information at the project, 

program, or, in the case of thresholds, basinwide scales.

Implementation

monitoring

Status and

trend monitoring

Effectiveness

monitoring

Types of 

Monitoring Questions Addressed

What are the conditions of 

natural resources and the 

environment and how do they 

compare to established 

environmental thresholds 

and goals? 

What are the projectspecific outcomes and how 

do they compare to 

planned outcomes?

Did project implementation

comply with regulatory 

conditions and meet 

mitigation obligations?

Informs management 

priorities and 

policy decisions

Informs design and 

implementation of capital

improvement projects

Management/Policy

Implications 

Verifies that implementing

agencies complied with

regulations and fulfilled 

mitigation obligations
23

An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

• Air quality. Improving Tahoe basin air quality monitoring is recommended 

by developing a comprehensive monitoring plan that addresses the criteria 

pollutants covered under the National Ambient Air Quality Standards 

(NAAQS) and those that impact human and ecosystem health (including 

water clarity) in an appropriate spatial and temporal framework. It is 

recommended that data collection use new technologies, including remote 

sensing and biomarkers. Once a plan is prepared, sampling locations can 

be chosen and appropriate air quality sensors can be deployed. Air quality 

and meteorological sensors would benefit from co-location to maximize 

efficiencies and information gain.

• Water quality. Initiate long-term status and trend monitoring of watershed 

hydrology and pollutant loads entering Lake Tahoe to (1) inform Lake Tahoe 

Total Maximum Daily Load (TMDL) land use and lake clarity models, and 

other water quality-related management models; (2) evaluate progress in 

meeting TMDL allocation requirements and other regulatory obligations; 

and (3) evaluate snowpack and snowmelt trends as they pertain to lake clarity. The Lake Tahoe Interagency Monitoring Program (LTIMP) partially 

meets these monitoring needs, but this program has eroded over the last 

decade owing to funding restrictions. The LTIMP does not include some key 

pollutant sources (i.e., urban stormwater and road runoff), and it does not 

include some key water quality constituents that directly affect lake clarity 

(i.e., particle number and particle size distribution). A regional stormwater 

quality/ best management practice (BMP) retrofit monitoring program is 

needed to assess the effects and effectiveness of capital investment projects 

at the project, watershed, and basin scales.

• Soil conservation. Develop and implement consistent and effective monitoring protocols for key soil properties and conditions. Monitoring protocols 

that are current, process-specific, and uniform across agencies and contractors would maximize effectiveness. Such a monitoring program is vital to 

informing the performance effectiveness of projects affecting soil properties 

and conditions throughout the Tahoe basin.

• Ecology and biodiversity. Develop a forest fuels reduction monitoring program to assess quantitatively the effects and effectiveness of fuels reduction 

projects at project, watershed, and basin scales. Establish a long-term, basinwide status and trend monitoring program for terrestrial and aquatic biotic 

communities. It is recommended that biodiversity monitoring focus on the 
24

GENERAL TECHNICAL REPORT PSW-GTR-226

composition of biotic communities and the distribution, frequency of occurrence, and abundance of native and nonnative species using direct measures 

and indicators to inform environmental thresholds and desired conditions. 

Monitoring data can provide a foundation for assessing the effects of climate 

change on terrestrial and aquatic biota of the Lake Tahoe basin. Status and 

trends monitoring are recommended for select focal species identified in 

regulations and basin management plans. Integration of focal species monitoring is recommended with biodiversity monitoring to the degree efficiencies can be gained, to provide reliable estimates of the population parameters 

targeted in those regulations and plans. 

• Social sciences. Regular and systematic surveys are recommended to document the use of existing recreation resources and to help inform priorities for 

future facilities. It is recommended that those surveys be based on common 

methods so that data can be aggregated among sites to obtain regional and 

basinwide estimates of use. 

▪ Develop and implement the infrastructure to collect basic social and 

economic data throughout the Lake Tahoe basin. Common sampling 

methods are necessary to ensure that data can be aggregated and 

analyzed to assess socioeconomic conditions within the various 

municipalities, within geographic regions, and throughout the basin. 

▪ Infrastructure is recommended to support the centralized collection, 

inventory, and distribution of transportation data at regional and 

basinwide levels. 

▪ Establishment of a spatially and temporally appropriate monitoring 

program is recommended to allow for assessing noise conditions 

relative to established threshold indicators.

Research 

Research can be defined as a structured process of inquiry that aspires to discover, 

interpret, and revise our knowledge of facts. Research aims to produce an evergreater knowledge of events, behaviors, processes, theories, and laws. Research 

may include laboratory or field experiments, or the development of models. 

Research is one of the fundamental ways in which science contributes to 

reducing uncertainties. Research and monitoring operate most effectively together. 

Monitoring, particularly status and trends monitoring, generates the knowledge 

necessary to pose new hypotheses about key cause-and-effect relationships, and the 

role of underlying processes in shaping ecosystems and their processes. Research 
25

An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

is the means by which new hypotheses are tested in order to elucidate and confirm 

the relationships and interactions that determine the environmental conditions we 

observe and experience. 

Research to support adaptive management often plays a relatively limited 

role in informing management decisions and policy choices, because obtaining 

new information is generally considered subordinate to demonstrating that policy 

choices and management actions have generated immediate results—such as 

on-the-ground projects, land preservation, or new regulations restricting a former 

activity—and that those results fulfill management objectives. 

The single highest priority for research is to establish a stable funding stream to 

(1) ensure that the level of scientific understanding of the Lake Tahoe basin continues to develop, (2) create the research tools and techniques necessary to best inform 

policy choices and management strategies, and (3) support timely and effective 

science delivery in terms of research findings and management implications. 

Over the past several decades, much of the available research funding has been 

allocated to address the factors and processes that specifically affect Lake Tahoe 

water quality, particularly lake clarity. Although great strides have been made in 

knowledge related to lake water quality, a similar level of progress in understanding 

other ecosystem elements, such as forest health, biodiversity, air quality, or socioeconomic dynamics has not occurred. Although it is known that terrestrial, aquatic, 

and atmospheric habitats are interconnected, and that all are influenced by human 

activities, the direction and intensities of interactions and effects are in many cases 

uncertain. We recommend balancing the distribution of new research funds across 

multiple theme areas so that we simultaneously increase our knowledge base across 

a diversity of issue areas. Research needs specific to each of the five theme areas 

are presented in chapters 3 through 7.

Data Application 

Data application includes the activities of analysis, reporting, management, and 

assessment in order to accomplish the following objectives: (1) manage data and 

information in ways that ensure their quality and availability; (2) complete analyses 

that convert data into information that can directly guide management; and (3) 

share that information with others, such that the information promotes knowledge. 

From a management standpoint, data application to real world problems may be the 

most important element of an integrated science program, because it is this activity 

that provides research and monitoring results in forms that managers can apply 

directly to management actions, decisions, and policy choices. To meet the growing 

data and information needs of resource management agencies and the public, it is 
26

GENERAL TECHNICAL REPORT PSW-GTR-226

important that data are translated into information and information is converted 

to knowledge as efficiently as possible to maximize its potential benefits. Ideally, 

a data management system works within existing institutional arrangements and 

policy requirements to meet agency communication and coordination needs, and 

allows for an integration of data and information sharing among a wide variety of 

entities inside and outside the Lake Tahoe basin.

Data Application Priorities 

A dedicated source of funds and resources is recommended for data management, 

analysis, and reporting to ensure that this element of an integrated science program 

is accomplished and maintained. The identification of responsible entities and 

allocation of sufficient resources to fulfill this function is essential to achieving 

success and minimizing costs by eliminating redundant efforts among institutions. 

In the course of developing this science plan, a number of high-priority investments 

in data applications were identified for completion over the next 5 years.

• Accomplishments report. Completed annually, an accomplishments report 

from the science community would synthesize the results of research and 

other science projects completed during the previous year; this report would 

then be combined with updated running totals of administrative outcomes 

(e.g., dollars spent, number of projects completed) and program outcomes 

(e.g., acres restored, volume of stormwater treated) from the management 

agencies. The accomplishments report would summarize scientific findings 

and relate the importance of those findings to ongoing and future management activities and policies. This report also could identify research needs 

that have emerged as a result of monitoring results, new environmental conditions, or changes in policy or regulation. Information in the accomplishments report would help agencies and scientists track projects completed or 

in progress, and develop or adjust near-term priorities for capital projects, 

research, monitoring, and analysis. Information in the report also could provide a snapshot of project outcomes.

• Central monitoring database. The value provided by a centralized information depository is manifold; however, a primary goal is to promote the 

management of both research and monitoring data in a manner that ensures 

their quality and accessibility. It is recommended that infrastructure should 

be developed and maintained so that basic data and summary information is 

stored, integrated, and accessible to a diversity of users. The Tahoe Integrated 

Information Management System (TIIMS) has begun efforts to address the 

needs associated with this activity.
27

An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

• Environmental management knowledge base. An environmental management knowledge base that documents our current understanding of conditions, interactions, threats, and desired outcomes can inform science and 

management priorities and activities. Integrated information systems can 

meet the array of information needs across institutions and disciplines in an 

efficient and transparent manner. Knowledge bases with their incumbent 

conceptual models provide valuable tools for integrated information storage 

and retrieval. A knowledge base of environmental quality for Lake Tahoe 

that serves both management and research would:

• Provide a transparent basis for evaluating desired conditions.

• Enhance the ability to communicate ideas and outcomes among stakeholders regarding management options at site and landscape scales.

• Facilitate rapid access by decisionmakers to needed information.

• Help prioritize information gaps to be filled and illustrate the basis of 

priorities.

• Compare outcomes associated with various management options to 

help determine priority objectives for specific locations and how to 

balance objectives across landscapes.

• Serve as a tool to interpret changes in desired conditions over time 

based on monitoring data. 

• State-of-the-Lake-Tahoe-basin report. It is recommended that a state-ofthe-basin report be completed every fifth year. This report could include 

a comprehensive synthesis of the research and monitoring that has been 

completed over the previous 5 years. Results could be framed in terms of 

an environmental report card for the Lake Tahoe basin, and would evaluate 

conditions relative to environmental goals and thresholds. This information 

could be used to evaluate and (if necessary) modify management strategies and implementation programs (i.e., inform the adaptive management 

process). This report also could alert high-level officials to emerging issues 

that may require new or alternative policies. The state-of-the-basin report 

would include at least the following subject areas: 

• Lake Tahoe

• Basin watershed condition 

• Basin airshed condition

• Living resources condition

• Human environment
28

GENERAL TECHNICAL REPORT PSW-GTR-226

Adaptive Management

An integrated science program is best applied to land and resource management 

efforts using structured adaptive approaches, which are referred to as adaptive 

management. Adaptive management has been dubbed, “learning by doing,” but 

that label understates the technical and institutional challenges in implementing the 

process and sustaining it over time. The essential elements of adaptive management 

include the clear articulation of management goals and objectives; the development 

of conceptual models that describe how managers believe the ecosystem works; the 

identification of alternative management responses; and an implementation strategy 

that describes management actions, tools, and expected outcomes. Successful application of adaptive management relies on various information inputs (e.g., scientific, 

legal, or economic) into each element along with processes that allow adjustments 

and adaptation. The aim is to develop policies and management strategies that are 

responsive to the best available knowledge and increase the chances of management 

success. 

A variety of processes and techniques have been advanced to guide those who 

wish to transfer available scientific information to policymakers and resource 

managers under adaptive management. In the Lake Tahoe basin, agency leaders 

have agreed to work together to develop and operate an adaptive management 

framework as a means to (1) integrate management actions across targeted 

resources, including air quality, water quality, lake clarity, soils and vegetation, 

and wildlife and fisheries; (2) coordinate monitoring, research, and ecological and 

hydrological modeling efforts; (3) define responsibilities among those contributing 

to adaptive management for data analysis, interpretation of study results, and the 

reporting of new information; and (4) identify protocols to facilitate collaborative 

decisionmaking in resource management and environmental restoration and in the 

updating of management strategies (TRPA et al. 2007). 

An operational science program for the Lake Tahoe basin should contribute 

pertinent and timely information to help guide management actions by informing 

data collection in monitoring and assessment efforts, and by building a foundation 

of reliable scientific information from directed research about the basin’s ecosystems (fig. 2.1). Interactions among science, management, and policy are typically 

complex and iterative. For example, agency-defined environmental thresholds that 

at least in part have been informed by scientific information guide many of the 

actions of the Tahoe basin’s capital improvement program, while at the same time 

regulations informed by the same and different information are intended to prevent 

further degradation of conditions relative to those thresholds (fig. 2.1). Management 

strategies and policy choices rely on iterative interactions among environmental 
29

An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

goals and implementation programs. An integrated science program is most effective if it operates to provide information at the intersections among these programs 

and the goals that guide them. An integrated science program can provide information that is useful in verifying cause-and-effect relationships, assessing program 

effects and environmental conditions, and influencing the selection of management 

strategies and policies that can ultimately translate into funding decisions and 

courses of action for program implementation. 

An adaptive management framework can provide links between management 

needs, scientific activities, and management actions, thus ensuring that the best 

available knowledge systematically informs planning and management activities 

(Manley et al. 2000). Incorporating a science program into an adaptive management 

framework explicitly acknowledges that critical uncertainties confront those who 

develop policies and implement management strategies. A functional adaptive 

management system will integrate the commitments to reducing uncertainty 

through research, monitoring, and data applications with the commitments to 

implement restoration actions. 

An adaptive management framework typically makes explicit the linkages 

between information acquisition, management decisions, and management action 

(fig. 2.3). In the purest form of adaptive management, uncertainties associated 

with management policies and strategies are treated as working hypotheses and 

Figure 2.3—An adaptive management framework illustrating the major steps in a decisionmaking 

process, and the various linkages by which scientific information can inform each step in the 

process. 

Take actions to

implement policy

choices and 

management strategies 

Identify issues and

objectives

Scientific activities:

1. Research to test

 hypotheses about 

 cause-effect relationships

2. Monitor to document

 activities and assess

 the outcome of actions

Explore policy

choices and

management

strategies

Select policy choices and

management strategies to

address issues and 

accomplish objectives

Reassess

the actions 

Reassess the

policies and strategies

Inform policy or

strategy options
30

GENERAL TECHNICAL REPORT PSW-GTR-226

are not permanent features (Lee 1993). Working hypotheses are tested through 

research and monitoring, with the outcomes of these data-gathering efforts (i.e., 

experiments) providing opportunities to acquire new knowledge that can inform 

subsequent policy and management decisions (Hollings 1978; Lee 1993, 1999). Both 

research and monitoring are essential elements in this form of adaptive management, framing policy choices as working hypotheses, and testing those hypotheses.

In an appraisal of adaptive management, Lee (1999) found that “adaptive management has been more influential, so far, as an idea than as a practical means of 

gaining insight into the behavior of ecosystems utilized and inhabited by humans.” 

As a result, adaptive management is frequently applied as a partial overlay to existing, frequently imbalanced infrastructure for problem identification, policy choices, 

and management actions. In such situations, opportunities to obtain new information are opportunistic endeavors that do not detract from the central objective of 

achieving management results. Accordingly, monitoring is viewed as the “science” 

needed to document the effects and effectiveness of management strategies. In 

either application of adaptive management, the role of science is to provide objective information that can tell us about the outcomes and consequences of actions 

or choices and that can help to improve future decisions.

Science Plan Conceptual Models

Conceptual models are an essential centerpiece of adaptive management and its 

integration of monitoring, research, and data applications into more effective, 

efficient, and accountable management actions. Their function is to communicate 

a shared understanding about the key elements, linkages, and stressors associated 

with ecosystems among those who are responsible for and contribute to their management. In essence, they document our hypotheses about how ecosystems function 

and thereby inform management planning, the development of assessment and 

monitoring programs, and the design of research efforts. Conceptual models that 

have been vetted through scientists, managers, and stakeholders can serve as the 

basis for numerical models that attempt to describe the consequences of alternative 

management actions. Without conceptual models, there can be no reliable adaptive 

management.

Conceptual models describe in graphical or narrative form the ecological 

system subject to management, allowing inference about how that system “works.” 

A model of riparian forest function in the Lake Tahoe basin, for example, would 

describe the relationships between the vegetation and the animals that depend on it, 

the hydrological and other physical processes that affect those relationships, and the 

roles of human activities in disturbing and sustaining the system. Such models help 
31

An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

to clarify the verbal descriptions of what we have observed in nature, and force us 

to think about ecosystem elements and interactions that we might otherwise ignore. 

In the formulation of a conceptual model, the combinations of parameters that drive 

ecological systems often become apparent, which in turn provides planners a more 

complete context to understand how various management strategies or regulations 

might affect system function and the condition of natural resources. Attributes 

of particular importance to ecosystem function and sustainability are strong candidates for management attention and assessment using monitoring. Conceptual 

models help us to assure that our current and future management actions target 

the correct ecosystem features and attributes, and increase the likelihood that our 

actions will produce the desired management outcomes. Acceptance of a common suite of conceptual models by agency and stakeholder representatives signals 

agreement regarding critical aspects of ecosystem well-being for which they share 

management responsibility.

A conceptual model should clearly identify key system elements and key 

physical and biological processes. The model should articulate how the system is 

impacted by stressors (i.e., disturbances and perturbations) from both natural and 

human-generated sources and how management can intervene to reverse undesirable conditions or trends. That description can take one or more forms, including 

box and arrow diagrams, cartoons accompanied by narrative descriptions, simple 

linear pathway illustrations, or even straightforward text descriptions. 

Conceptual models will always fall short of a full and accurate portrayal of 

ecosystem elements and processes—highly detailed renditions would be counter to 

the primary objective of serving as a communication device among stakeholders. 

Their very imperfection is part of their utility, in that they make our understanding 

of how natural systems work available for review and discussion; thus, they help to 

identify areas of uncertainty and direct efforts to the information necessary to make 

better management decisions. Once agreement is reached among stakeholders on 

workable conceptual models, they can begin to serve as a framework for decisionsupport systems that can help managers evaluate the relative risks and opportunities 

associated with various management options. If adaptive management is effective, 

conceptual models will improve in their accuracy and effectiveness as we learn 

more over time. 

We developed conceptual models for the Lake Tahoe basin Science Plan that 

had multiple objectives. They are intended to provide a roadmap to the key research 

needs identified in the theme areas considered in this science plan, orient the reader 

to the focal elements within each theme, and identify the interactions and linkages 

among these elements and the subthemes. Perhaps more importantly, they serve as a 
32

GENERAL TECHNICAL REPORT PSW-GTR-226

starting point for discussion and further development in collaboration with managers and other stakeholders in the Lake Tahoe basin. Here we provide an overview 

model of the primary elements of the research strategies and their linkages (fig. 

2.4). The primary biophysical and sociocultural elements of each theme—the 

subthemes—are shown in association with their respective themes. The themes and 

subthemes are arrayed around a circle to indicate that they are all part of a singlebasin ecosystem and thus affect one another to various degrees. Primary interactions between subthemes and across themes are indicated by arrows that span the 

circle. Interactions between subthemes within a theme are not shown—they are 

numerous and are addressed in the theme-specific models within each chapter. 

Old-growth and

landscape conditions

Special community

integrity 

Aquatic ecosystem

restoration 

Fire and fuels 

management

Soil properties 

and conditions

Soil conservation 

Air quality Water quality 

Atmospheric pollution

emission inventories 

Local vs. regional

transport

Fire impacts on

air quality 

Air quality criteria

pollutants 

Recreation

Transportation

Climate change

Tahoe basin

visibility

Atmospheric

deposition

Lake water 

quality

Near-shore water 

quality and ecology

Stream channel erosion 

Pollutant loads from 

urban watersheds

Water quality treatment 

and source controls

Drinking water 

protection

Noise

Economics

Scenic resources

Research

Monitoring

Modeling

Application

Research

Monitoring

Modeling

Application

Ecology and 

biodiversity 

Social

sciences 

Figure 2.4—Overview conceptual model of the integrated science plan for the Lake Tahoe basin. The model includes the five theme 

areas covered in this science plan and the subthemes within them for which knowledge gaps and research needs are discussed as part 

of each research strategy. Climate change is included as an example of an environmental stressor, and its linkages to various ecosystem 

components are indicated. 
33

An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

Although the subthemes arrayed around the circle are unique to each theme, 

the themes share many features. Most environmental stressors affect multiple 

biophysical and sociocultural elements in various ways. Climate change, for 

example, has the potential of affecting every theme area and every aspect of wellbeing in the basin. Thus, climate change is shown as a stand-alone element and 

the subthemes that it is likely to affect first and most directly are indicated via 

arrows. Certainly, many other stressors, such as air pollution, urban development, 

and invasive species, affect multiple themes and subthemes. These stressors are 

discussed within each theme, but the development of stressor-based conceptual 

models would be a valuable future investment. Science-based activities in support 

of adaptive management are noted in the center of the circle—research, monitoring, 

modeling, and data application—indicating that they span subthemes and themes, 

and in fact are most effective when designed to do so. 

More detailed theme-specific conceptual models appear in each theme chapter. 

They display the subthemes (focal points of the research strategy), the components 

of each subtheme, and what factors (drivers) affect them (fig. 2.5). The themespecific conceptual models identify the linkages between drivers and components. 

The conceptual models identify the linkages or elements that are priority areas for 

research, and for which specific research needs are specified in the strategy based 

on the combination of risk and uncertainty. The models also identify basic information needs where those needs can be a barrier to informing management, and they 

suggest useful monitoring indicators. Once a knowledge base is designed around 

the conceptual model, it can be used to assess conditions and derive information 

on relationships among its component parts. The relationships among any of the 

component parts of the knowledge base can be represented in the form of summary 

tables or figures akin to conceptual models. Using a knowledge base for this function alone can address a number of the commonly posed questions about ecosystem 

interactions from a management perspective. The real power of the knowledge base, 

however, is in evaluating ecosystem conditions, both across the entire planning area 

and by planning unit. 

The “box and arrow” conceptual models developed for this science plan are a 

common format for portraying basic parts and interactions of a system of interest. 

In the Lake Tahoe basin, the large number and scope of issues of interest and the 

complexity of their interactions make conceptual models a valuable and necessary 

tool—a means by which essential information can be displayed and understood. 

With such a complex problem set, however, it is challenging, if not impossible, 

to render a conceptual model that is sufficiently detailed to be representative, 

yet simple enough to be informative. Managers and scientists alike benefit from 
34

GENERAL TECHNICAL REPORT PSW-GTR-226

Figure 2.5—Elements and framework used to develop theme-specific conceptual models.

conceptual models, but models tend to fall short of the need to portray and analyze 

environmental interactions and complexities. It is likely that land and resource 

management agencies will be investing in knowledge base decision-support tools, 

and they will need conceptual models to serve as the roadmap for these knowledge 

bases. The theme-based conceptual models presented in this science plan are 

intended to contribute to this management need. 

Ideally, conceptual models serve as the basis for working models that can 

be used in an interactive decision-support process. Hierarchical knowledge-base 

models can be an effective approach to dealing with decisionmaking in a complex 

biophysical, sociocultural, and regulatory environment. In such models, the detailed 

1° Driver

2° Driver 

Definition: Primary categories of biological, physical, and social 

conditions of interest to agencies and the public.

Constituents: Interest areas are divided into their primary 

elements (i.e., air quality, water quality, soil conservation, 

ecology and biodiversity, and integrating the social sciences 

into research planning.

Condition: Degree of consistency with desired condition statements on condition of elements.

Definition: Categories within theme areas identifying the 

distinct subsets of interest.

Constituents: Subthemes are substantive topics that are 

important to consider within each theme area.

Condition: Degree of consistency with desired condition statements based on condition of individual constituents, or an

aggregation of constituents, depending on the desired condition.

Definition: Broad categories representing the substance of

each subtheme.

Constituents: Each component is represented by one or more

indicators and affected by one or more drivers.

Condition: Degree of consistency with desired conditions based

on condition of indicators. Information on indicators—what they

represent and how their values are derived—is contained within

this tier.

Definition: Drivers are agents of change, natural or humancaused, that affect the condition of improvements and indicator

values. Primary drivers affect change directly, while secondary

drivers affect primary drivers and often reflect the ultimate

causal factors for environmental change.

Constituents: Each component is directly affected by one or 

more primary drivers, which in turn are directly affected by one

or more secondary drivers.

1. Theme areas

2. Subthemes

3. Components

4. Drivers

Theme

Subtheme

Component
35

An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

complex interactions, as they are understood and refined over time, can be accurately represented and their relative influence on outcomes tested and quantified. 

Knowledge bases are a logical extension of conceptual models, and their development and application to decisionmaking in the basin will become increasingly 

important as scientists and managers face the substantial uncertainties about how 

to maintain and restore ecosystem resilience and sustainability. They are a form 

of metadatabases that represent a formal logical representation of a network of 

relationships among elements of interest. The structure of a knowledge base for 

Lake Tahoe would need to be designed to meet a spectrum of information storage 

and retrieval needs. It would need to contain information on broad categories of 

environmental quality and basic measures of their condition to meet the communication needs of managers and decisionmakers across institutions. It would also 

house the more detailed information associated with how the values of the measures 

are derived, and the uncertainties and risks associated with those values to provide 

a definitive link between information acquisition and application. 

References

Hollings, C.S. 1978. Adaptive environmental assessment and management. 

New York: Wiley Publisher. 377 p. 

Lee, K.N. 1993. Compass and gyroscope: integrating science and politics for the 

environment. Washington, DC: Island Press. 243 p.

Lee, K.N. 1999. Appraising adaptive management. Conservation Ecology. 3(2): 3. 

http://www.ecologyandsociety.org/vol3/iss2/art3/. (October 15, 2008).

Manley, P.N.; Tracy, J.C.; Murphy, D.D.; Noon, B.R.; Nechodom, M.A.; 

Knopp, C.M. 2000. Elements of an adaptive management strategy for the 

Lake Tahoe basin. In: Murphy, D.D.; Knopp, C.M., eds. Lake Tahoe watershed 

assessment: Volume I. Gen. Tech. Rep. PSW-GTR-175. Albany, CA: U.S. 

Department of Agriculture, Forest Service, Pacific Southwest Research 

Station: 689–735. Chapter 7. 

Suter, G.W. 1993. Ecological risk assessment. Boca Raton, FL: Lewis Publishers. 

538 p.

Tahoe Regional Planning Agency [TRPA]; USDA, Forest Service, Lake Tahoe 

Basin Management Unit; Lahontan Regional Water Quality Control Board; 

Nevada Division of Environmental Protection. 2007. Pathway 2007 draft 

evaluation report. Version 1.0. http://www.pathway2007.org/materials.html. 

(October 15, 2008).
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GENERAL TECHNICAL REPORT PSW-GTR-226
37

An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

A.W. Gertler,2 T.A. Cahill,3 T.M. Cahill,4 S.S. Cliff, 5 C. Emmett,6 J. Koracin,7

H. Kuhns,8 J.V. Molenar,9 J. Quashnick,10 and J.E. Reuter 11

Introduction

Air quality in the Lake Tahoe basin is known to affect lake water quality, forest 

health, and human health. The Lake Tahoe Watershed Assessment provided an 

initial summary of the status of our scientific knowledge regarding the factors 

leading to the observed decline in water quality (Reuter and Miller 2000) and steps 

that can be taken to restore the Lake Tahoe basin ecosystem (Murphy and Knopp 

2000). Among the factors contributing to the decline in water quality are nitrogen 

(N), phosphorous (P), and sediment flow into Lake Tahoe. Cliff and Cahill (2000) 

reported that atmospheric deposition accounts for approximately 55 percent of the 

N and 15 percent of the P load into the lake. No estimate of atmospheric particulate 

input was presented. These estimates are highly uncertain. Although there has been 

extensive water sampling in the basin, there has been minimal air sampling. Thus, 

despite atmospheric deposition possibly being a major source of N input, a significant source of P input, and a potentially important source of particle deposition and 

sediment loading, we lack knowledge regarding the sources of these nutrients, the 

contribution from in-basin vs. out-of-basin pollutant sources, the spatial and temporal distribution of pollutant deposition, and the factors contributing to the observed 

deposition. 

In addition to the issue of declining water clarity, there also are concerns 

regarding the impact of atmospheric pollutants on human and forest health and 

aesthetics. For example, the ozone (O3) levels in the Lake Tahoe basin exceed 

Chapter 3: Air Quality1

1 Citation for this chapter: Gertler, A.W.; Cahill, T.A.; Cahill, T.M.; Cliff, S.S.; Emmett, 

C.; Koracin, J.; Kuhns, H.; Molenar, J.V.; Quashnick, J.; Reuter, J.E. 2009. Air quality. In: 

Hymanson, Z.P.; Collopy, M.W., eds. An integrated science plan for the Lake Tahoe basin: 

conceptual framework and research strategies. Gen. Tech. Rep. PSW-GTR-226. Albany, 

CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station: 

37–81. Chapter 3.

2 Division of Atmospheric Sciences, Desert Research Institute, Reno, NV 89512.

3 University of California, Davis, Department of Physics, Davis, CA 95616.

4 Arizona State University, Department of Integrated Natural Sciences, Tempe, AZ 85287. 

5 University of California, Davis, Department of Applied Sciences, Davis, CA 95616.

6 Air Quality Program, Tahoe Regional Planning Agency, Stateline, NV 89449.

7 Division of Atmospheric Sciences, Desert Research Institute, Reno, NV 89512.

8 Division of Atmospheric Sciences, Desert Research Institute, Reno, NV 89512.

9 Air Resource Specialists, Inc., Fort Collins, CO 80525.

10 Sierra Nevada Environmental Consulting, South Lake Tahoe, CA 96150. 11 University of California, Davis, Department of Environmental Science and Policy, 

Davis, CA 95616.
38

GENERAL TECHNICAL REPORT PSW-GTR-226

current and proposed air quality standards that are designed to protect human 

health and welfare. In addition, elevated O3 leads to a decline in forest health that 

can increase the threat of fire owing to dead and dying trees. In terms of particulate 

matter (PM), PM2.5 (particulate matter with aerodynamic diameter less than 2.5μm 

or fine particulate matter) not only impacts in-basin visibility, but also is a concern 

because of its link to human health impacts. 

To address the continuing concerns about air pollution impacts and develop 

a sound scientific approach for mitigating these impacts, this chapter updates the 

work of Cliff and Cahill (2000) and builds upon the Lake Tahoe Atmospheric 

Deposition Study (CARB 2006) by delineating remaining knowledge gaps and 

defining the research needs and strategies to close these gaps. To support this 

approach, a number of Tahoe-specific subthemes related to air quality were identified, including: 

• Tahoe basin meteorology 

• Atmospheric deposition of N, P, and particles

• Local vs. regional transport of air pollutants

• Tahoe basin air quality: the criteria pollutants

• Air pollution emission inventories

• Atmospheric modeling of the Lake Tahoe basin 

• Impacts of fire on air quality

Climate change is likely to impact air quality through changes in emissions, 

activity, atmospheric transformations, and meteorology; however, at this time, these 

impacts are highly uncertain and have not been included in this summary. 

In this chapter, we summarize our current state of knowledge on the selected 

subthemes and present the research needs and strategies that are recommended 

to improve our understanding of the impact of air quality on human health, water 

quality, and ecosystem health. We note up front that the lack of long-term comprehensive air quality monitoring data substantially affects our ability to fully develop 

an understanding of the atmospheric processes leading to pollutant impacts. 

Addressing this shortcoming would necessitate a long-term commitment to routine 

monitoring coupled with data interpretation to address the identified air quality 

knowledge gaps outlined in this chapter.

Conceptual Model

There are a number of coupled processes leading to air and water quality impacts 

caused by air pollution in the Lake Tahoe basin. Initially, we have emissions from 

both human activities and natural processes. For example, motor vehicle use 
39

An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

results in the direct emission of carbon monoxide (CO), oxides of nitrogen (NOx), 

hydrocarbons (HC, also referred to as volatile organic compounds, VOCs), and PM. 

Biogenic sources (i.e., trees) are a major contributor to VOC emissions. Following emission of pollutants into the air, meteorological factors influence pollutant 

concentrations through transport, dilution, temperature/relative humidity, and precipitation. Simultaneously, chemical transformations take place in the atmosphere, 

leading to the formation of harmful species such as O3. 

One issue of concern is the deposition of N, P, and sediment (related to PM) 

into the lake, which contributes to a decline in water clarity. As pollutants are dispersed and react in the atmosphere, they can also deposit on surfaces through either 

wet or dry deposition pathways. Both of these are important in the basin; although, 

given the sporadic nature of rain and snow, it is likely the dry deposition pathway 

dominates. 

In addition to the water clarity issue, air pollutants can negatively impact both 

human and ecosystem health. One way to assess potential impacts is to monitor 

ambient pollutant levels and compare the results with ambient air quality standards 

that are designed to protect human health and welfare. Currently, the only basin 

pollutant in violation of the standards is O3. Based on the determination of negative 

impacts (i.e., violation of the air quality standard, impact on water clarity, declining visibility, etc.), management agencies can develop policies to reduce pollutant 

levels. In general, these involve the reduction of emissions.

To aid management agencies in their efforts to improve air quality and mitigate 

negative environmental impacts, we can make use of a conceptual model (fig. 3.1). 

The four major components contributing come from mobile, area, stationary, and 

biogenic sources. Each of these has a number of 1° (primary) and 2° (secondary) 

drivers. For example, land use and transportation systems and policies (2° drivers) 

affect the number and activity of on- and off-road vehicles and business operations 

(1° drivers), which control mobile source emissions. Similarly, fire risk and suppression (2° driver) influences naturally occurring fires and plant and animal outputs 

(1° drivers) that contribute to biogenic emissions. It is important to note there are 

overarching drivers that cannot be controlled: chemical and physical transformations and meteorology and climate.

As described above, emissions from the four components lead to air quality 

and environmental impacts. These impacts can be identified and assessed using 

various indicators. Potential indicators in the Lake Tahoe basin are vehicle miles 

traveled and activity, measurements of long- and short-range visibility, air quality 

standards, concentrations of nonregulated pollutants, and measures of human and 

ecosystem health. 
40

GENERAL TECHNICAL REPORT PSW-GTR-226

Figure 3.1—Conceptual model for Lake Tahoe basin air quality (AQ) subthemes. Identified are the primary sources of emissions in 

the Lake Tahoe basin airshed, the natural and human-caused drivers that affect these emissions, and arrows indicating the linkages 

among drivers and emission sources. These drivers and emission sources affect each of the air quality subthemes. Near-term air 

quality research priorities are indicated by symbols AQ1–AQ5 and correspond to the descriptions presented later in this chapter.

In this chapter, we’ve identified eight subthemes related to the various drivers 

and components outlined in the conceptual model that affect air quality issues in 

the Lake Tahoe basin. The sections that follow present the state of our knowledge 

regarding these subthemes and discuss measures that can be implemented to 

improve our understanding of the processes affecting air quality. By making use 

of the relationships among the various drivers, components, and indicators presented in this conceptual model, basin managers can develop effective strategies 

to improve air quality in the basin. 

Biogenic

emissions

2° Driver 1° Driver Component Indicator Subtheme Theme Linkage

Land use

Air quality

Subthemes Potential

indicators

Transportation

systems and 

policies

Chemical and

physical

transformations

Meteorology

and

climate

Fire risk

and

suppression

Road and 

bare soil 

dust

Business

operation

Human-caused

fires

Naturally

occurring fires

Plant and

animal 

outputs

On- and 

off-road

vehicles

Stationary

emissions

Area

emissions

Mobile

emissions

AQ2

AQ2

AQ2

AQ2

AQ2

AQ1

Tahoe basin 

air quality:

the criteria

pollutants

Atmospheric

modeling

Ecosystem

and

human 

health

State and 

federal

air quality

standards

Local vs.

regional

transport

Impacts

of fire on

air quality

AQ1

AQ3

AQ3

Atmospheric

deposition of

nitrogen,

phosphorous,

and particles

Air pollutant

emission

inventories

Tahoe basin

meteorology

Tahoe basin

visibility

AQ2, AQ4

AQ1

AQ5

Nonregulated

pollutant

concentrations

Vehicle miles

traveled

and activity

Long- and

short-range

visibility
41

An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

Tahoe Basin Meteorology

To understand the complex relationships among air quality, lake clarity, forest 

health, visibility, and human health in the Lake Tahoe basin, it is necessary to 

understand meteorology in and around the basin. Overall, our current understanding is limited and additional monitoring, research, and modeling efforts are needed. 

The unique physical attributes of the basin play an extremely important role 

in defining the basin’s atmospheric processes. There are generally two types of 

regimes that have the largest impact on atmospheric processes in the basin: 

thermal inversions and atmospheric transport.

Thermal inversions—

Because Lake Tahoe is located at a high elevation and surrounded by mountains on 

all sides, an atmospheric regime is created that, in the absence of strong synoptic 

weather systems, develops very strong, shallow inversions at all times of the year. 

Further, the rapid cooling at night generates downslope winds, which move from 

the ridgetops down and across populated areas and over the lake itself. Pollutants 

emitted by human and natural activities, along with those “carried in” with the 

downslope winds, will often become “trapped” at the surface (both land and lake 

surfaces) by thermal inversions, thereby creating areas of concentrated pollutant 

levels and increasing the chance for air pollutants to deposit on the lake and the 

surrounding watershed. 

Atmospheric transport—

The location of Lake Tahoe directly to the east of the Sierra Nevada Mountain crest 

creates the second most common meteorological regime: atmospheric transport 

of air pollutants from the Sacramento Valley/Bay area into the Lake Tahoe basin 

by mountain upslope winds. This pattern develops when the western slopes of the 

Sierra Nevada are heated, causing the air to rise in a chimney effect and move 

upslope and over the Sierra crest into the Tahoe basin. The strength of this pattern depends on the amount of heating, thus is strongest in summer, beginning 

in April and essentially ceasing in late October (Cahill et al. 1997). This upslope 

transport pattern is strengthened and made even more frequent by the alignment of 

the Sierra Nevada range across the prevailing westerlies common at this latitude, 

which combine with the terrain winds to force air up and over the Sierra Nevada 

mountains from upwind sources in the Sacramento Valley (Cliff and Cahill 2000). 

So, although this process is often discussed in terms of the out-of-basin air quality impacts from pollutants generated in the Sacramento Valley and Bay area, the 

atmospheric transport regime can also include pollutant contributions from largescale global atmospheric transport. For example, the atmospheric transport of dust 
42

GENERAL TECHNICAL REPORT PSW-GTR-226

from Asia has been detected in numerous areas across California’s west coast and 

in the Tahoe basin (CARB 2003). 

Another important meteorological regime, mediated by topography and low 

pressure systems, results in a summertime pattern that circulates moisture in from 

the east, often forming thunderstorms along the Sierra crest. In addition, strong 

high-pressure patterns north and northwest of Lake Tahoe can bring strong dry 

winds across the basin at almost any time of the year. Each of these meteorological 

regimes has a potential for concentrating anthropogenic pollutants within the Tahoe 

basin (Cliff and Cahill 2000).

Seasonal differences—

Seasonal differences in meteorology play a large role in air pollutant processes as 

well. In the summer months, nightly inversions are common, thereby contributing 

to higher pollutant concentrations in trapped areas. Additionally, summertime 

weather conditions such as higher temperatures and fewer storms create conditions 

favoring higher emission rates and concentrations of certain pollutants, such as O3. 

More specifically, warmer temperatures can increase emission rates of pollutants 

from motorized equipment and biogenic sources. Further, secondary effects also 

can result in increased vehicle emissions owing to an increase in the number of 

vehicles within the basin because of the increase in popular summertime activities 

and the greater use of second homes. Another important meteorological regime 

generally associated with the summer months includes the atmospheric transport of 

pollutants from areas outside of the basin (see details under “Atmospheric transport” above). 

Winter conditions in the basin are represented by cool temperatures and clear 

skies with periodic storms bringing precipitation in the form of rain and snow. 

These winter storms generally support strong vertical mixing in the atmosphere 

and the dilution of local and upwind pollutants while bringing in air from the very 

clean North Pacific sector (which accounts for the relatively low concentration of 

anthropogenic pollutants in the basin’s snowfall [Cliff and Cahill 2000]). Unlike 

the nightly occurrences in the summer months, inversions in the winter months 

may last into the daytime hours. As with summer night conditions, there are certain 

pollutants for which emissions are higher during the colder months, such as CO 

from motor vehicle exhaust. The increased emission rates coupled with thermal 

inversions can create localized areas of unhealthy pollutant concentrations. 

There are many weather stations of variable size and type, which have been 

operated by both private and public parties in the basin. These stations range 

from simple temperature and wind speed and direction monitors used by private 
43

An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

citizens or companies, including ski resorts, and marinas, to those operated by 

government agencies and researchers who require quality-assured information to 

meet regulatory requirements and research needs. Unfortunately, the existing 

quality-assured weather stations are not yet extensive enough to answer the 

meteorological questions that remain. Further, simple ambient temperature and 

wind speed and direction measurements cannot account for the conditions that 

occur from ground level to thousands of meters above—a space that encompasses 

a very complex variety of atmospheric processes. 

The most recent attempts to gather meteorological data at three locations in the 

basin occurred as part of the Lake Tahoe Atmospheric Deposition Study (LTADS), 

led by the California Air Resources Board (CARB 2006) in cooperation with other 

basin agencies. Unfortunately, this effort only continued for 1 year. This lack of 

consistent long-term meteorological measurements (and by extension, air quality 

monitoring) has hampered efforts to develop a better understanding of the factors 

influencing atmospheric processes in the basin. 

One other issue not fully understood is the impact of climate change on Tahoe 

basin meteorology. Although it is likely this will lead to warmer temperatures in 

the basin, it is unclear how this will affect other variables such as relative humidity, 

wind patterns, and precipitation. All of these potential changes can greatly influence pollutant emissions and atmospheric processes leading to the deposition of N, 

P, and sediment to the lake.

Knowledge Gaps

Given the limited amount of high-quality meteorological data, there are substantial 

gaps in our understanding of the impact of meteorology on air quality. This has led 

to substantial knowledge gaps in the following areas:

• Spatial coverage of data collection is scarce, limiting our knowledge of 

microenvironments and our ability to accurately determine deposition in 

the basin.

• Only a limited number of meteorological variables are measured, making it 

difficult to employ detailed atmospheric models to assess atmospheric processes. 

• There have been few upper air column measurements for use in evaluating 

chemical transformation and deposition pathways and validating atmospheric models.

• Advanced atmospheric models for use in evaluating management strategies 

have not been implemented for the Lake Tahoe basin, owing to the lack of 

adequate meteorological data.
44

GENERAL TECHNICAL REPORT PSW-GTR-226

Research Needs 

• An appropriate air quality model needs to be developed that can utilize a 

full suite of meteorological data to better assess air pollutant trends, estimate 

impacts, and support the development of regulations that will assist in meeting air quality and other environmental goals. This model needs to be linked 

with an appropriate emissions inventory and relevant deposition data.

• Although existing meteorological information has provided researchers with 

a general understanding of typical meteorological regimes in the basin, far 

more monitoring locations and instruments are recommended to address the 

basin’s air quality and lake clarity planning needs. Ideally this would include 

both ground-based and upper air column measurements.

• The seasonal impact of meteorology on pollutant levels with an emphasis 

on emissions from prescribed fires and wildfires should be evaluated using 

existing data.

• Seasonal overflights with light aircraft could be done to measure the concentrations of a number of air pollutants in the Tahoe basin and the meteorological conditions prevalent during these measurements. This will aid in 

the determination of meteorological conditions related to both deposition and 

in-basin vs. out-of-basin transport. 

• Perform gradient flux or eddy correlation studies to determine pollutant 

deposition dynamics. 

Atmospheric Deposition of Nitrogen, Phosphorus, 

and Particles

The atmospheric deposition of N, P, and particles to Lake Tahoe can be a substantial source of the pollutants contributing to declining lake water clarity (see section 

4.5 in the “Water Quality” chapter of this science plan). Using on-lake deposition 

buckets, Jassby et al. (1994), Reuter et al. (2003), and Hackley et al. (2004, 2005) 

demonstrated that atmospheric inputs of N and P are a significant source of the 

nutrients supporting algal growth. Recent studies of water clarity implicate insoluble particles, largely soil derived, as important contributors to declining lake clarity 

(Coker 2000, Losada-Perez 2002, Swift et al. 2006). Preliminary estimates suggest 

that on the order of 15 percent of the fine, soil-derived particles enters directly into 

Lake Tahoe via atmospheric deposition (LRWQCB and NDEP 2008). 

Deposition theory and validation have been summarized by Seinfeld and Pandis 

(1997). The dry deposition rate is codified by a net deposition velocity, which is, in 

turn, a sum of settling velocity for large particles and diffusion for small particles, 
45

An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

0.01 0.02 0.04 0.1 0.2 0.4 1 2 4 10

Particle diameter (μm)

0.6

0.5

0.4

0.3

0.2

0.1

0

Removal velocity (cm/sec)

with the minimum total deposition velocity and thus the slowest removal rate occurring for particles in the range of 0.04 to 2.0 μm diameter (fig. 3.2). 

This simple concept must be greatly modified in practice, when effects of 

boundary layer resistance and other factors are considered. Complex models have 

been developed to make these calculations, but direct in situ validation continues to 

be quite variable. No such model has been developed for Lake Tahoe, although the 

U.S. Forest Service (USFS) supported the development of a statistical model, the 

Lake Tahoe Airshed Model, in 2000 (Cliff and Cahill 2000). 

In the 1980s, those working to understand the water quality trends in Lake Tahoe 

took a renewed interest in airborne nutrients (especially P and N) critical to algal 

growth. Studies of deposition elsewhere in the country (e.g., the Great Lakes) gave 

added impetus to this idea, as did the Nation’s interest in acid rain and deposition 

of nitric and sulfuric acids. Airborne substances undoubtedly play a role in Lake 

Tahoe’s water quality dynamics, but what role, exactly, was unclear at that time. In 

1981 and 1982, the staff and consultants working on the Tahoe Regional Planning 

Agency’s (TRPA’s) threshold standards attempted to estimate the loading rate, in 

kilograms per hectare per year, of nitric acid that one might expect to see in the 

Sierra Nevada. Based on the responses received from scientific experts, TRPA 

estimated annual dissolved inorganic nitrogen load to the surface of Lake Tahoe on 

the same order of magnitude as the loads coming from surface streams and groundwater inputs. This conclusion—even without monitoring data to confirm it—influenced the development of TRPA’s threshold standards and subsequent regional plan. 

It caused TRPA to look beyond erosion and runoff control as methods to control 

eutrophication.

Figure 3.2—Plot of deposition velocities for particles. Above 0.3 μm, gravitational settling 

dominates, whereas below 0.2 μm, diffusion to surfaces dominates.
46

GENERAL TECHNICAL REPORT PSW-GTR-226

In 1999, researchers from the University of California Davis and the Desert 

Research Institute worked with TRPA to develop the Lake Tahoe Air Quality 

Research Scoping Document (Reuter et al. 2000). Following the publication of that 

document and in support of the Lake Tahoe total maximum daily load (TMDL) program, a series of recent studies have attempted to estimate atmospheric deposition 

of N, P, and soil-derived fine particles. This was intended to update earlier nutrient 

budget estimates and to include for the first time the fine, soil-derived particles. 

The principal contributors to this effort were the CARB (CARB 2006), the Desert 

Research Institute (Tarnay et al. 2000, 2005), the UC Davis Delta Group (Cliff and 

Cahill 2000), and the UC Davis Tahoe Environmental Research Center (Hackley 

et al. 2004, 2005; Reuter et al. 2003).

Results of these studies showed good agreement in estimated atmospheric 

deposition of N and P based on both modeling and direct-measurement approaches. 

Current estimates for loading directly to the lake surface from total N-deposition 

are about 200 metric tons per year (including inorganic and organic N), 6 to 8 

metric tons per year for total P, and about 750 metric tons per year for soil-derived 

fine particles (<20 μm in diameter). Based on the revised atmospheric deposition 

estimates, the percentage contribution via atmospheric deposition directly to the 

lake surface relative to the other major sources is about 55 percent for total N, about 

15 percent for total P, and about 15 percent for soil-derived fine particles. Recent 

studies shed light on the sources of P. Cahill (2005) found that most of the P mass 

was associated with local roadway soils and in particles > 10 μm diameter, and thus 

previously unmeasured. These data were put into Lake Tahoe Atmospheric Model 

(LTAM) and provided the deposition estimates reported by Gertler et al. (2006). 

Knowledge Gaps

• Spatial information of on-lake dry and wet deposition measurements is 

scarce, and almost all measurements are limited to the northern one-third 

of the lake. The difference between nearshore and offshore deposition (in 

terms of amount, chemical species, and particle size distribution) is not well 

understood, yet critical to whole-lake deposition estimates.

• Information on spatial and temporal distribution of aerosol measurements 

by size and composition is scarce and rarely matches the deposition sites for 

validation.

• Routine measurements of gaseous (nitrogen oxides, ammonia, and nitric 

acid [NOx, NH3, and HNO3, respectively] and organic nitrogen species are 

limited or nonexistent in the basin, leading to a large uncertainty in the 

N-deposition estimates.
47

An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

• Air quality models have not been developed that adequately incorporate 

Lake Tahoe’s complex meteorological conditions.

• The deposition of “black carbon” to the lake and its impact on water clarity 

have not been studied. Black carbon (also known as elemental carbon) is 

most evident as soot, and is formed through the incomplete combustion of 

fossil fuel, biofuel, and biomass.

• The distribution between the amount of N, P, and sediment deposited by 

wet and dry deposition pathways is uncertain.

• The greatest uncertainty in the deposition estimates is associated with soilderived particle deposition to the lake surface (only 1 year of limited data 

exist). In addition, estimates of wet deposition of particles are based on 

measurements that are less direct than measurements of dry deposition of 

particles.

• The impact of N and P atmospheric deposition to the lake surface in the 

summer is not well understood. This is a time when biologically available 

N and P are low and watershed loading is at its annual minimum.

• Atmospheric loading onto the water surface could have a disproportionate 

effect on lake clarity within the 20- to 30-m Secchi depth; however, this has 

not been evaluated.

• Atmospheric deposition studies have not accounted for the contribution of 

pollutant fallout to the land surfaces surrounding Lake Tahoe that is subsequently delivered to Lake Tahoe via hydrologic processes.

• Existing data are not readily available.

Research Needs

• Conduct focused studies (gradient or eddy-correlation studies, along with 

measurements of key species) of the sources and pathways of particle deposition to better inform models and restoration efforts. 

• Add deposition sites on the lake and in the middle and southern sections 

along the lake.

• Conduct deposition measurements along nearshore-offshore transects and 

data sets that allow averaging among seasons.

• Add gaseous monitoring capabilities to existing and future sites.

• Add size-segregated and chemically speciated aerosol measurements to 

quantify the relationship between deposition of particulate matter expressed 

in terms of weight and the particle chemical composition and size distribution most likely to affect lake clarity. 
48

GENERAL TECHNICAL REPORT PSW-GTR-226

• Pursue development of a detailed air quality model that includes transport 

and deposition modules specific to Lake Tahoe conditions. Develop these 

models for use as water quality management tools. 

• Determine the possible influence of “black carbon” deposition to the lake.

• Ascertain the relationship between pollutants entering via atmospheric 

deposition and the effects on lake clarity.

• Develop and sustain the infrastructure to combine data from all sources, 

archive results in easily accessible sources, and publish results.

Local Versus Regional Transport of Air Pollutants

Atmospheric pollutants deposited in the Lake Tahoe basin can come from both 

in-basin and out-of-basin sources (see “Conceptual Model” section and fig. 3.1 for 

details). In terms of hemispheric atmospheric circulations, California is within the 

latitude range of prevailing westerly winds. However, because of relatively weak 

synoptic forcing, wind patterns tend to be modified by differential heating between 

the land and ocean. Previous studies have described the typical summer flow pattern in California as the marine air that penetrates through the Carquinez Straight 

and bifurcates around the delta region into south and north branches (Frenzel 1962, 

Hays et al. 1984, Moore et al. 1987, Schultz et al. 1961, Zaremba and Carroll 1999). 

This primary pattern is superimposed by thermally driven daytime upslope and 

nighttime downslope flows, hence making pollutant transport possible from the 

more heavily polluted regions, such as the San Francisco Bay area and the Sacramento Valley, up into the Sierra Nevada mountains. 

To quantify how much HNO3 is transported from the Central Valley, Sacramento, and San Francisco Bay area to the Tahoe basin, Koracin et al. (2004) used 

advanced numerical atmospheric models (CALMET/CALPUFF, Scire et al. 2000 

and MM5, Grell et al. 1995) to estimate the contributions from both in-basin and 

out-of-basin N sources. Simulations of in-basin emissions and out-of-basin emissions were performed separately to determine their relative contributions. The 

overall simulation results indicated that pollutant transport from the Sacramento 

Valley and the San Francisco Bay area to the Lake Tahoe basin occurs; however, 

as indicated in previous studies (Bytnerowicz et al. 2002, Carroll and Dixon 2002, 

Dillon et al. 2002), pollutant concentrations are significantly diluted on the west 

slopes of the Sierras at increasing elevations (fig. 3.3). In short, the results of the 

Koracin et al. (2004) work suggest that although daytime pollutant transport from 

upwind of the Lake Tahoe basin appears to be likely, the amount of HNO3 transported into the basin is much less than that from in-basin sources. Note that estimates of the transport of additional N species (e.g., ammonia, ammonium, nitrate, 
49

An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

Figure 3.3—Nitric acid (HNO3) (μg/m3) plume evolution from the Central California Valley on August 28, 2000, beginning at noon 

(left panel) and continuing to 11 pm (right panel) from Gertler et al. (2006). Concentrations (filled contours overlaid with topography) 

are averaged over 3-hour intervals. The enclosed areas surrounding Sacramento and San Francisco designate the emission sources. 

Note the effects of daytime upslope flows (upper left panel) and nighttime downslope flows (lower right panel). Pollutant concentrations 

at elevated regions are low, implying minimal HNO3 transport to the basin. 

San Francisco

HNO3 concentrations—Aug. 28, 12 to 2 p.m.

Lake 

Tahoe

HNO3 concentrations—Aug. 28, 3 to 5 p.m.

HNO3 concentrations—Aug. 28, 6 to 8 p.m. HNO3

 concentrations—Aug. 28, 9 to 11 p.m.

Sacramento

Sacramento

San Francisco

Latitude

-122° -121° -120°

Longitude

38

39

Sacramento

38°

39°

Lake 

Tahoe

-122° -121° -120°

Longitude

San Francisco

-122 -121 -120

Longitude

38

39

μg/m3

0.05

0.1

0.5

1

3

5

10

20

San Francisco

Sacramento

San Francisco

Sacramento

Sacramento

San Francisco

Sacramento

San Francisco

San Francisco

Sacramento

38°

39°

Lake 

Tahoe

0.05

0.1

0.5

1

3

5

10

20

Latitude

38

39

Latitude

-122° -121° -120°

Longitude

38°

39°

Lake 

Tahoe

Lake 

Tahoe

Lake 

Tahoe

Lake 

Tahoe

Lake 

Tahoe μg/m3

0.05

0.1

0.5

1

3

5

10

20

Latitude
50

GENERAL TECHNICAL REPORT PSW-GTR-226

Figure 3.4—Distribution of ambient nitric acid (HNO3) concentrations (μg/m3) in the Lake Tahoe 

basin and its vicinity in the 2002 summer season from Gertler et al. (2006). Maximum levels (dark 

brown) are observed west of the lake and decrease with increasing elevation.

Interpolation by

Kriging (μg/m3

)

Sampling stations

(μg/m3

)

0.5–0.9

1.0–1.2

1.3–1.5

1.6–1.8

1.8–1.9

0.50–0.65

0.66–0.80

0.81–1.10

1.11–1.25

1.26–1.40

1.41–1.55

1.56–1.80

39°30' N

39°0' N

120°30' W 120°0' W

White Cloud

Forest Hill

Seed Orchard

Serene Lakes

Hobart Hill

Upper

Incline

Watson Creek

Clear

Creek

Baker Pass

Valhalla

Riverton Ridge

Loon Lake

Blodgett

Sly Park

Kelly Lake Tahoe

Regional

Park

Genoa

Park

Woodford’s

0 5 10 20 30 mi.

Echo Summit

64 Acres

Cave Rock

Sugar Pine

Point

nitrogen dioxide, or nitric oxide [NH3, NH4, NO3, NO2, and NO, respectively]), and 

the contribution from wet deposition were not determined as part of this study.

One of the great difficulties in evaluating air pollution transport into the 

Tahoe basin is the lack of data on the upwind western slope of the Sierra Nevada. 

Bytnerowicz et al. (2004) addressed this problem by using inexpensive passive 

samplers deployed throughout the region. Using a set of O3 and HNO3 concentration 

measurements, a spatial model of pollutant concentrations was constructed (Frączek 

et al. 2003). Frączek et al. observed a clear pattern in O3 and HNO3 concentrations 

over the course of the smog season with the lowest levels occurring in the first half 

of July and the first half of October, and the highest levels occurring in the second 

half of August. Elevated O3 and HNO3 concentrations southeast of the Tahoe basin 

were observed in the second half of August through the second half of September. 

Seasonal averages for HNO3 are shown in fig. 3.4. 

For all pollutants, Bytnerowicz et al. (2004) found decreasing concentrations 

entering from the west into the Tahoe basin, indicating minimal transport to the 

lake. They postulated that the mountain range west of Lake Tahoe basin (i.e., 

Desolation Wilderness) creates a barrier that prevents polluted air masses from 

the West (Sacramento Valley and foothills of the Sierra Nevada) from entering 

the Lake Tahoe basin. 
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

Cahill et al. (2004) used size-segregated chemically speciated measurements 

of chemical species to address the issue of in-basin versus out-of-basin sources 

of P. Measurements were performed in January 2002 and August 2002 using a 

particulate sampler developed at UC Davis (Cahill and Wakabayashi 1993) instead 

of using filter-based measurements. Samples were collected at the South Lake 

Tahoe site for 12 weeks in winter and 6 weeks in summer to allow analysis of 

synoptic weather patterns. They found that almost all the P observed occurs in the 

size fractions between 2.5 and 35 μm, consistent with the theory that resuspended 

road dust and soil is the major source. Previous studies in the area have used 

samplers with a 2.5-μm cut-point and thus would have missed the contribution from 

larger size fractions. This implies that most of the P comes from in-basin sources, 

as particles in this range tend to deposit rapidly.

Based on these findings, it appears that the majority of atmospheric species 

contributing to declining lake water clarity in Lake Tahoe come from in-basin 

sources. Using the emissions inventory estimates presented in the “Visibility in 

Lake Tahoe” section, the major in-basin sources of N, P, and fine particles are 

emissions from motor vehicles and areawide sources (e.g., paved and unpaved 

road dust and residential fuel combustion). In terms of the O3-forming precursors, 

the most likely sources are motor vehicles and biogenic emissions.

Knowledge Gaps

• To date, studies have indicated most of the N, P, and sediment deposition 

come from in-basin sources; however, out-of-basin sources contribute to N, 

PM2.5, and O3 levels in the basin. Quantifying these out-of-basin contributions is recommended. 

• The spatial and temporal resolution of airborne N, P, and particles is limited. Most measurements have been performed as part of intensive studies 

and do not cover all periods of the year or a range of meteorological conditions. In addition, many key species and parameters are not monitored 

(e.g., gaseous N species, P, or size-segregated aerosol chemistry).

• There are limited measurement sites both in the basin and leading up to 

the basin for use in assessing the sources of the pollutants.

• Direct measurements of plume transport to the basin and deposition have 

been limited or nonexistent.
52

GENERAL TECHNICAL REPORT PSW-GTR-226

Research Needs

• Perform air quality measurements of key species under a range of meteorological conditions. Include measurements for NOx, NH3, HNO3, size-segregated 

aerosol mass, particle number and particle size distribution, and aerosol 

chemical composition. Consider the use of advanced monitoring techniques 

such as differential optical absorption spectroscopy, remote sensing, and the 

use of airborne platforms to obtain additional data on key atmospheric species.

• Add air quality monitoring stations in the basin, on the ridges leading into the 

basin, and on the west side of the Sierra Nevada Mountains to assess in-basin 

vs. out-of-basin sources throughout the year and under different meteorological 

conditions. 

• Monitor plume transport using an aircraft platform to directly assess the 

pollutant contribution from out-of-basin sources and link these measurements 

to deposition.

• Develop a Tahoe-specific modeling system that includes appropriate emissions 

data to evaluate in-basin vs. out-of-basin sources of observed pollutants.

• Complete a meta-analysis of existing work on the conclusions for wet/dry and 

in-basin versus out-of-basin contributions. A fair amount of data and information exists, but no one has completed a comprehensive synthesis. Additional 

research questions would then come from this work.

Tahoe Basin Air Quality: The Criteria Pollutants

The Tahoe Regional Planning Agency Compact, amended in 1980, called for TRPA 

to adopt environmental threshold carrying capacities (“thresholds”) to protect the 

values of the Tahoe basin. The first set of comprehensive air quality thresholds were 

adopted by TRPA in August 1982; however, compliance with TRPA’s threshold 

standards as well as changing federal, state, and local air quality and visibility standards could require amendments to the thresholds. Currently, the TRPA is developing a new regional plan that may include new thresholds, indicators, standards, and 

regulations for air quality. Table 3.1 presents an overview of the attainment status 

and trend based on the recent analysis (TRPA 2005) of each criteria air pollutant 

where attainment of standards has been an issue. For reference, federal and California air quality standards for CO, O3, respirable particulate matter (PM10), and fine 

particulate matter (PM2.5) are shown in table 3.2. Note that monitoring locations in 

the basin have been chosen to provide information related to air quality standards, 

rather than to obtain data for deposition assessment. A discussion of the issues 

associated with each of these pollutants is presented below.
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

Table 3.1—Air quality indicator attainment statusa

Attainment status

Threshold criteria

air pollutant 1991 1996 2001 2006 5-year trend

Carbon monoxide Nonattainment Attainment Attainment Nonattainment Positive

Ozone Nonattainment Nonattainment Nonattainment Nonattainment Unknownb

Particulate matter (PM10) Nonattainment Nonattainment Attainment Nonattainment Unknown

a Results for 1991 to 2001 were obtained from Tahoe Regional Planning Agency (2005). 

b More stringent ozone standards became effective in May 2006. This may result in additional ozone violations in the future. 

Table 3.2—California and federal air quality standards

California Federal primary

Air quality constituent Averaging time standard standard

Carbon monoxide 8 hr 9 ppm 9 ppm

1 hr 20 ppm 35 ppm

8 hr (Tahoe) 6 ppm N/A

Ozone 8 hr 70 ppb 75 ppb

1 hr 90 ppb None

Particles less than 10 μm in diameter 24 hr 50 μg/m3 150 μg/m3

Annual arithmetic mean 20 μg/m3 None

Particles less than 2.5 μm in diameter 24 hr Same as federal 35 μg/m3

Annual mean 12 μg/m3 15 μg/m3

Note: The Nevada standards are the same as the federal standards.

Carbon monoxide—

Carbon monoxide is a tasteless, odorless, and colorless gas that is slightly lighter 

than air and is associated with substantial health risks especially at high altitudes. 

It affects humans by reducing the supply of oxygen to body tissues. The primary 

source of CO emissions is combustion of hydrocarbon fuels by motor vehicles; 

home heating devices such as fireplaces, stoves, and furnaces; and industrial 

processes. In the Tahoe basin, the primary source of CO emissions is from mobile 

sources such as motor vehicles and boats. For this reason, it is important to concentrate on transportation improvements within the basin as a control method for 

reducing CO levels. Owing to the substantial health risks posed by CO, the TRPA, 

California, Nevada, and the federal government have all adopted standards for 

this pollutant. 

Carbon monoxide is considered a “hotspot” pollutant, meaning elevated levels 

are very localized. Thus, it is necessary to use data from multiple monitoring 

stations within the basin to report on this pollutant. Currently, CO is only measured 

at one location (South Lake Tahoe), which does not provide the necessary data to 

either evaluate ambient conditions or make recommendations for improvements. 
54

GENERAL TECHNICAL REPORT PSW-GTR-226

Ozone—

Ozone is a secondary pollutant that is formed in the atmosphere by a photochemical 

process involving HC, NOx, and sunlight. This pollutant poses a substantial health 

risk especially to the young and elderly in the form of lung and other respiratory 

illnesses. Ozone also damages trees and plants, particularly ponderosa pines 

(Pinus ponderosa Dougl. ex Laws.), Jeffrey pines (Pinus jeffreyi Grev. & Balf.), 

and quaking aspen (Populus tremuloides Michx.) (Davis and Gerhold 1976, 

Miller et al. 1996). Ozone precursors are produced from human activities such 

as the combustion of fossil fuel, chemical processing, fuel storage and handling, 

and solvent usage. As with CO, the primary source of O3 precursor emissions in 

the basin is vehicle exhaust. Currently, O3 is measured at two locations (South 

Lake Tahoe Airport and Incline Village). However, as is the case for CO, this 

does not provide the necessary data to either evaluate ambient conditions or make 

recommendations for improvements.

Particulate matter— 

Particulate matter (PM) pollution consists of very small liquid and solid particles in 

the air. Two fractions of PM are generally measured: (1) PM10 (particulate matter 

with aerodynamic diameter less than 10 μm) and (2) PM2.5. The primary sources of 

PM10 in the basin include motor vehicles, sand, salt and road dust, smoke from both 

natural and human-set fires, and fugitive dust from construction and the landscape. 

PM10 can increase the number and severity of asthma attacks, cause or aggravate 

bronchitis and other lung diseases, and reduce the body's ability to fight infections. These effects are particularly harmful to children, exercising adults, and the 

elderly. PM10 was only measured at the South Lake Tahoe site from 2001 to 2005 by 

CARB. As with the other pollutants, PM10 measurements are inadequate.

Fine particulate matter, PM2.5, also is of concern. These particles have been 

linked to increases in human mortality and morbidity (Pope and Dockery 2006). 

PM2.5 is primarily generated from combustion processes and can contain significant amounts of carcinogens. California and the federal government have adopted 

standards and have increased efforts to study and control this pollutant. PM2.5 is 

measured at the two IMPROVE sites located at South Lake Tahoe and D.L. Bliss 

State Park. Coverage throughout the basin is inadequate to evaluate ambient conditions or offer recommendations for improvement.

There are a number of issues associated with the implementation of new air 

pollution standards and thresholds. These include:

• Not all agencies are in agreement that a single standard for the criteria 

pollutants is appropriate basinwide. Currently, there are different pollution 
55

An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

standards and measurement protocols for each state, local, and federal 

agency with jurisdiction in the basin. This leads to confusion and the 

expenditure of substantial amounts of resources to keep track of each of 

the multiple standards. 

• A permanent criteria pollutant monitoring program is lacking. The basin’s 

air quality monitoring program has suffered greatly in the last few years 

owing to reductions, relocation, or removal of various monitoring stations 

and the lack of adequate resources to implement this program. 

• An improved modeling system for criteria pollutants is needed. Although 

commonly available in other air basins, critical tools necessary to relate pollutant emissions to ambient and local air quality are lacking for the Lake 

Tahoe air basin. Specifically, these include updated activity data for each 

emissions source and an emissions model. Without these tools, the ability to 

assess the effectiveness of past or future emission reduction strategies is substantially limited. 

• There is a lack of information on ecosystem health effects of the criteria 

pollutants. Although numerous air quality standards have been established 

by the U.S. Environmental Protection Agency (U.S. EPA), California, 

Nevada, and TRPA, these standards were primarily implemented for human 

health concerns. Although it is understood that these standards provide some 

protection for the ecosystem, additional information is needed to ensure 

these standards adequately protect the ecosystem health of the basin. 

Knowledge Gaps

• What is the mechanism to adopt uniform standards for the criteria pollutant 

in the basin? 

• How many air quality monitoring sites are necessary and where they should 

be located to adequately monitor the basin for criteria pollutants and their 

sources and what species should be measured? What air quality monitoring 

is necessary to evaluate programmatic changes? Should monitoring capabilities be added to obtain deposition data?

• To improve and update the emissions inventory, what are the activity, 

population, and emission factors that control emissions from the various 

sources of criteria air pollutants in the basin?

• For the cases where standards are exceeded, what are the appropriate 

emission reduction strategies to use in the basin? What are the costs, 

effectiveness, and constraints of each strategy?
56

GENERAL TECHNICAL REPORT PSW-GTR-226

• To develop thresholds that protect ecosystem health in the basin, what are 

the appropriate ecosystem health standards for criteria pollutants that need 

to be considered? 

Research Needs

• Conduct studies to determine the number, distribution, and sampling 

frequency and protocol of a monitoring network that is adequate to 

obtain information related to air quality standards or other pollutants of 

interest (e.g., polycyclic aromatic hydrocarbon (PAHs), size-segregated 

and chemically speciated PM, and HNO3). As part of this study, consider 

alternative monitoring techniques such as remote sensing or 

biomarkers (i.e., lichens).

• Develop basin-specific activity rates, emissions inventory, and emission 

models necessary to evaluate the present and future conditions and programs 

in the basin. Develop emission estimates for transportation and other 

large-scale programs included in the various planning and programmatic 

documents produced by basin agencies. 

• Complete studies to inform implementation of the programs aimed 

at reducing the criteria pollutants emission levels in the basin. It is 

recommended that these studies also could provide information on 

cost, effectiveness, implementation issues, and constraints. 

• Complete studies to determine the appropriate standards for criteria 

pollutants that protect ecosystem health in the basin. Primary concerns 

would focus on water quality and the clarity of lakes, as well as vegetation 

and wildlife health. 

• Implement studies to assess the impact of pollutant levels (e.g., PM2.5

and PAHs) on the health of humans living at altitudes similar to that of 

the Lake Tahoe basin.

Visibility in Lake Tahoe Basin

Visibility is an indicator of air quality, and good visibility is a desired condition in 

and of itself. The original visibility thresholds for the Lake Tahoe basin were first 

developed by the TRPA in the early 1980s after analyzing data collected in a shortterm (June 1981 to May 1982) visibility monitoring program (Pitchford and Allison 

1984). Both regional (basinwide) and subregional thresholds were developed from 

this study. Regional visibility is defined as the overall prevailing visibility in 
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

the Lake Tahoe basin. The primary impact of regional visibility degradation is a 

general reduction in clarity, contrast, and color of vistas seen through the regional 

haze. Subregional visibility in the Lake Tahoe basin is characterized by a layer of 

perceptible haze that spreads over the urbanized areas, especially the south shore 

of the lake.

When the regional visibility thresholds were defined, it was thought that optical 

measurement techniques of the period (long-path horizon/sky contrast) would be 

unduly influenced by meteorological conditions, thus indicating below-standard 

conditions, when in fact the air in the basin was quite clean (low aerosol concentrations). Because the South Lake Tahoe aerosol consists of a large fraction of absorbing aerosols (e.g., aerosols composed of elemental carbon), it also was realized that 

basing the subregional standard on nephelometers that only measure the scattering 

coefficient, would significantly underestimate the true subregional visibility. As a 

result, an interesting hybrid was developed for both the regional and subregional 

thresholds. The standards were defined in terms of an optical property—visual 

range—but the results were calculated from high-quality speciated aerosol data. 

At the time these standards were developed, no guidelines existed for using the 

calculations. It was thought that proper algorithms would be developed as visibility 

research matured.

In 1989, TRPA instituted a visibility monitoring program to gather the data 

necessary to address its visual air quality standards. The program was fully operational and funded by 1991. Details of the program are discussed elsewhere (ARS 

1989, 2000). The monitoring program consisted of two major, fully instrumented 

sites—Bliss State Park and South Lake Tahoe—and one additional site that has had 

some periodic measurements: Thunderbird Lodge. The TRPA operated the Bliss 

State Park site from 1990 to November 1999. In December 1999, the Bliss State 

Park site was added to the national IMPROVE monitoring network, with funding 

provided by the US EPA. Both sites were operated by Air Resource Specialists, Inc. 

In June 2004, TRPA permanently shut down the South Lake Tahoe site. 

Results from detailed analyses of the limited 1981–82 measurements and monitoring data collected between 1989 and 1991, indicated that within the experimental 

uncertainty of the 1981–82 measurements, there was no statistically observable 

change in the visual air quality levels in the Lake Tahoe basin between 1981 and 

1991. Thus, in 1999 after further monitoring, TRPA restated visibility standards in 

terms of the current monitoring and data analysis techniques and set the baseline 

period as 1991–93 (table 3.3).
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GENERAL TECHNICAL REPORT PSW-GTR-226

Two additional TRPA standards were adopted in the early 1980s:

• Regional visibility: Reduce wood smoke emissions by 15 percent from the 

1981 base values.

• Subregional visibility: Reduce wood smoke emissions by 15 percent and 

suspended soil particles by 30 percent from the 1981 base values.

These stated reduction goals in wood smoke emissions and soil particulate concentrations appear to have been added as qualitative guidelines even though they 

are stated as specific reduction percentages. There are no existing valid estimates 

of wood smoke emissions for 1981, thus deciding if a 15 percent reduction has 

occurred is impossible. The reference to “soil” in the subregional visibility standard 

is not well understood. There is no existing record of what “soil” means, i.e., PM10

mass, reconstructed PM2.5 fine soil, or more probably coarse mass (the resultant 

PM10 to PM2.5 gravimetric mass). Thus, these additional standards have not been 

addressed in any meaningful fashion. However, given the new-found importance 

of the effects fine soil particle deposition on lake water clarity, the TRPA standard 

to reduce suspended soil particles by 30 percent from 1981 base values takes on 

increased importance.

Figure 3.5 shows the TRPA visibility standard cumulative frequency plots for 

the baseline period 1991–93 and 2001–03. As can be seen, subregional visibility has 

improved dramatically since the 1991–93 baseline. Regional visibility has improved 

on the cleanest and average (50 percent frequency) days, but has not improved much 

on the haziest (90 percent frequency) days. The TRPA is recommending through 

the Pathway process to replace the 1991–03 baseline with the 2001–03 period (table 

3.4). This is an attempt to prevent the loss of any visibility improvements that have 

occurred in the basin.

Table 3.3—1999 Tahoe Regional Planning Agency visual air quality environment 

threshold carrying capacities, 1991–93 baseline

Area Visibility threshold

Regional visibility Achieve a visual range of 156 km (bext = 25.1 Mm-1) at least 50

(Bliss to Round Hill) percent of the year as measured by particulate concentrations

Achieve a visual range of 115 km (bext = 34.0 Mm-1) at least 90

percent of the year as measured by particulate concentrations

Subregional visibility Achieve a visual range of 78 km (bext = 50.2 Mm-1) at least 50

(South Lake Tahoe) percent of the year as measured by particulate concentrations

Achieve a visual range of 31 km (bext = 126.2 Mm-1) at least 90

percent of the year as measured by particulate concentrations
59

An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

Figure 3.5—Tahoe Regional Planning Agency visibility standards: cumulative frequency plots of 

reconstructed extinction for 1991–93 (baseline year) and 2001–03 (from TRPA 2005).

Table 3.4—2006 proposed Tahoe Regional Planning Agency visual air quality 

environment threshold carrying capacities, 2001–03 baseline

Area Visibility threshold

Regional visibility Achieve a visual range of 188 km (bext = 20.8 Mm-1) at least 50 

percent of the year as measured by particulate concentrations 

Achieve a visual range of 116 km (bext = 33.7 Mm-1) at least 90 

percent of the year as measured by articulate concentrations 

Subregional visibility Achieve a visual range of 93 km (bext = 42.1 Mm-1) at least 50 

percent of the year as measured by particulate concentrations 

Achieve a visual range of 55 km (bext = 71.1 Mm-1) at least 90 

percent of the year as measured by particulate concentrations 
60

GENERAL TECHNICAL REPORT PSW-GTR-226

Currently there are a number of concerns regarding visibility monitoring in the 

Lake Tahoe basin. The Bliss State Park site has been in continuous operation since 

1990. Its operation and maintenance are currently funded by the US EPA as part 

of the IMPROVE monitoring network; however, funding and continued operation 

are uncertain. The South Lake Tahoe visual air quality monitoring station is now 

permanently shut down owing to loss of the property lease. This site was only 100 

m west of the earlier South Lake Tahoe location; thus data from the site are deemed 

appropriate for use in the TRPA Subregional Visual Air Quality standard calculations. There was no available location near the two past monitoring sites that would 

allow direct continuation of the subregional speciated aerosol monitoring record. A 

new monitoring site has been installed in South Lake Tahoe. However, it is quite a 

distance from Highway 50 and the old monitoring sites. No overlapping time series 

exists, so comparisons among the sites are not possible. 

Exhaust plume from a diesel truck located on a side road off of Highway 89, near Meyers, California. 

Scott Hinton
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

Knowledge Gaps

• Visibility measurement locations are limited in the Lake Tahoe basin compromising the ability to accurately estimate regional and subregional visibility.

• Standards have been adopted requiring percentage reductions in emissions, 

but the link between changes in these sources and the effects on visibility 

are unclear.

Research Needs

• Expand the spatial distribution of the visibility monitoring network 

to include locations in addition to Bliss and South Lake Tahoe is 

recommended. 

• Address the current lack of measurements related to the subregional 

standard. 

• Quantify the contribution of wood smoke and other sources in the basin.

Air Pollutant Emission Inventories

Optimally, emission inventories (EIs) describe the magnitude, along with when 

and where various pollutants are emitted in a regional domain. Because emissions 

are generated from numerous intermittent sources (e.g., fireplaces, disturbed land, 

vehicles, and commercial businesses), assembling a comprehensive emission inventory would include the use of careful assumptions, which result in a product that 

will achieve the necessary objectives. Uses of EIs include:

• Planning—As populations and activities change within a region, it is 

useful to know how this will affect emissions and ultimately atmospheric 

concentrations.

• Mitigation—Knowing the sources and magnitudes of pollutants allows for 

the design of cost-effective control measures that will reduce atmospheric 

concentrations.

• Simulation—EIs are used as inputs to air pollution dispersion models to 

simulate the impacts of sources on atmospheric concentrations within the 

modeling domain.

• Monitoring—Tracking changes in emissions is useful for interpreting other 

long-term time series such as lake sediments and atmospheric concentration records. Correlations between emission records and these time series 

provide strong empirical evidence to relate sources to the observed levels 

of pollutants in the air and lake sediments. 
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GENERAL TECHNICAL REPORT PSW-GTR-226

Because ambient concentrations of aerosols within the Lake Tahoe basin are 

below the National Ambient Air Quality standards, the primary objective of a 

Tahoe-specific EI is to focus mitigating efforts to protect human health and improve 

lake water clarity.

A preliminary EI was assembled for Lake Tahoe as part of the CARB LTADS 

(Kuhns et al. 2004). In this study, local emission factors for road dust, vehicle 

exhaust, and residential wood combustion were measured in the basin. Woodburning activity data were collected via a survey of local residents (Fitz and Lents 

2003). Vehicle exhaust emissions were derived from an estimate of the number 

of gallons of gasoline and diesel sold in the basin. Road dust was estimated from 

Department of Transportation published data of vehicle miles traveled. These data 

were supplemented with CARB county-level emission inventories and extrapolated 

to the portions of the Nevada counties that fell within the Tahoe basin. Emissions 

from wild and prescribed fires were not included in the EI owing to a lack of 

information on the quantity of fuel burned throughout the year. Table 3.5 shows 

the estimates of a variety of air pollutants that culminated from that study. 

Note that many of these sources are estimated with the goal of building a 

comprehensive California EI. As a result, some sources such as farming operations 

and unpaved road dust were estimated by scaling measurements collected in other 

counties to the Tahoe basin based on population or land area. These assumptions 

are unlikely to introduce a large error on the total statewide emissions; however, 

they are likely to be inappropriate for the specific needs of the Tahoe basin.

Airborne dust created during bike trail sweeping near Tahoe Pines, California.

Courtesy of Tahoe Regional Planning Agency
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

Table 3.5—Emission inventory results for the Tahoe basina

Source TOG ROG CO NOx PM PM10 PM2.5

Megagrams per year

Natural (nonanthropogenic) sources 0 0 30 0 5 5 5

On-road mobile sourcesb 1,019 935 2,489 148 7 7 4

Aircraft 112 99 998 73 34 30 30

Recreational boats 344 318 2,500 103 22 17 13

Off-road recreational vehicles 547 503 1,751 34 0 0 0

Off-road equipment 241 219 1,777 602 43 43 39

Fuel storage and handling 56 56

Residential wood combustion and campfiresb 570 251 6,400 187 726 680 653

Farming operations 392 30 60 26 4

Construction and demolition 366 176 39

Paved road dustb 628 287 48

Unpaved road dust 1,138 679 145

Fugitive windblown dust 17 9 4

Waste burning and disposal 202 90 1,162 30 133 129 120

Cooking 4 4 17 13 9

Solvent evaporation 422 387

Stationary sources 413 254 43 82 9 4 4

 Total 4,321 3,148 17,151 1,260 3,206 2,105 1,118

Definitions of emissions are as follows:

TOG = total organic gases, ROG = reactive organic grasses, CO = carbon monoxide, NOx = nitrogen oxide, PM = particulate 

matter, PM10 = particles less than 10 micrometers in diameter, PM 2.5 = particles less than 2.5 micrometers in diameter. a Emissions were estimated by scaling the California Air Resources Board Tahoe Air basin emissions with a multiplier based 

on land area, population, or vehicle kilometers traveled. 

b Sources measured as part of the Lake Tahoe Atmospheric Deposition Study. 

The Desert Research Institute recently completed a year-round monitoring 

program in the Lake Tahoe basin to measure the emissions of PM from roadways. 

The results of this study were integrated into a model to estimate emission factors 

based on the existence of emissions controls as well as meteorological and seasonal 

data for all road types in the Lake Tahoe basin. In addition, the study examined the 

effectiveness of emissions controls (i.e., sweeping, stormwater diversion systems, 

paved shoulders, and track-out prevention) for reducing particulate emissions 

(Kuhns et al. 2007). 

A new project to improve these estimates and allocate them spatially within the 

basin has been approved for funding via the US EPA Region IX with funding from 

the Southern Nevada Public Lands Management Act. This study has recently been 

completed and will provide a detailed emissions inventory for the criteria pollutants 

and other key species (e.g., NH3). Based on this inventory, the major contributors to 

ambient pollutants are as follows:
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GENERAL TECHNICAL REPORT PSW-GTR-226

• CO: Mobile sources and residential fuel combustion. There is a strong 

seasonal dependence in the residential fuel combustion source.

• PM10, PM2.5, P, and phosphate (PO4): Areawide sources, particularly 

residential fuel combustion and road dust resuspension. Emissions are 

significantly higher during the winter. Use of the road sediment data 

obtained with the DRI TRAKER (an instrumented vehicle developed 

to quantify silt loading on roads) significantly reduced the estimated 

resuspended road dust contribution when compared with the previous 

inventory.

• NOx and NH3: Mobile sources are the dominant contributor.

• VOCs: Mobile sources, biogenic sources, and areawide sources all 

contribute to VOC emission. There is a strong seasonal dependence 

in the biogenic and areawide source contributions

Knowledge Gaps

Linking air pollutant emissions to endpoints of interest (e.g., lake water clarity, or 

impacts to human or ecosystem health), creates some unique requirements for a 

Tahoe-specific EI. To develop cost-effective mitigating strategies that will improve 

water clarity, the following topics not generally included in EIs are recommended:

• The EI should account for the major species that are impacting the lake, 

specifically crustal particulates, N and P. 

• Evaluation of the uncertainty and measurement of the size and composition 

of the particulates emitted from different sources. 

• Accounting for emissions from events such as wildfires and prescribed 

burns, which can contribute to pollutant emissions.

• Addressing the lack of knowledge regarding the impact of wet deposition 

and scavenging by vegetation that may ultimately contribute to pollutant 

runoff into the lake. 

• Knowing what N and organic species are emitted locally and what are 

transported into the basin in order to accurately simulate the fate and 

transport of N in the basin.

Research Needs

• It is recommended that emissions be geo-referenced to their specific 

sources (i.e., roads, erodible hillsides, beaches, residences, and fire sites). 

Additional work on the emissions inventory may be necessary to evaluate 

the contributions from out-of-basin sources.
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

• It is recommended that emissions for specific events (i.e., wild and prescribed fires) be estimated on a case-by-case basis, based on acreage and 

fuel mass burned. Numerous specific measurements are needed to develop 

confident regional or basinwide estimates.

• For smaller ubiquitous sources (i.e., residential wood combustion and 

vehicle exhaust), the development of seasonal profiles is recommended to 

accurately simulate how their magnitude changes over the course of a year.

• It is recommended that source samples from major sources be reanalyzed 

using techniques that can better resolve the concentrations of bioavailable N 

and P, and distinguish the number and size distribution of fine soil particles 

that affect lake clarity.

• Development of mobile source emission factor models for Tahoe-specific 

conditions (e.g., vehicle age and model year distribution, altitude, or grade) 

is recommended.

• Regular updates of activity estimates (e.g., vehicle miles traveled) are 

recommended.

• It is recommended that the inventory estimates be compared with water 

and air concentrations to make sure the results are consistent with what can 

be empirically observed. When inconsistencies are observed, additional 

research could be recommended to improve estimates and reduce uncertainty.

• Determine size-segregated PM emissions to better understand transport 

deposition processes.

• It is recommended that the inventory be used to assess health impacts and 

additional information related to this need be included.

• Apply receptor modeling techniques (e.g., chemical mass balance and 

positive matrix factorization) to validate the emissions inventory.

Atmospheric Modeling of the Lake Tahoe Basin

Currently, the only available model specific to the Lake Tahoe basin is the LTAM. 

The LTAM is a heuristic model that was designed more to merge existing data into 

a self-consistent framework than derive results from first principles, a so-called 

deterministic model. The model was developed to analyze the effects of prescribed 

fires and wildfires on air quality (PM2.5 mass) and visibility, and would need some 

modifications and enhancements to handle deposition into Lake Tahoe for water 

clarity analysis.
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GENERAL TECHNICAL REPORT PSW-GTR-226

The LTAM is a gridded model, dividing the Lake Tahoe air basin into 1,500, 

2.59-km2 domains. Each domain has a land use type (e.g., forest, lake, or urban), 

potential sources (e.g., transport from upwind, fire smoke, urban emissions, or 

roadway emissions), meteorological transport (mean 12-hr day, 12-hr night, summer, winter), and particle removal rates (e.g., deposition to trees, lake surface). 

Sources generate aerosol masses that are passed downwind from cell to cell with 

lateral dispersion and removal rates included, based upon lateral wind variability 

from the South Lake Tahoe data. Cliff and Cahill (2000) provided a full description 

of the model development. 

One of the most difficult problems in developing a model at Lake Tahoe is that 

the data sources are widely dispersed in space and time. For example, there is excellent meteorology for 11⁄2 years at Tahoe City in 1967, daytime values from the South 

Lake Tahoe airport, and local metrology at the ARB Sandy Way Bliss, and TRPA 

SOLA sites.

Figure 3.6 presents an example of the LTAM calculation for smoke from a 101 

ha/day prescribed fire in the upper Ward Creek watershed. Nighttime downslope 

winds drive the smoke out over the lake, but note that relatively little smoke has 

penetrated to the southern end of the basin. The basic framework of LTAM was 

validated in 1999 via a 121-ha fire on Spooner Summit. The model output was compared to mass data from filter samples, with the results directly reflecting particle 

mass and indirectly reflecting visibility reduction. However, there are additional 

enhancements needed to meet the needs of water clarity. These enhancements are 

partially driven by improved water clarity models (Losada-Perez 2002, Schladow 

et al. 2004) that identify, in addition to the standard nitrate and phosphate inputs, 

airborne fine particles as an important factor in lake clarity. The most important 

enhancements involve incorporation of the new CalTrans data on soils, improved 

data on P from prior TRPA and LTAD studies, incorporation of the LTAD spatially 

and temporally dispersed data, and an enhanced particle deposition algorithm. 

These results can then be directly compared to the long-term record from the 

deposition buckets on and near the lake (Jassby et al. 1994). 

Recently, an extensive study was conducted on both sides of Highway 50 in 

order to examine emissions of particulate matter (and P) from roadways (Cahill 

et al. 2006). These data were used to update the LTAM and enhance its ability to 

predict P concentrations in the basin. Figure 3.7 shows an example of the updated 

LTADS predictions for a fire event incorporating both fire and traffic impacts. 
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

Figure 3.6—Example of Lake Tahoe Atmospheric Model prediction of the evolution of PM2.5 from 

forest fire smoke for 3 days during the Ward Creek prescribed burn. The z-axis represents PM2.5

concentrations in μg/m3

. The bottom graph is day 1 and the top graph is day 3.

μg/m3 μg/m3 μg/m3

200

100

0

100

0

100

0

N

N

N
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GENERAL TECHNICAL REPORT PSW-GTR-226

Figure 3.7—Lake Tahoe Atmospheric Model extended to include phosphorus concentrations (ng/m3) around and over 

Lake Tahoe. Courtesy of UC Davis Delta Group.

Winter phosphorus (ng/m3)

30.0–35.0

25.0–30.0

20.0–25.0

15.0–20.0

10.0–15.0

5.0–10.0

0–5.0

N

North–South

Distance from NW origin (km)

East–West

1

9

17

25

33

41

Knowledge Gaps

• Currently, the only modeling system developed for use in the basin is the 

LTAM, which is a heuristic gridded model that allows the merging of the 

limited amounts of meteorological, aerosol, and gas data into a mass conserving spatial distribution. No information on wet deposition or chemical 

transformations is included. Thus, it is no better than its limited data set, 

and is sensitive to gross assumptions.

• Additional limitations of the LTAM include limited grid size (2.59 km2

); 

includes only first-order deposition; and does not handle multiple inversion 

layers, including their impact on upwind sources. 

Research Needs

• Examine the literature for preexisting models that could be adapted to 

Lake Tahoe use. These models may supersede the LTAM.

• If continued use of the LTAM is going to occur, then the following 

improvements are recommended:

▪ Increase the spatial resolution of LTAM and add a near-roadway 

program model for the cells that straddle lakeside roads. 

▪ Add the information from LTADS study into the LTAM or any 

future modeling system.
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

▪ Add the LTADS aerosol data from the upwind sites into the LTAM 

or any future modeling system.

▪ Improve the deposition module in LTAM.

▪ Compare the LTAM sediment and P-deposition predictions with 

on-lake aerosol data from LTADS.

▪ Develop specific features in the model to address the potential 

effects of air pollutant control options that are spatially located 

within the basin. 

• Perform sensitivity analyses with the LTAM or any future modeling 

system to evaluate management strategies.

Impact of Fire on Air Quality

Atmospheric pollutants that contribute to overall air quality at Lake Tahoe derive 

from both natural and anthropogenic sources. For instance, wildfires, volatile 

organic compound emission from trees, and wind-blown dust from natural landscapes all are natural phenomena. On the other hand, automotive and industrial 

pollutants, prescribed fire smoke, and human-caused wildfire smoke all derive from 

anthropogenic sources. Fire sources can be broken into six forest regimes and one 

urban regime:

• Forest regimes:

▪ Natural wildfires. 

▪ Wildfire type 1—surface burn—close to natural wildfires, 

sometimes occurs after prescribed fires burn out of prescription.

▪ Wildfire type 2—passive crowning fire (e.g., the 1992 Cleveland 

wildfire at the maximum impact site).

▪ Wildfire type 3—active crowning wildfire (e.g., as in the early 

phases of the Oregon Biscuit complex fire).

▪ Prescribed fire type 1—pile burn, PF1 in which there is lofting of 

smoke (h) to greater altitudes (0.1 < h < 0.5 km).

▪ Prescribed fire type 2—surface burn, PF2, in which there is no 

lofting of smoke (h) (0 < h < 0.1 km), as in the 1992 Turtleback 

Dome (Yosemite National Park) prescribed fire.

• Urban regime:

▪ Residential wood fires.

It is surprisingly difficult to establish the effect of forest regime smoke sources 

on Sierra Nevada air quality. Smoke has a visual impact out of proportion with 

the mass of smoke present, so that smoke levels must be extreme before the record 
70

GENERAL TECHNICAL REPORT PSW-GTR-226

Smoke plume from the 2007 Angora wildfire in South Lake Tahoe, California. 

Courtesy of University of Nevada Cooperative Extension

of particulate mass reflects a major impact. Yet the 24-hour federal particulate 

standard for PM10 is not violated until visibility drops to about 3.2 km. Most of the 

air particulate sampling in the Sierra Nevada measures only PM10 mass, and thus is 

of limited use in identifying small and moderate smoke impacts. These sites only 

operate on a 1-day-in-6 cycle, and due to urban locations, are of little use to establish nonurban smoke levels. Further, the data on how many acres are burned each 

day from either wildfires or prescribed burns is often difficult to access. Meteorological measurements in the mountains are scarce, and terrain effects are major.

The IMPROVE (Interagency Monitoring for Protected Visual Environments, 

Malm et al. 1994) database is useful in several regards. The measurements are 

PM2.5, a better match to the size of smoke particles. The sites operate Wednesday 

and Saturday, in nonurban, nonvalley locations, and have full meteorology, 

chemical, and optical analysis. However, in 2002, there were only two such 

sites in the Sierra Nevada: Sequoia and Yosemite National Park. Fortunately, 

the paired stations at Lake Tahoe (Bliss and South Lake Tahoe), operated for 

the TRPA using full IMPROVE protocols, provide a very important third site, 

as well as an invaluable nonurban to urban comparison. Finally, data are extended 

by using similar sites in the Cascade and San Bernardino Mountains. This data set 

is used for long-term data on Sierran smoke, supplemented by local studies.
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

Figure 3.8—Lake Tahoe Atmospheric Model output for PM2.5 concentration distribution (μg/m3) in 

the Lake Tahoe basin (underlying map) from historical fire situation based on a 24-hour average. μg/m3

30

20

10

N

0

Impacts from “natural” wildfires are not seen today since this regime, (numerous small, noncrown fires in summer and early fall) ended in the mid 19th century. 

The expected air quality in the Tahoe basin under conditions of “natural” wildfires 

is for spotty but persistent smoke in relatively low concentrations around the basin. 

This regime has been modeled in LTAM (e.g., see the discussion in “Air Pollutant 

Emission Inventories” section), based upon the fire scars on Tahoe basin trees that 

yielded an average of 30 burned acres per day. The model results suggest the pollution maximum over the lake each morning did not exceed present (65 μg/m3

) and 

proposed (35 μg/m3) PM2.5 mass standards (fig. 3.8). 

Present day wildfires are often human caused and always enhanced by humans 

owing to fuel buildup. They are infrequent, but can and have had massive impacts 

on the Lake Tahoe basin, degrading visibility and probably violating state and federal air quality standards (based on Truckee data). However, the 202 300-ha Biscuit 

Fire in Oregon, which during many days was an actively crowning fire, delivered a 

maximum of only 20 μg/m3

 (PM2.5) into the Tahoe basin in August 2002. This was 

still adequate to largely obscure the visibility across the long axis of Lake Tahoe. 
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GENERAL TECHNICAL REPORT PSW-GTR-226

There are relatively few air quality data on the impacts of prescribed fire beyond 

the obvious smoke plumes seen near such burns. There are good reasons for this lack 

of data. First, filter measurements near a prescribed burn will often clog with the 

vapors from the burn. Second, it is difficult to obtain a close-in representative sampling site, especially when pile burns push the smoke up through the forest canopy. 

Analysis of aerosol data from several sites in the Sierra Nevada indicates that 

the most severe impacts on air quality occur from large wildfires, but shows little 

effect of controlled fires at remote locations (Cliff and Cahill 2000). In addition, 

relatively low levels of PM are seen during the subsequent fall season when the 

majority of agricultural waste burning occurs in the San Joaquin Valley as well as 

controlled burning in nearby forests for fire suppression and silviculture.

Current data suggest controlled forest burns are not a major source of particulate 

mass in populated areas of the Sierra Nevada, as compared to residential wood 

combustion and campfires (Cliff and Cahill 2000). Large wildfires produce severe 

short-term impacts on air quality. Prescribed or controlled burns are more common, 

but the amount of materials burned are more modest, and the measures to limit 

human smoke impacts are generally quite effective, leading to very low contributions to PM10 particulate loading in inhabited areas. Thus, it would appear that 

prescribed fires are usually performed in such a way as not to cause a substantial 

threat to regional air quality as measured by fine particulate mass. The obvious 

exception is for some local visibility reduction, but this would be offset by improved 

air quality from decreasing the fuel accumulation and resulting impacts of potential 

major wildfires that may occur. 

The best data on the impact of residential wood burning come from the TRPA 

sampling site at South Lake Tahoe. Based on these data and the results from D.L. 

Bliss State Park, located in a largely undeveloped area on the west shore of Lake 

Tahoe, it appears that residential wood combustion is a major source of PM in South 

Lake Tahoe. The only period in which occasional elevated levels of smoke are 

detected at both sites, indicating a source outside the basin, is the late fall when large 

amounts of cropland are being burned in the Sacramento Valley and controlled burning in the surrounding national forests is at its peak. But even in these conditions, 

the smoke levels are far less than the winter peaks in South Lake Tahoe (roughly 20 

percent of the total observed PM), and of much shorter duration.

Emissions from fire can impact lake clarity. Fires can be a source of P (Turn et 

al. 1997), but this appears to be very sensitive to the conditions in the burn as well as 

the type of vegetation. However, if there is extreme uplift of a catastrophic wildfire 

(active crowning), the P gets sucked up into the smoke plume and can be deposited 

many kilometers away. 
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

In summary, resolution of the questions regarding the impact of smoke in 

the Sierra Nevada mountains is difficult based on limited composition, size, and 

transport data for this source. In the Lake Tahoe basin, knowledge of meteorology 

for much of the basin as well as deposition to the lake surface is lacking, although 

the LTADS data set should help. Furthermore, few measurements have been made 

of emissions from wildfire and prescribed fires for both mass and chemistry. 

The LTAM would greatly benefit from increased knowledge of these parameters. 

Nevertheless, smoke from fires remains a major factor in visibility degradation in 

the Lake Tahoe basin (Molenar et al. 1994). Large wildfires are also reported to 

impact Lake Tahoe water quality by causing algal blooms in the lake (Goldman 

et al. 1990), although the impacts may be short-lived (TERC 2008). The impact 

of prescribed fire, however, is relatively unknown, but probably minor based on 

historical levels. 

Smoke from prescribed fire pile burning off Highway 267, north shore, Lake Tahoe. 

Scott Hinton
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GENERAL TECHNICAL REPORT PSW-GTR-226

Knowledge Gaps

• Understanding of how changes in prescribed fire regimens on the western 

slope of the Sierra Nevada will impact Lake Tahoe basin smoke levels. 

• Understanding of the uncertainty associated with the impact of out-of-basin 

wildfires on Lake Tahoe basin visibility.

• How different methods of prescribed fire within the Lake Tahoe basin 

impact basinwide visibility and air quality.

• A better understanding of the impacts of fires in general on deposition of 

particles and nutrients onto Lake Tahoe.

• The contribution of in-basin residential wood burning versus in-basin 

prescribed fires.

• The impact of fires on human health and emissions of toxic species such 

as polycyclic aromatic hydrocarbons (PAHs).

• Measures to effectively improve visibility and reduce deposition in 

smoke-impacted scenarios.

• What measures can effectively be taken outside but upwind of the Lake 

Tahoe basin to improve visibility and reduce deposition and air quality 

impacts in smoke-impacted scenarios in the basin?

Research Needs 

• Gather more detailed estimates of the frequency and location of prescribed 

fires along with measurements of prescribed fire aerosols by size, type, and 

composition by fire type (e.g., pile burn, or surface burn) and meteorology, 

and use cameras for vertical development of smoke, are recommended.

• Use impactors and continuous PM monitors (as opposed to filters) to 

quantify PM levels and the filter artifact from semivolatile organics in 

near-fire analyses.

• Measure the elemental carbon/organic carbon ratio as a function of the 

type of fire and fuel.

• Assess the impact of fires on air quality, deposition, and human health.

• Evaluate the effects transport of prescribed fire aerosols on the western 

slope of the Sierra Nevada have on Lake Tahoe, including nutrient deposition onto the lake.

• Evaluate the impact of wildfires by season, type, and transport, including 

nutrient deposition onto Lake Tahoe.
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

• Better establish and quantify the role of residential wood smoke in winter 

conditions. 

• Develop advanced measurement (e.g., satellite data) and visualization 

capabilities to aid with the assessment of fire impacts on air quality.

Near-Term Research Priorities

Air quality (AQ) near-term research priorities are as follows:

• (AQ1) Improving air quality and meteorological monitoring in the basin 

is highly recommended. The first step to addressing current deficiencies 

would include development of a comprehensive monitoring plan for the 

basin that addresses the criteria pollutants (covered under the NAAQS) and 

species impacting human and ecosystem health (including water clarity 

and forest health), along with the requisite spatial and temporal distribution 

of the measurements. Once this plan is prepared, sampling locations can 

be chosen and appropriate air quality and meteorological sensors can be 

deployed.

• (AQ2) Many of the key chemical species and physical parameters leading 

to secondary pollutant formation and/or deposition to the lake are not currently measured. Air quality measurements of key species under a range of 

meteorological conditions and averaging times are recommended following 

the development of a monitoring plan (see air quality monitoring description above). Parameters to measure include measurements for NOx, NH3, 

HNO3, size-segregated aerosol mass, particle number and particle size distribution, and aerosol chemical composition. Once these data are obtained, 

it will be possible to assess atmospheric impacts and air pollutant trends. 

This information will also provide the necessary input for developing an 

understanding of the processes controlling air quality and atmospheric 

deposition in the basin.

• (AQ3) The only model adapted for use in the basin is the LTAM, a heuristic 

model that is based on statistical input. An appropriate air quality model 

incorporating physical processes needs to be developed that can utilize 

the full suite of meteorological, chemical, and particulate data. This will 

enable managers and scientists to better assess air pollutant trends, estimate 

impacts, and support the development of effective regulations that will 

assist in meeting air quality and other environmental goals. 

• (AQ4) Atmospheric deposition to the lake is the major source of nitrogen 

and a substantial source of phosphorous and particulate matter. There is, 

however, significant uncertainty in deposition flux estimates. To reduce 
76

GENERAL TECHNICAL REPORT PSW-GTR-226

this uncertainty, conduct focused studies (gradient or eddy-correlation studies, along with measurements of key species) of the sources and pathways 

of particle deposition to better inform models and restoration efforts. 

• (AQ5) Mobile source emissions are a major source of pollutants in the 

basin. Improving our understanding of mobile source emissions would 

include obtaining Tahoe-specific vehicle model year distributions, emission 

factors, and activity data for use in mobile source emission factor models. 

These results will reduce the uncertainty in the emissions inventory and 

enable regulators to develop more effective strategies to reduce pollution 

in the basin.

English Equivalents:

When you know: Multiply by: To get:

Meters (m) 3.28 Feet

Kilometers (km) .621 Miles

Hectares (ha) 2.47 Acres

Square kilometers (km2

) .38 Square miles

Micrograms (μg) 3.527 × 10-8 Ounces

Micrometers (μm) 3.937 × 10-6 Inches

Tonnes 2,204.6 Pounds

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An extraordinary upwelling in a deep, thermally stratified lake. Geophysical 

Research Letters. 31: L15054.

Schultz, H.B.; Akesson, N.B.; Yates, W.E. 1961. The delayed “sea breezes” in 

the Sacramento Valley and the resulting favorable conditions for application of 

pesticides. Bulletin of American Meteorological Society. 42: 679–687.
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Scire, J.S.; Strimaitis, D.J.; Yamartino, R.J. 2000. A user’s guide for the 

CALPUFF dispersion model. Concord, MA: Earth Tech. 521 p.

Seinfeld, J.H.; Pandis, S.N. 1997. Atmospheric chemistry and physics, 

New York: John Wiley and Sons: 1326 p. 

Swift, T.J.; Perez-Losada, J.; Schladow, S.G.; Reuter, J.E.; Jassby, A.D.; 

Goldman, C.R. 2006. Water clarity modeling in Lake Tahoe: linking suspended 

matter characteristics to Secchi depth. Aquatic Sciences. 68: 115.

Tahoe Environmental Research Center [TERC]. 2008. Tahoe state of the 

lake report 2008. University of California, Davis. http://terc.ucdavis.edu/

stateofthelake/index.html. (October 15, 2008).

Tahoe Regional Planning Agency [TRPA]. 2005. Air quality supplement for the 

PATHWAY 2007 evaluation report. Stateline, CA. 51 p.

Tarnay, L.; Gertler, A.W.; Blank, R.R.; Taylor, G.E. 2000. Preliminary 

measurements of summer nitric acid and ammonia concentration in the Lake 

Tahoe basin airshed: implications for dry deposition of atmospheric nitrogen. 

Environmental Pollution. 113: 145–153.

Tarnay, L.; Johnson, D.; Gertler, A.W. 2005. Modeled inputs of atmospheric 

nitrogen to the Lake Tahoe basin due to gaseous pollutant deposition. Journal of 

the Nevada Water Resources Association 2: 41–57.

Turn, S.Q.; Jenkins, B.M.; Chow, J.C.; Pritchett, L.C.; Campbell, D.; Cahill, 

T.; Whalen, S.A. 1997. Elemental characterization of particulate matter emitted 

from biomass burning: wind tunnel derived source profiles for herbaceous and 

wood fuels. Journal of Geophysical Research. 102: 3683–3699.

Zaremba, L.L.; Carroll, J.J. 1999. Summer wind flow regimes over the 

Sacramento Valley. Journal of Applied Meteorology. 38: 1463–1473. 
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83

An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

John E. Reuter,2 James M. Thomas,3 and Alan C. Heyvaert4

Introduction

Lake Tahoe is a unique environmental treasure that has been designated by the 

state of California (in 1980) as an Outstanding National Resource Water under the 

federal Clean Water Act. However, the lake’s hydrologic and air basins are part of 

a changing landscape, with substantial portions of this once pristine region now 

urbanized. Studies during the past 40 years have shown that many factors have 

interacted to degrade the Lake Tahoe basin’s air quality, terrestrial landscape, 

and water quality. These factors include land and forest disturbance, increasing 

resident and tourist populations, increasing recreational use, habitat loss, air pollution, fire suppression, soil erosion, roads and road maintenance, and loss of natural 

landscapes capable of detaining and infiltrating stormwater and snowmelt runoff 

(e.g., Reuter and Miller 2000). As presented below, the progressive decline in lake 

water clarity has served as a key indicator of the decline in Lake Tahoe’s historical 

ultra-oligotrophic condition. Moreover, many consider lake water clarity a gauge of 

the watershed’s health as a whole.

Several decades of progressively greater disturbance in the Tahoe basin, along 

with increased pollutant loading, have been accompanied by a concerted effort 

to understand the processes that control water quality and to alert the public to 

the implications of allowing current trends to continue unabated. Simultaneously, 

during the past quarter century, numerous institutions have made substantial efforts 

to control these impacts, reverse the decline in lake clarity, and reduce pollutant 

loading to Lake Tahoe, its tributaries and its ground-water aquifers (e.g., CTC 2006; 

LRWQCB and NDEP 2008a, 2008b, 2008c; TRPA 2007). 

The watershed approach taken at Lake Tahoe recognizes that water quality 

is linked to upland watershed processes and air quality as well as to the legacy 

of adverse impacts to terrestrial and aquatic habitats. Consequently, successful 

implementation of land, air, and water quality restoration projects is considered 

key to arresting further decline in lake clarity. This understanding precipitated 

Chapter 4: Water Quality1

1 Citation for this chapter: Reuter, J.E.; Thomas, J.M.; Heyvaert, A.C. 2009. Water quality. 

In: Hymanson, Z.P.; Collopy, M.W., eds. An integrated science plan for the Lake Tahoe 

basin: conceptual framework and research strategies. Gen. Tech. Rep. PSW-GTR-226. 

Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research 

Station: 83–182. Chapter 4.

2 University of California Davis, Department of Environmental Science and Policy, 

Davis, CA 95616.

3 Center for Watersheds and Environmental Sustainability, Desert Research Institute, 

Reno, NV 89512.

4 Division of Hydrologic Sciences, Desert Research Institute, Reno, NV 89512.
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GENERAL TECHNICAL REPORT PSW-GTR-226

the formulation of the Environmental Improvement Program (EIP) by the Tahoe 

Regional Planning Agency (TRPA) and its partners in the federal, state, and local 

governments, and the private sector. The EIP is a regional restoration plan that 

describes programs considered necessary to achieve environmental goals in the 

Tahoe basin (TRPA 2007).

Science has played a key role in decisionmaking within the community of 

resource management agencies. Hundreds of scientific papers and reports have 

been written on many aspects of Lake Tahoe, its watershed, and its water quality 

since studies first began over 40 years ago. Many of these were reviewed in Reuter 

and Miller (2000). Active science involvement continues, and since 2000 many new 

findings have been produced. For example, the Lake Tahoe Total Maximum Daily 

Load (TMDL) program is serving as a science-based water quality restoration plan 

for Lake Tahoe (LRWQCB 2008) that addresses the following issues:

• Identify major pollutant sources and, where possible, quantify loading of 

nutrients and sediments to Lake Tahoe.

• Determine the extent to which the load of sediment and nutrients from the 

watershed and air basin can be effectively reduced by management and 

restoration activities.

• Understand how Lake Tahoe’s clarity will respond to environmental 

improvement efforts.

Sources of scientific information used to address these TMDL and other 

water quality policy issues include:

• Historical Tahoe data and analyses

• Scientific literature

• New and existing monitoring

• Laboratory experiments

• Field experiments

• Demonstration projects

• New statistical analyses

• Modeling 

• Best professional judgment based on scientific information

This chapter addresses science needs for water quality in the Lake Tahoe basin. 

It is intended to serve as a road map for discussions with resource managers to 

identify those research or science projects necessary to help guide water quality 

management strategies and understand related ecosystem processes.
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

Review of Important Background Elements 

Lake Tahoe is a well-studied feature of the Tahoe basin ecosystem. However, 

although a substantial amount of research and monitoring has been accomplished, 

it has only been in recent years that the institutional commitment has been made to 

focus this work on specific management issues, such as the Lake Tahoe TMDL 

program. Knowledge gaps and uncertainties about what is known still exist. Moreover, water quality restoration efforts in the Tahoe basin are expected to exceed $1 

billion, so it is critical that we continue to collect data and develop new information 

in an organized fashion in support of future investments. This scientific information is needed so that basin agencies know which management strategies are working and which strategies are not.

Based largely on the past investigations of the University of California at Davis 

(UC Davis), the Desert Research Institute, the U.S. Geological Survey, and the University of Nevada, Reno, there has been substantial effort since 1998 to integrate 

scientific efforts at Lake Tahoe, particularly in the area of water quality. Research 

institutions are pursuing the integration of information at the ecosystem level and 

among the scientific community and managers and decisionmakers. The focus of 

this collaboration has been to facilitate conversion of science information into management actions. Timely feedback of research findings for Lake Tahoe restoration 

activities is central to an adaptive management framework. This feedback relies on 

the completion and communication of basic and applied research, expanded monitoring, modeling, and best professional judgment. Such efforts are best guided by 

a more formalized research agenda. 

Our goals for this water quality research strategy are to:

• Update the Key Management Questions (KMQs) that relate to water quality 

on the basis of work accomplished to date, and integrate them in a manner 

that clearly defines how they apply to the programmatic needs of agencies.

• Identify sound science activities that will help answer remaining water 

quality KMQs. 

• Discuss the current or anticipated levels of certainty and areas of knowledge gaps of these water quality topics with respect to policy and resource 

management actions.
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GENERAL TECHNICAL REPORT PSW-GTR-226

Anticipated Water Quality Topics Requiring 

Additional Data, Research, and Modeling

On the basis of numerous discussions, workshops, and focused programmatic meetings between researchers and Tahoe basin agency representatives, the current water 

quality topics (i.e., subthemes) are listed below followed by the name(s) of the topic 

leaders.5

• Lake water clarity (John E. Reuter and S. Geoffrey Schladow, UC Davis)

• Near-shore water quality (Richard B. Susfalk, Desert Research Institute) 

• Pollutant loading from urban sources (Alan C. Heyvaert, Desert Research 

Institute)

• Stream channel erosion (Andrew Simon, USDA National Sedimentation 

Laboratory)

• Water quality treatment and source controls (Alan C. Heyvaert and 

James M. Thomas, Desert Research Institute, and Timothy G. Rowe,

U.S. Geological Survey) 

• Function of upland watershed with respect to hydrology and water quality 

(Mark Grismer, UC Davis)

• Water quality and forest biomass management practices (Wally Miller, 

University of Nevada, Reno and Sue Norman, U.S. Forest Service, Lake 

Tahoe Basin Management Unit [LTBMU])

• Drinking water protection (Michelle Sweeney, Allegro Communications, 

South Lake Tahoe)

• Water quality modeling (John E. Reuter and S. Geoffrey Schladow,

UC Davis) 

• Influence of climate change on hydrology and pollutant loading 

(Robert N. Coats, Hydroikos/UC Davis)

Below, we provide information on these subthemes with regard to what we 

know, the associated level of certainty, knowledge gaps, and ideas for research to 

address remaining key water quality issues. 

5 These sections were also informed by the following contributors: Brant Allen (UC Davis), 

Phil Bachand (Bachand & Associates), Clary Barreto (Tetra Tech, Fairfax, VA), Nicole 

Beck (2ndNature, Inc.), Sudeep Chandra (University of Nevada, Reno), Robert N. Coats 

(Hydroikos), Julie Etra (Western Botanical Services, Inc.), Scott Hackley (UC Davis), 

Michael Hogan (Integrated Environmental Restoration Services), Roger James (Water 

Resources Management), Theresa Jones (Nevada Department of Transportation), Steve 

Kooyman (El Dorado County Department of Transportation), Virginia Mahacek (Valley 

and Mountain Consulting), Sue Norman (U.S. Forest Service, LTBMU), Steve Patterson 

(Steve Patterson Consulting), Eric Strecker (GeoSyntec Consultants), Ed Wallace (Northwest Hydraulic Consultants), Russ Wigart (El Dorado County Department of Transportation), and Brent Wolfe (Northwest Hydraulic Consultants). 
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

Water Quality Conceptual Model

Prior to the arrival of European settlers, the Lake Tahoe Watershed was thought to 

have operated as a heterogeneous hydrologic system. Precipitation (both snow and 

rain) was distributed broadly through a variety of natural conditions defined by 

natural topography, habitat structure, and local meteorology. Natural features in the 

catchment determined the degree of surface water infiltration and surface groundwater interactions. Fire, floods, and other natural disturbances (e.g., earthquakes, 

landslides, or avalanches) were the major forces of disturbance and could generate 

major releases of pollutants such as fine sediment and nutrients. However, these 

were likely episodic in nature, with potentially substantial intervening periods 

between major events. More regular, low-intensity fires and a mature forest likely 

translated into low-nutrient stores on the forest floor. These were the watershed 

conditions that supported an ultra-oligotrophic Lake Tahoe: a lake with a sustained 

level of exceptional water clarity (≥30 m), a lake receiving low inputs of nutrients 

and therefore supporting low levels of primary productivity, and a lake containing 

a relatively simple food web that may have substantially relied on the recycling of 

nutrients and carbon, rather than new inputs from the surrounding watershed. 

Urbanization and other forms of infrastructure development in the Tahoe basin 

since the mid-1800s have contributed to a change in the natural hydrologic routing 

in many catchments. Development has also resulted in substantial areas of land 

disturbance and impervious cover, which directly affects runoff dynamics and 

inhibits infiltration. With this development comes a hydrologic system that tends 

to concentrate surface runoff and inhibit surface water–ground water interactions. 

Studies completed as part of the Lake Tahoe TMDL show disproportionately higher 

loads of fine sediment and nutrients coming from the urban-related land uses 

(LRWQCB and NDEP 2008a, 2008b, 2008c). Much of the urban development has 

occurred along the edge of Lake Tahoe, meaning that in most of these cases, there 

is little or no buffer between the highest source of pollution and the lake. Development, primarily inside the basin, is now thought to be responsible for many of the 

primary and secondary drivers of water quality (fig. 4.1).

From a water quality perspective, our contemporary understanding of the Lake 

Tahoe watershed is framed around the “pollutant pathway” concept. This concept 

follows a logical sequence of pollutant generation, transport, fate, and system 

response including (1) source identification, (2) transport within the watershed, 

(3) control and abatement, (4) loads to tributaries and the lake, (5) fate of pollutant 

material in the lake, and (6) assessment of water quality response. A water quality conceptual model illustrating this contemporary understanding is presented 

schematically in figure 4.1. This diagram is not intended to identify all the drivers 
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GENERAL TECHNICAL REPORT PSW-GTR-226

nor show all the linkages associated with water quality at Lake Tahoe. Instead the 

objective is to highlight select aspects of the “pollutant pathway” while emphasizing 

a number of key issues that will need consideration as resource managers develop 

and implement pollutant reduction strategies and evaluate resultant localized and 

basinwide effectiveness. 

Figure 4.1—Conceptual model for Lake Tahoe water quality subthemes. This model focuses on the pollutant pathway for fine sediment 

particles (<20 μm) and nutrients (nitrogen and phosphorus). Key processes in this pathway include source identification, transport within 

the watershed, control and abatement, defining loads to Lake Tahoe, fate in Lake Tahoe, and assessment of water quality response. For 

ease of viewing, only key linkages are shown. BMP denotes best management practice. Near-term water quality priorities are indicated 

by alpha numeric symbols (e.g., WQM2, PLT3) and correspond to the descriptions presented later in the chapter.

Geology

Urban land 

use and 

hard coverage

Precipitation

and hydrology

Climate 

change

Roadways

PLT3

PLT1,5

PLT2,5,6

PLT5

EPT1

Atmospheric

deposition

Water quality

treatment

Forest

management

Flood plain/

riparian 

modification

In-lake 

processes

2° Driver 1° Driver Component Indicator Subtheme Theme Linkage

Urban

hydrology

Sediment/nutrient

yield from forests

Urban

pollutant loads

Riparian/flood-plain

functions

Runoff and 

infiltration rates

Drinking water

quality

Lake water quality

Near-shore 

water quality, ecology, 

and aesthetics

BMP

effectiveness

Forest fuels

management

In-lake

particle loss

Erosion

Fine particle

and nutrient

sources

WQM2

CC1,2

CC3

WQM2

CC2

WQM2

EPT1

Water quality

Subthemes Indicators

Lake 

water clarity

Secchi depth

Lake oxygen

Periphyton

biomass

Near-shore

turbidity

Aquatic habitat

BMP

effectiveness

Drinking water

pathogens

Pollutant

concentrations

and loads

Fine particles

• Lake

• Runoff

• Air deposition

Near-shore 

water clarity

Stream 

channel erosion

Climate change

Pollutant loads 

from 

urban watershed

Water quality 

treatment and 

source control

Forest management and 

pollution control

Drinking water

protection

PLT4

EPT2

EPT2

NS1,2,3
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

Figure 4.2—Long-term Secchi depth data from Lake Tahoe. Each value represents the annual, 

time-weighted average based on approximately 25 measurements per year. Date collected by Tahoe 

Environmental Research Center, University of California Davis.

Lake Tahoe Water Clarity

Long-term monitoring of Lake Tahoe water quality since the early 1960s has documented a substantial decline in clarity (fig. 4.2). In contrast, the average summer 

Secchi depth measurements in oligotrophic, Crater Lake, Oregon, have remained 

consistent showing no declining trend over the long-term.6 The water quality 

standard and environmental threshold for Secchi depth7 in Lake Tahoe is 29.7 m 

and is defined as the mean of annual averages between 1967 and 1971. From 1968 

to 2000, there was a near-uniform decline in lake clarity as measured by Secchi 

depth. In some years, it seemed to improve, in other years it appeared to worsen, 

but invariably the trend was best defined by a straight line with an average loss in 

Secchi depth of approximately 0.25 m per year. However, in each of the 7 years 

since 2001, clarity has consistently been better than predicted by the historical data. 

This is unprecedented within the 40-year record. Based on the data available from 

6 Larson, G. 2006. Personal communication. Aquatic ecologist. USGS Forest and 

Rangeland Ecosystem Science Center, 777 NW 9th Street, Suite 400, Corvallis, OR 97330. 7 Secchi depth or Secchi disc depth is one technique to measure the clarity of a water body 

and has been used in limnology for over 100 years. Secchi depth is determined by lowering 

a 25-cm-diameter white disk into the water body. The mean depth at which the disk disappears and then reappears into view by a ship-board observer is taken as the Secchi depth.

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1965 1970 1975 1980 1985 1990 1995 2000 2005 2010

Secchi depth (m)

Year
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GENERAL TECHNICAL REPORT PSW-GTR-226

1968 to 1982, Goldman (1985) predicted that by 2007, the average annual Secchi 

depth in Lake Tahoe would be approximately 16.5 m, unless there was a change in 

the rate of clarity loss. During the period 2001–07, the actual annual Secchi depth 

measurements ranged from 20.6 to 23.7 m. Although these data do not pinpoint a 

specific cause for the recent change in trend, one possibility is that water quality 

improvement efforts targeting primary and secondary drivers (fig. 4.1) may be 

showing a benefit.

Secchi depth in Lake Tahoe is controlled by the light absorption and scattering properties of particles. The influence of particle number on clarity can be 

seen in data collected from Lake Tahoe (fig. 4.3). Earlier investigations focused on 

increased phytoplankton productivity as the primary source of these particles (e.g., 

Goldman 1994, Jassby et al. 2001). The long-term increase of primary productivity 

in Lake Tahoe has been attributed to increased nutrient loading acting in concert 

with the efficient recycling of nutrients (Goldman 1988). 

The finding that fine inorganic particles (<16 μm diameter) from soil and dust 

contributed to lake clarity decline is a fairly recent development (Jassby et al. 1999). 

This finding was immediately followed by the first comprehensive study of particle 

number, size, and composition in Lake Tahoe (Coker 2000). Typical particle size 

distributions for over 40 samples from long-term lake monitoring stations show that 

inorganic particles <5 μm in diameter compose the majority of inorganic material 

in the water column during both summer and winter (fig. 4.4). 

Clear, cobalt-blue-colored water of Lake Tahoe. 

Jim Markle
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

Figure 4.3—Relationship between in-lake particle number and Secchi depth (from Swift 2004). 

P < 0.001, r 2 = 0.57. Each point represents a single measurement of Secchi depth and particle 

concentration.

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0

0 4,000 8,000 12,000 16,000

Secchi depth (m)

Total particles (per ml)

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Figure 4.4—Particle size distribution in Lake Tahoe water samples showing dominance of particles 

<5 μm in diameter (from Swift et al. 2006).

0.1

1

10

100

1,000

10,000

0.1 1 10 100

Particles (per ml)

Winter

Summer

Particle size (μm)
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GENERAL TECHNICAL REPORT PSW-GTR-226

Coker’s (2000) investigation has been followed by a series of studies, examining the spatial and temporal distribution of particle concentration and composition 

in Lake Tahoe (Sunman 2001), characterization of biotic particles and limnetic 

aggregates in Lake Tahoe (Terpstra 2005), lake particles and optical modeling (Swift 

2004, Swift et al. 2006), and distribution of fine particles in Lake Tahoe streams 

(Rabidoux 2005).

Particle loss to the lake bottom through sedimentation is critical to any mass 

balance consideration of particle concentration in the water column. This was 

confirmed by Jassby (2006) who studied particle aggregation and developed a 

preliminary version of a particle loss submodel. Data from Sunman (2001) suggest 

fine particles can be transported through the upper 100 m of the water column in 

approximately 3 months. 

Because of efficient biotic mineralization and recycling, however, the nitrogen 

(N) and phosphorus (P) associated with the lake particles have a longer residence 

time in the water column than do the particles themselves. Mean settling velocities 

for N and P, as measured with large-sediment traps deployed in Lake Tahoe, were 

found to be 16.4 and 12.0 m per year, respectively (data from A. Heyvaert found 

in Reuter and Miller 2000). These rates correspond to decadal-scale settling times. 

With an average depth of over 300 m and a maximum depth of over 500 m, many 

of the nutrients associated with particles are mineralized by bacteria and effectively 

recycled before settling to the bottom (Paerl 1973). Note that although N and P are 

recycled back into the water column for use by algae, the inorganic particles that 

scatter light are not degraded and most settle to the bottom.

Swift (2004) and Swift et al. (2006) developed an optical submodel for Lake 

Tahoe to link particles and Secchi depth. The submodel takes into account algal 

concentration, suspended inorganic sediment concentration, particle size distribution, and dissolved organic matter to predict Secchi depth. It was found that both 

biological (e.g., phytoplankton and detritus) and inorganic (terrestrial sediment) particulate matter (PM) were contributors to clarity loss in Lake Tahoe. The high lightscattering properties of small inorganic particles mean they are the dominant cause 

of reduced light transmission. Specifically for Lake Tahoe, the optical submodel 

lends support to the earlier hypothesis (Jassby et al. 1999) that inorganic particles are 

the major determinant of clarity for most of the year. In winter, when mixing of the 

deep chlorophyll layer occurs, high algal levels in the surface waters result in greater 

attenuation by organic particles. Of the inorganic particles, it is the finer fraction 

(<16 μm) that is responsible for almost all of the light scattering (fig. 4.5). By relating 

organic and inorganic suspensoid concentrations in the lake to a predicted Secchi 

depth, the optical submodel has become a critical management tool.
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

Figure 4.5—Cumulative contribution to the scattering coefficient, bip, for each of the particle size 

ranges. The influence of the 16 to 32 μm size range was negligible and is not included in this plot. 

The full 0.5 to 16 μm size range shown represents the following individual size bins, 0.5 to 1, 1 to 2, 

2 to 4, 4 to 8, and 8 to 16 μm (from Swift et al. 2006).

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0

20

40

60

80

100

0 5 10 15 20

Cumulative scattering (percent)

Particle size (μm)

The Lake Clarity Model (LCM) released in 2006 yielded preliminary estimates 

on levels of nutrient and fine sediment reductions needed to achieve the water 

quality standard of 29.7 m. The LCM is a combination of the optical submodel, a 

hydrodynamic submodel customized for Lake Tahoe, an ecological submodel, and a 

particle fate submodel developed as part of the Lake Tahoe TMDL science program 

(Perez-Losada 2001, Sahoo et al. 2009). The model contains 31 parameters covering 

the general areas of algae, light extinction, nutrient utilization, settling, chemical 

reactions, sediment fluxes, zooplankton, and inorganic particles. Nutrient and fine 

particle loading inputs came from studies of ground water, atmospheric deposition, 

surface runoff from streams and intervening zones, and stream channel and shoreline erosion (LRWQCB and NDEP 2008a).

Based on model simulations and a quantitative investigation of pollutant load 

reduction opportunities, a reasonable load reduction target to reach the 29.7 m water 

quality standard for Secchi depth would combine a 65-percent reduction for fine 

sediment particles (from all sources combined) with a concomitant 35-percent and 

10-percent load reduction in P and N, respectively (LRWQCB and NDEP 2008c). 
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GENERAL TECHNICAL REPORT PSW-GTR-226

Changes in lake trophic (food web) status are now being documented (Chandra 

et al. 2005, Vander Zanden et al. 2003), and a significant shift in phytoplankton 

community structure has also been observed (Hunter 2004, Winder and Hunter 

2008). Microbial food web grazing impacts on phytoplankton density and size 

structure are not known. 

Lake Tahoe’s annual average clarity can vary substantially from year to year 

based on nutrient and fine sediment loading (Jassby et al. 2003). This type of 

variation has been observed at other times in the long-term data record and strongly 

suggests lake response to load reduction can be rapid, provided a substantial level 

of reduction is achieved. As reported by Heyvaert (1998), lake water quality was 

nearly restored to prehistorical conditions within about 20 to 25 years after mass 

disturbance from clearcut logging during the Comstock era ended in the late 1800s. 

As the basin was allowed to heal, lake conditions also recovered (fig. 4.6). Although 

there is evidence that the lake can respond to reduced pollutant loading; the Comstock era disturbance was a pulse disturbance, primarily owing to a single stressor, 

i.e. clearcutting, and the lake recovered when it ended. Currently, however, there 

are multiple stressors at play in the Tahoe basin and disturbance is chronic. Restoration of lake water quality will ultimately depend on an active program that reduces 

chronic loading from urbanized and disturbed landscapes and air deposition over 

the long term. 

Knowledge Gaps

A very large effort began in 2001–02 as part of the Lake Tahoe TMDL program 

to develop management tools for determining lake clarity response based on 

reductions in pollutant loads. Although this work has largely been successful, 

(LRWQCB and NDEP 2008a, 2008b, 2008c) knowledge gaps remain. Given 

the focus on restoration in the Tahoe basin, these initial recommendations apply 

primarily to improvement of the LCM and its application for management purposes.

Some of the key uncertainties regarding Tahoe’s water clarity include:

• Atmospheric deposition of particles onto the surface of Lake Tahoe, the fate 

of these particles upon entering the water, and subsequent impact on clarity. 

This topic and associated research needs are covered in more detail in the 

“Tahoe Basin Meterology” section of chapter 3.

• Characterization, distribution and dynamics of particles in Lake Tahoe’s 

water column, beyond the initial studies completed in the early 2000s. This 

includes new methodologies for measuring lake optical properties and in 

situ particle characterization, and developing a better understanding of the 

relationship between ultraviolet light transmission and lake particles.
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

Figure 4.6—Summary of paleolimnologic studies that reconstruct the recent water quality history of Lake Tahoe (from A. Heyvaert, 

Desert Research Institute). Mass sedimentation includes all material, both biogenic and abiotic. Biogenic silica is derived primarily from 

diatoms and provides an estimate of sedimentary material from this algal group. PPr denotes phytoplankton primary productivity.

• Rates of particulate matter (PM) sedimentation, mechanisms of nutrient 

loss, and dynamics of size-specific particle removal from the water column. 

This also includes the dynamics of very fine particles with regard to coagulation and aggregation, and lake-water equilibrium chemistry.

• Plankton and bacterial food webs and the importance of the “microbial 

loop” in regulating the fate of particulate organic matter. Macroscale 

food webs, i.e., large zooplankton, benthic invertebrates and fish, are 

considered in the “Aquatic Ecosystem Integrity” section of chapter 6.

• Biogeochemistry of biologically available nutrients.

• Mechanisms underlying the specific relationships between pollutant 

loading and load reduction, and lake clarity response.
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Research Needs 

Characterization, distribution, and dynamics of particles in 

Lake Tahoe’s water column—

Extensive measurements in Lake Tahoe during the first few years of the 2000s 

provided a new understanding of particle characterization and distribution. This 

was the first time such measurements were made and allowed us to ascertain some 

of the fundamental properties of these particles. Inorganic particles have been 

associated with the regulation of optical properties in lakes; however, with only a 

few notable exceptions worldwide, limnologists have not focused on this topic to the 

level needed at Lake Tahoe. Consequently, there is sparse literature to draw upon. 

Particles affect lake clarity as a result of the manner in which their number, 

composition, location, and shape affect scattering and absorption of light. The 

relationship between particle loading, biogeochemistry, physical forces that 

determine the position of particles in the water column, and processes that remove 

particles to the lake bottom are complex and have not been fully studied. There is 

also uncertainty associated with dry and wet deposition of particles onto the lake 

surface via atmospheric deposition. Further, the LCM is very sensitive to particle 

sedimentation processes, and the biological parameters were taken from literature 

values rather than from Tahoe-specific research. Our knowledge of phytoplankton 

and bacterial ecophysiology and lake nutrient cycling also is limited. 

The following investigations are considered important during the next 5 years. 

These investigations are intended to (a) update and refine the LCM, (b) determine 

baseline conditions for particles from which sound statistical assessments of longterm response to restoration efforts can be evaluated, (c) assist in the development 

of a sound environmental monitoring program, and (d) provide critical supporting 

data for concurrent studies of particle loss characteristics.

• Research is needed to establish a statistically based monitoring program to 

evaluate particle number, particle size distribution, seasonality of particle 

distribution, position of particles in the water column, particle composition, and particle shape as it affects clarity. In situ approaches for measuring particle distribution and lake optical properties are being developed. 

Application of such approaches (e.g., use of UV light attenuation profiles 

as a surrogate for particle density, or deployment of particle probes) would 

benefit from research and testing. Development of common methodologies 

is recommended to produce comparable data as part of any particle monitoring program, whether it be specifically for the water column in Lake 

Tahoe or other sources of particles (e.g., streamflow or urban runoff). 
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

• Remote sensing needs to be evaluated for large-scale (whole-lake synoptic) 

measurements of clarity, including particles and other factors affecting lake 

clarity. 

• Further investigation of particle loading from all upland sources is recommended to determine specific sources, loading rate characteristics based on 

size and composition, and characteristics of transport. Sources of particular 

interest include land use type, activities on the landscape (e.g., road sanding 

or sweeping), parent soil characteristics, vegetative cover, slope, and other 

factors. Determination of physicochemical fingerprints of particles for 

comparison to upland materials is one example of an approach to identify 

specific sources of loading to the lake.

• The influence of “black carbon” on lake clarity has not been quantified. 

Black carbon represents those particles that result from combustion of 

organic matter (e.g., biomass burning and diesel exhaust) and enter the lake 

through atmospheric deposition. Additional research is needed on the optical properties of these particles in water; their numbers, size and distribution in the water column; rates of dissolution and loss; and their ultimate 

effect on clarity. 

Mass sedimentation rates, nutrient loss, and mechanisms of size-specific 

particle sedimentation—

Loading and transport of particles to Lake Tahoe is an area where substantial new 

research, monitoring, and modeling is recommended. Focusing specifically on the 

lake itself and important processes in the water column, our knowledge of mass 

sedimentation rates, nutrient loss, and size-specific settling, is still not complete—

uncertainty exists. 

Previous studies of mass/bulk sedimentation in Lake Tahoe come primarily 

from work by Marjanovic (1989) and Heyvaert (1998). Installation and maintenance 

of in situ sediment traps is needed to evaluate long-term sedimentation rates and 

compositional characteristics. Chemical and biological analysis of the settled 

material allows us to better understand the quantitative and qualitative aspects of 

PM loss. For the mass balance approach taken in modeling, it is equally important 

to have sufficient information on particle loss as on particle loading. In the time 

allotted to develop the LCM for the TMDL, emphasis was placed on particle 

loading as it also gave insight into what control options would be most effective. 

It is recommended that additional scientific attention now be placed on the loss 

terms of the model.
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GENERAL TECHNICAL REPORT PSW-GTR-226

Particles typically enter Lake Tahoe as discrete units. The production of extracellular products, the formation of biofilms, and other biological processes (largely 

mediated by bacteria and algae) play a substantial role in the aggregation, coagulation, and settling of particle complexes (Logan et al. 1995). Coagulated material is 

able to settle much faster than individual particles. Results of the LCM show that 

these processes are crucial to the removal of particles from the water column and, 

in fact, the loss of aggregated PM can be rapid. The very initial aspects of these 

types of studies was recently started (Jassby 2006); however, more detailed investigations are recommended.

Bacterial and plankton food webs and their influence on biological particles—

An extensive literature has documented the importance of bacteria, picophytoplankton (0.2 to 2 μm), and the microbial food web in oligotrophic 

waterbodies (e.g., Callieri and Stockner 2002). The presence of the microbial 

food web in oligotrophic oceans and lakes was first documented about 20 years 

ago. A substantial portion of the nutrient and carbon cycling and energy flow in 

oligotrophic systems typically pass through this microbial loop (e.g., Azam et al. 

1983). It is suspected that an important portion of the lake’s primary productivity 

results from pico-phytoplankton, but definitive information is lacking.

The effect of bacteria, pico-plankton and the microbial food web could not be 

expressly quantified in the LCM, so assumptions were made. The influence of the 

microbial community on nutrient cycling, as well as the direct effect on biologic 

particles via production and grazing, and the indirect effect on inorganic particles 

(e.g., aggregation and coagulation processes) all warrant additional research. These 

studies take on additional significance given the recent findings that climate change 

and its effect on lake temperature may be influencing phytoplankton community 

structure (Winder et al. 2008).

Assessment of biologically available nutrients—

Biologically available phosphorus (BAP) was measured as part of the TMDL 

science program. Although this study included a variety of potential P sources, it 

was not extensive with regard to spatial and temporal characterization. However, 

the TMDL scope of work was intended to provide the LCM with values for BAP 

that were not simply taken from the literature. In this regard it was a successful 

project that for the first time provided an initial understanding of the importance of 

BAP (Ferguson 2005, Qualls 2005). Now that relationships have been established 

between BAP and chemical assessment techniques, BAP portioning for specific P 

sources are recommended. In addition, a better understanding of P availability and 

P cycling in Lake Tahoe would help improve the LCM.
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For P in particular, bioavailability can be affected by lake-water equilibrium 

chemistry. Depending on the in-lake concentrations and the magnitude associated 

with particulate matter, this nutrient can either be stored in PM and fine inorganic 

sediments, or it can leach into the surrounding water. Characterization of P-leaching rates associated with these processes is likely to be dependent on particle size 

and composition, and further research is recommended to update the relevant water 

quality components of the LCM. 

Organic N loading in the streams monitored through the Lake Tahoe Interagency Monitoring Program (LTIMP) typically accounts for >90 percent of the 

total N load, with about 50 percent of the organic N present in the dissolved form 

(Coats and Goldman 2001). Dissolved organic N is also abundant in wet and dry 

fallout from atmospheric deposition accounting for 25 to 30 percent of the total 

N load from this airborne source (LRWQCB and NDEP 2008a). Clearly, the 

fraction of the organic N pool that is bioavailable can have a substantial influence on algal growth as well as on our efforts to model this process. Biologically 

available N (BAN) is a difficult research area requiring experience, very specialized techniques, and a laboratory that is set up for these types of measurements. 

Only a limited number of research groups nationally are conducting such studies. 

Although an extensive BAN study is not necessarily recommended at this time, a 

feasibility study evaluating the potential impact of uncertainty associated with the 

lack of direct BAN measurements at Tahoe, vis-à-vis algal growth and lake clarity, 

is strongly suggested. 

Statistical relationships between pollutant loading and lake clarity response—

Statistical analysis of historical Secchi depth measurements, and the development 

of a statistically-based mechanistic model for evaluating long-term and interannual 

variability in Lake Tahoe’s clarity, have provided significant insights regarding 

changes in Lake Tahoe’s optical properties (e.g., Jassby et al. 1999, 2003). A statistical approach is recommended to determine when improvements in Secchi depth 

clarity and other measures of light transmission in Lake Tahoe occur. In particular, 

managers would benefit from science-informed criteria for determining the influence of pollutant load reduction on clarity. This would include determining the 

significance of short-term variation on clarity, and—given the natural degree of 

interannual variability—the number of years of data that would be required before 

agencies know if their management milestones have been met. 
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Update of Lake Clarity Model and linkage to other pollutant source and 

management models—

Based on the knowledge obtained from all research topics conducted in the 

Tahoe basin, we recommend specifically allocating funding to update the LCM 

to accommodate new data and insight. Additionally, linking existing and new 

management models (e.g., Tahoe Watershed Model, Water Erosion Prediction 

Program [WEPP], Lake Tahoe Atmospheric Model, Conservational Channel 

Evolution and Pollutant Transport System [CONCEPTS], Pollutant Load Reduction 

Model [PLRM] and/or ground-water modeling) to each other and to the LCM is 

desired by resource managers and the scientific community. As discussed in the 

“Climate Change and Water Quality” section (p. 155), the LCM is also considered 

an important research tool in evaluating the effects of climate change on lake 

stratification, water quality, and aquatic ecology.

Lake Tahoe Near-Shore Water Quality 

The near-shore zone of Lake Tahoe is one of the most visible components of the 

Tahoe ecosystem to both tourists and local residents, and a decline in near-shore 

water quality is more readily apparent to the largely shore-bound population. The 

near shore is part of the littoral zone: that portion of a lake where enough light 

reaches the bottom for macrophytes (rooted plants) and periphyton (attached algae) 

to grow. At Lake Tahoe, the littoral zone frequently extends to depths greater than 

40 m, and can extend 20 m to several kilometers out from the shore line, depending 

on bottom topography. Processes within the near shore exhibit spatial and temporal 

variability owing to their response to and integration of onshore activities, events 

within the near shore, timing and magnitude of channelized (stream), and unchannelized (surface) runoff, and the mixing with and dilution by mid-lake waters. 

The response of the near shore to pollutant loading is more immediate than 

mid-lake waters owing to the near shore’s proximity to the terrestrial environment 

and its shallow nature. Erosion and disturbance in the upper watersheds 

(including shallow ground-water flow) often manifests along the lake shore in 

terms of increased periphyton growth, decreased water clarity, higher nutrient 

concentrations, greater abundance of easily suspended sediments, and increased 

macrophyte growth. Near-shore water quality also influences higher order 

biological species that inhabit this region. Anecdotal information from long-term 

residents and visitors suggests near-shore aesthetics have substantially deteriorated 

over the last several decades, including but not limited to excessive periphyton 

growth, increased turbidity, and establishment and expansion of Eurasian watermilfoil (Myriophyllum spicatum) and other introduced species. 
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Undeveloped near-shore habitat near Sand Harbor, Lake Tahoe. 

Research on Lake Tahoe’s near-shore zone has not historically received the 

same level of attention that watershed and mid-lake processes have received. 

Studies that have investigated the near-shore zone have found processes and 

characteristics that are highly dependent on location and adjacent watershed 

activities and timing. For example, eulittoral periphyton communities located in 

the shallow area between high and low lake levels exhibit large seasonal variation, 

whereas sublittoral periphyton located in the remainder of the littoral zone exhibit 

less seasonality (Loeb and Palmer 1985). Some information is also available on 

near-shore water clarity, periphyton, fish and benthic communities, and currents 

and circulation patterns (e.g. Beauchamp et al. 1994, Hackley et al. 2007, Herold 

et al. 2007, Kamerath et al. 2008, Loeb et al. 1983). 

Studies that have assessed spatial variability in the near shore show decreased 

clarity and increased periphyton biomass associated with greater development and 

disturbance in the adjacent onshore watershed. Taylor et al. (2004) found near-shore 

water clarity along 7 km of shoreline ranged from moderately to highly impaired, 

and 4 km of shoreline was slightly impaired. These authors also reported large 

reductions in clarity near developed areas immediately after summer thunderstorms, during winter lake-level snowmelt events, and during the seasonal spring 

snowmelt. This observation highlights the adverse impacts of hydrologic events 

acting on urban landscapes. Increased nutrient loading from urbanized watersheds 

Jim Markle
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stimulates periphyton biomass (e.g., Hackley et al. 2004, 2005, 2007); however, 

there also have been instances of elevated periphyton biomass found off pristine 

(nonurbanized) watersheds, suggesting littoral zone currents also play a role in 

determining near-shore conditions. 

Questions exist as to whether current regulations are adequate, and if the 

regulations recognize the large spatial differences in near-shore water quality. 

There are also questions about our level of understanding of near-shore water 

quality: What are the trends in near-shore water quality, and what are the important 

processes controlling near-shore water quality? Ultimately these questions focus on 

generating the information necessary for determining the policies needed to protect 

this critical recreational resource and its natural resources. As discussed above, 

previous and current studies in the near-shore region of Lake Tahoe have provided 

us with some understanding of near-shore water quality. However, most of these 

studies have been done as separate investigations—a more holistic approach that 

is fully integrated with management information needs (similar to that being taken 

for mid-lake clarity) would provide resource and planning agencies with the type of 

knowledge they need when making policy decisions regarding the near shore. 

Metaphyton (Zygnema sp.) growing on the shells of nonnative clams (Corbicula fluminea) in the near 

shore at Elk Point, Lake Tahoe. 

Brant Allen
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

Knowledge Gaps 

Some of the key uncertainties regarding Tahoe’s near-shore water quality include:

• The lack of baseline data needed to develop a comprehensive understanding of the near-shore ecosystem that can inform management strategies 

and support environmental thresholds. The existing patchwork of studies 

investigating near-shore processes and stressors has not been sufficient to 

sustain a consistent baseline data collection effort. Continued research and 

monitoring is necessary to assess long-term trends, to better understand 

near-shore processes, and to develop quantitative near-shore water quality 

standards. Data from such a program are invaluable and could be used to 

inform the following knowledge gaps:

▪ Response of the near shore to watershed restoration and management 

activities. It is unknown if the reduction of nutrient and sediment 

sources currently being planned and implemented will mitigate nearshore impairment. Monitoring of near-shore characteristics is needed to 

better understand the effectiveness, results, and potentially unintended 

consequences of onshore treatments (e.g., stormwater infiltration basins 

installed near the lake shore).

▪ The potential direct and indirect effects of future near-shore development and boating scenarios (e.g., piers and buoys) on near-shore 

ecology and water quality. 

▪ The impacts that changing/managed lake level has on near-shore 

water quality, periphyton communities, and habitat.

▪ The information base agencies need to establish informed water quality 

standards, and environmental thresholds and indicators that will be 

protective of near-shore water quality and aesthetics.

• It is important to recognize and improve our understanding of the roles 

that spatial variation plays in near-shore processes. The spatial variability 

of conditions in the near shore is very complex owing to the interactions 

among a suite of land-based and lake-based processes. The near shore 

can be roughly divided on the basis of depth (eulittoral or splash zone and 

sublittoral) and urban versus undeveloped. Near-shore habitats also differ 

around the lake owing to factors such as embayments, marinas, open water, 

and bottom substrate type. Lastly, the near-shore zone is also impacted by 

the variability found within adjacent onshore watersheds (e.g., soils and 

land use) and the circulation and mixing patterns with the deeper waters of 

the pelagic zone. Examples of existing knowledge gaps include:
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▪ Limited information exists on the distribution and quantity of 

periphyton at a basinwide scale. This information is necessary 

to provide a regional context and sufficient basis for basinwide 

biomass estimates and predictive tools.

▪ There is little current knowledge on the degree to which urbanization 

has impacted the near-shore zone. Short- and long-term urbanization 

impacts may or may not exceed natural spatial and temporal variations. 

Information on the impacts of urbanization is needed so that thresholds 

and standards can account for the differences between pristine and 

urbanized areas.

• A greater knowledge of the linkages between processes within and external 

to the near-shore zone are needed to understand the ability of the near-shore 

zone to propagate watershed impacts into the pelagic lake zone. The nearshore zone is a dynamic buffer that integrates nutrient and sediment outputs 

from adjacent terrestrial watersheds, atmospheric deposition, processes 

within the near shore, and mixing with deeper lake waters. Specific knowledge gaps include:

▪ Limited understanding of the near-shore capacity to assimilate 

terrestrial outputs without adversely affecting water quality, clarity, 

and ecology.

▪ The inability to predict how near-shore physical processes will respond 

to onshore and littoral zone management actions. For example, will 

onshore erosion control differentially alter the loading of dissimilar 

particle size classes to the lake in a way that would negatively impact 

near-shore habitat characteristics?

▪ Limited understanding of the manner in which physical processes (e.g., 

currents, wave action) control the dispersion, accumulation, spatial 

distribution, and transport of pollutants and plankton, both within the 

littoral zone and between the littoral and pelagic zones.

Research Needs 

Research is needed to better understand near-shore processes at various temporal 

and spatial scales. This research is best accomplished so that it contributes to 

an integrated database, which can be used to determine trends and patterns for 

integrated, process-driven models. This research would further develop our understanding of the linkages between near-shore and mid-lake processes. Ideally, this 

new information would be linked directly to management decision models and 
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

could inform the development of appropriate thresholds and management strategies 

for Lake Tahoe’s near-shore environment. 

A science-based risk analysis of stressors to the near-shore environment is 

also an important research need. It is recommended that this analysis take a comprehensive, ecosystem approach, and evaluate the full suite of stressors that could 

affect near-shore water quality (both environmental and human health), as well as 

the ecology, recreation, and scenic values. Following are examples of more specific 

scientific inquiry and data collection efforts that are needed to inform Lake Tahoe 

near-shore water quality and ecology management.

The need for baseline data of near-shore characteristics to support 

management strategies and thresholds—

• It is recommended that these data could include nutrient sources and 

cycling (through tributary, direct runoff, ground water, lake mixing), 

physical processes affecting the littoral zone (e.g., lake circulation and 

currents, wave activity, changes in lake level, benthic substrate type), 

and biological variables (e.g., grazing, sources of algal colonization, or 

abundance and distribution of native and invasive species).

• Assess near-shore “hot spots” for water quality impacts, and evaluate contributing factors. Are “hot spots” related to upland activities (e.g., urbanization), or do they reflect impacts from near-shore facilities such as marinas 

or public beaches?

• Synthesize existing monitoring data to develop recommendations for 

numeric near-shore water quality targets for nutrients, fine sediment 

particles, lake optical properties, periphyton, and aquatic plants. 

Analysis of the roles that spatial variation plays in near-shore processes—

• Improve estimates of spatial variability in the near shore by developing 

remote sensing methods that can detect local and regional changes in water 

clarity and periphyton growth. Assess potential remote sensing methods for 

accuracy, limitations, and compatibility with existing methods.

• Determine the levels and spatial distribution of near-shore phytoplankton 

and periphyton production using field (observational and experimental) and 

modeling studies.

• Develop a basinwide database and predictive models to understand how 

physical, biological, hydrologic, and nutrient factors control periphyton 

communities in Lake Tahoe. For example, how is storm-water infiltration 

and loading to the lake via ground-water discharge affecting near-shore 

water nutrient loading?
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• Determine how changes in lake level affect near-shore processes, including 

shoreline erosion.

• Evaluate how hardening (reinforcement) of shore zones affects the nearshore environment. For example, how are erosion, deposition, and transport 

of near-shore sediments, hydraulics of wave run-up, and geomorphic processes affected by shore zone hardening?

• Develop a coarse-sediment mass budget for the near-shore environment. 

Is enough sediment being delivered to maintain beaches and near-shore 

habitat characteristics?

Define and assess the linkages between on-shore, near-shore, and mid-lake 

processes that affect water quality, clarity, and ecology—

• Define and create the tools to (1) simulate the transport of pollutants within 

the near shore and around the lake and (2) predict the ability of the near 

shore to buffer and propagate pollutant loading from onshore activities to 

mid-lake.

• Further refine/quantify the role of the near shore as an integrator of watershed, atmospheric, and mid-lake processes. Can a change in near-shore 

characteristics be used as an indicator for short-term, neighborhood-scale 

activities or as a long-term indicator for water impairment?

• Develop analytical approaches for generating quantitative water quality 

standards, thresholds, and indicators for the near-shore region.

Pollutant Loading From Urban Sources

Results of water quality monitoring show that for some urban land uses (e.g., roads 

and commercial development), the concentrations and loads of pollutants in surface 

runoff can be substantial. Although estimated flow from urban areas is only part 

of that from nonurban areas, the inputs of pollutants from these areas are 50, 60, 

and 72 percent of the total annual loads to Lake Tahoe for N, P, and fine particles 

(<16 μm), respectively (LRWQCB and NDEP 2008a).

Relative Contribution of Sediment Loading From Urban Sources

A number of studies have been completed over the past 30 years to address sediment delivery issues from various watersheds in the Tahoe basin. However, these 

studies have (1) generally focused on only a few streams within the watershed 

(Glancy 1988, Kroll 1976, Nolan and Hill 1991, Stubblefield 2002), and (2) usually 

considered only the larger category of total suspended sediments, rather than focusing on the finer fractions. More recently, studies have begun to examine the loading 
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of total suspended sediment mass, along with suspended sediment mass for material 

<63 μm, and the number of particles for <16 μm sediment fractions (LRWQCB and 

NDEP 2008a, Sahoo et al. 2007, Simon et al. 2003, Tetra Tech 2007). 

It is instructive to compare estimates of sediment loading to the lake from 

all sources, including the urban landscape (table 4.1). These results illustrate the 

importance of fine-sediment particle loading (number of particles) derived from 

urban sources. 

Table 4.1—Total suspended sediment (TSS) and fine particle loading to Lake Tahoe for the major 

pollutant sourcesa 

Pollutant source TSS TSS <63 μm Particle number b

Metric tons/year Percent Metric tons/year Percent <20 μm/year Percent

Urban upland runoff c 5,200 17 4,430 31 34.80 × 1019 72

Nonurban upland runoff c 11,700 40 4,670 33 4.11 × 1019 9

Stream channel erosiond 5,500 19 3,800 27 1.67 × 1019 4

Atmospheric depositione NA NA 750 5 7.45 × 1019 15

Shoreline erosion f 7,200 24 550 4 0.11 × 1019 <1

 Total 29,600 100 14,200 100 48.14 × 1019 100

a Values are expressed as metric tons/year (metric tons = 1000 kg) except for particle number, which is expressed as number of 

particles <16 μm/year (taken from LRWQCB and NDEP 2008a). 

b Percentage values refer to the relative portion of total basinwide load. 

c Upland runoff (urban and nonurban) is the annual mean value from measurements collected between 1994 and 2004. 

d Stream channel erosion is the annual mean value from measurements collected between 1983 and 2002.

e Atmospheric deposition is a combined estimate of wet deposition (annual mean value from measurements collected 

between 1992 and 2003) and dry deposition (measured in 2003 only).

f Shoreline erosion represents a 60-year mean annual value.

NA = not applicable.

The total suspended sediment (TSS) loading from all major pollutant sources 

was estimated to be approximately 29,600 metric tons per year (MT/yr) (LRWQCB 

and NDEP 2008a). Upland watersheds, including stream channel erosion, account 

for 22,400 MT/yr, and represent 75 percent of the TSS load. Upland watersheds 

without the contribution from stream channel erosion, deliver 16,900 MT/yr, of 

which 5,200 MT/yr or 30 percent was generated from the urban portion of these 

watersheds. Shoreline erosion contributes on average about 7,200 MT/yr; however, 

it is likely that this source is highly variable from year to year, and the total erosion 

rate between 1938 and 1998 was affected by some very large events (Adams and 

Minor 2002). Stream channel erosion from both urban and nonurban stream sections combined was independently estimated at 5,500 MT/yr (Simon et al. 2003). 
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For the TSS <63 μm size fraction (TSS<63 μm), it was estimated that average 

annual loading to Lake Tahoe was 14,200 MT/yr from all sources (LRWQCB 

and NDEP 2008a). This accounts for nearly 50 percent of the total TSS loading. 

Upland runoff contributed 9,100 MT/yr, or 63 percent of the TSS<63 μm load. The 

TSS<63 μm loadings from urban and nonurban areas were virtually identical, at about 

4,500 MT/yr. However, the loading ratio of TSS to TSS<63 μm differed between 

urban and nonurban land use categories. For urban areas, approximately 85 percent 

of TSS load was in the TSS<63 μm fraction, whereas only 40 percent of the TSS load 

from nonurban areas was contributed by this smaller size fraction. The contribution 

of TSS and TSS<63 μm from the other major measured sources is shown in (table 

4.1). 

The first estimates of fine particle loading to Lake Tahoe in terms of particle 

numbers have only recently become available (LRWQCB and NDEP 2008a). These 

estimates were based on studies by Heyvaert et al. (2008), Rabidoux (2005), CARB 

(2006), Tetra Tech (2007), Sahoo et al. (2009), and Adams (2002). As discussed 

previously, although loading estimates for TSS and the TSS<63 μm fractions are 

of interest, it is the fine particles <16 μm that are of greatest concern, as these 

have the most effect on lake water clarity. 

The average annual load of fine particles <16 μm from all major sources was on 

the order of 5 × 1020. Table 4.1 shows the estimated breakdown of loading by source 

categories for particle numbers in the <16-μm size range. Approximately 85 percent 

of the <16-μm size particle loading into Lake Tahoe is associated with surface 

runoff from urban and nonurban upland sources, including stream channel erosion. 

By far the most significant contributor was urban runoff, accounting for 72 percent 

of total lake loading for fine particles <16 μm. In contrast, the nonurban uplands 

only accounted for 9 percent of the fine particle loading, and stream channel erosion only accounted for 4 percent. It is very interesting to note that as the sediment 

size classification becomes smaller (i.e., from TSS to <63 μm to <16 μm particles), 

the relative contribution from urban areas increases substantially. Urban TSS load 

was estimated to be 17 percent. This nearly doubled to 31 percent for the <63-μm 

fraction and more than doubled again to 72 percent for the <16-μm particle number loads. Relative contributions from nonurban sources generally declined with 

decreasing particle size, except atmospheric deposition where loading from <16-μm 

size particles increased to 15 percent of the total.

Sources of fine sediments in urban areas of Lake Tahoe also include gully 

erosion from roadside ditches, upstream sediments that are more efficiently passed 

through urban drainage systems and concentrated in discharges, fine particulates 

washed off urban surfaces (including applied sanding materials, break-pad wear, 
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

or air deposition), and downstream erosion caused or accelerated by urban runoff 

discharges that exceed natural runoff conditions. 

Urban Hydrology 

Much of the existing urban stormwater drainage system around Lake Tahoe was 

installed as part of subdivision development many decades ago, when the main purpose of drainage system design was to provide efficient stormwater conveyance and 

flood control, with minimal design considerations given toward downstream water 

quality effects. Currently, insufficient hydraulic retention exists within many urban 

drainages. Instead, culverts and ditches generally concentrate and expedite runoff 

from urban watersheds into receiving waters. As a consequence, peak flows at most 

storm frequencies are much higher than would occur without urban development; 

these higher peak flows not only facilitate pollutant transport, they also enhance 

erosion within urban watersheds.

Runoff rates and volumes are sensitive to the amount of impervious surface 

area and its connectivity to drainage systems. Existing regulations on new development focus on the onsite retention and infiltration of runoff from impervious surfaces, and substantial investments are being made to retrofit existing development 

with new best management practices (BMPs) and public drainage improvements. 

Urban stormwater discharge to the Upper Truckee River at Highway 50.

Alan Heyvaert
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These BMPs are usually designed to accommodate the 20-year storm (equivalent to 

about 2.54 cm of precipitation in 1 hour). Although this approach may be adequate 

for designing individual BMPs, managing urban stormwater quality at Lake Tahoe 

will depend on a fully integrated understanding of the existing pathways of urban 

drainage on a subwatershed scale. Without a better understanding of the degree of 

connectedness between pervious and impervious parcels within the drainage in a 

subwatershed, estimating load reduction for the purpose of the TMDL will be done 

for individual projects in isolation and will ignore the connectivity along the pollutant pathway defined by regional hydrology. 

Land Use and Runoff Water Quality Relationships 

Our understanding of land use-water quality relationships in the Tahoe basin is 

based on five sources (1) the data collected since 1978 by the LTIMP (Coats et al. 

2008, Rowe et al. 2002); (2) the recently completed Lake Tahoe TMDL Stormwater 

Monitoring Study (Coats et al. 2008, Gunter 2005); (3) the Tetra Tech LSPC (Loading Simulation Program in C++) hydrology-water quality model recently developed 

for the Lake Tahoe TMDL (Riverson et al. 2005), (4) the Pollutant Load Reduction 

Model (NHC 2009), and (5) process-based studies of nutrient and sediment sources 

and transport in subalpine watersheds in and near the Tahoe basin (e.g., Coats and 

Goldman 2001; Hatch et al. 2001; Heyvaert and Parra 2005; Heyvaert et al. 2006; 

Johnson et al. 1997; Merrill 2001; Miller et al. 2005; Murphy et al. 2006a, 2006b; 

Sickman et al. 2002; Simon et al. 2003; USFS 2004). 

The designation of land use groups was a critical first step in developing 

watershed-scale estimates of pollutant loads under the Tahoe TMDL (LRWQCB 

and NDEP 2008a). Land uses have been grouped into 20 categories for the Tahoe 

basin. The second critical step in characterization of land use-runoff relationships 

was the estimation of runoff volumes for each land segment. This estimation was 

done using the Tetra Tech LSPC watershed model with hourly weather data and 

land-segment-specific hydrologic parameters that have been measured, estimated, 

or calibrated to reflect the unique characteristics of watersheds around the lake. 

In the Lake Tahoe watershed LSPC model, event mean concentration data from 

the TMDL Stormwater Monitoring study were used to model characteristic runoff 

water quality from different urban land use types. An event mean concentration 

(EMC) is a calculation used to provide a flow-weighted concentration for a pollutant in question that summarizes conditions during a defined stormwater runoff 

event. The EMC multiplied by total flow during a stormwater runoff event is 

taken as the estimated load during the event. For primary roads, the EMCs were 
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

developed using a combination of Caltrans and Nevada Department of Transportation (NDOT) monitoring data. Runoff sediment and nutrient loads were estimated 

by applying EMCs to modeled runoff volumes. The characteristic EMCs and load 

estimates can differ substantially by land use, region, soil type, and season of the 

year (e.g., Coats et al. 2008). 

Based on the studies described above, it is known that compared to undisturbed 

and naturally functioning forested lands, the urbanized areas produce much higher 

concentrations of ammonium-N, nitrate-N, total N, orthophosphate, total P, and also 

in some cases, suspended sediment. There are also large differences in the yield 

rates (loading per unit area) of nutrients and sediment for different watersheds in 

the basin (LRWQCB and NDEP 2008a, Tetra Tech 2007).

In general, for the 10 watersheds sampled as part of the LTIMP (Rowe et al. 

2002) the relative concentrations of N and P forms decrease in order: organic-N 

> nitrate-N > ammonium-N; and total P > dissolved P > orthophosphate. In some 

urbanized areas, the ammonium-N can be a significant fraction of total N load, 

whereas it is generally insignificant for watersheds containing little urbanization 

(Coats et al. 2008, Gunter 2005). 

The concentrations and loads of nutrients and sediment are directly related to 

impervious surface area. In developed areas, the concentrations of P (orthophosphate and dissolved P) are generally related to percentage of residential impervious 

area, although N concentrations (nitrate-N, ammonium-N, and organic-N) are 

directly related to the density of multifamily residential lots (LRWQCB and NDEP 

2008a). Suspended sediment concentrations are directly related to percentage of 

area in commercial-industrial-communications-utilities land uses (Coats et al. 2008, 

Gunter 2005). 

Stream channel erosion is a source of TSS in the Tahoe basin streams (see table 

4.1), and as noted above, channel erosion can be exacerbated by increased runoff 

from urbanized areas. Discharge-concentration relationships in basin streams differ 

greatly among constituents, and with season (Rowe et al. 2002). This variability 

strongly influences the accuracy and precision of load estimates. Orthophosphate 

concentrations are strongly controlled by equilibrium reactions with the substrate, 

and do not change greatly with discharge. Nitrate-N is influenced by biological 

release and uptake, and by an annual washout cycle, with high autumn and low 

spring concentrations. The concentrations of particulate constituents (e.g., total 

P and TSS) are both flow and supply-driven, and vary by orders of magnitude 

throughout the year. 
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GENERAL TECHNICAL REPORT PSW-GTR-226

Role of Highway Surfaces and Shoulders on Pollutant Runoff 

Highway surfaces represent about 15 percent of total impermeable surface area in 

the Tahoe basin. Yet, because of the large number of vehicle miles traveled on these 

surfaces and the hydraulic connectivity that these routes provide, they often contribute stormwater runoff concentrations that are much higher than observed from 

other distributed land use types. This is particularly true for fine-particle concentrations in roadside runoff. The Lake Tahoe TMDL has identified that over 50 percent 

of the fine particles (<16 μm) may originate from the basin’s extensive network of 

primary and secondary roads.

Highway and road runoff typically contain high concentrations of fine sediment, presumably from the abrasive action of traffic on roads and roadside soils and 

on winter traction materials applied to road surfaces. Traffic and parking on road 

shoulders in particular, causes considerable damage to the vegetation and the soils 

surrounding most roads. This compaction increases runoff as well as the loads of 

sediments and fine particles. 

The optimal conditions for fine particle settling include long hydraulic residence time and minimal water movement. Unfortunately, the size/area requirements 

for construction of optimal BMPs are usually not feasible alongside most roadways, 

given their limited rights-of-way, terrain constraints, and sensitive environments. 

Installation of large underground vaults generally is not practical either because of 

utilities, traffic, and maintenance requirements. These systems also require ready 

access and easy maintenance to ensure effective long-term functioning. Therefore, 

new approaches are being sought for treating highway runoff to the level required 

in the Lake Tahoe basin. 

Source control measures are necessary to help prevent sediment from becoming 

entrained in highway and urban runoff. Revegetation and soil restoration success, 

however, can be limited by factors that include the dry summer climate at Lake 

Tahoe, steeper topography, and nutrient-poor soils. In some cases, armoring of 

highway cut and fill slopes is a successful alternative with appropriate applications 

and dispersed runoff flows. 

Recreational Impacts 

There are three types of recreational facilities that can result in accelerated erosion 

and runoff. These consist of native-surface roads and trails, developed recreational 

facilities (e.g., visitor centers and trail heads), and established recreation sites (e.g., 

campgrounds and ski areas). Developed recreational facilities generally exhibit 

the same features and impacts typical of other urban development (e.g., parking 

lots and buildings). Forest roads and trails, ski runs, and campgrounds, however, 
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

all have unique characteristics that increase their potential sediment and nutrient 

loading into receiving waters if appropriate BMPs are not properly installed and 

maintained. 

The major risks to water quality from forest road and trail networks exist on 

those road segments that are hydrologically connected to stream crossings. So proximity of roads and trails to stream channels is the single greatest concern related 

to road/trail impacts on water quality. Between 2002 and 2005, the Forest Service 

implemented a forest road BMP retrofit program to reduce the connected length of 

roads, particularly at stream crossings. This program has decommissioned approximately 150 km of roads, and has conducted BMP retrofits on 241 km of roads. In 

2005, the USFS initiated a similar retrofit program for trails. The effectiveness 

of both programs has been under evaluation (USFS 2002). Based on this work, it 

is now understood that maintaining the efficacy of the BMPs typically applied to 

native surface roads and trails requires a high frequency of maintenance. However, 

preliminary monitoring results also indicate a substantial reduction in the potential 

for erosion and transport of sediments as a result of efforts to retrofit, upgrade, and 

decommission forest roads in the Tahoe basin.

A decade of monitoring at Heavenly Ski Area indicates substantial improvement can be achieved at ski areas in relation to water quality, soil condition, erosion 

control, and sediment transport (USFS 2004). 

Knowledge Gaps 

Urban hydrology—

Urban hydrology in the Tahoe basin is complicated by the fact that developed 

communities are relatively small, and highly influenced by runoff from wildland regions that are immediately adjacent and often located within the urban 

boundaries. Although models are used for uniform runoff computations for 

designing specific BMPs and runoff conveyance features in the Tahoe basin, no 

attempt has been made to comprehensively model flow from its upland source 

through all the urban features (including specific BMPs and water conveyance 

features) and finally into Lake Tahoe or one of its tributaries. Furthermore, 

project-specific models for BMP-related runoff computations are generally not 

well calibrated, and the effects of rain on snow have been difficult to estimate. 

Precipitation and flow monitoring data are scarce in urban areas of the basin. Our 

understanding of snow hydrology in urban areas and its contribution to seasonal 

runoff patterns also is limited. The Pollutant Load Reduction Model is a recent 

attempt to more accurately estimate runoff from project areas. However, it would 

benefit from more specific data.
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GENERAL TECHNICAL REPORT PSW-GTR-226

Land use and runoff water quality relationships—

The uncertainty associated with estimates for loads and concentrations is not 

well-determined. Specifications for two kinds of error are recommended. The 

first is sampling error associated with taking instantaneous samples of continuous 

concentration variables. The second is prediction error associated with regression 

estimates based on the relationships between concentration and discharge. Despite 

some published data (e.g., Loupe 2005, Meidav 2008, Miller et al. 2005) there is 

also considerable uncertainty associated with the runoff from vegetated land use 

categories, representing a large part of the Tahoe basin. 

Role of highway surfaces and shoulders on pollutant runoff—

The contribution of highway stormwater runoff and pollutant generation is not welldefined relative to total watershed impervious surface areas and the percentages 

of pollutants generated specifically within the road and rights-of-way. Although 

curb and gutter installations reduce road shoulder compaction and erosive runoff 

scouring, these structures generate larger runoff volumes at higher velocities. So 

effective alternative designs are needed to disperse roadway flows while preventing 

erosion. These designs also need to reduce the migration of soils and fine particles 

onto road surfaces, and their subsequent transport to receiving waters. 

Sources and transport of fine sediment—

Because the reduction of fine particles is considered key to improving the clarity of 

Lake Tahoe (LRWQCB and NDEP 2008a, 2008b, 2008c), more detailed information is needed on the specific sources of fine particles and the relationships between 

natural watershed characteristics (e.g., geology, aspect, slope, vegetation cover), 

anthropogenic features (e.g., roadways, transportation/traffic, land use), and fine 

sediment loads from watershed drainages in urban areas. 

Recreational impacts—

Increasing use and prevalence of trails around urbanized areas may be of concern, 

especially where hikers and mountain bikes are causing hillslope erosion, meadow 

disturbance, or streambank degradation. Roads may present even greater water 

quality concerns owing to their larger disturbance area, greater traffic volume, 

and larger stream crossings. Although ski run restoration has provided substantial 

increases in overall effective soil cover (and subsequent reductions in erosion), 

monitoring indicates that some areas of the mountain do not respond to current 

restoration practices. Therefore, better methods are still needed for maintaining 

vegetative cover in difficult conditions. 
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

Pollutant transport from urban areas—

Hydrologic adjustments are still being accommodated from historical disturbances 

in most drainages receiving runoff from urban land uses. We need a better understanding of stable equilibrium conditions in urban drainages and streams. On a 

basinwide or regional scale, the extent of specific urban area erosion problems (hot 

spots) and their relative contribution to long-term pollutant loads is unclear. Better 

spatial information is needed for all urban drainage systems in the Tahoe basin, 

especially for those that discharge directly to the lake or to potentially erosive 

channels, streams, and overland flow areas. 

Research Needs

Urban hydrology—

• Review and compile data from local runoff monitoring so that a Tahoe 

basin urban hydrologic monitoring network can be designed and implemented to provide consistent, comparable, and continuous urban flow data.

• Implementation of a Tahoe basin meteorological network is recommended 

to supplement existing long-term monitoring sites (SNOTEL, National 

Climate Data Center) and improve the utility of hydrologic monitoring and 

modeling. 

• Conduct modeling studies of monitored catchments to identify sensitive 

parameters and develop appropriate calibration and parameter estimation 

techniques. 

• Develop a coordinated modeling approach to simplify regional relationships 

for estimating runoff flow-duration characteristics and time series of flows 

from urban areas.

Land use and runoff water quality relationships—

• Conduct research to quantify fine particle and nutrient loading from 

nonurbanized, vegetated areas.

• With regard to urban stormwater flow, gain a better understanding of 

the temporal-and spatial-scale processes associated with hydrology and 

pollutant transport between pervious and impervious areas. 

• Better assess relative contributions from specific land use sources for 

particular pollutants of concern, including fractionalization into the 

dissolved, bioavailable, and total constituent concentrations. 

• Develop refined pollutant load estimates for the various land uses with 

a comparison of continuous and event-based modeling approaches for 

application to drainage and water quality project design. 
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GENERAL TECHNICAL REPORT PSW-GTR-226

• Conduct comparative water quality modeling studies in catchments where 

monitoring data are available or where monitoring is planned to improve 

our understanding of the relative importance of spring snowmelt, “first 

flush,” and extreme events. 

• Validate models that estimate sediment and nutrient loading/load reduction 

from urban land uses.

Role of highway surfaces and shoulders on pollutant runoff—

• Continue monitoring stormwater from highway runoff, including assessment of sediment loads and particle size distributions, to better understand 

spatial and temporal variability.

• Conduct quantitative evaluations to guide appropriate curb and gutter 

designs or alternatives that will reduce runoff volumes and pollutant loads 

while limiting roadside parking and soil compaction. These recommendations could address differences in roads at higher versus lower elevations, 

and between urban and rural areas, as well as between different types of 

road surfaces.

• Conduct studies on the effectiveness of different source control measures, 

such as paving of roadside ditches, placing riprap on cut and fill slopes, or 

the revegetation of disturbed areas, especially as these practices relate to 

mobilization and transport of fine sediments. 

• Develop better estimates for the pollutant load reduction of maintenance 

associated with road sand collection, sweeping, and sediment removal. 

Better estimates are particularly needed for fine sediments <16 μm. 

• Research the development of highway runoff treatment BMPs that remove 

fine sediment from stormwater runoff. This research could include methods 

that increase fine sediment capture with new types of hydraulic structures 

and with the retrofit of existing structures, e.g., drop inlets and small settling basins. 

• Conduct research to determine the specific size range for fine-grain sediments that are best targeted in treating highway runoff, and the amount of 

P that can be removed with this sediment.

• Gather more systematic information on the effectiveness of infiltration 

systems, given characteristics of Tahoe basin soils, and where these basins 

are optimally located to best treat highway runoff.
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

Sources and transport of fine sediment—

• Conduct comparative assessments of the various methods for determining 

and reporting particle-size distribution, with the goal of developing recommended uniform protocols for data analyses and for reporting results.

• Ascertain relationship between turbidity and the number of fine particles 

for monitoring load reduction.

• Compile and review the available monitoring data for particle-size distribution to improve estimates of loading by land use category, by drainage 

characteristics, and by catchment conditions. 

• Conduct studies to expand understanding of particle-size distribution loads 

from all land use categories, including source-specific studies on anthropogenic activities that produce and mobilize fine sediments. 

• Conduct trend analyses of particle-size data to describe the temporal and 

spatial contributions of fine-sediment loadings from dominant sources, 

including relative contributions.

• Complete studies on the mass of particles contributed annually from 

abrasive wearing of road surfaces, including the characterization of size 

distribution and the chemical properties of this material. 

• Increase measurements of the percentage of snow traction material removed 

by street sweeping activities, and the amount of material remaining. 

Conduct studies to better understand the drainage characteristics of 

primary and secondary roads as this flow transports fine particles and 

other pollutants to receiving waterbodies. Determine how road sand is 

pulverized by tires and quantify the conversion of large particles into 

smaller, more environmentally relevant sizes (i.e., <16 μm).

Recreational impacts—

• Revisit high- and moderate-risk road segments that did not improve with 

BMP retrofits to identify the causes of failure and possible improvements. 

This could also address the amount of maintenance required to maintain 

efficacy of BMP retrofits over the long term. 

• More intensively monitor ski resort impacts throughout the Tahoe basin, 

as is done currently at Heavenly Ski Resort. Ideally, this monitoring also 

would include the evaluation of soil restoration approaches being developed 

through the California Alpine Resort Environmental Cooperative. 

• Evaluate water quality effects from low-impact techniques currently 

used and proposed by ski resorts in removing trees to create new glades 

and runs.
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GENERAL TECHNICAL REPORT PSW-GTR-226

• Assess the distribution of impacts from trail development and use near 

urban areas (especially with respect to mountain bike use).

• Assess the distribution and impacts from fertilizer applications on 

recreational areas such as ball fields, golf courses, and public lawns.

Pollutant transport from urban areas—

• Conduct process-based studies to describe the sources, transport mechanisms, and sinks for nutrients and sediments on their journey from urban 

landscapes to Lake Tahoe. 

• Conduct studies to better understand pollutant accumulation, transformation, and transport processes in snow and snowmelt, especially for roadway, 

roadside, and parking lot snowpacks. These snow and snowmelt studies 

could include monitoring to better evaluate pollutant release from roadside 

snowpack under various settings and management strategies (e.g., highway 

versus secondary streets, heavy versus light traction abrasive application, 

shade versus sun, plowing and blowing versus hauling or moving). 

• Understand how new improvements will affect drainages, and what 

improvements are needed to allow watersheds to reach their desired equilibrium states is important. In addition to the hierarchical approach of 

source control, hydrologic design, and treatment is generally recommended, 

but alternative or complementary strategies also could be evaluated. 

Furthermore, it is recommended that monitoring, modeling, and project 

construction all occur on equivalent time scales to obtain validation data 

from “real” projects. 

• Develop a field classification scheme for identification of specific erosion 

problem areas (hot spots) as sources of sediment and nutrients. Watershed 

and drainage inventories that map the type, extent, and condition of hot 

spots according to an established classification scheme would help target 

mitigation projects. This would lead to research and monitoring to develop 

estimation techniques for evaluation of erosional hot spots as sources of 

sediment and nutrients to downstream waters. It would be useful to understand how much of the loads and concentrations measured in monitoring 

data (e.g., the land use-based characteristic concentrations) are associated 

with erosional hot spots versus distributed sources. 

• Complete of a basinwide inventory to provide detailed spatial information 

on the distribution of drainage systems and discharges, as well as existing downstream conditions. This inventory could address legacy as well 

as modern drainage and conveyance routes, and BMP installations. Local 

agencies have started this work.
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

• Develop better predictive models for evaluating the effects from disturbance and mitigation projects that change runoff paths, volumes, velocities, 

and patterns, especially as this relates to the potential loading of fine particulates from accelerated channel, stream, and overland flow erosion. These 

models could provide comprehensive watershed analysis for evaluating the 

best methods to reduce peak flows and pollutant loads from urban watersheds. 

Active streambank erosion along the Upper Truckee River, south of Highway 50. 

Peter Goin

Stream Channel Erosion 

In addition to affecting lake clarity, sediment derived from stream channels can 

affect localized hydrology, instream habitat for lotic aquatic biota, and stream-water 

quality. Streambank erosion has been identified as a source of TSS from several 

watersheds as a result of extensive reconnaissance-level field work throughout the 

Tahoe basin, and by resurveying monumented cross sections originally established 

in the 1980s (Hill et al. 1990, Simon et al. 2003). In particular, streambank erosion 

in Blackwood and Ward Creeks, and the Upper Truckee River was found to be 

the major contributor of TSS from these watersheds (fig. 4.7) (Simon et al. 2003). 

Overall, the contribution of sediment (<63 μm in diameter) from streambank 

erosion was estimated by developing an empirical relationship between measured 
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GENERAL TECHNICAL REPORT PSW-GTR-226

Figure 4.7—Fine-sediment loadings from 

streambank erosion (from Simon 2006).

or simulated bank-erosion rates 

(adjusted for the content of silt 

and clay in the bank material) 

with a field-based measure of the 

extent of bank instability along 

given reaches and streams (Simon 

2006). Measured unit values of fine 

sediment erosion rates ranged from 

12.2 m3 • y

-1 • km-1 for Blackwood 

Creek to 0.002 m3 • y

-1 • km-1 for 

Logan House Creek (Simon 2006). 

Based on this empirical relationship, 

the Tahoe Watershed Model (Tetra 

Tech 2007) predicted that the 

contribution of stream channel 

erosion to total sediment loading 

from all sources was 19 percent, 

although the contribution of stream 

channel erosion to the number of 

fine sediment particles <16 μm in 

diameter (the pollutant of concern 

for water clarity) is currently 

estimated to be <5 percent of that 

from all sources (see “Lake Tahoe 

Water Clarity” section). 

Knowledge Gaps 

The following discussion focuses on uncertainties and research needs associated 

with estimates of fine-sediment loadings from streambanks.

Stream channel cross sections—

• Estimates of sediment (<63 μm) loadings from streambanks are based on 

limited time-series cross-section surveys from a handful of streams and 

numerical simulations of three streams that were then extrapolated basinwide based on observations of the extent of bank erosion at about 300 sites 

across the watershed. Although these data represent good estimates based 

on available data, additional time series and site observation data could 

reduce levels of uncertainty. 

44

46 43

63

2

62

56

55

49

9

19

40

18

10

29

39

24

57

28

11

48

7

47

25

45

38

5

41

51

42

35

17

60

20

23

50

13

14

58 32

6

33

1

31

3

30

8

37

15

21

4

27

54

52 53

59

16

22

61

34

12

26

Tonnes/year

0–0.1

0.1–1

1–10

10–100

100–1000
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

Fine sediment sources and channel processes—

• The influence that urbanization in the Tahoe basin has on runoff and stream 

hydrology and ultimately stream channel erosion is not fully understood. 

• Uncertainties exist in the ability to quantify rates and volumes of bank 

erosion on stream segment and river-wide scales under historical, current, 

and future conditions. For example, measurements of flood-plain deposition 

rates in downstream areas of the Upper Truckee River and other streams will 

aid in determining the fate of fine sediment eroded from upstream areas.

• A better understanding of flood-plain processes, channel overbanking 

frequency and dynamics, and other geomorphic processes is needed to 

help guide restoration planning. 

• The legacy impacts upon current rates of stream channel erosion and sediment transport from major watershed disturbance that occurred during the 

timber clearcut in the late 1800s are not well understood.

Model improvement—

• The CONCEPTS model needs to be further calibrated (for specific conditions at Lake Tahoe) and validated to demonstrate its effectiveness 

in providing useful guidance for design-level planning at the project scale. 

• Improved erosion routines in the upland flow and sediment transport models 

are needed as they serve as the upstream boundary condition for running the 

CONCEPTS channel-evolution model and predicting upland contributions.

Research Needs 

Stream channel cross-sections—

• Establish, monument, and maintain additional cross sections for annual 

surveys. Collect field observations along stream courses, particularly 

in previously unsampled streams. Ideally, these surveys would include 

determination of the particle-size distribution of stream channel material 

susceptible to erosion. 

Fine sediment sources and channel processes—

• Separate natural from anthropogenic stream channel erosion loading to 

distinguish between baseline loads and treatable loads.

• Conduct sediment-tracking research using stable radionuclides or other 

elements to differentiate between the percentage of sediment and fine 

particles emanating from upland and channel sources.
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GENERAL TECHNICAL REPORT PSW-GTR-226

• Complete additional modeling of streams that are known to contribute 

substantial quantities of sediment from the channel margin using CONCEPTS to test alternative strategies for erosion control and to predict the 

amount of load reductions that could be obtained by these strategies.

• Determine the influence of the Comstock logging activities on current 

stream channel erosion; determine how (if) legacy impacts from the 

Comstock logging will affect the efficacy of future channel restoration 

work.

• Establish an understanding of recent (historical and late Holocene) 

streambank sediment sources, sizes, and relative rates of erosion.

• Link CONCEPTS modeling flood-plain water quality modeling to estimate 

load removal associated with channel overflow.

Model improvement—

• Develop improved hydraulic routines, in 2- and 3-D to support better 

channel model (CONCEPTS) predictions of lateral migration, bank erosion, 

planform change, and slope adjustment.

• Fully integrate the CONCEPTS channel evolution model with the upland 

flow and sediment-transport models AnnAGNPS and LSPC.

• Test and validate estimates of the shear stress required to entrain upland 

materials and the associated erodibility coefficient used in the upland 

models. Ideally, this work would be completed on different soils and 

geologic units using in situ field-based measurements.

• Collect calibration/validation data regarding ground water, surface water, 

seasonal ice, and vegetation interactions, with regard to bank failures in 

selected reaches that have been or are being modeled using CONCEPTS. 

• Collect field data regarding the in situ bank strength properties of stabilized 

or constructed bed/banks associated with recent stream restoration practice, 

such as excavated channels, placed riffles, or sod revetments.

Water Quality Treatment

Water quality treatment and source control measures are a key focus of management programs aimed at improving water quality in the Lake Tahoe basin. Although important research and monitoring has occurred in the past, future efforts 

will be most productive if they are hypothesis-driven to answer specific questions 

on the best methods for water quality treatment and source controls throughout the 

basin. Further, better integration of the research and monitoring results to date is 

recommended to address information needs at multiple spatial scales (e.g., at the 

project, watershed, and basinwide scales).
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

As with the other subthemes considered in this chapter, fine sediment and the 

nutrient forms of N and P are the primary pollutants of concern relative to water 

quality treatment and source control. Fine sediment and nutrients enter the lake 

from streams, atmospheric deposition, intervening areas, shoreline erosion, and 

by ground-water inflow (LRWQCB and NDEP 2008a, Reuter and Miller 2000, 

Reuter et al. 2003). Since 1970, several projects have been undertaken to reduce 

the quantity of sediment and nutrients entering the lake. Perhaps the most important project has been the pumping of treated effluent out of the Lake Tahoe basin. 

Additionally, the 1987 Clean Water Act Revision greatly affected transportation 

agencies by requiring more emphasis on wetland mitigation and stormwater 

management. Transportation agencies throughout the country began constructing wetlands and stormwater detention basins in response to provisions in the 

act. Numerous detention basins have been constructed in the Lake Tahoe basin 

(Fenske 1990, Reuter et al. 1992b). Constructing wetlands in or adjacent to stormwater detention basins has been shown to provide marked improvements in the 

quality of stormwater runoff (Martin 1986; Scherger and Davis 1982; Reuter et al. 

1992a, 1992b). However, some potential exists for degradation of the ground-water 

quality beneath detention basins and associated wetlands (Church and Friesz 1993, 

Granato et al. 1995, K.B. Foster Civil Engineering Inc. 1989, 2ndNature 2006a).

Planning for a comprehensive water quality treatment plan for the Tahoe basin 

has taken a substantial step toward the completion of a pollutant reduction opportunity evaluation (LRWQCB and NDEP 2008b) as part of the Lake Tahoe TMDL. 

This analysis estimated potential pollutant load reductions and associated costs at 

a basinwide scale. It was the first comprehensive analysis of possible load reductions from the major source categories of urban runoff and ground water, atmospheric deposition, forested uplands, and stream channel erosion. This analysis 

was done in three steps including (1) an evaluation of potential pollutant controls 

applicable within the Tahoe basin, (2) a site-scale analysis that included an evaluation of treatable areas and the potential level of treatment within each, and (3) an 

extrapolation to the basinwide scale. The uncertainty analyses conducted for each 

of the major pollutant source categories in that report highlighted where assumptions were made and best professional judgment applied. Much of the research 

discussed in this section will help fill those types of knowledge gaps and advance 

the TMDL as a living water quality improvement plan for Lake Tahoe.
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GENERAL TECHNICAL REPORT PSW-GTR-226

BMP Implementation, Operations, and Maintenance for 

Water Quality Treatment 

Implementation of BMPs in the Lake Tahoe basin has generally focused on soil 

restoration projects and hydrologic controls (installation of basins, culverts, and surface runoff conveyance). The overall effectiveness of these strategies has not been 

well evaluated for water quality improvements. Although a few individual BMPs 

have been extensively monitored for performance in the Tahoe basin, these tend 

to be the exception (e.g., the compilation of published studies in 2ndNature 2006b, 

Reuter et al. 2001). There is not yet a clear understanding of what BMPs work in 

subalpine environments like the Tahoe basin. It is likely that continued implementation and management of water quality BMPs in the Tahoe basin will be necessary 

over the long term. Thus, a better understanding of effective BMP design, operation, and maintenance will be essential for sustained water quality improvements. 

Effective BMPs for the protection and restoration of Lake Tahoe clarity will 

differ in some aspects from standard designs used in other parts of the country, 

largely owing to the unique climate characteristics in the Tahoe basin (e.g., rain and 

snow in winter, infrequent thunderstorms, and dry summers), as well as the fairly 

thin granitic soils, relatively low BMP influent concentrations, and very low desired 

BMP effluent concentrations. Thus, for Tahoe installations, it has been difficult to 

use with confidence the BMP design and effectiveness information from other parts 

of the country. Preliminary results from Tahoe-specific studies suggest, however, 

that in some cases, the overall effluent concentrations and efficiencies are similar to 

the national averages (Heyvaert et al. 2006b).

Most annual runoff into Lake Tahoe occurs from snowmelt and rain-on-snow 

events. These are typically due to large frontal storms that arrive as nominally 

uniform events around the Tahoe basin, where BMPs are usually designed to meet 

standards of hourly precipitation intensity (e.g., the 20-yr, 2.54-cm of precipitation 

in 1-hour storm event). In contrast, high-intensity thunderstorms typical of summer 

are much more variable in terms of spatial extent and intensity. A short-duration 

Tahoe summer thunderstorm of only 10 to 15 min can generate very high flows that 

scour online BMPs and exceed the first-flush capacity of offline BMPs. These summer thunderstorms are frequently a substantial source of sediment loading (Gunter 

2005, Jones et al. 2004). 

In the Tahoe basin, studies of BMP effectiveness have been largely confined to 

new or well-maintained projects. These studies may not capture the true range of 

performance for different types of BMPs or BMPs of different ages. Most treatment BMP designs in the Tahoe basin do not undergo comprehensive scientific or 
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

technical review by specialists. Common design problems, such as hydraulic shortcircuiting, could be avoided if this review practice were implemented as a standard 

operating procedure. 

Typical BMP evaluations can underestimate total particulate loads, especially 

the flux of very coarse material and debris that tends to fill basins and vaults. 

Capturing this information is important for developing maintenance programs. To 

address this issue, an alternative mass balance approach has been recommended, 

where feasible, for basins and vault evaluations (Heyvaert et al. 2005). This alternative approach substantially improves estimates of performance and life-cycle costs, 

particularly as related to maintenance requirements. To date, only limited data 

are available on the sustainability and life-cycle costs of BMPs. This is especially 

true for infiltration basins because of their high rates of failure caused by reduced 

infiltration capacity over time. 

Detention basins are one of the most common BMPs in the Tahoe basin for 

removing sediments, nutrients, and other contaminants from urban and highway 

runoff. Although this type of BMP may abate surface-water loads, any infiltrated 

stormwater can contaminate shallow ground water and potentially increase the 

ground water gradient and flows into the lake. In addition, contaminants associated 

with urban runoff often include organic compounds and metals that are potentially 

toxic when consumed with drinking water. Processes that affect ground water contamination from stormwater have only just begun to be considered (e.g., Prudic et 

al. 2005, Thomas et al. 2004), but understanding these details is important because 

a substantial number of EIP projects are planned that will increase infiltration of 

urban runoff to comply with TMDL regulations.

Pollutant loads in Lake Tahoe are highly variable under the normal range of 

annual precipitation and runoff (Heyvaert et al. 2008). Therefore, accurate estimation of long-term benefits from various management strategies will depend upon 

simulations over the full range of climate conditions and would include phased 

implementation for improvements, coupled with the effects from variable runoff 

quantity and quality. The first step toward this approach has been made with 

development of the LSPC watershed model and the LCM, as part of phase 1 in the 

Lake Tahoe TMDL program (LRWQCB and NDEP 2008a, 2008b; Perez-Losada 

2001; Riverson et al. 2005; Sahoo et al. 2007; Swift et al. 2006). Initial modeling 

results and monitoring data confirm, for example, that high runoff years are highly 

correlated with declines in lake water clarity. Better BMP designs, implementation, 

and maintenance will be needed to help reduce this effect.
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GENERAL TECHNICAL REPORT PSW-GTR-226

Urban Source Control

Hydrologic management is typically one of the first steps in sediment source 

control for urban landscapes. By reducing runoff intensity and volume, the 

downstream hydrographs show lower peak flows that are spread over longer 

periods and generally have lower pollutant loads. Hydrologic management also 

is an effective form of erosion control; however, some level of soil restoration is 

often necessary in disturbed areas to remediate compaction, protect surfaces, and 

restore soil function (see “Key Soil Properties” section in chapter 5). Other forms 

of source control include structures or materials, such as retaining walls or rock 

and vegetation coverage of unstable roadcuts or slopes. Hydrologic management in 

urban areas will be especially important for controling soil erosion from erosional 

hot spots, areas that produce large sediment loads during stormwater runoff.

New Treatment Technologies and Enhanced BMP Designs

Standard types of BMPs may not adequately achieve the pollutant load reductions 

necessary for improving Lake Tahoe clarity. A combination of standard BMPs and 

new types of BMPs will likely be needed to achieve low pollutant effluent concentrations destined for Lake Tahoe. Thus, additional efforts have been made over the 

last few years to explore alternative approaches and technologies for stormwater 

treatment in the Tahoe basin. Various mechanical and chemical methods for purifying water are well known. However, they tend to be expensive, energy intensive, 

and are not well suited for dealing with large volumes of stormwater runoff. In 

addition, there are certain factors at Tahoe that constrain potential solutions for 

stormwater runoff quality, including (1) extremely low nutrient and sediment target 

concentrations and (2) cool subalpine air and water temperatures that limit biological productivity. In wetlands, for example, the nutrient uptake by macrophytes and 

emergent vegetation tends to be lowest in winter and spring, which are times of 

maximum storm runoff and snowmelt (Reuter et al. 1992a). 

There are three broad areas of research on new BMP technologies that could 

be applicable to the Lake Tahoe basin. These include studies on (1) unit processes 

and treatment trains, (2) hybrid systems that provide chemical and mechanical 

augmentation of natural processes (e.g., co-precipitation of P with aeration and 

water pumping in constructed wetlands), and (3) novel ecological treatment systems 

(e.g., cultured periphyton, floating wetlands, submerged aquatic vegetation, clams 

and other filtering organisms, complex ecologies, and industrial food webs).
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

Coagulation is beginning to be more tested and applied for reducing stormwater 

turbidity (e.g., Harper et al. 1999). Laboratory and small-scale coagulations studies 

for the Tahoe basin have demonstrated that dissolved P and fine particles can be 

removed effectively with coagulation (e.g., Bachand et al. 2006a, 2006b; Caltrans 

2006). Ultimately, the new treatment technologies required to achieve compliance 

with TMDL and water quality standards in the Tahoe basin may approach or be 

equivalent to technologies employed in the drinking water and wastewater treatment industries, including reverse osmosis, membrane filtration, flocculation, 

sedimentation, and filtration.

Phosphorus removal by adsorptive media is another BMP technique currently 

under investigation, in part because soils in the Tahoe basin have relatively low P 

adsorptive capacity. Although the use of natural or engineered media (including 

derivatives of calcium, aluminum, iron, or lanthanum) can greatly increase available adsorptive capacity, they also cause changes in pH and other chemical characteristics of the treated water that may be detrimental (Bachand et al. 2006a, 2006b; 

Caltrans 2006). 

Existing BMP standards tend to result in designs that are not optimized for 

targeted pollutants in the Lake Tahoe basin. Most BMPs, for example, will not 

effectively remove fine particles and dissolved P from storm runoff when there 

is surface outflow through the systems (2ndNature 2006b, Strecker et al. 2005). 

Simply implementing current BMP designs, therefore, is not likely to meet the 

ultimate requirements for maintaining or improving lake clarity. Advancing BMP 

design standards at Tahoe will be important for achieving the TMDL and effluent 

standards.

Substantial efforts have been completed recently for enhancing BMP design in 

the Lake Tahoe basin. One such project is the Lake Tahoe Basin Stormwater BMP 

Evaluation and Feasibility Study (Strecker et al. 2005); another is the Pollutant 

Load Reduction Model (NHC 2009). These projects have assessed current design 

standards for BMPs in the Lake Tahoe basin, and have suggested potential refinements to BMP designs that would enhance performance. In addition, they have 

provided a draft methodology and associated spreadsheet tools for assessing options 

that maximize BMP effectiveness, both onsite and at the small watershed scale. 

Together, these projects have applied the latest BMP performance information from 

both the National BMP Database and from local Tahoe BMP performance studies, 

along with scientific knowledge on unit processes, to provide a reasonable evaluation of potential enhancements to BMP performance that could improve treatment 

for the pollutants of concern. 
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GENERAL TECHNICAL REPORT PSW-GTR-226

Knowledge Gaps 

BMP implementation, operations and maintenance for 

water quality treatment—

A uniform definition that quantifies BMP effectiveness in treating fine sediment 

and dissolved fractions of nutrients on an event, seasonal, and annual basis is 

recommended. This definition would be based on performance studies in the Lake 

Tahoe basin given the low effluent standards and the primary pollutants of concern. Ultimately, a much better understanding of effective treatment technologies, 

including treatment train approaches and infiltration practices is recommended to 

achieve compliance with Lake Tahoe TMDL requirements, particularly in areas of 

high-density development. 

The costs and components of oversight programs to ensure that BMPs are 

correctly monitored, maintained, or retrofitted to meet the fine particle and nutrient load reduction requirements have not been determined. Particular information 

needs include the long-term sustainability and life-cycle costs of BMPs, removal 

costs for particulate residuals, and disposal requirements for each type of BMP. 

A better understanding of the interactions among source categories and BMP 

implementation strategies is needed for accurate model simulations. When appropriately calibrated and verified, these simulations would provide opportunities to 

investigate the effectiveness of potential management actions on achieving environmental thresholds as part of the TMDL Program. Validating the results from these 

simulations is recommended to achieve confidence in the predictions of the longterm effects on water quality and lake clarity. 

New treatment technologies and enhanced BMP designs—

The potential effectiveness of modified or nonstandard BMP designs is under 

investigation, including coagulant-enhanced particle settling, filtration, biological 

treatment, and sorptive media. Anionic polyacrylamides show promise for turbidity 

treatment, but have not yet been studied for application in the Tahoe basin. Understanding and quantifying the toxicity effects of any potential chemical treatment is 

recommended before implementation in the Tahoe basin. 

With adequate surface area and hydraulic retention time, wetlands can be 

very effective at removing N, P and sediment from urban runoff. However, conventional designs for treatment of wetlands and detention basins only make use 

of a relatively small subset of the potentially useful biological and ecological processes. In many wetland and aquatic systems, for example, a higher percentage of 
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

P removal from the water column may occur via algae attached to plant stems and 

on surface sediments than through uptake by higher plants. Thus, for enhanced 

design opportunities, the treatment potential of other organisms and their combination in novel ecological systems should be investigated further in the Tahoe basin. 

Research Needs 

BMP implementation, operations and maintenance for 

water quality treatment—

• Improve BMP performance assessments based on standardized methods. It 

is recommended that these assessments focus on fine particles and nutrients 

and use our understanding of physical-chemical processes to improve the 

design of treatment systems through sound engineering principles. 

• Although not considered research per se, establishment of a centralized 

database of Tahoe BMP information support of stormwater and BMP 

research is recommended. Ideally, this database would include data on 

BMP location, area, and land uses; the type of BMP installed, its capacity, 

installation date, inspection dates, and maintenance information; as well as 

water quality data, and monitoring records. The available data also could 

include BMP design criteria (hydraulic loading, residence time, width/

depth ratio, aspect ratio, vegetation types), where the specified criteria 

are linked to predictable performance standards for specific pollutants 

of concern. 

• Complete applied research to create better design guidance and specific 

BMP requirements for the Tahoe basin to achieve uniform treatment and 

pollutant reduction targets. This could lead to a design manual that would 

be used to enhance BMP selection and design criteria in the Tahoe basin. 

This includes a process of standardized scientific review for BMP implementation, operation, and maintenance. Design criteria could link directly 

to specific BMP functional characteristics and processes that improve performance based on integrated results from monitoring and modeling. 

• Characterize the processes that influence nutrient transport from infiltration basins to shallow aquifers. This effort would include evaluating the 

removal (or addition) of pollutants during infiltration of stormwater into 

different underlying soils, as well as estimates of long-term seepage and 

pollutant loading at locations where ground water discharges into Lake 

Tahoe.
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GENERAL TECHNICAL REPORT PSW-GTR-226

• Identify and characterize emerging pollutants of concern in terms of their 

removal by and effects on treatment BMPs.

• Conduct additional BMP monitoring to improve the prediction capabilities of models that are under development. Testing these models at a variety 

of field sites and scales for comparison to actual monitoring data will help 

validate results and inform design practices. 

Urban source control—

• Conduct scientifically based effectiveness evaluations on different types of 

soil cover and erosion control materials, including longer term studies on 

soil restoration success and nutrient regimes.

• Develop appropriate metrics for evaluating the long-term success of urban 

source control projects in terms of their fine sediment particle and nutrientretention characteristics. 

New treatment technologies and enhanced BMP designs—

• Conduct replicated experiments to systematically assess the potential of 

new treatment technologies. A standardized, comparative approach will 

be needed to (1) predict and quantify performance; (2) understand the 

mechanisms of performance; (3) understand or identify ancillary effects, 

consequences, or benefits; (4) refine the logistics of application; and (5) 

understand inherent limitations. 

• Conduct research on enhanced methods for capturing fine sediment in 

the 0.5 to 16 μm range, especially for BMPs that can remove substantial 

sediment loads from surface runoff. 

• Evaluate the potential for engineered soil matrices that better adsorb 

nutrients, remove fine particles, and provide improved infiltration rates.

• Test anionic polyacrylamides in conjunction with other passive treatment 

technologies to determine their ability to reduce runoff turbidity and hence 

sediment loads. 

• Determine the toxicity of stormwater and different treatment technologies 

before large-scale implementation of new technologies is authorized for the 

Tahoe basin. 

• Determine what other stormwater pollutants could be treated by standard 

or enhanced BMPs. Examples of these pollutants include metals (i.e., 

cadmium, copper, and zinc), polyaromatic hydrocarbons, gasoline products 

(benzene, toluene, ethylbenzene, and xylenes and methyl tertiary butyl 

ether) and pesticides (pyrethroids).
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

• Develop a geographic information system (GIS)-based load reduction 

model to identify constituents and runoff volumes that could be most effectively addressed by different treatment methods, and to predict the results 

from different implementation strategies at various locations. It is possible, 

for example, that large regional advanced treatment systems would yield 

benefits from economies of scale and greater geographic flexibility compared to stormwater treatment applied through individual BMPs. 

• Conduct comprehensive studies to quantify and weigh the risks, benefits, 

costs, and maintenance requirements associated with new types of BMPs 

that may be introduced into the basin.

Upland Watershed Function—Hydrology and 

Water Quality 

More than 50 years of development in the Lake Tahoe basin has caused an 

increased flux of sediments and nutrients into the lake owing in part to soil disturbance and subsequent translocation. Road development and other forms of land 

disturbance, especially in areas of sloping topography, result in accelerated erosion 

and the accompanying loss of nutrient-containing topsoil. This is accompanied 

by the exposure of compacted, readily erodible decomposed granite, or andesitic 

volcanic subsoils, which are the dominant soil types in the Tahoe basin. Erosion 

and decreased infiltration rates in uplands are largely a result of soil disturbance by 

logging, grading, grazing, and related practices that result in loss of top layers of 

organic matter, established vegetation and nutrients, and subsequent soil compaction. The greater the disturbance in terms of soil impacts, the greater the erosion 

potential and loss of hydrologic function. As the physicochemical soil quality 

declines, vegetation growth is limited, soil stability decreases, and protective slope 

covers are lost that would otherwise minimize erosion. Efforts attempting to slow 

nutrient input to the lake have taken many forms, most of which focus on sediment 

source control including onsite retention. 

Upland source control is critical for reestablishing hillslope hydrologic function with respect to soil moisture retention and percolation to ground water. 

Improved hydrologic function enhances plant cover conditions, habitat, flood peak 

attenuation, and can result in greater amounts of fine sediment particles remaining 

onsite. For example, using GIS assessment methods, Maholland (2002) evaluated 

the sediment sources and geomorphic conditions in the Squaw Creek watershed 

northwest of Lake Tahoe, a mixed granitic and volcanic soils environment. He 

found that forest road ski runs subject to hillslope rilling were the greatest sources 

of sediment in the watershed. Unfortunately, and despite years of work, little 
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GENERAL TECHNICAL REPORT PSW-GTR-226

scientific information exists about the performance of road cut or hillslope erosion 

control measures employed in the Tahoe basin. However, there are numerous 

examples of anecdotal or visible failures in erosion control especially along road cut 

and ski run areas.

The nonurban landscape represents the largest land use in the Tahoe basin 

(about 90 percent). Although this land largely supports a forest biome, it is by no 

means unimpacted. The clearcut logging practices from the Comstock Era (1860 

to 1890s) affected the composition of forest vegetation: populations of major tree 

species and forest structure never returned to pre-Comstock conditions (Barbour et 

al. 2002). Additionally, erosion hot spots are readily visible on the landscape. These 

areas of increased erosion occur at a variety of spatial scales ranging from the Ward 

and Blackwood Canyon Badlands (subwatershed scale) to landslide/avalanches 

(catchment scale) and from large to small gullies.

Although there is an enormous amount of literature related to erosion control in 

agricultural and relatively humid environments, there are few statistically validated 

field evaluations of the performance of revegetation/restoration type erosion control 

efforts in semiarid, subalpine environments. Information that is available is often 

limited to the “grey” literature or “white” papers from agencies, consultant reports, 

or professional societies. Although erosion control work is not new in the basin, 

documented results when available lack the scientific rigor needed to provide 

credible information for management decisions. 

Selected examples of erosion studies that are relevant to the Tahoe basin 

include those of Fifield et al. (1988, 1989), Fifield and Malnor (1990), and Fifield 

(1992a,1992b) in western Colorado. In these studies, they evaluated the need 

for irrigation and runoff and erosion from plots “treated” with a variety of 

“natural” and geotextile covers on steep slopes. The “natural” treatments included 

hydroseeding, seed blankets, wood and paper hydromulches, straw, coconut, and 

jute materials. Generally, both runoff and sediment yields dramatically decreased 

as compared to bare soil conditions. Not surprisingly, the greatest sediment yield 

reductions were associated with the largest surface cover biomasses. What remains 

unknown are the long-term benefits of these erosion control strategies in the field, 

transferability to other locations, and what effects they have on infiltration rates and 

soil quality restoration. More recently, other efforts at assessing hydrologic effects 

of erosion control treatments at higher elevations or in nutrient-deficient soils have 

been reported. Montoro et al. (2000) described efforts to control erosion from 

anthropic soils on 40 percent slopes using 30-m2

 plots treated with vegetal mulch, 

hydroseeding with added humic acids and hydroseeding with vegetal mulch and 

humic acids. Runoff and erosion from natural rainfall events of 2 to 34 mm/h were 
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

significantly reduced from all treatments as a result of “protection against raindrop 

impact” and “general improvement in soil structure.”

In the Tahoe basin, rainfall simulation studies have provided a means by which 

to standardize evaluation of erosion control measures, in a more controlled setting, through replicated rainfall events of the same intensity (or kinetic energy) 

on multiple plots enabling statistical evaluation of treatment effects on hydrologic 

parameters of interest. Grismer and Hogan (2004, 2005a, 2005b) reviewed available 

literature associated with erosion control measures in subalpine regions and applied 

rainfall simulation methods to assess runoff and erosion rates for disturbed granitic 

and volcanic bare soils in the Tahoe basin. The most fragile and easily impacted 

soils are of volcanic origin. Erosion rates of volcanic soils, and to some degree 

infiltration rates, are slope dependent. They also found that sediment yield (kilograms per hectare per millimeter runoff) from bare soils is exponentially related 

to slope after a minimum threshold slope is exceeded. Although rainfall simulation 

measured infiltration rates were similar in both volcanic and granitic soil types (30 

to 60 mm/hr), sediment yields from granitic soils were several times smaller on 

average (from about 1 to 12 g • m-2 • mm

-1) than that from bare volcanic soils (from 

about 3 to 31 g • m-2 • mm

-1). 

Granitic soil particle sizes were greater than that of volcanic soils in both bulk 

soil and runoff water samples. Runoff sediment concentrations and yields from 

sparsely covered volcanic and bare granitic soils could be correlated to slope. 

Sediment concentrations and yields from nearly bare volcanic soils exceeded those 

from granitic soils by an order of magnitude across slopes ranging from 30 to 70 

percent. Similarly, granitic ski run soils produced nearly four times greater sediment concentration than adjacent undisturbed areas. Revegetation, or application 

of pine needle mulch covers to both soil types decreased sediment concentrations 

and yields by 30 to 50 percent. Incorporation of woodchips or soil rehabilitation 

that includes tillage, use of amendments (e.g., Biosol®,

8

 compost) and mulch covers 

together with plant seeding resulted in little, or no, runoff or sediment yield from 

both soils. Although mulch and grass covers provide some protection to disturbed 

bare soils, they alone do not improve hydrologic function and may only minimally 

reduce erosion and runoff rates depending on the extent or depth of coverage 

(Grismer and Hogan 2004, 2005a, 2005b).

Repeated measurements of sediment concentrations and yields for 2 years 

following woodchip or soil rehabilitation treatments showed little or no runoff. 

Revegetation treatments involving use of only grasses to cover soils were largely 

8 The use of trade of firm names in this publication is for reader information and does not 

imply endorsement by the U.S. Department of Agriculture of product service.
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GENERAL TECHNICAL REPORT PSW-GTR-226

ineffective owing to sparse sustainable coverage (<35 percent) and inadequate 

infiltration rates. It was suggested that a possible goal of restoration-erosion 

control efforts in the basin could be the re-creation of “native”-like soil conditions. 

“Native” soils below forest canopies with about 10 cm of duff, litter, pine needle 

mulch, or other organic matter have very high infiltration rates (>75 mm/hr) 

when the surface is not hydrophobic. When the surface is hydrophobic, runoff 

commences almost immediately with little infiltration in the first 10 minutes; 

although the runoff yields negligible mineral sediment, it can contain high nutrient 

concentrations (Miller et al. 2005). Soil rehabilitation (woodchip or compost 

incorporation) combined with revegetation appears to provide erosion control, 

increased infiltration rates, and restoration of hydrologic function for at least 3 

years, maybe more. 

Knowledge Gaps 

In comprehensive reviews of erosion control systems for hill slope stabilization, 

Sutherland (1998a, 1998b) noted that the “formative years” prior to about 1990 

resulted in a mass of information that lacked scientifically credible, standardized 

data. He argued for standardized evaluation methods that have field applicability 

and greater emphasis on the study of surface, or near-surface processes controlling 

erosion. Perhaps better still for the Tahoe basin, would be a greater emphasis on 

scientifically credible studies on the restoration of soil quality adequate to support 

hill-slope vegetation.

Scientific information about erosion control and soil stabilization methods is 

critical to local agencies, planners, and property owners who want to know which 

methods achieve the greatest reductions in erosion for the least amount of money. 

Forest management efforts related to water quality restoration planning lack both 

process-based erosion models (with the expectation of the use of the WEPP model) 

and the basic soil property/hydraulic information necessary for plausible prediction 

of streamflows and sediment (TSS) and nutrient concentrations. 

Understanding the relationship between mineral fraction particle sizes and fine 

sediment and nutrient transport is crucial for the design of effective sediment traps/

basins. For example, fine particle sizes (<16 μm) settle out extremely slowly and are 

not trapped in many cases, whereas larger particles of lesser water quality impacts 

can be trapped in smaller BMPs (e.g., drainage sediment cans or small detention 

basins). Further, contrary to prevailing thought, vegetation cover alone (e.g., grasses 

with 30 to 60 percent cover that look good) may have little effect on reducing 

erosion rates from disturbed soils. Scientists and managers in the Tahoe basin 

have posed numerous relevant questions, including: 
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

• What types of covers are effective for erosion control? 

• How effective are various types of cover relative to the level of effort 

required for implementation?

• Can effective covers be realistically deployed over large areas? 

• What are the maintenance requirements both short and long term?

• How do soil stabilization and erosion control approaches compare when 

applied at the project, watershed, and basinwide scales? 

• Is artificial replenishment necessary when using mulch covers? 

• Can soil be “rehabilitated” such that natural processes prevail (e.g., 

functional soil nutrient and microbial communities, growth of plants 

that leave a functioning litter layer)?

Research Needs

Erosion and pollutant loading—

• Study runoff particle-size distributions, sediment and nutrient loading from 

the undeveloped, yet disturbed forest landscape. This includes, but is not 

limited to (1) the mechanisms at play in the erosional hot spots (at the full 

spectrum of spatial scales at which this erosion occurs), (2) estimates of 

pollutant concentrations and loading from forest land and the hot spots, (3) 

the transport of nutrients and sediments from the forest floor to receiving 

water bodies that are tributary to Lake Tahoe, (4) the ability to better incorporate these processes into management models, and (5) an understanding 

of potential load reduction strategies for these pollutant sources.

• Determine the impact of existing and legacy roads, trails (e.g., hiking or 

biking), and other areas of the forest that have been disturbed to accommodate transportation (including recreation), vis-à-vis hydrology (including 

baseflow) and sediment/nutrient generation.

• Develop monitoring protocols to identify restoration methods that are the 

most effective in controlling runoff of fine particles and nutrients. These 

protocols also would provide data for the development of descriptive 

relationships between nutrients and particle size (for different soil types 

including granitic and volcanic).

• Quantify the particle size distribution in runoff from steep disturbed slopes 

for both granitic and volcanic soils. 

• Better characterize the possible effects of extreme hydrologic events and 

runoff from large tracts of disturbed land on erosion, sediment transport, 

and nutrient loading, given the risk of wildfire, avalanches/landslides, and 

other potential natural hazards in the mountainous terrain of the Tahoe 

basin, as well as future climate change. 
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GENERAL TECHNICAL REPORT PSW-GTR-226

• Quantify snowmelt-derived erosion across the basin, including monitoring 

and characterization of snowmelt-induced erosion rates from the different 

disturbed soils.

• Define the continuum, or threshold relationship between organic matter content of soil at Lake Tahoe and sediment yield (erodibility). Based on a better 

understanding of the nature of this relationship, there could be tremendous 

potential to develop a rapid assessment of potential erodibility based on litter 

layer thickness or percentage of organic matter in the surface soils.

• Develop process-based erosion models applicable to the basin under rainfall 

and snowmelt conditions—functional at scales ranging from the project scale 

to the entire basin—to help inform and guide management decisions related 

to watershed restoration. 

• Quantify nutrient concentrations in shallow interflow on hill slopes under a 

variety of hydrologic and cover conditions. Subsurface flow in the top 20 to 

30 cm of soil on the hill slope can be a major flow factor; however, it is not 

adequately considered in many erosion models. In the Tahoe basin, research 

shows significant shallow subsurface flow that essentially filters the "runoff" 

resulting in negligible sediment yields, but potentially substantial loads of 

dissolved nutrients. 

Processes related to soil rehabilitation—

• Determine what soil shear strengths are associated with rehabilitated soils 

and how they are affected by vegetative succession.

• Determine how soil rehabilitation affects soil aggregate stability, what 

aggregate stability occurs in “native” soils, and what aggregate stability 

value should be achieved in restoration projects.

• Study hydrophobicity of “native” soils to determine the extent of this condition, how rapidly it breaks down in summer storms, and whether “native” 

soils are a source or a sink of N or P.

Restoration effectiveness—

• Determine the ecologic and economic feasibility of treating erosion hot spots 

in the forest landscape. A better understanding of the occurrence of natural 

versus anthropogenic erosion hot spots is needed to assist managers in making decisions on implementation strategies.

• Determine which restoration methods will provide the greatest return in 

terms of hydrologic function per effort required, followed by how long, or if, 

the restoration method will last and if it is sustainable or self-sustaining.
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

• Determine whether successful soil restoration will result in permanent 

establishment of vegetation and “native”-like soil conditions in the sense that 

little, if any runoff or erosion is encountered. What microbial communities 

are involved?

• Determine restoration costs (per unit area) and quantified benefit in terms 

of erosion control and hydrologic function.

• Apply rainfall simulators with soil and runoff measurements to standardized evaluation of the variety of restoration techniques currently available to 

restore soil function.

Water Quality and Forest Biomass 

Management Practices

Forest biomass management practices can affect surface water and ground-water 

quality. As described below, although some initial research has been done to 

address this issue, a more complete program is needed. This is especially important in the Tahoe basin where forest fuel accumulation is high, biomass reduction 

programs are a high priority, and water quality protection standards are also high.

Fire Suppression

Fire suppression in forests of the Western United States throughout most of the 

20th century has resulted in extremely high fuel loads, reduced tree growth, 

increased disease and insect infestation, and increased risk of destructive wildfires 

(Bonnicksen 2007, Covington and Sackett 1984, Parsons and DeBenedetti 1979). 

In much of the eastern Sierra Nevada region, including Lake Tahoe and vicinity, 

these long-term impacts have been exemplified by a decline in forest health owing 

to the buildup of high tree densities and heavy understory, extensive ladder fuels, 

which provide vertical continuity between surface fuels and crown fuels, downed 

timber fuels, and deep organic layers on the forest floor (Johnson et al. 1997, Miller 

et al. 2006).

A common belief throughout the Tahoe basin and Sierra Nevada is that forests 

long protected by fire suppression contribute little in the way of water quality 

degradation via natural nutrient discharge, because nutrient uptake and interception 

are thought to be maximized by the thick vegetative understory (Reuter and Miller 

2000). Recent research, however, has identified the presence of high concentrations of biologically available N (ammonium nitrogen [NH4

+

-N], nitrate nitrogen 

[NO3-N]) and P (phosphate phosphorus [PO4

3--P]) in coniferous forest overland 

flow (Miller et al. 2005). This suggests that these nutrients may be derived from the 
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GENERAL TECHNICAL REPORT PSW-GTR-226

heavy accumulations of overlying forest floor surface organic layers (O horizons) 

and that there has been little biological uptake, leaching, or direct contact with the 

mineral soil where strong retention of NH4

+

-N and PO4

3--P would be expected. As a 

potential source of biologically available N and P, transport of these nutrients from 

terrestrial to aquatic habitats in the Lake Tahoe basin may therefore contribute to 

the already deteriorating clarity of the lake (Loupe et al. 2007).

Wildfire

The buildup of heavy understory fuels (~93 200 kg/ha of biomass) also has 

increased the potential for catastrophic wildfires in the Tahoe basin. It is well 

known that wildfire affects the various nutrient pools available for waterborne 

transport (Baird et al. 1999, Blank and Zamudio 1998, Johnson et al. 2004, Murphy 

et al. 2006b, Neary et al. 1999, Smith and Adams 1991). For example, wildfire typically results in large gaseous losses of system N owing to volatilization, but may 

often cause increases in soil mineral N owing to heat-induced degeneration of soil 

organic N (Murphy et al. 2006b, Neary et al. 1999). Conversely, wildfire effects on 

inorganic P are far more variable with some studies showing increases (Hauer and 

Spencer 1998, Saa et al. 1993) and others showing decreases (Carreira et al. 1996, 

Ketterings and Bighamm 2000) in available P depending upon fire intensity.

Wildfire has been found to increase the immediate mobilization of labile 

(readily available) nutrients. Murphy et al. (2006b) reported no significant differences in nutrient leaching prior to burning, but during the first winter following a 

wildfire, soil solution concentrations of ammonium, nitrate, phosphate, and sulfate 

were significantly elevated in the burn area. In addition, elevated concentrations of 

inorganic N and P also were found in surface runoff from the Gondola burn area 

above Stateline Nevada (Miller et al. 2006). The effect of wildfire was to increase 

the frequency and magnitude of elevated nutrient discharge concentrations during 

the first wet season following the wildfire event. At least some of this labile N 

and P may well have made it offsite during precipitation or snowmelt runoff, thus 

enhancing the nutrient loading of adjacent tributaries9

 and their discharge into 

Lake Tahoe.

Immediately following the 2007 Angora Fire that burned nearly 1255 ha in the 

Upper Truckee River watershed, the USDA Forest Service Burn Area Emergency 

Response team (USFS 2007) reported an elevated erosion potential of approximately 22 to 76 tonnes of sediment per hectare and that ash and sediment delivery 

9 Allander, K. 2008. Personal communication. Hydrologist, U.S. Geological Survey, 

2730 N Deer Run Rd., Carson City, NV 89701.
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

to Angora Creek, the Upper Truckee River, and ultimately Lake Tahoe could be 

high resulting in unacceptable water quality conditions. Monitoring is ongoing, 

however, loading was reduced in Water Year 2008 due to very low precipitation.

Prescribed Fire

Prescribed fire has become a popular management strategy in the Sierra Nevada 

for the removal of undesirable vegetation and heavy fuel loads (Neary et al. 1999, 

Reuter and Miller 2000, Rowntree 1998, Schoch and Binkley 1986). Controlled 

burning can remove large proportions of understory vegetation, litter layers, and 

larger surface fuels with minimal effects on the dominant tree vegetation. The 

treatments are generally mosaic in character and of much lower burn intensity than 

wildfires. Although carbon (C), N, and sulfur (S) remain susceptible to volatilization at lower burn temperatures, other elements such as P require higher burn 

temperatures to volatilize. Thus, substantial system losses of nutrients as a result of 

prescribed burning are generally the result of offsite particulate transport from ash 

convection, and waterflow runoff and erosion (Caldwell et al. 2002, Loupe 2005, 

Murphy et al. 2006a, Riason et al. 1985) rather than volitilization.

Whereas wildfire has been shown to cause a dramatic increase in labile nutrient mobilization (Johnson et al. 2004, Miller et al. 2006, Murphy et al. 2006b), this 

effect has not been identified for prescribed fires. Murphy et al. (2006a) found no 

significant increases in the leaching of ammonium, nitrate, phosphate, or sulfate 

following a prescribed Sierran burn on volcanic soils. Neither resin nor ceramic cup 

lysimeter data showed any effects of burning on soil solution leaching. Although 

Chorover et al. (1994) found increases in soil solution and streamwater ammonium 

and nitrate following a prescribed fire on granitic soils at a western Sierrian site, 

Stevens et al. (2005) reported that prescribed fire in the Lake Tahoe basin had no 

effect on soluble reactive phosphate and only minimal effects on nitrate in streamwaters. In support of this latter finding, Loupe (2005) found controlled burning to 

result in a net decrease of inorganic N and P concentrations in surface runoff at 

a site near north Lake Tahoe. On this basis, Murphy et al. (2006a) concluded the 

most ecologically significant effects of prescribed fire on nutrient status to be the 

substantial loss of N to the atmosphere from forest floor combustion. 

Mechanical Treatment

Mechanical treatment is a forest management approach that includes techniques 

such as tree removal, chipping, mastication, grinding, etc. to control slash and 

other undesirable biomass. Reduced biomass accumulations improve forest health 

while decreasing the threat of wildfire (Klemmedson et al. 1985). Such treatments 
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may temporarily increase litter mass from slash inputs; however, in the long term, 

mechanical treatment can (1) reduce new litter input by decreasing the number of 

young pole-sized trees, and (2) modify nutrient cycles through changes in plant 

uptake, substrate availability, infiltration ability, and soil temperature and moisture 

conditions (Parfitt et al. 2001, Smethurst and Nambiar 1990).

Although biomass reduction by fire has been shown to impact the nutrient pools 

available for waterborne transport (Baird et al. 1999, Blank and Zamudio 1998, 

Johnson et al. 2004, Miller et al. 2006, Murphy et al. 2006a, Neary et al. 1999, 

Smith and Adams 1991), much less is known regarding the effects of mechanical 

harvest. Hatchett et al. (2006a, 2006b) conducted a study on the west shore of Lake 

Tahoe to determine if heavy mastication equipment used for stand-density reduction would increase soil compaction, decrease infiltration, and thereby increase 

runoff and erosion: processes which would also be expected to increase nutrient 

and fine sediment discharge to adjacent tributaries. Data from cone penetrometer 

measurements indicated that the use of heavy mastication equipment did not cause 

significant compaction, regardless of the distance from the machine tracks. Furthermore, artificial rainfall applications showed erosion and runoff rates to be more 

dependent on soil origin, regardless of surface treatment (Hatchett et al. 2006).

Cut-to-length harvest/chipping mastication treatment in the absence of fire 

results in lower runoff concentrations of inorganic N, P, and S (Loupe 2005). Interactions between mechanical treatment and prescribed fire were more varied; however, the overall findings indicated that both prescribed fire and mechanical harvest 

management strategies have the potential to improve long-term water quality by 

reducing the nutrient content in surface runoff. Although prescribed fires have 

been typically reported to not result in P volatilization from organic combustion 

because of lower burn temperatures than wildfires, Murphy et al. (2006a) found the 

opposite to occur within the slash mats of the cut-to-length treatments, which would 

be expected to burn at higher temperatures. Surprisingly, some increases in soil C 

and N in both the slash mats of cut-to-length and skid trails of whole tree harvest 

were identified. Overall, however, the study by Murphy et al. (2006a) suggested the 

higher fuel loadings in the slash mats did not cause deleterious effects to either soils 

or water quality.

The USFS has been monitoring the implementation and effectiveness of timber 

harvest BMPs to protect soils and water quality using the USFS California Region 

BMPs evaluation program protocols developed in cooperation with the California 

State Water Quality Control Board. This qualitative assessment has found that since 

1992, Timber Harvest BMPs on the Lake Tahoe Basin Management Unit (LTBMU) 
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have been effectively implemented about 90 percent of the time in terms of 

soil erosion; however, this assessment did not include an assessment of nutrient 

concentrations.

Knowledge Gaps 

Fire suppression—

Comprehensive fire suppression has caused a shift from more frequent lowintensity fires, which were presumably prevalent prior to European settlement, to 

catastrophic, stand-replacing wildfires. Accurate assessments of the true nutrient 

status of pre-European pristine forest conditions are unavailable. Hence, the water 

quality effects of this paradigm shift are difficult to evaluate primarily because of 

the lack of prewildfire samples and suitable historical controls for assessing specific 

wildfire effects.

Comprehensive fire suppression has caused a decline in forest health, in part 

resulting in a buildup of excess organic debris that may now be an important source 

of biologically available N and P in naturally derived surface runoff. Litter mass is 

typically considered to be a nutrient sink; however, the equilibrium has apparently 

shifted such that the amount of nutrient mineralization within the excessive biomass 

has increased causing the release of large amounts of available nutrients into solutions passing through it—albeit the extent of which has not been fully quantified. 

Although the magnitude remains largely unknown, it now appears that overland 

flow from the forest may be an important source of dissolved nutrients discharged 

to nearby streams and lakes.

Wildfire—

Wildfire clearly has the potential to affect surface runoff water quality through 

enhanced mobilization of labile nutrients (likely through temperature-induced 

mineralization) and subsequent increased discharge concentrations. Whether or not 

these newly mobilized nutrients actually make it offsite and into adjacent tributaries 

and Lake Tahoe during precipitation or snowmelt runoff is unknown. The frequency and magnitude of such surface discharges cannot be quantified at this time 

because we have no means of determining the flow volume on an areawide basis. 

The long-term effects of wildfire on runoff water quality are unknown but may 

ultimately result in a decrease in discharge nutrient concentrations over time owing 

to the dramatic reduction of heavy surface deposits of decomposing organic litter.

Areas affected by wildfire are frequently prone to flooding, landslides, and 

debris and sediment flows as a result of increased postfire erosion owing to lack of 

vegetation cover, and fire-induced subsurface hydrophobic layers that can increase 

the mass wasting potential of overlying wettable soil. With the exception of a very 
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recent study (Carroll 2006), the degree and extent of fire-facilitated watershed 

erosion and accompanying nutrient discharge following the first major postwildfire 

precipitation event remains largely unknown throughout the Tahoe basin. Although 

it appears that the impact of a single erosion event following a wildfire may be at 

least an order of magnitude greater than the expected average annual erosion based 

on a 1,000-year projection, more accurate quantification of the specific source area 

is paramount to understanding the actual scale of erosion and potential nutrient discharge. In the case of the USFS-recommended water-quality-related BMPs following the Angora Fire, the primary focus was to reduce erosion and retain as much of 

the ash and disturbed soil onsite as possible. More research is needed to determine 

to what extent postwildfire BMPs can be designed to address nutrient mobilization. 

Prescribed fire—

There is considerable information on the immediate effects of prescribed fire on 

biomass reduction; however, there is much less information on both the short- and 

long-term impacts on site nutrient status and potential discharge water quality. The 

effect of prescribed fire on residual nutrient mobilization appears to be far less than 

that associated with wildfire, but the availability of comparative studies is limited. 

The few studies that do exist suggest prescribed fire may have negative impacts on 

soil fertility and site productivity because of N losses (and in some instances P), 

and therefore enhanced potential for improved surface runoff water quality. The 

full extent to which prescribed fire plays a role in affecting soil properties that may 

influence infiltration, percolation, surface runoff, and ground-water discharge also 

is largely unknown.

Mechanical treatment—

Mechanical biomass reduction is an alternative management strategy to offset 

the potential for catastrophic wildfire and to improve forest health. The overall 

environmental costs/benefits of treating forests with mechanical harvesters/masticators have not been adequately characterized. Specifically, the impacts of newtechnology mechanical harvesters and masticators on traditional soil and vegetative 

properties (e.g., compaction, infiltration ability, recovery, nutrient cycling) that 

can influence watershed erosion and surface runoff nutrient discharge have not 

been well characterized. Although short-term impacts in this regard appear to be 

minimal, impacts 1 to 3 years following treatment are uncertain and could be quite 

different.

The LSPC model and cumulative watershed effects analysis (using WEPP 

modeling) currently being conducted by the USFS is utilizing equivalent roaded 

acres (ERAs) coefficients developed by the USFS to estimate the area impacted by 
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various vegetation management practices (i.e., compacted/disturbed surfaces). The 

ERA coefficients are based on the professional judgment of Forest Service hydrologists, but they have never been verified by systematic field testing. Although 

regulatory approaches currently limit or prohibit the use of mechanical treatment 

methods within Tahoe basin stream environment zones (SEZs), the technology has 

vastly changed since these regulatory approaches were established. New research is 

recommended to determine whether or not innovative low-impact mechanical treatment technologies can be operated within some areas designated as SEZs without 

causing significant impact to soil/hydrologic function.

Research Needs 

Fire suppression—

• Further investigate soil and nutrient cycling parameters in pristine forested 

areas of the Sierra Nevada wherever possible to better establish treatment 

“control” scenarios; albeit the effects of fire suppression will be present to 

some extent.

• More fully quantify current nutrient contributions from the now thick 

O-horizon deposits throughout basin subwatersheds:

▪ Better delineate the distribution and thickness of O-horizon deposits 

throughout the basin.

▪ Quantify the potential contributions of inorganic N and P in kilograms 

per unit mass of dry matter; kilograms per unit area, and potential flux 

in kilograms per hectare per year.

▪ Determine the amounts of inorganic N and P contained in surface 

runoff that discharge into adjacent wetlands, tributaries, and ultimately 

Lake Tahoe.

• Stronger quantification of the true functionality of intervening wetlands and 

riparian areas in terms of N and P source/sink interactions. For example, can 

agencies effectively mitigate increased upland overland flow discharges of N 

and P using existing SEZs?

• Research is needed to identify pertinent restoration strategies that, to the 

extent possible, will allow us to mimic historical conditions and functionality.

• A quantitative comparison of water quality effects of wildfire, prescribed 

fire, and mechanical treatment is needed. This comparison will involve 

compiling the limited data that are available and collecting new data where 

needed to evaluate the effects of these three scenarios within watersheds 

having similar hydrologic and soil characteristics.
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Wildfire—

• Systematically study the effects of wildfire on nutrient and fine sediment 

status whenever possible where suitable adjacent control sites exist and especially in cases where, by happenstance, prefire data may be available. Further 

quantify and develop a better means of predicting short- and long-term 

changes in the amount of biologically available nutrients and fine sediment 

discharged from upper watersheds as a result of wildfire and during recovery.

• Apply spatial analysis models for balancing waterflow and nutrient budget 

parameters at the watershed scale to better assess the linkage between 

overland flow nutrient transport and discharge water quality as affected 

by catastrophic events such as wildfire and mass wasting.

• Evaluate the effectiveness of emergency treatments, typically applied to a 

burned landscape to control erosion, sediment/ash transport, and nutrient 

mobilization. 

Prescribed fire—

• More information is needed on both the short- and long-term effects of 

regular prescribed fire and cut-to-length harvest fires on soil and water 

nutrient status to determine the most beneficial and most ecosystem 

“friendly” return interval.

• Implement a long-term assessment to quantify the relationship between regular reductions in litter-fall biomass accumulation, and the N and P content in 

overland flow runoff and discharge water quality at the watershed scale.

• Determine the impact of burn frequency on soil and vegetative properties 

that influence infiltration, percolation, surface runoff, and ground-water discharge.

Mechanical treatment—

• More fully investigate the short- and long-term impacts of various mechanical 

treatments (e.g., cut-to-length, whole tree, or mastication) for fuels reduction 

on soil cover, bulk density, infiltration capacity (as measured by Ksat), 

site recovery, nutrient cycling, and surface runoff water quality. Better 

characterization of the impacts of new-technology mechanical harvesters 

and masticators and their influence on watershed erosion, surface runoff, 

and nutrient and fine sediment discharge is recommended. Currently, this 

type of information is very limited. Further-more, it is recommended that 

this research provide information that can be extended throughout the basin 

to account for the very large spatial area that will be affected by mechanical 

treatment and the extremely large volume of biomass that will be removed.
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• Further quantify residual and altered soil moisture status, soil cover, bulk 

density, and infiltration capacity to determine under what conditions innovative harvest technology can be safely applied within upland areas as well 

as those designated as SEZ using the existing SEZ indicators.

Both of the above research needs would benefit from demonstration projects 

and case studies that incorporate the different soil types and environments within 

the Lake Tahoe watershed.

Drinking Water Protection

Waters within the Lake Tahoe basin provide the drinking water supply for nearly 

a half million people living in the Tahoe-Truckee-Reno region, and over 50 million annual visitors to the region. In the Tahoe basin alone there are approximately 

90 water companies, utility districts, independent domestic suppliers, and private 

suppliers.

These water purveyors draw from both ground- and surface-water supplies. 

The federal Safe Drinking Water Act (SDWA) and the Clean Water Act together 

provide the umbrella of protections that the U.S. Environmental Protection Agency 

(US EPA) uses to govern the protection of drinking water supply. The SDWA 

emphasizes the use of comprehensive watershed protection as an important means 

of protecting drinking water.

The Lake Tahoe basin is a source of high-quality drinking water. However, 

despite Tahoe’s exemplary water supply, water purveyors and the state’s health 

protection agencies continuously seek ways to improve public protection against 

exposure to toxic and microbial contamination. Drinking water protection efforts 

typically focus on inhibiting the entry of potential toxic or pathogenic pollutants to 

the water supply, and on eliminating the potentially toxic byproducts of disinfection 

processes. 

Drinking water protection is crucial to human life and health. The U.S. EPA's 

Science Advisory Board (US EPA 1997 states:

Exposure to microbial contaminants such as bacteria, viruses, and protozoa 

(e.g., Giardia lamblia and Cryptosporidium) is likely the greatest remaining health risk management challenge for drinking-water suppliers. Acute 

health effects from exposure to microbial pathogens are documented, and 

associated illness can range from mild to moderate cases lasting only a few 

days to more severe infections that can last several weeks and may result in 

death for those with weakened immune systems.
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Research needs pertaining to drinking water protection focus on answering 

questions about the presence and proliferation of microbial contaminants and aim to 

inform managers in developing a watershed-protection approach to drinking water 

protection.

“From a watershed perspective, any practice that reduces runoff and erosion 

will reduce the transport of pathogen directly to surface water” (WSSI 2000). In 

this regard, efforts in the Tahoe basin to reduce runoff and erosion make a very 

substantial contribution to the overall efforts to protect drinking water. 

Although sediment-reduction efforts in the Tahoe basin benefit drinking water, 

opportunities to be more effective in the protection of drinking water are often 

overlooked. Improving knowledge of drinking water issues and including these 

issues in basin management discussions is essential to the environmental, economic, and social health of all who rely on the Tahoe basin as a source of drinking 

water.

The SDWA amendment (PL 104-82) includes requirements that contributing areas for drinking water supplies be delineated and that potential sources of 

contamination be identified within the delineated areas (US EPA 1997). This can 

be accomplished by watershed management programs, which comprise individual 

practices to manage various types and magnitudes of contaminant sources within 

the hydrologic boundaries of a watershed (Walker et al. 1998).

Knowledge Gaps

The SDWA directs attention to three activities for the protection of drinking/

source water: (1) characterize watershed hydrology and land ownership, (2) identify 

watershed characteristics and activities that may adversely affect source water 

quality, and (3) monitor the occurrence of activities that may adversely affect 

source water quality. Research is necessary at several levels to inform the 

development of a Tahoe-specific watershed management program comprising 

the most effective practices for managing drinking water contaminant sources.

Some of the key uncertainties regarding drinking water protection in the 

Tahoe basin include:

• The transport of pathogenic organisms (virus, bacteria, protozoa) 

in waterways and in Lake Tahoe.

• Pathogen viability.

• Animal waste and its effects on water quality.

• The role of natural and other bacteria in altering water quality 

through chemical and biological interactions.
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• The need for drinking water protection to include toxic substance control.

• The ability to predict pollutant dispersal of particulates, colloidal particles, 

and pathogenic organisms.

• Bio-fouling of treatment infrastructure.

Research Needs

• Investigate methods of stormwater management/treatment effectiveness in 

limiting conveyance of fine sediments (and accompanying pathogens) into 

drinking water supplies.

• Determine the risk of contamination from specific activities such as stormwater drainage, domestic animals, wildlife and human sources, in proximity to surface water intakes and wellheads. Characterize these potential 

sources in terms of the risk that they present to drinking water supply 

relative to their ability to perpetuate, preserve, reintroduce, and activate 

Giardia, Cryptosporidium, Escherichia coli, and other pathogens in the 

environment.

• Build upon efforts to characterize land and water uses and their potential to 

contribute to microbiological and toxic contamination of the water supply 

(TRPA 2000). 

• Utilize findings of the Lake Tahoe Basin Framework Study Wastewater 

Collection System Overflow/Release Reduction Evaluation (US ACE 2003) 

to hone in on potential “high risk” locations in the shore zone for wastewater contamination and investigate potential management practices that 

can minimize or eliminate risk. This also applies to toxic and nutrient 

contamination of drinking water sources.

• Build upon initial findings of the Detention Basin Treatment of Hydrocarbon Compounds in Urban Stormwater study (2ndNature 2006a) and 

Cattlemen’s Basin Infiltration of Stormwater study (USGS 2004) to better 

understand the potential impacts of stormwater contamination on groundand drinking-water sources.

• Develop pollutant dispersion models for particulates, colloids and 

pathogens in Lake Tahoe that focus on near-shore sources and water 

intake structures.

• Evaluate the potential applications of Tahoe TMDL modeling, tools and 

data to inform drinking water protection efforts. 
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Water Quality Modeling

Models are widely used in support of water quality and watershed research, planning, and resource management. In a diagnostic mode, they can be used to investigate cause-and-effect relationships by defining those critical factors that most 

determine how a water body or watershed responds to stressors and other ecological 

drivers. In a predictive mode, they can be used to forecast how a water body or 

watershed will most likely respond to management alternatives and environmental 

changes. They also provide an excellent framework from which we can assess our 

conceptual understanding of ecosystem function.

Rarely do scientists have the ability to assess ecological response to stressors 

based on ecosystem experimentation and large environmental manipulation studies. Although the combination of monitoring and process-based research allows 

scientists and resource managers to track environmental response over time and 

understand its causes, this approach is less than optimal because (1) it is slow; (2) 

researchers have less experimental or statistical control than in a laboratory or 

field experiment, so it can be difficult to detect a response from within the natural 

variability; (3) it is not possible to know a priori all the important variables to be 

measured, nor is it possible to measure them all; and (4) the ecosystem continues to 

change during the protracted period required to collect sufficient data. By describing the environment in quantitative or mathematical terms, models can provide 

invaluable management tools to help answer questions about stressors and ecosystem response and provide insight into current restoration efforts.

A mathematical model is an equation, or more commonly a series of equations that translates a conceptual understanding into quantitative terms (Rechow 

and Chapra 1983). Water-quality-related models are often broadly categorized as 

mechanistic and empirical. Mechanistic models attempt to mathematically define 

the actual ecosystem processes at play (e.g., in lake water quality models, these 

processes might include mixing and circulation, algal growth, food web dynamics, 

or nutrient cycling). Empirical models are based more on mathematical expressions 

of the relationships that appear in a set of data collected from the environment, and 

less on theoretical principles. For reference, the LCM (Perez-Losada 2001, Sahoo et 

al. 2007, Swift et al. 2006), used to evaluate Lake Tahoe’s response to nutrient and 

sediment loading, represents a mechanistic model and is based on linked algorithms 

describing lake processes. In contrast, Jassby et al. (2003) have developed an 

empirically based statistical time series model of Secchi depth variability based 

on actual field data measured over the historical period of record (>35 years). 
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Models can be useful tools for informing lake and watershed restoration. However, models have limitations. These include the ability to translate complex ecosystem processes into mathematical algorithms, the availability and quality of input 

data (both for initial conditions and boundary conditions), the technical capability 

of the model, and the expertise of the modeling team. “Blind” acceptance of model 

results is not recommended without careful evaluation of the models and modeling 

techniques. It is reasonable to expect that models and modeling approaches would 

require revision and updating as new data and new understanding of ecosystem processes become available through research and monitoring. At the same time, model 

results can frequently expose critical gaps in monitoring programs.

In the late-1990s, it was acknowledged that for the Tahoe basin, sufficient 

monitoring and research data were in place and the technical expertise available to 

begin development of a modeling “toolbox” for water quality/watershed management (Reuter et al. 1996). Furthermore, with the development of the EIP in 1997, 

it was understood that management models would be needed to help develop and 

evaluate alternative strategies.

Review of Tahoe Basin Resource Management Models

Selected models that are either currently in or under development/revision are 

briefly described in this section. Not all existing models are presented here, but this 

section does provide a relatively comprehensive overview of the models used to help 

evaluate and guide water quality restoration efforts in the Tahoe basin. Because the 

use of water quality and watershed models in the Tahoe basin is relatively recent, 

corresponding with development of the Lake Tahoe TMDL program, these models 

are currently at different stages of development.

Successful resource management models often are customized in one way or 

another to the specific conditions of the ecosystem under investigation. In some 

cases, an appropriate model does not exist and a new model would be recommended. These models are based on known principles of hydrology, earth science, 

water quality, biology, and chemistry, and are tailored for the ecosystem under consideration (e.g., LCM, LTAM, and PLRM). In other cases, algorithms and equations 

in an existing model are customized to reflect unique site-specific environmental 

conditions (e.g., LSPC as applied to the Tahoe basin). A third alternative is to 

populate existing models with site-specific input data to generate new results (e.g., 

CONCEPTS, WEPP, and Si3D). Each approach has pros and cons, and all three 

approaches have been used in the Tahoe basin.
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Lake Clarity Model—

The University of California, Davis has been developing the Lake Tahoe LCM 

based on the extensive data collected on lake processes by the Tahoe Environmental 

Research Center (TERC) and others over the last 40 years. The LCM is a unique 

combination of submodels including a one-dimensional hydrodynamic model, an 

ecological model, a water quality model, and an optical model. This model was 

developed to specifically identify Lake Tahoe’s response to pollutant loading and 

the pollutant reductions necessary for the protection of lake clarity (LRWQCB and 

NDEP 2008a, Sahoo et al. 2007).

Three-dimensional Lake Circulation Model (Si3D)—

The motion of water within Lake Tahoe determines to a large degree the fate of 

pollutants in the lake, and in the case of withdrawal of lake water for drinking 

purposes, the quality of that water. Si3D is a semi-implicit lake model that has been 

successfully used to describe the basin-scale motions within Lake Tahoe (Rueda 

et al. 2003). As originally developed, the model resolves the lake into 500- by 

500-m horizontal grid cells each with a depth of 5 m. Advances in computer power, 

together with new techniques for embedded subgrids, allows the model to be used 

with horizontal grid resolution as small as 20 by 20 m and vertical grid scales of 

1 m. Such resolution is compatible with processes in the near-shore zone, such as 

pathogen entrainment into drinking water intakes, pollutant tracking, and transport 

of invasive species. Coupling Si3D with water quality, ecological, and optical 

models of the LCM is also possible. 

Watershed Model (LSPC)—

In direct support of Phase 1 of the Tahoe TMDL, Tetra Tech, Inc. developed the 

Lake Tahoe Watershed Model using the Loading Simulation Program in C++ 

(LSPC). The watershed modeling system includes algorithms for simulating hydrology, sediment, and water quality from over 20 land use types in 184 subwatersheds 

within the Tahoe basin. This model has been used to estimate the current pollutant 

loading to the lake from surface runoff and for the exploration of various scenarios 

during development of an Integrated Water Quality Management Strategy as part of 

Phase 2 of the Lake Tahoe TMDL.

Pollutant Load Reduction Model—

The Pollutant Load Reduction Model (PLRM) was developed for use in evaluating 

and comparing pollutant load reduction alternatives for storm water quality 

improvement projects in the Tahoe basin. It uses publicly available software 

with the US EPA Storm Water Management Model as its hydrologic engine. 

The PLRM provides predictions of storm water pollutant loads on an average 
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annual basis for urbanized areas. The primary purpose of the PLRM is to assist 

project designers to select and justify a recommended storm water project 

alternative based on a quantitative comparison of pollutant loads and runoff 

volumes for project alternatives. Pollutant loads in storm water are highly variable, 

and notoriously difficult to predict with absolute accuracy at particular locations 

and times. The focus of the PLRM is to make use of best available Lake Tahoe 

storm water quality information to compare relative performance of alternatives 

over the long term. The recommended spatial scale of application for the PLRM 

is the typical Tahoe basin storm water quality improvement project scale (i.e., 

roughly 4.0 to 40.4 acres). The PLRM may eventually support broader objectives 

beyond prediction of the relative performance of storm water project alternatives 

(e.g., tracking TMDL progress, informing the Lake Clarity Crediting Program, 

and project prioritization). However, additional development, testing, and an 

institutional framework for supporting the PLRM are still needed.

Conservational Channel Evolution and Pollutant Transport 

System (CONCEPTS)—

CONCEPTS is a channel-evolution model developed by Langendoen (2000) with 

the USDA Agricultural Research Station. This deterministic numerical-simulation 

model is used to evaluate stream channel changes over time and simulate sediment 

loads from stream channel erosion. When used in concert with an upland watershed 

model (e.g., AnnAGNPS, LSPC, or WEPP), CONCEPTS can help in the quantification of the relative contributions of sediment from upland and channel sources. 

As part of Phase 1 of the Tahoe TMDL, Simon et al. (2003) used CONCEPTS to 

estimate fine sediment and total sediment loading to Lake Tahoe from General 

Creek, Ward Creek, and the Upper Truckee River. The importance of stream channel erosion to the loading of fine sediment was highlighted by Simon (2006) who 

found that stream channels provided about 25 percent of the annual sediment load 

for the <63 μm fraction. 

Water Erosion Prediction Project (WEPP)—

The WEPP erosion model was developed by the USFS and is based on fundamentals of stochastic weather generation, infiltration theory, soil physics, plant science, 

hydraulics, and erosion mechanisms (Flanagan et al. 1995). The WEPP is a processbased model that can be used to estimate both temporal and spatial distributions of 

soil loss. This model accommodates variability in topography, surface roughness, 

soil properties, vegetation, and land use conditions on hillslopes. The WEPP is 

currently used by the US Forest Service-Lake Tahoe Basin Management Unit for 

evaluation of erosion control projects in the general forest. 
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Lake Tahoe Airshed Model (LTAM)—

The LTAM is a heuristic eulerian model designed to provide predictive capabilities 

for environmental management in the Tahoe basin, vis-à-vis, air quality and atmospheric deposition. A heuristic approach is one where the most appropriate solution 

to a problem, of several found by alternative methods, is selected at successive 

stages. Although it is not specifically a water quality or watershed model, it is well 

established that atmospheric deposition of nutrients and fine particles both substantially contribute to pollutant loading of Lake Tahoe (CARB 2006, Jassby et al. 

1994, Reuter et al. 2003). Air pollution sources including automobiles, forest fires, 

and road dust can be put into the model. The model predicts pollutant transport 

and deposition across the basin and lake surface. The LTAM is an array of 1,248 

individual 2.56-km2 cells across the basin with a North-South range from Truckee 

to Echo Summit and an East-West range from Spooner Summit to Ward Peak. The 

LTAM is semiempirical in design, and incorporates available air quality measurements at Lake Tahoe, plus aspects of meteorological and aerometric theory. The 

model has two major immediate goals: (1) to predict the concentration of air quality 

pollutants in the Tahoe basin at spatially diverse locations where no data exist and 

(2) to predict the potential for atmospheric deposition of nutrients and fine particles 

to the watershed and lake by determining spatial concentration of pollutants within 

the basin. A thorough description of the LTAM, inputs to the model, and several 

output scenarios is given in Cliff and Cahill (2000).

Lake Tahoe Time Series Secchi Depth Model—

High year-to-year variability in lake conditions can obscure restoration actions 

and compliance with water quality standards. This is especially so when simple 

statistics are used to evaluate trends in long-term data. An overarching question 

for resource managers and scientists remains: How can we distinguish temporary 

improvements in lake clarity resulting from natural events from true and significant 

improvement as a result of restoration efforts? A time series model for Lake Tahoe 

Secchi depth was developed, incorporating a mechanistic understanding of interannual variability based on actual lake response over the historical data set (Jassby 

et al. 2003). The model focused on the summer when the lake is least transparent 

and most heavily used. The statistical model determined, with a very high degree 

of certainty, that interannual variability has been driven largely by precipitation 

differences. The model offers a tool for determining compliance with water quality 

standards when precipitation anomalies may persist for years, i.e., this model can 

help determine if the measured annual Secchi is simply climate-driven or represents a recovery of the lake based on restoration activities. 
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Knowledge Gaps 

As discussed above, the development and application of predictive models to help 

guide resource management in the Tahoe basin is a relatively recent but important 

trend. Consequently, although managers and scientists agree that there is great 

potential in the application of these tools, it is acknowledged that information 

gaps exist resulting in varying levels of uncertainty.

Assuming that models will continue to be developed and used by researchers 

and resource managers in the Tahoe basin, it is vital that the models have as much 

scientific validity as possible. All research and other avenues of scientific inquiry 

that reduce the uncertainty in any aspect of these and other applicable models is 

encouraged.

While it is beyond the scope of this science plan to critically review the 

specific areas of uncertainty associated with each of the management models, 

there are general topics that apply to models collectively. As the modeling efforts 

continue and are expanded, additional areas of uncertainty are bound to arise. 

Tahoe-specific numeric coefficients to support process-based 

modeling algorithms—

The important environmental driving forces captured in models are typically 

related to site-specific conditions. Each model uses a different set of modeling 

parameters, each with its own numeric characterization. Although literature 

coefficients are often used to support resource management models, they can 

add substantially to uncertainty. This is especially the case for the Tahoe basin, 

because of the unusual environmental conditions that exist (e.g., nutrient-poor 

granitic soils, mountainous topography, deep oligotrophic lake, and subalpine 

conditions) are not well represented in the literature. Research is needed to more 

accurately describe modeling algorithms and rate coefficients specific for the 

Tahoe basin.

Sufficient and appropriate model input data—

Models require reliable input data for initial conditions, boundary conditions, 

external sources (loads), and sinks (losses). Meteorological data is a critical 

category of input data for the water-quality-related models being used and 

developed for Lake Tahoe and the surrounding watershed. Meteorological 

conditions (e.g., temperature, precipitation, relative humidity, windspeed and 

direction, and solar radiation) are important forcing factors for erosion, hydrology, 

pollutant transport, lake currents, and vertical mixing. The mountainous terrain 

within the Tahoe basin is subject to both orographic effects and spatial variability 

in microclimatology. With climate change already acknowledged as an important 
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GENERAL TECHNICAL REPORT PSW-GTR-226

factor in the Sierra Nevada, maintaining a temporally and spatially extensive realtime network of meteorological data (both lake and watershed stations) is critical.

Other types of model input data also need more up-to-date and complete input 

data sets. Examples include expanded atmospheric deposition and urban runoff data. 

Validation of models using monitoring data from the Tahoe basin—

Model validation is a critical step in understanding uncertainty. During the 

validation phase of model development, the model run is compared to our actual 

understanding of the environment to determine if the model “got it right.” If not, 

the model can be revised and improved. However, validation data do not always 

exist, or may be insufficient. 

Model linkage—

By linking models, managers are better able to simulate environmental response on 

an ecosystem level. The importance of linked models is appreciated with the format 

of the Lake Tahoe TMDL (LRWQCB and NDEP 2008a, 2008b, 2008c). The initial 

step involved linking the output of sediment and nutrient loads from the watershed 

(LSPC) directly into the LCM. The TMDL Integrated Water Quality Management 

Strategy has recognized the need to link LSPC with CONCEPTS and LSPC with 

PLRM. Work is recommended to firmly establish these links and to investigate the 

feasibility of creating linkages between these and other models (e.g., LTAM and 

LCM).

Revision of existing models and development of new models—

Investigation of the applicability of existing or development of new models not 

yet under consideration for use in the Tahoe basin is recommended. For example, 

a model is needed to examine the growth response of near-shore periphyton to 

site-specific and basinwide nutrient loading, increasing water temperature, and 

invasive species.

Research Needs

• It is recommended that modelers work in close cooperation with scientists 

conducting field/laboratory research to ensure that the critical ecosystem 

drivers are incorporated into conceptual models and the mathematical 

expressions in predictive models.

• Collect a more comprehensive set of meteorological data to support models 

in the Tahoe basin. There is general agreement that current meteorological 

locations lack the spatial resolution to address the data input needs of models 

that operate from the project scale (hectares) to the entire watershed (about 

800 km2

). 
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

• Develop monitoring to support the goal of model validation as monitoring 

programs are redesigned or newly developed. This is especially important 

for the model(s) that will evaluate TMDL (load reduction) compliance.

• Revise models as new research addresses knowledge gaps and monitoring 

data are used to update input data and validate model output.

• It continues to be important to expand models to allow resource managers 

to evaluate the effectiveness of the EIP and TMDL compliance continue to 

be important. Examples of topics that would benefit from modeling include, 

but are not limited to, urban hydrology, pollutant loading from terrain 

impacts by fire, transport and reduction of fine particles (<16 μm) in natural 

environments and constructed BMPs, and linking near-shore and pelagic 

water quality and pollutant transport. 

• Link key models such as the LCM, pollutant load reduction models, and the 

Tahoe Watershed Model (LSPC) to increase the benefit of these models to 

water quality managers. 

• The PLRM model will be important to the TMDL Lake Clarity Crediting 

Program. Improvements to its calibration and validation will be critical to 

management.

Climate Change and Water Quality

There is now a strong consensus among climate scientists that (1) the Earth’s atmosphere and oceans are warming; (2) the primary cause is the anthropogenic release 

of greenhouse gases; and (3) the impacts to natural systems and human societies 

over the next century will fall somewhere between serious and catastrophic 

(Oreskes 2004). Over the last hundred years (1906 to 2005), the global average 

near-surface temperature has increased 0.18 to 0.74 °C (IPCC 2007). Based 

on various climate models and greenhouse gas emission scenarios, the U.N. 

Intergovernmental Panel on Climate Change (IPCC 2007) projected a global 

average temperature increase of 1.4 to 6.4 °C by 2100. More locally, Dettinger 

(2005) found a central tendency for the distribution of many modeled temperature 

increases for California of about 3 °C by 2050 and 5 °C by 2100. At that rate, and 

with an average environmental adiabatic lapse rate of 2 °C per 305 m, the end-ofcentury temperature regime at the elevation of Lake Tahoe would be comparable to 

the current regime at an elevation of about 1128 m (e.g., Grass Valley, California).

The impacts of climate change in the Tahoe basin are not merely theoretical, 

they have already been measured. The observed impacts include the warming of the 

lake itself, a shift toward earlier snowmelt, a shift from snow to rain, and a change 

in forest condition. Although any lasting remedy to the problem of global climate 
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GENERAL TECHNICAL REPORT PSW-GTR-226

change obviously would be global in scope, consideration of the local impacts by 

resource managers and scientists is appropriate for two reasons. First, the trends in 

climate may affect efforts to understand the causes of water quality changes in both 

streams and lakes in the Tahoe basin. Second, it may be possible to mitigate some 

of the impacts of climate change in the basin.

Knowledge Gaps

Direct hydrologic impacts—

Across the Western United States, the timing of snowmelt has shifted over the last 

half-century toward dates earlier in the water year (Cayan el al. 2001, Dettinger and 

Cayan 1995), with the snowmelt flood now running 30 to 40 days earlier in some 

rivers compared with the pre-1940s record. Using regression analysis of historical 

data together with a Parallel Climate Model (PCM) to forecast and hindcast air 

temperature and precipitation, Stewart et al. (2004, 2005) showed that the shift in 

snowmelt timing will accelerate during this century. This shift in snowmelt timing 

is largely in response to changes in air temperature rather than precipitation. The 

PCM, together with a Precipitation-Runoff Model System (PRMS) has also been 

used to simulate the hydrologic responses to climate change in the nearby Merced, 

Carson, and American River basins. The results show a recent and likely future 

shift in the timing of snowmelt runoff, and that the shift began in the early 1970s 

(Dettinger et al. 2004a, 2004b).

The shift in snowmelt timing is also occurring in the Tahoe basin. An analysis 

of daily discharge records for Ward, Blackwood, Trout, and Third Creeks and the 

Upper Truckee River shows an average shift in timing of the annual snowmelt 

peak discharge of 0.4 day per year since 1962 (fig. 4.8). The shift in timing of the 

snowmelt peak (after removal of the “total annual snowfall effect”) is correlated 

with the April–June Pacific Decadal Oscillation Index (see Mantua et al. 1997), but 

it is driven more directly by spring air temperature, which trends upward over the 

period 1914–2002 (Coats and Winder 2006).

Not only is the timing of snowmelt in the Tahoe basin shifting, but the fraction of precipitation that falls as snow rather than rain is decreasing. From 1914 to 

2002, the percentage of total annual precipitation falling as snow at Tahoe City has 

decreased at an average rate of 0.2 percent per year (fig. 4.9).

Although there is no discernible trend in total annual precipitation at Tahoe 

City, there is evidence that the frequency of intense rainfall is increasing. Modeling 

studies have shown that climate warming in the Sierra will increase the magnitude 

of the 95th percentile daily rainfall amount (3.9 cm/day for the period 1910–2007 

at Tahoe City) (Kim 2005). Figure 4.10 shows the trend for number of events 
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

Figure 4.9—The percentage of total annual precipitation falling as snow at Tahoe City, CA 

(from R. Coats). P < 0.001, r2

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Figure 4.8—Average date of snowmelt peak discharge, for Ward, Blackwood, Third and Trout 

Creeks, and the Upper Truckee River (from R. Coats). P = 0.01, r 2 = 0.14.

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GENERAL TECHNICAL REPORT PSW-GTR-226

Figure 4.10—Number of days per semidecade with daily rainfall exceeding the 95th percentile daily 

amount (from R. Coats). P < 0.1, r 2 = 0.14.

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exceeding 3.9 cm/day for half-decades since 1910. Most significantly, since 1975, 

deviations from the upward trend have increased. These changes will likely 

exacerbate surface soil erosion, especially where appropriate restoration and 

BMPs have not occurred. 

Indirect hydrologic impacts—fire frequency and vegetation—

Large wildfire activity in the West increased dramatically in the mid-1980s, in 

some regions owing more to climatic change than to land use history (Westerling 

et al. 2006). In parts of the West, simulations with the PCM have shown that the 

trend toward increased fire danger will continue at least through this century 

(Brown et al. 2004), and forest recovery following fire will be strongly influenced 

by climatic change (McKenzie et al. 2004). 

In the Tahoe basin, the threat of severe forest fires is increased not only directly 

by higher temperatures and lower humidity, but also by the indirect effects of 

climate and land use history on vegetation and fuel load. Heavy logging in the late 

1800s and subsequent fire suppression and exclusion led to the development of 

dense overstocked stands of Jeffrey pine (Pinus jeffreyi Grev. & Balf.), white fir 

(Abies concolor Gord. & Glend.) and red fir (Abies magnifica A. Murr. Lindl. ex 
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

Hildebr.). During periods of high moisture stress, these stands are vulnerable to 

bark beetle (Ips sp., Scolytus sp., and Dendroctonus sp.) attack (Manley et al. 2000); 

the potential growth rate in beetle populations is further enhanced by a warming 

trend (Logan et al. 2003). This issue is sometimes referred to as the “forest health” 

problem, but it is also a hydrology and water quality issue, as runoff in the first 

years following an intense wildfire can carry greatly increased loads of nutrients 

and fine sediment to the lake (Miller et al. 2006). 

Limnological (lake) impacts—

Since 1970, Lake Tahoe has warmed at an average rate of 0.013 °C per year (fig. 

4.11). This has increased the thermal stability and resistance to mixing of the 

lake, reduced the depth of the October thermocline, and shifted the timing of 

stratification onset toward earlier dates. The warming trend is correlated with both 

the Pacific Decadal Oscillation and the Monthly El Niño–Southern Oscillation 

Index, but it results primarily from increasing air temperature, and secondarily 

from increased downward long-wave radiation (Coats et al. 2006). Some of the 

resulting impacts to phytoplankton (Winder and Hunter 2008) and invasive warmwater fish (Kamerath et al. 2008, Ngai 2008) have been documented, but many of 

the water quality impacts from changes in lake thermal structure need more study.

Figure 4.11—The average daily temperature of Lake Tahoe, as deviation from seasonal norm (from 

R. Coats). P = 0, r 2 = 0.43. Average temperature values were calculated as the volume-weighted mean 

of daily measurements made at 11 depths from the surface to 450 m (near-bottom).

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GENERAL TECHNICAL REPORT PSW-GTR-226

Regional trends in climate change—

Analysis of regional trends in air temperature show that the warming rate at the 

Tahoe City station (adjusted for the effect of urbanization) is significantly higher 

(especially in late summer) than at nearby stations outside of the basin. It is also 

higher than the average for the Sierra region as a whole (see http://www.wrcc.dri.

edu/monitor/cal-mon/index.html). This is consistent with the findings for snowmelt 

timing. Of four streams outside of the Tahoe basin (Sagehen Creek, South Fork 

Yuba River, and East and West Forks of the Carson River), none showed a shift 

(1962–2005) in the date of the annual snowmelt peak discharge. The differences in 

warming rate inside and outside of the basin are striking, and suggest the lake itself 

may locally enhance the effect of increasing greenhouse gas emission. 

Research Needs

• How will the hydrologic changes associated with Tahoe basin warming 

affect flood frequency, channel change, and sediment/nutrient transport?

The hydrologic changes associated with the present warming trend likely will 

change the flood-frequency relationships for basin streams, increasing the discharge 

for a given frequency. The magnitude of the likely changes, however, is unknown. 

Floods of different recurrence intervals (e.g., the 2-yr flood vs. the 100-yr flood) 

may be affected differently, and these differences have important implications for 

channel erosion and sediment transport.

Anderson et al. (2002) showed how down-scaled historical climatic data can be 

used to analyze flood frequencies using Hydrologic Engineering Center Hydrologic 

Modeling System (HEC-HMS). For predicting future trends, the PCM output can 

be down-scaled and coupled to a hydrologic model, but at the extremes (infrequent 

high and low flow) it does not reproduce actual streamflow very well (Dettinger et 

al. 2004b). If a solution to that problem cannot be found, another General Circulation Model might be coupled with one of several watershed models (e.g., HECHMS, Hydrologic Simulation Program FORTRAN, PRMS) to model the effects of 

climate change on flood frequency in selected watersheds in the Tahoe basin. 

It is also recommended that existing management models applied in the Tahoe 

basin as well as new models be used to estimate changes in nutrient and sediment 

loading to Lake Tahoe. Ideally, such models would consider various surrounding 

land use types based on anticipated levels of precipitation and runoff predicted by 

climate change. The event mean concentrations for these pollutants are currently 

based on existing precipitation conditions. Updated estimates of event mean concentrations based on projected conditions of changes in total precipitation, rain/

snow regime, or timing are recommended. 
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

• How will the shift in snowmelt timing and sediment delivery interact with 

increasing lake temperature and thermal stability to affect lake biology and 

water clarity? How will the insertion depth of stream inflow be affected?

The LCM, used to model changes in lake water quality, will be used, together 

with down-scaled PCM meteorology, to address these questions. Future climate 

change scenarios will be used to generate input data sets to predict future trends 

in lake temperature, thermal structure, and mixing conditions. These results will 

then be combined with the output from the LSPC watershed model of the Tahoe 

basin (Riverson et al. 2005)—run under meteorological conditions defined by PCM 

output—to evaluate changes in water clarity and primary productivity that may 

result from simultaneous changes in lake thermal structure, watershed hydrology 

and sediment/nutrient loading. A sensitivity analysis of the combined model could 

help determine the extent to which lake clarity can be improved in an era of climate 

warming by efforts to reduce the input of fine sediment and nutrients.

Little is known about the likely direct and indirect effects of lake warming on 

lake ecology. Recent studies have shown some effects of lake warming on phytoplankton and fish. This work needs to be continued and extended to include the 

effects of lake warming on the microbial food web and zooplankton. 

• How will the increase in lake temperature and thermal stability affect 

Lake Tahoe’s dissolved oxygen (DO) profile? Is it possible for the lake 

to go anaerobic at the bottom?

The trends in lake temperature and thermal stability, combined with increasing 

primary productivity, will increase the fall/winter biochemical oxygen demand 

in the water column, while decreasing the solubility of oxygen, and possibly the 

downward transport of DO. The DO during spring and summer phytoplankton 

blooms may increase at some depths. Coupling the LCM to a climate model such 

as the PCM could help to assess how lake warming will affect the DO profile. 

The analysis could be combined with a study to sort out the impacts of combined 

lake warming and watershed change. The modeling could be combined with 

measurements of water column DO (ongoing), as well as careful measurement of 

redox potential across the sediment-water interface at the bottom of the lake.

• Does Lake Tahoe enhance the rate of climate change in the basin?

The trends in both air temperature and snowmelt timing indicate that the Tahoe 

basin is warming faster than the surrounding region. With a low albedo and high 

heat storage capacity relative to the land surface, much of the short-wave energy 

striking the lake surface is stored and released later as latent and sensible heat, and 
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GENERAL TECHNICAL REPORT PSW-GTR-226

long-wave radiation. The outgoing long-wave energy from the lake (and overlying 

atmospheric boundary layer) is thus higher than it would be absent the lake. As 

greenhouse gas concentrations increase, the rate of increase in energy absorption 

above the lake should exceed that above the land. A coupled lake-atmosphere 

climate model embedded in a General Circulation Model is needed to test this 

“lake climate change enhancement hypothesis.” If the results strongly support 

the hypothesis, they would indicate that the Tahoe basin is especially sensitive to 

the impacts of greenhouse gas emissions, and that planning is urgently needed to 

address the impacts of climate change in the basin.

• What impact will potential changes to watershed hydrology and pollutant 

loading have on current management strategies to restore Lake Tahoe’s 

water clarity?

Based on current and new research, resource managers will want to know how 

to address the potential for increased pollutant loading to Lake Tahoe as the result 

of changes in precipitation patterns. Such information is best obtained at the BMP 

project scale, the individual watershed scale, and the entire drainage basin scale.

Water Quality Research Priorities

Many of the current key management questions for water quality focus on the 

“pollutant pathway.” Topics include source identification, transport within the 

watershed, control and abatement, defining loads to the tributaries and the lake, 

fate of fine sediments and nutrients in the lake, and assessment of water quality 

response.

Research and monitoring efforts supported by the LTIMP, the Lake Tahoe 

TMDL Research Program, the Southern Nevada Public Lands Management Act, 

the Environmental Improvement Program, and many individual science projects 

funded by federal, state, and local governments, have resulted in a greater level of 

understanding of water quality in the Tahoe basin than at any previous time. Much 

of this information has been directly used in the development of new and innovative 

water quality management strategies (e.g., the Lake Tahoe TMDL).

As water quality improvement projects have been implemented and research 

and monitoring data have been collected, a number of future research needs have 

emerged in the area of water quality. Topics of research priority in this context are 

those that are needed by managers within the next 3 to 5 years to support current 

and developing water quality strategies.
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

For the water quality research priorities presented here, the authors have intentionally developed a series of topic areas rather than presenting detailed testable 

hypotheses because (1) researchers are making rapid progress in many of the water 

quality subthemes discussed above, (2) hypotheses and details change quickly, and 

(3) researchers and water quality agencies at Lake Tahoe have developed a flexible 

and dynamic approach toward setting priorities for specific investigations based on 

scientific merit and relevancy.

Based on all these considerations, and guided in part by the identification of 

the key drivers and linkages in the conceptual model (fig. 4.1), the water quality 

research priorities are as follows:

Pollutant loading and treatment (PLT) within the urban landscape—

PLT1. Develop a process-based understanding of sources, transport, and loading 

of fine sediment particles (<16 μm) from different urbanized land uses in the Tahoe 

basin. While this includes all features of the urban landscape, roadways appear to 

be particularly important and deserve focused attention.

PLT2. Quantify the effectiveness of BMPs and other watershed restoration activities on the control of fine sediment particle and nutrient loading to Lake Tahoe. 

Major load reduction approaches include hydrologic source control (HSC), pollutant 

source control (PSC) and storm water treatment (SWT). Although some data have 

been collected on BMP and restoration effectiveness in removing nutrients and 

fine sediment, these efforts have been for specific projects and have not provided 

basinwide process-based evaluations. A comprehensive basinwide watershed-scale 

evaluation of BMP and erosion control project effectiveness is needed, especially 

for the Lake Tahoe TMDL program.

PLT3. Conduct focused studies to understand the influence that altered urban 

hydrology has on pollutant pathways and determine how alternative hydrologic 

designs can enhance load reduction.

PLT4. Investigate longer term impacts from infiltration of stormwater runoff 

around the Tahoe basin, particularly as it relates to different soils, land uses, and 

ground-water quality. 

PLT5. Continue efforts to establish a Regional Storm Water Monitoring Program. 

Key elements of this program include (1) pollutant source monitoring; (2) pollutant 

reduction monitoring; (3) BMP design, operation, and maintenance monitoring; and 

(4) data management, analysis, and dissemination. Although this is not research per 

se, data collected under this program will be used to support research on BMPs and 

pollutant load reduction as described in this chapter. 
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GENERAL TECHNICAL REPORT PSW-GTR-226

PLT6. Validate pollutant reduction crediting tools that are currently being developed 

to track progress in implementing the Lake Tahoe TMDL. At the same time, develop 

a science-based adaptive management program to guide pollutant load reduction 

activities.

Near-shore (NS) water quality and aquatic ecology—

NS1. Research is needed to determine near-shore processes at various temporal and 

spatial scales. This research will contribute to an integrated data base that can be 

used to determine trends and patterns for integrated, process-driven models. From 

this information, construct a predictive model to help guide ongoing and future 

management strategies. It is recommended that this model include features such as 

nutrient loading, turbidity, localized and lakewide circulation patterns, wave resuspension, periphyton and macrophyte populations, introduced and native species, 

recreational uses, and activities within the near shore. 

NS2. Develop an aquatic invasive species research program with direct ties to water 

quality (e.g., risk of invasive species on native species composition and aquatic food 

webs, in-lake sources of drinking water, or water quality and stimulation of benthic 

algal growth in the near shore). 

NS3. Develop analytical approaches for establishing quantitative and realistic water 

quality standards and environmental thresholds for the near-shore region.

Erosion and pollutant transport (EPT)/reduction within the 

vegetated landscape— 

EPT1. Collaboration between researchers and agency representatives is recommended to evaluate fine sediment and nutrient loads resulting from forest fuels 

reduction activities. A major effort would include quantifying BMP effectiveness for 

controlling fine sediment and nutrient releases from wildfire, as well as from forest 

biomass management practices, such as prescribed fire and mechanical treatment.

EPT2. Fully evaluate the benefits and risks from using large areas of the natural 

landscape (e.g., forests, meadows, flood plains, wetlands) for treatment of urban 

runoff.

Water quality modeling (WQM)—

WQM1. Water quality management in the Tahoe basin has embarked on a pathway 

that will use science-based models to help guide management into the future. It is 

recommended that support continue for the development, calibration, and validation 

of these models.
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

WQM2. Develop appropriate linkages among the landscape, climate, and atmospheric and water quality models to provide more comprehensive assessment of 

primary and secondary drivers whose effects propagate through the ecosystem.

WQM3. Build decision-support modules for the linked ecosystem models that 

will support evaluation of effects from larger spatial scales.

Climate change (CC)—

CC1. Continue to document the effects of climate change on existing and future 

water quality conditions.

CC2. Apply predictive scenario testing for evaluating potential effects from 

climate change within the new and developing management models used for 

water quality in the Tahoe basin. In particular, it is recommended that models 

be used to evaluate basinwide BMP effectiveness and load reduction strategies 

based on the expected changes to temperature, precipitation, and hydrology.

CC3. Limnological processes in Lake Tahoe such as stratification, depth of 

mixing, particle distribution and aggregation, species succession, aquatic habitat 

based on water temperature, and meteorology could all benefit from reevaluation 

in light of climate change and possible management response to the impacts of 

climate change.

English Equivalents:

When you know: Multiply by: To get:

Millimeters (mm) 0.0394 Inches

Centimeters (cm) .394 Inches

Meters (m) 3.28 Feet

Kilometers (km) .621 Miles

Hectares (ha) 2.47 Acres

Square meters (m2

) 10.76 Square feet

Square kilometers (km2) .386 Square miles

Grams (g) .0352 Ounces

Kilograms (km) 2.205 Pounds

Tonnes or megagrams (Mg) 1.102 Tons

Kilograms per hectare (kg/ha) .893 Pounds per acre

Degrees Celsius (°C) 1.8 °C + 32 Degrees Fahrenheit
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GENERAL TECHNICAL REPORT PSW-GTR-226

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W.W. Miller,2 E.M. Carroll-Moore,2 H. Tretten,2 and D.W. Johnson2

Introduction 

Soil conservation as a research and management theme can mean many things to 

agencies and the general public. The historical concept was rooted in agriculture 

and agricultural production. Indeed, in 1957 the “modern” objectives of soil and 

water conservation identified by Stallings (1957) were to:

• Reduce the accelerated loss of soil that attends the use of land for 

agricultural crops.

• Find ways of reclaiming severely eroded land.

Soil conservation today conveys a much broader concept, and conservation 

management strategies go well beyond simply the protection of soil from processes 

of physical erosion to now encompass the protection and enhancement of overall 

soil quality. Improved soil quality prevents land resource degradation by resisting 

erosion and invasive species, strengthening the soil’s capacity to support plant 

growth, favoring species diversity, preventing environmental contamination, and 

conserving water (Brady and Weil 2008).

The intent of this chapter is to identify the most pressing management questions, uncertainties, and pertinent research needed to address a wide spectrum 

of plant-, soil-, and water-related issues in the Lake Tahoe basin. In developing 

the organization and content for the soil conservation theme, the original outline 

considered a number of topical subtheme areas such as soil ecology, soil erosion, 

etc. However, input received from Tahoe Science Plan participants indicated that 

they were interested in cross-cutting issues rather than the topic-specific categories 

originally presented. In response, suggestions from the workshop were grouped into 

four new subthemes considered by the constituency to be of high priority and in 

need of resolution relative to current management concerns. 

Chapter 5: Soil Conservation1

1 Citation for this chapter: Miller, W.W.; Carroll-Moore, E.M.; Tretten, H.; Johnson, D.W. 

2009. Soil conservation. In: Hymanson, Z.P.; Collopy, M.W., eds. An integrated science 

plan for the Lake Tahoe basin: conceptual framework and research strategies. Gen. Tech. 

Rep. PSW-GTR-226. Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific 

Southwest Research Station: 183–235. Chapter 5.

2 University of Nevada, Reno, Department of Natural Resources and Environmental 

Sciences, Reno, NV 89557.
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GENERAL TECHNICAL REPORT PSW-GTR-226

We provide a brief introduction of each subtheme, which includes a description 

of the subtheme, what we know about it, and its relevance (perceived or real) to the 

overall science theme area and management strategies in the Lake Tahoe basin. 

In the subsection entitled “Knowledge Gaps” we provide a problem statement that 

describes the general issue, then in bullet format identify specific aspects relative 

to that issue where we recommend enhancing the knowledge base. The subsection 

entitled “Research Needs” presents a nonprioritized list of identified research 

topics.

Beyond the specific discussions of subthemes, we provide a conceptual model 

and identify a prioritized listing of the three most immediate near-term soil conservation research priorities within each subtheme.

Conceptual Model

Figure 5.1 illustrates four primary tiers of hierarchy: theme area, subthemes, 

components (response-based ecosystem components), and drivers (e.g., stressors, 

management activities, other external forces). The following subthemes were 

adopted as the primary categories or conditions of interest to agencies and the 

public: 

• Key soil properties and conditions. 

• Development and application of predictive models.

• Effects of climate change.

• Policy implications and adaptive management strategies.

A number of coupled processes were next identified as a set of complex 

response indicators potentially affecting one or more aspects of the four 

subtheme conditions. These interactive components are used to identify and 

express the degree of consistency with desired conditions and include (1) changes 

in soil physical and chemical properties, (2) altered hydrologic function and the 

corresponding effects on erosion, (3) nutrient cycling and discharge water quality, 

(4) the collective and independent short- and long-term effectiveness of best 

management practice (BMP) strategies, and (5) the evaluation of current and future 

policy thresholds in the context of scientific information and an ever-expanding 

knowledge base. Finally, expected linkages between primary and secondary drivers 

are presented as agents of change, natural or human caused, which can affect the 

condition of the components specified and their indicator values.
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Figure 5.1—Conceptual model illustrating how the Soil Conservation research theme system is perceived, its scope, the primary factors, 

and how they relate to one another. Near-term priorities are indicated by alpha numeric symbols (e.g., CC1, PM3) and correspond to the 

descriptions presented later in the chapter.

Biomass

management 

strategies

Forest and

watershed

health policies

Climate

change

2° Driver 1° Driver Component Subtheme Theme Linkage

Natural

disturbance

Soil physical and

chemical properties

Soil quality, erosion, and 

hydrologic function

Policy 

implementation, validation, 

and assessment

BMP development,

effectiveness, and

management

Erosion, nutrient 

cycling, and 

hydrologic response

Fire

management

Urban development, land use, and

recreation

Impact response,

management and

evaluation

Vegetation

management

Watershed, 

stream, and riparian

restoration

CC1 Soil conservation

Soil properties

and conditions

Development and

application of

predictive models

Development and

application of

predictive models

Policy implications

and adaptive

management

strategies

Development and

application of

predictive models

Effects of 

climate change

Best

management

practices 

(BMP)

Land

development

regulations 

and policies

PM3 SPC1

CC2 PM1

PM3

PM3

PM3

CC3

SPC1

SPC3

PM1,2

SPC3

PM1,2

SPC1

SPC2

PI3

PI1,2

PI1

Key Soil Properties and Conditions

Soil conservation is a dominant factor in existing land use policies in the Tahoe 

basin. These policies can benefit from the incorporation of key soil properties and 

conditions that influence soil ecology, soil erosion, source/sink nutrient budgets, 

physical and hydrologic properties, and model use and application. This approach 

can yield land use policies that have a sound foundation in science.

The profile characteristics and physical and chemical properties of soils are 

determined by the five factors of soil formation; parent material, biota (both 
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vegetation and soil biota), topography, climate, and time (Jenny 1980). Primary 

minerals, or those that come with the soil parent material (e.g., volcanic or granitic 

origin), are generally unstable when exposed to ambient conditions at the Earth’s 

surface. They begin to disintegrate and dissolve at varying rates according to their 

resistance to processes of physical and chemical weathering. The soil skeletal 

fraction consists largely of sand and silts as a result of physical disintegration. On 

the other hand, ions derived from the dissolution of primary minerals, including 

essential nutrients such as phosphorus (P), potassium (K), calcium (Ca), magnesium 

(Mg), sodium (Na), and many micronutrients, enter the soil solution and either leach 

to greater depths, become adsorbed to the existing secondary mineral fraction, are 

taken up by biota, or form more stable secondary minerals themselves. These secondary minerals consist largely of the colloidal (clay) size fractions and contribute 

substantially to soil chemical properties such as surface reactivity, cation exchange 

capacity, and anion adsorption (Miller and Gardiner 1998). 

Two soil survey reports have been completed for the Tahoe basin; the original 

published in 1974 (USDA, NRCS, and FS 1974), and a more recent online "Web 

Soil Survey" published in 2007. The updated version (USDA NRCS 2007) of the 

soil survey includes some major changes that should be of interest to soil scientists 

and other specialists studying the Lake Tahoe environment. It presents information 

on a variety of key soil physicochemical properties, including particle size distribution (texture), structure, percentage clay, organic matter content, permeability, 

erosion hazard, pH, and cation exchange capacity, as well as delineations of origin 

(volcanic vs. granitic), flood frequency, frost-free days, mean annual precipitation, 

and depth to water table.

Soil Physical Properties

Soil texture is determined on the basis of the relative proportions of three general 

mass size fractions (USDA Classification); the sands (0.05 to 2.00 mm diameter), 

the silts (0.002 to 0.05 mm diameter), and the soil colloids or clays (<0.002 mm). 

The larger size fractions (sands and silts) typically settle rapidly and are fairly inert 

chemically. Soil colloids, however, can remain in suspension over long periods and 

can directly affect solution equilibrium chemistry as well (see later discussion). 

Water clarity in Lake Tahoe is largely controlled by absorption and scattering of 

light by biological and mineral particles that remain suspended within the water 

column. Of the inorganic fraction, it is the fine particle sizes (1 to 15 μm) that tend 

to have the greatest effect on lake water clarity because of their tendency to remain 

suspended within the water column and their light-scattering properties (Jassby et 

al. 1999, Swift and Perez-Losada 2006). 
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In terms of soil structure, particles may exist as unattached individual grains 

(single grain) at one extreme, or become tightly packed into large cohesive blocks 

(massive) at the other. In between, there exists an intermediate structural condition 

known as aggregates or peds (Hillel 1998). The interaction between soil texture and 

soil structure has a pronounced influence on a number of other physical properties 

such as erodibility, bulk density, porosity, water-holding capacity, and infiltration 

capacity. These properties in turn have a substantial influence on the soil’s ability 

to support plant growth, the cycling of nutrients, water retention and percolation, 

and the susceptibility to rill erosion. For example, in addition to being subject to 

transport mechanisms, nonaggregated or weakly structured soils are more easily 

compacted upon disturbance (natural or anthropogenic). Compact soils (soils with 

higher bulk densities) have a lower total porosity and smaller pore sizes skewing 

the overall pore size distribution throughout the matrix. This typically results in a 

negative effect by causing lower infiltration rates, which when subjected to highintensity precipitation, causes greater runoff. It is important to note, however, that 

for very sandy soils, compaction can also have a positive effect by increasing available water-holding capacity owing to a more compressed pore size distribution. 

The process of erosion is of paramount importance to the Lake Tahoe basin. 

Different soil textures, soil structures, and environmental conditions affect soil 

erosion. Particulate detachment and transport are the fundamental processes by 

which erosion takes place. Detachment may be the result of raindrop or particle 

impact, cycles of freeze/thawing and wetting/drying, overland flow, wind transport, 

Bank erosion in a Lake Tahoe basin neighborhood. 

Courtesy of Caiifornia Tahoe Conservancy
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GENERAL TECHNICAL REPORT PSW-GTR-226

or mechanical disturbance. Transport is usually the result of water or wind action 

with colluviation playing a role on steeper slopes. It is generally thought that 

increased erosion in upland watershed areas of the Sierra Nevada is largely due to 

mechanical disturbance from logging, grading, grazing, or other anthropogenic 

activities that cause loss of vegetation, organic matter, and nutrients from top soil 

layers; soil compaction; reduced infiltration; and enhanced runoff (Grismer and 

Hogan 2005a). Several erosion “hot spots” arising from natural and anthropogenic 

activities exist within the Tahoe basin. Some of the most notable hot spots include 

upper Ward and Blackwood Creeks, logged areas of the Upper Truckee River 

watersheds, and areas of high disturbance such as ski resorts and road cuts. Fire 

aftermath (natural or prescribed) may also result in increased erosion rates (Certini 

2005), especially if the burn area is left unattended. 

According to the soil erodibility factor (K factor) used for the Universal Soil 

Loss Equation (USLE), it is the larger sand-sized particles (0.05 to >2.0 mm in 

diameter) that are the most susceptible to transport because of their size and mass. 

Smaller size fractions (<0.05 mm in diameter) are less easily mobilized owing to 

their colloidal nature, but when eroded are more difficult to trap owing to their 

smaller size. 

Soil water repellency (often called hydrophobicity) is a common feature of soils 

in forested watersheds of the Sierra Nevada that has an effect on soil erosion and 

soil loss. These effects include reduced infiltration capacity, enhanced overland 

flow, and increased rainsplash erosion (Shakesby et al. 2000). Naturally derived soil 

water repellency in the Lake Tahoe area has been reported to be minimal or lacking 

during moist spring conditions, but most severe during the very dry conditions of 

late summer (Burcar et al. 1994). Fire-induced water repellency also is of concern. 

Following wildfire, a hydrophobic layer may exist either at the mineral surface or 

at some depth below. The depth and thickness of this layer is influenced by the 

amount of heat, ambient soil moisture content (Shakesby et al. 2000), and particlesize distribution, but does not often exceed 6 to 8 cm (Certini 2005). Whether 

natural or fire induced, hydrophobicity results in reduced infiltration, enhanced 

runoff, and, on occasion, mass wasting and deposition.

Recent research indicates that vegetative cover alone (i.e., grasses with 30 to 60 

percent cover) may have minimal effects on decreasing erosion from disturbed sites 

(Grismer and Hogan 2004, 2005a, 2005b). Using woodchips or compost in conjunction with revegetation as a soil restoration method appears to be more effective for 

providing erosion control, increased infiltration rates, and restoration of hydrologic 

function (Grismer and Hogan 2005b). The use of compost, however, may also 

enhance the growth of weeds while decreasing the growth of native vegetation.
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

Soil Chemical Properties

Soil chemical properties are directly related to solid particle (organic and inorganic) 

surface chemistry, geochemistry, fertility, mineralogy, and microbiology/biochemistry. Soil fertility considers soil as a medium for plant growth, mineralogy examines the structure and chemistry of the solid phase, and biogeochemistry studies 

soil microbial and biochemical reactions (Bohn et al. 2001). 

Soil solution is the equilibrium interface between the solid mineral and organic 

fractions and the atmosphere, biosphere, and hydrosphere. It is the source of 

mineral nutrients for all terrestrial organisms, and, as it mixes with ground water 

or drains to streams, lakes, or wetlands, can directly affect their chemistry. The 

integrating factor of time is also a factor: the importance of parent material to soil 

properties diminishes over time as the primary minerals disintegrate and are dissolved. Many Sierran soils are relatively young and thus the parent material still has 

substantial influence over soil properties. For example, soils derived from granitic 

sources are typically rich in basic cations such as Ca, K, Mg, and Na, whereas 

volcanically derived andesitic soils are typically more acidic and contain higher 

concentrations of ferromagnesium minerals (iron [Fe], aluminium [Al], manganese 

[Mn], Mg). Precipitation strongly affects soil leaching and mineral solubility, 

temperature affects mineral solubility, and both affect biota, which also has a major 

influence on soil properties. 

Rain- and hail-induced erosion after the 2002 Gondola wildfire above Stateline, Nevada. 

Jay Howard
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GENERAL TECHNICAL REPORT PSW-GTR-226

Biota has a strong influence on soil properties by adding organic matter, which 

imparts important chemical properties to soils such as cation exchange capacity and 

water holding capacity, and nitrogen (N), the most commonly limiting nutrient for 

plant growth. The organic content of soils is important to a number of soil properties, but especially to aggregate stability, water-holding capacity, plant nutrient 

supply, and ion exchange. Humus is an intermediate product following considerable 

decomposition of plant and animal remains (nonhumus). It is amorphous, dark 

brown in color, insoluble in water, and because of its large specific surface and ability to acquire a positive or negative electrostatic charge in response to soil reaction 

(pH), exhibits strong ion adsorption and exchange properties. Nitrogen is generally 

not added with parent materials and their primary mineral components; its source 

is from the atmosphere, which is 78 percent nitrogen gas (N2). Unfortunately, N2

is not available to plants and must first be converted to either ammonium (NH4) 

or nitrate (NO3) forms and added to the soil either naturally through atmospheric 

deposition or biological N fixation or through the anthropogenic processes of 

fertilization or air pollution. Nitrogen is therefore unique among nutrients in soil in 

that it is largely associated with organic matter and has only minimal involvement 

with secondary minerals or adsorption to colloids. 

Fire and fire suppression can have a substantial effect upon various soil chemical properties. A substantial portion of the Lake Tahoe basin is currently considered 

a high-risk environment for severe wildfire (Murphy et al. 2006b). This is due to a 

buildup of litter and “ladder” fuels that stimulate a high-intensity ground and crown 

fire. This shift may be the direct result of fire suppression, resulting in a change in 

fire regimes from regularly occurring low-intensity fires, characteristic of preEuropean settlement, to large often catastrophic stand-replacing fires (Neary et al. 

1999). 

Fire has a twofold effect on N in the forest ecosystem, and specifically in 

soils. First, most N contained in material that burns is lost to the atmosphere 

via volatilization because of its low volatilization temperature. Thus, fire (either 

wildfire or prescribed fire) causes net losses of N from the terrestrial ecosystem, 

which, over time, can be important because N is often a growth-limiting nutrient 

for vegetation. These N losses can be readily restored if N-fixing vegetation, such 

as snowbrush (Ceanothus velutinus Douglas ex Hook.), bitterbrush (Purshia sp. DC. 

ex Poir.), lupine (Lupinus argenteus Pursh), and whitethorn (Ceanothus cordulatus

Kellogg) is allowed to reestablish on the site, thereby providing N input to the 

system. In the case of shrub species such as snowbrush, however, it may take a 

decade for such inputs to reach peak values. It is important to note, however, that 
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

N losses from repeated prescribed fire designed to suppress understory vegetation 

such as snowbrush can equal or exceed those in a wildfire over time, and this may 

be of concern in terms of long-term forest nutrition. 

Secondly, heating of soil and partial combustion of the forest floor normally 

causes degradation of amino acids and proteins, resulting in an increase (sometimes 

quite substantial) in soil ammonium levels. This ammonium is basically equivalent 

to a dose of fertilizer and often contributes to lush growth of herbaceous vegetation 

after a wildfire. On the other hand, if the ammonium is converted to nitrate during 

nitrification, surface or ground-water pollution can occur. Wildfire has thus been 

found to increase the immediate mobilization of labile (readily available) nutrients 

(Miller et al. 2006, Murphy et al. 2006b). The effect is to increase the frequency 

and magnitude of elevated nutrient runoff concentrations during the first season 

following the wildfire event. At least some of this labile N may be transported 

offsite during precipitation or snowmelt, thus enhancing the nutrient loading of 

adjacent tributaries (Allendar 2002) and their final discharge into Lake Tahoe. 

Like N, elevated concentrations of soluble P also may be transported offsite during 

precipitation or snowmelt ultimately discharging into Lake Tahoe. 

Whereas wildfire has been shown to cause a dramatic increase in labile nutrient mobilization (Johnson et al. 2004, Miller et al. 2006, Murphy et al. 2006b), this 

effect has not been identified for prescribed fires. Murphy et al. (2006a) found no 

significant increases in the leaching of ammonium, nitrate, phosphate, or sulfate 

following a prescribed Sierran burn on volcanic soils. Neither resin nor ceramic cup 

lysimeter data showed any effects of burning on soil solution leaching. Although 

Chorover et al. (1994) found increases in soil solution and stream water ammonium 

and nitrate following a prescribed fire on granitic soils at a western Sierrian site, 

Stephens et al. (2005) reported that prescribed fire in the Lake Tahoe basin had no 

effect on soluble reactive phosphate and only minimal effects on nitrate in streamwaters. In support of this latter finding, Loupe (2005) found controlled burning to 

result in a net decrease of inorganic N and P concentrations in surface runoff at a 

site near north Lake Tahoe. On this basis, Murphy et al. (2006a) concluded the most 

ecologically significant effects of prescribed fire on nutrient status at his site was 

the substantial volatilization of N from forest floor combustion. 

Knowledge Gaps

Although a few broad comparative estimates are available (Carroll et al. 2007), 

historical erosion rates within the Tahoe basin are largely unknown. As a result, we 

have no means of knowing how current soil loss compares to historical conditions. 
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This information is recommended to help quantify the extent to which anthropogenic activities have impacted erosion and deposition. The alternative is to directly 

measure current impacts, but knowledge gaps also exist with this approach. For 

example:

• The basinwide distribution of developed pervious surfaces such as ski runs, 

unpaved roads, and gravel parking areas as potential sources of sediment 

have not been quantified. 

• We do not have a quantitative understanding of how developed impervious surfaces (e.g., buildings, roads, or parking lots) have altered natural 

hydrologic processes leading to increased surface and subsurface discharge 

runoff and erosion. 

• The extent to which jetties, piers, bulkheads, and dredging have altered 

natural littoral and eolian processes on near-shore soils are unknown.

It remains uncertain as to which erosion control and or site restoration 

methods are most effective at different locations throughout the Tahoe basin. 

• Overall effectiveness in terms of hydrologic function per input effort 

required is not quantified for most methods at variable site locations.

• How long control measures will remain effective, whether or not they 

are self-sustaining, and if sustainable, the frequency requirements for 

maintenance are unknown. 

• We do not know how the performance of erosion control methods varies 

among storm events (e.g., 20-yr vs. 50-yr vs. 100-yr) or changing hydrologic scenarios (e.g., rain-only events vs. rain-on-snow events). 

The methods that control the runoff of fine particle sizes most associated 

with nutrient source/sink loading are not clearly defined. 

• Whether or not all “native” soils are a source/sink for N and P remains 

unknown.

• Quantitative knowledge of equilibrium soil chemistry is recommended to 

support the proper design of control measures for the reduction of nutrient 

loading. 

The fate and impacts of fertilizer use in the basin are not well understood.

• Fertilizer may or may not be a significant source of nutrients to the lake. 

The impact of residential fertilizer use is unknown. 
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How native and nonnative vegetation respond to soil amendments and other 

restoration activities remains an issue. 

• It is unknown whether or not vegetative restoration practices will actually 

result in the permanent establishment of “native-like” conditions where 

little natural runoff and erosion takes place. 

• Long-term vegetative succession characteristics following the application of 

soil amendments and other restoration practices are unknown, particularly 

with respect to whether or not a normal progression of plant community 

succession can be established, and, if so, how many years may be required 

for succession. 

The presence or absence of vegetation may affect soil shear strengths and 

aggregate stability, but how and what shear strengths are associated with restored 

sites is an unknown issue. 

• The most effective type of vegetative cover for use at various locations 

relative to indigenous soil types has not been specified. 

• The effects of restoration efforts and vegetative cover on soil aggregate 

stability are poorly understood. 

• What natural aggregate stability is associated with undisturbed sites and 

what aggregate stability value should be most suitable are not known. 

Snowmelt-derived erosion is a poorly understood process and has not been 

quantified across the basin. 

• Most erosion studies have focused on situations and locations such as road 

cuts, skid trails, and construction, or extreme events. 

• Only a few studies (Granger et al. 2001, Riebe et al. 2001) have attempted 

to characterize baseline erosion from watersheds on an overall basis, 

including that derived from snowmelt cycles. 

The shift from low-intensity fire to catastrophic wildfire has the potential 

to affect many aspects of soil ecology. Obtaining sound data on the effects of 

prescribed fire is more practical, but the results may not apply to the effects of 

wildfire. 

• Wildfires have no prefire plan, therefore pretreatment and suitable control 

sites are rare. Hence, information on the effects of wildfire on organic matter, nutrient cycling, and biological response is scarce. 

• Fire suppression has increased organic matter accumulation above and 

within the soil, and the potential effect on nutrient cycling and discharge 

to Lake Tahoe is uncertain. 
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• Information gaps remain concerning postwildfire vegetation recruitment as 

well as long-term succession and ecological impacts. 

• A more comprehensive understanding of pre-European fire regime nutrient 

cycling conditions would be useful to the development of vegetation management strategies. 

• A comprehensive assessment of the effects of both wildfire and prescribed 

fire and postfire N-fixing vegetation on long-term N budgets is needed. 

Little is known about the management implications of soil water repellency 

in the Lake Tahoe basin. 

• The spatial distribution and intensity of soil hydrophobicity has important 

hydrologic recharge and nutrient “hot spot” implications in Sierran soils 

(McClain et al. 2003). 

• Temporal variability of the effect of soil hydrophobicity on overland flow is 

unknown, as are individual event-based effects (e.g., how rapidly the effects 

of soil water repellency dissipate during summer storms or snowmelt cycles 

remains unknown). 

• The role of soil hydrophobicity in soil erosion has not been well studied as 

it is challenging to isolate the effect of hydrophobicity from those of other 

erosion factors. 

• Most measurements are performed on small-scale areas (<5 to 10 m2

), 

which yield limited information on spatial variability. These small-scale 

measurements may not give a reliable indication of the detachment and 

transport of sediment and nutrients over an entire slope or catchment 

(Shakesby et al. 2000). 

There is a lack of information regarding potential impacts from trace 

element contamination in the basin. 

• Little is known regarding the status of trace elements. 

• Fate, behavior, and the related management options are largely unknown. 

Research Needs 

• Characterize or quantify historical vs. current and natural vs. anthropogenic soil loss on a watershed scale, including intervening zones. Detailed 

characterization of control site parameters coupled with predictive model 

estimations would facilitate such efforts.

• Identify the locations, practices, and features that have disrupted the natural 

littoral and eolian processes sustaining barrier beaches and marshes and 

influencing beach and dune formation. Where appropriate, develop and 

evaluate retrofit designs and restoration and management techniques. 
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• Quantify an aggregate stability index, the hydrologic function, and complete soil nutrient status for soils on native, disturbed, and restored sites. 

This would provide critical information on the need for restoration and the 

degree to which we can expect attainment.

• Do a quantitative assessment to determine which restoration methods are 

most effective in controlling runoff transport of fine particles most associated with nutrient loading and their equilibrium chemistry. 

• Evaluate amended or otherwise restored soils in terms of their ability to 

maintain hydrologic function, productivity, and erosion control over time. 

• Identify appropriate vegetation that will best achieve maximum erosion 

(and potentially nutrient) control at a realistic cost. 

• Structure project monitoring and assessment to provide relevant research 

data pertinent to erosion model development, improvement, calibration, and 

validation for the Lake Tahoe basin. 

• A quantitative assessment of erosion from snowmelt is recommended on 

different disturbed, native, and restored soils throughout the basin. 

• Conduct research on soils and nutrient cycles in natural forests to better 

understand fire suppression and its role in soil ecology, plant growth, and 

nutrient discharge. Ideally, sites would be established to measure nutrient 

cycling, which includes inputs such as plant-soil fluxes through litterfall, 

crown wash, and root turnover as well as losses from leaching, runoff, 

wind, or fire. 

• Conduct research to understand the effects of wildfire where a suitable 

adjacent control site is available, especially if prefire data are available (e.g., 

the Gondola wildfire, Stateline Nevada). Assessment of the immediate and 

long-term effects of prescribed fire on erosion, hydrophobicity, and runoff 

rates is recommended, particularly factors that affect the replacement of 

lost N such as through N-fixation. Ideally, long-term inventory plots would 

be established to include vegetation and soils to follow the progression of 

change over time. 

• Further quantify the spatial distribution of soil water repellency on a larger 

scale to determine the distribution of areas of recharge versus those that 

generate overland flow. Develop a more complete understanding of the factors (i.e., temperature, moisture, vegetation, and litter) that determine the 

formation, persistence, and dissipation of hydrophobicity to predict seasonal and long-term effects on recharge runoff and erosion. 
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• Complete, update, and continue refinement of Tahoe basin ecological site 

descriptions to assist agencies in the development of effective management 

strategies. An ecological site has specific characteristics that differ from 

other kinds of land in its ability to produce a distinctive kind and amount of 

vegetation. Ecological site descriptions include knowledge about site characteristics, plant communities, site interpretations, and other supporting 

information about such sites and their interrelationships to one another on 

the landscape. 

• Quantify the trace element status of plants, soil, and water in the Tahoe 

basin. 

Development and Application of Predictive Models 

as Related to Soil Conservation

The Bailey Land Classification System (Bailey 1974) was originally developed to 

group basin lands into broad classes of similar estimated erosion hazard, then rank 

each class as to relative environmental sensitivity to land development. It was soon 

apparent, however, that the Bailey System was inadequate to effectively evaluate 

the potential erosion hazard for individual parcels. The Individual Parcel Evaluation 

System (IPES) (TRPA 1986) was therefore developed to provide onsite sensitivity 

evaluation. Because no site-specific data were readily available, the IPES system 

used empirical models as the basis for estimating erosion potential. The use of such 

predictive models when properly applied and interpreted within the constraints of 

their inherent limitations can provide important tools to understanding and predicting the potential outcome of management strategies and programs. Models related 

to soil erosion, nutrient cycling, and hydrology can all help to inform actions aimed 

at soil conservation. The updated version (USDA NRCS 2007) of the soil survey 

includes a plethora of recent information on key soil physicochemical and biological properties that should be of interest for purposes of model updates to modeling 

specialists studying the Lake Tahoe environment. Advancing the development and 

standardized use of predictive models in the Tahoe basin remains a management 

priority. 

Soil Erosion Models

The USLE has been the dominant prediction tool for soil erosion by water for 

decades. Originally designed to predict sheet and rill erosion on croplands, it has 

been applied in the United States and abroad for conservation planning to estimate 

the impact of erosion on the quality of the surrounding landscape (Yoder and Lown 

1995). 
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The USLE equation, 

A = RKLSCP 

is based on empirical relationships where A is defined as the soil loss factor, R 

represents the rainfall erosivity factor, K is the soil erodibility factor, L is the slope 

length factor, S is the slope steepness factor, C is the cover management factor, and 

P represents the support practices factor (Nearing et al. 1989, Renard et al. 1991, 

Reyes et al. 2004). The USLE, however, was not designed to predict soil deposition, sediment yield from a watershed (as it does not include deposition), soil loss 

from single storm events, soil loss from concentrated channel flow in large rills 

or ephemeral gullies, gully or streambank erosion, or movement of sediment in 

streams (Hudson 1995; Renard et al. 1991, 1994). 

In the mid-1980s, the U.S. Department of Agriculture (USDA) worked to 

improve USLE by updating and revising the factors involved. This work resulted 

in the Revised Universal Soil Loss Equation (RUSLE). The RUSLE maintains the 

same equation structure of USLE, but each feature has been updated using more 

recent, larger, and more complete data sets (Hudson 1995; Nearing et al. 1989; 

Renard et al. 1991, 1994; Reyes et al. 2004). At the same time, the Watershed Erosion Prediction Project (WEPP) was being developed in order to produce a more 

process-based model that would eventually replace RUSLE for upper watershed 

applications on nonagricultural lands. 

Improvements to RUSLE included more data from the Western United States, 

providing more complete isoerodent maps and thus more accurate R-values for this 

region. In addition, the R-factor took into account ponding of water on flat slopes 

where intense rainstorms occurred. Ponded water decreases soil erosion owing 

to raindrop dispersion, thus the R-factor is decreased. The erodibility nomograph 

is often used to estimate K-values, but may not be appropriate for all soils. The 

RUSLE allows users to identify when the nomograph is applicable and when a different method should be applied. The RUSLE also allows K to vary seasonally and 

accounts for the presence of rock fragments in the soil (Hudson 1995; Renard et al. 

1991, 1994). Even so, K-value applicability in the Lake Tahoe basin is questionable. 

Hence, refining the data input for K-values to make them more pertinent to Lake 

Tahoe basin conditions would increase the applicability of RUSLE as a predictive 

tool for the basin.

A major change with the upgrade to RUSLE was the improvement of the 

L-factor. Choosing a slope length value was originally dependent on the judgment 

of the user, which resulted in different users choosing different slopes for similar 

conditions. The RUSLE provides better guidelines that help users to recognize 

important attributes of field conditions and generate greater consistency when 
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choosing slope length. The slope steepness factor was also revised, and, in most 

cases, resulted in reduced soil loss predictions (Hudson 1995; Renard et al. 1991, 

1994). Of all USLE factors, the P-factor was the least dependable as it represents 

broad effects of land use practices. Extensive data have been examined to reassess and improve P-factor values for the use of different land use practices (e.g., 

contouring, terracing, and rangeland practices) for RUSLE application (Hudson 

1995; Renard et al. 1991, 1994; Yoder and Lown 1995). Even with these important 

modifications, however, limitations remain. Like the USLE, RUSLE is unable to 

predict erosion rates for single storm events or for a single calendar year (Nearing 

1989, Yoder and Lown 1995). Furthermore, neither equation estimates deposition 

or sediment yield at a downstream site or clearly represents fundamental hydrologic 

and erosion processes (Renard et al. 1991). Ephemeral gully erosion is not included, 

nor are specific sediment characteristics (Renard et al. 1991). 

The WEPP was developed to provide a new technology for modeling erosion for prediction purposes based on what is currently understood about erosion 

processes. It is a process-based model that offers three versions, each suitable for 

a different scale. The profile version is the replacement of USLE as a predictor of 

uniform hillslope erosion, including deposition as an additional component. The 

watershed version is applicable at the field scale and incorporates areas where 

more than one profile version applies. The grid version can be applied to areas with 

boundaries that do not match watershed boundaries, or it can be broken into smaller 

areas where the profile version may be applied (Flanagan et al. 1995, Hudson 1995, 

Laflen et al. 1991b). 

Major determinants of the erosion process are soil resistance to detachment, 

stream power (transport), and deposition. Hydrologic processes included in WEPP 

are climate, infiltration, and a winter component that includes soil frost, snowmelt, 

and snow accumulation. Plant growth and residue processes estimate plant growth 

and decay above and belowground. The water balance component uses climate, 

plant growth, and infiltration to quantify daily potential evapotranspiration, which 

is necessary to compute water status and percolation. The hydraulic component 

computes shear forces exerted on soil surfaces. Processes that take place in the 

soil are also considered, including various soil parameters such as roughness, bulk 

density, wetting-front suction, hydraulic conductivity, interrill and rill erodibility, 

and critical shear stress. Tillage, weathering, consolidation, and rainfall impact are 

also considered (Flanagan et al. 1995, Hudson 1995, Laflen et al. 1991a). 

The WEPP, in this respect, is able to estimate spatial and temporal distributions of soil loss, although it has not necessarily provided more precise or realistic 

estimates of erosion rates. Because the model is process-based, it can extrapolate 
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a broad range of conditions and take into account the variability of topographic 

conditions of hillslopes (Flanagan et al. 1995). Although process-based models like 

WEPP may offer more power than an empirical model like the USLE and RUSLE, 

WEPP requires more complex technology, demanding expanded databases with 

more advanced computer resources (Renard et al. 1994). 

Nutrient Cycling Models

Two models have shown promise for the characterization of nutrient cycling in 

Sierran systems (Johnson et al. 2000): the Nutrient Cycling Model (NuCM) and the 

Nutrient Cycling Spread Sheet model (NuCSS). 

The NuCM is able to represent the cycling of N, P, K, Ca, Mg, and sulfer (S) 

on daily, weekly, and monthly time scales at the forest stand level. It also includes 

the fluxes of major cations (H+

, NH4+, Ca2+, Mg2+, K+, Na+), anions (NO3-, SO4

2-, 

PO4

3-, Cl-

, HCO3

-

 , and organic anions), and silicon (Si) in precipitation, soil 

solution, and throughfall. Throughfall describes the water that falls through the 

vegetation canopy to the ground surface generating erosive power. The NuCM 

was designed to simulate the effects of atmospheric deposition, harvesting, change 

in species composition, precipitation, elevated temperature, and increased carbon 

dioxide (CO2). It places heavy emphasis on elements that affect soil solution 

chemistry, tracking nutrient pools and fluxes associated with soil solution, the ion 

exchange complex, minerals, and soil organic matter. This model may be most 

appropriate for determining decadal-scale changes in nutrient pools and soils rather 

than intra-annual variations in soil solution chemistry (Johnson et al. 2000). 

The NuCSS model is a simplified model, but remains more complex than a 

simple nutrient budget calculation. This model simulates biomass production, 

organic matter decomposition, N mineralization, cation adsorption, weathering, 

and leaching. The model estimates a target biomass and associated nutrient uptake 

using the soil nutrient pools. The user can assess if the available soil nutrients will 

support the target biomass production (Verburg and Johnson 2001). A key feature of 

this model is the ease of calibration and, consequently, the simplicity of extrapolation over many sites. 

Hydrologic Models

Nutrient fluxes in the Sierra Nevada Mountains are both spatially and temporally 

variable (Johnson et al. 2001), and a better means of predicting changes in the 

amount of biologically available nutrients discharged from upper watersheds is 

needed. The ability to better assess what is occurring spatially in terms of surface 

runoff alone could offer a linkage in balancing the overall nutrient budget for 
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eastern Sierra Nevada watersheds. One approach is to develop a methodology for 

delineating a spatially explicit water balance that adequately estimates the potential 

for surface runoff over large areas using available input/output spatial data (Carroll 

2006). 

Most recent hydrologic applications involve some use of geographic information systems (GIS) for the ease of estimating model parameters. For example, GIS 

has been used to estimate and organize important hydrologic modeling parameters 

such as soil type, land use, slope, flow direction, elevation, drainage network, 

rainfall, evapotranspiration, and vegetation (Alemaw and Chaoka 2003, Bourletsikas et al. 2006, Jain and Singh 2005, Jain et al. 2004, Schumann et al. 2000). It can 

also be taken a step further: Liu et al. (2003) applied GIS to better estimate complex 

parameters requiring multiple spatial inputs such as a potential runoff coefficient 

as determined from slope, soil type and land use, and flow velocity and dispersion 

as determined from slope, hydraulic radius, and vegetation coverage. Much of 

this information may be found in the 2006 soil survey for the basin (USDA NRCS 

2007). 

Once data have been compiled, there are many variations on how they could be 

applied to hydrologic modeling. Fluvial transport of sediments occurs when water 

velocities are strong enough to overcome shear strength of the soil, transporting 

sediment downstream. Fine particles are often transported as suspended load, 

moving downstream while suspended in the water. Sufficiently larger particles 

may be transported as bedload, settling on the river or streambed, but still moving 

downstream. Areas that lack vegetation because of wildfire or other anthropogenic 

factors are more susceptible to water erosion and consequent downstream transport. 

Surface waters often transport these soils to a lower land area or receiving water 

body, such as Lake Tahoe. If possible, such a model would help researchers to more 

fully understand the magnitude and sources of nutrients and sediments that are 

being transported into tributaries and surrounding reservoirs and lakes (Carroll 

2006). 

Knowledge Gaps

Different agencies in the Lake Tahoe basin use different methods to predict soil 

erosion and to assess the need for erosion control measures. 

• The issue remains whether any of the erosion models and equations is 

adequate for use in the Tahoe basin. The USLE was originally developed to 

predict erosion rates on agricultural lands with relatively mild slopes, thus 

its applicability for the basin is questionable. The RUSLE may compensate 

for some deficiencies of USLE but may still be inadequate. 
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• Although WEPP may be the most inclusive model by incorporating a 

climate and soil database, it is by far the most complex. There needs to 

be consistency among agencies as to model use and application. 

• In the event that the current models are not adequate predictors of soil 

erosion for the Tahoe basin, appropriate modifications or adjustments 

are needed to make the existing models more functional. If this is not an 

option, starting from scratch and developing a new model that is simple, 

accurate, and appropriate for the basin may be necessary. 

There is a particular need for generating refined erosion estimates for singlestorm episodes as compared to seasonally weighted or seasonally adjusted annual 

erosion loads for purposes of programmatic modeling, BMP interpretive assessment, and design criteria. This is particularly true in the Lake Tahoe basin, where 

there is large variability among storm systems, and single events can result in major 

runoff. 

• Modeling single-storm episodes would be useful for planning and modeling applications, especially for BMP design and implementation associated 

with new development or redevelopment. 

From a management perspective, there is a lack of information on the effects 

of sediment transport and delivery, pertinent soil parameters, the effects of diminished impervious cover, fire effects (wildfire and prescribed), and the effects of 

fuel reduction on soil erosion. Information is also lacking on the need for specific 

erosion control practices. 

• Hydrophobic soil conditions have been shown to increase a soil’s vulnerability to erosion by increasing aggregate instability, reducing infiltration, 

and increasing overland flow (Robichaud 2000, Shakesby et al. 2000). 

Management options to deal with these vulnerabilities have not been 

defined. 

• The increased potential for soil erosion following fuel reduction activities, as well as site susceptibility and response to a subsequent catastrophic 

event, has not been quantified. 

• Fire suppression efforts, recreational development such as ski resorts, and 

recreational activities such as mountain biking or off-highway vehicle use, 

can all exacerbate soil erosion as well as soil compaction (Backer et al. 

2004). The significance of these anthropogenic activities is not well understood, especially with respect to erosion modeling and their importance as 

input parameters. 
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There is no hydrologic model specific to the needs of the Tahoe basin.

• A spatially explicit water balance model, specific to the needs of Sierran 

forested watersheds, is clearly needed to more fully understand nutrient 

and sediment transport. 

• Furthermore, there are several unusual factors that can affect surface water 

modeling and model output in the basin such as the spatial distribution of 

hydrophobicity, frequency and presence of frozen soils, and frequency and 

intensity of rain-on-snow events that are not addressed in most models currently in use.

Erosion, nutrient cycling, and hydrologic models have yet to be tested against 

the effects of fire and fuel reduction. 

• The effect of fire suppression, wildfire, and prescribed fire for fuel reduction is currently being studied to some extent, but the potential effects of 

impending climate change are a major unknown. 

Research Needs

• Evaluate the accuracy and application of models on a site-specific basis, to 

determine if the models currently in use are adequate in predicting soil erosion and sediment delivery within the Tahoe basin. 

• Determine the role and impacts on soil erosion of hydrophobic soils, fire 

and fire suppression efforts, and the effect of recreational activities. 

• Gain a better understanding of flow patterns, fine and coarse sediment 

loading, deposition, erodibility, infiltration, slope, and hydrophobicity. 

• Establish improved engineering guidelines, predictive tools, and cost-effective management approaches for the control of nutrient and sediment transport from upper watersheds and other high-impact areas of the Tahoe basin.

• Develop site-specific parameterization and model calibration. A prioritization is recommended of which soil parameters are important, what should 

be measured, and what information is needed to parameterize and calibrate 

the models. 

• Make models more applicable to the basin by improving data input whenever possible, such as the K-factor in USLE and RUSLE equations. 

• Understand and quantify the effects of fuels reduction activities on soil 

conservation. This necessitates recharacterization of infiltration, runoff, 

nutrient cycling, erosion, and general growing conditions for revegetation 

at the watershed and subwatershed scales. 
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

• Conduct quantitative evaluation and assessment of potential soil loss from 

hillsides and sediment trapping within stream environment zones as a result 

of catastrophic events such as wildfire, rain-on-snow, and excessive rainfall 

events. 

• Develop a spatially explicit water balance model, specific to the needs of 

Sierran forested watersheds, to more fully understand sediment and nutrient transport, the goal of which is to better assess the linkage between 

overland flow sediment and nutrient transport and discharge water quality. 

Applying the appropriate water balance information, localized point source 

surface runoff and nutrient data could then be used to quantify discharge 

loads at the watershed scale. 

Effects of Climate Change as Related to 

Soil Conservation

Global climate change is in part the result of greenhouse gases accumulating 

in the atmosphere. Predictions for the next century include a 3-°C rise in global 

temperatures (Roos 2005). Warmer temperatures would increase global evaporation 

and therefore increase global precipitation, much of which is predicted to occur in 

northern latitudes (Roos 2005). 

Depending on the models that are used, predictions for precipitation quantity 

and intensity are quite variable for the Sierra Nevada. In some predictions, precipitation events will increase in intensity leading to large-scale flooding. In other 

models, the Lake Tahoe area will be subject to warmer temperatures and more 

evapotranspiration, and increased precipitation will be more common farther to the 

north outside the basin. One analysis using General Circulation Models found Lake 

Tahoe to be one of the few areas where precipitation quantities remained about the 

same, even with a doubling of CO2 in the atmosphere (Phillips et al. 1993). 

There is general agreement, however, that with warmer temperatures, snow 

elevation levels will be higher. The Lake Tahoe basin has elevations ranging from 

roughly 1885 to 3070 m. Snow levels in the area typically range from 1520 to 2432 

m during the winter months. It is estimated that for every 1-°C rise in air temperature, the snow level will rise approximately 152 m (Roos 2005). As the basin sits 

right in the range of the average snow level, a rising snow level means less snowpack for the Lake Tahoe region. More rain and less snow throughout the winter 

will have important effects on watershed and regional hydrology. Soil quality and 

soil erosion are the two areas pertinent to soil conservation that will be affected 

by climate change and its effects on weather patterns and hydrology. 
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Soil Quality 

Snowpack in alpine areas can greatly influence microbial communities. A thick 

layer of snow acts as insulation to subsurface biota, keeping temperatures hovering 

around freezing instead of fluctuating with the air temperature to well below freezing (Nemergut et al. 2005). A thin snowpack or none at all leaves these communities vulnerable to the sometimes harsh Tahoe climate. During the winter months, 

these microbial communities are responsible for a large portion of the plant matter 

decomposition, thereby mineralizing and immobilizing N (Nemergut et al. 2005). 

Harsh weather diminishes their numbers and therefore the amount of decomposition 

that occurs. Slowed decomposition rates lead to fewer nutrients available for uptake 

the following year. Longer growing seasons will also lead to increased nutrient 

demand by vegetation, not only of N but carbon as well (Euskirchen et al. 2006). 

Ultimately, a diminished or more variable snowpack could lead to large duff layers 

and nutrient-deprived soils. 

Soil Erosion 

The main issue to confront when discussing soil erosion and climate change in the 

Lake Tahoe basin is the shift from large snowfall events to large rainfall events. The 

Lake Tahoe basin receives an average of 10.7 m of snow annually with an average 

of 3.2 m of annual snow at lake level. However, analysis presented in the “Water 

Quality” chapter found that the fraction of precipitation that falls as snow rather 

than rain is decreasing.

Current regulations require drainage structures and erosion control mechanisms 

to be able to withstand a 20-year storm, or a storm producing 2.54 cm of rainfall in 

1 hour. If more winter precipitation is received as rainfall rather than snow, current 

structures may not be able to withstand the strain. Large rainfall events could erode 

slopes and flood drainage control structures allowing surface water laden with 

sediments and nutrients to discharge directly into Lake Tahoe. More rain during the 

winter also increases the possibility of rain-on-snow events. Recent history, such as 

the flood of 1997, has shown how catastrophic a large rain-on-snow event can be. 

Another issue with variable rainfall is its effect on soil moisture. Rapid 

melting of a smaller snowpack would lead to a decrease in soil moisture during 

summer months owing to an earlier runoff. Sarah (2005) reported that less rainfall 

and therefore less soil moisture is associated with an increase in microparticle 

percentage and a decrease in aggregate stability. Hence, a decrease in snowpack 

could ultimately lead to increased instability of slopes and a higher incidence of 

erosion in the Lake Tahoe basin. Better characterization of amount, rate, duration, 
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

and seasonal occurrence of rainfall precipitation could help to delineate shifts in 

the local climatic precipitation phase and the potential impact on erosion, model 

estimations, and BMP design criteria. 

Knowledge Gaps 

How soil biota is affected by a decrease in snowpack as a result of climate change 

and the consequent result on soil nutrient content is uncertain.

• There is limited information about how climate change will affect microbial communities in the soil, their health, and their functionality. 

• There is limited understanding of how climate change will affect the 

breakdown of plant material. 

• Research is recommended to determine if soil nutrient contents will 

ultimately be affected by microbial community health and increased 

plant matter production. 

How a change in frequency and intensity of rainfall will affect erosion rates 

is unknown, specifically excessive rainfall and rain-on-snow events. 

• We do not understand how episodic events, such as flash floods, will affect 

Total Maximum Daily Loads (TMDLs) in Lake Tahoe. The ability to predict the recovery time from these natural disasters as well as anthropogenic 

disasters could benefit many agencies. 

• We do not know the relationship between increased intensity and frequency 

of precipitation events and the resulting erosion rates. 

A tool/data set is recommended to modify precipitation data used in sediment 

loading models to predict the effects of climate change. How this can be used to 

determine TMDL and the effect of different storm events has yet to be determined. 

• Several models are available, many predicting different futures for the Lake 

Tahoe basin. If model results contain uncertainty, then this uncertainty 

should be quantified and accounted for as part of the input to subsequent 

models. 

• We are not certain what will happen with climate change: therefore it is 

extremely difficult to plan for the future of the Tahoe basin. Development 

and evaluation of alternative scenarios is one objective approach that could 

help to inform managers of the future possibilities. 

• Current regulations only require new drainage control and erosion mechanisms to be capable of handling a 20-year storm event. What happens with 

a 50-year storm? What happens if there is frequent repetition of 20-year 

storm events?
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How soil stability is affected by climate change is unknown. 

• If soil moisture decreases during summer months owing to decreased 

supply from accelerated winter snowmelt, there could be a resultant 

decrease in soil aggregate stability. 

• If soil moisture increases owing to increased precipitation events throughout the year, soil aggregate stability could increase, but we could see an 

increase in surface soil removal or an overall decrease in slope stability. 

Research Needs

• Study microbial communities and their relationship to climate change and 

soil nutrient flux in the Lake Tahoe basin. Determine their niche and if 

climate change will remove certain species from the Tahoe region. This 

information would include a better understanding of their temperature 

range, soil moisture requirements, and tolerance to frozen soils. All of this 

ultimately would concentrate on understanding the effects microbial community health has on soil nutrient cycling. 

• Explore the relationship between possible intense summer and winter rainfall storms and erosion rates. Ideally, this research also would look into the 

rate of snowmelt during rain-on-snow events and the resultant erosion. Do 

frozen soils help to protect soil surfaces from erosion or increase erosion of 

surface layers? 

• Examine the ability of stormwater management practices such as BMPs, 

drainage control, and erosion control to diminish sediment and nutrient 

transport to Lake Tahoe. Do structures designed for a 20-year storm function to their intended design? Do these structures remain functional but at a 

diminished capacity during a 50-year storm, or do they fail entirely? Future 

designs could be developed to account for the possibilities that global climate change brings. 

• Run all models for the wide range of possible changes in precipitation that 

may accompany global climate change. New models such as the Channel 

Change GIS Simulation Model (CHANGISM) described by Hooke et al. 

(2005) can be used to determine the effect of global climate change on 

stream environment zones (SEZs) and the implications on nutrient and sediment loading into Lake Tahoe. 

• Test the implications of climate change on soil stability. Aggregate stability, 

surface soil stability, and overall slope stability would all be tested with 

different possible moisture regimes to determine what the ultimate effects 

of global climate change could be on soil erosion potential. In addition, 
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quantifying the sources of sediment would be useful for identifying and 

initiating management strategies at the most serious locations. 

• Explore future ecosystem scenarios. There is concern that long-term restoration may not be possible owing to the emergence of heretofore nontypical 

ecosystems. 

Policy Implementation and Adaptive Management 

Strategies as Related to Soil Conservation

Public input into the management and conservation of Lake Tahoe basin resources 

has taken many forms since decline in Lake Tahoe water clarity became a broadly 

recognized environmental concern. Surveys have been conducted regarding the 

public knowledge base on environmental issues, and regular news releases and 

reports have been issued to further educate the public on BMPs and other environmental issues in their community. 

Lake Tahoe also has been a longstanding subject of interest for researchers. 

Educational and research institutions such as the University of Nevada, Reno, the 

University of California, Berkeley, the University of California, Davis and the 

Tahoe Environmental Research Center (TERC), the Pacific Southwest Research 

Station, the U.S. Geological Survey, and the Desert Research Institute all have 

contributed time, money, and effort to monitoring and studying many aspects of the 

lake and surrounding watersheds. 

The Lake Tahoe Interagency Monitoring Program (LTIMP), a longstanding 

cooperative program created to acquire and distribute water quality information, 

has been collecting data on Lake Tahoe and its watershed since 1980. Many federal, 

state, and local agencies, as well as several nongovernmental agencies are active 

entities whose interests are directed toward protecting Lake Tahoe water quality 

by promoting preservation, restoration, sustainability, and environmentally sound 

development. 

The balance of Lake Tahoe’s ecosystem is delicate. Excellent communication 

among the scientific community, the public, and policymakers is recommended for 

Lake Tahoe to benefit from all of the efforts that go into protecting this precious 

resource. One aspect of adaptive management is the integration of current research 

into policy and management decisions and implementation. Current regulations in 

the Lake Tahoe basin regarding soil conservation, including BMPs to control erosion and sediment transport, protection of sensitive areas such as steep slopes and 

SEZ, and the mitigation of impervious coverage, can all benefit from a basinwide 

adaptive management framework. 
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Resource and regulatory agencies—

The monitoring of water quality management projects has been listed as a primary 

function of the Lahontan Regional Water Quality Control Board (LRWQCB). The 

Nevada Division of Environmental Protection (NDEP) and its water quality subsidiaries also oversee water quality issues on the Nevada portion of the basin (NDEP 

2007). Both the LRWQCB and the NDEP are leading the effort to develop TMDL 

pollution budgets for Lake Tahoe. However, both of these state agencies rely heavily 

on the ordinances set forth by the Tahoe Regional Planning Agency (TRPA) to 

enhance water quality in the Tahoe basin. 

The TRPA oversees much of the implementation and regulation of the water 

quality planning in the Lake Tahoe basin. Their established “Goals and Policies” 

call for application of land capability ratings in the planning for new construction 

and the authorization of additional impervious cover. Likewise, the “Code of Ordinances” ensures SEZ habitat is protected, and the repair of damaged SEZ habitat is 

authorized. Furthermore, chapter 25 of the TRPA Code of Ordinances calls for the 

BMP Retrofit Program, in which private and commercial landowners must install 

stormwater and erosion control BMPs with a capacity to assimilate the stormwater 

runoff from a 20-year storm event.

The U.S. Forest Service Lake Tahoe Basin Management Unit (LTBMU) 

manages soil conservation and water quality preservation on Forest Service lands. 

Monitoring is conducted on many LTBMU projects. The LTBMU has several 

monitoring programs such as the BMPs evaluation program, fuel reduction project 

soil monitoring, road decommissioning and BMP-upgrade program monitoring, 

trail decommissioning and BMP-upgrade monitoring, off-highway vehicle program monitoring, and urban erosion control grant program monitoring (USDA FS 

2005). Results from monitoring projects are compiled and reported anually. The 

2006/2007 Annual Monitoring Report was recently completed. This report presents 

a summary of data collected in calendar year 2006, as well as results of analysis 

conducted fall 2006 through spring 2007. The report contains much information 

regarding efforts related to monitoring of fuel reduction program, soil and water 

BMPs, restoration projects, and status and trend of biological resources. It is anticipated that the 2007/2008 Annual Monitoring Report will be completed on schedule, 

by late summer 2008. The findings identified in these reports have been used to 

make decisions in terms of which practices will be continued and what modifications in implementation or construction may be necessary. These Annual Progress 

Reports published by the U.S. Forest Service can be accessed by the general public 

through the following link, http://www.fs.fed.us/r5/ltbmu/ by clicking on the publications tab and then scrolling to LTBMU Monitoring Reports. 
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The U.S. Geological Survey (USGS) is another federal agency involved in 

the management of the Lake Tahoe basin. The USGS has provided numerous 

resources to help understand and conserve the basin’s ecosystems, and, among 

other things, they provide various geographic and cartographic data useful to 

resource managers. Such resources include a bathymetry map, a digital elevation model, a digital orthophoto quadrangle, a digital line graph, a digital raster 

graphic, numerous satellite images, a digital soil map, and a digital census map. 

The USGS is also one of the principal partners in the LTIMP stream monitoring 

program.

The California Tahoe Conservancy (CTC) is a state agency developed to create and implement programs that improve and protect the environmental quality of 

the Lake Tahoe basin. Protection of the clarity and quality of the lake’s waters is 

a primary objective of CTC. Additional goals and programs are geared toward the 

preservation and protection of wildlife and wildlife habitat, environmentally sensitive land, SEZ habitat, forest habitats, and recreation opportunities (CTC 2006). 

The CTC is only active in the California portion of the basin.

The Natural Resource Conservation Service (NRCS) is an agency within 

the USDA that provides data, information, and technical expertise on natural 

resources. Their primary customers are those who manage and use natural 

resources on nonfederal lands including farmers, private organizations, and nonprofit organizations (USDA NRCS 2007). An important recent contribution has 

been the 2006 update of the Soil Survey of the Tahoe Basin Area (USDA NRCS 

2007).

The Nevada Tahoe Conservation District (NTCD) is a subdivision of the state 

of Nevada whose mission is to “promote the conservation and improvement of 

the lake Tahoe basin’s natural resources by providing leadership, education, and 

technical assistance to all basin users” (NTCD 2003). The NTCD is active in the 

Nevada portion of the basin and is involved in a number of projects including erosion control, water quality, the Backyard Conservation Program, and a stormwater 

planning initiative. 

The Tahoe Resource Conservation District (TRCD) is a conservation district 

for the state of California that is active on the California side of the basin. Programs within TRCD include the Backyard Conservation Program, an invasive 

weeds program, a schoolyard restoration program, a watershed program, and most 

recently an aquatic invasive species program. The TRCD works in close cooperation with the NTCD (TRCD 2007).
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Best Management Practices 

Best management practices have been developed to minimize wind and water 

erosion, uncontrolled surface runoff from urban areas, and ultimately sedimentation and nutrient loading into Lake Tahoe. Urban runoff is considered a greater 

source of sediment and nutrients than undisturbed areas and is therefore of primary 

concern (LRWQCB and NDEP 2008a, 2008b, 2008c; Schuster and Grismer 2004; 

TRPA 2001a, 2001b). In 1992, the TRPA implemented the BMP retrofit program 

requiring all property owners to upgrade their property with BMP technology. Best 

management practices can be divided into three categories: temporary construction, 

permanent drainage control, and permanent surface stabilization. 

Temporary construction BMPs deal with the exposure and disturbance of 

soils and vegetation on a construction site. They are therefore temporary solutions 

designed to minimize the impacts of the immediate disturbance activities. Often 

included are temporary structures to stabilize and protect areas such as boundary 

fencing to protect environmentally sensitive areas from encroachment: fiber roll 

barriers, filter fences, drop inlet barriers, and gravel bags all of which act as sediment barriers; and erosion control blankets, mulches, and tackifiers to maintain 

unstable slopes (TRPA 1988). Construction sites also participate in activities 

such as traffic control where traffic is kept to areas that will have the same use 

postconstruction, stabilization of construction entrances to minimize transport of 

sediments outside of the construction zone by vehicle traffic, and dust control by 

maintaining surface cover by moistening, vegetating, or mulching exposed surfaces 

(TRPA 1988, White and Franks 1978). Construction sites disturbing more than 1 

acre of soil are required to file a Storm Water Pollution Protection Plan (SWPPP) as 

mandated by the National Pollutant Discharge Elimination System (NPDES) under 

the Clean Water Act (NDOT 2006, SWRCB 1999). The SWPPP specifies which 

BMPs will be applied to prevent all construction pollutants and erosion products 

from entering stormwater and exiting the construction site (SWRCB 1999). It also is 

required that this plan detail maintenance procedures and self-inspections that will 

be conducted to ensure the optimum performance of applied BMPs (NDOT 2006). 

The filing of the SWPPP is implemented and enforced by the LRWQCB and the 

NDEP. 

Permanent drainage control is used to minimize the effects of impervious areas 

and diminish the capacity of surface runoff carrying nutrients and sediments to 

move offsite. The continual growth of impervious area such as rooftops, driveways, 

compacted soils, patios, and decks is a side effect of urbanization. Numerous BMPs 

have been designed to address the problems associated with large areas of reduced 

or zero infiltration and the production of large quantities of surface runoff. The 
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first step is the installation of conveyance systems, such as slotted drains, swales, 

subsurface drains, gutters, downspouts, deflectors, gravel trenches, or gravel armor, 

which all intercept runoff perpendicular to the direction of flow and then divert it 

to an infiltration system (TRPA 1988). Next is the installation of surface infiltration 

systems such as naturally flat vegetated areas, infiltration trenches, drywells, gravel 

armoring layers, retaining ponds, and planter boxes (TRPA 1988). These systems 

retain water, allowing sediment to settle and the water to infiltrate into the soil, 

which has inherent cleansing abilities for some nutrient components. 

Permanent surface stabilization is designed to reduce the impacts of unstable, 

steep, or exposed soil surfaces. Compacted soils that do not infiltrate can be paved 

to avoid erosion or vegetated to improve infiltration (TRPA 1988). Vegetation also 

can be used to stabilize slopes. Other methods of slope stabilization include willow 

wattling, terraces, retaining walls, mulching, and erosion control blankets (TRPA 

1988). Which method(s) are most appropriate depends largely on the steepness of 

the slope. 

One of the most recent soil conservation issues is the defensible space program 

and the need for BMP implementation within the 0- to 30.5-m range by private 

homeowners. Guidelines are currently being developed for any person who owns, 

leases, controls, operates, or maintains a building or structure adjoining any mountainous area, forest-covered lands, brush-covered lands, grass-covered lands, or any 

land that is covered with flammable material. Such a defensible space perimeter 

would provide firefighters a working environment helping them to protect buildings 

and structures from encroaching wildfires as well as minimizing the chance that 

a structural fire would escape to the surrounding wildland. A key concern is the 

development of appropriate management strategies that maximize defensible space 

but at the same time function to minimize erosion and the degradation of runoff 

water quality. 

BMP Effectiveness 

To some extent, most BMPs are effectively reducing erosion and thereby reducing 

total sedimentation and nutrient loading to the lake. A study was conducted from 

1974 to 1977 comparing sediment yields from two development sites in or near 

the Tahoe basin: Northstar-at-Tahoe and Rubicon Properties Unit Number Two. 

Construction of one site (Northstar-at-Tahoe) included erosion control methods and 

the other (Rubicon Properties) did not. Sediment yield from Northstar-at-Tahoe 

increased by about 100 percent above the very low background levels associated 

with predevelopment conditions, whereas sediment yield from the Rubicon Properties site was found to be approximately 10,600 percent above background levels 
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well after development (White and Franks 1978). Evaluation of the sample plot at 

Rubicon Properties led researchers to believe that implementation of erosion control 

methods similar to those at Northstar-at-Tahoe would help reduce sediment yields 

to approximately 100 percent above background levels at Rubicon; however, data to 

this effect could not be found.

Identification as to which BMPs are most effective and to what degree for any 

given scenario is therefore important. Best management practice effectiveness is 

a function of three factors: design, construction, and maintenance. Choosing the 

most effective BMP requires sufficient knowledge of BMPs and their applicability depending on location and other site-specific conditions within the basin. Cost 

effectiveness is an additional issue to be considered when choosing the most appropriate BMP. A BMP that is the most effective in reducing sediment yield may not 

be cost effective owing to site conditions, materials, or labor requirements; hence, 

it may not be practically feasible. Alternatively, a BMP might be both cost and 

functionally effective in reducing sediment yield, but require excessive maintenance 

reducing its viability as a long-term method of erosion control. It is essential that 

each aspect be considered when selecting the most appropriate BMP. The question 

of which BMPs are most effective is best determined by having a full “toolbox” 

of BMPs from which to choose, and then having the knowledge and experience to 

apply the correct BMP to match onsite situations. 

Although it is considered best to minimize disruption and keep native vegetation intact whenever possible (Gray et al. 1980, Lynard et al. 1980, TRPA 1988), 

this is seldom feasible during construction activities. In such construction zones, 

revegetation and gravel mulches have been shown to be an effective means of 

erosion control on disturbed, bare soils. On the other hand, simple sprinkling has 

been found to be an inefficient practice (TRPA 1988). It requires large amounts of 

water, needs several applications daily, and creates sediment-laden water. In terms 

of sediment traps, straw bales do not trap much sediment and create a noxious weed 

hazard, and sandbags can tear and add additional sediment to flow (TRPA 1988). 

Fiber rolls and filter fences can be efficient sediment traps when installed correctly 

but unfortunately they seldom are (TRPA 1988). Therefore, temporary construction 

BMPs run the risk of frequently being ineffectual and require great vigilance on the 

part of authorities to make sure that they are being implemented and maintained 

correctly. 

For permanent drainage control, conveyance systems are used to direct surface 

runoff to a point of infiltration within the property line. Infiltration effectiveness is 

assumed but is unknown. Slotted drains are most effective for conveyance on driveways, but are quite costly, and swales cannot be installed on slopes greater than 
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5 percent (TRPA 1988). Berm construction is no longer a recommended practice 

because they lose functionality over time owing to the harsh Tahoe climate. The 

most appropriate infiltration systems for use are often determined by soil characteristics such as bulk density, water-holding capacity, hydraulic conductivity, and 

soil water repellency. Vegetation is often used to enhance the efficiency of certain 

drainage structures such as swales or channels by reducing flow velocities and 

thereby allowing time for greater infiltration. Gravel and other infiltration trenches 

are typically inappropriate structures on slopes, and gravel armor is only effective 

when used on highly permeable soils (TRPA 1988). 

The use of native vegetation has always been considered an effective means 

of retaining surface stabilization (Lynard et al. 1980). Once soil is bared, however, 

the combination of vegetation and mulch has been proven an effective method of 

stabilizing it (Grismer and Hogan 2005a, 2005b). Erosion control blankets, seeding 

and fertilizing, and wood fiber coating have all been found to be effective slope 

stabilizers; however, seeding alone was highly ineffectual (Leiser et al. 1978). 

Establishing vegetation on a steep slope can be quite difficult. It has been noted that 

high vegetation mortality can be the result of unstable slopes (Leiser et al. 1978). In 

this case, it is generally recommended that vegetation efforts can be supported with 

some kind of slope stabilization (Lynard et al. 1980). A combination of wattling 

and willow cuttings were found to decrease sediment yields from a road cut from 

approximately 83 m3/yr to almost nothing (Leiser et al. 1978). In fact, in a 1978 U.S. 

Environmental Protection Agency report it was noted that contour wattling should 

be applied more often (White and Franks 1978). 

BMP Maintenance 

Different erosion control structures require different levels of maintenance to 

remain effective, although regular maintenance is often important. For example, 

regular cleaning of conveyance structures is necessary to remove accumulated 

debris (TRPA 1988), and infiltration systems require regular cleaning to maintain 

maximum storage capacity. Although no clear maintenance schedule is given for 

conveyance and infiltration structures, it is recommended that they be examined 

after each storm to determine if cleaning is necessary. Organic mulches are best 

replaced yearly owing to decomposition loss. Mortality loss of vegetation coverage 

is best compensated for with the establishment of new vegetation (TRPA 1988). 

Vigilance is a major aspect to the maintenance of effective BMP structures. This 

is highly dependent on the level of devotion, regardless of whether the application 

is at the jurisdictional, the individual property owner, or the responsible regulatory 

agency scale. 
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Stream Environment Zones 

Stream environment zones are wetland, flood-plain, and riparian areas that influence the surface water quality in the Lake Tahoe basin. They have traditionally 

been considered an important means of reducing nutrient and sediment loads 

and surface water velocity before discharge into Lake Tahoe. In 1982, the TRPA 

stipulated that SEZ habitat should be protected in its natural state, that all disturbed 

SEZs in undeveloped areas would be restored, and that at least 25 percent of the disturbed SEZs in developed areas would be restored (TRPA 2001a, 2001b). From 1980 

to 1986, 61 ha of SEZ were restored; 28 ha were restored from 1987 to 1991; 41 ha 

were restored from 1992 to 1995; and 62 ha were restored from 1996 to 2001. In 

order to attain its SEZ habitat threshold, TRPA (2001a, 2001b) set a goal of restoring 253 ha of SEZ habitat by 2006, of which 148 ha of SEZ were restored (TRPA 

2006a, 2006b) by that date. Threshold attainment is reevaluated every 5 years. 

Impervious Coverage 

Impervious coverage is land that is unable to infiltrate surface water owing to paving, soil compaction, or structural coverage. Impervious coverage leads to increased 

surface water runoff, thereby increasing flow velocities, erosion potential, and 

sediment transport, as well as the production of other pollutants such as vehicle oil 

and grease, tire dust, and hydrocarbons from pavement. In 1982, the TRPA adopted 

the threshold policy that impervious coverage will comply with the land capability 

classification system set forth for the Lake Tahoe basin by Bailey in 1974 (TRPA 

2001a, 2001b). This land capability classification determined how much impervious coverage an area can handle based on its perceived hydrologic characteristics 

and is currently applied to all parcels developed before 1987. Bailey classifications 

were originally determined from soil and hydrologic features characterized by the 

original Tahoe basin soil survey report (USDA SCS and FS 1974), but may require 

revision based on the now updated soil survey (USDA NRCS 2007) to remain 

consistent. In 1987, the TRPA implemented the Individual Parcel Evaluation System 

(IPES) as its method for determining allowable impervious coverage on vacant or 

undeveloped lots. A numerical value is assigned to each parcel, which is calculated 

from its relative erosion hazard, runoff potential, degree of difficulty to access the 

building site, SEZ, condition of the watershed, ability to revegetate, need for water 

quality improvements in the parcel vicinity, and proximity to Lake Tahoe. The IPES 

field evaluation team determines the parcels most eligible for development (those 

with the highest IPES score) and ranks them accordingly for each jurisdiction. 

The Tahoe Environmental Geographic Information System (TEGIS) was 

developed in 1987 to track the amount of impervious coverage. Research with 
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the Desert Research Institute and TRPA has proved effective in being able to use 

satellite imagery and aerial photography to improve this database. However, it 

was not detailed enough to be applied in the 2001 TRPA monitoring of threshold 

attainment. Instead, project reviews, compliance records, and files were used to 

establish comparisons for the levels of new and old impervious coverage. From this 

it was determined that as of 2001, the Tahoe basin was not yet in attainment with 

the land capability threshold. Furthermore, it was believed that attainment could not 

be achieved by 2006. To our knowledge, no set schedule for attainment has been 

established (TRPA 2001b). 

Although attainment of the impervious coverage threshold has yet to be met, 

there is some evidence that the IPES program may be helping to reduce sediment 

loads discharging into Lake Tahoe. A study conducted by the Desert Research Institute in 1999 reported that 9 of 10 Lake Tahoe tributaries have seen a decrease in 

suspended sediments since the implementation of the IPES program in 1989 (TRPA 

2001b). This study has yet to be published in a peer-reviewed journal, and it is currently unknown whether or not the relationship was coincidental. Without detailed 

information, it is difficult to determine if the reduction in suspended sediments was 

actually related to the IPES program, or the result of other management strategies, 

climatic variation, or a combination of related factors. 

Implementation and Regulation 

Despite TRPA’s efforts in regulatory management for water quality protection, 

our experience in the Tahoe basin suggests there is a serious lack of community 

participation and understanding as to the importance of monitoring for BMP 

effectiveness and the relationship between impervious cover restrictions and SEZ 

protection that are pertinent to soil conservation (i.e., the reduction of nutrient and 

sediment loading) and the enhancement of stream and lake water quality. Although 

the monitoring of water quality management projects has been listed as a primary 

function of the LRWQCB, such evaluations have been difficult to conduct, and 

there does not appear to be much available to the general public regarding the efficiency of programs that have been implemented. As of June of 2006, an estimated 

36,000 properties of the approximately 42,000 developed parcels in the Tahoe basin 

remained in noncompliance (Fehd 2006). With a reported rate of 150 to 175 free 

evaluations available per month (Fehd 2006), and the limited construction season 

(May–October), it appears unlikely that this goal will be reached. The public might 

be more active in their participation if the required BMPs could be quantitatively 

demonstrated as the most effective of the BMP tools and, if applied, how much their 

individual involvement could directly improve water quality.
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Although there are gaps in monitoring and reporting of water quality protection efforts in the Lake Tahoe basin, there have been some successes as well. The 

LTBMU actively monitors management practices, facilitates the implementation of 

new research into policy, and actively participates in the dissemination of research 

results. The Annual Progress Report published by the USDA Forest Service, as well 

as the individual reports compiled for each monitoring project, allow interested 

members of the public the opportunity to educate themselves on topics concerning 

the preservation of Lake Tahoe’s water quality and surrounding soils. The use of 

monitoring results in decision and policymaking are fundamental to the success of 

adaptive management. 

Regional Planning 

Regional planning has been enhanced by the Environmental Improvement Program 

(EIP). The EIP was established in concert with the 1997 Lake Tahoe Presidential 

Forum. It outlined a 10-year plan investing $980 million into the Tahoe basin 

(TRPA 2006b). These investments have aided soil conservation efforts through 

the acquisition of sensitive lands, restoration of sensitive areas, treatment and 

removal of unpaved roadways in forested areas, funding of monitoring assessment 

programs, and the establishment of and technical assistance to EIP partners. For 

example, the new NRCS Soil Survey is counted as a federal contribution of technical assistance under the EIP. The list of EIP partners is extensive and includes 

almost all research, governing, and public institutions involved in maintaining 

Lake Tahoe basin environmental integrity (TRPA 2001b). 

The 20-year regional plan guiding TRPA programs expired at the end of 2006. 

It was subsequently determined that the public was very confused on environmental 

issues, that they lacked understanding of regulatory policy, and they did not grasp 

how their individual actions could have substantial impacts on the fragile Lake 

Tahoe ecosystem (Pathway 2006, 2007). Consequently, it is extremely important 

that members of all components of the community are involved in the development 

of a new regional plan.

Knowledge Gaps

Regulatory compliance and the effectiveness of management strategies on privately 

owned portions of the Lake Tahoe basin are unknown. 

• Government agency monitoring of the effectiveness of their programs is 

inconsistent and does not make public the results or findings of fact. 

• Because there is a lack of understanding as to the relationship between 

BMP effectiveness and the enhancement of stream and lake water quality, 
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Tahoe residents are not always predisposed to comply with existing 

regulations. 

• The agencies have not explored the full suite of opportunities available 

to regulate and monitor the effectiveness of existing policy ordinances.

• The status of the environment and resources relative to the existing 

thresholds is largely unknown. 

Agency identification of key management questions of specific interest and 

their relationship to short- and long-term goals based on specific management 

objectives is lacking. 

• Although a number of basinwide key management questions have been 

identified, each agency appears to have its own specific issues, goals, 

and agendas. These are often conflicting and repetitive when separately 

addressed. 

• Reexamination of the historical application of model-based policy is important, particularly in the context of new quantitative data and technologies. 

The foundation for current thresholds appears to reside in part within the 

context of the original Bailey Land Capability Classification (Bailey 1974). 

The Bailey system was based on the original “Tahoe Basin Soil Survey, 

California and Nevada” (USDA SCS and FS 1974), which has now been 

updated by the “Soil Survey of the Tahoe Basin Area, California and 

Nevada” (USDA NRCS 2007). We suggest this classification system and 

any related thresholds be reevaluated in the context and guidance of findings presented in the new Soil Survey

Underlying ecosystem processes of relevance to soil conservation are largely 

unknown, and such knowledge would help to facilitate the broad application of 

adaptive management strategies across projects within and among contributing 

watersheds. 

• Clear identification of where research and agency monitoring programs 

can be strengthened and integrated would benefit from full assessment. To 

implement more cost-effective data collection, better integration of science 

information (i.e., research and monitoring) into the decisions affecting the 

implementation of capital programs is recommended.

• There is no standard method of measurement and monitoring protocols 

from which data reporting and interpretive analysis can be applied basinwide. This makes it more difficult to compare results from different activities or similar activities at different locations throughout the basin. 
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The decrease in Lake Tahoe’s clarity has been largely attributed to a 

combination of fine sediment suspension and enhanced biomass production 

from N and P nutrient loading. 

• Most published erosion control studies in the basin have focused 

on reporting total sediment yields alone. More work is specifically 

recommended to determine how erosion control measures affect fine 

sediments and their related equilibrium nutrient chemistry. One erosion 

control study reporting on nutrients and fine sediment showed erosion 

control structures to be effective in removing coarse particulates, but 

not nearly as effective when dealing with fine sediments and nutrients 

(Garcia 1988). 

• Which colloidal-size particles, their associated mineralogy, and their equilibrium chemistry are most important to bio-available nutrient contributions and physical light scattering remains unknown. Better quantitative 

evaluation in this regard is critical to the development of programmatic, 

investigative, and management approaches that focus on what the literature 

now indicates as contributors to diminished water clarity. 

Revegetation of unstable slopes has had mixed success owing to a lack of 

uniform application of approaches that are based on well-founded principles. 

• Revegetating slopes has been more costly owing to the rarity and unpredictable availability of native vegetation seed stock and seedlings in nurseries. 

Broader knowledge of the overall effectiveness of multiobjective BMPs 

applied in the Tahoe basin is needed to educate the public and evaluate the 

suitability of regulatory policy. 

• Published research on basinwide BMP effectiveness is limited.

• Although BMP effectiveness research is currently being performed by 

several agencies in the Tahoe basin, consistency in parameter measurement 

and reporting of results is seldom achieved (Lynard et al. 1980, Schuster 

and Grismer 2004). 

Best management practice effectiveness is usually tested as a whole for 

several BMPs implemented on one site. 

• Determining which BMPs are truly the “best” would require (1) matching 

the correct BMP to site conditions and (2) testing them against one another 

rather than comparing their cumulative site effects. 

• Collective BMP effectiveness in different situations and combinations has 

not been evaluated. 
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Urban development and anthropogenic activities typically reduce native 

soil capacity. 

• BMPs for protecting and rehabilitating soils affected by urbanization are 

not well defined in terms of design, function, or effectiveness. 

Erosion control structures are being implemented throughout the basin on 

construction sites and residential and commercial lots. 

• The number of erosion control structures actually monitored for compliance 

and effectiveness is unknown. 

• Whether or not appropriate structures are being implemented in each site 

and location is uncertain. 

It has been suggested that many temporary sediment traps used on construction 

sites are frequently installed incorrectly. 

• Regular inspection of construction sites for compliance and real-time 

enforcement would assist in the performance evaluation of management 

strategies.

• Inspection, maintenance, and enforcement of monitoring protocols as 

outlined in the SWPPP would be helpful. 

Wood fiber, pine needles, and other organic materials are frequently used as 

mulch when stabilizing bare soils. 

• Thick pine needle mats have been shown to be a possible contributor to 

high levels of biologically available N and P in surface runoff. 

• Little is known about the nutrient release effects of woodchips when used 

as mulch layers or when incorporated into the soil.

• The creation of defensible space through vegetation management usually 

means reducing the amount of fuel around the building or structure, providing separation between fuels, or reshaping retained fuels by rearranging 

the trees, shrubs, and other fuel sources such as plant-residue groundcover 

in a way that makes it difficult for fire to transfer from one fuel source to 

another.

• Vegetation removal for defensible space can cause soil disturbance, soil 

erosion, regrowth of new vegetation, and introduce nonnative invasive 

plants. Areas up-gradient of water riparian areas, such as streams or ponds, 

are a particular concern for protection of water quality when developing a 

defensible space protocol.
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GENERAL TECHNICAL REPORT PSW-GTR-226

Tahoe Regional Planning Agency threshold standards remain in nonattainment status. 

• Goals for the attainment of established threshold standards continue to 

be set, but it remains difficult to achieve attainment. 

• Threshold indicators are typically monitored for attainment only, and 

not for determining the effectiveness of the threshold in protecting 

Lake Tahoe water quality. 

The application of new knowledge to improve management policies 

and practices would require active communication between researchers 

and policymakers. 

• Some agencies may be diligent in their monitoring and integration of 

results into management decisions, whereas others may not be so diligent. 

• There is inconsistency regarding the incorporation of pertinent new 

findings into the policymaking process.

Research Needs

• Include N and P nutrient forms, fine sediments, and their associated equilibrium chemistry as specific parameters in quantitative analysis for BMP, 

SEZ restoration, and impervious coverage reduction effectiveness studies.

• Develop standardized monitoring protocols for the monitoring of BMP 

effectiveness, SEZ restoration, and impervious-coverage reduction in the 

Tahoe basin. Findings could then be reported in peer-reviewed journals for 

enhanced credibility. 

• Measure the effectiveness of BMPs as separate entities rather than a program as a whole. Similar slope stabilization, infiltration, or sedimentation 

techniques could be tested against each other in similar and divergent environments as a means of ascertaining why some work better than others in 

one locale vs. another. 

• Determine the depth of understanding and actual participation of Tahoe 

basin residents in the maintenance of their privately managed BMPs. 

• Monitor and compare several combinations of BMPs in a variety of settings, including urbanized sites. This creates the perfect scenario in which 

to investigate the range of functions for commonly applied BMPs in the 

Tahoe basin and their ability to protect and restore important soil properties 

such as infiltrability, stability, soil moisture storage, and capacity to support 

revegetation. This could also be used to determine compliance maintenance 

schedules for BMP upkeep. 
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

• Monitor construction sites to determine compliance and ability to 

effectively install and maintain temporary BMPs. 

• Conduct research to quantitatively demonstrate how sound adaptive 

management practices, even at the individual scale (e.g., homeowner 

BMPs), benefits and enhances the Tahoe basin environment and improves 

water quality. 

• Develop a horticulture program geared toward the propagation, seed 

bank development, and seedling establishment techniques for native 

vegetation to stabilize slopes in the Tahoe basin.

• Monitor established environmental thresholds to assess attainment as well 

as effectiveness. Realistic goals could then be set for attainment of these 

thresholds when proven effective in the reduction of nutrient and 

sediment loading to the lake and the enhancement of water clarity. 

• Quantitatively compare Bailey’s Classification System and the IPES to 

naturally functioning systems with no impervious coverage to determine 

the level of effectiveness the reduction in impervious coverage is having. 

Furthermore, these studies could assist in integrating information from 

the new soil survey into these existing systems, and reassessment research 

could be performed to better understand the regulatory implications of any 

new findings.

• Determine the potential social and environmental benefits of relocating 

existing land coverage to hillsides or intervening zones having equal or 

greater infiltration capacity, deeper ground water, and greater potential 

for subsurface filtration. 

• Research and monitor existing regulatory programs such that their true 

effectiveness and applicability can be more quantitatively assessed. 

Research on BMPs and regulatory programs from other areas may be 

beneficial.

• Reevaluate or monitor the use of wood fiber, pine needles, and other 

organic materials and its relationship to defensible space, overall 

applicability and site-specific effectiveness. 

• Compile comprehensive data syntheses of results from previous findings 

and also current research projects, monitoring programs, and impending 

adaptive management strategies. A single database would be the optimal 

place to look for environmental information, management strategies, and 

problem-solving options in the Lake Tahoe basin. 
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GENERAL TECHNICAL REPORT PSW-GTR-226

• A standard protocol for the measurement and monitoring of Tahoe basin 

(eastern Sierran) ecosystems is recommended. Use of these protocols by all 

Lake Tahoe basin participants would facilitate the comparison of basinwide soil conservation projects and the compilation of results from projects 

conducted at different locations by different institutions and/or the private 

sector. 

Near-Term Soil Conservation Research Priorities 

Following are a prioritized listing of the three most immediate near-term soil conservation research needs within each subtheme category. This approach was taken 

because different agencies may be more focused on the components of a given 

subtheme rather than the broader category of Soil Conservation as a whole.

Key Soil Properties and Conditions (SPC)

Research Priority No. 1 (SPC1): Further quantify the distribution of various 

watershed properties such as soil water repellency, biologic and inorganic nutrient 

pools, infiltrability, and water balance parameters on a larger spatial scale. The 

impact of natural and anthropogenic activities such as development (impervious vs. 

pervious), forest management (fire suppression vs. mechanical or prescribed fire 

biomass reduction), vegetation (native vs. non-native species), restoration (physical and chemical amendments vs. reduced fertilization), and features that have 

disrupted natural littoral and eolian processes on soil health at the watershed scale 

(including the intervening zones) remains poorly understood. 

Research Priority No. 2 (SPC2): Assess which restoration methods are most 

effective in controlling event-based runoff. Also, the transport and equilibrium 

chemistry of fine particles most associated with nutrient and sediment loading 

should be more quantitatively assessed. Project success and longevity should be 

evaluated relative to the sustainability of hydrologic function, productivity, and 

erosion control over time. 

Research Priority No. 3 (SPC3): Characterize to the extent feasible and quantify where possible, historical vs. current and natural vs. anthropogenic induced 

declines in soil status and resulting soil loss at the watershed scale. Research on 

soils in natural as well as disturbed settings is recommended whenever possible, 

with sites established to measure soil conservation parameters including inputs 

such as plant-soil nutrient fluxes through litter-fall, crown-wash, and root turnover 

as well as losses from erosion, leaching, runoff, wind, or fire. Research would 

focus on sites where a suitable control portion is available, especially if event 

(e.g., prewildfire or pretreatment) data are available. 
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

Knowledge advancement potential—

The fate of Sierran ecosystems in a changing environment will have a direct impact 

on soil health, fire hazard, biomass mitigation strategies, erosion, and water quality. Manipulative research projects that include random assignment of treatments 

and replication are challenging to perform in the Tahoe basin. And yet a crucial 

research need is to identify and quantify key indicator parameters in a variety of 

historical and current ecological settings, under various manipulations, and over 

time. In such cases where robust experiments are possible, restoration methods 

that are most effective in controlling runoff and transport of fine particles, as well 

as those most effective in the reduction of nutrient discharge loading and its direct 

effect on water clarity, can be better assessed. This would allow a more complete 

understanding of the environmental factors (i.e., temperature, moisture, vegetation, 

and litter) that determine the formation, persistence, and dissipation of seasonal and 

long-term effects on runoff water quality and erosion. In this context, similar slope 

stabilization, infiltration, revegetation, or sedimentation techniques could be tested 

against each other in similar and divergent environments as a means of ascertaining why some work better than others in one locale vs. another. Research focused 

on the identification, monitoring, evaluation, tracking, and adaptive management 

of individual and collective BMP systems—and linking them to GIS layers at the 

watershed and basin scale—would go a long way toward addressing the concerns 

of stakeholders and agencies alike. Being able to track and revisit BMP strategies 

would further facilitate true adaptive management.

Development and Application of Predictive Models (PM) as 

Related to Soil Conservation 

Research Priority No. 1 (PM1): Successful model application dictates the need 

for site-specific parameterization and model calibration. Model use and application can then become more consistent and interpretive assessment more uniform 

among agencies basinwide. Research and monitoring protocols are recommended 

to provide relevant information for predictive model development, improvement, 

calibration, and field validation specific to the Lake Tahoe basin. A prioritization 

of which soil, vegetation, and hydrologic parameters are important, what should be 

measured, and what information is needed to parameterize and calibrate the models 

should be established.

Research Priority No. 2 (PM2): Develop a spatially explicit water balance, nutrient cycling, and erosion potential model to better understand current sediment 

and nutrient transport at the watershed scale and under conditions of potential 

changes in hydrologic and soil parameters. The role of hydrophobic soils should 
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GENERAL TECHNICAL REPORT PSW-GTR-226

be further studied to determine the spatial distribution of recharge areas vs. those 

that are overland flow generating and their influence on soil erosion model output 

estimates. A prioritization of which parameters are important, what should be 

measured, and what information is needed to parameterize and calibrate the models 

should be established.

Research Priority No. 3 (PM3): Develop a better understanding of how various 

factors or stressors change soil status in Tahoe basin watersheds to assist forest 

managers in preparing management plans and make predictions about ecosystem 

response to natural (e.g., fire, insect attack, drought, or erosion) and anthropogenic 

(air pollution, harvesting, development, or climate change) perturbations. For 

example, a comprehensive assessment of the effects of both wildfire and prescribed 

fire, and postfire vegetation, on long-term response in biological and physicochemical soil parameters is needed to better understand fire and its role in restoration 

ecology.

Knowledge advancement potential—

Because regulatory policies may be based on the subjective judgment of “risk 

potential” rather than on a sound quantitative decision-support system, the 

application of predictive models can provide important tools to understanding 

and estimating the potential outcome of management strategies and programs. 

Successful model application, however, is accomplished through site-specific 

parameterization and model calibration. Establishing a means for prioritizing 

which ecosystem parameters are important, what should be measured, and what 

information is needed to parameterize and calibrate the models. In the event that 

the current models are not adequate predictors, appropriate modifications or 

adjustments are needed to make the existing models more functional. If this is 

not an option, starting from scratch and developing a new model that is simple, 

accurate, and appropriate for the Tahoe basin may be necessary. Model use and 

predictive application is recommended to enable consistency among agencies 

basinwide wherein the acquisition of a more robust quantitative database could 

provide the foundation for policies of future management strategies.

Effects of Climate Change (CC) as Related to Soil Conservation

Research Priority No. 1 (CC1): There is concern that anthropogenic activities 

over the last century have resulted in nontypical ecosystem structure throughout 

the basin of which the distribution, character, variability, and potential response to 

climate change have not been evaluated. Consequently, strategic efforts directed 

toward long-term site restoration in response to a quasi-natural state will be the 
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

more likely scenario. Quantitative assessment of what we can and cannot hope to 

accomplish on a long-term basis is recommended. 

Research Priority No. 2 (CC2): More comprehensive localized point source 

precipitation, surface runoff, erosion, and nutrient transport data is recommended 

to quantify potential discharge loads as a function of amount, type (snow vs. 

rainfall), frequency, and precipitation intensity. Research and monitoring projects 

should be designed to address potential changes in hydrologic parameters as a result 

of climate change.

Research Priority No. 3 (CC3): Investigate the implications of climate change 

on slope stability parameters. Surface soil stability, compaction, soil structure and 

aggregate stability, infiltrability and runoff, and potential for mass wasting should 

all be tested under scenarios of different temperature and moisture regimes to estimate the potential effects of climate change on soil erosion in the Lake Tahoe basin.

Knowledge advancement potential—

The implications associated with climate change cannot be ignored. Predictions 

for changes in precipitation quantity and intensity are quite variable for the Sierra 

Nevada. One scenario is that precipitation will increase in intensity leading 

to large-scale flooding. Another is that the Lake Tahoe area will be subject to 

overall warmer temperatures and more evapotranspiration, whereas increased 

precipitation will be more common farther to the north. There is general agreement, 

however, that with warmer temperatures, snow elevation levels will be higher 

with less accumulation, which will lead to longer fire seasons. New approaches 

are recommended for examining shifts in the quality and amount of hydrologic 

inputs to support management strategies involving biomass reduction, drainage 

control, and practices to diminish sediment and nutrient transport to Lake Tahoe. 

It is recommended that future planning for the production of resilient, spatially 

heterogeneous and diverse forest structure be designed to account for potential 

changes in hydrologic function in response to different moisture regimes to 

determine what the ultimate effects of climate change could be on management 

protocols for sensitive areas at the watershed and locale scales.

Policy Implications (PI) and Adaptive Management Strategies 

as Related to Soil Conservation

Research Priority No. 1 (PI1): Monitor and study established environmental 

thresholds for attainment and for performance effectiveness. It is recommended 

that regulatory agencies and land managers develop a protocol for periodic review, 

verification, and update of processes, quantitative thresholds, and policy relevance. 
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GENERAL TECHNICAL REPORT PSW-GTR-226

More research and monitoring of existing regulatory programs is recommended, 

such that their overall effectiveness and applicability can be more quantitatively 

assessed relative to their actual reduction of nutrient and sediment loading to 

the lake and the subsequent enhancement of water clarity. Develop a basinwide 

protocol for “Standard Methods of Ecological Measurement and Monitoring in 

the Tahoe Basin.”

Research Priority No. 2 (PI2): Continued research that addresses critical natural 

resource issues and critical management questions relevant to soil conservation in 

the Lake Tahoe basin is essential. This exercise can begin by identifying a list of 

agency-specific management questions pertinent to soil conservation relative to 

key soil properties and conditions of interest. For example, what are the appropriate 

management strategies that maximize defensible space protection, but at the same 

time function to minimize erosion and the degradation of runoff water quality? 

It is then important to take advantage of unique opportunities and small-scale 

experimental field trials to quantitatively evaluate potential impacts. To ensure its 

credibility and applicability, such research would make every effort to be scientifically defensible, applicable to the Tahoe basin or similar ecological settings, and 

publishable in peer-reviewed journals. 

Research Priority No. 3 (PI3): Restoration and BMP strategies are generally 

implemented to mitigate known adverse impacts from either natural events or 

anthropogenic activities. Choosing the most effective strategy therefore will benefit 

from a thorough knowledge of the mitigation objective, process mechanics, both 

short- and long-term functionality, and whether or not these components will differ 

depending on location within a given watershed or the Tahoe basin in general. 

Performance evaluation is commonly assessed on a collective (e.g., projectwide) 

rather than individual (e.g., specific management activity) process basis. Complete 

evaluation of which strategies are truly the most effective in meeting specific 

restoration objectives would require testing each management activity against 

one another as well as assessing their cumulative effects. Management strategies 

implemented for one purpose (e.g., defensible space) may or may not have an effect 

on other issues of concern and ascertaining why some work better than others in 

one locale vs. another (or not at all) is a critical issue.

Knowledge advancement potential—

If greater confidence in performance effectiveness can be developed, consistency 

will likely follow. With new technology comes the opportunity for innovative soil 

conservation strategies that could alter or refine historical threshold values. In the 
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

past, technological advancement and expansion of the knowledge base was much 

slower. Today, it is not unusual for substantial new advancements to take place on a 

5-year rather than a 25-year cycle. Key to the success of any such approach, however, is the development of a consistent and effective monitoring protocol for key 

soil properties and conditions that is current, process-specific, and uniform across 

agencies and contractors. Hence, research is recommended to develop a standard 

protocol for ecological measurement and monitoring in the Lake Tahoe basin, 

which includes variable levels of intensity that can be applied to different types and 

scale of projects. In its absence, implementers and agencies frequently employ different techniques in attempting to evaluate the performance effectiveness of similar 

soil conservation activities. Comparative interpretive assessment is then difficult to 

impossible. Furthermore, evaluating which conservation and/or restoration methods 

are most effective is recommended in the context of a more comprehensive framework wherein each on-the-ground management strategy could be tested against 

one another in similar and divergent environments. Therein, lay key opportunities 

where new and unique soil conservation strategies could be explored. Finally, 

agency representatives can clearly identify agency-specific areas of concern, and 

then work with scientists and implementers to articulate the respective critical soil 

conservation issues. This would greatly assist in the development and design of 

successful monitoring, opportunistic, and/or experimental research programs that 

generate data and information directly applicable to agency needs.

English Equivalents:

When you know: Multiply by: To get:

Millimeters (mm) 0.0394 Inches

Centimeters (cm) .394 Inches

Meters (m) 3.28 Feet

Hectares (ha) 2.47 Acres

Square meters (m2

) 10.76 Square feet

Cubic meters (m3) 35.3 Cubic feet

Degrees Celsius (°C) 1.8 °C + 32 Degrees Fahrenheit

Acknowledgments 

Contributions to this document from M. Grismer (University of California, 

Davis), T. Hagan (Tahoe Regional Planning Agency), M. Hogan (Integrated 

Environmental Inc.), and D. Martin (Nevada-Tahoe Conservation District) 

are gratefully acknowledged.
228

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Reyes, M.R.; Raczkowski, C.W.; Gayle, G.A.; Reddy, G.B. 2004. Comparing 

the soil loss predictions of GLEAMS, RUSLE, EPIC, and WEPP. Transactions of 

American Society of Agricultural Engineers. 47: 489–493. 

Riebe, C.S.; Kirchner, J.W.; Granger, D.E.; Finkel, R.C. 2001. Minimal climatic 

control on erosion rates in the Sierra Nevada, California. Geology. 29: 447–450. 

Robichaud, P.R. 2000. Fire effects on infiltration rates after prescribed fire in 

Northern Rocky Mountains forests, USA. Journal of Hydrology. Vols. 231–232: 

220–229. 

Roos, M. 2005. Accounting for climate change. California Water Plan Update. 

Sacramento, CA: California Department of Water Resources. 4: 611–625. 

http://www.waterplan.water.ca.gov/docs/cwpu2005/vol4/vol4-globalclimateaccountingforclimatechange.pdf. (April 19, 2010).

Sarah, P. 2005. Soil aggregation response to long- and short-term differences 

in rainfall amount under arid and Mediterranean climate conditions. 

Geomorphology. 70: 1–11. 

Schumann, A.H.; Funke, R.; Schultz, G.A. 2000. Application of a geographic 

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Schuster, S.; Grismer, M.E. 2004. Evaluation of water quality projects in the 

Lake Tahoe basin. Environmental Monitoring and Assessment. 90: 225–242. 

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hydrophobicity: current problems and future research directions. Journal of 

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15, 2007).

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fire, soils, and stream chemistry in a watershed in the Lake Tahoe basin, 

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Ordinances. Individual parcel evaluation system. http://www.trpa.org/ 

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237

An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

Patricia N. Manley,2 Dennis D. Murphy,3 Seth Bigelow,4 Sudeep Chandra,5

and Lisa Crampton6

Introduction

The integrity of animal and plant communities serves as a critical measure of the 

effectiveness of policies designed to protect and restore ecosystem processes in 

the Lake Tahoe basin. The conservation of plants and animals in the Tahoe basin 

is utterly dependent on the conservation of its terrestrial and aquatic ecosystems; 

so, in many ways, the research agenda that follows builds on the other research 

described in this volume. Accordingly, successful integration of outcomes from 

research on water quality, air quality, and other natural attributes of the basin will 

contribute greatly to the recovery and persistence of biological diversity in the 

Tahoe basin.

A Lake Tahoe research agenda that considers biological diversity and ecological 

function is best based on data collected from across scientific disciplinary boundaries. In Tahoe’s intensively managed forests, there is an immediate and keen interest 

in linking forest fuel treatments to changing soil conditions, vegetation composition and structure, and the status of wildlife populations at multiple trophic levels. 

That immediacy noted, the Lake Tahoe basin actually is home to remarkably few 

imperiled species; however, that could change in short order through well-intended 

land and resource management actions that lead to unanticipated species declines 

(Manley 2005). Little species-specific information is currently available to guide 

land use and resource planning should changed circumstances lead to new listings 

under federal or state endangered-species statutes.

A number of policies direct and define management objectives for biological 

diversity in the Lake Tahoe basin. The Lake Tahoe Environmental Improvement 

Program identifies multiple restoration actions that are expected to benefit wildlife. 

Documents supporting the Tahoe Regional Planning Compact (1969), namely the 

1987 Regional Plan (see http://www.trpa.org), call out 20-year goals for wildlife and 

Chapter 6: Ecology and Biodiversity1

1 Citation for this chapter: Manley, P.N.; Murphy, D.D.; Bigelow, S.; Chandra, S.; 

Crampton, L. 2009. Ecology and biodiversity. In: Hymanson, Z.P.; Collopy, M.W., eds. 

An integrated science plan for the Lake Tahoe basin: conceptual framework and research 

strategies. Gen. Tech. Rep. PSW-GTR-226. Albany, CA: U.S. Department of Agriculture, 

Forest Service, Pacific Southwest Research Station: 237–301. Chapter 6.

2 Pacific Southwest Research Station, USDA Forest Service, Davis, CA 95618.

3 University of Nevada, Reno, Department of Biology, Reno, NV 89557.

4 Pacific Southwest Research Station, USDA Forest Service, Davis, CA 95618.

5 University of Nevada, Reno, Department of Natural Resources and Environmental 

Sciences, Reno, NV 89557.

6 University of Nevada, Reno, Department of Biology, Reno, NV 89557.
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GENERAL TECHNICAL REPORT PSW-GTR-226

fish, and identify environmental thresholds, indicator measures of those thresholds 

(including species indicators), and species and communities of special concern. The 

National Forest Management Act (1976) directives pertain to plants and animals 

on the more than 70 percent of Tahoe basin lands under U.S. Department of Agriculture, Forest Service jurisdiction. The U.S. Department of the Interior, Fish and 

Wildlife Service implements focal activities associated with several wildlife, fish, 

and plant species on National Forest System lands in the basin. Provisions of the 

Migratory Bird Act pertain to the entirety of the Lake Tahoe basin. Stringent state 

lands and parks rules are enforced on both California and Nevada sides of the lake. 

And both federal and state wildlife agencies are able to implement prohibitive policies if a threatened or endangered species listing becomes necessary.

Despite a demonstrated concern, our understanding of nearly all aspects of 

Lake Tahoe’s biodiversity—from species found in lakeside meadows, to those on 

alpine peaks above—is still rudimentary and would benefit greatly from implementation of the research agenda that follows. To varying degrees, management of all 

Tahoe basin ecosystems would be better informed by improved scientific knowledge about ecological characteristics, habitat associations, and species responses 

to management activities. Detailed status and management response information 

is available for very few species in the basin; and, in most management applications, it will be necessary to know the local status and responses to management 

given the unique configuration of wildlife habitats within the basin, and the basin’s 

relative isolation from the larger forested landscape of the Sierra Nevada. To that 

end, this chapter identifies focal management issues, associated uncertainties, and 

key research questions that, if answered, would encourage effective, efficient, and 

accountable resource management designed to maintain and conserve biological diversity, ecological function, and ecosystem services. Research in the Lake 

Tahoe basin will be most effective when designed in a manner that both decreases 

resource risk and uncertainty by closing information gaps, and directly informs 

management. For example, a research program to address uncertainties about the 

effects of forest management on biological diversity might start with a description 

of site-scale response patterns of biological diversity to various environmental 

changes associated with forest management. This program also might take steps 

to apply those data to the development of management tools that: 

• Apply that understanding to the basin as a whole to inform management 

about conditions throughout the Lake Tahoe basin.

• Identify system indicators that can be used to monitor progress toward 

management goals for forest ecosystems. 
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

• Determine thresholds of species and community responses that can inform 

how and when management actions should respond to monitoring results.

• Provide evaluation tools that managers can use independently.

• Provide basic data to enhance management and policy development. 

The ecology and biodiversity research agenda considers seven subthemes 

that represent various management activities and objectives: (1) old-growth and 

landscape management, (2) fire and fuels management, (3) special community 

management, (4) aquatic ecosystem restoration, (5) urbanization, (6) recreation, and 

(7) climate change. Within each subtheme, we provide a summary of issues and 

uncertainties and associated key research questions. The departure from historical 

conditions reflected in current ecosystem conditions in the basin presents many 

challenges to restoration. Most fundamental is the challenging fact that existing 

terrestrial and aquatic ecosystem conditions are unique in the history of the basin 

as are the current and projected future climate conditions. Thus, the objective of 

restoration is not to return these systems to an historical structure or composition, 

but rather to restore their biological diversity, function, and resilience. The key 

research questions span many types of information gaps including basic information gaps; effects and effectiveness of existing management approaches; models 

of past, current, and potential future conditions; and field and analysis tools to 

enhance the “toolbox” of methods available to managers to inform planning and 

decisionmaking. 

The ecology and biodiversity research agenda highlights the interactions 

between native species and communities and natural and human-caused stressors 

that present the greatest ecological and social risk, and for which research can 

reduce management-related uncertainties. Conceptual models are provided that 

show the primary linkages between native species and communities as components 

of ecosystems and the factors that affect their condition, including human-caused 

stressors (figs. 6.1 and 6.2). The two conceptual models group subthemes that 

share most of the same components and drivers: terrestrial ecosystem subthemes 

(old-growth forests and fire and fuels management) (fig. 6.1), and primarily aquatic 

ecosystems subthemes (special communities and aquatic ecosystems) (fig. 6.2). 

The subthemes identified in the ecology and biodiversity theme area represent 

focal elements for management planning and action in the Lake Tahoe basin (figs. 

6.1 and 6.2). The core components are those shared by most biological systems 

(e.g., species composition and abundance, vegetation structure), with differences 

expressed in the specifics of the components (such as associated species). The 

primary drivers of the condition of components also are often shared, given that 

human activities are pervasive and affect many biological components. Secondary 
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GENERAL TECHNICAL REPORT PSW-GTR-226

Figure 6.1—Conceptual model of the terrestrial ecosystem subthemes in the Ecology and Biodiversity theme. Identified are the primary 

components of terrestrial ecosystems in the Lake Tahoe basin, the natural and human-caused phenomena that affect their conditions, 

and the focus of research questions in the theme area, which are based on management concern and uncertainty. Thick arrows indicate 

especially important linkages between drivers and components. Research needs are indicated by alphanumeric symbols (e.g., CL3, OG3) 

and correspond to the descriptions presented later in the chapter.

FR1,3, 

5,6,8,9,10,11 

OG1,2,3,4, 

7,8;CL1,2

FR1,6, 

8,9,11

FR1,

3,4

FR2,7 OG5,

 8

OG6

OG3

UR1,2,5,11

UR3,4,14 UR6,

9,10

RE1–8

CL3

Forest health

and

biodiversity

policies

Fire risk and 

suppression

policies

Climate

change

2° Driver 1° Driver Component Indicator Subtheme Theme Linkage

Natural

disturbance

Vegetation

composition and

structure

Amount and 

distribution of habitat

Native species 

composition 

and abundance

Nonnative species

composition and 

abundance

Fire regime

Fuel

management

Human

disturbance

Vegetation

management

Ecology and

Biodiversity

Development and

application of

predictive models

Old-growth

and landscape

management

Development and

application of

predictive models

Fire and

fuels management

Recreation

FR13; OG8,9; 

UR2,12; RE9; CL4

Urbanization

drivers are typically external, broad-scale forces that act on human activities, such 

as regulations, policies, economic forces, and climate. It is important to recognize 

that stressors may have initial positive consequences for some species (e.g., habituation to human settlement by bears and geese) that then lead to undesirable secondary consequences (e.g., property damage or reduced diversity of native species). 

The majority of uncertainties associated with terrestrial ecosystems pertain 

to the linkages between vegetation management and climate change (i.e., natural 

disturbances) and their effects on vegetation structure, composition, and associated 

fire hazards (fig. 6.1). A more limited set of questions pertain to the effects of forest 

fuels treatments on plant and animal communities, populations, and habitats. The 
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

Figure 6.2—Conceptual model of the aquatic ecosystem subthemes in the Ecology and Biodiversity theme. Identified are the primary 

components of aquatic ecosystems in the Lake Tahoe basin, the natural and human-caused phenomena that affect their conditions, and 

the focus of research questions in the theme area, which are based on management concern and uncertainty. Thick arrows indicate 

especially important linkages between drivers and components. Research needs are indicated by alphanumeric symbols (e.g., R2, FM3) 

and correspond to the descriptions presented later in the chapter.

FM6

UR1,14

RB2

2° Driver 1° Driver Component Indicator Subtheme Theme Linkage

Vegetation

composition and structure

A2,3; FM2

Native species 

composition and abundance

R5; MB1; LT5,7; 

LM1; LT9; OE4,5

Fire regime

Aquatic chemical

composition

OE1,6

Ecology and

Biodiversity

Special 

community

management

Aquatic

ecosystem

restoration

A5; R6; FM7; 

MB2; LM2; 

OE7; UR2

Natural

disturbance

Fuel

management

Fire risk 

and 

suppression

Recreation

Aspen

Riparian

Lake Tahoe

Fens and

meadows

Marshes 

and 

beaches

Other 

aquatic

ecosystems

A3; R2;

FM3,5

A4; R3;

FM4

CL3

LT8,

10

Nonnative species 

composition and abundance

OE3

LT1,3;11; OE2

LT6

LT4

Amount and distribution 

of units and habitat

A1; R1; FM1; MB1;OE6,8 

Forest 

health

and

biodiversity

policies

Climate

change

Vegetation

management

Urbanization

Human

disturbance

Pollution

Hydrologic function

limited set of questions regarding biodiversity does not reflect lower uncertainty 

and risk, but rather it reflects the priorities of management, and the assumption that 

fuels treatments will not substantially alter the habitats of animals and herbaceous 

plants. Land and resource managers are interested in identifying key measures of 

conditions that can be used as indicators for progress toward desired conditions. 

These circumstances are not unique to the terrestrial ecosystems or the ecology 

and biodiversity theme; rather they recognize an area of substantial investment by 

management agencies at the present time.
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GENERAL TECHNICAL REPORT PSW-GTR-226

The majority of uncertainties associated with the special communities and 

aquatic ecosystems are weighted toward basic information, as opposed to linkages 

between activities and the condition of components (fig. 6.2). This is a function of 

limited investment in research and monitoring in aquatic ecosystems other than 

Lake Tahoe itself; so it is generally thought that basic information on current conditions is the first step toward informing management. Not all special communities 

are identified, but the suite that is identified represents those of greatest management interest. In addition to the pursuit of basic information, greater understanding 

is needed about the linkages between natural disturbances, and the role of fire and 

fuels reduction on vegetation composition and structure, particularly in riparian 

and meadow habitats. As with terrestrial ecosystems, indicators would be selected 

for the purposes of assessing the condition of each of the special communities and 

aquatic ecosystems.

The research questions that are identified here constitute the highest priority 

information needs over the next 10 years, based on the combination of three considerations: (1) uncertainty based on lack of knowledge, (2) current risk based on the 

current condition of biophysical components, and (3) potential future risk based on 

current or future management activities or climate change. The questions represent 

those that, if addressed through research, would make a substantial contribution 

toward reducing uncertainty and risk in conservation and restoration efforts targeting biological diversity and ecological integrity in the Lake Tahoe basin. Some of 

the questions presented are more time sensitive than others, either as a function of 

their placement in a sequence of discovery or the ease of answering the question 

relative to the value of the contribution to reducing uncertainty and risk. The most 

time-sensitive questions within each subtheme are indicated in bold.

Old-Growth and Landscape Management

Past management activities, particularly historical logging, followed by fire suppression, substantively shaped the amount, distribution, and condition of old-growth 

forests in the Lake Tahoe basin today. Current management activities continue to 

affect the character and distribution of the basin’s forest ecosystems. The combination of these anthropogenic and natural disturbances has determined the current 

distribution of seral conditions across the basin, including the distribution and 

abundance of the remaining old-growth forest patches (Manley et al. 2000). Forests 

in the Lake Tahoe basin now differ in a number of important aspects from their 

pre-Euro-American appearance. In the montane zone, the tree species composition 

and diameter distribution have changed greatly; there are far more small-diameter 

trees (e.g, < 30 cm diameter at breast height [DBH]) of shade-tolerant species 
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

(particularly white fir, Abies concolor (Gordon & Glend) Lindl ex Hildebr.) in the 

understory than there were formerly, and far fewer larger-diameter (e.g., ≥ 75 cm 

DBH), older trees (Barbour et al. 2002). Moreover, the proportion of the landscape 

in montane chaparral has diminished, having been converted to forest (Nagel and 

Taylor 2005). Both of these changes indicate a reduced proportion of the landscape 

in both early and late-successional stages relative to the pre-Euro-American Tahoe 

basin landscape. The majority of the landscape is single-aged, fire-suppressed, 

second-growth fir and pine, a condition that would not exist historically. 

Knowledge Gaps

The departure from historical landscape conditions in the Tahoe basin resulting 

from timber harvest and fire suppression has contributed to a reduction in 

terrestrial biodiversity, as well as apparent increases in fire risk (Weatherspoon 

and Skinner 1996). Debate exists regarding the extent of old-growth forests that 

historically occurred in the basin. A better understanding of historical old-growth 

reference conditions could help in assessing how the basin has changed over time, 

what those changes represent in terms of accompanying plant and animal diversity, 

and what targets might be set for future conditions in terms of the extent and 

condition of old-growth forests in the basin (Manley et al. 2000).

Old-growth mixed-conifer stand in the Upper Truckee watershed, Lake Tahoe basin (2006).

Seth Bigelow
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GENERAL TECHNICAL REPORT PSW-GTR-226

Central questions for Lake Tahoe basin forest managers pertain to how much of 

various forest conditions is desired, and how those conditions should be distributed 

spatially to ensure the persistence of associated species, functions, and services. A 

well-defined vision for desired conditions can be used to design forest management 

and speed the transition to desired conditions; alternatively, forest management 

lacking such a vision can impair progress toward desired conditions. Agency 

managers in the basin have identified species of special concern and interest. For 

example, species of special concern identified in the Pathway planning process that 

have an association with old-growth forests include northern goshawk (Accipiter 

gentilis), American marten (Martes americana), pileated woodpecker (Dryocopus 

pileatus), and California spotted owl (Strix occidentalis). Other species of concern 

associated with old-growth components include osprey (Pandion haliaetus), bald 

eagle (Haliaeetus leucocephalus), and black bear (Ursus americanus). The population of brown-headed cowbird (Molothrus ater), another species of concern based 

on its potential ecological impact, is also affected by forest management practices. 

Coyote (Canis latrans) and black bear are species of high public interest in the basin 

(Manley et al. 2000); their populations are likely to change in response to forest 

management, and given their status as top carnivores in the basin, changes in their 

populations are likely to precipitate changes in wildlife community composition 

and structure (Crooks and Soule 1999). 

There are many areas in the basin that cannot support certain forest structural 

conditions owing to physiographic constraints, such as slope, aspect, elevation, 

and soil depth, which affect vegetation growth rates and disturbance regimes 

(Taylor and Skinner 1998, Urban et al. 2000). Vegetation growth models can help 

define how those structural conditions will change over time across the basin, but 

a concrete understanding of how landscape configuration constrains the basin’s 

vegetation communities is needed. 

Disturbance fundamentally shapes forest structure and species composition in 

the Lake Tahoe basin. Current management activities with the greatest potential to 

affect old-growth forest vegetation conditions and landscape configuration are fuel 

treatments, fire suppression activities, and salvage logging. Wildfire, avalanches, 

and landslides are the most common natural disturbances shaping forest structure 

in the basin. Although we now understand the fundamentals about the role of fire 

in maintaining historical vegetation structure in the basin (Scholl and Taylor 2006, 

Taylor 2004, Taylor and Beaty 2005), relatively little is known about the role of 

avalanches and landslides, and their interactions with fire regime. Avalanches can 

break up landscape-level fuel continuity, and conversely, forested areas diminish 
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

avalanche risk (Kattelman 1996). A better understanding of the basin’s natural 

disturbance regimes and their interactive effects would aid forest restoration efforts. 

Recreation activities, both motorized and nonmotorized, can greatly affect the 

occurrence and abundance of wildlife species and thereby the structure of animal 

communities. Thus, recreation represents an added source of disturbance to wildlife, and its management is relevant to achieving desired forest conditions.

Research has begun to examine how forest restoration planning should consider 

a changing climate regime (Harris et al. 2006). Most research suggests that by 

the year 2070, a mean increase of two to five degrees centigrade in June–August 

temperatures will manifest in the Western States (Running 2006). This dramatic 

temperature change, and attendant changes in the hydrologic cycle, will predispose the basin to more extensive and intense wildfires (Taylor and Beaty 2005, 

Westerling et al. 2006) and change the distribution and interactions among plant 

and animal species. This means that desired-condition decisions and associated 

management strategies now informed and shaped by historical reference conditions 

also could be informed by current and projected future climatic conditions and 

disturbance regimes. 

These issues and uncertainties translate into the following broad management 

questions:

• What stand conditions should management create to ensure that forest 

health and resilience is restored in the future?

• How much and where should various stand conditions be located throughout the basin to ensure that populations and communities of native plant 

and animal species are maintained?

• What key measures of stand conditions and landscape configurations will 

be most effective and efficient in monitoring forest ecosystem health and 

informing forest management? 

Research Needs

In the subsequent research needs sections, and figures 6.1 and 6.2, research questions are identified by combined text and numeric codes. Text codes are defined 

as follows: OG for old-growth and landscape management; FR for fire and fuels 

management; FM for fens and meadows; A for aspen; R for riparian areas; LM for 

lakeside, beach, and marsh; LT for Lake Tahoe aquatic ecosystems; OE for other 

aquatic ecosystems; UR for urbanization; RE for recreation; and CL for climate 

change. Numbers refer to the sequence of questions presented in each subtheme. 

Bold codes indicate the most time-critical research needs. 
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GENERAL TECHNICAL REPORT PSW-GTR-226

Following are the old-growth and landscape management research questions:

(OG1) What more can we learn about pre-Euro-American settlement (prior to 1850) 

characteristics of forests in the Lake Tahoe basin with respect to plant species composition; diameter distribution of trees, snags and logs; and proportional representation of seral stages? How did these characteristics differ according to topographic 

position (slope, aspect, and elevation), longitude, and soil substrate? What is the 

relationship between historical stand structure and composition, and existing map 

products depicting “potential natural vegetation?” 

(OG2) Does the condition of the pre-Euro-American settlement forests in the Tahoe 

basin represent a satisfactory model for forest restoration (i.e., desired future condition), and if not, how should it be modified to account for factors such as climate 

change and irreversible changes in land use? What are the projected changes in 

range and elevation of dominant tree species within the Tahoe basin owing to 

climate change? 

(OG3) How did the historical disturbance regime (e.g., fires, landslides, avalanches, 

insect outbreaks) differ spatially, in intensity and extent, within the Tahoe basin? 

How did these disturbances shape the structure and composition of the forest? Did 

upper and lower elevation zones exhibit different spatial patterns of disturbance and 

resulting structure?

(OG4) What animal species are most closely associated with old-growth forests 

in the basin, and what are the relative effects of different stand conditions and 

landscape configurations on the persistence of these species and biodiversity, 

with particular emphasis on special-status species? How do closely associated 

species use old-growth stands, compared to other available areas, for foraging, 

shelter, dispersal, and reproduction, and what are the most favorable amount and 

configuration of forested conditions to support forest biological diversity and 

special-status species? 

(OG5) What were and are the effects of historical logging and fire suppression 

on forest-associated wildlife species, including composition, abundance, 

co-occurrence, and diversity?

(OG6) What are the likely spatial changes in range and elevation of sentinel animal 

and plant species (i.e., species that are sensitive indicators of change) within the 

Tahoe basin in response to climate change?
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

(OG7) What elements of old-growth forests are key to maintaining their biological 

diversity (including density of large trees, basal area, stand contiguity, tree age 

structure, or standing or fallen large woody debris)? What is an effective set of 

indicators of the physical and biological conditions of old-growth forests?

(OG8) What performance measures—including presence and abundance of plants 

and animals, and other ecological community metrics—can be used to assess the 

effectiveness of efforts to restore historical (or achieve desired) old-growth forest 

structure, composition, and function?

(OG9) What landscape features and locations (e.g., dispersal/migration corridors) 

play key roles in maintaining populations within the basin, and what species or 

measures can serve as indicators of the function of these key features? 

Fire and Fuels Management 

Fire was undoubtedly the most pervasive agent of ecological disturbance in the 

Lake Tahoe basin prior to its settlement by Euro-Americans in the latter half of the 

1800s. Reconstructions of the presettlement fire regime from cross-dated fire scars 

in old stumps and logs have shown that the historical fire-return interval ranged 

from an average of about 11 years in Jeffrey pine (Pinus jeffreyi Grev. & Balf.) 

and white fir forests (Taylor 2004), to 28 years in montane chaparral stands (Nagel 

and Taylor 2005), to 76 years in high-elevation red fir (Abies magnifica A. Murr.) 

and western white pine (Pinus monticola Douglas ex D. Don) forests. Fires then 

consumed surface fuels, thinned forest stands, and produced openings in the forest 

where shade-sensitive tree species could regenerate. Although historical fires were 

likely predominantly of low severity, the presence of extensive patches of montane 

shrubs in some areas indicates that stand-replacing fires of higher severity also 

occurred (Nagel and Taylor 2005). 

Forests that developed under fire suppression after extensive logging in the 

Tahoe basin during the middle and later 1800s are now very different than those 

historical forests. Tree densities, particularly in smaller size classes, are now much 

higher, and species composition has shifted to favor firs over pines (Barbour et 

al. 2002, Taylor 2004). The abundance of trees and lack of fire return has led to 

unnaturally high amounts of surface fuels (Barbour et al. 2002), and greater fuel 

continuity, contributing to high fire hazard and greater probability of stand replacement upon burning (Manley et al. 2000, McKelvey et al. 1996, Skinner and Chang 

1996). 
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Knowledge Gaps

Reducing surface and ladder fuels using prescribed fire or mechanical treatments 

has been shown to substantially improve the resilience of forest stands to wildfire 

(Agee and Skinner 2005, Pollet and Omi 2002); however, because of the importance 

of tourism, forest proximity to populated areas, and concerns about protection 

of natural resources, fuel management in the Lake Tahoe basin presents unique 

challenges. Smoke and liability issues may limit the use of prescribed burning in 

many areas. As a result, fuels in these areas are often treated mechanically or by 

hand, rather than through burning. Unfortunately, the extent to which mechanical 

forest treatments can mimic the ecological role of fire is poorly understood for 

many forest attributes (Weatherspoon and Skinner 2002). Because much of the 

excess forest biomass in the basin is in the form of small trees of low value, 

mechanical removal may not be cost-effective. As a result, new implementation 

strategies for reducing fire hazard, such as mechanical mastication or chipping, 

have been initiated. These methods leave the fuels on site, but alter their vertical 

profile, and have been shown not to result in soil compaction or erosion—yet 

concerns about fire effects such as soil heating, if the material should burn, remain 

(Busse et al. 2005, Hatchet et al. 2006; also see chapter 5, “Soil Conservation”). 

Even in areas where prescribed burning is a viable management option, smoke 

management and the narrow window available for prescribed burns in many years 

severely limit the number of acres that can be treated. Most fires in the Lake Tahoe 

basin historically occurred in the late summer or fall (Taylor 2004), and managers 

have often opted to conduct prescribed burns at that time of year; however, recent 

research has found that early-season prescribed burns, which typically consume 

less fuel, may have some benefits for at least the first burn in areas with heavy fuel 

loading (Knapp et al. 2005). Not only was the recovery of understory plant species 

more robust following early-season burns, but tree mortality was lower in earlyseason burns than late-season burns (Thies et al. 2005), although not significantly 

(Schwilk et al. 2006). Fire disturbance may also promote the invasion of exotic species (Keeley et al. 2003), and Merriam et al. (2006) and Kerns et al. (2006) reported 

a greater abundance of exotic plant species after late-season burns.

Forest fuels treatments can change forest habitat attributes required by many 

wildlife species, including vertical layering of vegetation, age structure of trees, 

tree composition, spatial distribution of remaining trees, snag and log densities and 

characteristics, and understory cover and species composition. The intensity and 

extent of fuels treatments and their objectives (i.e., only fuels reduction or some 

balance of ecological outcomes) can greatly differ among agencies and projects; 

thus, the magnitude of effects of treatments on plants and animals is directly related 
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to the intensity and extent of treatments. Treatments designed with fuel reduction 

as the primary objective tend to simplify and homogenize forest structure and 

composition. Further, they may extend impacts associated with urbanization farther 

into the forest by functionally extending edge effects. Simplified forest structure 

as observed in urban forest remnants (Heckmann et al. 2008) exhibited reduced 

biological diversity and ecosystem resilience (Manley et al. 2006; Sanford et al., in 

press; Schlesinger et al. 2008). Long-term environmental changes associated with 

fuel treatments are less certain, and will differ based on the combination of overstory treatments and postharvest treatments (chipping, pile and burn, prescribed 

burns) applied. 

The impact of fuel treatments on plant and animal populations in the Tahoe 

basin is not known because of the general lack of information on the distribution 

and status of wildlife and plant populations in the basin, and the unique combinations of understory treatment (including chipping), large extent of actions, and 

rapid implementation of treatments being employed in the basin. Populations of 

many forest-associated species, particularly those associated with the montane 

zone, could be at risk of habitat fragmentation and isolation as a result of forest 

alterations.

Prescribed pile burning as a means to reduce excess forest fuel loads. Tahoe Pines, Lake Tahoe basin. 

Peter Goin
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These issues and uncertainties suggest the following broad management 

questions:

• What vegetation management approaches will be most effective and efficient in meeting a variety of management objectives, including reducing 

fire hazard, restoring forest health, increasing the amount and integrity of 

old-growth forests, maintaining and conserving biological diversity, and 

restoring a more natural fire regime?

• What locations are the highest priority for management and what balance of 

objectives are most appropriate in each location and throughout the basin?

• What measures are most informative and efficient in determining the effectiveness of vegetation management approaches in meeting fire and fuels 

management objectives? 

Research Needs

Following are fire and fuels management research questions:

(FR1) How do current fuel treatments and future treatment scenarios simultaneously affect fire hazard and other values such as scenic and recreational amenity, 

water yield and quality, soil erosion, old-growth characteristics, and plant and 

animal diversity (including less-abundant species, narrowly distributed species, 

and forest and aquatic associates)? What are the effects of spatial distributions of 

fuel treatments on primary ecological management objectives in the basin, including (a) connectivity of populations of species expected to be most sensitive to 

changes in forest structure and understory conditions; and (b) maintaining quality 

habitat for aquatic species? 

(FR2) Are there fuel treatment solutions that are optimal with respect to the 

multiple forest management objectives that exist in the basin (see question FR1), 

including considerations of cost? (This question could be addressed within a 

multiobjective modeling framework; the quality of the answers would depend at 

least in part on data from the kinds of field studies outlined below and elsewhere 

in this plan.)

(FR3) How do sensitive and vulnerable animal species associated with montane 

forests and aquatic inclusions (e.g., ponds and streams) use treated (masticated 

versus prescription-burned) and untreated areas to meet various needs (e.g., reproduction, foraging, movement, and shelter)?

(FR4) What are the projected consequences of current and projected fuel treatments 

for landscape connectivity for sensitive and vulnerable animal species?
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(FR5) How does intensity of tree canopy thinning affect a range of ecosystem attributes? Is there a relationship between residual canopy cover after fuel treatments, 

and subsequent rates of surface and ladder fuel development? Do canopy openings 

and soil disturbance from fuel treatments favor establishment of shade-intolerant 

pine species? Is there a relationship between residual canopy cover and wildlife 

habitat value?

(FR6) How do alternative understory fuel treatments (e.g., canopy thinning followed by biomass removal, mastication and mulching, or prescribed burning) 

affect the trajectory of forest succession, including understory plant and animal 

species composition, relative abundances, and ecological community states and 

transitions? Do these treatments differ in resultant opportunities for invasive plant 

establishment? (It is recommended that the definition of forest succession include 

tree, shrub, herb, and grass plant forms, and that measurements include rate of fuel 

reaccumulation so that fire hazard can be calculated.)

(FR7) Mastication followed by mulching is a dominant mode of treatment of 

understory fuels currently used in the Lake Tahoe basin. Is the longevity of fire 

hazard reduction produced by mastication treatments related to vegetation type, 

resprouting potential, microenvironment, or chip depth? What are the ecological 

consequences of mulching compared to other treatment options? Will multiple 

cycles of treatment with mastication result in the buildup of unacceptably high 

amounts of surface fuels? 

(FR8) How do alternative techniques for prescribed burning that are currently in 

use in the Lake Tahoe basin (jackpot, piling, understory, and piling with understory 

burns) compare in terms of fuel consumption and fire hazard, soil heterogeneity, 

wildlife responses, and wildlife habitat?

(FR9) What are the ecological consequences of season of treatment (early or late) 

when applying fuel treatments, such as mastication, mulching and prescribed 

burning? Important response variables might include mortality of remnant trees, 

resprouting of shrubs, and germination of species that have seed banks, and effects 

on small mammals and birds. 

(FR10) What is the relative importance of ozone damage, soil depth, periodic 

drought, insect attack, and stand density in determining spatial patterns and 

temporal dynamics of tree mortality and subsequent surface fuel accumulation? 

What is the optimal range of temporal and spatial dynamics of tree mortality 

based on current and future climate conditions?
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(FR11) What fuel treatments, if any, are most appropriate for the higher elevation 

forests in the Tahoe basin? What are the considerations for protecting against the 

spread or reducing the prevalence of root rot in red fir through fuel treatments?

(FR12) How effective are current fuel treatments in altering fire behavior, improving fire suppression effectiveness, and reducing fire severity, under the range of 

fire-weather conditions likely in the Lake Tahoe basin?

(FR13) What performance measures—including presence and abundance of plants 

and animals, forest structure and composition, and other biotic metrics—can be 

used to assess the effects and effectiveness of fuel treatment success at various 

times after treatment? 

Special Communities Management

Biological diversity in the Lake Tahoe basin is a composite of species and the 

ecological communities of which they are members. Ecological communities 

that proportionally dominate the landscape typically are the primary focus of 

management. The Lake Tahoe basin, however, supports a number of classes or 

types of ecological communities that are limited in geographic extent but have 

great functional importance: among these are meadows, fens, aspen stands, 

riparian areas, and lakeshore marsh and beach communities. 

These communities support disproportionately large numbers and a high 

diversity of animal and plant species, and some serve as nodes linking upland 

ecosystems and Lake Tahoe. Each community has particular threats to its 

integrity. Ecological communities and species with high conservation value 

are addressed individually, including their status and the composite of potential 

effects of management activities. 

Aspen

Quaking aspen (Populus tremuloides Michx.) occurs in the Lake Tahoe basin 

in riparian areas, bordering meadows, as stand-alone groves in snow pockets or 

avalanche paths, or as disjunct patches interspersed with conifer forest (Shepperd 

et al. 2006). Aspen stands support high plant diversity relative to surrounding 

vegetation (Potter 1998), and use less water than conifer forests of equivalent 

area (Gifford et al. 1984). Many authors contend that in the semiarid West, aspen 

is second only to riparian habitats themselves in terms of the biodiversity they 

support and in importance as wildlife habitat. Aspen stands typically support a 

greater diversity and abundance of birds, mammals, and invertebrates than adjacent 

vegetation types ( DeByle 1985, Flack 1976, Salt 1957, Schimpf and MacMahon 
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1985). For example, several bird species have a strong affinity with aspen, including 

northern goshawk, red-naped and red-breasted sapsuckers (Sphyrapicus nuchalis/

ruber), dusky flycatcher (Empidonax oberholseri), warbling vireo (Vireo gilvus), 

Swainson’s thrush (Catharus ustulatus), and MacGillivray’s warbler (Oporornis 

tolmiei) (Finch and Reynolds 1988, Flack 1976, Heath and Ballard 2003, Richardson 

and Heath 2004, Salt 1957). Several mammal species also show affinities for aspen, 

including ungulates such as mule deer (Odocoileus hemionus), rodents such as 

pocket gophers (Thomomys), voles (Microtus), shrews (Sorex), and mountain beaver 

(Aplodontia rufa) (Beier 1989, Coggins and Conover 2005, Loft et al. 1991). The 

invertebrate communities associated with aspen in the Sierra Nevada are not well 

studied, but in Rocky Mountain National Park, 33 of 49 resident butterfly species 

were found in aspen, and 7 of those were unique to aspen forests (Chong et al. 

2001). 

Knowledge Gaps

In the absence of disturbance by fire, conifers have heavily encroached upon most 

aspen stands in the Lake Tahoe basin. Encroachment of conifers into aspen stands 

can have negative impacts on herbaceous cover, stand moisture, and invertebrate, 

mammal, and bird species richness and abundance. Many species of plants, birds, 

mammals, and invertebrates benefit from the thick herbaceous layer and deep leaf 

litter typical of aspen stands that experience periodic disturbance. In a recent inventory and assessment effort by the U.S. Forest Service, approximately 68 percent 

of aspen stands were designated as being at moderate to extremely high risk of 

extirpation (Shepperd et al. 2006). Restoration of decadent aspen stands elsewhere 

in the northern Sierra Nevada has met with considerable success (Jones et al. 2005). 

Information on the value of aspen in supporting animal populations in the Tahoe 

basin is still limited, but the few local studies that have been conducted suggest 

healthy herbaceous communities and limited conifer intrusion may be the optimal 

habitat condition for at least aspen-associated breeding birds (Richardson 2007, 

Richardson and Heath 2004). Clearly, approaches to managing aspen in the basin 

will directly affect many plant and animal species. 

These issues and uncertainties suggest the following broad management 

questions:

• Where and to what ecological condition should aspen stands be restored in 

the Lake Tahoe basin?

• What is the desirable extent, configuration, and distribution of aspen stands 

(patches) that will assure ecological benefits to wildlife and co-occurring 

vegetation?
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• What management actions can contribute to restoring and sustaining aspen 

stands in the Lake Tahoe basin?

Research Needs

Following are aspen research questions:

(A1) How well can we map and predict aspen existence from currently available 

methods (e.g., satellite imagery)? How well can stand condition be assessed with 

these methods, compared with ground surveys? What variables best predict the 

occurrence of plants of concern (e.g., physiographic, woody debris, indicator species, soil types, hydrologic regimes)? 

(A2) What was the historical versus the current ecological status of aspen communities and associated plant and animal populations? How have these communities 

changed in the absence of periodic disturbance from fire? What stand attributes 

(e.g., stand area, species composition) are critical to maintaining populations of the 

most closely associated species?

(A3) What management tools and actions can be identified that will best facilitate 

conversion of conifer forest to desired aspen conditions?

(A4) How does aspen restoration affect associated plant and animal populations, 

and ecological communities? Are species and communities responding to restoration efforts as expected?

(A5) What performance measures—including presence and abundance of plants 

and animals and other ecological metrics—can be used to assess treatment effects 

and effectiveness in restoring aspen biological diversity and ecological function and 

monitoring conditions over time?

Riparian Areas

Riparian areas support high diversities of plant and animal species owing to the 

presence of water, diverse vegetation composition and structure, and abundant 

food resources. Many riparian areas in the Lake Tahoe basin were degraded from 

overuse in the late 1800s, but current problems stem largely from lack of fire 

combined with the legacy of historical channel alterations. 

Knowledge Gaps

Riparian areas have been mostly excluded from forest fuel treatments because 

of concerns about soil disturbance resulting in nutrient and sediment deposition 

into streams and ultimately into Lake Tahoe. The limited management activity in 

proximity to stream riparian areas (also known as Stream Environment Zones or 
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

SEZs) has resulted in the invasion of shade-tolerant conifers into many riparian 

areas. Conifers are thought to compete strongly with riparian vegetation (Haugo 

and Halpern 2007, Jones et al. 2005, Lang and Halpern 2007, Stam et al. 2008). 

Consequences of the lack of fire in riparian habitats include a greater density of 

small-diameter trees and an overabundance of small woody debris in some areas. 

There are concerns that altered conditions in riparian areas translate into higher 

risk of high-intensity fire in these areas, substantially increasing sedimentation 

and nutrient inputs to Lake Tahoe. There also is potential for fire from lower elevations to expand into higher elevations via riparian corridors despite aggressive 

upland fuel treatment efforts. In addition, one special status species—the mountain 

beaver—is most closely associated with riparian areas, so riparian management is 

likely to directly affect the mountain beaver. The lack of information on the historical and current status of riparian ecosystems, including the status of associated 

plant and animal species, impedes determination of the ecological characteristics of 

natural community recovery, desired conditions, and opportunities for habitat and 

stream restoration. Management in these zones could be carried out with greater 

confidence if more information existed regarding historical vegetation structure 

and composition, and riparian area disturbance regimes. 

Riparian habitat along the Upper Truckee River, Lake Tahoe basin.

Scott Hinton
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These issues and uncertainties suggest the following management questions:

• What is the extent and condition of riparian ecosystems in the Lake Tahoe 

basin, and what conditions should management attempt to create through 

available techniques, including the use of fire?

• What measures are most informative and efficient in determining the condition of riparian ecosystems and their potential responses to management 

and environmental factors?

Research Needs 

Following are riparian research questions: 

(R1) How well can we map riparian vegetation using currently available methods 

(e.g., satellite imagery), and what is the current location, extent, and condition 

of riparian vegetation in the basin based on these methods? How effectively 

can riparian condition be assessed using these methods, compared with ground 

surveys? What variables best predict the occurrence of plants of concern (e.g., 

physiographic variables, woody debris, indicator species, soil types, or hydrologic 

regimes)? 

(R2) What was the historical versus the current ecological status of riparian plant 

and animal communities in the basin? What was the historical role of fire frequency 

and intensity in shaping riparian-area composition and structure in the basin? What 

was the historical composition and structure of vegetation in riparian areas, including the density of standing and downed woody debris?

(R3) Are riparian systems recovering naturally from historical anthropogenic 

disturbances? The need exists for a system to objectively classify riparian vegetation and its condition, compile and assess stream and wetland restoration efforts in 

the basin, review the efficacy of stream and wetland restoration techniques that are 

in use, and develop a system for assessing success of riparian restoration projects.

(R4) Does stream restoration have desired effects on riparian habitat and associated 

plant and animal species? How does restoration involving fire or fuel treatments 

differentially affect species richness or abundance? 

(R5) What is the distribution and abundance of the mountain beaver population in 

the Tahoe basin, with what habitat features are they most closely associated, and 

how can their populations be most efficiently monitored?

(R6) What performance measures—including presence and abundance of plants 

and animals and other ecological metrics—can be used to assess treatment effects 

and effectiveness in maintaining, restoring, and rehabilitating riparian biological 

diversity and ecological function, and to monitor conditions over time?
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Fens and Meadows

Fen and wet meadow communities are tightly linked to water-table attributes 

(Allen-Diaz 1991, Castelli et al. 2000, Kluse and Allen-Diaz 2005) and soil water 

chemistry (Atekwana and Richardson 2004, Bartholome et al. 1990). Many species 

of plants and some animal species, such as butterflies, fossorial mammals (e.g., 

gophers, moles, and marmots), meadow nesting bird species (e.g., willow flycatcher 

[Empidonax traillii] and mountain bluebird [Sialia currucoides]), and soil macroinvertebrates, are restricted to fens or meadows, which themselves are susceptible 

to impacts from human activities in the Lake Tahoe basin. 

Knowledge Gaps

Past land uses, including grazing and water diversions, have resulted in degraded 

resource conditions. Approximately half of the basin’s meadows have been permanently lost, fragmented, or altered in critical physical and biotic characteristics 

owing to these disturbances (Cobourn 2006, Elliot-Fisk et al. 1997). Grazing is no 

longer prevalent in meadows in the Lake Tahoe basin, but there may be substantial 

legacies of this former major land use (particularly altered plant and animal species 

composition), similar to circumstances elsewhere in the Sierra Nevada (Dull 1999). 

Meadows and fens also suffer current impacts primarily from recreation 

activities, which can result in soil compaction, desiccation owing to incision of 

streambeds, and conifer encroachment (Martin and Chambers 2004). Recreational 

activities in meadows primarily consist of hiking, biking, cross-country skiing, and 

snowmobiling, with some motorcycle and all-terrain vehicle (ATV) activity. These 

activities can have both direct and indirect negative impacts on plants and animals. 

Hiking, mountain biking, and off highway vehicle (OHV) use leads to proliferation of trails in heavily used areas, causing fragmentation and soil compaction and 

erosion. Trail use also disturbs many wildlife species, leading to increased stress or 

decreased foraging time, which may have negative consequences for survival and 

reproduction. Snowmobile use is prevalent in meadows during the winter (and on 

established routes through the forest). Snowmobile use compacts the layer of snow 

close to the ground where small mammals, particularly voles, move during winter, 

and commonly damages vegetation. Mammalian carnivores and raptors (including bobcat [Lynx rufus], northern goshawk, and bald eagle) tend to be sensitive 

to vehicle use, but also may use compacted snow for travel, changing the spatial 

pattern of their movements and predation. Preliminary results from recent research 

suggest summer and winter OHV use does not affect the probability of use of an 

area by marten, a species of concern in the Tahoe basin (Zielinski and Slauson 

2008). 
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Although we have a basic understanding of general cause-effect relationships 

between recreation and plant and animal responses, the information is not specific 

enough to inform the development of management thresholds. It is not clear which 

species are most impacted by recreation, the ecological and social consequences 

of those impacts in the basin, and how growing numbers of visitors may exacerbate those effects. Two special-status species are closely associated with fens and 

Meadow-stream complex, Angora Creek, Tahoe Paradise, Lake Tahoe basin.

Scott Hinton
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meadows: mountain yellow-legged frog (Rana muscosa) and willow flycatcher. The 

only robust population of mountain yellow-legged frogs in the basin is located in a 

fen (see “Special Communities” for more details). 

Stream restoration may reverse some losses of meadow habitat, and reconfigurations of channels may allow streams to meander more, and carry water to 

a greater area. Similarly, where some streams meet roads, they have historically 

been forced through a single culvert; planned additional culverts will increase the 

area “watered” by a stream (e.g., at Blackwood Creek). These restoration efforts 

may expand meadow habitat; influences on these populations could be detected by 

monitoring before and after restoration.

These issues and uncertainties suggest the following management questions:

• Where are the Tahoe basin’s fens and meadows located, and what are their 

current conditions?

• What management actions can contribute to restoring and sustaining fens 

and meadows in the basin?

• What measures are appropriate to assess the condition of fens and meadows 

and efficacy of management actions?

Research Needs 

Following are fens and meadows research questions: 

(FM1) Where are fens and meadows located in the Tahoe basin, and what are their 

current ecological characteristics and conditions? How important is water chemistry and ground-water hydrology in establishing and maintaining fen conditions?

(FM2) What are appropriate reference conditions and historical conditions for fens 

and meadows in the Lake Tahoe basin?

(FM3) How do current and potential future management and restoration practices 

in fens and wet meadows, including application of fire or fire surrogates, affect 

their susceptibility to invasion by unwanted plant species?

(FM4) How well do predictive models of meadow recovery, with and without 

restoration, apply to the Lake Tahoe basin circumstances? Which meadows should 

be used to validate these models, and what data need to be collected? How should 

meadows be assigned in a priority scheme for restoration?

(FM5) How are fens and meadows impacted by current disturbances, including 

water use, fire suppression, recreation, and beaver activities? Which meadows are 

most critical to maintaining populations of meadow-dependent species in the basin?
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(FM6) To what extent do recreation-associated impacts (both direct and indirect) 

in meadows change composition, abundance, and behavior of wildlife species? 

Do some species seasonally avoid meadow and riparian habitat because of 

snowmobiles, bike or foot traffic, or dogs?

(FM7) What performance measures—including presence and abundance of plants 

and animals and other ecological metrics—can be used to assess conditions, assess 

the effects and effectiveness of efforts to restore or rehabilitate meadow biological 

diversity and ecological function, and monitor conditions over time?

Lakeside Marsh and Beach Habitats

Marsh and beach habitats in the Lake Tahoe basin are limited in number and extent. 

The largest marshes occur in the southern part of the basin in association with the 

mouth of Upper Truckee River, Trout Creek, and Tallac Creek. Marshes provide the 

only suitable habitat for a large number of species in the basin, including many species of waterbirds (Manley et al. 2000). Beaches are numerous around Lake Tahoe, 

but they are limited in extent, particularly in years of high lake levels. 

Knowledge Gaps

Lakeside marsh and beach habitats have had their historical hydroperiods altered 

by the damming of the lake’s outlet. This has had adverse effects on Tahoe yellow 

cress (Rorippa subumbellata Rollins) (Pavlik and Murphy 2002), caused changes in 

marsh plant communities (Kim and Rejmankova 2001), hindered recent attempts to 

restore marsh habitat destroyed by lakeside housing developments,7 reduced populations of waterbirds, and may have fostered encroachment by lodgepole pine (Pinus 

contorta Douglas ex Louden) into lakeside areas. 

Tahoe yellow cress is a low-growing, perennial species endemic to the shores 

of Lake Tahoe. The species is listed as endangered by both states, is considered 

endangered or threatened by the California and Nevada Native Plant Societies, and 

is a candidate species for listing under the Endangered Species Act. The species 

has been the focus of a conservation strategy for the past 4 years, with the goal of 

restoring a self-sustaining metapopulation. Lack of access to certain privately held 

lakeshore areas has made it difficult to know whether this goal is being achieved. 

Additional uncertainty comes from lack of knowledge of seed bank dynamics, seed 

and rootstock longevity and dispersal, and genetic relationships among core and 

satellite populations.

7 Hunter, J. 2008. Personal communication. Senior ecologist, EDAW, Inc., 870 Emerald Bay 

Rd., South Lake Tahoe, CA 96150.
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Dairy Meadows in autumn, Taylor Creek watershed, South Lake Tahoe, California.

Peter Goin

Waterbirds (including ducks, shorebirds, and rails) are special-status species 

that find their primary habitat in lakeside marshes. Their populations have fallen in 

response to the loss of much of Pope Marsh to development in the 1960s (Manley 

et al. 2000). The TRPA has conducted surveys of key marshes around the lake 

for the past 7 years, and their findings are summarized in the Pathway planning 

documents.8

These issues and uncertainties suggest the following management questions:

• What management actions will contribute to restoring and sustaining 

desired ecological values and biodiversity in Lake Tahoe’s lakeside marsh 

and beach habitats?

• What ecosystem attributes should be subjected to monitoring to assess the 

effectiveness of management actions directed at lakeside marsh and beach 

habitats?

Research Needs

Following are lakeside, beach, and marsh research questions: 

(LM1) For shoreline plants of concern, does the spatial extent of existing populations support life-history requirements (including access to pollinators, disturbance 

regimes, seed dispersal)? What environmental factors most affect the persistence, 

8 Kelchlin, E. 2007. Personal communication. Wildlife biologist, Tahoe Regional Planning 

Agency, 128 Market Street, Stateline, NV 89449.
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extent, and reproductive success of populations at a given site? Are there genetic 

strains of shoreline plants that are more robust to environmental stressors, thus 

conferring enhanced survival? 

(LM2) What is the ecological status of marsh habitats in the basin, and what 

measures can be taken to retain and restore their ecological integrity?

(LM3) What performance measures—including presence and abundance of plants 

and animals and other ecological metrics—can be used to assess treatment effects 

and effectiveness in maintaining, restoring, and rehabilitating the biological diversity and ecological function, and to monitor conditions in marsh and beach habitats?

Aquatic Ecosystem Restoration 

The ecology of the aquatic ecosystems within the Lake Tahoe watershed has been 

altered dramatically over the last two centuries. Most of our knowledge of historical 

change has focused on alterations to Lake Tahoe itself; however, lakes, streams, 

and meadows within the upper watershed also have been altered resulting in the 

increased need to manage these ecosystems. In this section, we differentiate Lake 

Tahoe and other aquatic ecosystems to assist in interpreting the change and research 

needs for these distinctive ecosystems. We focus specifically on alterations from 

eutrophication, potential changes owing to atmospheric loading of nitrogen, and the 

influence of nonnative species (plant and animal) on the restoration or management 

of native biota. 

Lake Tahoe

Prior to large changes in community structure and conditions of nutrient loading 

brought about by human activities, Lake Tahoe’s community assemblage was relatively simple with 12 orders of zoobenthic taxa, 6 zooplankton species, and 8 fish 

taxa (Chandra 2003, Frantz and Cordone 1996, Juday 1906, Miller 1951, and Vander 

Zanden et al. 2003). The benthic invertebrate community supported one endemic, 

wingless form of stonefly. Beginning in the mid to late 1800s, species introductions 

combined with landscape disturbances started to alter the lake’s biology. 

The preinvasion food web (circa 1872) was dominated by a single predator, 

Lahontan cutthroat trout (Oncorhynchus clarki, subspecies henshawi), which 

fed primarily on pelagic tui chub (Siphateles bicolor pectinifer) and zooplankton 

(Chandra 2003, Juday 1906, Vander Zanden et al. 2003). Forage fishes obtained 

energy from a mix of benthic and pelagic primary production sources. By 1939, 

cutthroat trout were extirpated from Lake Tahoe, and a lake trout (Salvelinus 

namaycush) population replaced them as the top predator (Cordone and Frantz 
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1966). Three primary reasons for the demise of cutthroat trout were predation from 

introduced lake trout, the degradation of spawning stream habitat from increased 

siltation owing to watershed deforestation (Moyle 2002), and the hybridization 

of cutthroat trout with rainbow trout owing to hatchery propagation.9 There have 

been several attempts to reestablish both fluvial (stream form) and lacustrine (lake 

form) cutthroat populations in the Tahoe basin, all of which failed. As part of the 

U.S. Fish and Wildlife Service recovery plan for cutthroat trout (Coffin and Cowan 

1995) in its native range, efforts have begun to restore cutthroat in Fallen Leaf 

Lake, located in the southern end of the basin. 

Crayfish (Pacifastacus leniusculus) were introduced multiple times into Lake 

Tahoe and were established by 1936; they are now found in large numbers (55 

million in the late 1960s and 230 million by early 2000 (Abrahamsson and Goldman 1970, Chandra and Allen 2001). Studies suggest that, under low densities (0.16 

adult/m2), the crayfish stimulate periphyton productivity by removing old senescent 

cells (Flint 1975). Today, crayfish do not contribute to the energetics of nonnative 

lake trout except for the largest size classes (>50 cm). 

Chandra et al. (2005) investigated the effects of cultural eutrophication on the 

coupling between pelagic primary producers and benthic consumers in Lake Tahoe. 

At depths where ambient light levels equal 1 percent (which have shifted with time 

from 50 to 85 m), pelagic primary producer and zoobenthic consumer coupling 

was positive. Historically, the zoobenthos from this depth zone obtained 32 percent 

of their energy from phytoplankton sources; after 43 years of eutrophication, they 

obtained 62 percent of their energy from those sources. A simple model indicated 

increased pelagic production and resultant export of matter, combined with the 

loss of benthic primary production, has contributed to the change in zoobenthos 

energetics. Recent samplings of zoobenthos during 2008–09 suggest there may be 

a 50 to 80 percent loss in benthic invertebrate density and biomass in Lake Tahoe. 

Furthermore, there has been a substantial decrease in the density of native, endemic 

invertebrates such as the blind amphipod and more cosmopolitan invertebrates such 

as oligochaete worms (Chandra and Caires 2001). Whether this loss is due to a shift 

in pelagic to benthic coupling or from alterations to Mysid shrimp is unclear (see 

below).

The establishment of the invertebrate Mysis relicta, corresponded with shifts 

in the trophic niches of forage fishes (chubs) and the top predator lake trout, and a 

feeding shift of lake trout to pelagic energy sources. The resultant increase in lake 

trout may have increased predation rates on native forage fishes and decreased 

9 Cordone, A.J. 2007. Personal communication. Fisheries biologist, retired. California 

Department of Fish and Game, 1416 Ninth Street, Sacramento, CA 95814.
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their abundance (Vander Zanden et al. 2003). Growth rates of lake trout before and 

after mysid introduction do not appear to have changed, except in the smaller size 

classes. Post Mysis invasion studies in Lake Tahoe showed impacts on other biological components of the lake. A strong restructuring of the zooplankton community 

as a result of Mysis predation on native cladocerans occurred, shifting the lake’s 

pelagic environment to a Mysis and copepod-dominated system. Furthermore, 

modeling and empirical measurements suggest mysids may be influencing the 

carbon dynamics at the sediment-water interface as they feed in the deep part of the 

lake during the daytime and resuspend sediment particles through excretion during 

the nightime as they migrate to the pelagic zone (Chandra 2003). Thus the insertion of Mysis into the middle of the food web played a strong determining role in 

restructuring upper trophic level energetics, and in disrupting community dynamics 

in the middle and lower parts of the food web. Their role and impact at lower depths 

is unclear; however they may be playing a role in disrupting carbon dynamics in the 

deepwater and pumping particles back into the water column.

In the mid to late 1970s, and again in the late 1980s, a variety of nonnative 

species were discovered in the near-shore environment, primarily driven by the 

establishment and expansion of nonnative aquatic plants, which provided habitat 

and refugia for nonnative fishes. The warm-water fish introductions were illegal 

and thought to be the result of anglers eager to catch these fish. During that period, 

in the Tahoe Keys, a major rearing area of native fishes, warm-water fish species 

were rarely found, whereas native minnows remained abundant as evidenced by a 

snapshot sample obtained in 1999; however, by 2003, largemouth bass (Micropterus 

salmoides) were common, whereas redside shiner (Richardsonius balteatus) 

and speckled dace (Rhinichthys osculus) populations declined or were virtually 

eliminated from the Tahoe Keys (Chandra 2009, Kamerath 2009). The change in 

fish structure was substantiated by fishing guides operating out of the Tahoe Keys: 

within a decade they could no longer collect the minnows that were commonly 

used as bait by fishing charters on the lake. 

Until 1994, no lakewide surveys for rooted aquatic macrophytes had been 

conducted in efforts to document the presence of nonnative species. Early reports 

(1975) of water milfoil species near Taylor Creek did not identify the species of 

Myriophyllum, nor were vouchers or photographic records made. However, severe 

impacts from aquatic plants were observed in the Tahoe Keys by the end of the 

1970s and early 1980s, during which time mechanical harvesting was begun. 

Recent studies have documented the role of some of the invasion pathways and 

vectors (boats and boat trailers) for aquatic plants that are transported both to and 

away from Lake Tahoe (Wittmann 2008). These vectors contribute to issues of 
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continued reinfestation and potential new infestations of nonnative aquatic plant 

species.

In 2008, established populations of the nonnative bivalve species, the Asian 

clam (Corbicula fluminea), were discovered in the southeastern portion of Lake 

Tahoe by University of California, Davis researchers during regular near-shore 

periphyton surveys. Asian clams were first detected in Lake Tahoe in very low 

numbers at Timber Cove in 2002 (3 to 20 clams/m2—Hackley et al. 2008), and at 

Nevada Beach in 2003.10 Extensive field surveys during summer 2008 revealed 

much higher densities of Asian clams (50 to 3,000 clams/m2), suggesting evidence 

for local population growth and possible reintroduction from external populations. 

Asian clams in Lake Tahoe compete with other local native molluscan species, such 

as the montane pea clam (Pisidium spp.) and the ramshorn snail (Planorbidae). Its 

current known distribution (area ~1 million m2) is patchy along the southeast shore 

of the lake from Zephyr Cove to El Dorado Beach, and is rapidly expanding and 

colonizing a variety of physical circumstances.

Knowledge Gaps

Lake Tahoe’s ecological community has changed through the elimination of native 

trout, restructuring of food web energy flow, and introduction of species that occur 

in both the limnetic and littoral zones. It is unclear, however, how some of these 

introduced species are impacting near-shore and offshore water quality as well as 

native fish biomass and production. In addition, three special status species are 

primary participants in the Lake Tahoe food web—Lahonton cutthroat trout, osprey 

(Pandion haliaetus) and bald eagle. Populations of osprey and bald eagle are likely 

to be affected by changes in the relative and absolute abundance of fishes in Lake 

Tahoe.

Mysis shrimp are the lake’s dominant macrozooplankton, exhibiting a large 

(up to 400 m) diel vertical migration to the lake bottom (Rybock 1978). While on 

the bottom, mysids feed on sediment detritus and may actively pump detritus and 

nutrients into the lake’s limnetic zone (Chandra 2003). Research from other ecosystems suggesting mysids are supported by benthic detrital energy sources is supported by a number of studies (Lasenby and Lanford 1973, Lasenby and Vanduyn 

1992, Lester and Mcintosh 1994, Song and Breslin 1999, Viherluoto et al. 2000). 

Many ecotoxicological studies have determined that Mysis ingest heavy metals 

and organochlorines directly from sediment (Lasenby and Vanduyn 1992, Lester 

10 Herbst, D. 2003. Personal communication. Aquatic invertebrate ecologist, University of 

California Sierra Nevada Aquatic Research Laboratory, HCR 79, P.O. Box 198, Mammoth 

Lakes, CA 93546.
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and Mcintosh 1994, Song and Breslin 1999), and serve as a vector for contaminant 

transport to the pelagic zone. Gut content information also suggests mysids may 

derive a substantial amount of their energy from benthic resources, including 

zoobenthos and organic-rich sediment particles (Lasenby and Lanford 1973). 

The effect of warm-water invasive species on the native fish community 

and the potential for recycling nutrients in the near-shore habitat are important 

uncertainties. Recent surveys suggest warm-water fish such as bass (Micropterus 

salmoides), bluegill (Lepomis macrochirus), and catfish (Ameiurus nebulosus) 

are found around the lake. Recent assessment of their distribution indicated the 

densities are still very low compared to other ecosystems and can be variable over 

time. Overall the densities were low around most of the lake with higher densities 

in some locations such as Meek’s Marina and intermediate densities in the Tahoe 

Keys. Anecdotal observations indicated that bass may be in open water areas of the 

lake; however, it is unclear the extent to which bass have established in these areas 

or if they were moving through migration zones before they reach more enclosed 

sites such as marinas and embayments (Chandra et al. 2009). Although preliminary 

research suggests these fish are competing and/or preying upon native fishes in 

the near shore (Kamerath 2009), the role that nonnative crayfish and other physical 

factors may play in controlling warmwater fish establishment as well as recruitment 

around the lake remains unclear. Crayfish are a preferred food source for bass in 

their native habitats. Currently, the invasive crayfish in Lake Tahoe seem to have 

expanded in population since estimates were first made in the 1970s, with over 230 

million individuals in the lake estimated in 2001 (Chandra and Allen 2001). Current 

models that predict warmwater fish distribution (Chandra et al. 2009, Kamerath 

2009) do not account for crayfish as a food resource in the lake and how they may 

contribute to bass growth and maintenance. Furthermore, using Lake Tahoe fishes 

researchers have found that ultraviolet light penetration may control recruitment 

of nonnative fishes and allow the persistence of native fishes.11 Thus, there may 

be direct ties between the lake’s clarity and the distribution of warmwater fishes. 

These two resource controls of food availability and physical light constrains should 

be incorporated into existing models predicting warmwater fish establishment in 

the lake to refine areas for monitoring as well as management.

The recent invasion of the near-shore area by warm-water species such as bass 

species could lead to the remobilization of nutrients in this habitat. Examination 

of seasonal nutrient availability is recommended, particularly during low flow 

periods, to determine the biological contribution of nutrients to near-shore 

11 Williamson, C. 2009. Personal communication. Professor in ecology, Miami University, 

Department of Zoology, Pearson Hall 158, Oxford, OH 45056.
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production and lake clarity. The interactions between native and nonnative plants 

also are poorly understood. With the continued expansion of Eurasian water milfoil 

(Myriophyllum spicatum), and the newly expanded populations of curly leaf 

pondweed (Potamogeton crispus), the uncertainties of fish/plant interactions are 

even more complex. 

Eutrophication of Lake Tahoe has led to a shift in energy flowing to the 

bottom of the lake (Chandra et al. 2005). It is unclear, however, if increased 

coupling between pelagic to benthic energy flows along with carbon alteration 

due to mysid shrimp are altering benthic invertebrate community structure and 

production—an issue of particular importance when trying to manage native, 

benthic biodiversity such as the endemic, wingless stonefly (Capnia lacustra) 

or blind amphipod (Stygobromus sp.) and in evaluating the potential for the 

reintroduction of native species. 

Environmental impacts resulting from Asian clam establishment in Lake Tahoe 

related to water quality, benthic community structure and production, and the 

potential for the facilitation of invasion of other near-shore invasive species through 

habitat disturbance and localized increases in nutrient concentrations are uncertain. 

In particular, the Asian clam (1) excretes elevated levels of nitrogen and phosphorus 

into the water column and sediment substrate (Wittmann et al. 2008)—which can 

promote increased algal growth; (2) is able to filter extremely high volumes of water 

(Vaughn and Hakenkamp 2001)—potentially impacting both water quality and 

pelagic communities including Lake Tahoe sports fisheries; and (3) can increase 

levels of calcium through the concentration of dead shell matter—providing potential substrate and appropriate biochemical conditions for the establishment of other 

nonnative bivalve species such as the quagga (Dreissena rostriformis bugensis) and 

zebra mussel (Dreissena polymorpha). Current knowledge about the Asian clam 

in Lake Tahoe is limited because of the short time since its discovery. Continued 

efforts to assess the life history, environmental impact, and distribution, and to 

identify possible control and management actions in Lake Tahoe are underway.

Finally, little is known about the ecology and nutrient dynamics of Emerald 

Bay. This bay is an important destination for recreational boaters from various 

parts of the lake and particularly the Tahoe Keys, where most of Lake Tahoe’s 

nonnative species issues currently reside. At least 8 nonnative species have been 

observed in Emerald Bay including but not limited to Eurasion watermilfoil, largemouth bass, catfish, Mysid shrimp, lake trout, crayfish, Asian clam, and kokanee 

salmon. Efforts to integrate and assess the limnology and food web ecology of 

this bay are recommended as it is likely that future invaders will establish in this 

location owing to the amount of propagule pressure occurring through boat traffic, 
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warmer temperatures, and increased productivity. Furthermore, due to its isolated 

nature and increased productivity compared with Lake Tahoe, this may be an 

important area for restoring the native Lahontan cutthroat trout.

These issues and uncertainties suggest the following key management 

questions:

• What management actions are necessary to restore and sustain a desired 

food web in Lake Tahoe, and will those actions be consistent with efforts to 

reverse declines in the lake’s clarity? 

• What are the appropriate measures of management and restoration program 

actions to assess their effectiveness in meeting ecosystem objectives? 

Research Needs

Following are Lake Tahoe aquatic ecosystem research questions: 

(LT1) What is the interaction between nonnative and native species in the basin, 

and how does this affect our ability to manage native biodiversity?

(LT2) What is the linkage between habitats (i.e., profundal-pelagic, littoral-pelagic) 

for carbon, phosphorus, and sediment transport particularly with the introduction of 

nonnative species? How does this affect Lake Tahoe water quality and clarity and 

native benthic invertebrate biodiversity?

Kokanee spawning, Taylor Creek, South Lake Tahoe. 

Peter Goin
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

(LT3) What is the seasonal role of mysid shrimp in controlling native plankton 

and benthic invertebrate populations and reducing water clarity through the 

transport of benthic nutrients and sediment particles into the water column? 

Ideally, research would focus on understanding the life cycle, contemporary 

feeding behavior, and the role mysid shrimp may play in reducing water clarity 

in Lake Tahoe. 

(LT4) Can we predict future invaders (plant or animal) and the potential impacts 

to the lake’s water clarity or biodiversity?

(LT5) Will current limnological characteristics support the establishment of nonnative species or the potential recovery of native fish populations in Emerald Bay? 

(LT6) What is the variability of benthic algal production and does this affect 

near-shore production and clarity? Will nonnative species alter this production? 

Future research is recommended to examine the production of benthic algae and 

invertebrates such as invasive crayfish to determine if eutrophication is affecting 

ecological community structure.

(LT7) What is the status of osprey and bald eagle populations in the basin, and 

how do their distribution, abundance, and productivity track changes in fish 

populations in Lake Tahoe?

(LT8) How can we best ensure the survival of native fish and other desired 

aquatic vertebrates? What stressors are affecting native species, and what can be 

done to lessen negative impacts? 

(LT9) How can we restore native fishes and other aquatic vertebrates to the lake? 

What portions of Lake Tahoe are best suited to reintroduction efforts for native 

species? How will established nonnative species likely affect the success of 

restoration efforts?

(LT10) How can water quality and water clarity be protected from the effects of 

introduced species and human activities? What aspects of water quality and clarity are at most risk? What management actions might contribute to minimizing 

negative impacts from those sources?

(LT11) What is the current distribution of the Asian clam in Lake Tahoe, and 

what are its ecosystem-level impacts? How does it impact near-shore quality and 

the potential facilitation of the invasion of other nonnative aquatic species?
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Other Aquatic Ecosystems in the Lake Tahoe Basin

Much of the research carried out in the Lake Tahoe basin has focused on understanding the impacts of watershed development, nutrient loading, water quality, 

and aquatic ecology in Lake Tahoe itself. Very little effort has been placed on 

evaluating the response of other lakes, streams, and other aquatic habitat types to 

the array of human disturbances affecting them, including ground disturbance, 

increased inputs from atmospheric pollution, and the impacts of nonnative species 

(fish, amphibians, plants) introductions. 

The aquatic ecosystems research that has been conducted has been short term 

(seasonal or one-time assessments), primarily owing to funding constraints. For 

example, Marlette, Cascade, Fallen Leaf, Echo, and Spooner Lakes all have been 

evaluated for one or all of the following constituents: nutrient status (e.g., phosphorus, and nitrogen), basic physical and chemical measurements (e.g., dissolved 

oxygen, temperature, and pH), pelagic primary production, and zooplankton composition and biomass (e.g., Lico 2004, Reuter et al. 1996; University of California, 

Davis and University of Nevada, Reno 2003, 2006). In 2006, an effort was made 

to assess the nutrient concentrations via depth profiles and limitation (nitrogen, 

phosphorus, or co-limitation) for Upper and Lower Echo, Upper and Lower Angora, 

Fallen Leaf, Tahoe, Eagle, Spooner, and Marlette Lakes. Results showed the pelagic 

primary production in five lakes (Tahoe, Marlette, Fallen Leaf, Lower Echo, Lower 

Angora) was co-limited; Spooner Lake exhibited possible nitrogen limitation, and 

data from Eagle Lake were inconclusive (Chandra and Rost 2008).

Other research has focused on the ecology of some of the small lakes in the 

Tahoe basin. For example, Cascade Lake has a biological assemblage that closely 

resembles that of Lake Tahoe prior to the introduction of the two nonnative species (Mysis relicta and lake trout, Salvelinus namaycush). Vander Zanden et al. 

(2003) presented a food web structure for this lake in 2001, finding that hybridized 

cutthroat trout are a dominant predator feeding on pelagic energy sources (e.g., 

zooplankton). Since 2001, researchers from the Universities of California-Davis, 

Nevada-Reno, Wisconsin-Madison have been monitoring the biology and general 

limnology (nutrients, chlorophyll a) during spring, summer, and fall in Fallen Leaf 

Lake. This lake experienced the reintroduction of native Lahontan cutthroat trout 

(information on the bioenergetics, historical changes to the lake’s fishery, and 

limnology can be found in Allen et al. 2006). The California Fish and Game and 

U.S. Forest Service have also attempted to control nonnative brook trout populations on an annual basis to promote the persistence of cutthroat trout in the Upper 

Truckee River and Meiss Meadows watershed. This effort of more than 10 years has 

promoted the recovery of native trout; however, the effect of removal of book trout 
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on life history characteristics of cutthroat trout (including growth, survival, and 

condition) has not been evaluated on a regular basis. Limited food web and genetic 

information has been obtained for Stony Ridge and Gilmore Lakes. 

Most amphibian species in the basin are primarily associated with standing 

water bodies. Pacific treefrog (Hyla regilla), long-toed salamander (Ambystoma 

macrodactyla), and western toad (Bufo boreas) are all primarily associated with 

standing water, although the two frogs are also found in streams. Stocking nonnative fish creates large populations of predators that prey on larval amphibians. 

The U.S. Forest Service has conducted surveys of lakes throughout the watershed to 

determine the presence of fishes, amphibians, snakes, and waterbirds over the last 

decade. The limited distribution of most amphibian species has led to an analysis 

of genetic diversity by University of California, Davis, University of Nevada, 

Reno and U.S. Forest Service researchers12 of three species: long-toed salamander, 

western toad, and mountain yellow-legged frog. These data are being analyzed to 

better inform restoration efforts and promote amphibian populations that have been 

shown to be in decline in the Sierra Nevada.

Based on research to date in and near the Lake Tahoe basin, nonnative trout are 

likely to be a primary factor limiting the distribution and population size of native 

amphibians there (Knapp and Matthews 2000, Manley and Lind 2005). Although 

fish stocking has been discontinued on the California side of the basin, it continues 

on the Nevada side. Some streams in the California side of the basin have been designated “Wild Trout Areas” and are not (officially) stocked with nonnative fishes. 

It is not clear to what degree this management response benefits amphibians and 

stream-associated reptiles. Studies in the Sierra Nevada have shown that, without 

intervention, decades are required for trout populations to decline once stocking has 

ceased (see Knapp et al. 2001). Fish stocking could potentially benefit garter snake 

populations, as they can prey on fry. Bullfrogs (Lithobates catesbeianus) are also a 

potential threat to amphibian populations in the basin; however, they currently have 

a limited distribution, primarily in the mouths of streams in the southern basin. The 

number of sites occupied is fairly low, but where they exist, their populations are 

large and affect the native fauna (Manley and Lind 2005). 

Stream channel restoration is an active pursuit in the Lake Tahoe basin. Stream 

restoration and surveys are commonly conducted by the U.S. Forest Service as part 

of managing the national forest. Surveys have been conducted for most streams 

12 Manley, P., and Lind, A., research wildlife biologists, USDA Forest Service, Pacific 

Southwest Research Station, Sierra Nevada Research Center, Davis, CA; Shaffer, H.B., 

professor, University of California-Davis, Davis, CA; Peacock, M., assistant professor, 

University of Nevada-Reno, Reno, NV; and Vredenburg, V., research associate, University 

of California-Berkeley, Berkeley, CA. 
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in the basin over the past 10 years by the U.S. Forest Service; stream habitat types 

are mapped as are occurrences of fish and amphibian species. Stream restoration 

has been actively pursued by the U.S. Forest Service and the California Tahoe 

Conservancy for the past 5 years; in many cases, that work includes before and after 

measurements of responses of plant and animal species, including aquatic, riparian, 

and upland associates.13 The geomorphologic and water quality elements of these 

efforts are addressed in chapter 4, “Water Quality.”

In addition to biological threats, lakes and streams face physical degradation 

as well. Firefighting often involves the collection of water from lakes to deposit on 

the fire; associated siphoning activities can potentially directly affect amphibian 

populations. An evaluation of the ecological value and sensitivity of various water 

bodies in the basin has not been conducted; thus activities such as siphoning may 

occur in areas where impacts could be high (e.g., Watson Lake). 

Development has been responsible for the loss and fragmentation of marshes in 

the southern portion of the basin, specifically the Tahoe Keys development in the 

Upper Truckee Marsh. Surveys are being conducted to assess how development of 

this marsh has affected water birds and to evaluate the potential to restore affected 

species. Impacts to the physical condition of lakes, ponds, and marshes also are 

occurring in the basin, such as shoreline compaction and pollution from human 

uses. Anglers and hikers appear to have the greatest impact on the shoreline and 

nearby upland areas around existing lakes and ponds. The most common impacts 

include compaction of soil and removal of vegetation around the shoreline; however, 

some paved and dirt roads exist extremely close to shorelines creating the potential 

for erosion and conveyance of polluted runoff. Research has shown that degraded 

shoreline conditions can have a negative effect on the presence or abundance of 

aquatic species that occupy a site (Manley and Lind 2006). 

Knowledge Gaps

In the last decade a watershed management approach to restoration activities in 

the Lake Tahoe basin has dominated. Although nutrient limitations and their shifts 

have been studied in Lake Tahoe (Jassby et al. 1994), it is unknown how atmospheric nitrogen loading has shifted nutrient limitation in the other lakes in the 

basin watershed (see chapter 3, “Air Quality”). Understanding the nutrient limitation is critical if we are going to promote the persistence of native fish or amphibian 

species in these ecosystems. It also is important for us to understand the degree to 

which nonnative fish are limiting amphibian populations in small lakes, and what 

13 Romos, S. 2008. Personal communication. Science and evaluation program manager, 

Tahoe Regional Planning Agency, 128 Market Street, Stateline, NV 89449.
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

options exist for effective reductions in these negative interactions. Understanding the movement patterns of nonnative trout, including barriers and distribution 

mechanisms, would greatly inform effective options for conservation and restoration of native species. We still lack information on the habitat associations and 

population dynamics of Pacific treefrog and the two aquatic-system-associated 

garter snakes. Population models and spatially explicit landscape evaluations of 

habitat conditions and values have not been developed for any amphibian or aquatic 

snake species. Management agencies are considering attempting to reintroduce the 

mountain yellow-legged frog into multiple locations in the basin; additional assessment and evaluation are recommended to establish an information-rich foundation 

for a reintroduction plan. 

Uncertainties and concerns exist for native fish populations, as well. Restoration of native trout has been initiated at Fallen Leaf Lake. It is important to 

follow the effect of this restoration effort on all aspects of the lake’s ecology and 

limnology. In particular, measurements are recommended to determine the lake’s 

responses—nutrient, primary and secondary production—to the reintroduction. 

Overstocking of native trout in the lake, for example, could lead to trophic cascades 

and either increase or decrease the lake’s clarity. Most appropriately, this study 

effort would occur throughout the life cycle of the trout or until they are extirpated 

from the lake. Beyond the Lahonton cutthroat trout, little information exists about 

the status of native fishes (e.g., sculpin or redside-shiner). 

These issues and uncertainties suggest the following key management 

questions:

• Which lakes and other aquatic systems should receive priority management 

attention, and what actions should be undertaken to restore desired ecosystem values to each?

• What spatial and temporal strategy of restoration and management actions 

can be employed to maximize learning to inform future management decisions in aquatic systems with like conservation needs?

• What monitoring targets and sample techniques will best support adaptive 

management of Lake Tahoe’s aquatics systems and their biota?

Research Needs 

Following are other aquatic ecosystem research questions: 

(OE1) What are the limiting factors of production for other lakes in the Tahoe 

basin? Do variations in limitation affect secondary production and the ability to 

support fish and amphibians?
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(OE2) What are the ecological and limnological impacts of native fish reintroduction into Fallen Leaf Lake? What are the long-term changes to the lake owing to 

introductions and alterations to the lake’s biota? What impediments (e.g., stream 

habitat, or secondary production) need to be overcome to produce a self-sustaining 

population of native trout in the lake?

(OE3) What was the historical progression of occupancy of lakes and streams by 

nonnative fishes, and how does it correspond to changes in the distribution and 

abundance of native aquatic fauna?

(OE4) What is the status of populations of amphibians and aquatic snakes in the 

basin, including habitat needs, population dynamics (e.g., metapopulation structure), prevalence of disease (particularly chytrid fungus) and distributions that are 

important to maintaining or restoring populations?

(OE5) What is the distribution and abundance of native fishes in lakes and streams, 

and what factors regulate their populations?

(OE6) What is the chemical and physical status of lentic ecosystems in the Tahoe 

basin (other than Lake Tahoe), including measures of nutrients and pH?

(OE7) What performance measures—including macroinvertebrates, presence and 

abundance of plants and animals, and other ecological metrics—can be used to 

assess the condition and restoration effectiveness in maintaining, restoring, and 

rehabilitating the biological diversity and ecological function, and in monitoring 

conditions of lake and stream ecosystems?

(OE8) What is the limnological and ecological status of Star Lake and how has it 

changed in response to human stressors?

Urbanization

The urbanization of natural landscapes is a substantial factor in the erosion of biological diversity in the United States (Hansen et al. 2005, Theobald 2005). Urbanization imposes a suite of stressors for ecological communities, including habitat 

loss, alteration and fragmentation, reduced soil quality, increased soil erosion, water 

and air pollution, introduction of nonnative species, and human disturbance, all of 

which have negative consequences for native species (Baxter et al. 1999, Donnelly 

and Marzluff 2006, Fernández-Juricic 2000, McDonnell and Pickett 1990, McKinney 2002, Miller et al. 2003, Miller and Hobbs 2000, Pouyat et al. 1994, Steinberg 

et al. 1997). Urbanization can lead to lower diversity (structure and composition) of 

native plants and animals, losses of vulnerable species, and increases in exotic and 

generalist species. After the resource extraction era in the Lake Tahoe basin ended 
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in the early 1900s, many wildland areas in the lower elevation montane zone began 

undergoing conversion to urban uses, with subsequent changes in the amount and 

quality of habitat for wildlife. However, if well managed, it is thought the basin’s 

urbanized areas could maintain much of their native plant and animal diversity. 

Knowledge Gaps

Numerous impacts of urban development on plant and animal communities have 

been documented in the Tahoe basin (e.g., Heckmann et al. 2008, Manley et al. 

2006, Schlesinger et al. 2008). Nonetheless, many important uncertainties remain 

regarding the relative role of various urban-related stressors, such as habitat loss, 

fragmentation, or alteration, in affecting negative changes in population viability 

of species of concern or community integrity. The dynamic nature of native 

forest communities makes balancing social, ecological, and economic objectives 

a challenge (Folke et al. 2005). Stressors associated with urbanization act at both 

local and landscape scales; understanding the individual and interacting effects 

at multiple scales is key to managing future urban and recreational growth in a 

manner that conserves and maintains biological diversity and ecological integrity 

of native ecosystems. 

Wildlife—

Land development for housing, commercial enterprises, and infrastructure 

decreases the amount and changes the distribution and quality of habitat for 

wildlife. Habitat quality for wildlife species also may be affected by forest and 

fire management practices in and near urban areas, which can in turn lead 

to structural and compositional changes in those forests (see “Fire and Fuel 

Management” and “Old-Growth and Landscape Management” sections in this 

chapter). Wildlife species most likely to be negatively affected by these changes are 

those that are primarily associated with lower elevation montane forests, and those 

that have large area requirements and small populations in the Tahoe basin, such 

as northern goshawk, California spotted owl, spotted skunk (Spilogale putorius), 

and bobcat (Lynx rufus). Passerine bird species that are associated with old-growth 

forests or the understory habitats of older forests also may suffer population 

declines. 

Recent research conducted by Manley et al. (2006) has identified a number of 

species, species groups, and community metrics that respond to various aspects 

of urbanization, including development and human activity. They studied birds, 

small mammals, large mammals, ants, and plants. In general, birds and large 

mammals were most negatively affected by development, followed by individual 

species of small mammals and ants. Understory bird species were most sensitive 
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to surrounding development, as were mustelids and black bears. Coyotes showed 

no difference in frequency of occurrence with development, and domestic dogs 

were prevalent throughout all development areas. In contrast, few domestic cats 

were detected. Forest structure and composition did not change within undeveloped 

parcels in response to surrounding development, with the exception of lower 

snag and log densities and an increase in the richness of exotic plants with higher 

development. Not all relationships were linear; rather, in some cases sudden shifts 

in species abundance and composition were observed. But it is not known at what 

stage of development—earlier or later—that such responses may manifest. The 

study primarily identified patterns of richness and abundance, which suggest causeeffect relationships that can be confirmed and clarified through research focused on 

individual questions. 

In general, concentration of humans in urban environments leads to increased 

disturbance of wildlife habitats and mortality (from traffic and recreation), increased 

densities of exotics and domestic species (especially pets), and, in certain circumstances, habitat enrichment (including increases in food, cover, or water resources 

that can confer an advantage for certain species such as black bears and coyotes). 

Determining the site-specific impacts from high-intensity recreation and increased 

numbers of exotic species in the Tahoe basin’s urban forests is an outstanding 

information need. Pets and humans can contribute to the spread of exotic plants and 

diseases, with areas subjected to higher rates of human visitation at greater risk. 

Exotic plants pose a problem for wildlife species if they outcompete native plants 

that provide food or other essential resources, or if they lead to changes in habitat 

structure. Currently, the basin has few invasive exotic plants, so they do not pose a 

particularly high ecological risk to wildlife; however, effective measures to reduce 

the potential for future establishment of exotic plants are needed.

Habitat enrichment in the form of supplemental food and cover is varied 

but common in developed areas of the Lake Tahoe basin. It is likely that habitat 

enrichment has increased the prevalence of some bird species, coyotes, and black 

bears, and increased conflicts with humans (Manley et al. 2006). The effects of 

habitat enrichment on distributions and population sizes of these and other species 

are not clear. Habitat enrichment may lead to population growth in select species 

only in developed areas, or in the whole basin more widely, or it may simply cause 

shifts in animal species distributions, especially if animals abandon formerly 

suitable sites and move to urban areas. For example, Beckmann and Berger (2003a, 

2003b) found in a study of black bears in Lake Tahoe that urban bears had smaller 

home ranges and spent significantly less time foraging compared to wildland bears. 

Urban environments offer enriched and novel sources of food (e.g., garbage bins 
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and coolers) and cover (e.g., cabins and decks), making urban areas desirable for 

foraging and denning. Enriched environments typically have a greater carrying 

capacity than native ecosystems, with unknown long-term consequences.

Plants—

Forest structure and composition is affected by urbanization in the Lake Tahoe 

basin. Manley et al. (2006) found that on undeveloped forest fragments (most of 

which were >1 ha), snag and downed wood densities declined and exotic plant 

species increased with increases in the amount of surrounding development. 

McBride and Jacobs (1986) found even greater changes in the urban matrix, 

characterized by decreased tree density and cover and increased tree species 

richness and age-class diversity.

Exotic plant species are an immediate problem locally in certain areas of the 

Lake Tahoe basin. Elsewhere most exotic plants originally were introduced for 

horticultural uses by nurseries, botanical gardens, and individuals (Reichard and 

White 2001), but it is unclear whether plants used in horticulture are an important 

source of invasive species in the Lake Tahoe basin. Many of the established invasive plants in the basin (Donaldson 2004) appear to be plants that have spread into 

disturbed areas, particularly along roadsides, and that have no obvious horticultural 

application. 

The nutrient applications and water uses on residential and commercial landscapes can have adverse effects on local nutrient cycles, allowing nutrients in runoff 

and drainage to reach local water bodies (e.g., Bormann et al. 2001). Surprisingly, 

conventional turfgrass landscapes may retain applied nutrients better than multispecies landscapes that may have been designed for low nutrient and water inputs 

(Ericksen et al. 2001, 2005). Regardless of landscape type, having more knowledge 

about the nutrient status of landscape plants allows more efficient application of 

fertilizer (e.g., Scharenbroch and Lloyd 2004). 

Revegetation efforts on roadside edges are common projects within the Lake 

Tahoe basin, particularly because they are believed to decrease runoff and soil 

leaching. Not all revegetation projects have been successful; however, the use of 

locally adapted plant ecotypes may best support the steep elevation and dramatic 

precipitation gradients in the basin. In cases where native plant revegetation projects 

have been successful, there can be concerns about alteration of genetic structure of 

native plant communities (e.g., Gehring and Linhart 1992). 

Construction projects in the basin often occur close to large trees, and precautions are always taken to retain these trees as visual and ecological amenities. 

Regrettably, these trees often die prematurely, possibly from damage sustained by 
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Urban forest lot, South Lake Tahoe, California.

Peter Goin

roots during construction. Current practice is to protect the root zone that occurs 

inside the edge of the tree crown (the “critical root zone”), yet evidence from 

ponderosa pine (Pinus ponderosa Dougl. ex Laws.) excavations indicates that the 

maximum horizontal extent of conifer roots can be much greater than the crown 

edge (Berndt and Gibbons 1958, Curtis 1964, Greb and Black 1961, Hermann 

and Peterson 1969).

These issues and uncertainties suggest the following broad management 

questions:

• What aspects of urban development are most detrimental to conserving and 

restoring biological diversity and ecosystem integrity, and what facets of 

biodiversity are most vulnerable to urban stressors?

• What locations in the basin are most valuable for maintaining biological 

diversity, and what management approaches are most effective in minimizing the impact of urbanization?

• What risks do traditional landscaping and revegetation approaches pose to 

the introduction of nonnative native species to the basin, and what effective 

alternatives exist?
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Research Needs

Following are urbanization research questions: 

(UR1) What mechanisms determine observed declines in biotic diversity in the 

urban environments of the basin (e.g., habitat loss, habitat fragmentation, habitat 

alteration, disturbance, mortality from vehicles and pets)? 

(UR2) Are there threshold levels of development at which sweeping changes in 

wildlife species abundances and ecological community composition occur? 

(UR3) How does the current spatial pattern and extent of development affect connectivity of animal populations? Are there important areas (corridors, connectors) 

determined by the combination of fixed environmental characteristics (e.g., slope, 

elevation, or rock outcrops) and human development? 

(UR4) Which urban forest types and sites are most impacted by recreation and 

exotic species (including pets)? Impacts include creating functional barriers, 

ecological constraints, and limitations to habitat availability. Studies are needed to 

determine how these impacts can be mitigated. 

(UR5) Does habitat enrichment cause basinwide or local increases in geese (Branta

spp.), coyote, and bear populations, or shifts in their distributions? Is enrichment 

leading to changes in the survival, reproductive success or behavior (e.g., habitat 

use or response to humans) in these species? Are those changes likely to put those 

species or humans at demonstrably greater risk or otherwise affect area- and 

disturbance-sensitive species?

(UR6) Are roads serving as conduits for invasive species into the Lake Tahoe 

basin? Are plants used in residential and commercial landscaping contributing to 

invasive species problems in Lake Tahoe basin wildlands? 

(UR7) Which plant species, plant ecotypes, and planting techniques are best for 

enabling successful establishment of native species in disturbed roadside areas? 

What plants and planting techniques should be employed at the greater basin scale 

(e.g., by elevation and longitude), and at local scales (e.g., road shoulder versus 

exposed steep slope)? 

(UR8) Would increased use of water-efficient plants for residential and commercial 

landscaping result in lower demands on water supplies within the basin, and less 

runoff? Would use of nutrient-efficient plant genotypes for home and commercial 

landscaping result in lower fertilizer application rates?
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(UR9) What is the effectiveness of various conservation measures to maintain large 

trees in developed areas? What is the relationship between stem DBH and maximum horizontal extent of rooting for large trees (e.g., Jeffrey pine, sugar pine [Pinus 

lambertiana Douglas], incense cedar [Calocedrus decurrens Torr. Florin], red fir, 

and white fir) retained in developed areas of the Lake Tahoe basin? How should 

the critical root zone be designated for each species to preserve most surface roots, 

while acknowledging the realities of construction operations?

(UR10) What is an effective set of indicators—including plant, animal, and other 

ecological community metrics—that can be used to assess the effects and effectiveness of forest management efforts, and to monitor biological diversity in and 

adjacent to urbanized areas? How should urban parcels be prioritized for interventions to improve ecological function?

(UR11) What is the relative importance of potentially competing uses (e.g., reducing 

fire risks, or maintaining biological diversity) of urban lots in the urban-wildland 

interface? What are the tradeoffs among competing uses, both short and long term, 

including maintaining and restoring biological diversity?

Recreation

Outdoor recreation is a primary activity for residents and visitors of the Lake 

Tahoe basin. Many forms of recreation are available in the basin. In the summer, 

backcountry hiking, biking, mountain climbing, horseback riding, and fishing are 

popular. Activities on Lake Tahoe are numerous, including swimming, kayaking, 

sailing, speed boating, fishing, and jet skiing. Outdoor recreation is just as popular 

during the winter, including downhill and backcountry skiing, snowshoeing, and 

snowmobiling. 

Knowledge Gaps

Residents and visitors who hike or bike can disturb the activities of many vertebrate 

species, particularly species at higher trophic levels, such as northern goshawk, 

California spotted owl, American marten, and bobcat. Hiking and biking pose 

slightly different challenges and stresses to wildlife species—hikers have a 

longer residence time, thus having a greater impact on species sensitive to human 

presence, whereas bikes move quickly, posing a risk of physical impact, and 

some trails have a steady stream of users potentially posing barriers to wildlife 

movement. Dogs are common hiking companions in the Tahoe basin; they chase 

and sometimes kill wildlife species, particularly lower trophic-level species, such 

as mice, chipmunks, squirrels, and ground-dwelling birds.
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

Off-highway vehicle use in the Lake Tahoe basin during the summer and winter 

is restricted to relatively circumscribed areas; however, in the winter, snowmobile 

use can be widely dispersed in undesignated areas (e.g., the McKinney-Rubicon 

trail). Snowmobile use can affect resident wildlife species at times of their highest physical stress. The U.S. Forest Service recently completed route mapping for 

OHVs, and it is still evaluating designations. A study of the effects of summer and 

winter OHV use on American marten was conducted in the McKinney-Rubicon 

area, as well as at a southern study area on the Sequoia National Forest (Zielinski 

and Slauson 2008). Another study still underway is looking at community-wide 

responses of wildlife to summer OHV use, including study sites throughout the 

basin.14

Downhill ski areas have several potential adverse effects on wildlife: (1) forest 

losses and fragmentation (only shrub and grass layers remain on ski slopes), which 

affect late-seral associated species, such as American marten, northern goshawk, 

California spotted owl, and spotted skunk; (2) high human disturbance during 

daytime on ski slopes may create barriers to habitat use and between-habitat patch 

movement for diurnal species; (3) changes in forest cover and human disturbance 

may create “sink” habitat for American marten; (4) night lighting and grooming on 

14 Manley, P.; Campbell, L. 2008. Personal communication. Research wildlife biologists. 

USDA Forest Service, Pacific Southwest Research Station, 1731 Research Park Dr., Davis, 

CA 95618.

Ski slope in autumn, Mount Rose ski resort, Nevada. 

Peter Goin
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ski slopes may interfere with the behavior of nocturnal species; and (5) losses of 

snags in forested areas between ski runs owing to hazard tree removal can locally 

reduce wildlife habitat quality. In the Lake Tahoe basin, it is important to know the 

extent to which existing or potential ski resort expansions may affect the persistence of basin wildlife populations. 

The spatial extent of intensive cross-country skiing is limited, thus it does 

not appear to pose a major risk to wildlife. It is likely that although usage can be 

substantial locally, sufficient management structures are in place (including snow 

grooming, and bridges across streams), and monitoring to determine wildlife use in 

cross-country ski areas is probably the appropriate data-gathering investment at this 

time.

These issues and uncertainties suggest the following management questions:

• What recreational activities are the most detrimental to wildlife resources, 

and are there land management actions that can reduce impacts while 

accommodating those activities?

• What are appropriate measures of recreational impacts on wildlife, and how 

might those measures be integrated into monitoring programs? 

Research Needs

Following are recreation research questions: 

(RE1) What are the characteristics of key locations inhabited by animal species 

of concern that are sensitive to summer or winter recreation activities? Where do 

these key locations occur, for purposes of recreation planning and study design?

(RE2) What combination of summer recreation activities (motorized and 

nonmotorized; amount, timing, and location) and environmental factors 

present a risk of site abandonment by sensitive wildlife species? 

(RE3) What are the combined effects of snowmobile use (amount, timing, 

and location) in association with particular environmental factors that present 

a risk of site abandonment by resident wildlife species? 

(RE4) To what degree are dogs impacting wildlife populations and communities? 

(RE5) Are the locations of OHV routes (summer and winter) likely to pose biologically significant barriers to one or more species with large area requirements?

(RE6) Are existing ski areas predominantly occupied by male martens, and if so, 

does the extent of this population response pose a threat to the persistence of this 

species in the Tahoe basin?
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

(RE7) To what degree may existing and potential expansions of ski areas fragment 

the landscape mosaic for species that have large home ranges and are dependent on 

closed-canopy forest conditions for nesting, foraging, and movement?

(RE8) What tools can be developed to assess how best to manage recreation and 

habitats to reduce people-wildlife conflicts?

(RE9) What tools are most effective and efficient in measuring recreation use 

of various types in a manner that informs interpretations of effects on biological 

diversity and ecosystem function?

Climate Change

Conservation planners and managers have acknowledged the reality of climate 

change and incorporate expected changes into their land and resource planning 

efforts (McCarty 2001). Despite uncertainty in many aspects of climate predictions, there is widespread agreement that in California and Nevada, mean summer 

temperatures will increase, there will be more extreme heat events, residual summer snowpacks will decrease, and consequently the ranges of organisms that are 

restricted to higher elevations will shrink (Hayhoe et al. 2004, Kim et al. 2002). 

Disturbance regimes that are climate dependent also will be subject to changes. 

Fires in the Lake Tahoe basin are expected to be more frequent and intense under 

higher average temperature regimes (Taylor and Beaty 2005, Westerling et al. 

2006). Organisms will respond to these changes in species-specific ways, creating 

communities that may have no modern analogue (Ibáñez et al. 2006, Millar et al. 

2006). 

Knowledge Gaps

The span of elevations in a relatively small geographic area makes the Lake Tahoe 

basin particularly vulnerable to change in species distribution and abundance 

because of the limited amount of suitable habitat for many species. It also makes 

the basin a valuable test case for how plants and animals may respond to climate 

change. The first challenge is to obtain precise and accurate measurements of 

climatic conditions through meteorological monitoring stations. Currently, three 

weather-monitoring stations are located in the basin, and a Global Observation 

Research Initiatives in Alpine Environments (GLORIA) monitoring site was established on Freel Peak in 2006 (see http://www.gloria.ac.at/ for more information). 

The proper targets or desired conditions for ecosystem management are not 

obvious given the complex realities of organism responses to climate change 

(Harris et al. 2006). Research for such a contextual stressor as climate change 
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can be approached in multiple ways. One approach to answering questions regarding species responses to climate change is to build statistical models of species 

occurrences (as climate and soil envelopes) by relating present-day distributional 

ranges to climate and soil variables. The models are then applied to climate scenarios that have been generated by global circulation models or regional climate 

models (e.g., Ibáñez et al. 2006, Kueppers et al. 2005, Sala et al. 2001). An alternative approach, most suitable for intensive work on individual species, is to build 

process-based, mechanistic models of species responses to the environment, and to 

apply these models to climate scenarios. That approach may be particularly advantageous when multiple environmental values are being considered (e.g., carbon 

sequestration by trees) or there are strong feedback effects.

These issues and uncertainties suggest the following broad management 

questions:

1. What are the anticipated responses of wildlife, fish, and vegetation communities in the Lake Tahoe basin owing to climate change? What are the 

appropriate responses of land and resource managers to changes in these 

natural communities?

2. How can ongoing monitoring programs and research efforts be adjusted to 

provide the information necessary to allow managers and decisionmakers 

to integrate climate change into management planning and implementation?

Research Needs

Following are other climate change research questions: 

(CL1) How is climate changing in and around the Lake Tahoe basin? (Note: this 

basic information, coupled with question 2 above, can also be used to support 

research projects proposed in the “Water Quality” and “Soil Conservation” 

chapters)

(CL2) How is climate change predicted to change the elevational boundaries 

between ecosystem types (e.g., montane and subalpine forest, and subalpine 

and alpine zones) in the Lake Tahoe basin over the next 10 to 100 years?

(CL3) How is climate change predicted to change the ranges and populations 

of plant and animal species of concern over the next 10 to 100 years?

(CL4) What are an effective set of indicators of the physical and biological 

changes that may occur as a result of climate change?

(CL5) How might management practices be altered in response to the 

projected environmental effects of climate change?
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

Summary of Near-Term Research Priorities

The research needs identified under the ecology and biodiversity theme represent a 

coherent set of information that is needed to reduce uncertainties and improve the 

probability of achieving desired conditions for living resources and their habitats 

in the next 5 to 10 years. The near-term value of some information is greater in 

some cases than in others. In many cases, steps toward building a knowledge base 

are most efficient when pursued in a particular sequence. In other cases, individual 

pieces of information are highly valuable in their own standing in that they can 

positively contribute to meeting management objectives as soon as they become 

available. Research needs that match one or both of these situations are considered 

near-term research priorities, and given equivalent opportunities for funding, they 

are recommended for funding and implementation first. Near-term research needs 

and priorities are synthesized below.

Subtheme 1: Old-growth and landscape management—

The ultimate objective of forest management is to restore and maintain forest health 

and resilience such that forests maintain and restore their full range of functions 

and native biological diversity, and thereby retain their resilience and assocated 

ecosystem services upon which human communities in the basin depend. The 

primary uncertainty limiting management’s ability to meet this objective is a clear 

understanding of desired environmental conditions, and when and where to create 

them. Specific questions pertain to the historical amount and distribution of forest 

structural conditions, associated plant and animal species composition, and how to 

translate historical conditions into target conditions for the future that will enable 

forested ecosystems to adapt to future environmental stressors without the loss of 

function or biological diversity. Old forests are of particular concern and interest, 

because, despite the maturity of existing forests, it is apparent that extant forests 

have lost some ecological complexity associated with old forests, and therefore 

species and functions restricted to old forests are rare and most at risk from uninformed management. Finally, development of robust measures of forest biological 

diversity and resilience are recommended to enable simple and effective tracking of 

management progress and success. 

Subtheme 2: Fire and fuels management—

One of the greatest ecological risks associated with fire is the uncertainty associated with the effects of fuel reduction treatments—both their effectiveness in 

changing fire behavior, and the unintended consequences of treatment effects on 

other ecological conditions, such as biological diversity and forest ecosystem resilience. The near-term research needs pertain to addressing risks and uncertainties 
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posed by current management activities, which target fuel reduction treatments on 

tens of thousands of acres without a specific understanding of the ecological consequences. Therefore, near-term research needs include improving the understanding 

of the effects of various types and intensities of treatments on the spectrum of 

ecosystem management objectives, including but not restricted to fire behavior. Of 

primary concern is the fate of terrestrial species and processes, because they are 

directly affected by forest conditions. It would be most efficient to develop and test 

silvicultural prescriptions in the course of addressing near-term research priorities, 

as opposed to after ecological risks are more clearly understood. Once the primary 

ecological objectives at risk related to actions of reducing the threat of catastrophic 

wildfire are understood, it is important to develop simple and informative measures 

of their status for long-term monitoring. 

Subtheme 3: Special communities management—

The conservation and restoration of special communities in the basin are best 

served by similar sets of information: (1) maps of current location and condition 

throughout the basin, (2) reference conditions based on historical data and other 

relevant data sources, (3) evaluation of the effectiveness of restoration approaches, 

and (4) the development of performance measures to assess status and restoration 

effects. A few unique information needs are associated with individual special 

communities. In aspen communities, techniques for converting conifer-encroached 

stands back to aspen-dominated habitats is a primary information need. Fens and 

meadows are under an unknown level of threat from various human activities. 

Information on the current status of marshes is lacking. Finally, detailed information on genetic and environmental sensitivities of Tahoe yellow cress are needed to 

aid population restoration efforts of this endemic species. 

Subtheme 4: Aquatic ecosystem restoration—

The emphasis of aquatic ecosystem research is on conservation and restoration of 

vertebrate biota in Lake Tahoe and the conservation of species in the rich array of 

other aquatic ecosystems around the basin. In Lake Tahoe, the uncertainties with 

the greatest potential impact on management are those associated with the interactions between nonnative and native plant and animal species. These interactions 

have potential consequences for biodiversity, lake clarity, and near-shore aesthetics. 

Research on measures to control established nonnative species is urgently needed, 

and is best pursued through an adaptive management approach using information 

from careful assessment of pilot projects to guide longer term management strategies. Restoration of native fishes in Lake Tahoe presents a steep challenge, and 
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

information on the ecological interactions is key to making progress. The other 

aquatic ecosystems are in need of more basic information: (1) the status of vertebrate populations and communities, and (2) factors limiting the ability of sites to 

support native species. Once these kinds of information are developed and better 

understood, it would then be appropriate to develop efficient measures that can be 

used to track conditions over time. 

Subtheme 5: Urbanization—

Research recently conducted in the Lake Tahoe basin identified substantial and 

unexpected effects of urban development and human activities on various elements 

of biological diversity (Manley et al. 2006). The patterns observed differed by taxonomic group (i.e., birds, small mammals, mammalian carnivores, ants, and plants), 

among species (some were sensitive, whereas others were not), and by type of 

human disturbance (e.g., habitat loss, habitat alteration, habitat enrichment, or different types of human activities). The results of that work suggest the need to understand mechanisms of key responses such that development and management can be 

conducted in a manner that minimizes and mitigates negative effects on biological 

diversity. Questions of particular priority pertain to better understanding thresholds 

of change in habitat loss, habitat alteration, or habitat use indicated by past research, 

specifically changes observed at 30 to 40 percent development. Above this threshold, it is unclear what happens, but sites likely become sinks, traps, or abandoned 

by a wide array of species. In addition to site-scale mechanisms, landscape-wide 

modeling is needed to understand the implications of existing results and facilitate 

the rapid evaluation of the implications of new information on landscape design and 

management priorities. Finally, as in other subthemes, the development and testing 

of effective measures for use in monitoring and assessment are recommended, as 

vulnerable species and target ecological objectives in more urban areas are clarified. 

Subtheme 6: Recreation—

Recreational uses have been identified to have substantial effects on the occupancy 

and abundance of many and diverse species, based on multiple research projects. 

Alternatively, species thought to be impacted by certain recreational activities (e.g., 

effects of OHV use on occurrence of American marten) did not exhibit negative 

responses. Specific uncertainties in the Tahoe basin pertain to the effects of dogs 

on retaining biological diversity in more urban environments, and the effects of ski 

areas on montane obligates, namely American marten. Although population sizes 

of northern goshawk and bald eagle are limited, their sensitivity to the presence 
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of people has important implications for the management of events and ongoing 

recreational uses in the vicinity of known use sites. Effective measures of use and 

effects are lacking and their development will be important for monitoring. 

Subtheme 7: Climate change—

Climate change is perhaps the ultimate source of uncertainty, and arguably poses 

a high environmental and economic risk. Under such circumstances, information needs start at the basics. In the case of ecological elements and processes, 

this translates to applying existing and new information to modeling potential 

responses—plant and animal ranges and associated effects on population sizes, 

species interactions, and ecological services—to predicted or potential climate 

change and associated broad-scale environmental responses. It is important for 

modeling to be conducted in the basin, as opposed to relying on modeling outside 

the basin or at larger scales because detailed information will be needed to inform 

management agencies about how they can respond to potential threats. As with 

all other subtheme areas, as information is accrued, the development of effective 

and reliable measures of key population and community metrics for monitoring is 

recommended. 

Acknowledgments

A number of individuals contributed substantially to the development of this theme 

area. Malcolm North (USFS Pacific Southwest Research Station) contributed 

ideas and text to the old-growth and landscape management subtheme, as did 

Eric Knapp (USFS Pacific Southwest Research Station) to the fire and fuels 

management subtheme. William Richardson (University of Nevada, Reno), Brant 

Allen (Tahoe Environmental Research Center, University of California, Davis), 

and Bibit Traut (City College of San Francisco) contributed substantially to the 

special communities management and aquatic ecosystem restoration subthemes. 

Mary Cadenasso (University of California, Davis) provided valuable contributions 

to the urbanization subtheme. Finally, Alison E. Stanton (BMP Ecosciences, San 

Francisco), Tamara Sasaki (California Department of Parks and Recreation, Sierra 

District), and Lars Anderson (USDA Agricultural Research Station Exotic and 

Invasive Weed Research) provided text and helpful feedback on various subtheme 

topic areas. 
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Derek Kauneckis2 and David Halsing3

Introduction

The social sciences were included in the science plan to explicitly recognize that 

the processes underlying environmental degradation, as well as environmental 

conservation and restoration, are fundamentally based on human decisionmaking. 

Effective management of the natural environment relies on a well-grounded understanding of human behavior, the interaction of social and natural processes, and 

linking information from the social sciences to policymaking. This has become an 

explicit component of environmental management in the Lake Tahoe basin with the 

inclusion of the concept of the “Triple Bottom Line” in the Pathway planning process. This stresses that management should consider the well-being of the environment, the economy, and the community in the development of new regional plans 

(Pathway 2007). The term social science is used here to refer to the study of human 

behavior, typically through the use of the scientific method. The definition has 

been kept as broad as possible and includes different disciplinary approaches such 

as economics, sociology, and political science, as well as interdisciplinary fields 

of study such as community planning, decision sciences, policy evaluation, and 

socioeconomic development. It also includes applications of social science methods 

and knowledge in management decisions such as data collection methods, monitoring, evaluation, modeling, organizational behavior, and policy process evaluation. 

Although there have been numerous academic studies calling for integrated 

environmental assessment that explicitly include the social sciences (see Cortner 

2000, Endter-Wada et al.1998, Mascia et al. 2003, Meyer 1997, Redman et al. 2004), 

to our knowledge, this is the first attempt to include a social science chapter in a 

science plan. We have attempted to develop both a broad framework for understanding the collection and use of the social sciences as well as specific types of information requested by stakeholders in the Lake Tahoe basin. There are a wide variety of 

methods in the social sciences that can be brought to bear on the types of research 

identified in this chapter. These range from qualitative interviews to quantitative 

surveys, cost/benefit and cost-effectiveness analyses, program evaluation, public 

Chapter 7: Integrating the Social Sciences 

in Research Planning1

1 Citation for this chapter: Kauneckis, D.; Halsing, D. 2009. Integrating the social sciences 

in research planning. In: Hymanson, Z.P.; Collopy, M.W., eds. An integrated science plan 

for the Lake Tahoe basin: conceptual framework and research strategies. Gen. Tech. Rep. 

PSW-GTR-226. Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific 

Southwest Research Station: 303–367. Chapter 7.

2 University of Nevada, Reno, Department of Political Science, Reno, NV 89557.

3 U.S. Geological Survey, Menlo Park, CA 94025.
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GENERAL TECHNICAL REPORT PSW-GTR-226

management, and policy analysis. All methods involve tradeoffs in terms of the 

reliability, validity, replicability, and overall utility for answering specific management needs. For some questions, such as conflict resolution and community vision 

development, qualitative interviews and consensus-building workshops are likely 

most appropriate. In others, such as ranking cost-effectiveness of various projects 

or models of projected fiscal revenues, more quantitative methods are preferred. 

The method to be used depends on the specific research questions and the methods 

ability to provide useful information for management decisions. 

The goals of this chapter are to (1) identify social science data and research 

needs for management objectives that are not directly environmental, (2) describe 

the research needs necessary to improve policy design and implementation for 

managing environmental conditions, and (3) develop a framework for prioritizing social science research needs in areas that focus on the interaction between 

the human and natural environments. In many ways, the social sciences could be 

considered an overarching theme that encompasses all human-caused drivers of 

environmental change. However, in following the organization of issues set out in 

various management plans and process documents (see table 1.1, chapter 1), this 

chapter focuses on five subthemes determined as unique to Lake Tahoe basin social 

sciences: recreation, transportation, economics, noise, and scenic resources. Additional areas of research interests and cross-cutting issues identified by the various 

stakeholders are discussed throughout the chapter. Issues that were common across 

the subthemes have been organized into a section of “meta-themes” and include the 

following topics: collaborative information management; community management, 

program evaluation, policy design and policy process evaluation, management of 

fire and natural hazards, and climate change impacts.

Methodology

Social science research on the Lake Tahoe basin is limited in comparison to the 

amount of natural science investigation and data collection. The 2000 Lake Tahoe 

Watershed Assessment states that “relatively little comprehensive or integrated 

social and economic analysis has been done in the basin to date. Nearly all the 

recent economic studies conducted in the basin have been focused either on 

a specific sector or geographic region” (Murphy and Knopp 2000). There are 

challenges to even simple data collection and analysis in a watershed that spans 

two states, five counties, and one incorporated municipal area, and includes 

federal, state, and utility district lands as well as several special government 

units and private landholdings. 
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

Information on the social science research needs for the Tahoe basin was 

gathered using a number of different methods. First, relevant literature on the 

various subthemes was reviewed. Documents included general background work 

on the role of social science in environmental management (Cortner 2000, Imperial and Kauneckis 2003, Mascia et al. 2003, Redman et al. 2004, Sturtevant et 

al. 2005). Reports specific to the Tahoe basin included the Lake Tahoe Watershed 

Assessment (Murphy and Knopp 2000), the Pathway Process documents (TRPA 

2006c, 2006d), the Tahoe Regional Planning Agency (TRPA) Threshold Evaluations (TRPA 2006a), the Environmental Improvement Program (EIP) (TRPA 2001b, 

Tahoe Federal Interagency Partnership 1997), and regional work such as the Sierra 

Nevada Ecosystem Project (Centers for Water and Wildland Resources 1996, 1997; 

Elliott-Fisk et al. 1996; Nechodom et al. 2000a, 2000b) and other basinwide assessments of management and data needs (Halsing et al. 2005, Nechodom et al. 2004, 

Tracy 2004). Private sector activities and concerns related to economic health and 

community well-being were examined in various economic assessments, regional 

strategic plans and community development forums (Jones and Stokes 2000; North 

Lake Tahoe Resort Association 2004, 2007; TRPA 2006b). Particular attention was 

paid to studies that inform the subthemes considered in this chapter. 

From the literature review, a list of topics for discussion was sent to a wide 

range of Tahoe stakeholders including federal, state, and regional resource management agencies; local governments and utility districts; representatives from 

chambers of commerce and other business associations; and environmental organizations. A letter was sent explaining the Tahoe Science Consortium’s (TSC) purpose in developing a Tahoe basin science plan, asked the recipient to comment on 

the relative importance of the items on the list, solicited additions or modifications, 

and requested followup meetings and conversations. Written and verbal responses 

to the letter were compiled and are discussed in the relevant sections below. 

The third step involved a series of focused meetings and workshops held 

with two important stakeholder groups: natural resource management agencies 

and chambers of commerce.4

 Each of the five subthemes was discussed with the 

resource management agencies to determine the science needs of the participants. 

Workshops held with both the north and south shore chambers of commerce were 

less explicitly oriented on the goals of this chapter, but rather sought input on developing measures and indicators of the more broadly defined concept of sustainable 

communities. The input of all stakeholders was reviewed to discern common areas 

4 The full array of participating organizations is too numerous to report here; however, the 

“Acknowledgments” section of this chapter lists the most substantive contributions and key 

participating organizations. 
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GENERAL TECHNICAL REPORT PSW-GTR-226

of interest for social science research and those that might best serve the widest 

audience. Emphasis was placed on those topics that were within the regulatory 

mandate of one or more Tahoe basin agencies, or that could best serve the needs of 

environmental management and offered opportunities for collaboration in funding, 

data collection, and distribution of information. 

The first draft of this chapter was sent to registered attendees of the TSC’s 

Tahoe Science Symposium, which took place in October 2006. Major themes and 

findings were presented to attendees and discussed in a 3-hour breakout session. 

Input was included in a second draft of the chapter, that was then sent to social scientists and other stakeholders who work in the Tahoe basin. Thus, key local participants have had a number of opportunities to correct, modify, and add contributions. 

A draft of the science plan was sent for external review by the U.S. Environmental 

Protection Agency, with comments received in July 2007. Reviewer comments and 

suggestions have been fully addressed through chapter revisions, in the response to 

reviewer’s comments, or with additional analysis. Based on the information collected at this stage, a list of research needs was identified and is presented in the 

following tabulation. These included a broad array of issues related to monitoring, 

research, and application of social science data and methods. Individual items are 

listed within subthemes and organized according to whether the issue represented 

data collection and consolidation, monitoring for a specific management need, public agency management and collaboration, or program evaluation and policy design. 

Summary of social science research needs identified

Recreation subtheme

Data collection and consolidation:

• Development of a set of core questions to be used in all recreational surveys within 

the Tahoe basin (R1)a

• Development of a 3-year recreational survey cycle as suggested by the Tahoe 

Regional Planning Agency (TRPA) (R2)

• Track recreation accomplishments and needs over time at the basin level (R3)

• Collection and consolidation of recreation data at larger spatial scales (e.g., 

regional and basinwide) (R4)

Monitoring for management needs:

• Research recreational capacity at the site and regional level (e.g., South Shore/basin) 

(R5)

• Quantitative evaluation of adequacy of current measures (e.g., TRPAs PAOT system 

and indicator) (R6)

• Statistical analysis of the factors associated with current measures of recreational 

quality (R7)

• Investigations of the spatial and temporal variation of the environmental impacts 

of recreational activities (R8)
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

Recreation subtheme (continued)

• Investigations of the link between recreational activities and associated 

infrastructure (R9)

• Research on crowding-out owing to price, incompatible activities, and 

infrastructure (R10)

• How vulnerable are recreational facilities to natural hazards? Can resiliency to 

such hazards be improved? (R11)

• How vulnerable are recreational facilities to climate change? Can resiliency to 

such hazards be improved? (R12)

Public agency management and collaboration:

• Develop common data-sharing infrastructure and coordination of data collection 

efforts (R13)

• Knowledge-sharing about effective approaches used in other alpine resort 

communities (R14)

Transportation subtheme

Data collection and consolidation:

• Increase the consistency, spatial and temporal coverage of current data collection 

efforts (T1)

• Increase collection frequency of congestion and vehicle-hours of delay (VHD) data (T2)

• Collection of bicycle use data at the basin level (for paved and unpaved trails) (T3)

• Collection of public transit ridership data to and within basin (T4)

• Study public perception of congestion, traffic levels, and transportation issues (T5)

Public agency management and collaboration:

• Increased collaboration in data collection to serve needs beyond transportation 

planning, such as associated infrastructure and community development (T6)

• Centralize the collection, inventory, and distribution of transportation data at the 

basin level (T7)

• Develop a Web-based information source about all basin-level public transportation 

systems (e.g., route, fares, ridership, etc.) (T8)

Program evaluation and policy design:

• Development of a transportation indicators for environmental impact, person-hours 

of delay and other measure of basinwide transport (T9)

• Statistical study of the link between changing demographic characteristics and 

travel patterns (T10)

• Feasibility study of a commuter bus from South Shore to Minden and 

Gardnerville areas and increased ferry service (T11)

• Research on travel behavior and consumer choice within the Tahoe basin (T12)

• Research on the impact of increased second-home ownership on transportation 

planning (T13)

• Research on transportation options for lower-income workers and the link to 

affordable housing (T14)

• Evaluation of effective policy options for increasing public transit ridership (T15)

• Evaluation of effective policy options for managing illegal parking at recreational 

facilities (T16)

• Evaluation of transportation research and use of alternative transportation in 

other alpine resort communities (T17)
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GENERAL TECHNICAL REPORT PSW-GTR-226

Transportation subtheme (continued)

• Examination of the linkages between transportation and recreation, land use, 

economics, and other management issues (T18)

• How vulnerable is the transportation infrastructure to natural hazards? Can 

resiliency to such hazards be improved? (T19)

• Develop long-term predictive models of vehicle use, transit ridership, and 

demographics (T20)

Economics subtheme

Data collection and consolidation:

• Develop a reporting system of business/construction permits and permitting costs (E1)

• Increase data collection and analysis of housing trends at the community and 

basin level (E2)

• Increase data collection and analysis of employment trends at the community 

and basin level (E3)

• Increase data collection and analysis of tourism trends at the community and 

basin level (E4)

• Increase data collection of community quality of life variables (e.g., school 

enrollment, arts funding, etc.) (E5)

• Collect and consolidate basin-level demographic data (e.g., part-time residents, 

homeownership, commuters, etc.) (E6)

• Increase data collection on socioeconomics of populations such as low-income 

workers, Latinos, and Washoe Tribe members (E7)

• Increase data collection on affordable housing for year-round residents, public 

sector employees, and service workers (E8)

• Research on the development and use of community sustainability and well-being 

indicators for directing long-term planning (E9)

Monitoring for management need:

• Research the impact of redevelopment projects on housing, employment, and 

tourism (E10)

• Investigate the linkages between housing and land values, environmental quality, 

environmental regulations, tourism, economic health, and community well-being (E11)

• Research on the competitiveness of Tahoe-based businesses and leveraging the 

local Tahoe businesses (E12)

• Predictive, simulation modeling and forecasting of interaction of housing prices to 

job, wages, tourism, regional economy, and environmental regulations (E13)

• How vulnerable is economic and community well-being to natural hazards? Can 

resiliency to such hazards be improved? (E14)

• How vulnerable is economic and community well-being to climate change? Can 

resiliency to climate change be improved? (E15)

Public agency management and collaboration:

• Develop a community and basinwide information sharing infrastructure on 

economic and community well-being data (E16)

• Research into the system of collaborative funding involving both public and 

private sector for economic research (E17)

• Develop knowledge-sharing system on property valuation and housing dynamics 

in similar alpine tourism communities (E18)
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

Economics subtheme (continued)

Program evaluation and policy design:

• Evaluation, prioritization, and coordination of EIPs projects (E19)

• Feasibility studies of new environmental policy approaches (e.g., emissions 

trading, carbon markets, land coverage banking) (E20)

• Program evaluation of the effectiveness and cost-efficiency of existing 

environmental programs and regulations (E21)

• Research into effective policy options for increased affordable housing (E22)

Scenic resources subtheme

Data collection and consolidation:

• Data collection on public perceptions of the degree of light pollution and 

potential policy options (S1)

• Data collection on public perceptions of the scenic quality of the natural 

environment (e.g., forest, lake, ecosystems, etc.) (S2)

• Data collection on public perceptions of the scenic quality of the built 

environment (e.g., viewsheds, architecture, public space) (S3)

• Develop a meaningful TRPA Scenic Quality Indicator (S4)

• Develop community character indicators of the built environment (S5)

• Develop a scenic carrying capacity rating system (S6)

Monitoring for management need:

• Research into the links between redevelopment investment and the associated 

built environment (S7)

• Research on public valuation of scenic resources and link to associated 

management goals (S8)

Public agency management and collaboration:

• Increase the integration of TRPA indicators with other land management agencies 

(e.g., U.S. Forest Service, state, local governments) (S9)

Program evaluation and policy design:

• Feasibility study of new electric utility infrastructure being placed underground 

(S10)

• Feasibility study of a scenic easement and scenic development rights trading 

system (S11)

• Evaluation of effective policy options for increasing building design diversity (S12)

• Evaluation of effective policy options for improving scenic quality overall (S13)

• Evaluation of effective policy options for increasing investment in commercial, 

rental, and public facilities (S14)

• Evaluation of effective policy options for mitigating visual impact of private 

home development (S15)

Noise subtheme

Data collection and consolidation:

• Increase spatial and temporal extent of monitoring to capture single-event noise 

levels (N1)

• Research into the public perception and acceptability of noise levels (N2)
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GENERAL TECHNICAL REPORT PSW-GTR-226

Noise subtheme (continued)

Program evaluation and policy design:

• Evaluation of effective policy options for use of noise reduction technologies in 

off-road vehicles (N3)

• Evaluation of effective policy options for decreasing violations of current noise 

levels (N4)

• Evaluation of effective policy options for decreasing conflict over incompatiblenoise level activities (N5)

a Codes in parentheses correspond to text in this chapter. 

External reviewers explicitly requested prioritization of social science research 

needs and the inclusion of a more quantitative analyses. To develop the subset of 

higher priority items from the list identified in the tabulation above, an additional 

source of information was obtained through a Web-based survey of stakeholders 

conducted as an ancillary project by the University of Nevada, Reno (UNR). 

Survey questions were based on all comments received, as well as specific issue 

areas identified through conversations with data managers within each of the major 

resource management agencies, chambers of commerce, local governments, and 

environmental organizations. Invitations to participate in the survey were sent 

to approximately 350 natural scientists, resource managers, private and public 

interests, and other stakeholders identified as those who either attended the Tahoe 

Science Symposium or actively participate in public meetings with resource 

management agencies. Sixty-two responses were received from a wide variety of 

stakeholders (fig. 7.1). Details of the methods used and questionnaire format are 

available in Kauneckis and Copeland (2008). 

Results from the survey are presented throughout this chapter and confirm 

earlier stakeholder input, as well as provide new information. Four types of 

information from the survey are presented in this report: (1) the relative importance 

of each subtheme, (2) the type of data needed, (3) the spatial scale of the data, 

and (4) the temporal scale of the data. The relative ranking of the importance of 

each issue area within each subtheme is presented in fig. 7.2. Because the purpose 

of the survey was to better understand social science data and research needs, 

questions were also asked about specific areas of research. For example, within the 

subtheme of transportation, the most common research topics of interest were (1) 

transportation choice by tourists, (2) highway use, (3) shuttle bus use, (4) alternative 

transportation demand, and (5) vehicle miles traveled (VMTs) (see fig. 7.3). Within 

each subtheme, a series of questions were asked about the type of data required for 

each topic, and the spatial and temporal scale necessary to make the data useful for 

professional decisionmaking. Results are presented in this chapter. Respondents 
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

Figure 7.1— Distribution of participating stakeholders by type (n = 61). (Source: Kauneckis and 

Copeland 2008).

Federal agency

California state agency

Private business

Local government

Regional agency

Community association

Utility district

Nevada state agency

Individual citizen

Business association

Environmental organization

Public safety

Stakeholder

0 2 4 6 8 10 12 14

Number of responses

Figure 7.2—Ranked importance of recreation issues (n = 3). (Source: Kauneckis and Copeland 2008).

4

3

2

1

0

Research topic

Number of responses

Carrying capacity 

Urban bicycling 

Skiing/snowboarding 

Watercraft use

Shoreline access

Somewhat important Most important
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GENERAL TECHNICAL REPORT PSW-GTR-226

were given the following list of potential data types along with their definitions: 

current status—information on current use; future projections—expected future 

activity; general trends—direction of change; “what-if” scenarios—expected 

change if a single factor is changed; risk assessment—evaluations of potential 

risk; efficiency improvements—evaluations of cost effectiveness; alternative 

futures scenarios—expected change when multiple factors are changed; and online 

quick reference—single-page dashboard presentation of information. The various 

spatial scales of social science data were presented as follows: individual—data 

on individual consumer, citizen, or private landowner behavior; local political 

jurisdiction—information at county or municipal government level; community—

information at aggregate community level (such as South Lake, North Lake, or 

West Shore); administrative jurisdiction—according to the government agency 

responsible for an area (such as U.S. Forest Service, utility district, state parks); 

watershed—information at the natural watershed scale (such as Emerald Bay, 

Truckee River, Incline Creek); habitat—information at the level of natural habitat 

Figure 7.3—Ranked importance of transportation issues (n = 12). (Source: Kauneckis and Copeland 

2008).

14

12

10

8

6

4

2

0

Research topic

Number of responses

Transportation 

choice by 

 tourists 

Highway use 

Shuttle bus use 

Alternative 

transportation 

 demand

Vehicle miles 

traveled

Less important Somewhat important Most important
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

area (such as shoreline, forest, grassland); Tahoe basin—information at the level 

of the entire basin; state—information at the level of state (such as California or 

Nevada). Finally, in terms of temporal scale, respondents were presented with the 

following choices: daily, weekly, monthly, seasonally, annually, biannually, every 5 

years, every 10 years. Only the most frequent responses are reported here. For the 

full survey results and methods, see Kauneckis and Copeland (2008). 

Because the survey was designed to derive types of information different 

from that gained in the meetings and workshops, not all of the subthemes have a 

direct set of equivalent questions in the survey format. This chapter is organized 

on the subthemes of recreation, transportation, economics, scenic resource, 

noise, and five cross-cutting metathemes. The survey focused on six areas of 

social science research: transportation, public agency management, community 

well-being, economic health, public information and program participation, and 

recreation. So although information needs on economic activity are consolidated 

in one heading in this report, the survey attempted to broaden this out to include 

economic health of the region, various noneconomic aspect of community wellbeing, and cost-effectiveness of public policies. Similarly, although collaborative 

information management and policy evaluation are discussed in this report as 

separate metathemes, in the survey, aspects of public management were combined 

under public agency management. Survey results would be considered supplemental 

to those derived from the more focused stakeholder meetings, and both sources of 

information are complementary in informing social science research needs. 

We stress that although the survey presents discrete results, it would be a misinterpretation to accept these as representing the full scope of social science needs, 

the priority, or even necessarily the key areas of concern by survey participants. 

The survey is one type of information that should be combined with others to get an 

accurate picture of priorities. There are important limitations of Web-based survey 

data collection. The survey was anonymous, and there was no attempt to screen the 

level of expertise in a specific issue area or the level of management responsibility. 

Similarly, the intensity of stakeholder preferences was not measured (the ranking 

does not accurately represent the weight different stakeholders assign to each item). 

Additionally, the survey was available for a period of 2 months, and some important 

stakeholders may have not had the opportunity to respond, or may have simply 

chosen not to respond. There are other limitations common to those familiar with 

online-survey methods, but the main point is that these results should be taken as 

evidence of interest and issues to address, not as definitive answers about the needs 

and priorities of research. 
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GENERAL TECHNICAL REPORT PSW-GTR-226

Based on the various sources of information examined, there was an attempt 

by the authors to prioritize specific research needs. These are presented in table 7.1 

along with an explanation of the reasons for prioritization. Items were prioritized 

based on the following criteria: consistent requests across multiple information 

sources (publications, stakeholder meetings, and survey data), degree to which 

a specific item will meet a need identified by the broadest range of stakeholders 

(public agencies, private organizations and environmental groups), relative ease 

and costs through which a research priority can meet an immediate management 

need, and a direct link of social science research to environmental management 

goals. There was no attempt to reach common agreement on the priority of various 

research needs among the different stakeholders included in this process. Given the 

unique nature of questions regarding human behavior, public policy, and economics, and the different interests and management responsibilities of the diverse set 

of stakeholders, additional work would be recommended to find areas of common 

research priorities among all relevant stakeholders. The authors are of the opinion 

that selecting immediate priorities is best left to stakeholders and funding agencies. 

The priorities given here are intended only to help guide that effort. 

A legitimate concern was raised by some stakeholders that by addressing the 

broad array of social science research questions covered in this chapter, it may 

distract agency effort away from their central mission of environmental management. Some of the social science issues discussed here may be better left to the 

private sector and local government rather than natural resource management 

agencies. However, research conducted during this study indicates there is more 

overlap between public and private sector interests than may be recognized by 

stakeholders, and there are multiple issue areas where research effort can be 

coordinated and conducted with greater collaboration. This chapter is intended to 

assist in that effort by summarizing the current state of knowledge, suggest some 

research priorities, and provide a broad framework for discussing future social 

science research. 

Approach

The social science research discussed in this chapter can be arrayed along a 

continuum of simple data collection and consolidation, to more complex statistical 

analysis and modeling, and the process of innovation and knowledge production. 

Specific research topics of interest within each subtheme have been organized 

to indicate this complexity and placed in one of four general categories of 

social science research: (1) data collection and consolidation, (2) monitoring for 

management need, (3) public agency management and collaboration, and (4) policy 
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

Recreation subtheme:

Development of consistent, basinwide 3-year recreational 

survey cycle to track seasonal changes, crowding 

behavior, and long-term patterns of recreational activity 

change (R1, R2, and R4)

Statistical analysis of the factors associated with 

recreational quality in order to determine visitor 

preference and direct future infrastructure investment 

(R5 and R7)

Analysis of recreational capacity in terms of ecological 

impact and infrastructure at both the specific site level 

and basinwide (R8, R9, and R10)

As the central economic activity of the basin, recreation 

is of general interest to the public as well as private 

stakeholders. Additional information on the quality of 

recreational experience, the factors perceived as increased 

quality experiences, and information at a basin scale can 

assist in planning efforts in both the public and private 

sectors. 

Transportation subtheme:

Collection and analysis of current bicycle use data in 

order to determine road and trail use, and to determine 

adequacy of current bicycle routes and the potential for 

use as an alternative transportation mode (T3)

Collection and analysis of public transit ridership data 

to and within basin (T4)

Centralized collection, inventory and distribution of 

transportation data at a basin level (T6 and T7)

Centralized Web-based information source about all 

basin-level public transportation systems (T8)

Analysis of effective policy options for addressing 

parking at high-impact areas (T16)

Predictive modeling on vehicle use, transit ridership, 

demographics and linkages between economic health 

and emergency preparedness (T18, T19, and T20)

Identified as important information to a broad selection of 

stakeholders; including the local governments, resource 

management, and the private sectors. Items outlined here 

can provide baseline data for other research on public 

transportation.

Economics subtheme:

Increased data collection and analysis of housing trends 

at the community and basin level (E2 and E5)

Increased data collection and analysis of employment 

trends at the community and basin level (E3 and E5)

Increased data collection and analysis of tourism trends 

at the community and basin level (E4 and E5)

Estimate affordable housing availability for year-round 

residents, public sector employees, and workers in service 

and hospitality industry (E8)

Evaluation, prioritization, and coordination of 

Environmental Improvement Program projects (E19)

There is a strong interest by the private sector in 

collaborating on core economic data and research 

questions. Affordable housing, the impacts of the growth 

of second-home ownership and community-well-being 

continue to be common issues of interest to basin 

stakeholders.

Table 7.1—Social science research priorities 

Research need Justification for prioritization
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GENERAL TECHNICAL REPORT PSW-GTR-226

Table 7.1—Social science research priorities (continued)

Research need Justification for prioritization

Economics subtheme (continued):

Development of community sustainability and well-being 

indicators (E9)

Program evaluation of the effectiveness and costefficiency of existing policies (E21)

Analysis of effective policy options for increasing 

affordable housing options (E22)

Scenic resources subtheme:

Examination of public perceptions of the scenic quality 

of natural and built environments (S3 and S4)

Development of new Tahoe Regional Planning Agency 

scenic quality indicators (S4)

Analysis of effective policy options for increasing 

building design diversity (S12)

Analysis of effective policy options for improving 

overall scenic quality (S13)

Scenic resources are directly linked to recreational 

experience. Although of minimal environmental impacts, 

they are of high importance to individual communities and 

planning officials. Substantial resources have already been 

dedicated to outlining management questions and new 

indicators. Additional research could assist in informing 

improved policy design. 

Noise subtheme:

Increasing spatial and temporal monitoring to capture 

single-event noise levels (N1)

Research on public acceptability and perception of 

current noise levels (N2)

The research outlined here is relatively easy and directly 

related to processes already underway.

Collaborative information management metatheme:

Information synthesis and knowledge-sharing about 

effective approaches for recreation infrastructure, and 

about mitigating environmental impacts in other alpine 

resort communities.

Developing a collaborative information sharing network 

can facilitate data sharing across research areas, act as 

an information portal for both researchers as well as the 

public, and is the first step toward decision-support system 

and complex systems modeling.

Tahoe basin community management metatheme:

Consolidation of existing information, design of new 

collection instruments, and examination of methods for 

upscaling data to the Tahoe basin level and downscaling 

to the community level.

Research activity needs to be able to address issues at both 

the basin level as well as local communities. Scaling across 

spatial units is critical to many of the research issues in 

this chapter. 

Program evaluation, policy design and policy process 

evaluation metatheme:

Development of policy conflict resolution mechanisms for 

programs working at cross purposes. Target research on 

policy evaluation and program design of costly programs.

Because of the highly regulated environment at Tahoe, 

evaluation and design issues are important both for the 

purpose of developing more effective and efficient policy 

instruments, but also for maintaining public support. 
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

analysis and program design. Data collection issues are the simplest type and 

involve the use of sampling methods rather than actual research and hypothesis 

testing. The most commonly identified social science research needs in the basin 

fell within this category. These tended to revolve around four issues: (1) developing 

more consistent questions across current data collection efforts, (2) increasing the 

frequency of collection efforts, (3) up or down scaling existing data to the basin 

and community level, and (4) expanding data collection to include new emerging 

management issues. Monitoring for management needs generally falls within the 

middle range of complexity and involves research directed at commonly identified 

regulatory and nonregulatory management responsibilities. Research on public 

agency management and collaboration is slightly more advanced and involves 

methods of organizational behavior and public management. Most topics mentioned 

in this area involve ways of increasing interagency cooperation, communication, 

and coordination of research results, and the development of mechanisms to 

mitigate perceived policy conflicts. Attention also was directed at improving the 

relationship between public agencies and the public. Policy analysis and program 

design included questions on the evaluation of current policies and programs in 

terms of their effectiveness and efficiency, methods to improve implementation and 

adoption, and examination of potential new approaches. By organizing according 

to relative complexity, the framework presents a tool to understand the relative ease 

by which the various research needs identified here can be achieved, as well as the 

likelihood that results can be directly incorporated into management decisions. 

For example, the question of whether visitors are satisfied with the selection of 

recreational activities available can be answered in a relatively straightforward 

manner through basic survey research, whereas the question of how climate change 

Table 7.1—Social science research priorities (continued)

Research need Justification for prioritization

Fire and natural hazards metatheme:

Development of a basinwide emergency communications 

network, implement reforms to fuels management on 

private parcels, and begin extensive interagency review 

of basinwide coordination of fuels management.

If acted on quickly, substantial reforms could be made to 

public agency processes and private sectors in the basin 

following the 2007 Angora Fire.

Climate change metatheme:

Development of predictive models of climate change 

impacts on the Tahoe environment. Focus on outputs that 

can communicate potential outcomes to local government, 

the general public, and agency personnel.

Given the expected impacts of climate change to the Tahoe 

ecosystem and correspondingly the recreation-based 

economy, it would be advantageous for planning efforts to 

be aimed to account for projected climate impacts.

Note: Codes in parentheses reference research needs shown in figure 7.5.
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GENERAL TECHNICAL REPORT PSW-GTR-226

is likely to impact recreation is quite complex and involves various projections of 

probable impacts and estimates of the resulting human choices based on a changing 

natural environment. Integrating this information into the formal planning process 

at a basinwide level is one step more complex as it involves multiple organizational 

units and the policymaking process itself. The social sciences do have methods 

and techniques that can address each issue, but each requires a different level of 

commitment and resources by the various stakeholders.

Subthemes are organized according to five topical areas and five cross-cutting 

metathemes. Across participant groups, the majority of the discussion focused on 

economics and recreation, with less attention paid to transportation, and only a few 

stated social science research needs on noise management and scenic resources. 

This deviates slightly from the ranking of priorities from the survey (fig. 7.4), but 

not in significant ways given the relatively small sample size and differences in 

the organization formats. 

Figure 7.4—Ranked importance of social science research areas (n = 52). (Source: Kauneckis and 

Copeland 2008).

Most important Second most important Third most important

Ranking

12

10

8

6

4

2

0

Number of responses

Transportation

Public agency management

Community well-being

Economic health

Public information

Recreation
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Conceptual Model

To develop a broad framework of how the social sciences directly impact the natural 

processes discussed in the other chapters, a conceptual model (fig. 7.5) is used 

to illustrate relations among the various subthemes and research needs discussed 

here. The primary drivers of the social processes impacting management at Tahoe 

involve broad demographic changes and dynamics of the regional economy, both 

of which are beyond the impact of policy decisions in the Tahoe basin. These 

influence visitor and resident behavior, generally referred to as choice preferences 

in the policy literature. Both residents and visitors make decisions regarding such 

Figure 7.5.—Conceptual model for Lake Tahoe basin social sciences subthemes. Key socioeconomic components are indentified. 

The arrows indicate how these components are influenced by local and regional drivers as well as conditions and approaches pursued 

through the various subthemes. The alpha-numeric codes identify the social science research issues considered highest priority (table 

7.2). Code locations indicate the conceptual model element that research would most directly influence. Near-term priorities are 

indicated by alpha numeric symbols (e.g., R1, T3) and correspond to the descriptions presented later in the chapter.

Policy interventions

R8,

R9,

R10

E2, E3, E4, 

E5, E8, E9, 

E19, E21, E22 

T6, T7, T8, 

T16, T18, 

T19, T20

S4,

S12,

S13

N1

Recreational

choice

Lodging

choice

Aesthetic

valuation

Spending

choice

Transportation

mode choice

Social 

sciences

Demographic

changes

Regional

economy

Recreation

Transportation

Economics

Noise

Scenic

resources

N2

S3, S4

Visitor

preferences

Resident

preferences

R1, R2, 

R3

T3, T4

2° Driver 1° Driver Component Subtheme Theme Linkage
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GENERAL TECHNICAL REPORT PSW-GTR-226

things as recreational activities (e.g., type, frequency, and location), transportation 

mode (e.g., bicycle, bus, or auto), lodging (e.g., North Shore, South Shore, or a 

specific hotel), and spending decisions (e.g., location of spending, amount, and 

item). The subtheme of recreation is impacted by a variety of different decisions, 

from direct recreational choices to mode of transportation, spending choice, and 

the aesthetic valuation of recreation sites. There are complex interactions among 

each of the drivers and subthemes, and the model presented here is a simple 

representation. Rather than outlining all the potential paths of causality, the figure 

is intended to illustrate the potential points of potential policy intervention by 

decisions made within the Tahoe basin. For example, both visitors and residents 

make transportation mode decisions. There are various types of interventions 

that can result in incentives or sanctions to influence the choice these two types 

of publics make. If the goal is to improve parking at specific sites for residents 

only, allocating parking permits to residents and issuing a threat of fines for those 

without permits can serve that purpose. Likewise, if increasing ridership on public 

transportation during periods of peak use is a goal, increasing the availability and 

frequency of shuttle buses makes that choice more likely. Program activity can 

also include providing services to account for existing patterns of behavior rather 

than attempting to change behavior, such as understanding the current demand 

for alternative transportation modes rather than explicitly attempting to increase 

ridership. The conceptual model is intended to explicitly outline how a policy or 

program change can impact the system as a whole. 

Recreation

Recreation opportunities by residents and visitors are the driving force of the 

regional economy. Tahoe contains a wide variety of natural attractions, and tourist 

amenities. The natural environment provides an exceptional background for a range 

of multiseason activities. Managing recreation for the estimated 56,000 permanent 

residents, 200,000 visitors during peak holiday periods, and an estimated 15 million annual visitors—without degradation to the natural environment—presents a 

number of challenges. Understanding the vulnerability of recreational opportunities 

to both anthropogenic and natural hazards from such issues as wildfire and climate 

change, as well as changing consumer and citizen preferences, are important 

components of the long-term management of the Tahoe basin.
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Day use recreation at Sand Harbor State Park, Lake Tahoe, Nevada.

Courtesy of Tahoe Regional Planning Agency

Knowledge Gaps

The most comprehensive source of data on recreational activities are the National 

Visitor Use Monitoring (NVUM) surveys conducted by the U.S. Forest Service 

(USFS) with the 2005 report for the Lake Tahoe Basin Management Unit (LTBMU) 

being the most recent (USDA Forest Service 2006a). A USFS document issued in 

preparation for a forest plan update noted that recreational visits to the LTBMU 

have doubled since 1988 to almost 4 million per year (USDA Forest Service 2001). 

These public surveys are conducted for each management unit within the USFS 

and are used to estimate visitation as well as activities on USFS lands. Because 

they are standardized for use across national forest units, they are less useful for 

Tahoe-specific management questions (USDA Forest Service 2006b). 

The TRPA also measures recreation quality at various sites as part of their 

quintannual Threshold Evaluation Report process. The TRPA measures both the 

quality of recreation and the amount of recreation provided; the metric for this 

is a capacity-based system called the Persons-At-One-Time (PAOT) system. It is 

intended as a planning tool for examining design capacity and the provision of a 

fair share of recreation for public use. The PAOT works well for closed systems like 

ski resorts, but not as well for open areas like hiking trails. Nevertheless, PAOT 
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GENERAL TECHNICAL REPORT PSW-GTR-226

(i.e., recreation capacity) and recreation quality are both indicators of the TRPA recreation threshold. The recreation capacity indicator was attained in 2001, although 

the recreation quality indicator was not (TRPA 2004b); however, both indicators 

were attained in 2006 (TRPA 2006a).

The Lake Tahoe Watershed Assessment also contains data on the number of 

visitors to Tahoe and their recreational activities (Nechodom et al. 2000b). These 

activities include casino gaming, ski area use, dining, boating, camping, shopping, 

and hiking. Transportation data collected by the TRPA also contain useful information on recreational activities involving destination points. The various business 

organizations in the basin also use private consultant studies to collect data on 

specific recreational activities, spending, and the tourism market (e.g., Nozicka 

2001, 2003; NuStats 2004). Much of this has been shared and published in various 

public reports, but to our knowledge, there has not yet been a full inventory and 

analysis of existing data. The Pathway planning process has begun the process of 

establishing new metrics and standards by developing new indicators to measure 

recreation quality, access, and education. These are still under refinement and 

discussion (Design Workshop Incorporated 2004). 

Research Needs

Fourteen areas of potential research were identified for the recreation subtheme in 

the tabulation on page 306. These are categorized into three areas of research: data 

collection and consolidation, monitoring for management need, and public agency 

management and collaboration. Data collection and consolidation issues included 

requests for increased consistency and consolidation of data toward understanding 

trends in recreational activity across the entire basin (indicated by R1, R2, and 

R4 in the tabulation on page 306). Current data collection is focused on specific 

agency needs, and no agency is gathering adequate data for basinwide planning. 

Specific items discussed include more complete, frequent, and consistent surveys of 

recreational quality and access, recreational area usage, occupancy rates in various 

types of tourist accommodation units (motels, campgrounds, rented cabins), and 

demographic data about who partakes in various types of recreational activities and 

where. The TRPA has requested a 3-year cycle of surveys wherein winter recreation 

users are surveyed one year, summer recreation users the next, and the third year 

would be used for special “focus surveys” targeted to specific questions or issues 

(R2). It also is critical to detect differences between visitor and resident populations’ opinions about recreational experiences, and usage patterns. This includes 

one of the most requested items, the development of a core set of survey questions 

to be included in all surveys conducted within the basin regardless of their specific 
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purpose (R1). This standardization of questionnaire data would allow for a more 

complete picture of general information (e.g., demographics, origin, destination, 

and travel cost) over a longer period and at a broader scale than what is currently 

collected (R4). This would enable better trend analysis and the ability to develop 

predictive models. Because surveys are conducted for different purposes, each 

survey can still have a section specific to those needs, but a subsection could be 

standardized. Better information is needed specifically for the capacities of open 

and closed recreation areas for planning parking and public transit opportunities 

across the basin (R4). Finally, some importance was placed on tracking accomplishments like miles of new trails or trailheads and the amount of shoreline available for 

public recreation (R3).

The second aspect of recreation research was characterized as issues involving 

specific monitoring for management need (R5–R12). There is a perceived need to 

go beyond simply monitoring use of recreation areas, toward understanding the 

environmental impacts of various types of recreation in locations with differing 

sensitivities (R8). For example, there is a perception that there has been a decrease 

in low-impact recreational activities (such as snowshoeing and hiking) and an 

increase in high-impact activities (such as snowmobiling and mountain biking). It 

would be useful to determine the different impacts of a single day of peak usage 

versus many days of high nonpeak usage. A very common discussion point was on 

exploring ways of balancing conflicting recreational use demands (R10). The potential conflicts between protecting fragile environments while providing high-quality 

recreational opportunities warrants research to determine the tradeoffs and means 

of mitigating negative impacts. Some stakeholders expressed the importance of 

capturing unmet demand. They wanted to know who did not participate in various 

recreational activities at Tahoe and why. Were they priced out, crowded out, unable 

to access the recreation facility, or was there some other reason (R10)? 

The types of research questions presented above include statistical analysis of 

the factors associated with current measures toward developing a better understanding of what factors increase recreational satisfaction and how well current indicators capture the relevant management questions (R6, R7, and R8). Examples include 

identifying a system for capacity planning that could augment or replace the PAOT 

system. Specific topics for capacity-related research were parking, access, facilities, 

and transportation to and from the recreation areas. For example, would it be an 

effective strategy to design the capacities (parking, in this case) of natural recreation areas to accommodate average demand, and then augment that with free public transit on peak demand days? It was suggested that effort be made to understand 

the capacity differences (available and needed) between peak demand and average 
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GENERAL TECHNICAL REPORT PSW-GTR-226

demand, as well as how to choose accompanying facility designs (R9 and R10). 

The USFS, in particular, requested research and modeling of recreation capacity 

throughout the entire basin, with an emphasis on South Shore recreation areas (R7). 

Example research topics included the status and trends related to visitor use, visitor 

experience, and visitor preference on national forest lands within the Lake Tahoe 

basin. A study exploring the relationship between recreation capacity and recreation experience also was of particular importance to the USFS. Another proposed 

topic of research is the quantification of “high-quality recreational experiences” 

(a TRPA regulatory threshold and a major goal of the USFS and other organizations participating in the Pathway planning process) through statistical analysis of 

survey responses with particular quantifiable conditions in that recreation area (e.g., 

amount of crowding, restroom availability, interpretative signs, parking spaces, 

and other elements of facility design). From such analysis, predictive modeling 

could be completed to quantify the likely recreation experience without having to 

rely directly on survey responses. This type of research also included new areas of 

crosscutting research such as the vulnerability and resilience of tourism to natural 

hazards and climate change (R11 and R12), which are further discussed as crosscutting metathemes below. 

The third most discussed area for social science research was the creation of 

a data clearinghouse or other system whereby survey results related to recreation 

could be better distributed to a wider variety of organizations (R11). The need for 

communicating information to citizens was highlighted in transportation and access 

to recreational areas, particularly during periods of peak use for heavily impacted 

areas. The dissemination of information on what other recreational communities have done to manage similar problems is another use of a data clearinghouse 

discussed among stakeholders (R12). The section below on “Collaborative Information Management” elaborates further on the potential uses of a data clearinghouse. 

Results from the survey serve to highlight some specific areas of interest; although 

it should be kept in mind that there were very few respondents, many respondents’ 

likely subsumed recreational areas of interest into community well-being and 

economic health. 

Table 7.2 illustrates the relative importance of the top five interest areas for 

recreation. All five issue areas were ranked the same in terms of overall importance, with the development of an indicator of carrying capacity for recreational 

use having the highest overall level of interest. The other areas of interest included 

measurement of bicycle use on trails in urban areas, skiing/snowboarding activity, 

watercraft use, and shoreline access. Table 7.2 presents the preferred type and format of the data. In terms of data to be collected, the majority of respondents were 
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

Table 7.2—Recreation information 

Carrying Urban Skiing/ Watercraft Shoreline 

Recreation capacity bicycling snowboarding use access

Number of responses

Type General trends 1 0 0 0 1

What-if scenarios 1 0 1 0 0

Efficiency improvements 2 0 0 1 0

Spatial Local political jurisdiction 1 0 1 1 0

Administrative jurisdiction 1 1 0 0 1

Tahoe basin 1 1 1 0 1

Temporal Daily 0 0 0 1 1

Seasonally 0 0 1 0 0

Annually 3 1 1 1 1

Note: Items in bold represent the most frequent responses in each row.

interested in recreation carrying capacity and improvements in the efficiency of 

recreational offerings. Recreational carrying capacity is an important issue for both 

public and private sector interests as it impacts the use of public lands, associated 

public infrastructure such as parking and transportation, and the regional economy. 

Carrying capacity includes two aspects; the ability of infrastructure to meet public 

demands, and the environmental impact of recreational activities. There was a 

diversity of spatial extents of interest. Research on urban bicycling, skiing/snowboarding, and shoreline access was of interest at the basin level. Annual collection 

of data was consistently the temporal scale of interest across all recreation issues. 

Transportation

The movement of people, goods, and services is an important component of the 

Tahoe economy and community. Creating appropriate infrastructure and mitigating 

the negative environmental impacts of transportation is one of the most challenging 

issues in the basin. Transportation issues include improving travel times for residents and visitors to the basin, reducing congestion, air pollutant emissions, runoff 

from road surfaces, operations and maintenance of impervious surfaces, and risk 

of accidents. The transportation subtheme focuses on the social science needed to 

understand specific patterns and modes of transportation, and their interaction with 

employment, recreation, and other aspects of environmental, economic, and community health. Motor vehicle transportation does generate air and water pollution, 

and the physical processes are described in the “Air Quality” and “Water Quality” 

chapters of this science plan. This section focuses on the behavioral aspects of 

transportation. 
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GENERAL TECHNICAL REPORT PSW-GTR-226

Knowledge Gaps

The major aspects of socioeconomic interest with regard to transportation are 

VMT and alternatives to private vehicle use. Also of concern are travel patterns 

and congestion-related delays, typically measured in vehicle-hours of delay (VHD). 

Lake Tahoe transportation planning is done partly by the TRPA, which has certain 

transportation planning responsibilities, and by the Tahoe Metropolitan Planning 

Organization (TMPO), which is primarily responsible for federal transportation 

planning. The TRPA and TMPO goals are to maintain environmental protections, plan for growth in the major population centers, and ultimately support the 

economic vitality of the basin. Obtaining these goals would require a functional 

multimodal transportation system including roads, bicycle and pedestrian paths, 

public transit options, and reduced traffic congestion. 

The TRPA has a goal of reducing VMT by 10 percent relative to the 1981 

levels, a target that has never been met. The VMT has actually increased by 

somewhere between 3 percent (TIIMS 2006) and 8 percent (TRPA 2004a) in the 

last 20 years. These rates of increase are in line with the basin’s rate of population 

growth, but much lower than surrounding communities in California and Nevada. 

Growth of both human population and VMT in the Tahoe basin are constrained by 

limits on additional housing and roads. Despite the small increase in VMT, peak 

traffic volume seems to have leveled off or even declined since 1981 (TRPA 2008). 

The TRPA and the California Department of Transportation measure peak traffic 

volumes on the U.S. 50 corridor as part of their ongoing program of traffic counts. 

The VMT is not directly measured but rather modeled based on a program of traffic 

counting using both automatic permanent counters and spot counts.

The 2004 Regional Transportation Plan provides much of the available data on 

VMT, origin/destination, occupancy, and public transit ridership (TRPA 2004a). 

It also discusses those EIP that are currently underway, planned, and funded that 

address transportation concerns. Other sources of information are contained in the 

Threshold Evaluation Reports issued by TRPA every 5 years and consultant reports 

(see LSC Transportation Consultants 2003, 2004, 2005; Tahoe Regional Planning 

Agency 2008). NuStats consulting has recently conducted intercept surveys at 

specific locations to gather origin-destination and other data necessary for TRPA’s 

transportation planning purposes (NuStat 2004), but nearly 30 years has passed 

since the same set of information was collected,5

 and some business leaders have 

expressed concerns about the methods and its use. A common source of tension 

5 Norberg, Keith. 2006. Personal communication. Senior planner, Transportation Department, Tahoe Regional Planning Agency (TRPA), 128 Market Street, Stateline, NV 89449.
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between business and transportation planners is that a reduction in the availability 

of common transportation modes (typically automobiles and associated parking) 

may restrict economic activity. Finding the correct balance is an ongoing process.

Although there are no regulatory threshold standards for transportation, there 

are air quality thresholds that rely on VMT and U.S. 50 traffic volume. As a result 

of the Pathway planning process, a revised set of transportation indicators were 

presented to the TRPA governing board for approval. These proposals include an 

emphasis on multimodal transportation systems, viable alternatives to private automobiles (i.e., public transit that is accessible and useful to a wide range of people), 

and the replacement of VMT with an environmental vehicle impact indicator. If 

adopted, assessing the status of these proposed indicators would obviously require 

specific monitoring efforts (TRPA 2006c, 2006d), and these needs are discussed in 

the research needs section below. 

The TRPA has recently replaced its transportation planning model, TRANPLAN, with another modeling software tool called TransCAD, which is being 

parameterized by Parsons-Brinckerhoff, Inc. The inputs are numbers and locations 

of occupied homes, number of workers and jobs, and travel origin and destination 

information for residents and visitors. The outputs are total VMT, overall delay, 

and alternative transportation mode splits (TRPA 2008). 

Tahoe Trolley, a seasonal public transport service running between the north and south ends of 

Lake Tahoe. 

Courtesy of Tahoe Regional Planning Agency
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Although 90 percent of Tahoe visitors arrive by private vehicle and most local 

residents own and regularly use cars, there is a large network of public transit 

options, including BlueGo, the Tahoe Area Regional Transit system, the Tahoe 

Trolley, and private ski area shuttle buses. Ridership numbers and both temporal 

and spatial use patterns are available from these systems, although they have not 

been consolidated into a central reporting location. Overall, the network appears 

to be underutilized, but with “spiky” high levels of use in certain locations and 

periods. A system of bicycle and pedestrian paths for both recreational and transportation use exists in portions of the basin, but it is spatially incomplete enough 

that it does not provide a completely motor-vehicle-free experience. Finally, there 

are some watercraft-based transit options, including the Tahoe Queen, which ferries 

passengers—primarily skiers—between the north and south shores during part of 

the ski season, and a limited water taxi service that operates along the south shore 

during the summer months. There is interest in exploring expanded ferry use to 

mitigate congestion during other seasons. 

Research Needs

Twenty areas for potential research were identified for transportation (tabulation 

on page 307). These were categorized into three areas of research effort: data 

collection and consolidation, public agency management and collaboration, and 

program analysis and policy design. The issue of data collection and consolidation 

duplicates many of the same issues in other sections, with general needs for 

increased volume, consistency, and replication of transportation data (T1–T5). 

Enhanced monitoring efforts were recommended with increased spatial and 

temporal coverage (T1). Data of interest included traffic counts, vehicle occupancy, 

origin-destination, trip purpose, number of trips per day, and others. Management 

agencies have stated that the base data on these topics are so spotty that there is 

no basinwide consensus on whether traffic volume and congestion are increasing, 

flat, or decreasing in recent years. Specific areas of interest include data on bicycle 

use on both paved and unpaved trails, in bicycle lanes, purpose of use (commute 

vs. recreation), miles ridden, origin-destination, and hours spent riding (T3). For 

mountain bike trails, interest was expressed in knowing whether people drove their 

cars or rode their bicycle to the trailhead. The Tahoe City Public Utility District 

has regularly surveyed users on its multiuse trail network over a number of years, 

and the methodology may serve as a model for the basin (Tahoe City Public Utility 

District 2007). More detailed and integrated data about public transit ridership 

could better inform transit operations. Particularly important would be the numbers, 

demographics, and transportation choices of basin visitors who did not arrive in 
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

private vehicles. Just as important is the use of public transit to get to recreation 

areas or commercial cores. Better monitoring of congestion and VHD would be 

useful to the TRPA, as very little of that information is regularly collected (T2). 

It also was suggested that a metric of “person-hours of delay” be developed to 

include the delays experienced by people riding buses (T9). 

An important addition to the collection of transportation patterns is detailed 

demographic information of who is going where and for what purpose, and 

linking this to statistical analysis of behavior (T10). Knowing the interactions 

between demographics and travel patterns would increase the utility of the information for general planning purposes. Similarly, it would be useful to know the 

differences in transportation patterns between day-use visitors, overnight visitors, and residents, especially with regard to their travel to recreation areas and 

commercial core areas. Collecting and analyzing these kinds of data on regular 

intervals is critical for presenting overall trends. 

The TRPA’s transportation models indicate that many of the traffic problems 

have stabilized or even decreased in recent years (see footnote 5), and yet prevailing public opinion is that things continue to worsen. Research into the source of 

these perceptions was considered useful to the TRPA and other organizations to 

determine to what degree this represents specific transportation infrastructure 

problems or just changing aesthetic values (T5). Particularly important was 

research to guide the development of a Transportation Environmental Impact 

Indicator, should it be approved. This would relate vehicle impacts directly to 

goals for water and air quality, wildlife, noise, and other resource areas (T7).

As with other sections, there was substantial discussion about improving 

interagency collaboration and developing a basinwide information infrastructure 

(T6–T8). It is possible that much more data exist than is acknowledged here, but 

there is no centralized collection, inventory, or distribution of that information. A 

research step of primary importance would be to review all available studies and 

planning documents for the purpose of compiling existing data and identifying 

and filling data gaps (T7). A centralized Web site with information about all 

public transit systems and options including route, fare, and ridership data would 

be tremendously useful. Broad support for this information distribution system 

came from both the business community and planning agencies (T8).

Many of the research issues were directly related to addressing specific 

questions of program analysis and policy design (T9–T20). The types of questions 

included determining how people and their travel behavior would respond to 

changes in prices of parking or gasoline, special vehicle use fees, increased 

or decreased congestion, mandatory public transit to certain heavily visited 
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GENERAL TECHNICAL REPORT PSW-GTR-226

destinations, and incentives to ride public transit and rideshare (T15, T16, and T17). 

There was discussion of the importance of understanding the linkages between 

transportation, recreation, land use, and population, as well as how transportation 

affects social, economic, and equity issues (T18). Feasibility studies of many 

different transportation and transit options were recommended to understand the 

effectiveness of various types of incentives or disincentives. There is also interest in 

understanding the intra- vs. interbasin traffic trends and effects on travel dynamics 

of job locations (T18). The trend toward second-home ownership has important 

impacts on transportation planning as fewer year-round residents are present (T13). 

Some participants requested studies of how these trends affect Tahoe basin town 

centers and common areas; whereas others were concerned about the impact on 

transit options for low-income workers (T14 and T18).

Specific studies of what it would take to increase transit ridership and reduce 

private vehicle use also were discussed (T15). One example given was to explore 

the feasibility of a commuter bus into the South Shore area from the communities 

of Minden and Gardnerville. Another was to study the demand and feasibility of a 

ferry across Lake Tahoe (T11). Currently, the approach is to increase transit options, 

and although many stakeholders want and value increased public transit, there 

has been little research done to establish the consumer demand or willingness-topay for public transit or the intermodal transit hubs that could accompany them. 

Research on the most important destinations (commercial and recreation), key 

locations for transfer nodes, and similar information would enable transit operations 

to be tailored to increase total ridership. 

Parking restrictions and enforcement of violations are contentious issues in the 

basin. Access to recreational facilities is limited by parking capacity, yet many park 

illegally to access the area (Franz and Nozicka 2003). This has negative effects on 

the recreation destination, but it also leads to overuse and congestion on transportation corridors and to direct impacts on air quality and water quality and degradation 

of soil and vegetation. Research into the proper balance between increasing parking 

capacities, raising fees, and enforcing restrictions would benefit local communities 

and law enforcement, which are often at odds about this topic (T16).

The vulnerability and resilience to natural hazards (including large forest 

fires, earthquakes, earthquake-triggered seiche waves, and landslides) of the roads 

and other transportation systems in the Tahoe basin would benefit from further 

study both in terms of public safety and infrastructure vulnerability (T19). Finally, 

management would benefit from predictive modeling based on the data and insights 

described above: insights about long-term dynamics in vehicle use, transit ridership, 
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

or population demographics would all improve the ability to plan for transportation 

infrastructure needs farther down the road (T20). 

The stakeholder survey gives additional insight into the need for transportation research. Survey results (table 7.3) suggest the most important research issue 

is alternative transportation demand. Similar in priority were transportation mode 

choice by tourists, highway use, shuttle bus use, and VMT. Highway use statistics 

were considered of particular concern to the business community as there was 

a desire to distinguish destination traffic to Tahoe from that of nondestination 

highway traffic. 

Table 7.3—Transportation information 

Transportation Alternative 

choice by Highway Shuttle transportation Vehicle miles 

Transportation tourists use bus use demand traveled

Number of responses

Type Current status 6 6 6 8 6

Future projections 7 6 6 7 7

What-if scenarios 5 4 4 6 4

Spatial Local political jurisdiction 3 3 2 2 3

Community level 3 3 3 3 3

Tahoe basin 3 4 3 3 3

Temporal Seasonally 4 3 4 2 3

Annually 2 3 2 2 2

Biannually 1 1 2 2 1

Note: Items in bold represent the most frequent responses in each row.

The most important types of data for transportation were current status and 

future projections (table 7.3). General trends and various forms of scenario analysis 

were of equal frequency depending on the specific transportation issue. Table 7.3 

shows the spatial extent of concern—the Tahoe basin, community level, and local 

political jurisdiction—in about that order. Unsurprisingly, the temporal range for 

collecting transportation data across issues was seasonal, with some interest in 

annual data for highway use and alternative transportation demand. 

Economics

For the purpose of this chapter, economics was defined as the management, use, 

allocation, and flow of fiscal resources. It includes aspects of the broad regional 

economy as well as that of specific sectors and allocation of resources among public 

and private organizations. Although direct interventions in the private economy 
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GENERAL TECHNICAL REPORT PSW-GTR-226

Local businesses, east side of Highway 50, South Lake Tahoe, California. 

Peter Goin

should always be approached with caution, indirect impacts in terms of reducing 

negative externalities, producing public goods, understanding the impact of 

regulations, and managing the public economy are important areas of management 

concern. 

Because economics is explicitly one-third of the “Triple Bottom Line” concept 

(i.e., environment, economy, and community)—and is a major part of another third 

(i.e., human communities)—it is not surprising that this subtheme dominated many 

of the workshops, symposium breakout sessions, and literature reviews. The “Triple 

Bottom Line” and other sustainability framework concepts arose out of the recognition that environmental management is inextricably linked with human economies 

and communities, and is appropriately considered in comprehensive planning 

strategies such as the Pathway planning process. Within the private sector, information on sector-specific economic trends can assist in improving the sustainability 

and competitiveness of the business community, aid in short- and long-term planning, help in developing new markets, and help in adjusting to broad demographic 

changes. Based on recent activity by the Chambers of Commerce, there is substantial overlap in the information demands of public and private sector organizations 

in the Tahoe basin. 
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Knowledge Gaps

There have been a number of recent attempts to identify what is known about economic activities, trends, and sensitivities at Lake Tahoe. Nechodom et al. (2000b) 

provided status data on the population, demographics, spending, and visitation at 

Lake Tahoe. However, they noted that “the broad and inconsistent range of socioeconomic data that does exist has been gathered in a piecemeal fashion, funded by 

the private sector or by public agencies whose missions are to support tourism and 

recreation.” They noted that little baseline socioeconomic information is available 

beyond non-Tahoe specific county-level data. Other reliable information comes 

from the California Employment Development Department; the Nevada Departments of Employment, Transportation, and Rehabilitation; other state agencies like 

the Nevada Gaming Control Board; industry groups like the Reno/Sparks Convention and Visitors Authority or the California Ski Industry Association. 

The TRPA releases Threshold Evaluations Reports every 5 years, which 

contain some information about employment and earnings distributions and housing stocks. Although socioeconomics is not one of their regulatory thresholds, 

various aspects are monitored as indicators of social and economic well-being 

(TRPA 2001a). An Affordable Housing Needs Assessment was conducted in 1997, 

not repeated since, which showed that 77 percent of basin employees fell into the 

low or very low income categories (Tahoe Regional Planning Association 1997). 

The Regional Travel Impact Model (RTIM) developed for the TRPA, focuses on 

the impacts of visitor spending both for the Lake Tahoe basin and the Greater Lake 

Tahoe area (which includes the city of Truckee, and ski resorts and recreational 

areas just outside the Tahoe basin). The RTIM model was used in the visitor spending analysis for TRPA’s 2001 Threshold Evaluation Report to provide greater detail 

on visitor spending by visitor type and specific activities. The direct economic 

impacts associated with visitor spending were generated using RTIM, but were 

again one-time results, not trends (TRPA 2001a).

The USFS typically conducts NVUM surveys every 5 years. These surveys 

gather the demographics, activities, and spending data of forest visitors, but are 

designed for managing recreation, not for collecting economic data. The Pathway 

planning process draft documents set out proposed desired future conditions and 

proposed indicators to measure them. As part of the Pathway process, a series of 

place-based visioning workshops were conducted in each community around Lake 

Tahoe in the summer of 2006 (Regional Planning Partners 2006). The aggregated 

vision statements from these group discussions are available online (http://www.

regionalplanningpartners.com). In addition to these Tahoe-specific efforts, countyand state-level data, as well as national census information can be brought to bear 
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GENERAL TECHNICAL REPORT PSW-GTR-226

on understanding economic trends in the basin, with various levels of difficulty 

involved in scaling it to Tahoe-specific aspects. 

The economic and community health of Tahoe communities has also begun 

to be discussed as an important research need. Recent work conducted by the 

U.S. Army Corps of Engineers in connection with the North and South Lake 

Tahoe Chambers of Commerce has produced a working document outlining data 

collection needs for developing a set of “community sustainability indicators” 

that focus specifically on the human environment (U.S. Army Corps of Engineers 

2008). The set of 73 initial indicators was reduced to a subset of 13 measurements 

deemed the most feasible and cost-effective for pursuing. The selection process 

used and method of prioritization provides an important model to build on for 

other social science research. These 13 measurements were considered in the social 

science priorities outlined here. Based on all of these information sources, a number 

of key insights emerge. 

First, there is a perception that the trend toward increasing second home 

ownership and decreased year-round residency has serious implications for the 

economic viability and community character of Tahoe. The declining school 

enrollments have been read as a loss of families living in the basin. As in much of 

the country, housing prices have escalated dramatically at Tahoe. A nearly-fixed 

stock of housing units combined with a trend toward second homes has led to a 

shortage of affordable housing as less of the housing stock is available for yearround residents. Further, environmental regulations limit the ease with which 

additional housing can be built for lower income residents. These types of lower 

income workers are an important population to consider because most employment 

and earnings are in the service and hospitality industries. Recreation and tourism 

drive the local economy. 

Research Needs

Twenty-two areas of research were identified for the economic subtheme (see 

tabulation on page 308). These are discussed in terms of the four general categories 

of research used in earlier sections: data collection and consolidation, research 

related to specific management needs, collaborative research across organizations, 

and program evaluation and policy design. As with the other sections, the greatest 

demand was in the area of basic data collection and consolidation (E1–E9). There 

is a need for more standardized and regularly collected data about three main 

topics: housing (E2), employment (E3), and tourism (E4). Given the trends toward 

second-home ownership, the high cost of housing for either purchase or rent, and 

the difficulty of adding to the housing stock, it is important to obtain the following 
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

information in more detail and at more frequent intervals: the stocks, locations, and 

prices of various housing types; the numbers of full- and part-time residents; home 

sales figures including time-on-market and unsold inventory; number of rental 

properties occupied and vacant; rental rates; turnover rates of commercial and residential properties; percentage of residents who own their homes; percentage of local 

workers who reside in the basin; number of occupants in a residence; percentage of 

homes with children (and their school enrollment); percentage of median income 

spent on housing; percentage of households that can afford a median priced home; 

living conditions of low-wage earners, and assessed values/property taxes. For all of 

these topics, it is critical to develop estimates of the historical trends to provide the 

ability to project future dynamics. Some stakeholders noted that a formal and complete housing assessment, at 5-year intervals, could collect this data in a consistent 

manner, although others said this is akin to a local version of what the U.S. Census 

Bureau already does and is therefore too large an effort to conduct locally every 5 

years. Further, a number of different stakeholders expressed interest in knowing 

how many of the current public servants (e.g., police officers, firefighters, teachers, 

and government employees) are able to live in the basin (E8). 

With regard to employment (E3), the development of status and trends data is 

recommended for earnings and employment distribution, number and type of jobs, 

permanent vs. seasonal dynamics, place of residence of workers, poverty rates, 

health-insured households, the number of mid-sized businesses (10 to 25 employees), business startups and closures, and numbers of sole proprietorships (especially 

run out of a home). As for tourism and visitation (E4), more regular and consistent 

collection of community-specific data such as the number of hotel nights, total 

transient-occupancy tax (TOT) receipts, means of arrival and transportation around 

the basin, duration of stay, campground use vs. hotel stays vs. cabin rentals, the 

amount of uncollected TOT from cabin rentals, what activities tourists engage in 

and how much they spend on them are some of the data that was requested. 

Community well-being indicators are included in this section as they tend to 

have substantial overlap with economic research needs. Survey results for this subtheme are discussed below. Many community workshop participants requested—

and the proposed revised TRPA indicators would require—tracking and reporting 

of school enrollment, arts funding, local donation ratios (E5), and number of 

business/construction permits, and permitting cost distributions (E1). Interest also 

was expressed in understanding the socioeconomic situation of specific populations living and working in the Tahoe basin, including low-income workers, Latino 

populations, and Washoe tribe members (E7). Generally, this kind of information 

included aspects of both community well-being and interactions with other broader 
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GENERAL TECHNICAL REPORT PSW-GTR-226

areas of concern such as housing, service and tourism industry employment, tax 

base, and transportation (E11). A number of organizations also requested developing alternative methods of measuring economic and social well-being (E9). The 

Sierra Business Council developed a metric called the “Wealth Index” that includes 

common economic factors along with social indicators like literacy, health care, and 

others (Sierra Business Council 2006). There were discussions that a similar index 

could be developed for the Tahoe basin. This idea has moved forward in terms of 

the recently presented Community Sustainability Indicators (U.S. Army Corps of 

Engineers 2008). 

Research topics related to specific management needs include an array of 

questions having to do with the impact of changing conditions on the basin 

economy (E10–E15). Topics generally revolved around understanding the elasticity 

(marginal change) of demand for various activities, goods, and services. This 

would inform decisionmakers about the expected changes in visitation, spending, 

home ownership, employment levels, commercial or residential rents, that might 

result from a change in one or more of the conditions affecting them (E10, E11, and 

E13). For example, how susceptible are tourism levels to changes in hotel rates or 

ski lift tickets? At what level of rental home increase will seasonal workers stop 

seeking work at Tahoe? What will be the change in property values if the air quality 

(visibility) or lake clarity degrades? There are a number of methods common in 

social science research for generating objective evidence on such questions. 

A number of parties were interested in economic “leakage” of basin revenue to 

communities outside of the Tahoe basin. This includes workers taking their wages 

to homes located elsewhere, or to locals spending their income in stores in Carson 

City, for example. These dynamics seem to exist but have not been measured, and 

may provide an economic justification for expanding affordable housing in the 

region (E11). There also was interest in the economic impacts of special events like 

festivals, triathlons, or concerts (E11) and the competitiveness of Tahoe-based businesses (E12). Others mentioned broader studies on the impact of regulations on the 

regional economy and determinations of the marginal benefit of additional environmental quality gains (E13). This is probably the most difficult research need as it 

involves very complex economic interactions (discussed further below). Other complex research questions were also expressed, such as the impact of climate change 

and natural hazards on visitation, employment opportunities, housing values and 

prices, business activity, ski resort use, hotel room rentals, and the overall Tahoe 

economy (E14, E15). Predictive modeling and forecasting, the logical progression of 

many of these research efforts, might be developed to project housing prices, jobs 

and wages, or visitation responses to guide policy decisions more accurately (E13). 
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

One example is projecting how redevelopment would proceed given certain incentives and market conditions (E10). Although simulations of this sort are relatively 

complex tasks, often producing highly uncertain results, some types of simulation 

modeling might produce useful information and the concept is worth exploring. 

As with the other subthemes, a broad crosscutting topic was the potential 

for collaborative research across organizations (E16, E17, and E18). Much of the 

primary data are already collected periodically by various entities—the counties, 

the school districts, the chambers of commerce—but there is no systematic way of 

accessing or sharing data. The most thorough survey of existing economic data was 

recently conducted as part of the Community Sustainability Indicators study (U.S. 

Army Corps of Engineers 2008). A data clearinghouse would be particularly useful 

in this case, where so much of the information needed could be gathered and shared 

quite easily (E16). The utility of a “library” or database of land/property market 

valuation studies for other communities similar to Tahoe also was discussed as a 

component of such a clearinghouse (E18). Collaborative funding, collecting, and 

sharing of data will allow much more of it to be gathered at lower cost, and coordinating research efforts will allow for academic or government researchers to direct 

their efforts more efficiently toward needed research topics (E17).

The last category of research questions involves issues of program evaluation 

and policy design (E19–E22). Additional research and more extensive application of 

policy analysis tools were broadly included under economic research. This included 

consistent methods and practices for evaluating, coordinating, and prioritizing EIP 

projects (E19) in order to implement the projects with the greatest marginal effect 

first. This also included developing an implementation framework for tracking, 

auditing, and quantifying the benefits of various projects to maintain ongoing 

funding as well as broad public support. A frequently voiced concern was how to 

provide incentives to encourage or discourage certain behaviors. Specific research 

topics included understanding and quantifying incentives to increase compliance 

with best management practice (BMP) requirements on private lands, stimulate 

redevelopment of commercial or residential properties, leverage public and private 

investment in community spaces and green building construction (E21), provide 

low- and moderate-income housing (E22), slow the trend toward second-home 

ownership, and retain businesses in the Tahoe basin. Other research topics included 

studying the feasibility of trading systems for air or water pollution emissions and 

the expansion of land coverage “banking” systems (E20). Affordable housing issues 

were mentioned in this section as well as determining what housing bonuses would 

be needed to keep teachers or firefighters living in Tahoe communities (E22). 
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GENERAL TECHNICAL REPORT PSW-GTR-226

Overall, a number of stakeholders expressed a desire for establishing a consensus 

vision—or a process to establish one—for identifying and managing economic 

goals and ongoing monitoring of the community’s social and economic well-being.

The survey of social science data needs helps to narrow the focus of this broad 

array of issues. The three most important items of interests were housing affordability, the economic impact of different types of visitors, and gaining access to 

information on similar resort economies. This was followed by redevelopment 

investment and general visitor profile/behavior information (fig. 7.6). 

Figure 7.6—Current source of social science information (n = 50). (Source: Kauneckis and 

Copeland 2008).

The types of information requested differed according to the specific research 

areas (table 7.4). The most important housing affordability information was on 

current status and alternative futures (projections of alternative scenarios), followed 

by general trends. Types of information on the economic impact of visitors included 

general trends, current status (interpreted as more information about the economic 

10

9

8

7

6

5

4

3

2

1

0

Number of responses

Housing 

affordability

Economic impact of 

types of visitors 

Comparative information on similar economies 

Redevelopment 

investment

Visitor profile/ behavior

Somewhat important Most important
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

impact of current visitation), and alternative futures. Information types on comparative information on similar resort communities included general trends, and current 

trends and alternative futures ranked equally. 

The spatial extent of data was focused on community level for housing affordability, local political jurisdiction for the economic impact of visitors, and local 

political jurisdiction and the Tahoe basin for comparative data on other resort 

communities. The temporal extent was generally annual and seasonal. 

Scenic Resources

There are a number of aspects to the management of visual resources and the 

scenic characteristics of the Tahoe basin. Views of, and from the lake, aspects of 

the built environment, and forest appearance all contribute to the overall scenic 

quality. Scenic quality directly impacts property values and underlies the choice of 

recreational activity within the basin as well as in choosing the basin as an overall 

travel destination. Important aspects of scenic quality are covered in other parts 

of this science plan, and are not directly discussed in this chapter. For example 

the issue of visibility loss because of air pollution is covered in the “Air Quality” 

chapter; and lake clarity, in the “Water Quality” chapter. It should be noted that the 

single greatest threat to basin scenic resources is a catastrophic forest fire, a topic 

considered in several other chapters of this science plan. However, the question of 

the social acceptance of various treatment options for forest fire fuels is relevant to 

this section and is discussed below and in the natural hazards metatheme.

Table 7.4—Economics Information 

Visitor Economic 

Housing Redevelopment profile/ impact of Comparative 

Economic affordability investment behavior visitors information

Number of responses

Type Current status 3 3 3 4 4

General trends 1 3 5 5 5

Alternative futures 3 2 2 2 4

Spatial Local political jurisdiction 2 4 3 4 4

Community level 4 4 2 3 2

Tahoe basin 2 1 3 2 5

Temporal Seasonally 2 0 3 3 3

Annually 4 3 3 4 4

Every 5 years 0 3 0 0 1

Note: Items in bold represent the most frequent responses in each row.
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GENERAL TECHNICAL REPORT PSW-GTR-226

Emerald Bay, Lake Tahoe, in autumn.

Jim Markle
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

Knowledge Gaps

The TRPA currently has indicators for four types of scenic resources: (1) travel 

route rating, (2) scenic quality rating, (3) public recreation areas and bike trails, 

and (4) community design. These indicators have numerical scores representing 

the visual appeal of each spatial unit. Each indicator has a regulatory threshold 

that the TRPA tries to maintain, and measurements are taken every 5 years. None 

of these indicators have achieved attainment of threshold values during any of the 

four previous evaluations (TRPA 2006a). In addition, the USFS conducts scenic 

class inventories on their lands. A recent document prepared by the USFS as part 

of preparing its next forest plan addresses changes since the 1988 plan in the scenic 

conditions as measured by the 1997 Scenery Management System (SMS). It notes 

that there has been no recent monitoring of existing visual conditions, which means 

that there are no data sufficient for trend analysis on USFS lands. It calls for more 

monitoring, a reorganization of TRPA’s Scenic Resources threshold system, and an 

upgraded SMS for scenic inventories (USDA Forest Service 2006b).

In terms of the built environment, there has been a trend toward larger residential structures, which are more visible around the basin and block views of and from 

the lake. The Place-Based Planning and Forum portions of the Pathway planning 

process and workshops with the Tahoe Chambers of Commerce all revealed a 

perception of “urban blight” in portions of the basin, where insufficient investments 

have been made in redevelopment of commercial properties, rental residential 

properties, tourist accommodations, public spaces, and infrastructure (Regional 

Planning Partners 2006). As a result of the Pathway planning process, the TRPA 

Scenic Resources staff has recently proposed new scenic quality indicators to their 

governing board. These indicators explicitly recognize issues like community 

design and the built environment, the importance of improving the scenic quality 

and integrity rating systems, and even light pollution.

Research Needs 

Fifteen areas for potential research were identified for the scenic resource subtheme 

(see tabulation on page 309). These are discussed in terms of all four categories 

of research; data collection and consolidation, monitoring for management need, 

public agency management and collaboration, and program evaluation and policy 

design. Basic data collection and consolidation needs are focused on determining 

a better estimate of the public demand for improved visual resource management 

(S1–S6). Of interest to management agencies is the public perception of the scenic 

values and appearance of environmental qualities like a healthy forest, lake, or 

ecosystem (S2). The public’s perception of a healthy forest, or the visual impact of 
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fuels management, do not match that of a professional forester, and this can lead to 

conflict between staff on the ground and the general public. Issues of light pollution in the Tahoe basin were widely discussed at the science symposium, and it was 

recommended to develop a sense of how big an issue it actually is, how the public 

perceives it, and how it might be addressed (S1). The USFS document for updating 

the forest plan noted a similar demand by residents and visitors for night sky viewing and reductions in light pollution. 

There is a broad perception (supported by TRPA Scenic Resource ratings) of 

increased private investment in larger single-family homes that block views and 

dominate viewscapes, and a decline in the visual appeal of commercial areas and 

rental communities owing to low investment in public and commercial areas. Some 

research into the accuracy of these perceptions is recommended (S3), as would be 

work on possible regulatory and economic incentives to address them. The draft 

Pathway report (TRPA 2006d) describes some details of desired future conditions 

and indicators for scenic qualities, in relatively vague language. The TRPA and 

other management agencies recommended obtaining more complete public input 

as a way of refining these goals and there has been increased discussion of more 

frequent and thorough scenic inventories (S5). One suggested way to do that is 

through a broader and more detailed public perceptions study or survey (S1, S2, and 

S3). This would provide systematic knowledge of how people want their communities to look (S6). This can be weighed against the various environmental regulations 

and other regulations to make policy. To date, limits to land coverage on a developing parcel have been guided almost entirely by ideas about soil capability. However, 

it is possible that alternatives to soil-based limitations would be useful. A “scenic 

quality carrying capacity” was proposed as a limiting factor to developments of all 

types, but particularly directed toward larger single-family residences (S6).

Monitoring for specific management needs was primarily directed toward 

understanding the link between questions about reinvestment and improving the 

built environment (S7) and studies of the public value of increased scenic resource 

protection (S8). Most stakeholders agree that the high scenic beauty of the Lake 

Tahoe region is a large part of why people live, visit, and recreate in the area, yet 

the actual value of scenic resources, and the degree to which public spending should 

be dedicated to preserving and improving them is largely unknown. A study of 

the value of scenic resources, both in economic and noneconomic terms (S8) was 

suggested. A combination of contingent valuation, hedonic valuation methods, and 

travel-cost studies could be conducted to provide some information on these values. 

The only question regarding interagency collaboration was that of integrating 

the TRPA’s visual resource management tools and indicators with those of the 
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

USFS (S9). Although the different management mandates of the two agencies may 

not allow a single system, increased examination of the correspondence of the two 

systems and coordination with other public and private entities interested in scenic 

resource management are recommended. 

As with the other subthemes, there are numerous questions related to program 

evaluation and policy design (S10–S15). A simple request in the community workshops was the question of whether it was feasible to put power lines underground 

as part of the same construction processes that will be undertaken to put in storm 

drainage systems and sewers as a means of improving scenic quality (S10). Overall, there were broad discussions on examining the means of directing public and 

private investment to those communities with the greatest visual resource declines 

(S14). Various participants expressed concern about homogenous residential 

development being a threat to “local community character.” The assertion was 

that TRPA restrictions on coverage and other regulations have resulted in a more 

uniform home style: with new homes designed to maximize profit while meeting all 

the regulations and the outcome leading to the decline in unique visual composition 

of communities around the lake. Requests were made to understand the extent to 

which this is true, and, if it is, what might be done to induce more diverse designs 

(S12). Research was requested on possible regulatory and economic incentives to 

address various scenic resource issues involving both public and private development (S13); this included possible methods for mitigating the decline of viewscapes 

owing to private residential development (S15). One issue raised was examining the 

feasibility of a scenic easement system—analogous to conservation easements—

and purchase of scenic development rights or a scenic rights trading system were 

also recommended as ways to balance the public demand for high-quality scenery 

with private property rights (S11). Visual resources were not examined in the 

stakeholder survey. 

Noise

The noise subtheme provoked substantially less input than the other social sciences 

subthemes. However, some research needs were identified. Noise is an issue in the 

basin largely for human aesthetic reasons, although some instances of noise pollution also may affect wildlife and the quality of their habitats. For example, the 

northern goshawk (Accipiter gentilis), requires quiet and undisturbed nesting sites 

for breeding and is discussed as a management issue in the ecology and biodiversity 

chapter. Noise in the Tahoe basin is primarily anthropogenic and includes private 

vehicle traffic, boats, airplanes, construction, snowmaking, off-highway vehicles, 

and certain special events. 
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Knowledge Gaps

The TRPA uses a metric called the Community Noise Equivalent Level (CNEL) 

to assign and evaluate noise levels based on land use compatibility. The CNEL is a 

weighted average of all noise in a place within a 24-hour period. The CNEL standards are assigned based on land use categories and transportation corridors. A few 

examples of existing CNEL standards based on land use are (1) 65 decibel average 

(dBA) for industrial areas, (2) 60 dBA for hotel and commercial areas, and (3) 55 

dBA for high-density residential and urban outdoor recreation areas. The airport 

CNEL value of 60 dBA applies to approved flight paths. However, TRPA Noise 

Resource Area staff recently presented a set of modified noise indicators to the 

TRPA governing board. In addition to somewhat modified decibel levels for CNEL, 

the proposed indicators include effects on wildlife and single-event noise levels 

such as low-flying aircraft.

Noise levels are monitored in only a few locations once every 5 years. This creates a temporally and spatially incomplete data set, making it hard to assess trends, 

adjust for temporary noise sources like construction, capture site-specific noise 

sources, or test actual traffic noise against the levels predicted by current noise 

models. In contrast, the Tahoe airport maintains a monitoring system and reports 

exceedances and complaints monthly (Brown-Buntin Associates, Inc. 2004).

Research Needs

Five areas of research were identified for the noise subtheme, falling into two 

categories of research: data collection and consolidation, and program evaluation 

and policy design (see tabulation on page 309). The single largest issue with regard 

to noise appears to be the need for a monitoring system capable of providing a more 

spatially and temporally complete and uniform coverage (N1). The current CNELdriven system does not capture one-time violations and does not provide a good 

estimate of average noise levels. The proposed single-event and wildlife-related 

TRPA noise indicators also would need more thorough and effective monitoring 

systems. A study into the feasibility of remote sensors to monitor noise levels over 

extended periods could be useful. There also is an issue of the difference between 

the actual noise levels and the perceived noise levels of private vehicles, off-highway recreation vehicles (like snowmobiles), and other motorized transportation and 

recreation vehicles. Technological improvements may have reduced the actual noise 

production of these vehicles, but people may have over the same time become more 

sensitive and perceive them as incompatible with the Tahoe experience. A number 

of agency personnel expressed an interest in understanding the real public demand 

for increased noise regulations (E2). 
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

In terms of policy evaluations, interest was expressed in examining subsidies or 

other public investment in noise abatement technologies that can reduce noise levels 

from vehicles while still allowing their current levels of use (N3). Feasibility and 

effectiveness studies of mitigation opportunities were mentioned as potential topics 

for study (N4). Studies of enforcement options could be helpful in addressing noise 

issues and finding resolutions to conflicts about them (N5). Noise was not explicitly 

addressed in the stakeholder survey. 

Metathemes and Emerging Areas of Interest

There were a number of overarching research needs that cut across many of the 

social science subthemes and even other thematic areas in the natural sciences. 

Where there were very clear subtheme research questions, they are included in 

the sections above; however, some issues were raised frequently enough that they 

warrant separate discussion. This section presents information and research needs 

for five metathemes: (1) collaborative information management; (2) Tahoe basin 

community management; (3) program evaluation, policy design, and policy process 

evaluation; (4) fire and natural hazards; and (5) climate change impacts. 

Collaborative Information Management 

A conclusion of this research process was the recognition that many types of data 

and research results would be useful for a variety of different regulatory and management agencies, advocacy groups, private businesses, and community interests. 

There was a broad consensus that it would be beneficial to the entire Tahoe basin 

community if there were increased collaboration on funding, collection, dissemination, and storage of social science data, rather than piecemeal data collection 

for specific organizational needs. There has already been substantial effort made 

toward the archiving and storage of social science data through the establishment 

of the Tahoe Integrated Information Management System (TIIMS). The TIIMS was 

recently established as a data clearinghouse and information hub for Lake Tahoe 

related data. However, based on the results of the 2008 Social Science Data Needs 

Assessment for the Lake Tahoe basin (Kauneckis and Copeland 2008), TIIMS 

appears to be underused as a data clearinghouse (fig. 7.7). 

The underuse of TIIMS is likely due to its relatively recent implementation and 

current redevelopment of its user interface; it is possible that TIIMS will become 

an important component of the information infrastructure for Tahoe if sustained 

funding is established. At least some members of the business community have 

expressed a willingness to join with natural resource management agencies in 

sharing the cost of gathering, maintaining, and distributing mutually beneficial 
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GENERAL TECHNICAL REPORT PSW-GTR-226

scientific information at regular intervals. Numerous regulatory and management 

agencies and local governments have similarly expressed the potential utility of a 

clearinghouse where data could be held and distributed for common use. The Tahoe 

Chambers of Commerce in conjunction with the U.S. Army Corp of Engineers have 

recently formed a working group of stakeholders to scope the potential for developing a set of community sustainability indicators for the Tahoe basin (see U.S. Army 

Corps of Engineers 2008). Given the very high level of communication and collaboration already occurring among stakeholders in the basin (Kauneckis and Imperial 

2007), focusing this attention on the collection and dissemination of social science 

information is the next logical step. Although there are challenges to designing a 

data product useable by such a diverse array of stakeholders, this is a broad-based 

interest worth pursuing. 

Increasing access to the broad array of information on specific management 

issues adopted in other alpine-tourist-based economies was also a common point of 

50

40

30

20

10

0

Source of information

Number of responses

Public agency reports 

Consultant studies 

Tahoe Regional Planning Agency

Own organization

Academic publications

Other organizations

Tahoe Integrated Information 

Management System

Other

No Yes

Figure 7.7—Current source of social science information (n= 50). (Source: Kauneckis and 

Copeland 2008).
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

discussion throughout the science workshop. For example, there was strong interest 

in the development of a “library” of consumer choice on transportation, vehicle use, 

and alternative modes (T17). In another example, managers were interested in better 

understanding how other areas have dealt with recreational conflicts (R12) and 

with affordable housing in resort communities (E19). Fundamentally, decisionmakers feel that they may be “reinventing the wheel” in dealing with many of the 

challenges at Lake Tahoe and that access to information on policy options created 

in other areas would facilitate the policymaking process. 

Creating such a useable knowledge base is a substantial challenge. However, 

there are a number of immediate options available. The most common means of 

learning what has been done elsewhere is through informal professional networks 

and formal professional associations and publications. This assumes continued 

engagement with other practitioners and would reallocate attention that otherwise 

goes toward more immediate tasks. An alternative to Tahoe-based managers going 

outside the basin is to bring in outside experts and use consultant services to summarize current understanding of a specific topic area. This is a common occurrence 

at Tahoe and when combined with a focused workshop with resource management 

agencies can be a very effective means of getting immediate questions answered. 

However, there are disadvantages to this method of knowledge diffusion. Typically, 

the input by one person is less useful and reliable than a broader array of opinions. 

Given the already high level of collaboration at Lake Tahoe, scheduling by top 

managers tends to be overallocated and unless immediately relevant, attendance 

to events is often low. Additionally, most managers prefer information available 

on their own schedule, thus the preferred source of social science information is 

standard compilations in existing reports (fig. 7.7). 

With the advent of Web-based knowledge systems, there are a number of 

technical options that may be worth exploring. The TIIMS represents an important 

first step in collecting Tahoe-related research by developing and cataloging reports 

and data in a searchable format. However, there have been complaints about difficulties in accessing data stored on TIIMS. Information consumers are increasingly 

demanding immediate usability of the information output and TIIMS structure 

as a data clearinghouse does have some limitations. Given the specificity of the 

knowledge needs for particular policy issues, and the underlying complexity of 

many of the questions public and private stakeholders are interested in addressing, 

there are no immediate simple technical fixes. Web-based delivery of information 

would require substantial upfront investment in an information infrastructure and 

continued resources dedicated to upkeep and management. There are, however, 

some potential technology-based experiments that could be considered. One of the 
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GENERAL TECHNICAL REPORT PSW-GTR-226

most-used sources of Web-based information is open-source discussion groups 

dedicated to specific topics. If Tahoe-based decisionmakers are having problems 

finding information on the impact of something like parking fees on recreational 

experience, it is likely that other transportation and recreation managers have as 

well. Lake Tahoe is often on the forefront of emerging management issues and this 

could potentially be leveraged to develop new discussion groups with resource 

managers facing similar issues. It is recommended that this be done experimentally 

and in close association and perhaps co-sponsorship with professional associations. 

This might involve only a subset of alpine lake resource management associations, 

or even just agency officials within the basin. The challenge is to design a means of 

leveraging the experience and knowledge of other alpine-resort-based communities. 

Other options include various forms of information and knowledge architectures. 

For example, an open discussion and contribution-based Web site such as a “Tahoewiki’ where general questions could be posted with response options open to other 

resource managers or the general public. Many Tahoe residents do tend to be highly 

educated and can bring time and technical expertise to management questions. 

There are obviously numerous challenges with designing digital knowledge systems 

that produce useful results; however, Tahoe’s unique environmental characteristics, 

national prominence, and proximity to the epicenter of the digital economy may 

give it an advantage in being at the forefront of the development of new knowledge 

systems for managing natural and human-made environments. 

Tahoe Basin Community Management

A consistent point of discussion was moving management questions from a central 

focus on environmental management toward broader issues of basinwide community management (TRPA 2007a, b). This was not directed at weakening environmental regulations, as most stakeholders have come to accept the importance 

of environmental quality as central to the character of Lake Tahoe, but rather an 

attempt to bring a similar commitment to social aspects as well. This represents the 

natural evolution of public management questions in the Tahoe basin as businesses 

and communities have adapted to the regulatory structure necessary to preserve 

lake water quality and the broader ecosystem, they increasingly understand the 

interdependence of the Tahoe basin in terms of a regional economy and a set of 

interconnected communities. 

Examples of immediate areas of research integrated with community decisions 

included basinwide methods for prioritizing and improving local infrastructure 

that impact local quality of life. Although the allocation of infrastructure projects 

is partially discussed in the EIP plan, some stakeholders felt the central focus on 
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

environmental aspects may weaken some community priorities. A related infrastructure issue was that of the lack of DSL or other high-speed Internet access 

basinwide. This may be an important business diversity issue for likely a large 

percentage of Tahoe-based professionals who work out of home offices. Discussions of the lack of a basin- or regionwide communication infrastructure that would 

help emergency response in the aftermath of a natural disaster, extreme weather 

event, or other emergency situation were an additional concern. Coordination of 

the various local government units could improve the response, but there is little 

direction or infrastructure to support it. It is recommended that alternative energy 

and environmentally sustainable design be considered part of the ongoing Tahoe 

basin management, and yet they do not fit into any thematic category. There has 

been some local interest in the development of regional forest biomass conversion 

facilities for renewable energy. Additionally, participants in workshops held by 

the chambers of commerce expressed the need to further develop public gathering 

places, community and nonoutdoor recreation centers, increase the level of arts and 

culture, town centers, and other community-related amenities. Information on the 

return to public investment as well as how to better target the type of public space 

infrastructure investments was a shared concern by both local planners and regulatory agencies, and private businesses. 

A number of recent activities suggest there is a substantial movement toward 

redirecting and coordinating management decisions at the community level. The 

Pathway process involved a number of direct engagements between the regulatory 

agencies and communities. The Community Enhancement Program produced a set 

of community visions (from which many of the questions examined in the survey 

and discussed in this chapter were drawn). The recent effort toward the development of a set of sustainability indicators (U.S. Army Corps of Engineers 2008) 

suggests substantial room for collaborative work between the public and private 

sectors.

The discussion above presents the primary issues brought forth during meetings 

and workshops, yet additional insight is revealed in the stakeholder survey results. 

Based on a review of the literature and discussion with local stakeholders, the 

survey was designed to specifically capture other community-based research issues 

not directly addressed in the subthemes above. Community well-being consistently 

ranked high among issues addressed in the survey (fig. 7.4). In terms of specific 

community well-being issues, school quality ranked as most import, followed by 

fire vulnerability, the population living below the poverty line, housing affordability, and public space (fig. 7.8). 
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GENERAL TECHNICAL REPORT PSW-GTR-226

Figure 7.8—Ranked importance of community well-being issues (n = 10). (Source: Kauneckis and 

Copeland 2008).

10

9

8

7

6

5

4

3

2

1

0

Research topic

Number of responses

School quality 

Fire vulnerability 

Housing affordability 

Population below poverty line

Public space

Less important Somewhat important Most important

An examination of the type of data needed for each of the above (table 7.5) 

reveals that the most important need is for information on current status, followed 

by future projections and general trends. The spatial extent of the data varies 

according to the specific issue (table 7.5). School quality data were both important 

at the basin and local political jurisdiction level. Fire vulnerability was included in 

the survey under community management, and interestingly the spatial extent of 

the necessary data was at the individual level, likely a recognition of the importance 

of establishing defensible space on private parcels. Fire risk was also considered 

important at all other spatial extents. In terms of the temporal extent of data, the 

results were predictable, with school quality and the population below the poverty 

line important annually, fire vulnerability and public space seasonally, and housing 

affordability biannually and seasonally, probably recognizing the unique nature of 

the seasonal employment flows to Tahoe. 
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

Program Evaluation, Policy Design, and Policy 

Process Evaluation

Many participants in the various workshops and meetings, from the public, private, 

and nonprofit sector expressed interest in a variety of public policy evaluation 

issues. These are grouped here into three overlapping areas of policy research: (1) 

program evaluation, (2) policy design, and (3) policy process evaluation. Program 

evaluation involves research questions on the efficacy, efficiency, and equity of 

current public programs and regulatory policies implemented in the basin. Policy 

design addresses questions of the specific components of existing and new programs and how these can be adjusted to improve desired outcomes. Policy process 

evaluation includes research and methods for improving the decisionmaking and 

collaborative process itself. Typically this includes such issues as coordinating 

policy implementation across different agencies, public management, managing 

program implementation in networked environments, developing conflict resolution 

mechanisms, and increasing public participation. 

In terms of program evaluation needs, a number of participants, including 

management agencies and representatives of the Tahoe Chambers of Commerce, 

expressed interest in identifying conflicts between various regulatory policies. 

One example discussed was installation of a flashing light to warn motorists of a 

crosswalk, which was proposed and planned but could not be implemented owing to 

its direct violation of TRPA scenic regulations. Another is the perceived incompatibility of vegetation management for fuel reduction and habitat enhancement versus 

the impacts these actions can have on soil and water quality. These interventions 

produce positive public outcomes, but they present challenges for reconciling local 

Table 7.5—Community management

School Fire Population below Housing Public 

Community quality vulnerability poverty line affordability space

Number of responses

Type Current status 5 6 6 6 4

Future projections 2 5 6 5 4

General trends 3 3 2 4 3

Spatial Local political jurisdiction 5 4 5 5 3

Community level 3 4 4 2 3

Tahoe basin 4 3 3 2 3

Temporal Seasonally 1 4 2 2 3

Annually 2 1 3 1 1

Biannually 1 0 0 2 0

Note: Items in bold represent the most frequent responses in each row.
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GENERAL TECHNICAL REPORT PSW-GTR-226

Figure 7.9—Ranked importance of public agency management issues (n = 11). (Source: Kauneckis 

and Copeland 2008).

12

10

8

6

4

2

0

Research topic

Number of responses

Public/agency relationship

Interagency cooperation 

Effectiveness of 

 evaluation policies

Cost-effectiveness 

 of policies

Conflict reduction mechanism

Less important Somewhat important Most important

and regional goals and clearly communicating actions to the public. Developing an 

explicit means for the systematic investigation of these types of clashes was also 

discussed. The fact that the request was not to resolve specific conflicts, but rather 

for a process to frame future discussions, suggest various methods of policy process 

evaluation could be useful. This incompatibility is further discussed in the recent 

emergency fire management report (California-Nevada Tahoe Basin Fire Commission 2008). 

The social science data needs survey was designed explicitly to address various 

aspects of the policy environment at Lake Tahoe and included questions about a 

variety of public agency management issues (fig. 7.9). The highest ranked items of 

interest were the relationship between government agencies and the public, followed 

by improving interagency cooperation, evaluation of the effectiveness of current 

policies, cost-effectiveness, and conflict reduction tools. 
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

With regard to the types of data, current status was highest in importance in the 

areas of public/government relations, interagency cooperation, and conflict reduction (table 7.6). Evaluation of program effectiveness was directed toward future 

projections suggesting an emphasis on improving the participation and compliance 

with existing policy programs. Across all issue areas, the spatial extent of interest was the Tahoe basin, with some attention to administrative jurisdiction for 

effectiveness evaluation of policy and cost-effectiveness. The temporal extent was 

generally focused on annual or biannual evaluations (table 7.6). 

Table 7.6—Public agency management and collaboration

Public/ Effectiveness Cost Conflict 

agency Interagency of evaluation effectiveness reduction 

Agency relationship cooperation policies of policies mechanisms

Number of responses

Type Current status 5 4 1 2 4

General trends 2 2 2 6 2

Efficiency improvements 1 1 2 3 3

Spatial Local political jurisdiction 2 1 0 0 3

Administrative jurisdiction 0 1 3 4 2

Tahoe basin 8 7 5 5 6

Temporal Monthly 2 2 1 1 3

Annually 3 2 4 3 1

Biannually 1 0 1 1 3

Note: Items in bold represent the most frequent responses.

A related issue discussed by some public agency representatives was the extent 

to which the public understood the environmental quality issues at Lake Tahoe and 

the impact of the various policies. Although some research has been conducted on 

public attitudes and perceptions of the TRPA and its regulatory policies (Kauneckis 

2008, TRPA 2005a, Weible 2007), very little work has been done pertaining to the 

role of public education in helping organizations and agencies in the basin promote 

collaboratively defined desired conditions (for an exception see Ward et al. 2003). 

This includes understanding the impact of current public information and interpretive programs, as well as how these can be designed to be more effective. 

One of the best examples of the crosscutting nature of policy evaluation is the 

current program of private and commercial property BMPs. In the Lake Tahoe 

basin, BMPs are structural and landscape design components intended to reduce 

soil erosion and polluted runoff from private parcels. Best management practices 

include building infiltration systems for stormwater runoff from impervious 
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surfaces such as driveway and rooftops, mulching and revegetating bare or 

disturbed soils, stabilizing steep slopes and loose soils, and paving dirt driveways 

and roads. Best Management Practices were required on new construction 

and remodeling projects beginning in the 1980s. The TRPA instituted BMP 

requirements as part of its basinwide ordinances in 1992 and in 2002 created the 

BMP Retrofit Program targeting existing structures as well as new constructions. 

As of 2003, TRPA ordinance requires BMPs on all private residential, commercial, 

and industrial parcels. Implementation of BMPs on existing private parcels was 

prioritized into three watershed groups, and deadlines have been set for compliance 

and certification. The deadlines for compliance in priority 1 and 2 areas passed in 

2000 and 2006, respectively, and are set for 2008 for priority 3 areas. Despite active 

public outreach via TRPA, university cooperative extension programs, resource 

conservation districts, and the Natural Resources Conservation Service, average 

rates of compliance for the BMP retrofit program are at 16 percent for California 

and 25 percent for the basin as a whole (TRPA 2009: 25). So far there has been 

little attempt at enforcing BMP retrofit requirements on existing structures and 

few actual sanctions imposed. Compliance has occurred on a voluntary basis and is 

likely linked to home renovations (Kauneckis 2008). 

The environmental aspect of BMPs is the primary management strategy for 

dealing with water quality and soil management issues in the Tahoe basin. Their 

environmental aspects are discussed in the “Water Quality” and “Soil Conservation” chapters. Although relatively limited, some work has evaluated the water 

quality benefits of private and commercial BMPs (see Schuster and Grismer 2004). 

However, BMPs also are important to understand as a component of the social 

sciences, as the primary questions raised by stakeholders involved increasing 

implementation on private parcels, the efficiency and cost-effectiveness of current 

designs and location selection, and means of coordination and prioritizing public 

investment in community-scale BMPs for targeting the most effective projects. 

The most salient contemporary program design issue is that of modifying 

regulations that work at cross purposes for protecting environmental quality and 

reducing excess forest fuels following the Angora Fire (California-Nevada Tahoe 

Basin Fire Commission 2008). This included clarifying the private homeowner’s 

responsibility to clear vegetation and pine needle debris to create defensible space 

and requirements to maintain native vegetation and ground cover to minimize 

erosion and polluted runoff. This issue also extends into undeveloped areas such 

as fuel management of areas in stream environment zones, and balancing forest 

thinning practices and biomass management with water quality and erosion control 

functions. 
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

Social science applications for improving environmental management include 

aspects of the management organizations as well as those of the public. Issues of 

engaging public participation, compliance, information, education, and general 

relationships between public agencies and private citizens also were addressed 

explicitly in the stakeholder survey. Public willingness to participate in programs 

was ranked as the most important, followed by public understanding of environmental issues at Lake Tahoe, public perceptions and understanding the effectiveness 

of current policies, access to information on BMPs, and general knowledge of 

public policies (fig. 7.10). In terms of the types of information needed, the majority 

of social science research needs were around items of current status, with some 

attention to future projects (participation and perceptions of policy effectiveness), 

and efficiency improvements (table 7.7). The spatial extent of the data requirements 

were almost universally at the basin level, with some interest in community level. 

7

6

5

4

3

2

1

0

Research topic

Number of responses

Participation

Environmental understanding 

Perception of effectiveness

Information on best 

managment practices

Knowledge of policies

Less important Somewhat important Most important

Figure 7.10—Ranked importance of public information issues (n = 7). (Source: Kauneckis and 

Copeland 2008).
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GENERAL TECHNICAL REPORT PSW-GTR-226

Table 7.7—Public information and outreach 

Information 

on Best 

Environmental Perception of Management Knowledge 

Public Participation understanding effectiveness Practices of policies

Number of responses

Type Current status 3 2 3 3 6

Future predictions 3 1 3 2 2

Efficiency improvements 2 0 2 2 2

Spatial Community level 2 0 0 1 3

Administrative jurisdiction 0 1 1 1 2

Tahoe basin 4 3 5 3 4

Temporal Seasonally 3 2 3 0 2

Annually 0 0 0 3 1

Every 5 years 2 1 2 2 1

Note: Items in bold represent the most frequent responses in each row. 

Although the expected relationship was that this information would be most useful 

on an annual basis, respondents indicated that seasonal was more important. This is 

tied to the winter/summer recreational seasons and likely the need to understand the 

new dynamic of second-home ownership. 

With much of the policy focus at Tahoe on issues of private land management, 

such as BMP implementation and fuels management, the role of public participation is becoming increasingly important. However, the growth of second-home 

ownership presents challenges to maintaining public involvement in community 

and basinwide issues. Yet there are a broad array of methods for informing and 

engaging the public in policy decisionmaking including traditional forms of public 

engagement through community workshops, consensus visioning, public education, and interpretation. There also are a number of methods for more explicitly 

addressing the role of the public. Community-based mapping, using Web-based 

geographic information systems or other visualization techniques were discussed 

in various meetings. The TIIMS is currently developing an online map service to 

show the location and relevant information about EIP projects, and progress on 

defensible space on a parcel-by-parcel basis. Commercial and other open-source 

data visualization programs also are available for better presenting information to 

the general public. Other technology-based methods include such approaches as 

alternative futures and consensus modeling. Alternative-futures modeling presents 

various possible scenarios in land, water, and natural resource management and 

permits stakeholder discussion around the state they would prefer for their community. Alternative-futures modeling has been useful in a number of environmental 
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

management applications (Baker et al. 2004, Van Sickle et al. 2004). Consensus and 

alternative-futures modeling involves simulations of different scenarios in order 

to engage both stakeholders and the broader public in terms of the consequences 

of their actions (or inaction). This typically presents visual output that is easily 

interpreted by the public and allows various parameters over which decisions can be 

made to examine outcomes (Costanza 1998, Van Den Belt 2004). 

Although the history of the creation of the current regulatory structure at 

Tahoe accompanied a high degree of contention, today it may offer a competitive 

advantage for marketing Lake Tahoe as a center of businesses and community 

sustainability. There are numerous areas of potential collaboration between the 

public and private sectors for not only data collection that serves a common interest, 

but also creative partnerships that move beyond regulatory policy and toward forms 

of voluntary and market-based policy instruments. There has already been some 

success with relatively creative programs such as transferable development rights 

and the BMP programs (Kauneckis and Reid 2006, Reid and Kauneckis 2008). 

Other areas worth exploring might include formal recognition of environmentally 

friendly activities by Tahoe-based businesses through green business certification 

programs. There are a number of well-established industry-specific, regional and 

international environmental management and certification systems (ISO 14001 and 

Europe’s Green Dot are the best known). There is good reason to consider creating 

something like a “Tahoe Blue Dot” system that can simultaneously engage and 

reward the business community and allow the marketing of regulatory compliance 

as an asset and sustainable business practices. Other nonregulatory approaches can 

include various forms of “social marketing.” Social marketing programs represent a 

method of communicating public goals, targeting programs to citizens, and focusing the distribution of information to specific population subsectors. Rather than 

regulatory policy and negative sanctions serving as the principal tools to increase 

participation in public programs, it relies on information and positive psychological 

rewards using methods from the private sector to gain brand loyalty and understand 

market niches. Successful applications of social marketing toward public goals 

have been noted in public health and human services delivery (Goldberg et al. 1997, 

McKenzie-Mohr and Smith 1999). 

Managing Fire and Natural Hazards

The effects of wildfire and fuels management were crosscutting themes in many 

of the discussions. Following the 2007 Angora Fire, fuels management became 

particularly salient to the public and government agencies. Additionally, the presence of faultlines in the basin has also alerted stakeholders to the possible threat 
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of seismic events. Both hazards generate important social science needs as well. A 

major forest fire in or near the Tahoe basin would have broad effects on all of the 

theme areas considered in this science plan, including most of the social science 

subthemes. Fire impairs both the quality and quantity of recreation and scenic 

resources, and a major fire could do so for an extended period. This would have 

broad implications for the regional economy, both in terms of direct and indirect 

costs. The more immediate economic concerns have to do with paying for fuels 

management on public, private, and state lands, and prioritizing fuel reduction 

projects. Other concerns discussed in stakeholder meetings include the social 

acceptance of various treatment options for forest fuels. 

Transportation issues are directly linked to community safety questions and 

most discussions regarding public agency management and natural disaster management were directed at creating a basinwide communications network for public 

officials. The Tahoe basin is a seismically active area with potential for large-scale 

events including earthquakes, landslides, and earthquake-triggered seiche waves 

in the lake that would act like a tsunami and flood major portions of the near-shore 

areas (Ichinose et al. 2000). Assessment of the vulnerability and resilience of 

transportation networks, recreation areas, and communities to these natural hazards 

is recommended, so that mitigation strategies can be developed and their likely 

effectiveness understood. This will certainly aid management agencies and local 

governments in long-term planning goals.

Climate Change Impacts

Finally, global climate change represents a relatively new research area that 

deserves some discussion. Because the projected changes are so broad-based, there 

are impacts on all the subthemes discussed in this chapter. The most immediate 

effect would be in terms of the interactions between recreation, scenic resources, 

and the local economy. 

If changes occur as projected by most climate models, the Sierra Nevada is 

likely to experience substantial reductions in the amount of snowfall, and the snowpack that does exist is expected to melt off earlier in the year (Cayan et al. 2006). 

Both of these effects will lead to a reduced areal extent and season length for ski 

resorts and backcountry snow-based recreation while simultaneously increasing the 

season of some forest-based recreation such as hiking and mountain-biking, all of 

which are important parts of the recreation experience at Tahoe. Changes in precipitation and temperature will likely affect vegetation and pest dynamics and increase 

the threat of major forest fires (Ibañez et al. 2006, Westerling et al. 2006). The 

short- and long-term economic responses to, for example, a ski season shortened 
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

by the reduced snowpack expected from climate change or summer recreational 

opportunities impacted by increased wildfires are important to assess now so that 

mitigation and response plans can be developed. Reduction in snowfall, the risk 

of wildfire, and resulting impact on water quality and scenic resources related to 

forest cover all suggest major changes for the regional economy, which could be 

examined through alternative futures modeling. 

Near-Term Social Science Research Priorities

The most consistently discussed topics related to issues of transportation, economics, recreation, and the quality of the built landscape. Common metathemes were 

the development of a collaborative information management infrastructure, increasing the evaluation and effectiveness of current policy, a refocusing of management 

decisions toward the community level, and management of fire hazards and climate 

change impacts. Table 7.1 illustrates items identified as the highest priority.

The selection of this subset of priorities was based on a number of criteria. 

First, a social science research issue area had to reappear across the multiple 

information collection methods used here: literature review, stakeholder meetings 

and workshops, focused discussions with key public and private representatives, 

and the stakeholder survey. Second, there was an estimation of the complexity of 

the research issues and the likelihood that resources dedicated to a specific topic 

would lead to immediate improvements in environmental management decisions. 

This was balanced against a movement toward more complex tasks and laying out 

the necessary information infrastructure for improving long-term decisionmaking 

as discussed in more detail below. Finally, emphasis was placed on research issues 

that served the broadest array of stakeholders. Those that were requested by a mix 

of private, public, and community interests were considered more important than 

those of a smaller set of agency representatives. The specific justification for the 

selection of priority research items are indicated in table 7.1. The conceptual model 

in fig. 7.5 illustrates the relationship between the highest priority social science 

research needs and the causal driver and linkages to specific management components. 

In the opinion of the authors, the crosscutting metatheme of collaborative 

information management is the highest overall priority. This is based on three 

rationales. First, this was the most common request across all the subthemes 

and by that fact alone can be ranked as the highest priority. Second, in terms of 

practically advancing the goals of this document, strategically allocating resources 

toward data collection will allow a common focal point for stakeholder input by 
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creating a platform for distribution of results from other research areas. Third, 

there has already been substantial activity by stakeholders in this direction. Input 

from the chambers of commerce has enthusiastically expressed support for joint 

data collection and a subset of priorities for data inventory and consolidation has 

recently been released (U.S. Army Corps of Engineers 2008). Taking advantage 

of the current level of interest and the resources that have already been dedicated 

should move this to the top priority. Creating a platform for a small subset of 

common topics of interest will make data collection and consolidation in each of 

the individual subthemes easier. There are exceptional opportunities for creative 

public-private cooperation. 

There also has been interest expressed toward the creation of a Tahoe basin 

decision-support system. Decision-support systems (DSS) use a common computer 

platform that brings together multiple data sources on management issues to better 

inform decisionmakers about the relative tradeoffs of various policy options. These 

have most commonly been used for natural resource management agencies making 

decisions for multiple-use areas; however, there is considerable potential for using 

DSS for integrating the social sciences with environmental goals. The advantage 

of a DSS is that it creates a library of integrated data sets, models, and methods. It 

can act as a focal point of discussion for research across different disciplines and 

stakeholders. Additionally many are scalable and present data in a useable visual 

output that implicitly incorporates uncertainty and highlights missing and needed 

data (Reynolds et al. 2000). The concurrent discussions on the implementation of 

DSS and community indicators by very different stakeholders presents an opportunity for better integrating the social sciences with management decisions. 

It is worth repeating a couple of final comments and caveats. The process 

outlined here for scoping the need and form of various types of social science data 

was intended to develop a framework for further discussion, not the priorities of 

data collection needs. These priorities are best determined by specific stakeholders 

and focused on direct policy needs. This is particularly true in collaborative 

situations involving multiple organizations as much of the academic literature 

stresses that collaboration works best when there are substantive outcomes that 

each participant recognizes as useful (Kauneckis and Imperial 2007, Lubell 2004, 

Singleton 1998, Wondolleck and Yaffee 2000). This document outlines a broad 

array of social science data needs and perceptions of the relative utility of various 

forms of data in terms of the type and spatial and temporal scales. However, 

it does not measure the costs of obtaining the data in its most useful form, nor 

the willingness by stakeholders to commit resources toward the collection, 

management, and distribution of useful information products. 
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An Integrated Science Plan for the Lake Tahoe Basin: Conceptual Framework and Research Strategies

There may be opportunities to advance the research needs discussed here 

by establishing closer ties with one or more of the regional campuses. The University of California, Davis and the Desert Research Institute already have close 

research connections with resource managers in terms of the natural sciences. A 

similar effort can be made in the social sciences. University of California, Davis 

has numerous departments—particularly the Department of Environmental 

Policy—that can bring expertise and student resources to many of the research 

issues discussed in this chapter. The University of Nevada, Reno (UNR) likewise 

has seen recent activity in expanding its research capacity in the social sciences. 

New research centers and programs established in the past 5 years include the UNR 

Academy for the Environment, the School of Journalism’s Interactive Environmental Journalism program, in addition to well-established programs in cooperative 

extension and other departments. Graduate programs on both campuses include 

business and economics, resource economics, land use planning, public administration, and policy analysis. Much of the research discussed here involves relatively 

straightforward data collection, consolidation, and statistical analysis, all of which 

are amendable to graduate student research. 

Finally, it is worth returning to the concepts discussed earlier about the distinction between the collection and consolidation of data, making that data relevant and 

contextualized for informing policy, and turning that information into knowledge 

about the social dynamic of Lake Tahoe. Although the feasibility of data collection 

and research on the various topics of interest listed here has not been explicitly 

measured, few of the issues are beyond the scope of contemporary social science 

research methods. However, some of these studies would require substantial costs 

and are best addressed in terms of the relative benefits to informing policy decisions. Although the majority of input from stakeholders was on data collection, 

most of the subsequent discussion was about its priority as useful information and 

bringing the relevant knowledge to bear on policy decisions. Because of the high 

costs of primary data collection and continued monitoring, it is recommended 

that the starting point of prioritizing research efforts be on the direct application 

of the information for a specific management need, rather than only a generalized unspecified perception of the importance of more information on a topic. 

Stakeholder meetings often included interest in having information on an issue of 

concern without the necessary followup discussion of how that information can be 

put to direct use. Focusing on the management use of prior data collection efforts 

will help focus the priority of even basic data collection and consolidation, as well 

as assist in honing it down to those pieces of information that will best inform the 

broadest array of decisions and serve the most stakeholders. 
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Acknowledgments

A number of people helped schedule and coordinate the community workshops, 

agency meetings, and breakout session at the Tahoe Science Symposium: Steve 

Teshara, Carl Ribaudo, and Andrew Strain of the Tahoe Chambers of Commerce; 

Zach Hymanson of the Tahoe Science Consortium; Neil Crescenti, Keith Norberg, 

and Lisa O’Daly of the Tahoe Regional Planning Agency; Ken Anderson of California State Parks; Cindy Gustafson of the Tahoe City Public Utilities District; Susan 

D. Kocher of the UC Davis, Cooperative Extension, El Dorado County; Christy 

Prescott of the U.S. Forest Service, Lake Tahoe Basin Management Unit. Marlene 

Rebori was the facilitator for the breakout session at the Tahoe Science Symposium; 

Kevin Drake was the notetaker. A number of anonymous external reviewers at the 

U.S. Environmental Protection Agency offered valuable input and comments. 

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