EPA/600/R-06/037F 

March 2008 

Application of Watershed Ecological Risk 


Assessment Methods to Watershed Management


National Center for Environmental Assessment


Office of Research and Development


U.S. Environmental Protection Agency


Washington, DC 20460

DISCLAIMER 

This document has been reviewed in accordance with U.S. Environmental Protection 

Agency policy and approved for publication. Mention of trade names or commercial products 

does not constitute endorsement or recommendation for use. 

ABSTRACT 

Watersheds are frequently used to study and manage environmental resources because 

hydrologic boundaries define the flow of contaminants and other stressors. It is a challenge to 

incorporate scientific information in watershed management and planning. Ecological assessments 

of watersheds are complex because watersheds typically overlap multiple jurisdictional 

boundaries, are subjected to multiple environmental stressors, and have multiple stakeholders with 

diverse environmental and socioeconomic interests. Ecological risk assessment (ERA) is an 

approach that has successfully been used to increase the use of ecological science in decision 

making, by evaluating the likelihood that adverse ecological effects may result from exposure to 

one or more stressors, yet its application to watershed assessment is limited. The purpose of this 

report is to provide suggestions and examples for making scientific information more relevant to 

the needs of watershed managers by using ERA principles to help structure ecological 

assessments of watersheds. 

This report supplements the Guidelines for Ecological Risk Assessment (U.S. EPA 

1998a) by addressing issues commonly encountered when conducting watershed ecological 

assessments. Suggestions and examples to follow are provided based upon lessons learned from 

prior watershed ERAs. This report is of potential use to ecologists, hydrologists, watershed 

managers, risk assessors, landscape ecologists, and other scientists and managers seeking to 

increase the use of environmental assessment data in decision making. 

Each activity and phase of the watershed ERA process is explained sequentially in this 

report. Guidance on how to involve stakeholders to generate environmental management goals 

and objectives is provided. The processes for selecting assessment endpoints, developing 

conceptual models, and selecting the exposure and effects pathways to be analyzed are described. 

Suggestions for predicting how multiple sources and stressors affect assessment endpoints are 

also provided; these include using multivariate analyses to compare land use with biotic 

measurements. In addition, the report suggests how to estimate, describe, and communicate risk 

and how to evaluate management alternatives. 

Preferred Citation: 

U.S. Environmental Protection Agency (EPA). (2007) Application of watershed ecological risk assessment 

methods to watershed management. National Center for Environmental Assessment, Washington, DC; 

EPA/600/R-06/037F. Available from National Technical Information Service, Springfield, VA, and online at 

http://www.epa.gov/ncea. 

ii 
CONTENTS


LIST OF TABLES .......................................................... v 


LIST OF FIGURES .......................................................... v 


LIST OF ABBREVIATIONS AND ACRONYMS ................................. vi


AUTHORS, CONTRIBUTORS, AND REVIEWERS ............................... vii


1. EXECUTIVE SUMMARY ................................................. 1 


2. INTRODUCTION ........................................................ 5 


2.1. WHO WILL USE THIS REPORT ........................................ 7 


2.2. WHAT IS WATERSHED ECOLOGICAL RISK ASSESSMENT? ............... 7 


2.3. THE WATERSHED ERA PROCESS ..................................... 8 


3. PLANNING PHASE ..................................................... 12


3.1. IDENTIFYING AND INCLUDING STAKEHOLDERS ...................... 12


3.2. SETTING ENVIRONMENTAL MANAGEMENT GOALS AND OBJECTIVES ... 14


3.3. DEFINING MANAGEMENT OPTIONS .................................. 17


3.4. DETERMINING THE FOCUS, SCOPE, AND COMPLEXITY OF THE RISK


ASSESSMENT ..................................................... 17


3.5. PRODUCTS OF THE PLANNING PHASE ............................... 18


4. PROBLEM FORMULATION PHASE ........................................ 19


4.1. IDENTIFY STRESSORS AND SOURCES ................................ 20


4.2. SELECTING ASSESSMENT ENDPOINTS ............................... 21


4.3. DEVELOP CONCEPTUAL MODEL .................................... 22


4.4. EVALUATE ASSESSMENT ENDPOINTS ............................... 26


4.5. CREATE ANALYSIS PLAN ........................................... 27


5. RISK ANALYSIS PHASE ................................................. 29


5.1. STRESSOR IDENTIFICATION EVALUATION (SIE) AND CAUSAL


EVALUATION: IDENTIFY STRESSORS AND EVALUATE CAUSES OF


IMPAIRMENT ..................................................... 30


5.2. EVALUATE STRENGTH OF ASSOCIATION BETWEEN CAUSES 


AND EFFECTS ..................................................... 31


5.3. WORK WITH CHEMICAL STRESSORS ................................. 32


5.4. WORK WITH NONCHEMICAL STRESSORS ............................. 33


5.5. WORK WITH DATA LIMITATIONS .................................... 35


5.5.1. Use Categorical Data ............................................ 35


5.5.2. Use Source Data ............................................... 36


5.5.3. Rank Risks .................................................... 38


5.6. DEAL WITH UNCERTAINTY ........................................ 40


5.7. RECOGNIZE THAT ANALYSIS IS AN ITERATIVE PROCESS .............. 41


5.8. UNDERSTAND SECONDARY OR INDIRECT PATHWAYS ................ 42


iii 
CONTENTS (continued) 

6. RISK CHARACTERIZATION PHASE ....................................... 44


6.1. RISK DESCRIPTION ................................................ 44


6.2. DESCRIBING UNCERTAINTY ........................................ 49


6.3. RISK CHARACTERIZATION COMPONENTS ............................ 51


7. RISK COMMUNICATION ................................................ 53


7.1. THE RISK COMMUNICATION PROCESS ............................... 53


7.2. ADDRESS THE TARGET AUDIENCE .................................. 54


7.3. COMMUNICATE EFFECTIVELY ...................................... 55


8. RISK MANAGEMENT ................................................... 56


8.1. EVALUATING MANAGEMENT ALTERNATIVES ........................ 56


8.2. APPLICATION TO DECISION MAKING IN THE EPA CASE STUDIES ....... 58


8.3. APPLICATION TO THE TOTAL MAXIMUM DAILY LOAD PROCESS ........ 60


8.4. APPLICATION TO THE UAA PROCESS ................................ 61


9. CONCLUSIONS ........................................................ 67


GLOSSARY .............................................................. 69


REFERENCES ............................................................ 72


iv 
LIST OF TABLES 

1.	 Prominent aspects of the watershed approach and comparable aspects of


the ecological risk assessment process ...................................... 8 


2.	 Effects matrix summarizing assumed strength of relationships between assessment


endpoints identified (e.g., percent eelgrass cover) and stressors in the Waquoit Bay


watershed .......................................................... 21


3.	 Middle Snake River case study: integration of stressors, responses, and recovery


potential for the reproduction, growth, and survival of the mountain whitefish


population .......................................................... 46


LIST OF FIGURES 

1.	 The ecological risk assessment framework ................................... 9 


2.	 Relationships between management or ecological risk assessment (ERA) goals 


and objectives, management options, the planning phase, and subsequent


problem formulation in watershed ERA .................................... 13


3.	 Elementary conceptual model ............................................ 23


4.	 Conceptual model of the Waquoit Bay watershed ............................. 25


5.	 Significant relationships between urban land use activities and invertebrate ephemeroptera,


plecoptera, and trichoptera (EPT) score .................................... 37


6.	 Historical changes in nitrogen (N) loading predicted by the nitrogen loading model ... 47


7.	 Fish index of biotic integrity (IBI) or insect ephemeroptera, plecoptera, and 


trichoptera (EPT) values in relationship to proximity to coal mining sources 


in the Clinch Valley ecological isk assessment ............................... 48


8.	 Thermometer scale for relative ranking of management options .................. 57


9.	 General steps involved in conducting a use attainability analysis (UAA) 


using a watershed ecological risk framework ................................ 64


v 
LIST OF ABBREVIATIONS AND ACRONYMS


BPJ Best professional judgment 

BMP Best management practices 

CADDIS Causal Analysis/Diagnosis Decision Information System 

COLD Conditions suitable to support coldwater ecosystems 

CWA Clean Water Act 

CWAP Clean Water Action Plan 

EIS Environmental Impact Statement 

EPA U.S. Environmental Protection Agency 

ERA Ecological risk assessment 

ESA Endangered Species Act 

EPT Ephemeroptera, Plecopera, and Trichoptera 

FIFRA Federal Insecticide, Fungicide, and Rodenticide Act 

FWS U.S. Fish and Wildlife Service 

GIS Geographic information system 

HSI Habitat Suitability Index 

IBI Index of Biotic Integrity 

ICI Index of Community Integrity 

LOE Lines of evidence 

LIT Literature search 

Miwb Modified Index of Well-Being 

MOS Margin of safety 

NCEA National Center for Environmental Assessment 

NEPA National Environmental Policy Act 

NRC National Research Council 

RRM Relative Risk Model 

SAB Science Advisory Board 

SIE Stressor Identification Evaluation 

TMDL Total maximum daily load 

TNC The Nature Conservancy 

TSCA Toxic Substances Control Act 

TVA Tennessee Valley Authority 

UAA Use Attainability Analysis 

WQS Water quality standards 

vi 
AUTHORS, CONTRIBUTORS, AND REVIEWERS


The National Center for Environmental Assessment within EPA’s Office of Research and 

Development was responsible for preparing this document. This document was written by the 

primary author and several co-authors from Tetra Tech, Inc., under contract number #3W-2881­

NATX. EPA staff and external experts reviewed this report. Three of the external reviewers 

were independently selected by Versar. 

Project Officer: 

Victor Serveiss, U.S. EPA, National Center for Environmental Assessment, Washington, DC 

Authors: 

Victor Serveiss, U.S. EPA, National Center for Environmental Assessment, Washington, DC 

Jerome Diamond, Tetra Tech, Inc., Owings Mills, MD 

Jonathon Butcher, Tetra Tech, Inc., Owings Mills, MD 

Jeroen Gerritsen, Tetra Tech, Inc., Owings Mills, MD 

Benjamin Jessup, Tetra Tech, Inc., Owings Mills, MD 

Reviewers: 

EPA Reviewers: 

Barbara Mazur, Region V, Chicago, IL 

Leela Rao, formerly with NCEA, Washington, DC 

Gina Ferreira, Region II, New York, NY 

Randy Bruins, formerly with NCEA, Cincinnati, OH 

Katherine Sciera, Region VI, Dallas, TX 

Jeff Frithsen, NCEA, Washington, DC 

External Reviewers: 

Wayne Landis, Western Washington University, Bellingham, WA 

Peter DeFur, Environmental Stewardship Concepts, Richmond, VA 

Bruce K. Hope, Oregon Department of Environmental Quality, Portland, OR 

Mark T. Southerland, Versar, Inc., Columbia, MD 

Barbara S. Washburn, Office of Environmental Health Hazard Assessment, Sacramento, CA 

David Dow, National Oceanic and Atmospheric Administration, Woods Hole, MA 

Acknowledgments: 

Technical Editing Contributors: 

Julie Blue, The Cadmus Group, Inc., Watertown, MA 

Roy Schiff, Milone and MacBroom, Inc., Cheshire, CT 

Chi Ho Sham, The Cadmus Group, Inc., Watertown, MA 

Karen Swetlow, KBM Group, Silver Spring, MD 

vii 
1. EXECUTIVE SUMMARY 

Environmental managers are increasingly using a watershed approach for making 

environmental decisions. These environmental managers include state, federal, or local regulatory 

or resource agency staff; local and county officials; watershed associations or councils; and river 

watch citizen groups. Many environmental decisions are made with much uncertainty, and the 

quality of such decisions would increase if more science were used in the decision making 

process. Ecological risk assessment (ERA) is a process to collect, organize, and analyze scientific 

information in order to evaluate the likelihood that adverse ecological effects may occur or are 

occurring as a result of exposure to one or more stressors (U.S. EPA, 1998a). This document 

discusses the merits of applying ERA principles to watershed management and describes how 

more science can be used in decision making. 

The watershed approach is a framework for coordinating environmental management that 

focuses public- and private-sector efforts on addressing the highest priority problems within a 

hydrologically-defined geographic area (U.S. EPA, 1996). Watershed ERA integrates ERA 

methodologies with the watershed approach to improve the use of environmental monitoring and 

assessment data in watershed and regional decision making. This document is written primarily to 

provide guidance and examples for ecological risk assessors, scientists, and managers performing 

ecological assessments of watersheds. Some of the principles also should be useful to landscape 

or seascape ecologists performing spatial-scale environmental assessments and to others seeking 

to increase the use of ecological science in decision making. The document is structured around 

the elements of watershed ERA (planning, problem formulation, risk analysis, risk 

characterization, risk communication, and risk management) and is based primarily on lessons 

learned from EPA-sponsored case studies. 

Watershed ERAs are complex because a watershed typically overlaps multiple 

jurisdictional boundaries and has multiple stakeholders with diverse environmental and 

socioeconomic interests. Planning is essential in ensuring that watershed ERAs address locally 

based socioeconomic and scientific challenges. Key elements of planning include identifying and 

involving relevant stakeholders, developing environmental management goals and objectives, and 

agreeing on the focus, scope, and complexity of an assessment. Appropriate goals and objectives 

should be based on existing watershed management plans, environmental organizations’ mission 

statements, survey results, and other stakeholder opinion data. An interdisciplinary team of 

scientists and managers who understand ecological principles and possess local ecological and 

socioeconomic knowledge is needed to develop a set of scientific assessment objectives and 

management options. 

1

During watershed assessments, the needs of managers and stakeholders may change and 

managers may need to take action before an assessment is completed. Thus, flexibility on how the 

ERA process is implemented, along with regular and recurring interactions between scientists and 

managers throughout the assessment process, are essential. ERA, and especially watershed ERA, 

is an iterative process. It is unlikely that everything will proceed precisely in the chronological 

manner presented in this report. 

Watershed ERA problem formulation provides an organizing framework for the entire 

assessment. Assessment endpoints are identified and conceptual models and an analysis plan are 

developed. Assessment endpoint selection translates abstract environmental management 

objectives into specific, well-defined, and identifiable attributes of the system. Establishing a 

linkage between abstract management objectives and specific system attributes is particularly 

important in watershed assessments because of the ultimate need to choose between competing 

management objectives. Several assessment endpoints may be identified, but only a subset of 

them may be analyzed because resources, data availability, or realistic management control 

options may be limited. To select a subset of assessment endpoints to be analyzed, an assessment 

team composed of risk assessors, scientists, engineers, policy analysts, managers, and other 

professionals may use their best professional judgment to assess stressor impact and risk reduction 

opportunities. 

Conceptual models are especially valuable in watershed assessments because they describe 

multiple physical, chemical, and biological stressors, their sources, and the pathways by which 

they affect various valued ecological resources. Due to the large number of pathways to be 

considered, the conceptual model can make it easier to identify causal pathways and establish 

priorities for analytical efforts. The model may also evolve as the assessment progresses and a 

better understanding of pathways and impacts is acquired. 

The final product of problem formulation is an analysis plan documenting the proposed 

approach for the assessment. Sometimes, when assessment resources are limited or a decision 

must be made quickly, it may not be practical to complete a quantitative and formal assessment. 

In these instances, the qualitative information from problem formulation can be used for decision 

making. 

Assuming the assessment team proceeds with the ERA, the next step is the risk analysis 

phase. It is ideal to develop stressor-response relationships that relate the magnitude, duration, 

frequency, and timing of exposure to the biological effects. However, generating such 

relationships for multiple interacting sources, stressors, and endpoints for a watershed assessment 

is not trivial. Geospatial analysis using geographic information system (GIS) tools usually play a 

pivotal role in analyzing land use, land cover, and other stressor and response data. To be cost 

effective and efficient, a watershed assessment might need to focus on assessing the relationships 

2

of one or a few major stressors (or their sources) on a small set of ecological effects rather than 

on attempting to quantify each stressor’s exposure and effect on each endpoint. For instance, the 

focus could be on studying the relationship between percentage of urban land and fish species 

richness. Another approach is to examine relationships between groups of stressors, habitat data, 

and impact data (i.e., groups of responses). High relative risk occurs in the spatial areas where 

source, habitat, and impact co-occur. Categorical data or scores from one to five may be used 

rather than numerically modeled or predictive estimates. Moreover, it may be necessary to 

extrapolate results from a similar watershed as long as uncertainties are adequately documented. 

When data on the assessment endpoint are not available, a measure of effect on a surrogate may 

be commonly used instead. Finally, exposure and effects data may need to be aggregated, thus 

implying that the typical risk analysis phase is performed in conjunction with the next phase—risk 

characterization. 

ERA risk characterization consists of risk estimation and risk description. Risk estimation 

is a difficult task because stressors can affect valued ecological resources in different habitats via 

multiple interacting pathways. The exposure and effects characterizations (or their aggregated 

estimates, as may be necessary in a watershed context) developed in the prior phase should be 

used as lines of evidence to support particular risk estimates. Lines of evidence also may include 

models, field work, and analyses performed elsewhere. All the lines of evidence need to be 

brought together to reach a final conclusion about the likelihood and the consequences of effects. 

The spatial distribution of exposure and effect is important to consider and is often assessed and 

presented using GIS data layers. 

Risk description summarizes and presents information so that choices among alternative 

courses of action can be made with knowledge of the outcomes. Descriptions can be qualitative 

statements such as “option A causes more risk than option B.” Effects data also can be colorcoded onto maps of the watershed, allowing stakeholders to see potential impacts in the lakes and 

rivers that matter most to them. A data table can be used to show the various lines of evidence 

and to summarize the conclusions of risk and recovery potential to various life stages. Risk 

assessments need to be peer reviewed to help convince skeptics. Assumptions and uncertainty 

can be summarized in data tables or described in narrative form. 

Successful communication of risk assessment findings requires that communication occur 

throughout the assessment process to ensure the results reflect the management objectives, 

decision context, and environmental conditions that exist at the time the information is needed. 

Managers need to tell assessors what they need, and scientists need to describe what they can 

provide. Effective risk communication must accurately translate the best available and most 

useful scientific information in a manner understandable to managers and stakeholders. 

Depending on the audience, the risk assessment information may need to be provided in a 

3

nontechnical manner. Clear presentation of results (e.g., maps, figures with simple dose-response 

curves) will enhance understanding of findings and foster consensus among stakeholders. 

Watershed ERA principles can help watershed managers make more informed total 

maximum daily load (TMDL) decisions or take a wiser approach to identify or attain beneficial 

use designations. ERA principles can also assist managers in resource planning, making land use 

zoning decisions, and implementing best management practices (BMPs). 

4

2. INTRODUCTION


This report discusses the merit of applying ERA principles to watershed management and 

describes how science can be better integrated into decision making. Environmental managers 

are increasingly using a watershed approach for making decisions. Environmental managers 

include regulatory or resource agency (state, federal, local) staff and any party with authority to 

implement a management plan (e.g., local and county officials with zoning oversight), watershed 

associations or councils, and river watch citizen groups. Many environmental management 

decisions are based on uncertain information and would benefit from having a stronger scientific 

basis. 

ERA is a process to collect, organize, and analyze scientific information in order to 

evaluate the likelihood that adverse ecological effects may occur or are occurring as a result of 

exposure to one or more stressors (U.S. EPA, 1998a). ERA principles are relevant to 

environmental management because even though most decisions are made without a risk 

assessment, the decisions are fundamentally about managing ecological risks. 

EPA and others, through various regulations, have reduced the impact of point source 

pollution and it is now widely recognized that further substantial improvements in the quality of 

U.S. lakes, streams, rivers, creeks, and ponds depend on controlling nonpoint source pollution 

(U.S. EPA, 2006). Nonpoint source pollution problems are not as well addressed by existing 

regulations because of the difficulties in identifying diffuse sources and ill-defined pathways. 

Efforts to address nonpoint source pollution rely heavily on voluntary compliance, stakeholder 

involvement, and a scientific understanding of the cumulative impacts of multiple physical, 

chemical, and biological stressors over a broad range of spatial scales. 

Consistently incorporating science in watershed management decisions, however, is 

challenging. Multiple physical, chemical, and biological stressors result from human activities. 

These stressors, when combined with a network of interrelated environmental conditions, cause 

diverse impacts on numerous ecological resources. Tradeoffs among environmental, political, 

economic, and social factors often result from subjective value judgments that occur as part of the 

decision process. It is sometimes difficult to implement scientifically supportable actions because 

of conflicting interests, lack of certainty (or public trust) in the science, and poor communication 

of the risks and benefits, particularly to those who must face the risk. As a result of these 

challenges, data from many monitoring and assessment efforts frequently do not play a major role 

in management decisions (Ward, 1996; Ward et al., 1986). 

Helpful suggestions and frameworks have been developed to increase the use of science in 

watershed and regional environmental management (Rhoads et al., 1999; Maxwell, 1998; U.S. 

EPA, 1996; Armitage, 1995; MacDonald, 1994; Slocombe, 1993; Ward et al., 1986). Guidelines 

5

for Ecological Risk Assessment (U.S. EPA, 1998a), written for all types of ecological risk 

assessment, generally addresses the application of ERA to watershed and regional assessment. 

Yet the application of ERA to these larger scales is still somewhat limited (Serveiss, 2002; U.S. 

EPA, 2000a). This report supplements the ERA guidelines by providing additional guidance for 

those performing watershed and regional ecological assessments. 

In the mid-1990s, EPA’s Risk Assessment Forum and Office of Water cosponsored the 

development of five demonstration watershed ERAs to test application of the ERA Framework 

(U.S. EPA, 1992) to Office of Water programs. The five watershed assessments were performed 

in the Clinch and Powell Valley, VA, Middle Snake River, ID, Waquoit Bay, MA, Big Darby 

Creek, OH, and the Middle [segment of the] Platte River, NE. All five selected sites had valued 

ecological resources, multiple stressors, an existing data set, and willing assessment participants. 

Using these assessments, many key aspects of the watershed ERA process, including challenges 

encountered and lessons learned, were documented. Several researchers (Diamond et al., 2002; 

Serveiss, 2002; U.S. EPA, 2000b; Butcher et al., 1998) reviewed these assessments and 

concluded that use of the risk assessment approach can add significant value to watershed 

management. 

Butcher et al. (1998) described the benefits of applying the integrated watershed risk 

assessment approach in a review of these five case studies, including the following: 

•	 The risk assessment framework can add significant value to watershed-scale 

management programs that follow the watershed approach, particularly when 

addressing problems caused by multiple and nonchemical stressors. 

•	 The watershed approach is expected to benefit from use of the formal and scientifically 

defensible methods of risk assessment for prioritizing and evaluating risk. 

•	 Although best professional judgment may arrive at the same conclusions as an ERA, 

the process helps people to carefully examine what led them to their conclusions and 

document their findings. 

•	 Simpler methods will be required for applying watershed ERA to environmental 

management on a widespread basis. 

In addition to the EPA case studies, many other watershed assessments have been 

published; several of these are referenced in this report. A comprehensive review of watershed 

assessment literature would require a much larger report and is beyond the scope of this effort. 

6

2.1. WHO WILL USE THIS REPORT? 

This report is principally written for environmental scientists and risk assessors seeking to 

provide scientific information for watershed management. Watershed managers, coordinators, 

and others participating in watershed assessment may also find this report useful. The report also 

provides valuable information to researchers, educators, and students in the fields of aquatic 

ecology, landscape and seascape ecology, ecological risk assessment, and watershed management. 

Although the focus of this report is on watershed assessment, many of the principles can be 

applied to any environmental monitoring and assessment activity intended to inform 

decisionmakers. Finally, with its examples and descriptions of watershed ERA, this report also 

serves as an easy-to-understand introduction to ERA. 

2.2. WHAT IS WATERSHED ECOLOGICAL RISK ASSESSMENT? 

Watershed ERA combines the watershed approach with ERA. Before describing EPA’s 

watershed ERA approach, we need to first describe the watershed approach. 

Watershed Approach 

The watershed approach is organized around the guiding principles of partnerships, 

geographic focus, and management based on sound science and data. The watershed approach is 

a framework for coordinating environmental management that focuses public- and private-sector 

efforts on addressing the highest priority problems within a hydrologically-defined geographic 

area (U.S. EPA, 1996). The watershed approach should involve the pertinent levels of 

government, users of watershed resources, environmental groups, those believed to cause 

environmental problems, and the public and should help them better understand the specific 

problems in hand, identify and agree on goals and priorities, and choose and implement solutions. 

Because watersheds often cross political boundaries, it is important to involve stakeholders across 

these boundaries. 

EPA and other agencies are increasingly using watersheds for environmental management 

because watersheds are naturally cohesive hydrologic units that are spatially appropriate for 

management actions (Maxwell, 1998; U.S. EPA, 1995a). For example, the Tennessee Valley 

Authority has River Action Teams, in which members share monitoring information with key 

stakeholders (i.e., regulatory agencies, state and local governments, businesses and industries, 

citizen-based action groups, and watershed residents) to obtain their support in developing and 

implementing protection and mitigation plans (U.S. EPA, 2002a). The Canaan Valley Task 

Force in West Virginia produced an inventory of environmental stressors associated with 

ecosystem problems and their causes, determined whether the problems are getting worse, and 

developed solutions (U.S. EPA, 1997a). In addition, many other states have implemented 

7

watershed restoration actions in response to the Clean Water Act’s Section 319, nonpoint source 

pollution program (U.S. EPA, 2002b). These strategies focus management actions on geographic 

regions rather than on specific media (e.g., air or water). 

The prominent aspects of the watershed approach and comparable aspects of the ERA 

process are shown in Table 1. Assessment is one of the most critically important parts of 

watershed management because it attempts to transform scientific data into policy-relevant 

information that can support decision making and action. A watershed assessment may be 

initiated by any concerned party in the watershed, including citizen groups, regional coalition 

members, municipal officers, decisionmakers, litigants, ecological scientists, and a number of 

other stakeholders regardless of title or association. Watershed ERA provides a basis for 

estimating ecological effects as a function of exposure to various sources of stress in the 

watershed. This approach helps environmental managers focus analyses on the highest priority 

problems in the watershed. 

Table 1. Prominent aspects of the watershed approach and comparable 

aspects of the ecological risk assessment process 

Watershed Management Ecological Risk Assessment 

Geographic focus The scope of the assessment is identified during 

problem formulation. 

Partnerships and stakeholder 

involvement 

Interactions with managers and stakeholders are 

encouraged, particularly during problem formulation 

and risk characterization phases. 

Continuous improvement based on 

sound science 

The analysis and risk characterization phases provide 

and organize scientific information relevant to 

management decisions. 

Source: Serveiss (2002). 

2.3. THE WATERSHED ERA PROCESS 

Watershed ERA integrates the central aspects of the watershed approach, hydrologically 

defined geographic boundaries, stakeholder involvement, and sound management, with ERA. 

Although the ERA process is presented in a linear fashion, in reality it is an iterative process that 

includes a regularly occurring dialogue between scientists and managers. This iterative process is 

represented by the two-sided “arrows” in Figure 1 and by the vertical box at the right of the 

diagram denoting tasks that may need to be revisited along the way. Stakeholders should be 

involved to ensure relevancy to their concerns. The iterations and risk communication take on 

8

Planning

(Risk Assessor/

Risk Manager/

Interested Parties

Dialogue)

As Necessary: Acquire Date, Iterate Process 

Monitor Results 

Ecological Risk Assessment 

PROBLEM FORMULATION ANALYSIS 

RISK CHARACTERIZATION 

Risk Management and 

Communicating Results to 

Interested Parties 

Characterization 

of 

Exposure 

Characterization 

of 

Ecological 

Effects 

Communicating Results 

to the Risk Manager 

Figure 1. The ecological risk assessment framework. 

Source: U.S. EPA (1998a). 

9 
greater importance in a watershed assessment with multiple stressors, pathways, ecological 

resources, and diverse managerial and stakeholder interests (Serveiss, 2002). Based on the 

nature and timing of risk management needs, scientific findings, and resources available for the 

assessment, various phases of watershed ERA may need to be revisited repeatedly or skipped 

entirely. For presentation purposes, this report covers the six steps of ERA sequentially: 

planning, problem formulation, risk analysis, risk characterization, risk communication, and risk 

management. 

Planning 

During planning, the stakeholders are identified and assembled with the goal of sharing 

information. Their input is needed to establish the focus, scope, and complexity of the risk 

assessment. The workgroup needs to identify the boundaries for study (see Chapter 3). 

Problem Formulation 

During problem formulation, assessment objectives are refined and information about the 

watershed and resources at risk is collected and shared. In addition, assessment endpoints are 

selected. A conceptual model is created showing pathways between sources, stressors, effects, 

and assessment endpoints. The associated risk hypotheses are described and environmental 

management options are developed based on stakeholder input. Finally, an analysis plan is 

developed to guide future steps of the assessment (see Chapter 4). 

Risk Analysis 

The objectives of the risk analysis phase are to gain a better understanding of (1) the 

extent to which ecological resources have been or will be exposed to the most important 

environmental changes resulting from human activities, and (2) what effects are likely to occur 

or have already occurred as a result. Characterizations of exposures and effects can take the 

form of graphs, models, maps, or other illustrations of the relationships among sources, stressors, 

and measures of effect. These characterizations can be complex because of multiple interactions 

among the various watershed ecosystem components (see Chapter 5). 

Risk Characterization 

Risk characterization describes the linkages between exposure and potential effects. The 

lines of evidence supporting findings need to be presented along with the uncertainty. Ideally, 

risks are associated with the various management alternatives to support the risk management 

process (see Chapter 6). 

10
Risk Communication 

Risk communication begins with planning, then continues through the assessment, and 

concludes with the presentation of results. Scientists or assessors share information with 

managers or stakeholders to ensure that the most fruitful analyses are performed to provide 

meaningful information for environmental management (see Chapter 7). 

Risk Management 

Risk management is the decision making step of the process. Attempts are made to select 

management options that will reduce or minimize risk and achieve objectives that have societal 

value (see Chapter 8). 

11

3. PLANNING PHASE


To make sure ERA results support decision making, risk assessment participants plan their 

activities at the start of the assessment process. During planning (Figure 1), scientists, managers, 

and stakeholders discuss the focus, scope, and complexity of the risk assessment. Participants 

should determine whether watershed ERA is an appropriate tool to address environmental 

management concerns because a sound scientific approach is not without cost. 

Scientists, managers, and stakeholders all play a role in watershed ERA. Scientists (e.g., 

risk assessors, hydrologists, and ecologists) need to communicate what they can realistically 

provide to the managers and the level of uncertainty. Managers must describe why the risk 

assessment is needed, its relevance to regulations, and what they expect to do with the 

information they will receive. They must also identify and include the opinions of all appropriate 

stakeholders, such as ecological and socioeconomic concerns. Managers then need to develop a 

list of environmental goals and objectives that reflect the stakeholder input. Without this 

information, assessment results may not be useful to decision making. The success of a watershed 

ERA strongly depends on the quality of communication that occurs during this initial planning 

process. 

The watershed approach (U.S. EPA, 1995a) defines a similar, although less linear and 

more problem-oriented, planning process (Figure 2). The watershed approach is contingent on 

building a project team and obtaining public support. One of the team’s initial activities is 

defining the problem: generating a mutually agreeable problem statement that defines future 

activities. Specific goals and environmental objectives are then developed to address perceived 

problems. Goals and objectives are based on the condition or vulnerability of valued resources 

and beneficial uses, the needs of the ecosystem, and stakeholder’s needs (U.S. EPA, 1996). 

The key aspects of planning, as outlined in this chapter, include: 

• Identifying and including stakeholders 

• Setting environmental management goals and objectives 

• Defining management options 

• Determining the focus, scope, and complexity of the risk assessment 

3.1. IDENTIFYING AND INCLUDING STAKEHOLDERS 

The planning phase for a watershed assessment is especially complex because a 

watershed typically overlaps multiple jurisdictions that are managed by organizations with 

divergent goals and responsibilities and inhabited by numerous stakeholders with varied 

interests. 

12

Figure 2. Relationships between management or ecological risk 

assessment (ERA) goals and objectives, management options, the 

planning phase, and subsequent problem formulation in 

Watershed ERA. 

Depending on the perspectives of the stakeholders involved, a different set of goals and 

objectives may emerge. Stakeholder participation may become complex if there are numerous 

stakeholders or some of them are interested in only a limited range of issues (Glicken, 2000). 

The inadvertent exclusion of a stakeholder group may influence a group’s decision to accept or 

reject the outcome of the process. Excluded stakeholders could even take legal actions, such as 

filing a citizen lawsuit under the Clean Water Act to challenge the process outcomes. Therefore, 

stakeholder involvement needs to be balanced against the limited resources available for 

watershed assessment and management (U.S. EPA , 2001a). 

Participants in the watershed ERA process should include all regulatory or resource 

agencies (state, federal, local) with responsibilities for protecting and managing the water body 

and any parties whose authority will be needed to implement a management plan (e.g., local and 

county officials with zoning oversight). Nongovernmental organizations (such as watershed 

associations or councils, river watch citizen groups, volunteer monitoring group, educational and 

research institutions, industries, and agricultural associations) all have a stake in watershed 

management. Besides organized groups, other stakeholders include landowners, those who use 

the watershed, and those whose participation is essential to successful management. In some 

instances, stakeholders may be hundreds of miles away from the assessment (e.g., bird watchers 

13

concerned about migratory waterfowl). The nongovernmental and unaffiliated stakeholders may 

have objectives that are very different from those of the regulatory agencies (e.g., minimizing 

restrictions on land use, resource development, or waste disposal). Although such social, legal, 

and economic objectives may be in conflict with some environmental objectives, they are still 

relevant concerns that need to be considered (Stahl et al., 1999). To help assess these tradeoffs, it 

may be helpful to involve environmental economists. 

Participants on a watershed assessment team contribute different resources that include 

socioeconomic information, historical data, scientific expertise, and assets to conduct the 

assessment. Team members of the Big Darby Creek assessment project included biologists, a city 

planner, and environmental scientists (Cormier et al., 2000). Team members performed literature 

reviews and frequently consulted or interviewed experts in other disciplines. The team interacted 

regularly with the Big Darby Partners, a group of state agencies, representatives from The Nature 

Conservancy (TNC), and farmers concerned about the future of the working landscape. 

The various EPA-sponsored watershed assessment teams found that continuity of project 

contact people, face-to-face contact, and formal and informal group discussions were important to 

building the trust necessary to engage stakeholders. Continuity with federal or state leaders is 

also important. A watershed assessment coordinator is generally needed to keep participants 

focused and on track, and to communicate progress to the stakeholders (U.S. EPA, 1997b). 

Stakeholder exclusions may be acceptable if a trusted member of another group 

adequately represents their opinion. In the Clinch and Powell Valley assessment (U.S. EPA, 

2002a), for example, an interdisciplinary workgroup was established with representatives from the 

U.S. Fish and Wildlife Service (FWS), Tennessee Valley Authority (TVA), TNC, EPA, and other 

state and federal agencies. Although many other stakeholders were not directly involved in the 

Clinch and Powell assessment, TNC was able to represent their concerns because TNC interacts 

regularly with the public and used results from a recently completed survey. EPA provides 

further guidance on identifying and engaging stakeholders (U.S. EPA, 2001b, 2000c). 

3.2. SETTING ENVIRONMENTAL MANAGEMENT GOALS AND OBJECTIVES 

Managers must implement decisions to achieve environmental management goals. 

Selected decisions often begin as one of several management options identified during the 

planning phase. Clear management goals must be set before the assessment team jumps into 

analysis. It is quite common that assessments often hastily include a few obvious goals and then 

devote most of the effort to data gathering and analysis (Reckhow, 1994). Valuable data may be 

collected, but if the goals are poorly defined, the data may not be useful in management decisions 

(Ward, 1996). 

14

Before beginning the actual assessment, managers, with input from scientists and 

stakeholders, need to develop watershed management goals. Some goals are clearly spelled out in 

legislation and regulation (e.g., support for designated uses under the Clean Water Act). Other 

times, goals may not be defined, such as “apply an ecosystem approach to management.” 

Elements of existing goal statements from watershed councils, conservation plans, or local growth 

planning strategies should be incorporated where appropriate. Goals may relate to protecting, 

restoring or maintaining a species, community, or ecosystem, and they should be explicit and 

quantifiable. 

When goals are broad, it may be useful to break them down into multiple management 

objectives. Objectives may translate the goals–which may be very general, abstract, and 

impossible to measure–into specific characteristics that are useful for deciding among 

management alternatives. The process of defining objectives generally reflects a stakeholder 

vision of the future condition of the watershed (U.S. EPA, 1995a), and plentiful examples are 

available from successful watershed projects (U.S. EPA, 1997a). 

It may be useful to develop lists of draft objectives representing all stakeholder 

perspectives using a facilitated interview process with individual members of a stakeholder 

advisory group (McDaniels, 2000; Gregory and Keeney, 1994). A watershed coordinator (or 

watershed assessment coordinator) can then condense the lists of draft goals and objectives into a 

single coherent set. Frequently, stakeholders have differences, and sometimes they have 

contradictory perspectives. Some goals may be more politically favorable than others. A group 

could start with general environmental goals (e.g., control eutrophication and protect salmonid 

fishery) and then make them into specific objectives (e.g., prevent nuisance blue-green algal 

blooms and provide habitat for successful trout spawning). Reckhow (1994) provides an example 

of the objectives hierarchy process for Lake Okeechobee. At the top level is the overall goal to 

“manage the eutrophication of Lake Okeechobee.” The next level is subdivided into nine specific 

goals, ranging from “minimize costs” to “protect threatened and endangered species.” These 

goals are further subdivided by defining specific objectives, such as maintaining the populations of 

specific wading bird species. As the objectives become more refined, they are likely to naturally 

point the way to useful measurement endpoints for use in the analysis. 

To illustrate the process used in establishing a goal and objectives, Big Darby Partners, 

composed of managers and a stakeholder group, developed a management goal, “protect and 

maintain native stream communities of the Big Darby Creek ecosystem” (Cormier et al., 2000). 

Three objectives for accomplishing that goal were established: 

1. Attain water quality criteria for designated uses throughout the watershed. 

15

2.	 Maintain exceptional warmwater criteria for stream segments that were designated 

between 1990 and 1995. 

3.	 Ensure the continued existence of native species in the watershed. 

These were very useful objectives because Ohio’s Water Quality Standards specifically 

link water quality to the ability of a stream to support and maintain native species. For instance, 

several streams in the watershed, including the Big Darby and Little Darby, are classified as 

Exceptional Warmwater Habitat waters. These water bodies have species composition, diversity, 

and functional organization comparable to the 75th percentile of statewide designated least 

impacted reference sites. 

The Waquoit Bay interdisciplinary and interagency assessment team developed an 

environmental management goal for the watershed through a multistep planning process (Serveiss 

et al., 2004). The process included a public meeting to initiate the assessment and understand 

public values, a meeting of team members to develop the goal and more specific objectives, and a 

meeting with local resource managers to refine the goal and objectives. The following goal was 

mutually agreed upon: Reestablish and maintain water quality and habitat conditions in Waquoit 

Bay and associated wetlands, freshwater rivers, and ponds to (1) support diverse, self-sustaining 

commercial, recreational, and native fish and shellfish populations and (2) reverse ongoing 

degradation of ecological resources in the watershed. 

The Waquoit Bay assessment team identified 10 objectives that more explicitly stated the 

kinds of management results implied in the general goal (U.S. EPA, 2002c): 

1.	 Reduce or eliminate hypoxic (low oxygen) level or anoxic (no oxygen) events. 

2.	 Prevent toxic levels of contamination in water, sediments, and biota. 

3.	 Restore and maintain self-sustaining native fish populations and their habitat. 

4.	 Reestablish viable eelgrass meadows and associated aquatic communities in the bay. 

5.	 Reestablish a self-sustaining scallop population that can support a viable fishery. 

6.	 Protect shellfish beds from bacterial contamination that results in bed closures. 

7.	 Reduce or eliminate nuisance macroalgal growth. 

8.	 Prevent eutrophication of rivers and ponds. 

9.	 Maintain diversity of native biotic communities.

 10. Maintain diversity of water-dependent wildlife. 

Ideally, these objectives would be quantified to evaluate environmental improvements 

from an established baseline. For instance, objective 1 above could have been quantified as 

16

reducing the number of days per year in which dissolved oxygen in any part of Waquoit Bay 

declined to below 2 parts per million within 5 years. For objective 4, the quantified objective 

could have been to increase eelgrass cover from 10% to 20% of the bay’s water surface area 

within 5 years. 

Although reaching agreement on watershed goals may delay the start of the assessment, 

reaching agreement among diverse interests is valuable for performing the most useful analyses 

and for selecting and implementing the most relevant management options. 

3.3. 	DEFINING MANAGEMENT OPTIONS 

For assessment findings to be useful, there should be a set of management actions or 

options that could be implemented pending the results of the assessment. Managers usually must 

take action to achieve environmental management goals, although no action and passive 

approaches are also available. A preliminary identification of management options or alternatives 

should take place as part of the planning phase (U.S. EPA, 1998a). 

The Clinch and Powell assessment team identified several management actions that could 

be implemented to help attain their environmental management goal (Serveiss, 2002), including 

containing and treating mining runoff, implementing agricultural best management practices, 

installing roadside spill protection devices, and improving treatment of wastewater discharges. 

Inclusion of management options early in the ERA process helped them select the spatial scale on 

which to focus the assessment (i.e., upstream of Norris Dam, TN) and the impacts from mining 

and agriculture. For more complex watershed assessments, it may be necessary to determine 

baseline risks as an initial goal, with subsequent iterations evaluating risks relevant to specific 

management alternatives. 

Identification of management alternatives in the planning phase is consistent with the 

watershed management process, in which the stakeholder objectives and management options 

together define the context for the management effort. The watershed approach (U.S. EPA, 

1995a) further emphasizes this linkage by combining “setting goals and identifying solutions” into 

a single step in the process. 

3.4. 	DETERMINING THE FOCUS, SCOPE, AND COMPLEXITY OF THE RISK

 ASSESSMENT 

For most natural resource management decisions, ample time and resources to conduct a 

rigorous assessment of the impacts of multiple stressors are rarely available, thus limiting the 

potential scope of the assessment. The focus and scope of the assessment should be determined 

by how sure a manager needs to be to choose an appropriate management action. Some 

decisions, due to limited resources or urgency, may require only information obtained during 

17

problem formulation, while more costly or controversial decisions may require more detailed 

evaluations. Spatial and temporal boundaries need to be established at the start of the process, 

and these should be both relevant to stakeholder objectives and practical from an analysis 

standpoint. The level of effort necessary to reach a decision ideally would be agreed upon early 

during planning, before conflicts arise over selecting the analyses to be performed. The key to 

this challenge is obtaining consensus on where (what size watershed), when (what time period), 

and how much (cost) to study. 

Using a tiered approach can help control the level of effort. Each tier would represent 

increasing levels of complexity and investment and decreasing levels of uncertainty. When tiers 

are used, specific descriptions of management questions and decision criteria for each tier should 

be included in the analysis plan (U.S. EPA, 1998a). 

Another approach for reducing the level of effort is to concentrate on one portion of the 

watershed. Only the free-flowing upper portion of the Clinch and Powell Rivers was analyzed 

because this was believed to be the major refugia for a number of native aquatic species (U.S. 

EPA, 2002a). It was also decided to focus limited resources on analyzing previously collected 

data and not to collect additional samples. 

3.5. PRODUCTS OF THE PLANNING PHASE 

Planning is complete when participants have reached consensus on the following: 

• Environmental management goals and objectives for the watershed 

• Potential management options to be considered pending assessment results 

• Objectives for the risk assessment, including criteria for success 

• Focus and scope of the assessment, including the geographic boundary 

• Technical and financial resources to be invested in the assessment 

Ongoing planning activities that will sustain the assessment include coordination of 

participants, assignment of administrative or data collection tasks, scheduling regular meetings, 

and setting due dates for assessment products. Much planning occurs early, but because this is an 

iterative process, planning activities are often revisited. Management goals, objectives, and 

options along with the assessment, focus, scope and complexity can all change based on new 

information. 

18

4. PROBLEM FORMULATION PHASE 

Problem formulation develops and evaluates potential causes of ecological effects. This 

phase provides the organizing framework upon which the entire ERA depends (U.S. EPA, 

2000b). Problem formulation begins by reviewing available information and describing and 

delineating the place and system of interest. Relevant information includes habitat and ecosystem 

type, general geography, major animal and plant groups, location, boundaries, and dominant land 

uses. The goal is to assemble information on the potential stressors contributing to either 

observed or potential alterations in watershed conditions and processes. The assessment will 

strive to improve the understanding of these relationships, provided resources to do so are 

available. In some cases, for complex systems such as watersheds, completing just the problem 

formulation phase may yield an effective standalone management product without proceeding to 

the risk analysis, risk characterization, risk communication, or risk management phases (Serveiss, 

2002). 

Problem formulation leads to causal analysis (i.e., what processes could have caused 

deleterious environmental effects). A comprehensive understanding of the ecological interactions 

within a watershed is one of the most powerful tools for informed planning, participatory decision 

making, and logical problem formulation. The paradigms that have been developed to explain the 

structure and function of stream and river ecosystems serve as examples of the basic knowledge 

needed by the practitioner. These paradigms include the river continuum concept (Vannote et al., 

1980), the flood pulse concept (Junk et al., 1989), influences of the riparian corridor (Stewart et 

al., 2001), the nutrient spiraling concept (Newbold et al., 1982), the 

serial discontinuity concept (Ward and Stanford, 1983), and the patch dynamics concept 

(Townsend, 1989). 

The following basic elements of problem formulation are outlined in this chapter: 

•	 Identifying stressors and sources 

•	 Selecting assessment endpoints and other measures that will be needed to quantify 

risks 

•	 Developing a conceptual model describing predicted relationships among sources, 

stressors, and assessment endpoints 

•	 Evaluating assessment endpoint 

•	 Creating an analysis plan to guide the next phase of the assessment 

19

4.1. IDENTIFY STRESSORS AND SOURCES 

Stressors are defined as any chemical, physical, or biological entity that can cause an 

adverse effect on an assessment endpoint. Examples of stressors include pesticides, altered 

stream flow, and invasive species. Typically a wide range of stressors may affect a given resource 

or assessment endpoint. Stressors may originate from a variety of sources, including different 

human activities and natural processes. 

Watersheds are challenging from an ERA standpoint because they generally have multiple 

potential stressors and sources of stressors. Sources are actions that release or impose a stressor 

on the environment. The sources of the stressors are often more manageable than the stressors 

themselves in watershed ERA and are therefore the focus of management options. 

When multiple sources are identified, it may be efficient to focus only on sources that 

management can control. Nutrients (nitrogen in particular) were identified as the cause of 

observed algal blooms in Waquoit Bay (U.S. EPA, 2002c), resulting in declines in commercial 

fisheries. Management options focused on limiting excess nutrients originating from septic 

systems or from various land uses in the watershed, rather than from atmospheric deposition, 

because nitrogen inputs within the watershed were found to be more controllable. 

Depending on the properties of the watershed and the assessment endpoints, a large 

number of stressors may be identified. The decision about which pathways and stressors to 

analyze should be made by using the collective best professional judgment of an interdisciplinary 

team with expertise encompassing the spectrum of the system under study (Foran and Ferenc, 

1999). Foran and Ferenc (1999) summarize a variety of techniques, the vast majority of which 

involve having team members score the impact of stressors on valued ecological resources. Each 

individual team member can enter a score, or groups of individuals can arrive at a consensus 

through discussion. Scores can then be summed or averaged to determine impacts, and a matrix 

can be devised to determine potential remediation options (Harris et al., 1994). 

This scoring approach to stressor prioritization was used in the Waquoit Bay assessment, 

in which a preliminary list was developed showing the major stressors that included nutrients, 

toxic chemicals, suspended sediments, and physical habitat alterations (Serveiss et al., 2004). An 

impact matrix, with stressors as rows and assessment endpoints as columns, is derived from the 

conceptual model (Table 2). The interdisciplinary workgroup used their best professional 

judgment to rank the stressors and their perceived impact on assessment endpoints on a scale of 1 

to 5. This decision analysis method for ranking alternatives according to multiple criteria is 

known as fuzzy-set logic (Harris et al., 1994; Wenger and Rong, 1987). Going through this 

stressor ranking process helped the assessment team justify use of limited resources to analyze 

impacts on the two stressor-assessment endpoint pathways related to effects of nutrients on 

scallops and eelgrass. Assessment endpoints are discussed in the next section. 

20

Table 2. Effects matrix summarizing assumed strength of relationships 

between assessment endpoints identified (e.g., percent eelgrass cover) and 

stressors in the Waquoit Bay watersheda 

Stressor 

Assessment endpoints ranking 

Percent 

eelgrass 

Finfish 

diversity 

Scallop 

abundance 

Anadromous 

fish 

Wetland 

birds 

Piping 

plovers 

Fish/ 

shellfish Totals 

Chemical pollution 1 1 1 1 1 1 3 9 

Altered freshwater 

flow 

1 1 1 2 3 1 1 10 

Nutrient enrichment 5 5 5 3 2 1 1 22 

Physical alteration 

of habitat 

2 1 2 1 2 3 1 12 

Fishing pressure 1 1 2 3 1 1 1 10 

Pathogens 2 1 1 1 1 1 3 10 

Totals 12 10 12 11 10 8 10 73 

a

Each cell represents the relative effect of a stressor on an endpoint. The ranking (1 = minor, 5 = severe) reflects 

experience with the likely effects specifically for the Waquoit Bay watershed. 

Source: Adapted from Serveiss et al. (2004). 

Stressor Identification Evaluation (SIE) is a logical scientific process that has been 

developed to evaluate available information, identify relevant stressors, and determine the 

stressors that are most likely causing observed biological impairments. SIE (U.S. EPA, 2000d) 

was specifically designed to determine probable causes(s), given that an undesirable effect has 

already occurred. Drawing from standard ecological risk principles, the SIE process works 

backward from the observed effect to identify possible pathways (causal linkages) that could 

cause such an effect, the stressors that would lead to those causal linkages, and the sources that 

would elicit those stressors. As part of this process, potential relevant stressors in the watershed 

are identified, which, along with the proposed causal linkages and sources, are used to construct a 

conceptual model as discussed in Section 4.3 of this report. A recent addition to SIE is the 

Causal Analysis Diagnosis Decision Information System (CADDIS) which can be accessed at 

http://www.epa.gov/caddis. CADDIS helps users apply SIEs to streams. CADDIS includes 

information and many examples of stressor-response relationships that can help ecologists and 

assessors identify possible stressors in their systems (see Section 5.1). 

4.2. SELECT ASSESSMENT ENDPOINTS 

In the planning process, participants identified general goals and specific environmental 

management objectives. Assessment endpoints translate these objectives into something 

21

scientifically important and measurable. They also provide direction for future analyses. 

Assessment endpoints consist of an entity and an attribute. The entity is the species (e.g., 

eelgrass), functional group (e.g., piscivores), community (e.g., benthic invertebrates), or 

ecosystem (e.g., lake) that must be protected. The attribute is the characteristic of the entity that 

is important to protect. For example, for eelgrass it could be acres of coverage, and for a lake it 

could be frequency and severity of algal blooms. 

It is a challenge to select the most useful assessment endpoints. Watersheds have a 

diverse array of stakeholders who may have competing management objectives. To the extent 

possible, assessment endpoints should represent the range of objectives of all stakeholders. They 

should be selected based on three criteria: their relevance to the management objectives, their 

importance in the ecosystem, and their susceptibility to stressors (U.S. EPA, 1998a). 

For example, in the Clinch and Powell assessment, protection of native and endangered 

mussels and fish was an ecologically relevant management objective. Two assessment endpoints 

were examined in that ERA: recruitment and reproduction of native mussels; and richness, 

distribution, and abundance of native fish (Diamond et al., 2002; U.S. EPA, 2002a). Native 

mussels and fish were assumed to be sensitive to the multiple stressors in the watershed. 

It is important to tie the assessment endpoint to the overall management goal to make the 

results directly applicable for resource management. If the overall goal is protection and 

restoration of native and endangered mussels, for example, the assessment endpoint might specify 

a relevant well-described species and an attribute representative of the number of individuals of 

this species in the area of interest. Such specificity in the assessment endpoint can reveal the types 

of analyses or models needed to determine causality and risk in the risk analysis phase, and will 

help diffuse potential controversies that may arise among different stakeholders. 

In the Waquoit Bay ERA, all of the assessment endpoints were potentially vulnerable to 

certain watershed stressors (see Table 2), and they reflect the management objectives and overall 

goals: (1) support diverse, self-sustaining commercial, recreational, and native fish and shellfish 

populations; and (2) reverse ongoing degradation of ecological resources in this watershed 

(Serveiss et al., 2004). However, only two assessment endpoints (eelgrass cover and scallop 

abundance) were analyzed in detail because of their relationship to the predominant stressor. 

4.3. DEVELOP CONCEPTUAL MODEL 

Conceptual models describe key causal relationships that will be evaluated in the risk 

assessment. They include a diagram of the predicted relationships between ecological effects and 

stressors, and ideally they also include a written description of the relationships. Conceptual 

models are necessary for watershed and landscape assessments because they describe the 

multiple physical, chemical, and biological stressors and their sources in a system, as well as the 

22

pathways by which they are likely to affect 

assessment endpoints (Suter, 1999) (see Figure 3). 

At this point in the watershed ERA, these potential 

causal relationships are based on professional 

judgments and knowledge and are not yet 

quantified. 

Sources, stressors, and effects are 

interconnected; thus, it is a challenge to evaluate 

their relationships to one another. Furthermore, 

each pathway and each endpoint may have 

professional advocates and detractors. Therefore, 

achieving a focus for the assessment may be 

difficult. Conceptual models help address this 

challenge. 

Developing the conceptual model provides 

a forum for discussing causal pathways, a 

framework for explaining the hypothetical 

relationships and the scope of the assessment, and a structure for the forthcoming analyses. Many 

specialists may find it easier to see the big picture with the aid of a conceptual model. This 

systematic process provides a forum and documentation to management actions and helps to 

efficiently bring new personnel up to speed. U.S. EPA (2005) provides a more in-depth 

discussion on the applicability of conceptual models 

Conceptual model development has been identified as the single most valuable component 

of EPA’s watershed ERA prototype assessments (Serveiss, 2002; Butcher et al., 1998). For 

instance, in the assessment of the middle segment of the Snake River, the conceptual model and 

associated risk hypotheses provided a common basis for coordinating the concerns of EPA, FWS, 

the Federal Energy Regulatory Commission, and the public (U.S. EPA, 2002d). In the Big Darby 

Creek assessment, group efforts to develop the conceptual model and risk hypothesis were 

particularly valuable for communicating expectations within the technical workgroup and to the 

stakeholders. 

An interdisciplinary scientific team, including individuals with local ecological 

knowledge, should develop the conceptual models for the watershed based on previously 

collected information and their best professional judgment. Early models may be simple, but they 

provide a basis for predicting relationships between sources, stressors, and effect, and for 

identifying knowledge gaps. The predicted relationships provide qualitative forecasts of the 

impact of stressors on resources, and qualitative hypotheses may also be helpful for decision 

23

making. These predictions may be used to prioritize problems and help address TMDLs, which 

are pollution budgets to reduce loadings of pollutants that exceed water quality criteria. 

Figure 4 is an example of a complex conceptual model with various pathways organized 

around stressors. Each stressor has a patterned line that illustrates a pathway connecting its 

sources to possible effects and selected endpoints. Each model component is represented by a 

different geometric shape to aid interpretation. This is a broad-based model that provides a 

framework for the risk assessment and an overview of ecosystem processes. The diagram shows 

only stressors and effects thought to be potentially important in the Waquoit Bay watershed. The 

exposure pathways, from the source of stressors to valued resources, are the possible risk 

hypotheses to be analyzed as part of the risk assessment. 

Initial efforts at producing a conceptual model may result in a highly complex set of 

pathways and linkages. These models are necessarily complex to capture all the pathways 

between sources, stressors, and effects. It is a challenge to balance complexity with the clarity 

necessary to understand a diagram with a complicated web of boxes and arrows. One way to 

achieve this balance is to use hierarchical nested conceptual models (Suter, 1999). Hierarchical 

nested conceptual models are more detailed and may contain more pathways or information than 

space permits viewers to see clearly in a general model. Depending on the need, a hierarchical 

model could be constructed for just one stressor or just one assessment endpoint. These singlestressor and single-endpoint models can be used to provide viewers with a simplified overview 

along with the capability to examine particular pathways in more detail. 

Another option for achieving clarity and complexity is to have the model evolve as more 

information is developed during an assessment (U.S. EPA, 2005). The initial conceptual model in 

the Clinch and Powell assessment provided a framework that was used to track the progress of 

analyses. Areas in need of further data collection or analysis were identified and prioritized. 

Additional complexity was added over time as the assessment team developed a better 

understanding of source-stressor-endpoint relationships. 

Different forms of communication need to be used for those involved. Scientists will need 

all the details. For the nontechnical audience, it is better to produce a simplified version of the 

conceptual model showing only significant linkages. For example, the generalized version of the 

Waquoit Bay conceptual model (U.S. EPA, 2002c) proved to be a powerful communication tool 

during the risk assessment phase of the process. Its perceived value to stakeholders is illustrated 

by the decision to display the conceptual model at the Waquoit Bay National Estuarine Research 

Reserve Visitor Center. The more complex conceptual model can be held in reserve and used to 

help answer more specific questions. 

Another advantage of developing a conceptual model, such as Figure 4, is that it helps 

elucidate cascading or secondary effects that otherwise may not be immediately apparent. 

24

Figure 4. Conceptual model of the Waquoit Bay watershed. 

Rectangles represent sources of stressors, hexagons are the specific stressors to the system, 

trapezoids represent the effects of those stressors, and the ellipses indicate specific endpoints that 

are affected. 

Source: U.S. EPA (2002c). 

25

For example, in Waquoit Bay, excess nutrient input may exert indirect effects on commercial 

fisheries by causing algal blooms that reduce light levels to the point that some submerged 

aquatic plants important as habitat for juvenile fish cannot survive (Serveiss et al., 2004). In this 

case, habitat loss is the stressor impacting the fish, but knowledge of the whole chain of events is 

necessary to take cost-effective corrective action (U.S. EPA, 2002d). 

4.4. EVALUATE ASSESSMENT ENDPOINTS 

Assessment endpoints need to be evaluated using observations or modeled predictions. 

Choosing between management options typically involves a prediction of the response of an 

assessment endpoint to the proposed management action. However, watershed assessment 

endpoints themselves are not easily measured, and the response of endpoints to changing 

conditions is often difficult to predict. For instance, an assessment endpoint of a fishery might be 

the spawning success of salmonids. This is difficult to measure directly for an entire watershed, 

although associated information, such as counts of young-of-year from a set of monitoring sites, 

may be feasible. 

Instead of measuring and predicting responses of the assessment endpoint directly, it is 

often necessary to identify surrogate measures or measures of effect that can stand in for the 

assessment endpoint. Measures of effect are quantifiable changes in an attribute of an 

assessment endpoint or its surrogate in response to a stressor to which it is exposed. Meaningful 

measures of effect will have the following characteristics (Butcher and Craig, 1998; U.S. EPA, 

1998a): 

•	 They will link objectives to stressors (and changes in stressors) and thus correspond to 

a risk hypothesis defined in the conceptual model. 

•	 They will be sensitive to changes in the condition of the objective. 

•	 They will reflect susceptibility to individual stressors. 

•	 They will be meaningful to decisionmakers. 

•	 They will be measurable, or at a minimum, risk assessors will be able to semiquantitatively rank them. 

•	 Risk assessors will be able to predict them in response to management options. 

Measures of effect are selected for their suitability in reflecting changes likely to occur in 

the assessment endpoint and for their ability to be measured accurately, consistently, and 

economically. For example, in the Clinch and Powell assessment, an assessment endpoint was 

26

the recruitment and reproduction of native and endangered mussel species. Because data were 

not available in many parts of the watershed, a surrogate measure of effect was used, mussel 

species richness. In the Big Darby watershed ERA (Cormier et al., 2000), an assessment endpoint 

was “biotic integrity of the macroinvertebrate community.” The applied measure of effect was the 

macroinvertebrate Index of Community Integrity (ICI), a quantifiable indices with a standardized 

protocol that defines the quality of the macroinvertebrate community. 

Quantified target values of assessment endpoints should be specified, where applicable, as 

a means of evaluating and communicating whether a given management alternative meets 

management objectives. This is particularly important in watershed management, where defined 

target values can serve as a shorthand to communicate how different management options 

compare in their ability to meet multiple management objectives. Target values for some 

measures are already established directly in regulation. For instance, numeric water quality 

criteria must be met for a management plan to be acceptable, although it may be determined that a 

more stringent target is appropriate for specific objectives. Where target values are not already 

defined, a variety of mechanisms can be used to determine an appropriate value. In many cases, 

the target value will emerge from the ERA itself. Sometimes a narrative standard can be used. It 

is also possible to set target values by comparing the conditions in the watershed to conditions in 

an appropriate reference (relatively unimpacted) watershed (U.S. EPA, 1999a), conducting user 

surveys (e.g., Smeltzer and Heiskary, 1990), or using an existing classification system (e.g., 

Vollenweider and Kerekes, 1980). 

It is desirable to select assessment endpoints and determine whether alternative measures 

or target values are available before collecting additional data or analyzing them. The next step is 

to plan the analysis before moving into the risk analysis phase. 

4.5. CREATE ANALYSIS PLAN 

The analysis plan must describe the objectives of the analysis phase, which are to gain a 

better understanding of (1) the extent to which assessment endpoints are exposed to important 

environmental changes from human activities and (2) the effects that are likely to occur as result 

of exposure. The pathways more likely to impact assessment endpoints or achieve management 

objectives, along with those more amenable to corrective action, should be investigated. The plan 

should describe possible results and uncertainties and how this information will be communicated. 

An integrated and comprehensive data analysis plan also will ensure that all the data needed to 

define exposure scenarios and pathways to assess ecological risk are clearly identified (Cook et 

al., 1999). The costs of data collection and analysis have to be weighed against the anticipated 

risk reduction benefits. 

27

The analysis plan provides a scientifically defensible process to select specific pathways 

between sources and assessment endpoints for further study in developing meaningful 

conclusions. The analysis plan should describe the quantifiable (either ordinal or continuous) 

measures (i.e., both assessment endpoints and stressors/sources) that will be analyzed. 

The analysis plan could also include a pilot study intended to test the proposed approach. 

The Copper Creek subwatershed was studied before the risk assessment team moved on to assess 

the entire Clinch and Powell watershed (U.S. EPA, 2002a). The analysis plan was modified in 

response to the pilot study (see Section 5.7 for more details). 

When problem formulation is complete, the risk assessment team and stakeholders should 

have a clear understanding for the assessment and a plan for the analysis phase. Even if the 

remaining assessment phases are not carried out, problem formulation alone is extremely 

valuable to watershed management because it summarizes existing relevant ecological 

information for the watershed, significant data gaps, and potential risks in an organized manner 

(Serveiss, 2002). 

28

5. RISK ANALYSIS PHASE 

The risk analysis phase investigates the most important stressors and exposure pathways 

and predicts how assessment endpoints respond to stressors under different exposure scenarios. 

Risk analysis tasks may include collecting data, interpreting historical data, modeling, and 

statistical analysis. The risk analysis phase ultimately characterizes exposure and effects. 

During exposure characterization, the assessor seeks to describe the intensity of the 

contact or co-occurrence between the stressor and the assessment endpoint (U.S. EPA, 1998a). 

This involves linking stressors with their sources and then describing where, how, and when the 

stressors occur in the environment. In traditional ERAs, exposure is commonly estimated by 

measuring or modeling amounts of stressors and combining these estimates with assumptions 

about the route of exposure to the assessment endpoint. Spatial and temporal distributions of 

both the endpoint and the stressors are considered. 

During ecological effects characterization, stressor-response data and cause-and-effect 

relationships are evaluated and extrapolations are made from one set of data to the system being 

studied. Statistical techniques or mathematical models may be used to quantify and summarize 

the relationship between stressor and effect. No matter which approach is used, biological 

monitoring data are very pertinent in identifying the biological consequences of human actions and 

provide an essential foundation for assessing ecological risks (Karr and Chu, 1997). 

The analysis phase of watershed ERA is challenging, because watershed assessments must 

deal with direct and indirect effects on multiple endpoints from multiple physical, chemical, or 

biological stressors (Suter, 1999; Hunsaker et al., 1990). The traditional risk assessment 

paradigm was developed for evaluating risks to single species, or risks from specific chemicals, 

for which quantitative information is often available or feasible to collect as part of the ERA. For 

these kinds of single stressor and effect assessments, during the risk analysis phase calculated 

relationships may be expressed as a stressor-response curve. Although desirable, it may be 

impractical to develop such a curve in a watershed assessment because of a host of confounding 

factors, including the presence or absence of other stressors, abiotic influences on the stressor 

effect (e.g., dissolved carbon and pH affects the toxicity of many metals), and spatial and temporal 

heterogeneity of the assessment endpoint itself. In many cases, it will be too costly to attain the 

data for each exposure and effects pathway (Serveiss, 2002). Thus, risk analysis for watershed 

ecological assessment is likely to require some deviation from the ideal ERA process. 

For watershed ERAs, rather than assessing exposure and effects separately, these analyses 

are usually aggregated. This can lead to merging risk analysis with risk estimation (typically part 

of the risk characterization phase) because the assessor frequently compares source or stressor 

data with biological effects data to draw conclusions. As a result, in watershed ERA the 

29

distinction between risk analysis and risk characterization can become blurred. Many of the 

points presented in this chapter on risk analysis could also apply to risk characterization. 

Ecological effects are analyzed by describing stressor-response relationships, evaluating 

evidence for causality, and linking measurable effects to the assessment endpoints identified 

during problem formulation (Norton et al., 2002a). These components can be developed in any 

order, and the emphasis may be different depending on whether the objective of the assessment is 

to predict the effects associated with future change or to retrospectively analyze the causal factors 

influencing the current state of ecological resources. Tools and challenges associated with 

watershed risk analysis include the following: 

•	 Identifying stressors and evaluating causes of impairment and associations between 

causes and effects (Sections 5.1 and 5.2) 

•	 Working with chemical or nonchemical stressors (Sections 5.3 and 5.4) 

•	 Using categorical data or source data or ranking risks because data are limited 

(Section 5.5) 

•	 Dealing with lots of uncertainty (Section 5.6) 

•	 Recognizing that analysis is an iterative process (Section 5.7) 

•	 Understanding secondary and indirect pathways on valued resources (Section 5.8) 

5.1. 	STRESSOR IDENTIFICATION EVALUATION (SIE) AND CAUSAL

 EVALUATION: IDENTIFY STRESSORS AND EVALUATE CAUSES OF

 IMPAIRMENT 

SIE is a tool that helps organize scientific information and provides a system to help 

document the stressors and pathways considered in a risk analysis. The SIE process (U.S. EPA, 

2000d), provides a clear and consistent method to identify and compare candidate causes and 

determine which causes are best supported by the evidence. The process uses a series of three 

methods. First, candidate causes that are not logically possible given the evidence are 

eliminated. Then, conventional medical/veterinary diagnostic approaches are used to identify any 

causal agents that have produced characteristic symptoms. For the remaining causes, a strengthof-evidence analysis is used to evaluate and score all available evidence for each candidate. 

Based on current experiences in using SIE in several state TMDLs, diagnostic information is 

often lacking (i.e., data demonstrating a specific stressor or mechanism of effect are often 

unavailable). Instead, different types of association analysis (temporal and spatial associations 

between a given stressor and effects), as well as relevant data from the published literature (e.g., 

tolerance values for a given species and stressor, Habitat Suitability Index [HSI] information, or 

30

toxicity endpoints for a given species and chemical) are used to help support or refute 

hypothesized causal linkages between candidate stressors and observed effects. 

Thus, like ERA, SIE promotes risk analyses that are transparent to all stakeholders. In 

addition, SIE helps ensure that stressors and pathways are not overlooked or prematurely 

dismissed from consideration, forcing the assessors to consider and evaluate all reasonable 

sources, stressors, and pathways. SIE has been incorporated into a causal assessment to 

determine probable causes of biological impairment (Suter et al., 2002), and this methodology has 

been applied in several places, including the Little Scioto River in Ohio (Norton et al., 

2002a, b). 

Causal evaluation can identify factors that might be controlled by management actions to 

improve environmental conditions (Norton et al., 2002a, b; Suter et al., 2002). CADDIS (U.S. 

EPA, 2004) is a Web-based decision support system that can help scientists in the regions, states, 

and tribes find, access, use, and share information to determine the causes of biological 

impairments in aquatic systems. CADDIS is a tool that applies the SIE process to streams and 

enables scientists to make causal determinations more quickly, cheaply, and defensibly. It helps 

scientists bring together relevant knowledge on physical, chemical, and biological stressors that 

may be affecting the aquatic system of concern. This tool is particularly useful for watershed and 

regional ecological assessments in which scientists must often rely on stressor-effect relationships 

reported for other sites or in laboratory studies. CADDIS can also help scientists more easily 

identify relevant data from scientific studies conducted elsewhere and help them organize and 

compare the evidence across all the candidate causes. Finally, some scientists must be able to 

clearly communicate the logic of their causal conclusions. 

5.2. EVALUATE STRENGTH OF ASSOCIATION BETWEEN CAUSES AND EFFECTS 

Both SIE and watershed ERAs evaluate the strength of associations between a particular 

source or stressor and the effects of concern. The strength of the association (typically evaluated 

using multivariate analysis) between the stressors (or sources) and measures of effect may 

indicate cause-and-effect relationships, especially if supported by other lines of evidence 

(Serveiss, 2002; U.S. EPA, 2000b). In most retrospective watershed ERAs, causes of effects are 

often inferred from spatial relationships derived using relatively current information. For 

example, in the Clinch Valley ERA, a strong negative relationship existed between coal mining 

activities (particularly coal preparation plant discharges) and both fish IBI and number of native 

mussel species. The particular stressors in this relationship were believed to be both pollutants 

and sedimentation from coal fines, based on published research in the watershed. The analyses 

did not suggest that every coal mining activity in the watershed would cause a certain effect on 

native species, but they did suggest a relatively high probability of risk to native species living in 

31

close proximity to coal mining activities. Landis et al. (2000), using a relative risk model 

(RRM), also inferred cause-effect relationships by associating sources with important habitats in 

various subareas of the Willamette River Basin in Oregon. This model of analysis yields a 

ranking of stressors and habitats based on the relative likelihood of occurrence, not on the 

relative consequence (or effect) of occurrence (Landis et al., 2000). 

The Big Darby Creek assessment sought associations between stressors and impacts 

(Cormier et al., 2000) by relying on current and past land use practices and biological 

measurements taken at specific sites. Researchers used the ICI for macroinvertebrates and the 

fish IBI to represent ecological status within stream segments in the watershed. Multivariate 

analyses were used to determine relationships between index results, instream stressors, and land 

use patterns in the watershed. The analysis identified community components that were 

associated with specific types of stress. For example, the percent of Tanytarsini midges and 

Glypotendipes increased at sites with low and high biological oxygen demand, respectively. 

Also, the percentage of darters increased at sites with high scores for stream corridor structure 

and low concentrations of inorganic nutrients. 

5.3. WORK WITH CHEMICAL STRESSORS 

Exposure and effects analysis that relate to multiple chemical stressors in a watershed 

ERA can be handled in much the same way as in single-chemical ERAs. Both types of ERA use 

nationally developed chemical criteria (e.g., U.S. EPA, 1995b), if available, or effects thresholds 

obtained from laboratory or field studies are used. A challenge in many watershed ERAs is the 

likelihood of having multiple chemicals present simultaneously, any or all of which may have 

caused, or could cause, effects on valued resources. Laboratory-based chemical criteria or 

thresholds are generally developed independently of other chemicals, or any other stressors in 

percent, and therefore do not necessarily take into account synergistic or additive effects of 

multiple chemicals. Some field-based chemical thresholds, such as sediment quality thresholds 

(Field et al., 2002; Long et al., 1998), may incorporate chemical interactions in their threshold 

values. Several different tools have been developed to help address the issue of multiple chemical 

contaminants, the most common of which is to express each chemical concentration as a 

proportion of its criterion or threshold concentration. These proportions are then added; i.e., the 

assessor assumes that the effects of the different chemicals are additive. If the sum of the 

proportions is >1.0, some assessors have interpreted this as a potentially toxic condition. Limited 

research suggests that additivity may be a reasonable assumption in many cases, but a fair degree 

of uncertainty is associated with the results of this approach. 

One major uncertainty is the concentration of each chemical that is being compared to its 

respective criterion or threshold. Ideally, one has sufficient data to calculate frequency, duration, 

32

and magnitude of concentrations exceeding a given threshold so that the entire record of 

measurements is used to calculate risk. However, such comprehensive data are rarely available, 

and the assessor often needs to estimate an average or maximum value with which to analyze risk 

for the suite of toxic chemicals that might be present. 

Another source of uncertainty in analyzing risk of multiple chemicals is the often large 

variability in sensitivity to the chemicals for different species of interest. While the measures of 

effect selected to represent the assessment endpoint may help to focus this issue in risk analysis, 

chemical criteria and thresholds are typically based on many species, and the sensitivity of the 

types of species that occur in the watershed may be fundamentally different. One tool that has 

been beneficial, particularly in ERAs involving chemicals for which there are criteria, is a species 

sensitivity distribution approach (Bossuyt et al., 2005; Newman et al., 2000). This approach 

examines the sensitivity of all species for which relevant effects data exist and evaluates the 

distribution of species sensitivity to help predict the portion of the distribution of species that is 

likely to be affected by a given chemical concentration. For multiple chemicals, such species 

distributions of risk can be overlaid to obtain an estimate of risk due to chemicals to the aquatic 

assemblage. The species distribution can also be tailored in some cases to a given region by 

including only those species, or those representative species, that could reasonably occur in the 

watershed. In practice, however, this approach is often limited due to a paucity of relevant 

toxicological data for species of interest. In addition, certain types of species, such as freshwater 

filter feeders (e.g., unionid mussels) are currently under-represented in toxicological databases 

because reliable test methods were unavailable. Thus, the database of species sensitivity values 

available for certain chemicals may be limited and may not include many species or genera of 

concern in a given watershed. 

5.4. WORK WITH NONCHEMICAL STRESSORS 

Another challenge regarding stressors in risk analysis, particularly for watersheds, is the 

fact that many common stressors are nonchemical (e.g., sediment/turbidity, habitat), or are 

chemicals in media for which readily available criteria or thresholds may not exist (e.g., 

pesticides in sediment), or are chemicals that are naturally available and necessary for life (e.g., 

nutrients). Thresholds for these types of pollutants typically require some type of reference 

condition approach. A reference approach has been commonly used to set thresholds or criteria 

for biological assemblage condition (e.g., U.S. EPA, 1999a; Karr and Chu, 1997) and is now 

being extended to other types of parameters that are strongly influenced by natural regional 

conditions, such as nutrient criteria (U.S. EPA, 2000e). The reference condition approach relies 

on having measures for the parameter of interest from multiple sites in the same ecological 

region or ecoregion (Omernick, 1987) and that are minimally or least affected by human-caused 

33

activities. Ideally these sites are located in the same watershed or basin as the one being 

investigated. Thus, these measures ideally reflect natural variability, without the stressor of 

interest present. 

The threshold developed using reference conditions is a statistical attribute of the 

measures obtained from the various reference sites. A variety of statistical parameters have been 

used to formulate a threshold. For example, EPA’s Rapid Bioassessment Protocols (U.S. EPA, 

1999a) recommend, and many states use, the 25th percentile IBI value from the distribution of 

reference site IBIs as the threshold for reference biological condition. IBIs from a site that are 

significantly lower than this threshold suggest an undesirable biological condition (i.e., an effect). 

Clearly, this approach is sensitive to proper selection of reference sites. If more truly impaired 

sites are identified as reference sites, the established threshold will be less sensitive, and the 

associated risk analysis will be more uncertain. Also, this referenced threshold approach works 

best when there are many appropriate reference sites with which to measure natural variability. 

Biased estimates of natural variability are more likely as fewer reference sites are available. 

Finding reference sites is a common challenge in certain regions of the United States (e.g., 

agricultural areas of the Midwest and highly urbanized areas in southern California or the midAtlantic coast). 

As described in Chapter 3, biocriteria thresholds used in the Big Darby watershed ERA 

were based on ecoregional reference conditions (Cormier et al., 2000). EPA’s nutrient criteria 

guidance for streams and rivers suggests using a reference approach for developing nutrient 

criteria (U.S. EPA, 2000f). The EPA report recommends using either the 75th percentile of 

concentrations observed in ecoregional reference sites or the 25th percentile of measurements 

obtained from all sites in an ecoregion as a concentration threshold for a given nutrient. 

In some ecoregions and watersheds, reasonable reference conditions may be difficult to 

identify as a result of large-scale human-caused changes to the landscape, hydrology, and aquatic 

habitats. In these cases, appropriate stressor thresholds may need to be developed using other 

tools, such as modeled reference conditions, based on multivariate analyses of existing stressor 

and biological condition data. For example, using quantile regression (Cade et al., 1999; 

Koenker and Bassett, 1978) of macroinvertebrate Ephemeroptera, Plecoptera, and Trichoptera 

(EPT) taxa richness (a metric that includes many of the pollution-sensitive macroinvertebrate 

species) and associated urban intensity (a combination of urban land cover and population and 

road density), resource agencies in southern California were able to identify best attainable 

biological conditions despite the occurrence of high urbanization and associated stressors. Such 

analyses can help elucidate causes of biological impairments and provide the means to identify 

working “thresholds” for a given stressor. 

34

Risk analysis can often take the form of simulation or numerical modeling, depending on 

the stressors of concern, availability of appropriate criteria or thresholds, and extent of stressor 

data available. For the middle segment of the Snake River in Idaho, a life cycle analysis was 

conducted for three highly valued coldwater fish (rainbow trout, mountain whitefish, and white 

sturgeon), aquatic macrophytes, and endemic molluscs (U.S. EPA, 2002d). These representative 

species from three major trophic levels (fish, invertebrates, and plants) were chosen as assessment 

endpoints for the analysis. A simulation model was used to develop quantitative risk estimates of 

the likelihood of exceeding water quality criteria, macrophyte biomass, and habitat suitability 

indices for the fish species. The system dynamics were simulated using information on 

meteorological conditions, hydrological and hydraulic conditions, biological oxygen demand, 

dissolved oxygen, phytoplankton biomass, various forms of nitrogen and phosphorus, 

temperature, coliform bacteria, and water depth, all obtained from several locations in the river. 

Risks to fish and macroinvertebrates were estimated by determining the likelihood of being above 

or below coldwater biota tolerance limits. Tolerance limits are generally the natural level to which 

most native species have adapted. In addition, qualitative analyses of exposure and effect were 

completed using best professional judgment and comparisons with field studies under similar 

conditions. The results of these individual analyses were combined to draw conclusions in the risk 

characterization phase. 

5.5. WORK WITH DATA LIMITATIONS 

5.5.1. Use Categorical Data 

All of the EPA-sponsored case studies, and most other large-scale ERAs that have been 

reported, had to work with some types of data limitations, and therefore uncertainties in their risk 

analyses. Given the multiple stressors in watersheds, and the relatively large spatial scale of 

watersheds, most analytical approaches will require fairly large sample sizes to account for the 

many different potential sources of variation. As a result, data requirements can be enormous for 

developing meaningful associations between sources, stressors, and effects in watersheds. 

While regression-based relationships between a stressor and effect or a source and a 

stressor are desirable in any ERA, watershed analyses often lack sufficient and appropriate data 

with which to derive meaningful “stressor-response” curves. One alternative that has been used in 

many watershed ERAs is to categorize the data before conducting analyses. Categorical data may 

depict relationships more effectively than continuous (gradient) data in some cases. Both stressor 

and effect data may be categorized. For example, in the Clinch and Powell ERA, few if any 

gradient relationships were evident between sources, stressors, and effects. However, 

relationships became more evident when the measure of effect (i.e., fish IBI), was categorized as 

impaired or unimpaired (based on ecoregional reference conditions) rather than using the IBI 

35

values themselves (Diamond et al., 2002). Similarly, relationships between habitat quality factors, 

such as sedimentation, and effects were more apparent if habitat quality (the stressor) was 

categorized as “satisfactory” or “unsatisfactory,” based on the resource agency rating system. 

Categorizing data for a factor, as opposed to relying on the individual data values themselves, 

often results in increasing the power of the analysis; i.e., there is a higher probability of seeing a 

relationship if one is there. However, stressor-response relationships may or may not be feasible 

using categorical data. If only two or three categories of a source, stressor, or effect are 

appropriate, analyses will often take the form of an analysis of variance (e.g., ANOVA, 

MANOVA, ANCOVA) or analogous nonparametric analysis to determine statistical differences 

between categories. 

Categorizing data works best when either criteria or consensus-based thresholds for the 

parameter are available. For example, if the bioassessment data available are based on an index 

that discriminates four or five categories of condition such as the fish IBI , then it is appropriate to 

categorize the bioassessment data accordingly for analysis. Analysis of more categories in this 

case would not be appropriate and could yield misleading results because the index was not 

calibrated to distinguish between a greater number of categories. Categorizing stressor or source 

data can be more uncertain because consensus thresholds are typically unavailable. For example, 

effects on assessment endpoints as a function of distance from a potential source (a surrogate for 

stressor intensity) is often a useful analysis in watershed ERAs. However, distance from a source 

may not lend itself to obvious categories unless much is known about the fate and transport of the 

stressors assumed to be present. In this case, it may be useful to examine the cumulative 

distribution of the distance of each relevant biological sampling location to its nearest source type 

of concern (e.g., distance from the nearest active coal mine was examined in the Clinch Valley 

ERA). Based on this distribution of distances, the assessor can then identify a few categories 

(e.g., <1 km, 1-5 km, and >5 km), for which a sufficient number of sites exist and can be 

statistically analyzed in a meaningful way. Again, care should be taken to ensure that the levels or 

categories are appropriate for the measure being analyzed and the particular watershed. 

5.5.2. Use Source Data 

Another common data limitation in watershed ERA is that stressor data may not be 

available or in a useful form. To deal with this limitation, risk assessors have often relied on 

source data as a substitute. Source data include such parameters as the number of a certain type 

of point source or the percentage of a particular type of land use. These data are typically fairly 

easy to obtain for a watershed, and they are readily mapped for analysis and presentation 

purposes. Source-effect analyses were used both for the Big Darby Creek and the Clinch Valley 

assessments where land use (source) data were compared with fish and macroinvertebrate 

36

(effects) data. Originally, the assessment team for the Clinch and Powell Valley assessment 

sought associations between fish or mussel data, and stressor data such as sedimentation or water 

chemistry data, yet data were too sparse for quantitative analyses (U.S. EPA, 2002a). Instead, 

emphasis was focused more on probable stressor sources and their effects on resources (Diamond 

et al., 2002) (see Figure 5). 

Figure 5. Significant relationships between urban land use activities and 

invertebrate ephemeroptera, plecoptera, and trichoptera (EPT) score. 

Relationships between the biological measure (EPT in this case) and land use cover 

were often difficult to evaluate using linear stressor-response analyses (upper left 

graph). Relationships were more apparent when examined on a categorical or 

threshold basis (upper right and lower graphs). 

Source: U.S. EPA (2002c). 

37

By comparing a group of sources or stressors to a group of effects, potential exposure of 

biota to sources of stress can be examined in a variety of ways, depending on the types of data 

and tools available and the types of sources being analyzed. As watershed ERAs are necessarily 

place based, spatial analysis tools and GIS are essential to watershed assessment work. Land use 

(e.g., agricultural and urban) constitutes a major sources of stress identified in watersheds, and 

these data can be estimated using digital satellite images (e.g., Landsat) and GIS. Typically, 

percent area cover in a watershed or other defined spatial area is used to quantify the “exposure” 

of a given land use (e.g., urban development). The way in which land use percentages are 

calculated may need to be considered carefully. The Clinch and Powell ERA demonstrated that 

land uses within the riparian corridor were more indicative of exposure to aquatic life than land 

uses calculated for the watershed as a whole (Diamond et al., 2002). Others have shown similar 

significance of riparian land uses in watershed studies (Stewart et al., 2001; Hunsaker et al., 

1990). 

Exposure stemming from different land uses may be inferred from available data in the 

watershed and exposure-effects information from the literature. For instance, in the Clinch and 

Powell assessment, the fish community was consistently poor when the surrounding riparian 

zone included all four main sources of stress: mining, urbanization, major roads, and pasture 

areas (Diamond and Serveiss, 2001). The strong association of adverse effects on fish in the 

presence of these nearby land uses was meaningful, even in the absence of more specific, 

quantitative profiles of exposure and effects (Diamond et al., 2002). However, assessing 

relationships between land use and ecological resources is especially complex in a watershed, 

and interactions among land uses could mask relationships between a particular land use and 

ecological effects. The effects of pasture land could be masked by coal mining effects (Diamond 

and Serveiss, 2001). In addition, land cover estimates have a certain amount of error associated 

with them, both spatially and thematically. Thus, depending on when effects or stressor 

characterization data were collected, land coverages may not be an accurate reflection of source 

information. These uncertainties need to be recognized and discussed either during this risk 

analysis phase or during the next phase, risk characterization (Chapter 6). Land coverage data 

may require “ground-truthing” to ensure that risk assessors are dealing with correct estimates of 

specific land coverages in a given location or region. 

5.5.3. Rank Risks 

Occasionally, insufficient data are available to analyze an assessment endpoint or to assess 

a surrogate measure of effect. In these instances, less precise measures (scalar, ranked, or 

qualitative) could be used. Relative ranking of risks associated with management options may be 

sufficiently robust to provide meaningful information to decision makers. The RRM (Landis and 

38

Wiegers, 1997) is one approach that produces a “quantitative” assessment using best professional 

judgment of exposure and effect rankings. 

While the focus of this report primarily has been on EPA-sponsored case studies, a 

number of regional ecological risk assessments have been developed using the RRM (Walker et 

al., 2001; Landis et al., 2000; Wiegers et al., 1998). Rather than looking at the traditional risk 

assessment components of stressor, receptor, and response, the relative risk regional approach 

examines relationships between sources (groups of stressors), habitat (groups of receptors), and 

impacts (groups of responses). Stressors are ranked in terms of their occurrence and importance. 

High relative risk occurs where a source, an impact, and a habitat strongly overlap; where there is 

little overlap, the relative risk is low. Relative risk can also be summarized via numeric indices, as 

shown in Wiegers et al. (1998) for the Fjord of Port Valdez. This approach has the clear 

advantage of prioritizing what are thought to be the main stressors and major habitat types at risk. 

This analysis can then focus subsequent data collection and analytical efforts on managers 

concerns. 

With the RRM approach, a source can serve as a group of stressors, a habitat can serve as 

a location for a set of organisms or receptors, and an ecological impact can serve as a group of 

receptor responses. Each source and habitat is ranked for each area to indicate high, moderate, 

low, or no risk within the context of the study site. Ranks are assigned using site-specific criteria. 

RRM ranks sources or stressors in different locations in a region, evaluates or filters if the habitat 

is affected by sources or stressor, and ranks the impacts. The region is divided into several similar 

areas based on land use, basins, clusters of similar stressors, habitats, or land uses. Ranks or 

scores to represent high, medium, low, and none are used to reflect the magnitude of the source 

and the amount of habitat that could be affected in each subarea. The ranks assigned for each 

combination of source, habitat, and subarea are multiplied together. If the interaction between the 

exposure and effect is likely to occur, the two scores are multiplied by one another. The results 

for each subarea are summed to determine the relative ranking of risk within the region. Thus, by 

quantitatively determining the relative risks in subregions, the contribution of risks from sources 

or stressors, risks in various habitats, and risks to assessment endpoints can be calculated. The 

results can then be portrayed easily in graphical form with maps depicting the relative risks within 

the study region. 

This approach is simple to use and can include stressors about which little is known 

quantitatively. This approach uses a qualitative, graphical approach for sorting out the many risk 

pathways that may be present in a regional-scale ERA. 

39

5.6. DEAL WITH UNCERTAINTY 

Another source of uncertainty in using source data as a surrogate for stressors is that some 

land uses or habitat features may be correlated to natural physiographic factors that may 

themselves have a pronounced effect on biota. In the Clinch Valley ERA, for example, elevation 

was related to the number of aquatic species present, even in the absence of stressors. Thus, 

elevation effects had to be “factored out” of the analysis to examine source-effect relationships. 

This made it necessary for risk assessors to use a smaller subset of the available sites that were 

within a relatively narrow (i.e., nonsignificant) elevation range for analyses. Further partitioning 

to account for other factors, such as land uses, would further reduce the number of appropriate 

sites available for analysis. Depending on the stakeholder concerns, predicted results, and 

acceptable levels of uncertainty, data may be insufficient for any type of statistical analysis. 

The inclusion of natural variability in risk analyses is especially critical at the watershed 

or larger spatial scales. Aquatic life habitats, for example, are known to vary in quality and 

quantity depending on natural factors, such as precipitation patterns and geology. At larger 

spatial scales, these natural factors (including the elevation example described above) are likely 

to vary across the watershed, necessitating proper assessment of these factors. Typically, natural 

variability is addressed through the identification of appropriate reference conditions as discussed 

previously. Measurements of the assessment endpoint, as well as natural factors that affect that 

endpoint, across several reference sites under a given set of conditions (e.g., precipitation range, 

elevation), will yield an estimate of natural variability. This variability becomes the basis for 

determining whether a given source or stressor tends to be associated with a non-natural (i.e., 

impaired) effect. In some regions, biological data collected over more than a year or season are 

needed at reference sites to adequately characterize natural variability. This is particularly the 

case for more arid regions where “wet” years can yield a different biological condition than “dry” 

years. Because habitat features are often sensitive to natural regional factors, it is important to 

define natural variability for the watershed in which one is working. The assessor should 

critically evaluate reference condition data obtained elsewhere to be confident that the 

information is appropriate for the watershed being evaluated. 

An additional uncertainty associated with using source data as a surrogate of stressor 

information is that all members of a given source class may not have an equal probability of being 

associated with a hypothesized stressor (or condition). For example, certain agricultural land uses 

may be associated with sedimentation and excess nutrient input into streams. However, 

depending on particular BMPs that may have been implemented at a given site, the intensity of 

these stressors (and therefore predicted ecological effects) may not differ much from that 

observed under reference conditions (where few or no stressors are present). If such 

differences among sites are not identified and taken into account in risk analysis, results will be 

40

“noisy,” masking any real relationships that might exist between certain source types, stressors, 

and effects. A solution to this challenge is to involve people in the ERA who are knowledgeable 

about the watershed and can help “ground-truth” risk analyses. 

Literature reviews often play an important role because site-specific information about 

ecological effects, or interpretive benchmarks (e.g., effect or impairment thresholds), is often 

unavailable for the watershed under study. For example, researchers elsewhere may have 

demonstrated relationships between the ecological measure of interest (e.g., the abundance and 

diversity of certain wetland plants) and certain stressors (e.g., urban stormwater runoff) that also 

occur in the watershed of concern. It may be appropriate to extrapolate results obtained in those 

other locations to the subject watershed as long as uncertainties are adequately documented. 

5.7. RECOGNIZE THAT ANALYSIS IS AN ITERATIVE PROCESS 

Another issue of importance is that the collection and analysis of data in the analysis phase 

may be iterative. Analyses may bring new information to light (e.g., one pathway between 

stressor and effect may have much greater impact than originally predicted). In addition, 

management’s needs may change or the environmental situation may become dramatically 

different (e.g., following an oil spill). Many recent recommendations have emphasized that 

stakeholder and manager involvement needs to be initiated in the planning step, and recurring 

rounds of deliberations and analysis are necessary throughout the process to make the findings 

most useful (U.S. EPA, 2000a; Foran and Ferenc, 1999; PCCRARM, 1997; NRC, 1996). New 

information obtained through literature review, field data, peer review, or environmental changes 

since the beginning of the assessment may trigger these iterative loops. 

As the analysis proceeds, the assessors should present interim findings to managers to 

ensure that the assessment is targeting the appropriate problems. A lesson learned from the 

prototype assessments is that regular discussions between the assessors, managers, and 

stakeholders during the analysis phase are very useful for refocusing the ERA and for 

reprioritizing pathways for evaluation (Serveiss, 2002). The Clinch Valley ERA demonstrates 

two particular concepts related to the benefit of dialogue among participants during analysis. The 

initial risk analysis showed that certain measures were not available or were spatially distributed in 

a way that made some of the proposed analyses infeasible. Active dialogue among assessors, 

managers, and stakeholders during this process resulted in modifying the analysis plan and 

switching to determining associations between land use and biotic measures. 

The value of pilot studies also illuminates how watershed ERA is an iterative process. 

Given the often complex interactions in watersheds, data limitations, and costs, it also may be 

useful to perform a pilot analysis first to determine that the analytical approach is likely to be 

completed successfully. Analyzing a subwatershed is one approach for pilot testing the analysis 

41

plan (Diamond et al., 2002; MacDonald, 1994). In the Clinch and Powell Valley assessment, the 

Copper Creek subwatershed was examined first to assess how to apply the analysis plan to the 

entire watershed (U.S. EPA, 2002a). Copper Creek was chosen for this pilot analysis because it 

was the most data-rich subwatershed and because it was a relatively simple case in that 

agricultural uses were the major source of anthropogenic activity. The pilot study was needed to 

address two analysis objectives central to this assessment: (1) to identify the appropriate spatial 

scale to test relationships between land use activities or stressors and measures of effect and (2) to 

identify whether the benthic macroinvertebrate measure (i.e., the EPT index) or the fish IBI would 

be a reliable surrogate measure of effect for predicting the status of native mussel assemblages. 

Achieving the latter objective was especially desirable because it was known at the outset of this 

study that available native mussel data were more limited than either EPT or IBI values. 

5.8. UNDERSTAND SECONDARY OR INDIRECT PATHWAYS 

In watershed ERAs, more than one source of a given stressor may lead to secondary 

pathways. For example, toxic chemicals can originate from nonpoint sources such as agriculture 

(pesticides), point sources (wastewater treatment plants), urban runoff (oils, metals), and many 

other human activities. The conceptual model should identify the major sources of a given 

stressor to reveal all pathways. Some stressors, such as temperature, acid runoff, and low 

dissolved oxygen, may have a seasonal component or may be associated with certain geological 

or hydrological characteristics of the watershed. The analysis phase should attempt to address 

such variation if possible to better reflect stressor-response associations. 

Secondary stressors may be even a greater concern than primary stressors. Some stressors 

may have direct results that impair valued resources, such as nutrients resulting in nuisance algal 

blooms. Nutrients may have secondary effects, however, such as decreased dissolved oxygen, 

due to increased bacterial respiration and algal die-off. In this case, a secondary stressor from 

one source can be similar to a primary stressor from another source; e.g., decreased dissolved 

oxygen may also be a primary stressor resulting from release of oxygen-demanding material 

(biological oxygen demand) from wastewater effluents or agricultural runoff. These secondary 

stressors may need to be considered as well. For instance, in Waquoit Bay, nutrient input is a 

stressor that caused excess macroalgal growth (a secondary stressor) to shade eelgrass and inhibit 

its growth (Figure 4). Fate and transport models were then used to predict environmental 

nitrogen loading from the watershed into the bay (U.S. EPA, 2002c). In a similar manner, a 

model of phosphorus loading in Wister Lake in Oklahoma allowed estimation of changes in lake 

trophic conditions (Hession et al., 1996). 

42

Risk analysis is complete when quantitative stressor-response characterizations have been 

completed and assumptions, uncertainty, and data limitations have been discussed. A few 

iterations may be required to progress from start to finish. Finally, since watershed ERA 

combines risk analysis with risk characterizations, the necessary iterations may become blurred. 

43

6. RISK CHARACTERIZATION PHASE 

The exposure and effects data generated in risk analysis are integrated to form 

conclusions in the risk characterization phase. Risk characterization should use multiple lines of 

evidence from models, field work, and statistical extrapolations to increase the confidence in 

conclusions (U.S. EPA,1998b) and to help provide answers for risk management (see Chapter 8). 

Finally, uncertainties originating in both the problem formulation and analysis phases should be 

described (Suter, 1998). The final product of this phase is the risk assessment report prepared for 

managers to support defensible management decisions. 

Risk characterization includes two major steps: risk estimation and risk description. 

Risk estimation integrates the exposure data and stressor-response estimates from the analysis 

phase while addressing uncertainties that arose throughout the assessment. Methods used to 

integrate exposure and effects data include comparing single values of effect and exposure, 

comparing statistical distributions of exposure and effect values, or conducting simulation 

modeling. As previously presented, in watershed ERA different techniques may be used to 

accomplish the integration of exposure and effects data (e.g., causal analysis, risk ranking, using 

source data) and risk analysis may merge with risk estimation. As a result, risk estimation was 

discussed in the preceding chapter, and risk description is the focus of this chapter. 

6.1. RISK DESCRIPTION 

During risk description, risk assessors prepare a summary of ecological risk and interpret 

ecological significance. Summarizing risk involves making a bottom-line estimate of risk. 

Ideally, a risk summary would be in the form of a quantitative statement: for example, “There is 

an 80% chance of a 50% die-off of certain valued fish species in the watershed due to 

sedimentation from nonpoint sources under current conditions.” Such a statement, when 

combined with information on the reliability of the statement (i.e., the uncertainty), would allow 

analysis of the relative costs and benefits of one of several management options. Unfortunately, 

often it is not possible to produce a precise quantitative estimate for a watershed ecological 

assessment because risks typically involve multiple stressors, from multiple sources, across a 

range of habitats. Instead, probability estimates can be expressed in high, medium, or low terms. 

For example, there is a high probability that shorebird populations in the immediate vicinity of a 

filled wetland will decline as a result of habitat elimination. Every effort should be made to state 

the likelihood of the outcome. Risk descriptions also can be expressed in relative terms, such as 

option A poses more risk to the assessment endpoint than option B. This is the kind of risk 

description derived from the RRM that ranks risks by habitat areas and stressors (Wiegers et al., 

1998). 

44

Because exposure to stressors is typically complex and possibly synergistic in watersheds, 

these relative data may be more useful in characterizing risks than single dose-response estimates 

derived from laboratory data. The nature and magnitude of predicted effects should be 

considered with regard to the structure and function of the affected ecological system, its 

resistance and resilience, spatial and temporal effects, indirect effects or cascading interactions, 

and the life-cycle of organisms. 

The risk description interprets ecological significance, translating possible risk estimates 

into a discussion of their consequences for the watershed. The description may address the nature 

and magnitude of effects, the spatial and temporal patterns of effects, and the potential for 

ecosystem recovery if effects have already occurred. For instance, the factors limiting mountain 

whitefish survival and reproduction include elevated water temperatures, loss of lotic habitat, 

fluctuating water flows, and excessive sedimentation (U.S. EPA, 2002d). The evidence for these 

effects is demonstrated by evaluating favorable spawning conditions at two conditions in the river, 

comparison with habitat suitability indices, and review of the literature. The risks are described in 

Table 3. 

The significance of the consequences of predicted effects may vary considerably for 

different types of ecological systems or habitat types. For example, in the Waquoit Bay 

assessment (U.S. EPA, 2002c), atmospheric deposition of nitrogen on loam soils far from a water 

body was deemed less of an impact than deposition occurring on sandy soils near an estuary. The 

effect of a herbicide may be quite different in a stream that derives most of its organic carbon 

energy from plants as compared with the effect in a stream that utilizes predominantly 

detrital-based organic carbon. The loss of wetland area may be highly significant if it represents 

the only habitat available in an area for waterfowl but may be negligible if it occurs among a much 

greater wetland area. 

Sometimes risks can be described in relationship to a management option that is being 

considered. A review of historical data was used to describe the relationship between increased 

nitrogen loads and decreasing levels of eelgrass cover over time (U.S. EPA, 2002c). Managers 

then could select a particular percentage of eelgrass bed coverage as a management target. For 

example, if the restoration target was to support the growth of eelgrass on 30% of the main 

estuary, this would be comparable with 1970 conditions in Waquoit Bay. Thus, the nitrogen load 

would need to be reduced to 18,000 kg N per year (Figure 6). 

In watershed assessments, the spatial distribution of exposure and effect is important to 

consider and can be assessed using GIS overlays of the two types of information. GIS maps can 

be helpful as analytical and communication tools. When large areas are parsed into small subunits 

or representative subsets are selected for analyses, key differences can be lost. GIS 

45

Table 3. Middle Snake River case study: integration of stressors, responses, and recovery potentia

reproduction, growth, and survival of the mountain whitefish population 

Factor Stressor 

Line of 

evidence Risk Uncertainty Assumption 

Number of spawning fish 

Loss of adult habitat, restrictions on movement to feeding and overwinter areas 

LIT, BPJ An increase in the 

population size is not possible with low or no reproduction 

Low, very few adult fish are present in the Middle Snake River or nearby tributaries 

Available fish surveys are a true indication of the scarcity of fish in this area 

Spawning Water temperature too 

high for successful egg development, loss of lotic habitat 

HSI, LIT, WQS, BPJ 

Population cannot recover without successful reproduction 

Low, water temperature requirements for spawning are known 

Low spawning success attributed to poor water quality conditions 

Fry High water temperature, 

fluctuating water levels, loss of lotic habitat, and predation 

HSI, LIT, WQS, BPJ 

Population cannot recover without successful recruitment 

Low, temperature requirements are known, but rearing habitat not identified 

Fluctuating water levels push fry into deeper water where increased predation occurs 

Food supply Sedimentation and 

increased water temperature 

LIT, Field, BPJ 

Invertebrate fauna not adequate to support an increased mountain whitefish population 

Moderate, adequate analysis of sampling information has not been completed 

Food supply is limited 

Movement Dams prohibit seasonal 

migratory movement by adults 

LIT, Field, BPJ 

Adults unable to reach upstream tributaries or refugia used for spawning or rearing 

Moderate, limited information on migratory population in the Middle Snake River 

Adult fish reported at Lower Salmon Falls Dam in the 1950s represented a migratory population 

BPJ = best professional judgment LIT = literature search Field = field survey in the Middle Snake River WQS = water quality standards HSI = habitat suitability index Source: U.S. EPA (2002d). 

46
Figure 6. Historical changes in nitrogen (N) loading predicted by the 

nitrogen loading model. If the management target is a return to 30% 

eelgrass cover, then managers must reduce N loads to 18,000 kg N per year, 

loads comparable to those of around 1970. 

Source: U.S. EPA (2002c). 

approaches help address that problem by providing a mapping tool that can capture basic 

descriptive information and link calculations for subareas to produce an overall picture. 

With a GIS, data layers can be added to identify other more specific potential sources of 

stressors, such as wastewater discharges and mining activities. Although certain types of land use 

may not be amenable to calculation on a percent area basis, they may be analyzed on a distance or 

density basis (i.e., how close a given mine is to aquatic life habitat or how many mines per square 

mile are within a certain area) (see Figure 7). Either of these calculations could provide useful 

estimates of mine “exposure.” Before using GIS, ground-truthing of data is necessary. 

The RRM projects described previously rely on GIS-based approaches to evaluate 

potentially important stressors in watersheds or regions. These approaches use maps to display 

major stressors and associated degree of potential biological impacts on maps. The maps greatly 

facilitate communication of the information to the public and managers. Stressors can also 

47

Figure 7. Fish index of biotic integrity (IBI) or insect ephemeroptera, 

plecoptera, and trichoptera (EPT) values in relationship to proximity to coal 

mining sources in the Clinch Valley ecological risk assessment. If the location 

of a source of potential stressors can be clearly defined (as in the case of point 

sources or those that can be spatially defined), it may be useful to relate measures 

of effect to the distance from the source (in this case active coal mines) to assess 

potential effects of a given type of source. 

Source: U.S. EPA (2002a). 

48

combine to influence the quantity and quality of habitat for species, and therefore habitat can be 

used as a basis for analyzing and showing how effects are integrated. Because habitat 

requirement vary by species, the range occupied by particular species, or biological communities 

needs to be considered. For example, land use impacts can be analyzed and shown according to 

the various habitats occurring across the landscape. 

Besides GIS, graphs are also a helpful analytical and communication tool. In assessments 

where timing of events is critical (e.g., as in acid precipitation, or in the assessment of episodic 

events), graphs that show the timing and distribution of stressor events in comparison to an 

effects threshold may be a useful way to present the information. For prospective risk analyses 

supporting watershed planning, risk estimation derives the estimates of management measures for 

comparison between different management options. In EPA’s TMDL protocols, this is referred 

to as a “linkage analysis” (U.S. EPA, 1999b). When exposure and effects data are limited or are 

not easily expressed in quantitative terms, qualitative evaluation techniques may be used to rank 

risks using subjective judgment and categories such as low, medium, and high. 

It can be useful to incorporate results of risk analyses, and the probable co-occurrence of 

stressors and assessment endpoints, into the initial conceptual model. Pathways can be shaded or 

color-coded (e.g., green = no or few risks likely; yellow = risk possible; red = risk likely) on the 

conceptual model to help characterize the risk to assessment endpoints (Serveiss and Ohlson, 

2006). This “updated” conceptual model could also be used to highlight where major 

uncertainties lie and/or additional sampling or analyses would be especially worthwhile. 

Agreement among multiple lines of evidence increases the confidence in the conclusions, 

although any discrepancies warrant discussion. The Clinch Valley assessment used a table to 

show the various lines of evidence and to summarize the conclusions of risk and recovery 

potential to various life stages of mussels and fish (U.S. EPA, 2002a). In the Middle Snake River 

ecological risk assessment (U.S. EPA, 2002d), a water quality simulation model was used to 

predict attainment of water quality standards and habitat requirements for the life-cycle stages of 

coldwater fish (Table 3). Simulations were used to modify the model or provide information on 

uncertainty. The data from the model were supplemented with field data and literature review. 

6.2. DESCRIBING UNCERTAINTY 

There are two general sources of error in ecological assessments—uncertainty and 

variability. Uncertainty from lack of knowledge can be expressed in many ways during the 

analysis phase, such as insufficient data or spatial coverage of required data, inaccurate model 

inputs, GIS spatial resolution errors, and investigator bias (intended or unintended). Natural 

variability includes spatial and temporal heterogeneity in measures of interest, background 

49

ecological system factors (e.g., geology, precipitation), and natural disturbances (e.g., floods, 

fires, hurricanes), all of which can affect the ecological effects and relationships observed. 

Uncertainties encountered throughout the assessment, including the basic data set, should 

be summarized in an uncertainty analysis (Suter, 1998; U.S. EPA, 1998a). There are likely to be 

many sources of uncertainty, such as measurement error (inappropriate, imprecise, inconsistent, 

or sparse measurements), conditions of observation (such as extrapolating from laboratory tests to 

field predictions), and limitations of models (oversimplifying complex ecological processes, 

incorrect assumptions). Uncertainties exist in every aspect of a risk assessment, especially in 

estimates of exposure, estimates of effects, and the integration of exposure and effect. The 

relationship between the measure of effect and the actual assessment endpoint is always subject to 

uncertainty. Watershed and regional assessments may contain more uncertainty than smaller scale 

assessments because of difficulties in obtaining sufficient data for multiple stressors, sources, and 

measures of effect for the entire geographic area. 

Uncertainty can be described in either narrative form, with statistical distributions, or with 

sensitivity analysis. Narrative descriptions are especially suited for qualitative types of 

uncertainties, such as unclear communications, descriptive errors, and professional judgment and 

any assumptions made throughout the assessment. Depending on how data gaps are addressed, 

their associated uncertainties may be quantifiable. Natural variability and sampling error may be 

described statistically. Some uncertainties, such as those associated with modeling, can be 

addressed in terms of sensitivity analyses, in which certain model parameters (or inputs) are 

adjusted and the outputs compared (Suter, 1998; Hession et al., 1996). Other uncertainties, such 

as those associated with sparse data, cannot be reduced without additional data collection but may 

lead to identification of those data that greatly reduce uncertainties for future risk assessments. 

Uncertainties may increase if the appropriate temporal scale is not evaluated. Episodic pollution 

events, for example, can have dramatic, persistent ecological effects that might not be recognized 

using a short temporal horizon. In the Clinch and Powell ERA, episodic chemical spills from 

transportation or industrial accidents resulted in long-term reductions in native mussels 

downstream (Diamond and Serveiss, 2001). Long-term data sets, if available, are desirable to 

address such issues (Hunsaker et al., 1990). 

The Middle Snake River assessment (U.S. EPA, 2002d) used Table 3 not only to 

summarize risk descriptions but also to describe the uncertainties and assumptions associated with 

each effect. In successive model simulations, input parameters were varied to provide information 

on uncertainty in model output. The data from the model were supplemented with field data and 

literature review. Sources of uncertainty included limited knowledge of stressor-effect 

relationships, variability in field measurements, model error, and estimation error. Each source of 

uncertainty was further described in narrative form. 

50

When data are sufficient to proceed with a quantitative assessment, as when focusing on 

single pathways of the conceptual model (e.g., Serveiss et al., 2004; Hession et al., 1996), then 

quantitative measures of uncertainty are possible for the exposure and effect measures. 

Uncertainties associated with interactions among stressors and sources that are not considered in 

models only can be addressed qualitatively. RRM addresses multiple stressors and effects by 

using qualitative rankings in quantitative models. Uncertainty of model output can be represented 

quantitatively in sensitivity analyses, although the input is qualitative (Walker et al., 2001; 

Wiegers et al., 1998). Describing risk and uncertainty are two of the major features of risk 

characterization. Since there will always be uncertainty, it is important to have assessments peerreviewed by other scientists in the field. Peer-review comments and a description of how the 

report was revised in response to comments should be made available. 

6.3. RISK CHARACTERIZATION COMPONENTS 

Risk assessors prepare a report for the risk managers using concepts outlined in Chapter 7, 

Risk Communication. Risk managers use this information to make decisions (see Chapter 8). 

Although these steps are presented chronologically in this report, these activities may occur 

throughout a watershed assessment. A watershed manager may need to make a relatively quick 

decision to control the impact of a spill and may ask the assessor for interim information before 

the assessment is completed. Thus, some elements of risk characterization and risk 

communication described in this and subsequent chapters may apply to the manner in which 

interim findings are presented earlier in the assessment. 

Risk characterization can take the form of an isolated product by summarizing the earlier 

phases or it can be one part of the entire report. Either way, somewhere in the risk assessment 

report, the following information should be included: 

•	 A description of risk assessor/risk manager planning results 

•	 A discussion of the major data sources and analytical procedures used 

•	 A review of the stressor-response relationships 

•	 Estimates and descriptions of risks to the assessment endpoints 

•	 A summary evaluation of the relative risk of different management scenarios for use as 

a basis for risk management decision making 

•	 The major sources of uncertainty and the approaches used to address them 

•	 Documentation of science policy judgments or default assumptions used to bridge 

information gaps and the basis for these assumptions 

51

• Lines of evidence and the rationale for reaching the conclusions that were reached 

• A description of the peer review process and a list of peer reviewers 

Effective risk characterization must accurately translate the best available information 

about a risk into a language nonscientists can understand (NRC, 1996). The risk characterization 

should clearly communicate to the risk manager the major risks at some level of biological 

organization (organism, population, community, ecosystem), the ecological significance of the 

findings, and the level of uncertainty. It is important to clearly describe the ecological resources 

at risk, their value, and the costs of protecting (or failing to protect) the ecological resources. 

The ultimate success of risk characterization depends on successful risk communication. 

52

7. RISK COMMUNICATION 

Risk communication is the translation of scientific information in a form that is useful to 

managers and the public. Risk communication should occur throughout the assessment process, 

but ultimately what counts is how the results of the assessment are used. Hence, this is one of the 

latter chapters in this report. 

When risk characterization is complete, a report is prepared showing estimates of risk. 

Clear presentation of the strengths, limitations (including uncertainties), and conclusions of the 

risk assessment will greatly enhance the assessment’s usefulness in decision making (Serveiss, 

2002). The challenges for risk communication include establishing a process for risk 

communication, targeting the audience, and communicating effectively. 

7.1. THE RISK COMMUNICATION PROCESS 

Regular and recurring communication between scientists and managers is needed to 

refine management objectives and to ensure that the risk assessment is relevant to the decision 

problems. The management objectives, and the relationships between management objectives, 

stressors, and management options, are often poorly defined at the start of the process (Reckhow, 

1994). When the assessment is completed, the manager and key stakeholders should have a full 

and complete understanding of the assessment. 

Involving stakeholders throughout the process increases the likelihood that findings will 

be useful and helps prevent confrontation or litigation. Watershed management requires an 

interactive, participatory approach (Glicken, 2000; U.S. EPA, 1995a), in which risk assessors 

both impart information to and gain information from stakeholders. In the ERA paradigm, 

stakeholders work as partners with scientific risk assessors at every stage of the 

process—defining the problem to be solved, prioritizing the risks posed, and evaluating and 

ranking the remedies (NRC, 1996). 

Many types of risk cannot be assessed in a completely objective way because the 

evaluation of risk depends on the interpretation of goals and objectives, which is a value-laden 

process. A stakeholder-based decision process provides a means to make value judgments that 

are supported by the scientific evidence. The Delaware River TMDL for polychlorinated 

biphenyls was established as such a structured decision process. A Web site was used to describe 

the time and place for business and public meetings and the topics to be discussed (Delaware 

River Basin, 2006). 

While stakeholder involvement is crucial, there are also reasons to refrain from involving 

all participants. An adequate treatment of science is possible in a stakeholder-driven 

process—but only if sufficient high-quality staff, time, and resources are available (U.S. EPA, 

53

2001a). Where resources are not sufficient, or when a decision must be made quickly to address 

an imminent threat, stakeholder processes may not be the solution. The SAB also warns against 

handing off processes to a stakeholder group as a means to address mandates for which an agency 

has insufficient resources. 

If stakeholder groups are too large (or intractable in their opinions or desires), it may be 

difficult to reach closure. There are likely to be different stakeholder groups with widely varying 

levels of interest in the process, and varying ability to commit time to the process. One solution 

appropriate for many risk assessments is to form a stakeholder advisory group consisting of a 

smaller group of well-respected and representative stakeholders (about 10 members). The 

advisory group serves as a communication pathway between the risk assessment team and the full 

set of interested parties in the community, serving both to identify values and concerns of the 

community at large and to communicate progress in risk assessment and risk management efforts 

back to the community. 

An important challenge for the participatory approach lies in balancing meaningful 

stakeholder participation with the need to make progress in managing risks. Rather than allowing 

the stakeholders to have access to the risk management team and process at all times, it is 

important to strike a participation/management balance by providing well-defined, strategic time 

frames for stakeholder input. 

7.2. ADDRESS THE TARGET AUDIENCE 

Watershed assessments have multiple audiences and stakeholders. There are managers or 

groups with defined decision making authority, such as a governmental agency with regulatory 

authority over water quality or a county staffer charged with developing a land use plan. In 

addition, individuals or groups may actively participate in the watershed management process 

(e.g., watershed associations, river watch citizen groups, volunteer monitoring groups, industries, 

and agricultural associations). Other stakeholders include participants who are influenced by and 

have the ability to affect (if only through lifestyle and land use choices) the area of interest (e.g., 

land owners in the watershed). Communication needs differ accordingly with each stakeholder 

group. Glicken (2000) makes the further point that stakeholders or interested parties are 

identified and defined relative to specific issues. A characteristic of most watershed management 

exercises is that there are multiple issues to be addressed, and thus the participating stakeholders 

do not constitute a single, well-defined set but, rather, a complex network that shifts in 

relationship to issues. Efforts should be made to engage the participants on all issues and at every 

phase, as neglect may lead to alienation and contention (also refer to Section 3.1). 

54

7.3. COMMUNICATE EFFECTIVELY 

Risk characterizations should be prepared in a manner that is clear, transparent, 

reasonable, and consistent with other Risk Characterizations of similar scope prepared across 

programs in the Agency (U.S. EPA, 1998a). For effective risk communication, scientific 

information should be characterized in the manner most appropriate for addressing the major 

concerns. Since watershed risk assessments are place-based studies, maps are often important 

tools to aid in summarizing and conveying information (Serveiss, 2002). With the advent of 

powerful and accessible GIS and database tools, it has become feasible to analyze and display 

spatial information. Maps can provide easily grasped visual relationships between sources, 

stressors, and effects to which the public can relate. In the Clinch and Powell assessment, maps 

were especially useful in visualizing cumulative sources of stress in relationship to locations of 

threatened and endangered mussel species (U.S. EPA, 2002c). Such maps can then be used to 

evaluate vulnerability and prioritize management strategies. 

Communication can be misleading if uncertainty is not described. At each step of the 

assessment process, what is not known should be presented along with what is known. 

Clear presentation of the strengths, limitations, and conclusions of the risk assessment will 

greatly enhance the assessment’s usefulness in decision making. Describing the association 

between a range of stressor levels and various risk estimates can be especially useful for managers 

who may need to choose among options with varying capabilities for stressor reduction. 

Successful risk communication leads to a higher likelihood of selecting optimal risk management 

actions. 

55

8. RISK MANAGEMENT


The purpose of a watershed ERA is to provide managers with information to help address 

environmental risks (i.e., make risk management decisions). The goal of risk management is to 

take scientifically sound and cost-effective actions to reduce or prevent risks, while taking into 

account social, cultural, ethical, political, and legal considerations (PCCRARM, 1997). 

8.1. EVALUATING MANAGEMENT ALTERNATIVES 

Decisionmakers must select a particular management action, including no action. When 

environmental management goals and risk assessment endpoints are clearly defined and agreed 

upon by assessment participants, the risk assessment process is more likely to produce useful 

information for choosing an action. 

Watershed management typically includes multiple objectives, and watershed degradation 

typically involves multiple sources and multiple risks. In an ideal case, the final management 

plan would be endorsed by all participants. However, when conflict arises between stakeholder 

groups with opposing goals, selecting appropriate management alternatives may be difficult. 

Some watershed projects may prove too controversial and reach a stalemate from time to time. 

In the worst cases, some stakeholders may choose to selectively ignore parts of the science or to 

withhold relevant information. Despite these challenges, some stakeholder involvement is 

always necessary (as discussed in Sections 7.1 and 7.2), especially before money is committed to 

possible solutions. 

An important characteristic of many watershed risk management processes is the role of 

stakeholders in identifying and evaluating potential management options. It is generally not 

appropriate to use open stakeholder processes to make regulatory decisions that are explicitly 

mandated to agencies (U.S. EPA, 2001a). However, when selecting among management 

alternatives that are consistent with the law, stakeholder involvement increases acceptability of the 

outcome—particularly where social value judgments are required to choose the best level and 

approach for environmental protection. 

Evaluation of the relative performance of candidate management options can be made on 

the basis of their abilities to achieve management objectives, usually through the use of an 

appropriate indicator or measurement endpoint. In some cases, this is a relatively straightforward 

process. For instance, the objective might be to reduce fish mortality from ammonia inputs, with 

an associated indicator of predicted un-ionized ammonia concentrations. If the modeling tools are 

present to evaluate un-ionized ammonia concentrations associated with each management 

alternative, then a direct comparison may be made. Designing assessment endpoints by relating 

them to management objectives enables them to be useful in risk management decisions. 

56

Quantitative management objectives are always desirable and sometimes management 

options can be tested in a prospective risk analysis. In these instances, there may be a wide array 

of management options that can be considered (e.g., limit types of development, limit 

development density, use stormwater controls). In circumstances where there are no competing 

objectives, and if all possible management objectives can be ranked quantitatively using an agreedupon scale, selecting the best management options is relatively straightforward. However, this 

rarely happens. 

In most watershed management exercises, there are competing objectives and a lack of 

consensus. Objectives may be pre-established and 

unfocused (e.g., control future nutrient-related 

eutrophication in a lake). The management options also 

may change after the results of early analyses are 

presented. 

When a quantifiable management objective may 

not be available or agreed upon by various stakeholder 

groups, a subjective ranking approach may be used. A 

simple value scoring approach that requires each 

stakeholder group to identify the best and worst of the 

candidate scenarios (combinations of management 

alternatives) can be used (Stewart and Scott, 1995). 

Best and worst scenarios are defined as the top and 

bottom of a subjective scale running from 100% to 0%, 

with intermediate scores relating to the relative “gaps” 

between the two well-defined extremes. The resulting 

scores obtained by a stakeholder group for a set of 

scenarios may then be displayed on a “thermometer” 

scale (Figure 8). In the figure, the stakeholder group Figure 8. Thermometer scale for 

relative ranking of management 

ranked five scenarios, judging Scenario II the best and options.


Scenario III the worst. A compilation of rankings


submitted by multiple stakeholder groups would reveal the alternatives that are least objectionable


to the participants.


While the actual decision process is likely to vary on a case-by-case basis, some 

guidelines may help promote a successful outcome. First, as stated earlier, it is important to 

ensure that stakeholder groups are present at the table from the start of the process, particularly 

those with the potential to present obstacles to management. Second, the decision context and 

the role of stakeholders in the process should be made clear from the onset. Third, it is important 

57

for all parties to realize that evaluating alternatives is an iterative process and that scientific 

information that emerges during the process may require rethinking the nature and significance of 

risk pathways and, potentially, the management goals for a watershed. Finally, one must 

recognize that risk management may take time and money. 

8.2. APPLICATION TO DECISION MAKING IN THE EPA CASE STUDIES 

The three completed EPA case study reports each include a section on how the 

assessment assists with decision making. For the Clinch and Powell Valley assessment (U.S. 

EPA, 2002a), the ERA process advanced a better understanding of environmental problems. In 

particular, the conceptual model and multivariate analysis helped clarify the interrelationships 

between various components of the ecosystem and the manner in which human activities 

contribute to environmental problems within the watershed. In assessing environmental risk, a 

number of federal, state, and local environmental agencies and organizations came together to 

share data, explore and develop solutions, and undertake actions within the watershed. Risk 

assessment findings can also help direct the efforts of the Upper Tennessee River Basin 

Roundtable, which is composed of various individuals, agencies, and organizations that have an 

interest in protecting the watershed. ERA findings will be useful to the roundtable as it begins 

comprehensive strategic planning for watershed protection. Additionally, the numerous 

watershed coalition groups within the basin can use the risk assessment findings to direct their 

efforts to protect and improve water quality within their watershed. 

The process of risk assessment helped lend further credence to what many professional 

resource managers had long conjectured about problems within the watershed, thereby providing 

more scientific support for taking actions to address identified problems. For example, there is 

now a better understanding of what sediment from cattle grazing contributes to the river. Such 

risk assessment findings will be useful to FWS and TVA personnel, who can now share this 

information with farmers and encourage them to take actions, such as building fences to keep 

cattle out of streams. 

Examples of management actions that FWS and TVA have considered on the basis of the 

overall risk assessment findings include the following: 

•	 Restoring additional abandoned mine lands throughout the watershed 

•	 Further study of the chemical makeup of discharges from coal mining and processing 

facilities and the toxicity of these discharges to aquatic species 

•	 Increasing the extent of forested riparian areas adjacent to and upstream of critical 

aquatic habitat sites for mussels and fish 

58

•	 Implementing better spill control mechanisms on roadways and railroads near sensitive 

streams and more spill contingency plans for the watershed, which will enable the 

Virginia Department of Transportation and other agencies involved in constructing 

highway projects on or near waterways to design those projects to reduce catastrophic 

events and minimize impacts of accidental spills 

•	 Installing BMPs for pasture and agricultural land to reduce sediment loading and 

implementing better treatment of wastewater discharges 

In the Waquoit Bay ERA (U.S. EPA, 2002c), nitrogen loading from the watershed led to 

water quality problems and diminished populations of valued biotic resources. Nitrogen loading 

and estuarine loading models were developed as part of the assessment. These models helped 

managers to better understand nitrogen impacts and to make more informed decisions in 

addressing the problem. The ERA process also helped engage local citizens in the dialogue on 

mitigation strategies (including validating model predictions, long-term monitoring, and adaptive 

management). 

Another valuable product of the Waquoit Bay ERA is the conceptual model, which allows 

citizens and local managers to view the watershed stressors and their associated ecological effects 

from a more holistic perspective. It emphasizes both point and nonpoint sources of pollution and 

shows how valued ecological resources are impacted by multiple stressors. Before formulation of 

the model, the focus was on analyzing single stressors or single species or on managing fisheries 

on a stock-by-stock basis. The conceptual model helped develop evaluation and management 

strategies that considered the wider effects of pollution on habitats. 

EPA used simulation techniques developed for the middle segment of the Snake River 

(U.S. EPA, 2002d) and the associated conceptual model to determine impacts from a proposed 

new hydroelectric facility. The Federal Energy Regulatory Commission was convinced by 

EPA’s risk assessment that cumulative impacts should be addressed when reviewing new or 

continued licensing for hydroelectric facilities. The assessment also provided community groups 

with a more robust analysis of the possible impact of impoundments on the river. Risk 

assessment participants were able to gain an improved understanding of the interrelationships 

between various components of the ecosystem and how human activities contribute to 

environmental problems within the watershed. For example, there is now a better understanding 

of the contribution of sediment to changes in the ecosystem, including eutrophication and decline 

of fish species. This information is useful to managers as they begin to regulate the sediment 

sources. 

The simulation models developed for the Snake River can be used to test management 

scenarios. Those interested in land use activities and resource protection can better explore 

59

alternative management options for the watershed. The methods developed through this risk 

analysis can also be applied to other watersheds with similar conditions. 

The benefits of watershed ERA for the three EPA-sponsored watershed assessments have 

been summarized and demonstrated above. Watershed ERA can also improve the organization 

and quality of several other types of watershed analyses, including TMDL analyses and use 

attainability analyses (UAAs), as described in the following sections. 

8.3. APPLICATION TO TOTAL MAXIMUM DAILY LOAD PROCESS 

The TMDL process includes activities similar to ERA risk analysis. For instance, the 

objective of source assessment is to “characterize the types, magnitudes, and locations of sources 

of sediment loading to the waterbody,” while the objective of linkage analysis is to “Define a 

linkage between the selected water quality targets and the identified sources to determine total 

assimilative capacity for sediment loading or total load reduction needed” (U.S. EPA, 1999b). 

The source assessment objective is consistent with exposure characterization, while the linkage 

analysis is similar to effects characterization. 

An example of how watershed ERA can be applied to a TMDL watershed management 

exercise is the clean sediment TMDL for Redwood Creek, CA (U.S. EPA, 1999b), which was 

summarized in Serveiss et al. (2005). Redwood Creek had a designated use to support a 

coldwater fishery. The water body was listed as impaired on the state’s 303(d) list because 

salmonid stocks were declining. Upland sediment loading from forestry was believed to have 

salmonid impaired habitat quality. Changes to pool, substrate, and channel structure were 

apparent and also contributing to increased temperatures. Collectively, these changes affected the 

spawning and food production of salmonids. The TMDL listing and associated problem 

statement mimic ERA problem formulation activities. 

Numeric water quality standards for sediment do not exist in California’s regulations, and 

the narrative criteria (support of the coldwater fishery) and a pollutant (clean sediment) made it 

necessary to select measures and target values for the analysis. The TMDL identified eight 

instream and seven “hillslope” indicators and targets. The instream indicators were measures of 

exposure, and they have been selected in a way that provides reasonable surrogates for effects. 

For instance, the first instream indicator was “Percent fines <0.85 mm in riffle crests of fishbearing streams.” This indicator represents a critical node along a stressor-response pathway: that 

excess fine sediment clogs spawning gravels and results in reduced spawning success of 

salmonids. For the purpose of the analysis, the first instream indicator (and the seven other 

indicators) served as a surrogate for what would be the assessment endpoint (spawning success of 

salmonids) and thus was also used as a measure of effect. The hillslope indicators (e.g., number 

of road crossings with diversion potential) served as measures of exposure. Management 

60

(implementation) scenarios could then be compared and defined in terms of the hillslope indicators 

and their estimated effect on sediment loading. 

Target values for the instream indicators were based on a combination of literature values, 

professional judgment, and comparison of impaired reaches within the watershed to unimpaired 

reference reaches, both within the watershed and in similar terrain. For instance, the target value 

for percent fines in riffle crests was determined to be less than 14%. 

The linkage analysis for this TMDL was performed by estimating upland sediment loading 

rates for the impaired water body and the unimpaired reference streams. Based on this 

comparison, it was estimated that a 60% reduction in average annual sediment loading in 

Redwood Creek was needed to support uses. Specific allocations were made by comparing 

management scenarios and their impacts on erosion processes, which were mostly associated with 

specific land use activities and were expressed as long-term average annual loads per square mile. 

The individual load allocations were based on assessment of the controllability of different source 

categories, that is, the extent to which sources are associated with human activity and will 

respond to mitigation. The analysis indicated that the application of reasonable reductions to the 

controllable load would be adequate to meet the TMDL. 

The Redwood Creek TMDL did not follow all the ERA steps because of time and budget 

constraints. Yet, this TMDL has most of the components of a watershed risk assessment, 

including problem formulation; identification of environmental management goals, options, and 

objectives; assessment endpoints and associated measures; and the equivalent of exposure 

characterization and effects characterization. In other words, the TMDL was performed in a 

manner consistent with the watershed ERA approach. The scientific underpinnings of TMDL 

development can be improved using several features of watershed assessment (Serveiss et al., 

2005). 

8.4. APPLICATION TO THE UAA PROCESS 

Beneficial uses of water bodies are designated by states under the Clean Water Act. 

Common beneficial uses include warm or coldwater aquatic life uses, which are ecologically based 

goals of a watershed that are subject to enforcement, if necessary. The designated use dictates the 

physicochemical water quality standards that need to be maintained in a water body. Therefore, 

proper identification of uses is a critical component of all water quality programs. A UAA is 

defined as the process of determining attainable beneficial uses, or modifying designated uses that 

are inappropriate (U.S. EPA, 1994). The Federal Water Quality Standards Regulation, 40 CFR 

131.10(g), defines a UAA as “a structured scientific assessment of the factors affecting the 

attainment of a use which may include physical, chemical, biological, and economic factors.” 

61

A UAA consists of three general steps: (1) identifying and defining existing proposed or 

designated beneficial uses that were present as of November 1975, (40 CFR 131.3(e)); (2) 

determining whether uses are appropriate for the water body, and (3) identifying constraints on 

these uses by examining physical, chemical, and biological characteristics of the water body as 

well as socioeconomic constraints. The Federal Water Quality Standards Regulation defines six 

factors, any one of which, if demonstrated, could be used to remove an inappropriate use or 

establish subcategories of a use (see below). The documentation necessary to support any of 

these factors has been unclear and a source of confusion in UAAs to date, limiting the use of this 

essential tool. This is especially true for substantially modified watersheds (e.g., urban or 

agricultural watersheds and streams that are effluent dominated) in which “human caused 

conditions” predominate (see item 3 in list below). The following six factors are associated with 

modifying or removing a 101(a) use within a use attainability analysis (UAA), as cited in 40 CFR 

131.10(g): 

1.	 Naturally occurring pollutant concentrations prevent attainment of the use. 

2.	 Natural, ephemeral, intermittent, or low-flow conditions or water levels


prevent the attainment of the use.


3.	 Human-caused conditions or sources of pollution prevent attainment of the use and 

cannot be remedied or would cause more environmental damage to correct than to 

leave in place. 

4.	 Dams, diversions, or other types of hydrologic modifications preclude attainment of 

the use, and it is not feasible to restore the water body to its original condition or to 

operate such modification in a way that would result in attainment of the use. 

5.	 Physical conditions related to the natural features of the water body, such as the lack 

of a proper substrate, cover, flow depth...unrelated to water quality preclude 

attainment of aquatic life protection uses. 

6. 	 Controls more stringent than those required by Sections 301(b)(1)(A) and (B) and 306 

of the Clean Water Act would result in substantial and widespread economic and 

social impact. 

The watershed ERA framework offers many advantages for conducting UAAs, because: 

(1) stakeholders in the watershed (including resource agencies and wastewater dischargers) 

participate in defining the assessment process; (2) the assessment explicitly treats multiple 

stressors, including physical and biological, and other water quality stressors; and (3) the 

assessment is necessarily place based because it deals with a specific watershed or basin. UAAs 

have been thwarted in many cases because it was often difficult to reach consensus on the types 

62

of information needed and how multiple lines of evidence should be interpreted. Watershed ERA 

can greatly improve the performance and transparency of UAAs by providing the scientific 

structure necessary to evaluate relevant information in a comprehensive and efficient manner. 

Figure 9 shows a generalized watershed ERA process that lends itself well to UAAs. 

Using the watershed ERA approach, the UAA methodology relies on the identification and 

evaluation of specific indicators (i.e., assessment endpoints) and measures that represent salient 

characteristics of a given use (i.e., measures of effect) as defined by the standards. Through 

objective analysis of these measures (i.e., risk analysis), and comparison of those measures with 

known minimum criteria, thresholds, or requirements needed to support the use, an assessment is 

made as to the attainability of that use (i.e., risk characterization). Explicit in these analyses is a 

determination of specific water body attributes that are preventing attainability of a given use 

(stressor identification evaluation or causal analyses). These attributes are evaluated to determine 

whether certain modifications or controls would allow the use to be attainable and, if so, the 

feasibility or reasonableness of those options (i.e., risk management). 

The first step in a UAA is problem formulation, which includes determination of the 

assessment endpoints indicative of the use in question, and conceptual model development. 

Assessment endpoint selection (as well as measures of effect) should be identified using a 

stakeholder process as described earlier in this document. Because UAAs depend on at least one 

of the six factors to be legally defensible, these factors become central to conceptual model 

development. In practice, socioeconomic constraints (item 6 listed above) are handled differently 

from ecological constraints (items 1 through 5 listed above) and are addressed outside the ERA 

process. The remaining five natural or anthropogenic factors then become categories of stressors 

that help define relevant potential stressors and their sources in the watershed. The assessment 

endpoints identified during problem formulation give rise to specific measures of effect that can be 

analyzed and compared with what is minimally needed to maintain that endpoint in the water 

body. An example endpoint would be salmonid abundance and distribution for a beneficial use 

that identifies coldwater aquatic life habitat. 

63

Step 1: Problem Formulation: 

•	 Translate proposed use into a goal and objectives. 

•	 Develop conceptual model. 

•	 Identify assessment endpoints and measures of effect. 

Step 2: Risk Analysis 

•	 Evaluate stressors, pathways, and measures of effect. 

•	 Use reference conditions and watershed data to determine effects on 

assessment endpoints. 

Step 3: Risk Characterization 

•	 Compare stressor levels and physicochemical regimes with minimum 

thresholds or criteria for assessment endpoints. 

•	 Determine relevant causes of nonattainability; stressor identification 

evaluation. 

Step 4: Risk Management 

•	 Identify control options if applicable. 

•	 Consider feasibility of meeting use given candidate causes. 

•	 Conduct stakeholder discussions and management option evaluation. 

•	 Modify use if appropriate. 

Figure 9. General steps involved in conducting a use attainability analysis 

(UAA) using a watershed ecological risk framework. 

64

In a UAA context, risk is analyzed in terms of the ability of an indicator or assemblage 

maintaining itself given the five ecological factors as they exist in the water body. Later steps in 

risk characterization and risk management will determine whether present constraints in the water 

body can and should be remediated to better meet the use. 

A UAA of two small central valley streams in California followed a watershed ERA 

framework. The conditions suitable to support coldwater ecosystems (COLD) include 

preservation or enhancement of aquatic habitats, vegetation, fish, invertebrates, or wildlife. Both 

salmonid habitat suitability indices and available habitat information for coldwater stoneflies were 

used to assess suitability of the COLD use. Measures were compared with criteria or “threshold 

values” to determine whether each indicator is supported and the use is attainable. 

Using published habitat suitability parameters for salmonids, a simple conceptual model 

was constructed to delineate potentially important pathways for determining attainability of the 

COLD use with respect to fish. Physical habitat and chemical data indicative of habitat 

requirements for assessment endpoints were collected and combined with available literature 

information for the region to identify likely stressors affecting the maintenance of obligate cool- or 

coldwater aquatic life. The relative strength of each cause-effect relationship was then determined 

by considering the degree to which the indicator measurements differed from minimum thresholds 

needed to maintain coldwater fauna. For aquatic life uses, such as COLD, comparisons between 

measured values and thresholds could be quantitatively determined in part using fish HSIs and 

their various component values. The greater the disparity between the measured HSI component 

value for a given environmental factor (e.g., sediment particle size) and the minimum threshold, as 

defined by the HSI, the more limiting that particular factor is in preventing attainment of the use. 

These quantitative assessments were often supplemented with qualitative evaluations. 

A decision tree was developed that presents the process used to determine whether the 

COLD use is attainable. If analyses indicated that water body conditions do not meet minimum 

thresholds for a particular use, the causes for nonattainment were then evaluated. The poorer a 

particular factor actually is, in comparison with minimum threshold criteria, the more limiting that 

factor is in attaining the use and, therefore, the less likely it is that the use is attainable. A weightof-evidence approach was used to synthesize results of the conceptual model approach to 

determine overall attainability of the COLD use. In this example, indicator results (i.e., trout and 

stonefly results for COLD use) were consistent. More consistency in results among indicators 

(e.g., all indicators demonstrate a low probability of beneficial use attainment) suggests a higher 

degree of certainty in the overall assessment regarding use attainability. The more factors that do 

not meet minimum threshold criteria, the more certain that the use is not attainable under current 

conditions. 

65

Using a watershed ERA framework, regulatory and resource agencies were able to quickly 

and efficiently weigh management options and the feasibility of making the COLD use attainable 

in these streams. These UAAs confirmed that COLD use was not attainable in the streams 

examined, given current hydrological and physical conditions. 

66

9. CONCLUSIONS 

This report advocates integrating the watershed approach with ecological risk assessment 

to make more informed environmental management decisions. This integration offers numerous 

benefits for those faced with the challenge of structuring environmental monitoring and analysis 

efforts and using such information for decision making. The process fosters regular interactions 

between scientists and managers to increase the likelihood that the optimal suite of scientific data 

is collected, analyzed, and considered in decision making. 

Developing a management goal for a watershed ERA encourages organizations and the 

community to work together to develop a common vision, share information, and understand 

problems and ecological concerns. The usefulness of a risk assessment is enhanced when 

scientists and managers communicate regularly throughout the process. Agreeing on the focus, 

scope, and complexity of an assessment increases the likelihood that the collected and analyzed 

data will be of value to and used by managers. 

The analysis phase of risk assessment addresses, to the degree technology and resources 

permit, the spatial and temporal co-occurrence of exposure of ecological resources to stressors 

and the direct and indirect effects of such exposures to valued ecological resources. Based on a 

few watershed ecological risk assessments conducted to date, risk analysis is the most challenging 

phase of the process because there are typically multiple sources and stressors that may vary over 

time and space. Tools such as GIS and CADDIS can assist with deciphering causes of ecological 

degradation. Often it is useful to simplify the analytical approach in a watershed setting by 

focusing on the impacts of one stressor or to use multivariate analysis to describe associations 

between sources or stressors and effects. Risk characterization integrates multiple lines of 

evidence into a clear summary of the risks to ecological resources. The risk characterization may 

predict the magnitude of changes in environmental quality associated with alternative management 

actions. It also identifies critical knowledge gaps and describes the level of certainty of the 

findings. 

Interim findings need to be presented to managers to provide a clear view of the 

significance of the findings and to enable conceptualizing and possibly implementing potential 

solutions. The analysis can also be refined based on new information. Maps, graphs, and tables 

are among the best analytical tools for explaining relationships between stressors and effects. The 

presentation needs to be targeted for a specific audience, and more than one final presentation 

format may be necessary. 

Documenting the impacts to valued ecological resources and the importance of their 

protection helps justify management actions. Even without direct regulatory authority, 

environmental improvements can be attained because the increased awareness that occurs as a 

67

result of documenting the consequences of human activities on the environment will encourage 

positive behavioral changes and management actions. In addition, improvements in the 

communication and coordination associated with the process can reduce duplication of effort. 

Elements of the watershed ERA process can be used alone or in conjunction with other 

analytical decision making frameworks (e.g., TMDLs and UAAs). Scientists, managers, and 

stakeholders should communicate to determine the goals, objectives, focus, scope and complexity 

of an assessment. Developing a conceptual model requiring an interdisciplinary team of scientists, 

preferably with some local knowledge helps to better understand and communicate how potential 

stressors might affect valued ecological resources. The analysis plan helps to focus the 

assessment and to determine which data to collect and analyze. It should be based on perceived 

importance of stressors on assessment endpoints, corrective action potential, and resource 

constraints. The analysis plan should be considered an iterative process, where modifications are 

made as results are obtained. 

Scientists, managers, and stakeholders should meet periodically to discuss interim findings 

and modify or refine the thrust of analysis efforts based on intermittent deliberations as necessary. 

With multiple stressors and multiple pathways, multivariate analyses or categorical ranking 

schemes may be needed to compare land use, stressors, and biotic measures. Conclusions need to 

be based on integrating multiple lines of evidence and discussing the degree of uncertainty in the 

findings. 

Presentation of assessment findings needs to be targeted for particular audiences. As a 

result, more than one report may be needed. Maps, GIS, and other visual-based graphics are 

often very useful for communicating risk assessment results. All these steps will increase the 

likelihood that environmental monitoring and assessment data will be used in decision making. 

68

GLOSSARY


The following definitions are derived from EPA’s Watershed Academy training module on 

watershed ecological risk assessment (U.S. EPA, 2000b), Guidelines for Ecological Risk 

Assessment (U.S. EPA, 1998a), and Terminology Reference System 

(http://www.epa.gov/trs/index.htm). 

Adverse ecological effects – Changes that are considered undesirable because they alter valued 

structural or functional characteristics of ecosystems or their components. An evaluation of 

adversity may consider the type, intensity, and scale of the effect as well as the potential for 

recovery. 

Assessment endpoint – An explicit expression of the environmental value that is to be protected, 

operationally defined by an ecological entity and its attributes. For example, salmon are valued 

ecological entities; reproduction and age class structure are some of their important attributes. 

Together salmon reproduction and age class structure form an assessment endpoint. 

Characterization of ecological effects – A portion of the analysis phase of ecological risk 

assessment that evaluates a stressor’s ability to cause adverse effects under a particular set of 

circumstances. 

Characterization of exposure – A portion of the analysis phase of ecological risk assessment 

that evaluates the interaction of the stressor with one or more ecological entities. Exposure can 

be expressed as co-occurrence or contact, depending on the stressor and ecological component 

involved. 

Coal fines – Fine particulate manner from coal mining operations. 

Comparative risk assessment – A process that generally uses an expert judgment approach to 

evaluate the relative magnitude of effects and set priorities among a wide range of environmental 

problems. Some applications of this process are similar to the problem formulation portion of an 

ecological risk assessment in that the outcome may help select topics for further evaluation and 

help focus limited resources on areas with the greatest risk reduction potential. 

Conceptual model – A conceptual model in problem formulation for ecological risk assessment is 

a written description and visual representation of predicted relationships between assessment 

endpoints and the stressors to which they may be exposed. The conceptual model also describes 

the sources of stressors, the ecosystem potentially at risk, the relationships between measures of 

effect and assessment endpoints, and exposure scenarios. 

Dose-response – How a biological organism’s response to a toxic susbstance quantitatively shifts 

as its overall exposure to the substance changes (e.g., a small dose of carbon monoxide may cause 

drowsiness; a large dose may be fatal). 

69

Ecological risk assessment – A process for analyzing environmental problems intended to 

increase the use of ecological science in decision making. The process evaluates the likelihood 

that adverse ecological effects may occur or are occurring as a result of exposure to one or more 

stressors. 

Exposure – Co-occurrence of or contact between a stressor and a receptor. 

Exposure pathway – The course a chemical or physical agent takes from a source to an exposed 

organism. Each exposure pathway includes a source, an exposure point, and an exposure route. 

Air or water may be transport/exposure media if the source and endpoint are not in contact. 

Lines of evidence – Information derived from different sources or by different techniques that 

can be combined to reduce uncertainty and increase defensibility of a finding. 

Linkage analysis – Terminology used in EPA’s total maximum daily load (TMDL) protocols to 

describe a quantitative analysis of stressor response, typically accomplished with a simulation 

model. 

Management scenario – A structured combination of management options selected for 

evaluation. 

Measure of effect – A change in an attribute of an assessment endpoint or its surrogate in 

response to a stressor to which it is exposed. 

Measure of exposure – A measure of stressor existence and movement in the environment and its 

contact or co-occurrence with the assessment endpoint. 

Problem formulation – The first phase of ecological risk assessment, which includes a 

preliminary description of exposure and ecological effects, scientific data and data needs, key 

factors to be considered, and the scope and objectives of the assessment. This phase produces 

the risk hypotheses, conceptual model, and analysis plan, which serve as the basis for the 

assessment. 

Receptor – The ecological entity that is exposed to the stressor. 

Risk analysis phase – A phase of ecological risk assessment consisting of two main parts: (1) 

characterization of exposure—evaluating the interaction of the stressor with one or more 

ecological entities, and (2) characterization of ecological effects—evaluating the ability of a 

stressor(s) to cause adverse effects to an assessment endpoint under a particular set of 

circumstances. 

Risk characterization phase – A phase of ecological risk assessment that integrates the 

exposure and stressor response profiles to evaluate the likelihood of adverse ecological effects 

associated with exposure to a stressor. Lines of evidence and the adversity of effects are 

discussed. 

70

Risk estimation – Ideally, the conclusions of the risk characterization phase preferably expressed 

in a quantitative manner (e.g., there is a 20% chance of 50% mortality under the circumstances 

assessed), but also if expressed as a qualitative statement (e.g., there is a high likelihood of 

mortality occurring). 

Risk hypothesis – A candidate description of the relationship between a particular stressor and an 

assessment endpoint. A risk hypothesis formulates proposed answers to questions risk assessors 

have about what responses assessment endpoints will show when they are exposed to stressors 

and how exposure will occur. 

Risk management – The process of evaluating and selecting action alternatives in response to 

risk assessment findings. The goal of risk management is to recommend scientifically sound, costeffective actions that reduce or prevent risks while taking into account social, cultural, ethical, 

political, and legal considerations. 

Seascape – The marine equivalent of landscape ecology and tied into the designation of special 

areas as marine protected areas. 

Source – An action that releases to the environment or imposes on the environment a chemical, 

physical, or biological stressor or stressors. 

Stakeholder – An individual or group influenced by, and with an ability to significantly affect 

(either directly or indirectly), the topical area of interest. 

Stressor – Any physical, chemical, or biological entity (agent) that can induce an adverse 

response. 

Stressor-response curve – A graphic, quantitative representation of the relationship between a 

stressor (such as a pesticide concentration in the water column) and an ecological effect (such as 

mortality of a given fish species if exposed to a certain concentration of that pesticide). 

Tiered approach – An approach to assessment that commences with simple scoping analyses and 

proceeds to additional tiers of more complex analyses as needed based on the results of earlier 

tiers or information that becomes available as the assessment progresses. 

Watershed approach – A framework for coordinating environmental management that focuses 

public- and private-sector efforts on addressing the highest priority problems within 

hydrologically defined geographic areas, taking into consideration both ground-water and surfacewater flow. 

Watershed ecological risk assessment – An ecological risk assessment carried out at the 

watershed level, usually in support of watershed management efforts. 

71

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