United States

Department

of Agriculture

Forest Service

Pacific Southwest

Research Station

General Technical

Report PSW-GTR-141

Research and Cumulative

Watershed Effects

Leslie M. Reid
Publisher:

Pacific Southwest Research Station

Albany, California

(Mailing address: P.O. Box 245, Berkeley, CA 94701-0245

Telephone: 510-559-6300)

March 1993

Reid, Leslie M. 1993. Research and cumulative watershed effects. Gen. Tech. Rep. PSW-GTR141. Albany, CA: Pacific Southwest Research Station, Forest Service, U.S. Department of

Agriculture; 118 p.

The mandate for land managers to address cumulative watershed effects (CWEs) requires that

planners evaluate the potential impacts of their activities on multiple beneficial uses within the

context of other coexisting activities in a watershed. Types of CWEs vary with the types of landuse activities and their modes of interaction, but published studies illustrate both descriptive and

predictive evaluations of many of these types. Successful evaluations have generally used geomorphological and ecological approaches based on the understanding of the processes involved. In

contrast, most generalized “cookbook” analysis procedures are shown to be unable to assess

accumulations of impacts through time, usually cannot evaluate the range of activities and uses that

are necessary, and are rarely validated. A general approach to evaluation is proposed, and the types

of information available for assessments are reviewed.

Retrieval terms: watershed, cumulative impact, land-use planning, water quality

The Author:

Leslie M. Reid is a research geologist with the Station’s Hillslope Processes/Fisheries Research

Unit, and is stationed at the Redwood Sciences Laboratory, 1700 Bayview Drive, Arcata, CA 95521.

Foreword and Acknowledgments:

This report was prepared under cooperative agreement PSW-88-CL-009 (CDF 8CA74571)

with the California Department of Forestry and Fire Protection, P.O. Box 944246, Sacramento, CA

94244-2460. This version differs from the final report to the Department primarily in editing

details, omission of two appendices, addition of two figures, revisions to tables, and inclusion of

more detail about cited studies. Trade names and commercial enterprises are mentioned solely for

information and do not imply the endorsement of those products or organizations. This publication

neither recommends any pesticide uses reported nor implies that they have been registered by

appropriate government agencies.

John Munn, Peter Cafferata, Jennifer Nielsen, and Larry Costick provided extremely helpful

reviews of the manuscript, and innumerable scientists and land managers (listed, in part, in the

Appendix) gave freely of their time for lengthy discussions of ideas, research, and the challenges

confronting conscientious land management.

Note: This publication was reprinted December, 1998 from a reconstructed layout. It closely

resembles the original printing, but a difference in text style has caused a slight shift in pagination.

The Table of Contents has been revised to reflect the changes. Also, Figure 4, (previously on page

29) now appears on page 28, and Tables 5 (previously on page 28) appears on page 29.
Research and Cumulative Watershed Effects

Leslie M. Reid

Contents

List of Figures ....................................................................................................................................... v

List of Tables ........................................................................................................................................ v

Glossary of Acronyms ......................................................................................................................... vi

In Brief ................................................................................................................................................ vii

1. The Problem of Cumulative Watershed Effects ......................................................................... 1

Definitions ................................................................................................................................... 1

History of the Issue ..................................................................................................................... 3

The Major Issues ......................................................................................................................... 4

Goals of the Review .................................................................................................................... 4

Scope of the Review.................................................................................................................... 4

2. How Cumulative Watershed Effects Occur ................................................................................ 6

Watershed Function .................................................................................................................... 6

Runoff Generation ............................................................................................................. 6

Channels ............................................................................................................................. 6

Sediment Production.......................................................................................................... 6

Interactions Between Hillslopes and Channels ................................................................ 7

Physical Basis for Cumulative Watershed Effects ........................................................... 7

Ecosystem Function .................................................................................................................... 7

Constraints on Individuals and Species ............................................................................ 8

Habitat ................................................................................................................................ 8

Biological Communities .................................................................................................... 8

Response to Change........................................................................................................... 9

Biological Basis for Cumulative Watershed Effects ........................................................ 9

A Framework for Understanding CWEs .................................................................................. 10

Issues that Complicate the Understanding of CWEs ............................................................... 11

3. Research Methods for Understanding Cumulative Watershed Effects ................................. 12

Requirements of CWE Research .............................................................................................. 12

Research Approaches and Methods .......................................................................................... 12

Descriptive Studies .......................................................................................................... 12

Extensive Surveys ............................................................................................................ 13

Experimentation ............................................................................................................... 14

Modeling .......................................................................................................................... 15

Monitoring ....................................................................................................................... 16

Statistics ........................................................................................................................... 16

Scale and Focus of Studies ....................................................................................................... 17

Process Studies ................................................................................................................ 17

Watershed Case Studies .................................................................................................. 17

Paired- and Multi-Basin Studies ..................................................................................... 17

The System as Focus ....................................................................................................... 18
ii USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993.

4. Assessment of Cumulative Watershed Effects .......................................................................... 19

Types of Cumulative Watershed Effects ............................................................................... 19

On-Site and Off-Site CWEs .......................................................................................... 19

Interactions that Generate CWEs.................................................................................. 19

Retrospective Assessments of Cumulative Watershed Effects ............................................. 20

Examples of Retrospective Analyses ........................................................................... 20

Same-Influence Effects .......................................................................................... 20

Complementary Influences .................................................................................... 22

Cascading Influences ............................................................................................. 23

Interdependent Influences ...................................................................................... 23

Patterns of Retrospective Understanding ..................................................................... 24

Prediction of Cumulative Watershed Effects......................................................................... 25

Examples of Predictive Studies .................................................................................... 25

Same-Influence Effects .......................................................................................... 25

Complementary Influences .................................................................................... 26

Cascading and Interdependent Influences ............................................................. 27

Patterns of Predictive Understanding ........................................................................... 27

General Methods for Predicting Cumulative Watershed Effects .......................................... 27

Examples of General Predictive Methods .................................................................... 28

Equivalent Clearcut Area ....................................................................................... 28

Klock Watershed Cumulative Effects Analysis .................................................... 30

Equivalent Roaded Area ........................................................................................ 30

R-1/R-4 Sediment-Fish Model .............................................................................. 31

California Department of Forestry Questionnaire ................................................ 31

Water Resources Evaluation of Non-point Silvicultural Sources ........................ 32

Limiting Factor Analysis ....................................................................................... 32

Rational Approach (Grant) .................................................................................... 33

Discussion of General CWE Assessment Procedures.................................................. 33

 Methods Under Development ....................................................................................................... 35

Sequoia National Forest Method ............................................................................................ 35

TFW Method ........................................................................................................................... 35

NCASI Method ....................................................................................................................... 35

Prospects for a General CWE Evaluation Procedure ............................................................ 35

Attributes Required of a General Method .................................................................... 36

Development and Use of a General Method ................................................................ 36

5. Effects of Land-Use Activities on Environmental Parameters ............................................... 38

Roads ....................................................................................................................................... 38

Roads and Vegetation ................................................................................................... 38

Roads and Soils ............................................................................................................. 39

Roads and Topography ................................................................................................. 39

Roads and Chemicals .................................................................................................... 39

Impoundments and Water Development ................................................................................ 39

Timber-Management Activities ............................................................................................. 40

Timber Management and Vegetation ........................................................................... 40

Timber Management and Soils ..................................................................................... 41

Timber Management and Topography ......................................................................... 42

Timber Management and Chemicals ............................................................................ 42

Indirect Effects of Timber Management on Environmental Parameters..................... 42

Grazing .................................................................................................................................... 43

Grazing and Vegetation ................................................................................................ 43

Grazing and Soils .......................................................................................................... 45

Grazing and Topography .............................................................................................. 45

Other Effects of Grazing ............................................................................................... 45

Indirect Effects of Grazing on Environmental Parameters .......................................... 45

Interactions Between Grazing and Timber Management ............................................ 46
USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993. iii

Mining ..................................................................................................................................... 46

Mining and Vegetation.................................................................................................. 46

Mining and Soils ........................................................................................................... 46

Mining and Topography ................................................................................................ 47

Mining and Chemicals .................................................................................................. 47

Interactions Between Mining and Timber Management ............................................. 47

Agriculture .............................................................................................................................. 47

Urbanization ............................................................................................................................ 48

Urbanization and Vegetation ........................................................................................ 49

Urbanization and Chemicals ......................................................................................... 49

Other Effects of Urbanization ....................................................................................... 49

Interactions Between Urbanization and Timber Management .................................... 50

Flood Control and Navigation ................................................................................................ 50

Recreation and Fishing ........................................................................................................... 51

General Patterns of Change .................................................................................................... 53

Cumulative Effects of Land Use on Environmental Parameters........................................... 54

Research Needs for Understanding Changes in Environmental Parameters ........................ 54

Understanding Environmental Parameters ................................................................... 55

Understanding the Effects of Land Use........................................................................ 54

6. Effects of Land Use and Environmental Change on Watershed Processes .......................... 56

Relation Between Watershed Processes and CWEs .............................................................. 56

Effects of Environmental Change on Hydrology .................................................................. 57

Water Input .................................................................................................................... 57

Runoff Generation ......................................................................................................... 60

Water Transport on Hillslopes ...................................................................................... 62

Water Transport in Channels ........................................................................................ 62

Water Budgets and Runoff Modeling ........................................................................... 63

Effects of Environmental Change on Sediment ..................................................................... 64

Erosion and Sediment Transport on Hillslopes ............................................................ 64

Surface Erosion ...................................................................................................... 64

Gullies .................................................................................................................... 65

Landslides ............................................................................................................... 65

Soil Creep ............................................................................................................... 66

Treethrow ............................................................................................................... 66

Sediment Delivery to Streams ...................................................................................... 66

Erosion, Transport, and Deposition in Channels ......................................................... 66

Bank and Channel Erosion .................................................................................... 66

Channel Sediment Transport ................................................................................. 67

Sediment Deposition in Channels ......................................................................... 68

Land Use and Sediment Yield ...................................................................................... 69

Sediment Budgeting and Modeling .............................................................................. 69

Effects of Environmental Change on Organic Material ........................................................ 72

Production of Organic Material from Streamsides ...................................................... 73

In-Channel Production of Organic Material................................................................. 74

Modification of Organic Material................................................................................. 74

In-Channel Transport of Organic Material ................................................................... 74

Organic Material Budgets ............................................................................................. 75

Effects of Environmental Change on Chemicals ................................................................... 75

Input and Removal of Chemicals ................................................................................. 75

Transport of Chemicals ................................................................................................. 76

Alteration of Chemicals ................................................................................................ 77

Accumulation of Chemicals .......................................................................................... 77

Nutrient Cycling, Chemical Budgets, and Modeling ................................................... 77

Effects of Environmental Change on Heat ............................................................................ 78

Research Needed to Understand Watershed Processes ......................................................... 79
iv USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993.

7. Impacts on Beneficial Uses and Values ...................................................................................... 81

Impacts on Fisheries Resources and Aquatic Communities .................................................... 81

Flow Characteristics ........................................................................................................ 81

Channel Morphology ....................................................................................................... 82

Water Temperature .......................................................................................................... 83

Water Quality ................................................................................................................... 84

Food Resources ................................................................................................................ 85

Predation and Fishing Pressure ....................................................................................... 85

Other Effects .................................................................................................................... 86

Impacts on Timber Resources................................................................................................... 86

Loss of Land Base ........................................................................................................... 86

Forest Productivity and Mortality ................................................................................... 86

Impacts on Recreation and Esthetic Values ............................................................................. 87

Recreational Needs .......................................................................................................... 87

Physical Site Characteristics ........................................................................................... 88

Crowding and Ambiance ................................................................................................. 88

Impacts on Water Supply and Power Generation .................................................................... 89

Water Quality ................................................................................................................... 90

Water Quantity and Runoff Timing ................................................................................ 90

Water Storage .................................................................................................................. 90

Impacts on Floodplain and Channel Use .................................................................................. 90

Stream Flow ..................................................................................................................... 90

Channel Morphology ....................................................................................................... 91

Other Changes.................................................................................................................. 91

Impacts on Other Resources and Values .................................................................................. 91

Research Needed to Understand Impacts ................................................................................. 92

8. Summary of Research and Cumulative Watershed Effects .................................................... 93

A Framework for Understanding CWEs .................................................................................. 93

State of Present Knowledge ...................................................................................................... 93

Methods for Evaluating CWEs ................................................................................................. 94

A Framework for CWE Evaluation .......................................................................................... 95

The Role of Research in Assessing CWEs ............................................................................... 95

Appendix: People Interviewed During Report Preparation ....................................................... 96

References .......................................................................................................................................... 97
USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993. v

List of Figures

1. Types of System Response....................................................................................................................................................... 2

2. Framework for Understanding Cumulative Watershed Effects ............................................................................................ 10

3. Combinations of Influences That Can Generate CWEs ........................................................................................................ 20

4. Cumulative Effect of Differing Recovery Times for a Driving Variable and an Impact .................................................... 28

5. Influences That Generate On-Site and Off-Site CWEs......................................................................................................... 93

List of Tables

1. Advantages and Disadvantages of Research Approaches and Methods .............................................................................. 13

2. Examples of Studies Diagnosing Cumulative Effects .......................................................................................................... 21

3. Approaches Used by Studies Described in Table 2 .............................................................................................................. 24

4. Examples of Predictive Cumulative Effect Studies .............................................................................................................. 25

5. General Procedures for Evaluating CWEs ............................................................................................................................ 29

6. Attributes of CWE Evaluation Procedures ............................................................................................................................ 34

7. Potential Direct Effects of Various Land-Use Activities on Watershed Properties ............................................................ 52

8. Interactions Between Altered Environmental Parameters .................................................................................................... 53

9. Effects of Altered Environmental Parameters on Watershed Processes .............................................................................. 56

10. Interactions Between Watershed Processes ........................................................................................................................... 57

11. Studies of the Hydrologic Effects of Roads and Timber Management ................................................................................ 58

12. Studies of Sediment Production from Roads and Timber Management .............................................................................. 70

13. Sediment Budgeting Studies .................................................................................................................................................. 72
vi USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993.

Glossary of Acronyms

BKD Bacterial kidney disease

BMP Best management practice

CDF California Department of Forestry and Fire Protection

CEQ Council on Environmental Quality

CREAMS Chemicals, Runoff, and Erosion in Agricultural Management Systems

CWE Cumulative watershed effect

ECA Equivalent Clearcut Area

ERA Equivalent Roaded Area

GIS Geographic information system

LFA Limiting Factor Analysis

KWCEA Klock Watershed Cumulative Effects Analysis

NCASI National Council of the Paper Industry for Air and Stream Improvement

ORV Off-road vehicle

PCB Polychlorinated biphenyl

PRMS Precipitation Runoff Modeling System

RA Rational Approach (Grant)

R-1 Region 1 of the USDA Forest Service

R-4 Region 4 of the USDA Forest Service

RV Recreational vehicle

TFW Timber/Fisheries/Wildlife group

THP Timber harvest plan

TOC Threshold of concern

USGS U.S. Geological Survey

USLE Universal Soil Loss Equation

WEPP Water Erosion Prediction Project

WRENSS Water Resources Evaluation of Non-point Silvicultural Sources
USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993. vii

In Brief . . .

Reid, Leslie M. 1993. Research and cumulative watershed effects.

Gen. Tech. Rep. PSW-GTR-141-WWW. Albany, CA: Pacific

Southwest Research Station, Forest Service, U.S. Department

of Agriculture; 118 p.

Retrieval terms: watershed, cumulative impact, land-use planning

Cumulative watershed effects (CWEs) include any changesthat involve watershed processes and are influenced by

multiple land-use activities. CWEs do not represent a new

type of impact, and almost all land-use impacts can be

evaluated as CWEs. The CWE concept is important primarily

because it identifies an approach to impact evaluation and

mitigation that recognizes multiple influences. The

significance of a CWE varies with the type of resource or

value impacted and is determined on political, economic, and

cultural grounds. In contrast, impact magnitude can be assessed

objectively by measuring physical and biological changes.

Most CWEs are incremental results of multiple controlling

factors; rarely can a single threshold value be identified for

provoking a response. (Chapter 1)

Watersheds are topographical forms that concentrate runoff.

They are sculpted by production and transport of water and

sediment. These media transport other watershed products,

such as organic material, chemicals, and heat. If one watershed

process or product is altered by land use, others change in

compensation. Changes often influence or are influenced by

biological communities, and most biological changes have

repercussions throughout a biological community. Both

physical and biological systems continually undergo change

even in an undisturbed state. (Chapter 2)

CWEs are caused by changes that accumulate in time or

space. Land use directly affects only a small number of

environmental parameters, including vegetation, soil properties,

topography, chemicals, and fauna. These parameters, in turn,

influence production and transport of water, sediment, organic

matter, chemicals, and heat. Onsite impacts can occur if the

triggering change or the resulting impact is persistent in time.

Off-site impacts can occur only if watershed processes or

products are altered, because something must be transported

for a remote effect to occur. (Chapter 2)

The manifestation of CWEs is complicated by lags in

system response to change, geographic decoupling between

cause and effect, site-specific variations in impact expression,

accumulation of innocuous changes to the point that a

catastrophic change is triggered, the ability of high-magnitude

events such as storms or earthquakes to trigger delayed

impacts, and interaction between changes that modify their

expression. (Chapter 2)

CWE evaluation must be based on the basic understanding

of watershed and ecological processes already provided by

studies in hydrology, geomorphology, forestry, ecology,

and many other fields. These studies have employed a variety

of approaches, including qualitative description, statistical

comparison, monitoring, experimentation, and modeling.

Each approach has advantages and disadvantages, and

methods must often be combined to solve a particular

problem. Studies also vary in their selected focus: some

concentrate on the mechanics of an isolated process, while

others compare multiple sites or watersheds through time.

Research that specifically addresses CWEs usually must

consider large temporal and spatial scales and often needs to

include interdisciplinary work. (Chapter 3)

Cumulative effects can be caused by repeated, progressive,

sequential, and coexisting land-use activities. They can occur

because of a single type of influence on an environmental

parameter (for example, many types of land use can compact

soils), complementary influences (for example, both increased

compaction and altered snow accumulation can affect flood

peaks), cascading influences (one type of land use can

influence a second to cause an impact, as when urbanization

increases recreational pressure, increasing trail erosion),

and interdependent influences (for example, two introduced

chemicals can react to produce a third). Many studies

document the occurrence of such impacts, and a few attempt

to predict them at particular sites. (Chapter 4)

Several methods have been developed to evaluate potential

CWEs from particular land-use activities. However, methods

rarely address accumulation of effects through time, few are

adequately validated, and rarely do they address more than

one type of land use or impact. Approaches fall into three

categories: procedures for calculating values or indices

(including the Equivalent Roaded Area, Equivalent Clearcut

Area, and Region-1/Region-4 methods), a collection of

analytic procedures (the WRENSS procedures), and a

checklist of issues to consider during evaluation (the method

developed by the California Department of Forestry and

Fire Protection). None is adequate for a complete CWE

analysis. (Chapter 4)

A valid general approach to CWE analysis would be

capable of assessing the full variety of impacts and land-use

activities at all potentially impacted sites. To remain credible,

it would need to incorporate new analytical methods as they

become available, and it would require comprehensive

validation and monitoring of component methods and results.

A useful format might use a CDF-style checklist to select

relevant analytical methods from a WRENSS-type collection

of procedures. Enough is already known of most environmental
viii USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993.

parameters, watershed processes, and impacts to develop

preliminary procedures. (Chapter 4)

The effects of land use on specific environmental parameters

have been widely documented, and a selection of these is

described in Chapter 5. Modification of environmental

parameters, in turn, provokes changes in watershed processes.

These influences have also been widely studied (Chapter 6).

The impact of altered environmental parameters and watershed

processes on particular uses and values is more poorly

understood (Chapter 7). Research like that described in

Chapters 5, 6, and 7 provides the foundation for future methods

of evaluating CWEs.
USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993. 1

Chapter 1

The Problem of Cumulative Watershed Effects

to include the effects of any combination of uses, and this was

considered by some to undermine the intent of the definition.

They feared that cumulative effects would become simply a

synonym for land-use effects, because no land-use activity is

carried out in isolation. “Too broad,” the synergists complained.

However, breadth appears to be an intent of the definition,

and was certainly a goal of those driving the legal process.

The CEQ definition is useful not in circumscribing a certain

class of environmental impacts, but in identifying an approach

to land management and impact mitigation that had not been

employed in the past. Over time, the literal version of the CEQ

definition has been accepted by most researchers and jurists.

Impact assessment has traditionally been carried out on a

project scale. In the case of timber resources, this approach

has taken the form of best management practices (BMPs).

The BMP approach is based on the premise that if on-site

effects of a project are held to an acceptable level, then the

project is acceptable, regardless of activities going on around

it. Interactions between projects are beyond the scope of

BMP analysis, and operational controls are applied only to

individual projects. In addition, BMPs are designed to be

“practical”: the interests of the impactee and impactor are

balanced in such a way as to make the remedy economically

palatable to the impacting user, rather than having it be

based primarily on the needs of the impacted resource.

In this context, the CEQ definition was revolutionary and

lent legal credence to injuries sustained by other uses. A

land-use activity would now be judged according to its

contribution to the overall impact from all activities in a

watershed. Efforts to narrow the definition of CWEs thus

skirt an important aspect of the cumulative effects concept

by concentrating on the form of impact rather than on the

philosophical approach to impact evaluation. In addition,

land managers do not have the luxury of limiting the problem’s

scope by constraining the definition. Injured parties will

continue to sue when damages can be attributed to activities

taking place in their watersheds, and courts will continue to

apply the legal definition.

For the purposes of this paper, a cumulative effect is any

environmental change influenced by a combination of landuse activities. Redd siltation affected by accelerated

landsliding in a managed basin is thus considered a cumulative

effect, whether the preponderance of sediment is natural or

management-induced. In contrast, individual effects can be

traced to a specific activity at a specific site, and high

sediment levels in gravels immediately downstream of an

isolated landslide might be evaluated as an individual effect.

However, if the landslide resulted from a combination of

activities (e.g., oversteepening of a slope by excavation,

combined with an altered soil moisture regime caused by

Cumulative watershed effects (CWEs) present a potentchallenge to land managers. Land must now be man-aged

so that the combination of activities within a watershed does

not significantly impact other beneficial uses. Managers are

required to evaluate the interaction of their activities with

those of the past, present, or future, and to assess their combined

effect on other existing or potential uses or values. Even

though the effect of an activity might be minimal when

considered in isolation, it may combine with the effects of

adjacent or future projects to cause unacceptable impacts.

Meanwhile, the definition of “acceptable” is becoming narrower.

Land managers must be able to predict the environmental

effects of planned activities if they are to avoid CWEs.

Before prediction is possible, however, we need to understand

how CWEs come about, how they are expressed, and how

systems respond to them. The answers to these questions lie

in an understanding of how physical and biological systems

work, and this is also the general goal of science.

This report outlines issues underlying CWE concerns,

discusses mechanisms by which CWEs are expressed,

describes methods used in their analysis, and summarizes

research relevant to their evaluation.

Definitions

The definition of CWEs provided a controversial issue for

conferences and workshops of the late 1970s and early 1980s.

In 1971, the Council on Environmental Quality (CEQ) agreed

on a restatement of the principles expressed in the National

Environmental Policy Act of 1969 (CEQ Guidelines, 40

CFR 1508.7, issued 23 April 1971):

“Cumulative impact” is the impact on the

environment which results from the incremental

impact of the action when added to other past,

present, and reasonably foreseeable future actions

regardless of what agency (Federal or nonFederal) or person undertakes such other actions.

Cumulative impacts can result from individually

minor but collectively significant actions taking

place over a period of time.

This definition still left room for interpretation. Were additive

impacts CWEs? Or did impacts have to be synergistic, such that

the effect of two activities is more than the sum of their independent

effects? Were decrementally synergistic changes included?

Acceptance of the additive interpretation would allow CWEs
2 USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993.

of the resource or value affected. For example, a high sediment

load might be identified as a problem because it increases

water treatment costs in a community, but the same impact

might go unnoticed if the community were not present.

Similarly, a change can be beneficial to some uses and

harmful to others: gravel mines may benefit from the same

increases in sediment load that damage fisheries. Thus, a

CWE can be considered “good” or “bad” only with respect

to particular resources. Negative effects are most commonly

considered, both because positive effects do not motivate

lawsuits, and because a change from existing conditions

usually proves detrimental to at least some of the resources

that have equilibrated to the status quo.

Impact significance is sometimes evaluated against a “threshold

of concern,” where levels of impact below a certain value are

considered acceptable and higher levels are not. However, physical

and biological systems usually respond incrementally to change

(fig. 1a) rather than experiencing true thresholds (fig. 1b).

Even when a response involves a dramatic change in

state, factors causing the change rarely exhibit a simple

threshold. For example, an abrupt shift in channel pattern

rarely results from a single year’s sediment input; annual

variation in sediment input under undisturbed conditions

would likely have included values characteristic of the altered

input rate (fig. 1c). Chronically increased inputs can increase

the probability of channel disruption, but there is rarely a

definable value at which response is certain.

Many sudden changes, such as landsliding or gully incision,

result from the interaction of multiple factors. For example,

the rain intensity capable of triggering slides varies with

antecedent moisture, soil depth, vegetation cover, and other

conditions that vary with time. As soon as conditions change,

the threshold rainfall intensity also changes.

logging), or altered runoff promoted sediment accumulation

from the slide, or sedimentation combined with other impacts

to contribute to a decline in salmon populations, then even

this well-constrained impact may be considered either a

CWE or a part of one. Almost all effects of land use can be

evaluated as cumulative effects because most are influenced

by more than one aspect of land use. In addition, air pollution

and other systemic changes are present at most sites and thus

interact with other use-related changes.

There is also disagreement about the meaning of

“watershed” in “cumulative watershed effect.” Some interpret

this to include any changes occurring within the bounds of a

drainage basin, so the watershed is simply the location of an

impact and does not necessarily play a role in its generation.

By this definition, decreased habitat for narrowly ranging

species might be considered a CWE, while similar decreases

for species whose individuals range beyond the watershed

are not. This interpretation produces inconsistencies resulting

from watershed scale. For example, fragmentation of spotted

owl habitat is neither meaningful nor assessable in a 5-ha

headwater catchment, but may be extremely important in a

large river basin.

Others interpret “watershed effects” to include only those

changes occurring to resources influenced directly or indirectly

by watershed processes, so processes of water and sediment

transport are functionally linked to the expression of impacts.

To provide a consistent definition that avoids problems of

watershed scale, this definition will be used in this paper.

Definition of the “significance” of particular CWEs is

also a focus of controversy, and Thompson (1990) reviewed

methods for evaluating impact significance. Some aspects of

significance can be defined economically. If oyster beds are

damaged, for example, costs and damages sustained by all

dependents of the oyster industry, including oyster farmers,

wholesalers, service industries, restaurant patrons, and others,

can be summed to estimate economic impact.

However, impacts on noneconomic values must also be

evaluated. Some resources have widely recognized intrinsic

and cultural worth that cannot be translated into dollars:

What price tag can be attached to potential medical discoveries

from the Pacific yew? How much is the esthetic integrity of a

trout stream worth? And how does one compensate in dollars

for cultural losses incurred by inundation of a Native American

ceremonial site? These impacts cannot be easily expressed as

economic effects on the primary users or on the public as a

whole, yet they are of concern to many who will never see or

experience them. If the Arctic National Wildlife Refuge were

valued only by those likely to visit it, use of that land would

not be controversial. Value in such cases is culturally defined,

so significance is a political or cultural issue rather than a

scientific or economic one. Economists have only recently

begun to develop methods of assessing “existence” values

(e.g., Pope and Jones 1990, Sanders and others 1990).

The significance of identical changes differs with the

locale and types of beneficial uses present, because

significance of an impact is defined from the point of view

Figure 1—Types of system response. “Response” may be percent

of channels gullied, smolt population, water treatments costs, and

so on. “Driving variable” may be sediment input, peakflow discharge, length of active roads, or some other parameter.

A. INCREMENTAL NON-LINEAR

 RESPONSE

C. RESPONSE TRIGGERED BY CHRONIC CHANGES

 RATHER THAN INSTANTANEOUS ONES

response might

threshold

response might

B. THRESHOLD RESPONSE

DRIVING VARIABLE DRIVING VARIABLE

TIME

DRIVING VARIABLE

RESPONSERESPONSE
USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993. 3

For these reasons, thresholds of concern can rarely be

equated to physically or biologically meaningful numbers.

Instead, a threshold must be defined as either a tolerable

level of injury to impacted resources or an acceptable

probability of detrimental response. It is the goal of research

to develop an understanding of CWEs, the physical and

biological processes driving them, and the physical and

biological processes they affect. Scientists are thus concerned

with quantifying the impacts, but judgments of significance

and tolerance are left to political and legal arenas.

History of the Issue

Cumulative effects have been with us since humans began to

manipulate their environment. Progressive salinization of

irrigated land contributed to the breakup of Sumerian civilization by 1700 B.C. (Jacobsen and Adams 1958), and the

combination of agriculture, grazing, and logging in ancient

Greece may have promoted sedimentation that forced relocation of ports (Kraft and others 1975). Deforestation and

land use in Great Britain is thought to have contributed to

transformation of forest land and pasture into peat bogs over

the past 5,000 years (Moore 1975), and the cumulative effects of this vegetation conversion are only now being reversed through reforestation. The British later became known

for their efforts to control cumulative impacts in their colonies. In East Africa, for example, restrictions were put on

herd size, agricultural practices, and cutting of fuelwood,

often over the protests of indigenous peoples (e.g., Berry

and Townshend 1972).

Cumulative watershed effects were recognized in the

United States by the 1860s, when coarse sediment produced

by hydraulic mining in the California foothills began to

choke downstream channels, overrun productive agricultural

lands, and contribute to flooding of valley towns. These

impacts provoked legislation that banned hydraulic mining

and led to what may be the first rigorous CWE assessment.

In his 1917 report, “Hydraulic-mining debris in the Sierra

Nevada,” G.K. Gilbert tracked the movement of mining

sediment and evaluated its likely impact on navigation in

San Francisco Bay.

Gilbert explicitly described examples of CWEs. To explain

increased flood frequencies in the Central Valley, he cited

construction of levees in agricultural areas in combination

with aggradation of mining debris: “If these changes had been

independent of those wrought by mining debris they would

have resulted in the automatic deepening and widening of the

channels.” But because they occurred in combination, channel

flow capacities decreased and downstream cities were flooded.

Gilbert then evaluated the influence of mining debris on

growth of the San Francisco bay-mouth bar. His calculations

showed that reclamation of tidal marshes has a more profound

effect on bar sedimentation than the influx of mining debris,

and he noted that “every acre of reclaimed tide marsh implies

a fractional reduction of the tidal current in the Golden Gate.

For any individual acre the fraction is minute, but the acres of

tide marsh are many, and if all shall be reclaimed, the effect at

the Golden Gate will not be minute.”

Gilbert was well-equipped to evaluate such problems

because, as a geomorphologist, he was experienced in dealing

with complicated, interacting systems. Geomorphologists

frequently must decipher complex causal relations, disentangle

interactions between multiple driving variables, and predict

the effects of environmental change. Typical

geomorphological problems include: What determines the

extent of channel networks? What controls channel form?

How does a channel adjust to a change in watershed

conditions? These questions are also fundamental to

understanding and predicting CWEs.

Ecology provides a biosciences analog to geomorphology.

The focus of ecological research is communities of interacting

organisms responding to each other and to changes in their

physical environment. As with geomorphology, ecology was

initially a descriptive science. It was not until the middle of

this century that process-oriented and experimental approaches

to ecology became widely practiced, resulting in insights that

interested a lay audience. This was the era of One Day on

Beetle Rock (Carrighar 1943), The Immense Journey (Eiseley

1946), and A Sand County Almanac (Leopold 1949). Soon

after came Silent Spring (Carson 1962), and capital-E Ecology

was suddenly part of the public consciousness. Interdependency

and ecological complexity became familiar ideas; noneconomic

values of wildlands were accepted as legitimate; and, within

a decade, CWEs became a recognized issue.

Thus, even though the term was recently coined, cumulative

watershed effects are not a new problem. Cumulative impacts

were suffered at least 4,000 years ago, and questions central

to an understanding of CWEs are the basis of sciences such

as ecology and geomorphology. The only “new” elements

are the legislative mandate to address the problem, the

widespread popular concern about it, and the social and

economic changes that are required to fulfill the legislative

directives.

Because of their ubiquity and complexity, CWEs have

been used as a legal tool for delaying controversial

management activities and as a generic reason for downstream

damages. A “solution” to the CWE problem requires a valid

method for predicting the environmental impacts of land use.

Such a solution would benefit all those interested in resources,

whether that interest be primarily as a resource user, a land

manager, or an environmentalist. If the costs and delays of

litigation were obviated, resource production costs would

decrease and budgets that now contribute primarily to

supporting the legal profession could be devoted to other

pressing needs. A valid method for predicting land-use effects

would allow selection of land-use plans that minimize impacts

while providing for truly sustainable industries. Impacts to

off-site users would be reduced, and the environment would

be better protected.
4 USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993.

The Major Issues

Discussions and investigations of CWEs usually involve

several topics:

1. Do CWEs exist? The general question is easy to answer:

studies have described CWEs resulting from many land-use

practices, including forestry (Lyons and Beschta 1983),

urbanization (Hammer 1972; Leopold 1973), and mining

(Touysinhthiphonexay and Gardner 1984). In each case,

progressive land-use changes caused increasing impacts on

water quality, runoff volume, channel morphology, or habitat

quality. The question is harder to answer on the scale of

particular projects, and methods for detecting CWEs will be

examined in this review.

2. Which sites are susceptible to CWEs? Scientists have

long been interested in factors controlling a system’s response

to change. This research is directly applicable to the CWE

problem, and relevant studies will be described in this report.

3. How significant are the impacts? The magnitude of

changes caused by land use is widely measured, and such

studies will be discussed. But the significance of the damage

to a resource is often defined on social or political grounds,

and these will not be considered.

4. Who is responsible? CWEs result from multiple or

progressive activities that are often carried out by different

users. Effects are also complicated by the occurrence of

large storms or other natural events that make distinction

between natural changes and land-use impacts difficult.

Contributions to watershed response from different land

uses can be assessed using methods such as water and sediment

budgeting, and these will be discussed.

5. How can impacts be predicted? Several methods of assessing

CWEs have been developed and implemented, and their scientific

basis, application, and limitations will be described.

6. How can land be managed to avert or redress impacts?

This has been a major research focus for land management

agencies, and the answer requires a basic understanding of

how watersheds and ecosystems work. Once out of the realm

of basic understanding, however, the topic becomes one of

engineering design. Basic principles will be discussed, but

specific implementations will not.

Goals of the Review

Many CWE reviews are already available. Most are compendia of symposium papers that either report research results (Callaham and DeVries 1987; NCASI 1984, 1986) or

outline perspectives on CWEs (Standiford and Ramacher

1981). Other volumes describe likely CWEs and review

relevant literature (Coats and others 1979, Geppert and others 1984, Beanlands and others 1986), and some outline

needed research (Peterson and others 1987b, Sonntag and

others 1987).

The present review differs from these in several respects.

First, it describes many relevant studies in geomorphology,

hydrology, and ecology that have not been discussed elsewhere

because they do not overtly focus on CWEs. Inclusion of

these studies shows the types of background information

available for development of evaluation procedures and

indicates productive paths for future research. Similarly,

gaps and weaknesses in our overall understanding of watershed

function are revealed when published work is fitted into a

framework to describe watershed function. Second, this review

describes a variety of methods that are now being used to

evaluate cumulative watershed effects. Methods are critiqued

in the context of the working definition of cumulative

watershed effect, and a procedure for CWE evaluation is

proposed. Finally, much has changed since the most recent

reviews were published. New frameworks and methods for

understanding CWEs have been developed, and new

procedures for their evaluation are being implemented.

Scope of the Review

This report reviews research that adds to our understanding

of CWEs, whether or not it was specifically designed to

address them. Much of our understanding of CWE mechanisms emerged from basic research in the fields of hydrology and geomorphology. Resources affected by change are

often biological, so ecological investigations also produce

fundamental information on how and why impacts are expressed. Chapter 2 provides an overview of watershed and

ecosystem function and describes CWE mechanisms. CWE

evaluation can be simplified by understanding the interactions that generate them: most land-use activities affect only

a few environmental parameters, which, in turn, alter watershed processes and affect beneficial uses.

Our present knowledge of CWEs is based on many years

of research in several fields, and this research has taken a

variety of forms. Chapter 3 describes the strengths and

weaknesses of research approaches used in understanding

watersheds, ecosystems, and CWEs.

Chapter 4 discusses a variety of CWEs and categorizes

them according to the types of interactions generating them.

Studies that describe or predict CWEs are outlined, as are

existing methods for assessing land-use impacts. A program

for developing and validating a general CWE assessment

procedure is outlined. The proposed evaluation procedure

depends on a fundamental understanding of how CWEs

occur, and the remaining chapters outline the present state

of our knowledge.

The focus of this review is CWEs generated by use of
USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993. 5

of how altered environmental parameters affect watershed

processes.

Resources and values influenced by CWEs are also numerous.

Of particular concern are impacts on fisheries resources,

timber use, recreation, water supply, and floodplain use, and

each of these resources has characteristic environmental

requirements. Chapter 7 outlines research that evaluates

environmental requirements and effects of altered conditions

on specific beneficial uses.

forest lands in California. Because CWEs include the effects

of any combination of activities occurring in a watershed,

methods for predicting CWEs must be able to assess the

effects of all land uses in a watershed and their relation to

naturally occurring processes. Interactions between impacts

related to timber management and those arising from other

uses must therefore be considered. Chapter 5 describes

research that evaluates the effects of various land-use activities

on environmental parameters, and Chapter 6 reviews studies
6 USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993.

Chapter 2

How Cumulative Watershed Effects Occur

CWEs can result from the accumulation of effectsthrough time, through space, or both. They occur because

of fundamental properties of systems with interacting

components, and because of special qualities of watersheds

and ecosystems. To understand the basis for CWEs, it is

necessary to understand how watersheds and ecosystems work.

Watershed Function

A watershed is a topographical form that concentrates runoff. The form is molded by transport of sediment or dissolved material from one part of the landscape to another,

generally with the aid of water.

Runoff Generation

Surface runoff from a watershed is produced by groundwater seepage and by water falling onto the land surface. When

water accumulates on a surface faster than it can infiltrate,

the excess runs off. Some infiltrated water may be conveyed

rapidly to channel networks through soil pipes, but the remainder percolates through soil and bedrock until it encounters the water table and is incorporated into the groundwater

reservoir. Along the way, it may be absorbed by vegetation

and transpired back into the atmosphere, or adsorbed onto

soil particles and later evaporated. Springs and bogs form

where a water table intersects the ground surface, and seeps

can subtly augment base flow along a channel. Overland

flow contributes directly to runoff only during or immediately after snowmelt or rain; otherwise, streamflow is supported only by groundwater.

By dissolving chemical constituents of soil and bedrock,

percolating water speeds their physical and chemical

breakdown. This contributes to formation of transportable

sediments, aids weathering, and physically removes much of

the original bedrock mass. Limestone caves and sinkholes

are obvious evidence of chemical dissolution, but the process

also contributes to sculpting of most other landforms. Chemicals

produced by human activities can also be redistributed by

groundwater flow.

Channels

Water exerts force on surfaces it flows across, and this shear

stress increases with flow depth and gradient. High shear

stress enables runoff to mobilize sediment and excavate

channels. Elevated discharges transport more sediment than

lower discharges at a site, but the pattern is more complicated along a channel’s length. Downstream increases in

flow depth usually compensate for characteristic decreases

in channel gradient. In an equilibrated stream, each channel

segment is precisely adjusted to carry off sediment contributed from upstream and from tributaries. Such a channel is

described as “graded.”

A channel’s cross-sectional form and long profile depend

on the balance between its ability to transport sediment and

the type and amount of sediment provided to it. Sediment

accumulates where input rates are higher than transport rates

or where contributed sediment is too large to move, and

deposition can locally increase transport by steepening a

channel or broadening its transport zone. Where flow can

carry more sediment than is provided, excess shear stress

acts on the bed and may cause incision. Down-cutting decreases

channel gradient locally but increases it immediately upstream,

increasing potential sediment transport into the reach while

decreasing transport out.

Channels adjust when conditions change, but conditions

change continually. Introduced sediment may accumulate

during low flows and be washed away when flows increase,

and an uprooted tree may cause local erosion by forcing

flow against a bank while allowing sediment to accumulate

in the slackwater it forms. Undisturbed channels continually

adjust to the distribution of high and low flows. Bars are

molded and channels realigned to accommodate flood

discharges, but subsequent low flows more weakly readjust

channels according to their own patterns of shear stress.

Channel form usually most strongly reflects moderate flows,

which are frequent enough to reassert their effects but large

enough to move sediment efficiently. Localized scour and

deposition may occur, and channels may migrate by bank

erosion and bar deposition, but the overall form of an

undisturbed channel generally changes little through time.

Fallen trees accumulate in streams until removed by floods,

debris flows, or decay. Woody debris traps sediment and

creates plunge pools and obstructions, which expend flow

energy that might otherwise contribute to erosion or sediment

transport. Sediment traps formed by debris can moderate the

impact of large inputs of coarse sediment by releasing stored

sediment gradually over decades or centuries.

Sediment Production

Several processes transport sediment to channels. On steep

slopes or in weak materials, gravity may nearly overcome

the resistance produced by inter-particle friction, cohesion,

and anchoring roots, and landslides can then be triggered by

increased pore-water pressures or loading by rain. Susceptibility to sliding can change with soil age as weathering alters

cohesion, frictional properties, and soil depth. Shallow debris avalanches commonly occur in bedrock hollows, where
USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993. 7

pore-water pressures tend to be high and soils are deep.

Debris avalanches that land in steep channels may flow

downstream and incorporate channel sediment to form a

debris flow or debris torrent. Debris flows can continue to

move for several kilometers and often come to rest only on

entering lower gradient channels, where they form debris

jams of logs and sediment. Other types of landslides are less

influenced by topography. Clay-rich or sheared bedrock may

form earthflows over a kilometer long and hundreds of

meters wide. Earthflows move slowly during the dry season,

but they can move several meters in a month when saturated

by winter rains.

Gravity gradually pulls the entire soil mantle downslope

by a group of processes known as soil creep. Some soils may

flow plastically, much like a very slow earthflow. Others

move because of the downslope component that gravity

introduces to most soil disturbances. Soils expand

perpendicularly to the soil surface when wetted or frozen, so

they expand preferentially downslope. As they dry or melt,

upslope contraction against gravity does not fully compensate

for the original downslope motion. Similarly, when soil

particles fall into an animal burrow or empty root hole, they

move slightly downslope because they fall toward the center

of the earth rather than perpendicularly to the slope. Soil

creep can move a soil profile a fraction of a millimeter to

several centimeters each year.

Several transport processes primarily affect the soil surface.

Rainsplash launches particles preferentially downslope or loosens

them for entrainment by overland flow, and surface runoff

dislodges and transports sediment. Soils are protected from

both rainsplash and sheetwash erosion if they are covered by

vegetation, organic debris, or immobile particles.

Interactions between Hillslopes and

Channels

Sediment transported down a hillslope eventually encounters a stream channel. Entry of sediment into a channel

(“sediment production”) may result directly from hillslope

transport processes, as when a debris avalanche falls into a

channel. In contrast, some creep processes cause channel

banks to encroach gradually on a channel. As banks move

inward, channel flows deepen and increase the force exerted

on banks, and sediment production occurs by bank erosion.

Where stream channels are lined by floodplains, much of the

sediment removed from hillslopes accumulates on the floodplains until channel migration entrains it.

A channel aggrades if its ability to remove sediment

cannot keep pace with the amount introduced. Aggradation,

in turn, modifies sediment input rates: not only do sediment

deposits buttress hillslopes and reduce their transport rates,

but they also trap more hillslope sediment. In contrast, if

channels remove sediment more quickly than it is produced,

then they incise, hillslopes steepen, and hillslope erosion

rates increase to equal removal rates. Meanwhile, these

morphological changes affect the ability of channels to

transport sediment. Aggradation provokes changes that usually

increase channel transport rates, whereas incision may

decrease gradients downstream and thus reduce transport

rates out of a reach.

A watershed thus modifies its form by eroding or aggrading

to balance rates of sediment input from hillslopes against

rates of removal by runoff. This adjustment is realized by a

simple feedback mechanism: what does not get carried off

remains, and this alters both input and export rates. Aggradation

tends to increase transport capability and decrease input rate,

while incision increases the input rate and decreases transport.

The implications of this balance are extremely important for

land management. When one factor in a system changes, that

factor alters others to compensate for the change, and both

form and processes are likely to be modified in response.

Physical Basis for Cumulative

Watershed Effects

Most land use directly alters only a few environmental factors: vegetation, soils, topography, and chemicals. All other

changes result from alterations to these factors. CWEs are

caused by changes that accumulate in time, space, or both.

Accumulation of effects through time requires that individual changes be persistent, so that one effect is not fully

healed when the next occurs. This mechanism can generate

on-site CWEs, but off-site impacts can be caused only if

changes accumulate through space, and thus off-site changes

must involve altered transport processes or watershed products. Land-use activities that cause persistent changes or

affect transport of watershed products thus have the potential for causing CWEs.

The importance of a cumulative change depends on what

resources or values are affected by the change. In many

cases the impact of concern is a physical one: accelerated

channel migration may destroy agricultural land, enhanced

peak discharges may increase flood frequencies, or increased

erosion may fill reservoirs. Off-site effects require alteration

of transport processes or rates, and these alterations often

affect channel morphology.

Ecosystem Function

Most land use alters biological communities, which can then

modify physical conditions by influencing production, quality, and transport of water and sediment. Watershed processes, in turn, mold the physical habitat of biological communities, and the resulting impacts on biological resources

are often a major concern. Biological processes affect both

the generation and expression of CWEs and must be understood if CWEs are to be evaluated.

Because the network of interactions between organisms is

intricate, a biological change can affect an entire community
8 USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993.

of organisms. Even those interested only in the abundance of a

particular species must take into account its community

interactions. A population change may result indirectly from

changes to another organism, and the resulting change may

affect still other organisms. To understand how biological

changes lead to CWEs, it is necessary to understand how

biological communities function, and this is the focus of ecology.

Krebs (1978) defined ecology as “the scientific study of

the interactions that determine the distribution and abundance

of organisms.” Organisms respond to their physical

environment, to the assemblage of species they interact with,

and to other members of their own species. Ecology is thus

concerned with extremely complicated interacting systems of

biological and physical influence. These influences are usually

evaluated at the scale of individuals, species, or communities.

Constraints on Individuals and

Species

Physical constraints provide a clear example of limits to

distribution of a species, because each species has physiological requirements for temperature, moisture, light, and

environmental chemistry. However, within these broad limits are a series of less-well-defined constraints. An individual’s

ability to reproduce, grow, compete, or survive disease is

often impaired near physical limits. The species may survive

under marginal conditions, but it may not thrive.

Physical constraints are complicated by other factors.

Physiological demands often vary seasonally or with

developmental stage, and the ability to compete under

particular physical conditions may depend on the identity of

competing species. Apparent physical constraints can also

reflect behavioral influences. Animals may frequent an

environment because it provides opportunities for nuptial

display, a vantage for spotting prey, or simply because of

historical happenstance. For example, dense seabird colonies

may occupy particular islands while adjacent ones remain

inexplicably barren.

Organisms interact with others and so are influenced by

their biological environment. Most animals are affected by

species dependent on them for nutrients, and in turn depend

on nutrients derived from other organisms. Both plants and

animals compete with others for resources and living space.

Habitat

An organism’s physical and biological environment is its

habitat. Sedentary species use habitats of fairly uniform

character, but migratory and wide-ranging species often have

a variety of habitat requirements. For example, some salmon

begin life in a stream, over-winter in a lake, and spend

several years at sea before returning inland to spawn. Some

organisms require different environments for different uses,

such as feeding and escape from predators. Welfare of a

species depends on the condition of each component of its

habitat. Characteristic and identifiable habitat components,

such as pools in a stream environment, are described as

“habitat units.”

For an environment to be habitable by an organism, it

must fall within the range of conditions the organism can

tolerate. This view of habitat has led to the concept of “limiting

factor”: abundance of an organism in a given environment is

assumed to be limited by a dominant constraint. Thus, smolt

production in some rivers is thought to be restricted by lack

of clean spawning gravels, while insufficient rearing habitat

limits production at other sites. If the limiting constraint is

removed, then abundance is expected to increase until it is

bounded by a different constraint. “Carrying capacity” is the

potential abundance of a species, given the physical and

biological environment at a site. Both of these concepts are

widely used in impact analysis, yet both are controversial.

A limiting factor is usually viewed as a single parameter,

although abundance is actually modified by interactions between

many physical and biological factors. Occasionally a single

factor may dominate, as when chemical spills or elevated

stream temperatures kill fish. However, abundance more

commonly reflects not only the local habitat, but also the

history of changes at a site, the history of the population, and

influences throughout the organism’s life cycle in every

component of its habitat. One factor may produce the highest

mortality one year, while the following year a different one

may dominate. In other cases, a factor may become important

only because an interacting factor changes. For example,

elevated turbidity or toxins might harm fish only while water

temperature is high. The concept of limiting factors can be

applied either to a population, in which case all influences

throughout the population’s life cycle must be considered, or

to an organism’s tenure in a particular habitat unit, where only

local influences are of concern.

“Carrying capacity” also can be applied in different

contexts. Carrying capacities have been defined for particular

habitat units during specific seasons, but have also been

measured as time-averaged values in systems containing

many habitat units. Carrying capacity for a system must

account for the range of habitat conditions present through

time; it cannot be defined simply by characteristics observed

at a particular time. System capacity must also reflect the

full range of environments used by the species. Whether

carrying capacity is defined for a system or a habitat unit, it

is usually poorly correlated with abundance. Abundance

also depends on nonhabitat factors such as disease, abundance

of the previous generation, and recent history of disturbance.

Biological Communities

Some biological environments are recognizable entities: in

California, a redwood forest in Del Norte County looks like

a redwood forest in Humboldt County, and is very different

from a red fir forest in the Sierra Nevada. Once the forest

type is identified, its plant and animal species can often be

predicted. These environments are biological communities.

A community is usually described using the types and rela-
USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993. 9

tive abundance of species comprising it and the structure of

the community. Communities have a physical structure imparted by the size and forms of plants, and a trophic structure

defined by who eats what or whom. The physical characteristics of a site strongly influence which biological community develops there.

Biological communities are opportunistic collections of

individuals brought together by overlapping sets of physical

and biological requirements, and how a community functions

depends on how its organisms interact. Each organism is

part of the environment of others, and each has a particular

role as a consumer and provider of resources. These roles

must be filled if the community is to remain stable. Functional

roles within a community are called “niches,” and organisms

with overlapping niches compete with each other. If a habitat

change depletes a species, competitors better able to tolerate

the new conditions may benefit from its absence and multiply

to fill the niche. Removal of a habitat unit or species can also

destroy a niche, as when a prey species is replaced by an

unpalatable competitor.

Community members thus participate in a complex network

of interdependencies. If abundance of a species changes, it

will usually cause adjustment throughout the network. When

an accustomed predator is removed, prey populations may

increase to the point that food resources are depleted and

their competitors impacted, while a reduction in the population

of a prey species may decrease the abundance of its predators.

Response to Change

Biological communities undergo continual change. Organisms and species respond to changing environmental conditions on scales of minutes to millennia, and as they vary in

abundance, so do organisms dependent on them. Until humans arrived in the Americas, large-scale community changes

in the New World generally resulted from climatic shifts.

These shifts provoked slow migrations of plant and animal

species and changed both the composition and distribution

of most communities.

In contrast, short-term and localized disturbances have

more restricted effects. Forest fires, windstorms, and floods

alter the physical structure of communities and allow

colonization by successional species, but the communities

that are eventually reestablished are usually similar to the

original. Successional assemblages can even be viewed as

parts of the community, just as forests include species that

colonize windthrow gaps. Physical disturbance is often

essential for maintaining the character of a community and

preserving species diversity.

On an even shorter scale, organism requirements and

interactions vary by season. Food sources and predation risk

change regularly through the year, and behavioral patterns

have developed to take advantage of predictable habitat changes

in stable systems. For example, wildebeest migrate to Tanzania’s

Serengeti Plain just as new grass is ready to support them.

The response of a biological community to changing

conditions depends on the nature of the trigger and the types

of organisms and interactions in the community. Some

communities can shift through time in response to gradually

changing conditions without experiencing major disruptions.

“Resilient” communities may change radically to adjust to

short-term changes, yet can later reestablish themselves in

their original form. Others, described as “stable” or “resistant,”

can absorb major changes in conditions without being altered.

Still others respond with a complete change in character.

Most communities are fairly stable or resilient under

natural conditions. Catastrophes like floods or fires may

occasionally destroy segments of a community, but its

biological and physical context is unchanged so the regenerated

community is much like the original. However, if the biological

or physical context is altered by land use, or if the frequency

of disturbance changes, then populations will adjust to reflect

the new conditions. As populations change and provoke

compensating changes in interacting populations, the nature

of their communities also changes. Biological change is thus

measured not in terms of a single species’ abundance, but by

characteristics of its community.

Sensitive “index species” are often monitored to detect

changes in communities for impact analysis. The example of

coal-mine canaries is frequently used to illustrate the concept:

toxic fumes killed the canary before they affected people,

allowing timely evacuation. However, survival of an organism

depends on many physical and biological factors. The canary

was a successful index species for air quality because its

reactions were understood and its limitations recognized;

canaries would have been unsuitable for warning of flooding

or collapse. Similarly, index species cannot provide warning

of all potential impacts, because some species are more

sensitive to some changes than others are.

Like watersheds, biological communities are intricate systems

of interdependent components, and when one component is

altered, those interacting with it must change in response.

Biological Basis for Cumulative

Watershed Effects

Land use usually alters vegetation. If this change is persistent or affects transport processes or rates, it is capable of

generating CWEs. Persistence of a vegetation change depends on the lifespan and age of the species affected, the

successional sequence, the type and persistence of impacts

generating the change, and the response mode of the vegetation community. In general, altered distributions of longlived species persist longer than those of short-lived ones,

and thus carry a greater risk of contributing to on-site CWEs.

Vegetation influences the transport of watershed products.

Hillslope and riparian vegetation affects generation of runoff,

sediment, and organic debris. As these watershed inputs

change, channel morphology adjusts to accommodate them,

and change propagates along channels as aggradation, incision,

bank erosion, changes in bed material, or altered channel

pattern. These changes, in turn, affect water velocity, flow
10 USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993.

National Park threatened the stability of Tall Trees Grove,

and a desire to control this impact was an important motivation

for expansion of the park.

Reid and Miller (1989) discussed the causes for

documented species extinctions, and noted that most

ecosystem disturbances and extinctions are the result of

cumulative effects: “Any factor that leads to a decline in the

population size of a species makes it more vulnerable to

extinction” (Reid and Miller 1989, p. 45). They identified

habitat loss, degradation, and fragmentation as the most

important factors in extinctions, followed by overexploitation,

species introductions, and pollution.

A Framework for Understanding

CWEs

Many on-site and downstream interests may be affected by a

single land-use activity, and many activities may occur within

a watershed (fig. 2). If the effect of each potential combination of activities on each potential downstream resource had

to be measured, prediction of CWEs would be intractable.

Figure 2, for example, includes more than 50,000 combinations of multiple land uses and impacted interests. The problem can be greatly simplified by focusing on the relatively

few mechanisms by which impacts occur.

All impacting activities directly affect only a few

environmental parameters, including chemical content,

vegetation, topography, soil properties, and fauna. Different

activities often affect environmental parameters in the same

way, and changes can be measured for each activity. The

persistence of each type of change and its effects on the transport

Figure 2—Framework for understanding cumulative watershed effects. Land-use activities affect a small number of environmental

parameters, and these can alter production and transport of watershed products. Off-site impacts can result only from a change in

transport. Arrows indicate an influence of one category on another.

depth, baseflow discharge, intergravel flow, water temperature,

water chemistry, and flood frequency.

The habitats of all aquatic and riparian organisms are

affected when channel morphology changes. Alterations may

occur within habitat units (e.g., changes in temperature or

flood frequency), in their distribution (e.g., fewer undercuts),

or in their variety (e.g., infilling of pools). As the habitat

changes, so do the populations of organisms dependent on it.

Any change in species abundance affects populations of the

organisms it feeds on and those that feed on it. In this way,

altered channel morphology can propagate changes throughout

biological communities.

Impacts on biological communities can be cumulative in

several ways. Different land-use activities can all contribute

to the same impact, as when urbanization, road construction,

agriculture, log-milling, and other activities each

independently destroy off-channel rearing habitat for fish.

Although each activity may be responsible for only a 10

percent decrease in rearing area, a combination of ten such

activities would result in its complete loss.

Different activities can also produce complementary

impacts. Gravel mining may decrease spawning success by

destabilizing channels at the same time that timber management

activities decrease over-winter survival and fishing decreases

adult populations. Each activity affects the fish population in

different ways, but the cumulative effect is decreased abundance.

Repeated impacts at the same site can cause a cumulative

effect. A species’ mechanisms for coping with impacts are

often keyed to the characteristic frequency of disruption

with which it has evolved. If the disturbance frequency is

increased (as by logging) or decreased (as by fire control),

the community will change in response. Anadromous salmon

cope with occasional catastrophes in part by having some

variation in the age of returning spawners. If a year class is

destroyed by excessive scour during a storm, a few returns

from other age classes ensure that the year class is eventually

reestablished. However, if sequential age classes are destroyed

by repeated catastrophes, as might occur if land use increases

scour frequency, the species’ coping mechanism is defeated,

and the population may collapse.

Cumulative impacts can also occur when one activity makes

a population vulnerable to a different type of change. Loss of

tributary habitat might force young fish to over-winter in

mainstem channels at the same time that hydrologic alterations

are increasing flood frequency and decreasing in-channel survival.

Several types of biological impact have attracted notice

in the past. The most frequent trigger for popular concern is

decreased abundance of a desired species. Decreased duck

and geese populations provoked organization of Ducks

Unlimited, which established refuges to offset the cumulative

impacts of progressive wetland conversion. Changes can

also affect the characteristics of a species, as where high

fishing rates and large hatchery releases have combined to

decrease the genetic diversity of some anadromous fish

species. In other cases, concern arises from a site-specific

alteration. Aggradation of Redwood Creek in Redwood

LAND-USE

ACTIVITIES

ENVIRONMENTAL

PARAMETERS

TRANSPORT OF

WATERSHED

PRODUCTS

OFF-SITE

BENEFICIAL

USES

ON-SITE

BENEFICIAL

USES

grazing

mining

roads

power

water use

recreation

fishing

navigation

agriculture

urbanization

flood control

logging

etc.

chemicals

vegetation

topography

soil

water

fauna

water

organic

 material

sediment

chemicals

heat

power

grazing

fishing

logging

flood control

esthetics

habitation

recreation

wildlife

agriculture

water use

transportation

etc.

logging

esthetics

wildlife

habitation

grazing

recreation

agriculture

etc.
USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993. 11

of watershed products (water, organic material, sediment,

chemicals, and heat) can then be evaluated independently of

the triggering land uses. Other resources can be impacted only

by changes in environmental parameters or by altered transport,

and these impacts can also be defined independently of triggering

land use. Off-site CWEs must result from changes in transport,

because impacts can be propagated from one location to another

only if something is moved there. At each stage, environmental

parameters and transport mechanisms can interact to alter the

expression of impacts.

The problem of understanding CWEs thus becomes: (1)

understanding the effects of specific land-use activities on

physical and biological environmental parameters, (2)

quantifying the influence of altered environmental parameters

on watershed products and transport mechanisms, and (3)

understanding how changes in environmental characteristics

and transport mechanisms affect particular resources and values.

CWEs can be caused by repeated, sequential, or coexisting

land-use activities, or by activities that progress either through

time or across a landscape. Activities may produce the same

type of change to environmental parameters, cause different

changes that contribute to the same impact, influence another

activity to cause an impact, or provoke changes that interact

with each other to produce an impact. These mechanisms

will be discussed in Chapter 4.

Issues that Complicate the

Understanding of CWEs

Physical and biological systems that generate and experience CWEs are extremely complex and interact intimately

with each other. This complexity creates a variety of problems for CWE analysis.

Off-site CWEs result from altered transport of watershed

products. Because transport usually involves redeposition and

re-entrainment, watershed response usually lags behind the

changes driving it. Time lags between cause and effect can

obscure the reasons for environmental change. In some cases,

ongoing changes result from activities carried out centuries

ago. In the Appalachian Piedmont, for example, today’s high

sediment loads include sediments originally loosed by

exploitive agricultural practices of the 1700s (Costa 1975).

Also because transport is involved, impacts may occur far

from the activities triggering them. Gilbert (1917) recognized

this in his study of effects of hydraulic mining in the Sierra

Nevada on navigation in San Francisco Bay, 200 km

downstream. This geographic decoupling also masks reasons

for change and causes time lags in impact expression.

Causes of change are also obscured because different

sites respond to a particular environmental change in different

ways. Low-gradient reaches aggrade more readily than steep

reaches, for example, so coarse landslide sediment may have

little effect until it encounters a susceptible reach. Such sites

are common where mountain channel gradients decrease on

entering intermountain valleys. Lyons and Beschta (1983)

associated historical changes in channel pattern of Oregon’s

Upper Middle Fork Willamette River at such a site with

forestry-related landslides high in the basin.

The progress of change can be recognized as it occurs if a

system responds incrementally, as when reservoir siltation

rates increase with increasing watershed disturbance.

However, responses such as landsliding and gully formation

involve discrete changes in state and are not evident until

they occur. Discrete changes are often presumed to be triggered

when a threshold value is exceeded, but they usually represent

the combined effects of multiple influences. For example, a

slide might occur because bedrock has weathered to the

point that the amount of water added to the soil during a

storm, with the addition of culvert outfall, can permit the

accompanying wind to topple a tree and trigger the failure. A

threshold has been surpassed in that a response has been

provoked, but the threshold cannot be identified as a discrete

value of a single parameter.

Apparently benign changes may accumulate that require

a trigger of sufficient magnitude to exhibit their full impact.

Culverts designed to carry 25- or 50-year flows are common

in the western United States, for example, but the cumulative

impact of this practice will not be apparent until a larger

storm occurs. Slopes destabilized by land use can show a

similar response: nothing may happen until a large storm

occurs, and then sliding may be widespread. This response

confuses interpretation of the role of management in generating

impacts. The storm of December 1964 was the largest on

record in parts of California. Landsliding and channel

disruption were widespread, but how much would have

occurred before land-use disturbance? Analysis is difficult

without precedents or undisturbed catchments for comparison.

In other cases, chronic changes may take a long time to

accumulate to the point that adverse effects become evident.

Siltation of a large reservoir may not affect us, but it may

economically cripple a region 100 years from now. Similarly,

global warming began with the industrial revolution, but its

potentially devastating effects are not yet conclusively measurable.

Different changes interact and modify a system’s response,

further complicating CWE analysis. Thus, toxicity of an

introduced chemical might increase with acidification of the

stream system (Cleveland and others 1986), or decrease with

addition of suspended sediment (Hall and others 1986).

Finally, principles of equifinality and indeterminacy

undermine the confidence of prediction and understanding.

Equifinality refers to the concept that different causes may

result in similar responses. Thus, the cause of an impact

cannot be diagnosed simply from its nature, just as a virus

cannot be identified by the fever it produces. Indeterminacy

expresses the opposite idea: a particular change may not always

elicit the same response. The expression of an environmental

change reflects many modifying conditions, and those conditions

must be understood if a response is to be predicted.
12 USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993.

Watershed Effects

Cumulative watershed effects will no longer be anopera-tional issue when the combined effects of landuse activities can be anticipated, causes of particular impacts

diagnosed, and effects of mitigation or prevention measures

predicted. Progress toward resolution of the CWE problem

is measured by how well these tasks can be carried out, and

CWE research includes any research that contributes to these

goals. Three phases of research are required:

1. Provide a basic understanding of how and why CWEs

occur.

2. Construct a method for evaluating CWEs.

3. Test the effectiveness of evaluation methods and

mitigation programs.

The most challenging phase, and that on which the others

depend, is to develop an understanding of how CWEs occur,

and thus of how physical and biological systems work. Most

research carried out in the fields of geomorphology, hydrology,

and ecology contributes to our understanding of CWEs, but

very little research has specifically focused on CWEs. This

chapter discusses the strengths and weaknesses of common

research approaches as they are applied to understanding

CWEs, and is intended to provide a basis for evaluating

studies described in later chapters.

Requirements of CWE Research

The CWE perspective introduces three issues of special

importance: large spatial and temporal scales, complexly

interacting systems, and an interdisciplinary focus. Each of

these is considered a difficult problem, and past studies were

often designed to avoid them. Studies that have dealt explicitly with these issues are particularly relevant to CWEs.

CWEs involve the accumulation of impacts over time or

through space, and these time or space scales may be large.

Research must account for long-term variation in conditions

and processes, and for variations over wide areas. The effects

of infrequent events, such as large storms or fires, need to be

understood, as well as the influence of the spatial distribution

of activities and impacts. Studies often use data collected at

a variety of scales, but methods for reconciling multiple

scales are poorly developed.

Many CWEs are caused by interactions between different

processes. Such interactions have often been excluded from

studies as secondary complications, or are overlooked because

they fall between disciplines. Most experiments are designed

to simplify systems by avoiding interactions.

Evaluation of interactions is even more difficult when

they are between physical and biological systems, as is

usually the case with CWEs. These evaluations require

interdisciplinary work, but this has also often been avoided.

Multidisciplinary work, where researchers from different

disciplines work at the same site, is occasionally substituted

for interdisciplinary studies. Multidisciplinary work often

provides a broad perspective on conditions at a site, but

without close understanding between disciplines and a research

plan that specifically addresses interdisciplinary problems,

it is no guarantee of interdisciplinary understanding.

Research Approaches and

Methods

Some research approaches are more productive than others

for addressing particular aspects of CWEs. Which approach

is selected depends on the nature of the problem, how it is

posed, the type of solution desired, and the level of funding.

A system can be viewed as a collection of parts, where the

function and response of component parts explains the system

response. Systems can also be represented by a series of processes

that are activated to produce a response. Alternatively, systems

can be viewed as black boxes: certain stimuli provoke a given

response, but the mechanisms producing that response are

irrelevant. A system can be studied from any of these points of

view by selecting an appropriate research approach. A single

investigation often uses several approaches, as when a study of

a watershed process includes monitoring, manipulation, and

modeling phases. In other cases, studies using one approach

require data produced using others. Each research approach

has inherent strengths and weaknesses (table 1).

Descriptive Studies

The early phase of most sciences is descriptive and often

makes use of case studies, in which particular incidents or

sites are examined in detail. Phenomena are cataloged, measured, and described to disclose patterns of occurrence. Descriptive studies might address questions such as: “In what

ways did this stream change when land use in the basin

changed?” or “How much did salmon populations change

when this dam was constructed?”

Chapter 3

Research Methods for Understanding Cumulative
USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993. 13

Table 1—Advantages and disadvantages of research approaches and methods

Approach or

 method Applications and advantages Disadvantages

Descriptive Useful for exploring a new problem Results cannot be generalized to other

 study Can identify interactions sites or times

Helps define research questions Nonroutine methods are often required,

Allows detailed observations so personnel must be knowledgeable

Requires little prior knowledge

Extensive Often used for inventories or to develop Requires large sample sets

 survey databases for statistical analysis Often expensive unless data already exist

Reveals patterns of occurrence Can show association, but not cause

Defines spatial distribution of effect or Results cannot be generalized beyond

 process sampled area or time

Identifies associated variables Implementation is usually methodical

Design of sampling plans requires prior

 knowledge

Experiment Defines cause-effect relations Need prior understanding to design an

Variables can be isolated to make a effective study

 problem tractable Results using isolated variables may be

Identifies controlling variables misleading if variables interact

Produces generalizable results

Can be inexpensive and quick

Modeling Can predict results for hypothetical Requires sophisticated knowledge of the

 scenarios system to be modeled

Reveals data requirements Complexity makes model opaque

Shows sensitivity to variables Calibration may hide defects in model

Helps organize information Often requires a lot of data

Reveals gaps in understanding Results of complex models are hard to

 validate

Monitoring Reveals change Long-term sampling is often required

Defines baseline conditions Can show association, not cause

Provides data for comparison Cannot generalize to other sites without

Reveals temporal association other information

Implementation is methodical Must be carefully planned to provide

 valid results

Requires long-term budget commitment

Statistics Useful for describing systems Often misapplied: wrong methods are

Reveals patterns of association used; or method is applied to cases that

Measures strength of association violate assumptions; or results are

Distinguishes effects of multiple extrapolated beyond sampled range

 variables

Can use to calculate probability

Extensive Surveys

Extensive surveys measure parameters or make qualitative

observations at many sites to detect patterns of occurrence.

These are commonly used to map distributions of processes

or attributes, determine their significance, or reveal factors

associated with them. Extensive surveys include both

inventories, such as soil or vegetation surveys, and sampling

for statistical analysis. These techniques can be used either

to describe a phenomenon or to associate occurrences with

particular conditions. The approach might be used to address

such questions as: “How much sediment do debris flows

contribute to streams in this area?” or “What soil,

topographical, and land-use variables might explain the

distribution of landsliding in the region?” Extensive surveys

can be used to identify either the responses associated with a

Descriptive studies identify processes and interactions active

at a site, often with the expectation that they will be found

elsewhere in similar situations. These studies frequently identify

questions to be addressed later using other techniques.

Descriptive studies have the disadvantage that numerical results

cannot be applied to other sites unless patterns of variation

are understood, and how widely applicable even qualitative

results are is unknown without further survey work.

Dietrich and Dunne’s 1978 study of sediment production

in a forested watershed is a descriptive case study. Although

numerical results are relevant only to that case, the patterns

of process interactions that were identified, the approaches

taken in analysis, and the framing of the problem all provide

useful guidance for other studies.
14 USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993.

particular environmental change or the environmental changes

associated with a particular type of response.

Inventories and statistical surveys are fundamentally

different. Inventories sample an entire population and so

produce databases that fully represent the population. A soil

map, for example, provides information at every point within

the sampled area. Computer-based Geographic Information

Systems (GIS) are useful for managing inventory information.

In contrast, statistical surveys sample a subset of a

population and use the information to infer the full

population’s characteristics. Precise spatial information is

usually not needed, although results are often used to infer

distributions. For example, a statistical survey might show

landslides to be most common on shale slopes steeper than

30°. Bedrock and slope maps could then be used to map

landslide potential in the sampled area.

Samples must represent the range of spatial variation likely

to occur in a study area. Sample selection may be completely

random, or random within identified strata, as when 20 samples

are randomly selected on each of three rock types. Stratified

random sampling usually requires fewer samples to produce

the same precision as completely random sampling, because

sample variance is reduced by segregating samples according

to meaningful strata. The potential efficiency of a sampling

plan thus increases with understanding of a feature’s controlling

variables. Sampling is often carried out at fixed intervals, as

when soil depths are measured at intervals along road cuts,

but interval sampling may produce invalid results if the

measured property is not randomly distributed.

Many statistical methods are available for disclosing

patterns among survey data. Multivariate techniques are

particularly useful for exploratory studies, which often

measure many variables to identify those most closely

associated with a phenomenon. Discriminant analysis

identifies the combination of variables best able to predict

membership in selected categories. Cluster analysis defines

groups having similar characteristics, and factor analysis

identifies associations between variables.

Extensive surveys are often used to identify patterns,

associations, or distributions for use in predicting response

to change. Exploratory studies frequently use the method to

define important issues or to reveal possible controlling

variables. However, statistical surveys merely identify

association; they cannot demonstrate causality.

It is tempting to generalize results of extensive surveys to

wider areas or longer time periods, but generalization is

tenuous unless the reasons for statistical association between

variables are known. Valid generalization also requires that

a wide enough variety of conditions be sampled that variations

in driving variables average out. Qualitative results are more

readily generalized than quantitative ones, but even they

cannot be applied to areas with different site characteristics

or to the sampled area if conditions change.

Extensive surveys have several drawbacks. Many samples

must be measured if surveys are to provide valid statistical

results. If controlling variables are not already identified, the

sample size must be enlarged further to test the range of

potential controls. In many cases, a system’s prior history is

an important control, but this can be difficult to assess. For

example, effects of the 1964 storm lasted for at least a

decade in many west-coast watersheds, and surveys carried

out during the recovery period reflect the storm’s influence.

Local events also introduce variation in system response and

further increase the sample sizes required to distinguish

results from noise.

Extensive surveys also have several strong advantages.

They can be used to identify possible controlling variables

and assess spatial distributions, and survey implementation

is relatively routine once sampling protocols are defined.

Anderson (1954) used an extensive survey to determine

the effects of several land-use activities and environmental

parameters on sediment yields. Multiple regression showed

which land uses are associated with increased yields, and

allowed mapping of erosion potential in western Oregon.

More recently, Touysinhthiphonexay and Gardner (1984)

measured channel geometry in 29 basins sustaining different

intensities of strip mining and demonstrated an association

between proportion of area mined and the magnitude of a

channel’s response. Lewis and Rice (1989) measured attributes

of 655 logged units in California and used discriminant

analysis to identify parameters associated with erosion events.

In each case, the study’s goals were to provide generalizable

information for use in predicting watershed response to land

use and to identify factors that control the response.

Experimentation

Experiments are used to identify and quantify relations between cause and effect. Experimentalists control the context

of a system’s response by holding all variables constant

except those being tested. Test variables are then manipulated, and responses are attributed to their influence. Experiments usually are interpreted by comparing results from an

untreated sample or from the same sample before treatment.

Questions addressed by experiment might include: “How

does clearcutting affect baseflow?” and “What pore pressures will induce landsliding?”

Experimentation is more effective than monitoring or

surveys for revealing cause-effect relations, but success

depends on how well variation can be restricted to specific

variables. If other parameters vary, results become less

interpretable. For example, watershed-scale experiments often

test the effects of land use on basin response by comparing

either pre- and post-treatment measurements or measurements

in similar treated and untreated basins. In the first case,

differences in the character of storms during the two study

periods introduce magnitude-dependent and carry-over effects.

These effects are avoided if coexisting manipulated and

untreated basins are compared, but the basins will not be

physically and biologically identical. Small differences in

basin character can provoke different responses, and the

chance occurrence of an infrequent event in one basin (e.g.,
USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993. 15

a wildfire or landslide) will confuse the interpretation of

observed differences. Grant and Wolff (1991) used a longterm monitoring record from a paired-basin experiment in

the Oregon Cascades to illustrate these effects.

Well-designed experiments can be used to isolate and

study parts of a complex system in ways that would be

impossible in an unmanipulated system. In addition, it often

takes less time and money to use a battery of experiments to

approach a general problem than to employ extensive surveys

or long-term monitoring.

Design of effective experiments requires knowledge of

the system being tested. Many parameters might influence a

system’s response, and an experimentalist either needs to

know which are the more important or must be willing to test

the lot. In addition, the researcher should know how controlling

variables interact, because isolation of a variable from a

closely correlated one may cause atypical responses. Response

of a complex system is usually determined by many linked

changes, so it is often difficult to isolate a particular aspect for

study. Some problems may require one set of experiments to

identify interactions among changing variables and a second

set to test response of the system to changes. Experimental

work is rarely routine, and participation by experts is often

required at each step.

Many CWE studies include laboratory and field

experiments to isolate interactions or processes for study.

For example, Hall and others (1986) and Cleveland and

others (1986) explored the effects of zinc and low pH,

respectively, on aquatic organisms and tested the influence

of other variables on those effects. Work was carried out in

laboratories to control for habitat variables. At a larger scale,

Collins (1987) controlled grazing and burning on field plots

to examine the effect of interactions of these disturbances on

prairie plant communities. In the field of geomorphology,

Bradford and Piest (1977) irrigated a gully wall to measure

pore pressures necessary for failure, and Wolman and Brush

(1961) used a laboratory flume to explore the effects of

several controlling variables on channel form in sands. Many

watershed manipulation studies have been carried out to

quantify various effects of land use (e.g., Harr and McCorison

1979, Rice and others 1979).

Modeling

Modeling uses physical or mathematical analogs to simulate

systems and to explore relationships between controlling

variables and system responses. Physical modeling has been

used to solve problems for millennia, and mathematical

modeling is becoming increasingly common with the spread

of personal computers. Problems addressed by modeling are

similar to those approached by experimentation, and include: “How would a change in baselevel affect gully erosion?” and “How would a 30 percent increase in ground

cover affect runoff hydrographs?”

Physical modeling is often used as a tool in experiments,

as when studies use flumes as analogs for channels. If physical

models are of a different scale than the features modeled, they

must be carefully designed to ensure that changes in scale do

not introduce aberrant responses. A small-scale model of a

stream, for example, might require use of a fluid other than

water if sediment transport measurements are to be representative.

Of the approaches considered, mathematical modeling

requires the most sophisticated understanding of a system if

it is to be used successfully. In the ideal case, driving variables

are fully understood and are described mathematically, and

these relations are combined to predict system response to a

stimulus. Predicted results are often compared to measured

results to test a model’s validity.

Mathematical modeling is easy to misuse. Modeling results

are only as good as the data used to construct the model, and

data are frequently inadequate. Data needs are usually large,

and may include parameters that are rarely measured. These

requirements tempt non-field-oriented modelers to estimate

values for missing data. Results obtained using estimated

parameters may still be useful if estimates are supported by

field measurements from other sites, and if the sensitivity of

results to inaccuracies is determined. If several values are

estimated, sensitivity analyses must be carried out for

combinations of parameter values.

Missing data can also be managed by constructing models

that require calibration. However, calibration can hide flaws

in deterministic parts of a model, and simulations become

little more than regression exercises unless estimated variables

are identified and their effects understood.

Model complexity has grown with access to sophisticated

computers. The probability of error increases with complexity,

and errors become increasingly difficult to detect. Even

commercial software generally contains errors discovered

only after long use, and the frequency of errors in

noncommercial, unreviewed software is higher still. Because

errors are usually identified from anomalous results, those

contributing to results that fit users’ preconceptions are

particularly hard to recognize. A model’s validity also depends

on the validity of the assumptions it incorporates. Rarely are

these described in enough detail for other users to evaluate

them fully, and implicit assumptions may be hidden among

thousands of lines of computer code.

Proper model validation requires that predictions be

compared to measured responses throughout a model’s

intended range of use. If a model must be calibrated, then

data sets used for validation and calibration must be

independent of one another. Some models are intended to

predict effects of hypothetical changes, so validation over the

range of intended applications is not possible. Fully

deterministic models are most acceptable under these conditions,

but even these results are suspect.

Valid models are useful for predicting responses to

particular stimuli, as long as the inherent uncertainty of the

results is well understood. Modeling is often the only way to

predict response to unprecedented conditions.

Although a model’s predictive power depends largely on

its validity, modeling has other applications that do not. The
16 USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993.

attempt to construct a model often produces its most useful

results: linkages between processes must be identified and

understood, information organized, important parameters

identified, and data needs clarified. Simulations can test a

system’s sensitivity to particular variables, further identifying

critical linkages and factors. Modeling also provides a measure

of how well a system is understood. Because anomalous

results reveal gaps in understanding, even unsuccessful models

can provoke useful questions and identify directions for

further study.

Predictive equations are also a type of model, and are

widely used to estimate flood flows (e.g., Muskingum routing

method, McCarthy 1940) and sediment production (e.g.,

Universal Soil Loss Equation, Wischmeier and Smith 1978).

In these cases, user groups are large enough that the models’

limitations are well defined (e.g., Wischmeier 1976), and

results have proved extremely useful. Hirschi and Barfield

(1988a) combined several physical relations into a model to

predict rill erosion. These authors explicitly presented their

model as a research tool, and, in a companion paper (1988b),

carefully described validation of the model.

Some simple models are distributed for their heuristic

value, and these may be useful tools for generating hypotheses.

For example, Ahnert (1976) described a 3-dimensional

landscape evolution model that can be used to explore the

effects of climate change on landforms. More complex models

are often used primarily by those constructing them, both

because others are hesitant to use products that may not have

been fully tested and because many models address problems

with stringent constraints.

Monitoring

Rather than being a research approach, monitoring is a method

that can be applied to many research approaches. Monitoring studies measure attributes through time and are often

used to define site characteristics such as rainfall, distribution of flood peaks, and average sediment yield. The success

of future research depends on continued gathering of longterm records, both to define baseline conditions and to detect

and measure responses to change. Many studies require

short-term monitoring. Time-sequence measurements define the response to an experimental manipulation, for example, and modeling results are often tested against monitoring records. Monitoring provides answers to questions

such as: “Is global warming occurring?” and “How often is

this site likely to experience flooding?”

Like the spatially distributed samples used in extensive

surveys, monitoring samples may be selected according to

identified strata (e.g., storms and nonstorms), at fixed intervals,

randomly, or randomly at frequencies weighted by subclass

importance (as when turbidity samples are collected at a

frequency weighted by discharge). Most monitoring has used

fixed sampling intervals. These intervals determine resolution

(e.g., daily precipitation is of little use for predicting sheetwash

generation), and can introduce spurious results if the properties

being measured are periodic. A daily 6:00 A.M. stream

turbidity measurement, for example, is likely to underestimate

the effects of truck traffic.

To be valid, a monitoring plan must represent the range of

conditions present, provide enough measurements that shortterm fluctuations are averaged out, and ensure uniformity of

sampling technique. The particular research application

defines the precision, accuracy, and sample size required.

Data are usually analyzed statistically to identify trends,

describe the variance, and estimate probabilities and recurrence

intervals for events of particular magnitudes. Time series or

other sequential analyses are often appropriate but require

an understanding of possible interactions between sampling

and sampled periodicities.

Monitoring may quantify a site’s response to a change,

but response of an adjacent site cannot be predicted unless

the mechanisms for response are known. Monitoring produces

only circumstantial evidence of causality, because it can

demonstrate only temporal association between events. Many

of the existing short-term records of process rates are unusable

because their relation to long-term seasonal or annual variations

is unknown. Large data sets are usually necessary for adequate

sampling of the temporal variability at a site, and good baseline

data require long-term commitments of time and funds.

Despite its drawbacks, monitoring is often the most

effective method for identifying baseline conditions and

measuring changes through time, and short-term monitoring

is essential for quantifying experimental results. Once a

monitoring system is in place, sampling is usually routine.

Monitoring is an important part of many studies. Anderson

(1954) based his extensive statistical survey on monitoring

records, and Lyons and Beschta (1983) used existing streamgauge records to associate changes in peak flow with changes

in land use in the Upper Middle Fork Willamette River

basin. Emmett (1974) used a network of monitored crosssections to describe morphological changes in arroyos of the

southwestern United States. Megahan and Kidd (1972a)

installed catch troughs to monitor sediment eroded from

logged sites in Idaho.

Most experimental watersheds provide both long-term

baseline data and shorter-term records to document results of

watershed manipulations. Rice and others (1979) and Harr

and McCorison (1979) used both of these types of data to

assess hydrologic response to altered land use in experimental

watersheds in California and Oregon, respectively. The two

functions for experimental watersheds are not always

compatible, however, because basins must remain undisturbed

if they are to provide continuous baseline records. Thomas

(1990) described problems arising from use of experimentally

manipulated basins as controls in later experiments.

Statistics

Statistics, like monitoring, is more of a tool than a research

approach, and most studies use statistical analysis to identify

and quantify relationships between variables and responses
USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993. 17

and to measure the strength of those relationships. Statistical

methods are required to answer such questions as: “How

often is this event likely to occur?” “Is this response atypical?” and “Which variable is most influential in producing

this result?”

No matter how sophisticated, statistics only describe

information already contained in a data set, so their usefulness

depends on data quality and study design. Enough samples

must be measured that treatment effects can be distinguished

from system noise, and enough variables must be controlled

that interpretation of the result is possible.

Because computers have made statistical methods easy to

use, statistics have also become easy to abuse. Computers

can lead naive users through complicated analyses without

addressing the methods’ applications, assumptions, and

limitations. As a result, inappropriate techniques are often

used, assumptions violated, and outcomes predicted beyond

the original data range. Sediment rating curves are extrapolated

to predict sediment loads during major floods, for example,

and recurrence intervals are estimated for unprecedented

storms. Mark and Church (1977) reviewed use of linear

regression in earth sciences, and found that in only two of 24

studies was the method applied appropriately. Benson (1965)

and Williams (1983) reviewed other types of statistical errors

common in hydrology and earth sciences. Some statistical

packages for personal computers contain undocumented errors

or faulty algorithms, and calculated results may not be valid

(Dallal 1988).

However, if the methods to be used are thoroughly

understood by the researcher, statistics are invaluable for

making sense of research results. Statistical methods allow

identification of patterns of response, calculations of

probability or risk of a particular occurrence, and evaluation

of the strength of relationships.

Scale and Focus of Studies

In designing a study, researchers select not only the approaches to be used but also the organizational strategy.

Strategies differ primarily in how problems are framed and

in the spatial and temporal scales considered.

Process Studies

Complex systems are often studied by isolating individual

physical or biological processes and measuring factors affecting them. As more components are investigated, the

system is conceptually reassembled. Process studies have

employed many research approaches. To investigate causes

for landsliding, for example, descriptive case studies have

detailed the characteristics of specific slides (Fredriksen

1963); extensive surveys have identified variables associated with landslide distribution (Furbish and Rice 1983,

Rice and Lewis 1986); laboratory experiments have revealed

the effect of soil moisture on shear strength (Yee and Harr

1977); mathematical models have explored the mechanisms

that trigger slides (Swanston 1970); and monitoring programs have tracked pore pressures at likely slide sites

(McGreal and Craig 1977).

Some system responses are governed by interactions

between several components, and studies of isolated elements

may not provide enough information to understand the

response. In these cases, process studies are expanded to

focus on interactions instead of on events or species. Predicting

a channel’s morphological response to flooding, for example,

requires understanding its response both to altered sediment

load and to altered flow, and also to the combined effects of

sediment and flow. Physical and biological components also

affect each other, so both geomorphological and ecological

process research require knowledge of the other discipline.

Watershed Case Studies

Most research that documents CWEs has been in the form of

case studies. An impact is observed, the situation is described, and the causes are investigated. This procedure defines the issues and types of mechanisms involved, and has

traditionally been the first step in exploring a new type of

problem. Quantitative results of case studies are not generalizable, but the processes and effects they describe often are.

Watershed case studies usually include reconstruction of

original watershed conditions and documentation of changes.

Changes are evaluated either by monitoring ongoing responses

or by finding evidence of intermediate conditions, and aerial

photographs often provide an important information source.

Effects of particular land-use activities are estimated or

measured, and these provide a link between land use and

watershed response. Without this information, association

between land use and watershed response is only

circumstantial.

G.K. Gilbert (1917) provided the first comprehensive

analysis of CWEs in a case study of hydraulic mining impacts

in the Sacramento River watershed. Gilbert’s circumstantial

linkages between land use and impacts were strengthened by

field measurements and by his understanding of component

processes. Subsequent CWE studies followed a similar pattern

by combining historical reconstructions with process analyses.

Collins and Dunne (1989), for example, used this approach

to evaluate cumulative changes from gravel mining in rivers

of the Olympic Peninsula, Washington.

Watershed experiments may also be case studies. Basin

attributes are monitored to describe the system, changes are

made, basin response is monitored, and pre- and post-change

records are compared to define the magnitude of the response.

This method ensures that attributes such as bedrock and

topography are identical for both states, but it cannot account

for differences in temporal variables. Occurrence of a major

storm during one study period, for example, may either alter

the watershed’s response to manipulation or introduce changes
18 USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993.

that cannot be distinguished from treatment effects. Similarly,

changes caused by a fortuitous natural landslide may

overshadow the effects of land use. Case studies can describe

a particular basin’s response to conditions it has experienced,

but results cannot be generalized unless the response mechanism

is understood.

Paired- and Multi-Basin Studies

Many watershed experiments manipulate one basin and preserve a second as a control. This strategy accounts for temporal variations, but differences in basin characteristics may

affect the basins’ responses, and occurrence of a landslide or

other natural event in one basin confuses inter-basin comparisons. The effects of these complications can be estimated only if response mechanisms are understood. Some

paired-basin studies are descriptive rather than experimental

and compare conditions in existing pre- and post-disturbance watersheds.

Paired-basin studies produce site-specific results that

cannot be generalized without other information. It is difficult

to recognize and control for all potentially important variables

in these experiments, and linkages between imposed change

and observed response are often treated as a black box,

further hindering interpretation. Nevertheless, these studies

define the magnitudes of potential responses and often identify

the most significant problems for further study.

Multivariate statistics often can sort out the relative

importance of variables if multiple basins are compared, and

these results are more readily generalized. However, any

collection of sample watersheds includes many variables. If

these are to be evaluated adequately, then the sample set

must include the likely range of each variable, the variables

must be independent, and enough samples must be evaluated

to provide statistically sound results. Multi-basin comparisons

are also useful in settings where no undisturbed watershed is

available for paired-basin comparisons. In this case, multiple

basins are selected that represent a gradient of treatment

intensities. If the basins’ responses show a trend that correlates

with the treatment intensity, then an association between

treatment and response can be inferred even in the absence

of a standard “control” watershed.

Multiple-basin comparisons can identify associations

between watershed treatment and response and can reveal

patterns of occurrence, but they cannot demonstrate causes.

These studies may lead to hypotheses of why a particular

response occurs, but the evidence they provide in support of

those hypotheses is circumstantial. Callaham (1990) discussed

practical aspects of carrying out paired and multi-basin studies.

The System as Focus

Researchers have recently begun to study watersheds as

coherent systems. Tangible results from these studies have

so far been few, because appropriate methods are still being

developed. Some such methods have arisen from general

systems theory (Von Bertalanffy 1968), a field that studies

the properties of systems. According to systems theory, a

system’s behavior is determined by the interactions comprising it, so its response might be predicted from knowledge of its structure. Geomorphologists have translated geomorphological ideas into the vocabulary of systems theory

(e.g., Chorley and Kennedy 1971), and this provides a useful

way to describe interactions, but it has not yet produced

major advances in understanding watersheds.

Recent developments in chaos theory (Prigogine and

Stengers 1984) aid a watershed-level approach by classifying

and explaining styles of response that might be generated by

complex systems. Chaos theory has not yet been applied to

watershed studies, although geomorphologists occasionally

use its concepts to interpret study results.

The discipline of landscape ecology (Naveh and Lieberman

1984) is most widely recognized in Europe. Landscape ecology

views landscapes as collections of interdependent physical

and biological processes, so studies of landscape-scale

problems incorporate both geomorphological and ecological

components. This approach has occasionally been adopted

in the past (e.g., Hack and Goodlett 1960) and does not

represent a new field, but its formal recognition will encourage

interdisciplinary work and legitimize landscape-scale studies.

Problems are also being explored at the scale of watersheds.

Parsons (1982) defined basin-wide trends in hillslope

morphology, for example, and Benda and Dunne (1987)

found that debris-flow mobility could be predicted from

channel network geometry. Methods are being developed to

characterize watersheds using information gathered at a

process scale. For example, sediment budgeting provides a

framework quantifying rates of sediment production, transport,

and redeposition throughout a basin (Dietrich and others

1982). These values can be combined according to the

distribution of rate-controlling variables to estimate process

rates and distributions in the sampled area. Processes are

influenced by changes in land use and environmental

parameters, and sediment budgeting allows estimation of the

effects of such changes on basin response (Reid 1989, 1990).

Work is also being done to incorporate spatially explicit data

into sediment budgeting (e.g., Reid 1989), and this will

eventually enable sediment budgets to be used to model

landform evolution and spatially distributed responses to

environmental change.

Recent advances in computer software and remote sensing

also aid watershed-scale studies. Geographic Information

Systems (GIS) now provide an efficient way to organize,

manipulate, and evaluate spatial data. GIS is being used in

watershed simulation models and sediment budgeting to

produce spatially distributed results. New methods of remote

sensing and image analysis permit rapid mapping of watershed

attributes over large areas in a form that can be incorporated

into GIS databases. This combination allows rapid

characterization of watersheds for modeling.
USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993. 19

The ultimate goal of CWE research is to be able topredict the effects of multiple land-use activities. If the

magni-tude of a physical or biological change can be predicted,

then its potential economic, philosophical, and political

significance can be more easily assessed. CWEs have been

described and evaluated for many years, but under different

labels. This chapter examines published descriptions of CWEs,

CWE assessments, and methods of predicting CWEs.

Literature specific to CWEs is not extensive, but many

other studies examine the effects of land-use activities on

environmental characteristics, the response of watershed

function to environmental changes, and the impacts of altered

watershed function on resources and values. This work is

described in following chapters, and is the foundation for

most of the CWE research outlined in this chapter.

Types of Cumulative Watershed

Effects

Most watershed impacts can be evaluated as cumulative

effects because they occur in the context of multiple landuse activities. CWEs can be generated either at the site of

land-use disturbance or downstream.

On-Site and Off-Site CWEs

Environmental changes can cause on-site CWEs if they

persist long enough for sequential or progressive effects to

accumulate, or if changes generated elsewhere are transported

to a site and interact with on-site changes. Recognized onsite CWEs include impacts on soils, nutrient cycling, and

soil moisture recharge. These changes affect long-term

productivity, and can have major impacts on timber

management, range use, and agriculture.

Off-site CWEs assume many forms, but each occurs

because environmental change alters the production or

transport of watershed products (water, sediment, organic

material, chemicals, and heat). Off-site CWEs can occur

when processes are altered for long enough that changes can

accumulate through time, when responses from multiple

sites are transported to the same site, or when a transported

response interacts with an on-site change at another site.

Altered hydrology can modify the time distribution and

amount of runoff and thus change baseflows, peakflows, and

flow seasonality. These changes can then alter flood

frequencies; aggrade, incise, or widen channels; and change

the size distribution of stream-bed sediment. Changes in

sediment input can trigger similar responses through different

mechanisms. Altered flood frequencies and channel

morphology affect flood-plain land use, fisheries resources,

and navigation.

Changes in the input and transport of organic material are

important both because large organic debris affects channel

morphology, and because organic matter is a food source for

aquatic animals. Off-site impacts from altered basin chemistry

generally appear as water quality problems affecting aquatic

biota and water supplies. Changes in stream temperature are

important primarily because of their effects on biota.

Interactions That Generate CWEs

A single type of land-use activity can generate CWEs if it

occurs repeatedly or persistently at a site or if it occurs over a

progressively larger area. Different land-use activities can

produce CWEs if they occur sequentially or if they coexist

within a watershed. Impacts are cumulative when activities

reinforce the same watershed response, when multiple responses disturb the same resource, when one response provokes another, or when responses interact to produce another.

Repeated, progressive, sequential, or coexisting activities

can cause CWEs if the activities cause the same type of

environmental change. For example, logging, road use, and

grazing all compact soil, and the total area compacted in a

basin may include contributions from each activity. Because

compaction can be persistent, sequential performance of

these activities at a site can cause on-site cumulative

compaction. In these cases, both activities (A and B in the

following examples) cause the same type of change to an

environmental parameter (Y), and the resulting CWEs (Z)

can be evaluated by understanding the extent of the change

in Y. Activities A and B contribute to the same type of

impact whether acting alone or together (fig. 3a).

CWEs can also occur when different activities affect the

same resource by different mechanisms but contribute to the

same response; these are called complementary effects.

Increased water pollution and increased fishing pressure

both decrease salmon populations, so the overall population

decrease is a cumulative result of both mechanisms. Similarly,

flooding might be aggravated both by increased quickflow

runoff and by channel diversion. Complementarity implies

that activity A causes Y1, B causes Y2, and both Y1 and Y2

contribute to cumulative impact Z. Either activity acting

alone also contributes to Z (fig. 3b).

Both mechanisms described above involve changes that

can be recognized by examining the effects of each land use

independently. CWEs can also result from responses that interact

in ways not predictable from analyses of individual effects.

CWEs can be generated by cascading influences, where one

Chapter 4

Assessment of Cumulative Watershed Effects
20 USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993.

type of use influences a second to provoke a CWE. For example,

lack of shade after clearcutting might concentrate cattle in

riparian buffer zones, thus increasing bank erosion rates; or

construction of logging roads might improve access to an area

and increase recreational impacts. In these cases, activity A

modifies activity B to cause impact Z, and Z does not necessarily

occur unless both A and B are present (fig. 3c).

In other cases, impacts result from interactions between

different environmental changes, and influences are

interdependent in causing the impact. For example, toxic

compounds may form when two introduced chemicals mix.

CWEs can also occur if one type of change alters the

importance of another. For example, hatchery-related

introduction of Bacterial Kidney Disease to wild fish

populations may decrease the ability of wild stocks to survive

environmental change. In these cases, response Z happens

when both activities A and B occur. Either A or B acting

alone does not necessarily provoke Z (fig. 3d).

Retrospective Assessments of

Cumulative Watershed Effects

Most published CWE studies evaluate the reasons for observed problems. The impact has already occurred and is

well defined, so the scope of the problem and the possible

controlling elements are known. This problem is less complex than that of predicting possible CWEs from unspecified

land-use combinations. Retrospective studies evaluate scenarios that are already complete, and this is more attractive

to researchers than prognoses of future change. Explanations of an existing condition are immediately testable; predictions of the distant future are not.

Examples of Retrospective Analyses

Four types of land-use combinations (repeated, progressive,

sequential, and coexisting activities) can generate CWEs by

four types of mechanisms (combinations of the same, complementary, cascading, or interdependent influences). These

combinations and mechanisms form a 4x4 matrix that is a

convenient framework for describing CWE studies (table 2).

Same-Influence Effects

If an environmental parameter is altered in the same way by

repeated or multiple activities, those activities all contribute

to the watershed’s response, and response is a cumulative

effect of the activities. The change may impact a resource

directly or alter watershed processes and cause an indirect

impact, but evaluation is relatively straightforward in either

case. If the general relationship between an environmental

change and an impact is known, then the effect of a land-use

activity can be estimated by evaluating its effect on environFigure 3—Combinations of influences that can generate CWEs.

Types are described in the text. A and B are activities, Y is an

environmental parameter, and Z is an impact.

mental parameters. These effects often are easily measured.

Repeated or prolonged activities can augment an impact

by repeatedly perturbing the same environmental parameters.

Collins and Dunne (1989) examined air photos, gauging

records, and mining records to demonstrate that streambeds

downstream of mined gravel bars are incising at rates that

account for the cumulative gravel deficit left by mining.

Sediment budgets suggest that incision will continue if gravel

mining continues at present rates. Froehlich (1979) and Geist

and others (1989) found that soils required long periods after

logging to recover from compaction, and inferred that recovery

may not be complete before the next logging cycle. Compaction

may thus accumulate through multiple cutting cycles.

In the case of gravel mining, the activity occurred annually,

so demonstration of a CWE required evidence of continued

change. In contrast, logging was to be repeated at long intervals,

so the recovery period for compaction had to be shown to be

longer than the logging rotation. Each study used surrogate

evidence to measure response trends through time. Collins

and Dunne (1989) used historical air photos, while Froehlich

(1979) and Geist and others (1989) compared soil densities

on sites logged at different times. Monitoring of changes

through time would have provided the same information, but

studies would have required a half-century of monitoring.

Both of these problems were approached at a process scale,

and researchers focused on mechanisms driving the changes.

Activities that progress through an area progressively

broaden their effects on environmental parameters. CWEs

from a progressive activity can be demonstrated by comparing

TYPE OF CUMULATIVE

EFFECT

EFFECTS OF

INFLUENCES

ACTING ALONE

EFFECT OF COMBINED

INFLUENCES

EXAMPLE

B. Complemntary

effects

A. Same-influence

effects

C. Cascading

effects

D. Interdependent

effects

A B

Y Y Y

Z Z Z

A B logging

compaction

altered peak flow

road

traffic

logging

compaction diverted

channels

altered peak flow

road

building

road

building

recreational

use

increased chemical

& nutrient inputs

impaired

water quality

industrialization

input of

chemical 1

input of

chemical 2

formation of chemical 3

mining

increased

access

A B

Y1 Y2 Y1 Y2

Y1 Y2 Y1 Y2

Y1 Y2

Z Z Z

Z

Z

A B

A B A B

A B
USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993. 21 Table 2—Examples of studies diagnosing cumulative effects. Brackets indicate hypothetical or undocumented examples. Activity type Same influence Complementary influences Cascading influence Interdependent influences Activity repeated Repeated gravel mining lowers the channel Channel form changes because woody [Traffic degrades ORV track and alters runoff Acid and arsenic from mining combine or progresses bed (Collins and Dunne 1989) debris is destabilized by logging and future over widening area] (Mok and Wai 1989) through time inputs will be low (Hogan 1987) Repeated logging progressively compacts soil (Geist and others 1989, Froehlich 1979) Decreased flows and sediment alter bed (Pemberton 1976) Activity Amount of fine sediments in redds increases Streams incise when overflow is restricted [Erosion severity increases disproportionately progressive with road density (Cederholm and Reid and flow increases (Nanson and Young as logging progressively enters steeper areas] through area 1987) 1981) Peak flows increase as logged area increases Increased sediment and flows from logging (Dietterick and Lynch 1989) alter streams (Lyons and Beschta 1983) Channel change increases with mining (Touysinhthiphonexay and Gardner 1984) Urban runoff and conveyance enlarge channels (Hammer 1972) Channel change increases with logging (Ryan and Grant 1991) Sequential Logging, burning and herbicide increase Fire risk increases from grazing after [New Melones Dam destroys river run, Grazing and burning alter vegetation activities nitrogen loss (Feller 1989) logging (Zimmerman and Neuenschwander increases crowding nearby] (Collins 1987) 1984) Logging and leaching deplete magnesium [Construction of logging roads improves access Vegetation change makes forest supply (Roberts and others 1989) and leads to increased recreational impact] susceptible to smog (Mazurski 1986) Conservation after erosion forms gullies (Costa 1975) Coexisting Cropping and grazing increase Fishing and habitat loss reduce fish [Urbanization is accompanied by increased Disease introduced from hatcheries activities sedimentation (Reid 1990) populations (Salo and Cederholm 1981) recreational demands, increasing recreational decreases ability of fish to survive impact] environmental stress (J. Nielsen, personal communication) Reduced flow and high sediment aggrade U.S. demand for beef encourages conversion of a stream (Petts 1984) rainforest to range (Miller and others 1991) Aggradation and channelization increase Socio-economic conditions lead to grazing floods (Gilbert 1917) practices that accelerate Tanzanian soil erosion (Reid 1990) Wetland conversion lowers water quality (Johnston and others 1988)
22 USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993.

impact magnitudes at different stages in its progress. Dietterick

and Lynch (1989) measured changes in peak discharges as

clearcutting progressed through watersheds. Incremental

increases were greater than those predicted from the area

cut, implying a synergistic interaction.

Cederholm and Reid (1987) described a series of studies

that associated spawning gravel siltation with percentage of

a basin in roads. The relationship was developed from

measurements of fine sediments in gravels in many watersheds

with different road densities. However, statistical association

does not demonstrate causality, and a sediment budget was

used to identify logging roads as a dominant sediment source.

Results were translated to an impact on fisheries using

laboratory and field measurements of survival-to-emergence

as a function of siltation level.

CWEs from sequential activities are demonstrated by

measuring impact levels after each activity. Feller (1989)

measured nitrogen loss by streamflow in catchments that

had been clearcut, clearcut and burned, and clearcut with a

follow-up herbicide application. He found that herbicide use

mobilizes more nitrogen than slashburning, and that addition

of either of these treatments produces more than logging

alone. Measurements were continued over a 5-year period to

assess recovery rates. Causes could not be demonstrated

because only outflow was measured, but they could be inferred

because earlier process-based studies examined nutrient loss

from each of these activities. Without this background of

research, results from a study of this type would be difficult

to interpret or generalize.

CWEs are relatively easy to evaluate at a process scale

when coexisting activities cause the same type of

environmental change: each environmental change can be

assessed independently and the changes summed to estimate

their combined effect. Reid (1990) used this approach to

estimate sediment yield from multiple land-use activities in

Tanzania. Soil-loss rates on each type of surface were

estimated from published relations and field measurements,

and loss rates were summed according to the distribution of

land use to estimate total input from the combination of

land-use activities.

Complementary Influences

Complementary effects occur when land-use activities contribute to the same result through different mechanisms, and

their evaluation requires that the link between altered environmental parameters and watershed response be established

for each activity. As a result, few evaluations of complementary effects are “complete”: rarely does a study demonstrate

every link in an association, and most use results of earlier

studies to infer some linkages.

Hogan (1987) evaluated the effect of logging on the

frequency of debris jams on the Queen Charlotte Islands,

British Columbia. His measurements and estimates of debris

input, storage, and output showed that debris is less stable

after logging than before. Inputs of logging slash and decreased

channel stability both contributed to this change, and future

logging cycles will prevent regrowth of large trees and ensure

that organic debris inputs continue to be mobile. This study

measured the immediate complementary effects of logging

and inferred future effects as logging is repeated in the

future. In contrast, Pemberton (1975) provided a complete

assessment of one type of complementary effect from the

operation of Glen Canyon Dam. The dam decreased both

downstream sediment load and peak discharge, and both

effects contributed to channel-bed coarsening. Pemberton

used bedload equations that take both influences into account

to successfully predict the observed changes.

Progressive activities that cause complementary effects

are widely studied, but the component influences are usually

not disentangled. Touysinhthiphonexay and Gardner (1984),

for example, surveyed channels in 29 watersheds that had

different proportions of their area strip-mined and defined a

relationship between channel destabilization and proportion

mined. Channel enlargement was attributed to the combined

effects of increased peak flows and increased sediment input,

because earlier studies had shown that both of these changes

can alter channel morphology. Hammer (1972) used a similar

approach to explore the effects of progressive urbanization

on channel form. Hammer attributed changes to increased

peak flows from altered drainage networks and increased

runoff but did not evaluate the relative importance of these

causes. Ryan and Grant (1991) measured the disruption of

riparian vegetation on sequential aerial photographs in basins

that had been logged to different extents and showed a

correlation between riparian area disturbed and basin area

logged. Lyons and Beschta (1983) associated channel changes

in the Middle Fork Willamette River with increased sediment

loads and peak flows caused by timber management and

roads. Progressive changes were measured using sequential

aerial photographs and stream gauge records, and the effects

of altered sediment and flow were inferred from earlier

process studies. Nanson and Young (1981) compared

urbanized and undisturbed channels in southeastern Australia

to demonstrate enlargement associated with urbanization.

They attributed changes to increased peak flows from the

growing impervious area and from levee construction that

prevented storage of water on floodplains. The progressive

nature of the change was inferred from the effects of increasing

urbanization on the component mechanisms.

Sequential uses can also produce complementary effects.

Zimmerman and Neuenschwander (1984) compared vegetation

in grazed and exclosed selection cuts to measure the effects

of grazing on fuel loading after logging. Cattle removed

cool-burning, herbaceous fuels while increasing the volume

of downed wood, a hot-burning fuel, and thus increased fire

hazard and decreased the effectiveness of controlled burns.

These changes compounded the effects of logging, which

had increased fuel loading and provided routes for ground

fires to reach the forest canopy by promoting the growth of

spindly, sub-canopy trees. Roberts and others (1989) associated

the decline of some European forests with magnesium

deficiencies produced by the sequential effects of (1) export
USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993. 23

of nutrients in logs, (2) leaching from acid precipitation, and

(3) increased magnesium demand as second-growth forests

reached a growth stage requiring large nutrient inputs.

Salo and Cederholm (1981) presented a clear analysis of

complementary influences in an evaluation of the effects of

coexisting land-use activities on coho populations in the

Clearwater River, Washington. The authors used experimental

results, field measurements, and fishery records to estimate

the effects of road-related redd siltation, logging-related

habitat modification, and fishing pressure. Each of these

land uses increases fish mortality through a different

mechanism, so incremental impacts could be summed to

estimate their combined effect. Gilbert (1917) described

complementary effects from coexisting land uses in the

Sacramento River basin. Construction of flood-control levees

increased flood peaks by reducing floodplain storage, while

upland hydraulic mining increased sediment input and

aggraded the channels. Both effects increased downstream

flood risk. Gilbert inferred causes for observed changes

using his understanding of river mechanics and results of a

sediment budget he constructed for the watershed. Petts

(1984) described channel aggradation from the combined

effects of reduced sediment transport capacity below a dam

and increased sediment input from silvicultural site preparation

on a tributary. Aggradation occurred only at the mouth of the

affected tributary, where the altered flow regime could not

remove the additional sediment. Johnston and others (1988)

used a geographic information system to evaluate the

combined effects of wetland loss and increased pollution on

water quality near Minneapolis.

Because of the complexity of the interactions involved,

most studies of complementary influences draw inferences

from prior studies or basic principles, and thus produce

qualitative results more readily than quantitative ones. Effects

may be additive if different mechanisms do not interact, but

many changes do affect each other. For example, increases

in both sediment production and flood peaks can destabilize

channels, and increased peak flows also usually increase

sediment input, so the combined effect cannot be predicted

simply by adding the expected response to each influence. In

addition, responses may not be linear. Touysinhthiphonexay

and Gardner (1984) found that combined increases in peak

flows and sediment input gradually enlarge channels up to a

certain impact level, but then a major shift in channel stability

occurs. The existence of a threshold cannot be predicted by

adding incremental changes from peak discharges to those

from sediment loading unless the mechanisms producing the

threshold response are understood.

Cascading Influences

CWEs also occur when one activity influences another to

provoke a watershed response. Miller and others (1991) described such a case in the Brazilian rainforest, where deforestation contributes to loss of soil productivity. In this case,

deforestation is in part caused by conversion of forest to

rangeland, and an underlying cause for the conversion is the

demand for cheap beef in North America. Analysis of the

magnitude of future impacts from deforestation would thus

require forecasts of beef demand in international markets.

Land-use impacts are usually evaluated in terms of their

immediate cause, so the nature of the interaction between the

land-use activities is rarely examined even in cases where an

impact results from a cascading influence. Studies of cascading influences are thus uncommon, but hypothetical examples

are described in table 2 to illustrate potential influences.

Repeated activities or those that continue through time

may produce a cascading influence if early use influences

how later use is carried out. For example, repeated use of a

muddy trail or off-road vehicle track eventually produces a

mire and forces later traffic to broaden the impacted area by

forming parallel tracks. An activity that progresses through

an area can also modify its impact as it proceeds. Many types

of land use begin on optimal sites and spread to more sensitive

ones. Disproportionate responses from sensitive sites then

cause the overall impact to increase more rapidly than the

proportion of area affected.

An activity can also influence subsequent use if it alters

the environmental parameters that control the second use.

For example, construction of New Melones Dam destroyed

a popular river run as demand for whitewater recreation was

increasing in central California. Impact intensity on the

remaining whitewater runs in the area increased due to the

decreased resource availability. Road construction also

frequently produces cascading impacts: roads constructed

for timber management open areas to recreational access

and lead to increased recreational impact.

Coexisting activities can influence one another in a similar

way. For example, recreational demand increases as

urbanization proceeds, increasing all types of recreational

impacts in nearby wildlands. Similarly, Reid (1990) attributed

high soil erosion rates on Tanzanian rangelands ultimately

to the socio-economic conditions in the area that inadvertently

promote high stocking rates.

In each of these cases, observed impacts could be evaluated

simply as results of the activity that directly caused them:

campsite degradation could be analyzed as a function of

recreational pressure without acknowledging the role of dam

construction or increased urbanization, and timbermanagement impacts are usually expressed as a per-unit-area

average rather than accounting for changes in site sensitivity

as logging progresses. Prediction of future impacts, however,

would require a full understanding of the cascading influences.

Interdependent Influences

Influences are interdependent when changes interact to create

an impact or when one change alters the importance of another.

An interaction between introduced chemicals is a clear example

of an interdependent influence. Mok and Wai (1989), for example, described chemical changes caused by an Idaho mine:

mine dumps leached arsenic into streams while mine drainage

increased stream acidity, and acidic conditions decreased the

impact of arsenic by altering it to a less mobile form.
24 USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993.

Mazurski (1986) inferred interdependent sequential

influences to explain forest dieback in the Sudetes Mountains

of Poland. Five centuries of logging depleted hardwoods in

montane forests and replaced them with conifers, which are

more sensitive to acidic conditions. Industrial smog then

acidified the environment enough to injure conifers. Costa

(1975) found that high sediment loads in streams of the

southeastern United States reflect the sequence of land use

there: sediment eroded during two centuries of intensive

agriculture accumulated in valleys, but channels began to

incise and export the sediment when declining agricultural

use decreased incoming sediment loads. In both of these

cases, neither of the most recent changes operating alone

would have produced the present impacts. Collins (1987)

documented another example of interdependent sequential

influences by evaluating community composition on natural,

grazed, burned, and grazed and burned prairie plots. Grazing

was found to increase species diversity on burned plots, while

burning reduced species diversity on ungrazed plots. Interaction

between disturbances produced the highest diversity.

Coexisting land-use activities also generate interdependent

effects. For example, Bacterial Kidney Disease has been introduced

to wild anadromous fish populations in California from hatchery

stock. Although the disease does not kill the fish directly, it makes

them more sensitive to environmental stress, and so less able to

survive the types of environmental change induced by other landuse activities (J. Nielsen, USDA Forest Service, Pacific Southwest

Research Station, personal communication).

Interdependent influences must be evaluated together to

assess the nature of their interdependency. The processes of

interaction often control the nature and occurrence of

watershed or population response.

Patterns of Retrospective

Understanding

Most research approaches described in Chapter 3 have been

applied to CWE studies (table 3). In addition, some studies

used published equations to calculate process rates, others

used existing evidence for past conditions (e.g., air photos)

or surveyed sites of a variety of ages, and several predicted

the nature of effects using a basic understanding of how the

component processes work. No method dominated among

the studies surveyed, and most used multiple approaches.

Most examples either used process studies to define impacts

or used the results of earlier process studies to infer the causes

of impacts (listed as R in table 3), and the importance of

process studies increased with the complexity of interactions

causing the impacts. Key research for interpretation and

assessment of CWEs thus is not limited to studies that focus

Table 3—Approaches used by studies described in table 2 to diagnose CWEs. Studies are listed in the same order as in

table 2, and symbols are explained below.

Same Complementary Cascading Interdependent

influence influences influences influences

ADEMQRST/BCHMP ADEMQRST/BCHMP ADEMQRST/BCHMP ADEMQRST/BCHMP

Activity A.EM..S./.C..P .D...R../B...P .....R../..H.. .DE...S./.C..P

 repeated A....RS./...MP ...M..../.C..P

A....R../...MP

Activity ..E..RS./...MP .D....S./...M. .....R../..H..

 progressive ..E.Q..T/B.... AD....../.C...

....Q.S./...M.

A...Q.S./...M.

AD...RS./...MP

Sequential ..E....T/B.... AD...R../B...P .....R../..H.. ..E...../...MP

 activities .....R../..H.P .....R../..H.P

AD....S./.C..P

Coexisting A..M..S./.C..P ..E..RS./..H.P .....R../..H.. .....R../..H..

 activities .D...R../.C... .....R../..H..

AD.M.R../.C..P .D...R../.C...

....Q.S./...M.

Approach or method Scale

A reconstructs past using air photos or B paired basins

records, or by sampling age distribution C case study

D based on descriptive measurements H hypothetical or general case

E experimental M multiple basins

M modeling or calculated results P process study

Q based on statistical analysis

R reasoned from basic principles

S survey of multiple sites

T long-term temporal monitoring study
USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993. 25

specifically on CWEs, but also includes basic research on

geomorphological, hydrological, and ecological processes; many

of these background studies are described in Chapters 5 and 6.

Most studies that compared watershed-scale responses

sampled more than two basins so that the cumulative nature

of a response could be demonstrated. Both the studies that

employed temporal monitoring compared paired sites, but

paired basins were otherwise rarely used.

Influences are more easily identified than quantified. Many

of the cited examples produced only qualitative results because

they depended on inference or on general process information,

and many were intended merely to indicate how an effect

occurred. Even when results were quantified, the relative

contributions of particular influences often were not.

It is easiest to quantify impacts from coexisting and

progressive land uses that have the same effect on

environmental parameters (table 2, column 1, rows 2 and 4),

because individual effects can often be summed and their

combined effect evaluated. This approach may also be applied

to complementary effects (table 2, column 2) if influences do

not interact with one another. In contrast, responses to repeated

and sequential use often change as an impact progresses. For

example, soil compaction increases with increased traffic, but

most compaction occurs with the first few passes. Compaction

from 100 passes thus cannot be calculated as 100 times the

compaction from a single pass. The effects of cascading and

interdependent influences can be evaluated and interpreted

only if the interactions causing them are understood, so

assessment requires knowledge of the processes involved.

Prediction of Cumulative

Watershed Effects

Research studies rarely focus on predicting future effects because predictions are not testable in the framework of the

study, and predictions described in published research usually

are by-products of present effects. However, many predictive

CWE assessments are carried out by nonresearchers in support of particular land-use plans. Federal and State agencies

are required to predict CWEs for planned projects, as are

many private developers. These reports are often in the form

of environmental impact statements, environmental assessments, and timber harvest plans, and they do not constitute

research. Other predictive studies include proprietary consulting reports that detail anticipated impacts from planned projects.

Examples of Predictive Studies

The few predictive studies available can be organized in the

same way as retrospective studies (table 4). None of the described studies evaluates cascading or interdependent effects.

Same-Influence Effects

Interactions between processes rarely need to be evaluated

when impacts are caused by alteration of a single type of

parameter. As described earlier, however, the response

Table 4—Examples of predictive cumulative effect studies

Activity Type Same Influence Complementary Influences

Activity Effects of soil conservation on productivity Effects of dam construction on

repeated or (Benson and others 1989) delta erosion (Pickup 1980)

progressive

through time Effects of diverted flows on channel

Effects of erosion on productivity form (Jackson and Van Haveren

(Christensen and McElyea 1988) 1987)

Effects of repeated logging on productivity Effects of cloud seeding on channel

 (Waide and Swank 1976) form (Rango 1970)

Effect of increased flow on fish

production (Johnson and Adams 1988)

Activity Effects of tidal marsh reclamation Effects of urbanization on local

progresses on aggradation of bay-mouth bar climate (Lein 1989)

through area (Gilbert 1917)

Effects of ozone depletion on

agricultural production (Adams and Rowe 1990)

Effects of logging pattern on

channel stability (Ziemer and others 1991)

Sequential [Predictions of the effect of

activities planned mitigation works]

Coexisting Effects of land uses on estuary Effects of riparian vegetation and

activities sedimentation (Dickert and Tuttle 1985) land use on fish production (Theurer and others

1985)

Effects of pesticides and sediment

on fish production (Braden and

others 1989)
26 USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993.

generated by an incremental environmental change often

changes as an impact develops.

Many recent studies have evaluated the cumulative impact

of progressive erosion on crop yields. Benson and others

(1989) used a published model (EPIC: Erosion Productivity

Impact Calculator) to predict the long-term effects of erosion

on soil productivity, while Christensen and McElyea (1988)

developed a procedure for calculating marginal changes in

productivity using field measurements. In these cases,

cropping is repeated through time, and cumulative effects

are on-site. Waide and Swank (1976) modeled the cumulative

impact of multiple logging cycles on nitrogen budgets and

forest productivity in a southeastern forest.

Gilbert (1917) predicted the effects of progressive marsh

reclamation on shipping through the Golden Gate by

calculating the resulting change in volume of the tidal prism,

and thus of the transport capacity across the bay-mouth bar.

These calculations were based on fundamental research on

sediment transport mechanics.

No published examples predicting the effects of sequential

uses were found, but these are commonly carried out during

planning of rehabilitation and mitigation projects. The effects

of current practices are usually estimated with reference to

undisturbed conditions, and changes expected from the

planned work are then predicted. Dickert and Tuttle (1985)

calculated percentage of bare ground for coexisting land

uses in the Elkhorn Slough watershed and used these as

erosion indices to aid in planning mitigation work to reduce

estuary sedimentation.

All studies in this category employed process-based

analyses. Prediction implies that conditions or results change

through time, and processes are often altered as they progress.

Where processes are not altered, results are calculated by

summing the process rates produced by each activity (e.g.,

Gilbert 1917). In contrast, where progress of a change modifies

its rate, the marginal increase in impact due to an incremental

change in driving parameter must be evaluated either

empirically (e.g., Christensen and McElyea 1988) or through

a basic understanding of the processes involved (e.g., Benson

and others 1989).

Complementary Influences

Most predictive studies concern land-use activities that alter

multiple environmental parameters and watershed processes,

and changes may be modified by interactions with one another. These changes can cause complementary impacts if

they affect a target resource through different mechanisms.

Interactions must be evaluated if complementary impacts are

to be predicted, but interactions are often complex, and predictions of complementary effects are usually qualitative.

Pickup (1980) used a flow-routing model to predict the

flow regime resulting from a proposed dam in New Guinea.

He then used a basic understanding of river mechanics to

assess qualitatively the river’s long-term response to the new

regime and to the accompanying decrease in sediment input.

Jackson and Van Haveren (1987) addressed a similar

problem: they needed to predict the effects of a proposed

Alaskan flow diversion on channel morphology. The

researchers used a regional relation between flow

characteristics and channel geometry to predict the channel

form that would be in equilibrium with the proposed flows.

Rango (1970) used a similar approach to estimate the effects

of cloud seeding on channel geometry in the western United

States. In both cases, flow was to be altered without a major

change in sediment input, but the altered flows would modify

bank erosion rates and indirectly change sediment input. The

studies implicitly accounted for these adjustments by using

statistical descriptions of equilibrium channel forms.

Johnson and Adams (1988) also evaluated the effects of

changing flow regime, but they focused on the impact to a

particular resource, a recreational steelhead fishery. They

used a published stock-recruitment model to estimate the

number of returning adults for a given smolt production rate,

and used measurements of juvenile survival as a function of

flow level to assess smolt production. Impact magnitudes

could then be estimated by calculating the number of returning

adults as a function of flow regime. A questionnaire that

measured anglers’ valuation of the resource was used to

translate impact magnitude into a dollar value. This approach

allowed both the biological magnitude of changes and their

economic significance to be estimated, but did not address the

existence value of steelhead to other sectors of the population.

In each of these cases, cumulative effects were expected

from continuing activities that alter channel flows. A onetime change in flow is similar to an isolated storm or drought

and usually has only a short-term effect, but a chronic alteration

cumulatively alters channel morphology and biological

communities to reflect a long-term change in hydraulic regime.

The cumulative impacts of activities that progress through

an area are the focus of many predictive studies. In this case,

too, influences are usually complementary and interacting.

Lein (1989) used an energy balance model to predict local

increases in average temperature of 0.16 °C from urbanization

in Ohio. The study accounted for the effects of altered

albedo, surface roughness, and evapotranspiration rates on

six types of land cover. Records of changes for 1969 to 1979

were used to calibrate the model and to identify land-use

changes that accompany urbanization.

Adams and Rowe (1990) predicted the economic impact

of stratospheric ozone depletion on agricultural production

in the United States. Ozone depletion reduces crop production

by increasing ultraviolet-B (UV-B) radiation and tropospheric

ozone, and altered yields were calculated from existing

information on the relations between pollutant inputs and

ozone depletion, ozone depletion and UV-B penetration,

ozone depletion and tropospheric chemistry, UV-B and crop

yield, and tropospheric ozone and crop yield. Finally, an

economic model used yield estimates to predict annual losses

of $1.3 to $2.5 billion for a 15 percent decrease in stratospheric

ozone. The researchers emphasized that results are preliminary

estimates, and that accurate prediction requires research to

better define the relationships employed by the model.
USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993. 27

Ziemer and others (1991) combined equations describing

landslide probabilities, sediment transport, scour depth, and

channel morphology to construct a model that predicts the

effects of logging patterns on channel stability. The model

was presented primarily as a heuristic tool for indicating

qualitative trends.

Theurer and others (1985) predicted the effect of altered

coexisting land uses on fish. The Tucannon River basin in

Washington includes grazed and cropped land, and riparian

revegetation is planned for salmon-producing reaches to

mitigate temperature increases. The study used an existing

temperature model in conjunction with models of water and

sediment yield (Garbrecht and DeCoursey 1986), sediment

intrusion into gravels, dissolved oxygen, and juvenile survival

to compare the effects of revegetation options. In Michigan,

Braden and others (1989) modeled the combined effects of

agricultural erosion and pesticides on fish populations by

using published chemical and sediment transport models and

habitat suitability indices.

Complementary effects usually involve interactions between

processes that modify each others’ rates. Statistical relations

between driving variables and responses can implicitly account

for consistent interactions, and studies that use regional

hydraulic geometries to predict the effects of altered flow on

channel geometry employ this approach (e.g., Jackson and

Van Haveren 1987, Rango 1970). More complex interactions

are better described using process-based computer models,

and all of the cited examples that evaluated progressive and

coexisting activities employed models.

Cascading and Interdependent Influences

No studies were found that predicted the effects of cascading

or interdependent influences. Some of the previously described studies clearly reflect cascading influences: agricultural development and urbanization usually are the reasons

that dams are built, clouds seeded, and flows diverted. However, each of these immediate causes was evaluated without

reference to the underlying land-use change that provoked

the activity. This approach is adequate for evaluating single

projects, but a full prediction of impact duration and future

changes would have to take into account the patterns of

change in the driving land use. For example, the cumulative

effects of a flow diversion designed to service a short-term

mining effort are quite different from those of a diversion

that supplies water to a rapidly growing city. Prediction of

CWEs from cascading influences will require coupling of

socio-economic models with physical and biological models. The socio-economic models would predict demand trends

in the underlying land use, while physical and biological

models would assess the effects of activities implemented to

satisfy the underlying demands.

Prediction of interdependent effects requires a clear

understanding of the underlying processes. Except in the

case of chemical interactions, interdependent effects are

rarely recognized.

Patterns of Predictive

Understanding

Published predictive studies addressed a single type of impact or several closely related impacts. No studies were

found that provided a general overview of possible impacts,

although this task is often carried out in a planning context.

Each study isolated particular processes or mechanisms of

change and evaluated impacts in the context of those processes, and none depended on analogy to paired-watershed

results. This is probably because the conditions driving the

change were unprecedented (e.g., ozone depletion), or the

unique nature of the setting prevented a suitable analog from

being found.

Studies were more likely to use computer modeling to

evaluate process interactions as the complexity of interactions

increased. Modeling results were usually used as indicators

of qualitative response and expected orders of magnitude,

and quantitative results were rarely accepted without

qualification. Most authors stressed that results were

preliminary and recommended additional basic research to

improve the models.

In contrast to retrospective studies, many predictions were

carried from the physical system to the biological because

the studies were often motivated by concern for a particular

biological resource. In other cases, the nature of physical

changes was predicted, but biological interpretation was left

to those interested in the biological resources. This approach

to biological impacts is a common one among physical

scientists for several reasons: biological responses are often

complex; interpretation usually requires a biologist; and a

physical change can influence many biological elements.

General Methods for Predicting

Cumulative Watershed Effects

Because of the importance of the CWE problem and the

mandate to predict CWEs, several researchers and land managers have developed methods for assessing potential CWEs.

These procedures are usually designed for a particular land

use in a particular area, and they predict responses of a

particular target resource. Many produce disturbance indices that can be used to compare sites or management alternatives, and most are in the form of a series of steps that can be

used reproducibly by resource professionals and technicians.

Although the methods’ limitations are recognized by their

developers, demands for ready-to-use estimators often propel

a model into general use, regardless of whether its assumptions

are appropriate at other project sites or whether it has been

independently tested or validated. Model results are occasionally

interpreted as a measure of likelihood for all possible impacts,

rather than for those which the model was developed to assess.
28 USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993.

Examples of General Predictive

Methods

The most common CWE evaluation procedures used in California are reviewed here, as are several models developed

elsewhere. In addition, a few procedures have been published as general tools for evaluating particular aspects of

CWEs, and these, too, are reviewed. Methods are compared

in table 5.

Equivalent Clearcut Area (ECA)

One of the earliest CWE analysis procedures was developed

by the USDA Forest Service for use in northern Idaho and

Montana (USFS 1974, Galbraith 1975). The primary impact

of concern was channel disruption, and this was assumed to

be caused primarily by increased peak flows from reduced

transpiration due to logging. Channel disruption was assumed to be an index of impacts to many beneficial uses, so

specific impacts were not considered.

Application of the model first requires calibration for an

area. The extent to which each management activity increases

water yield is determined as a function of vegetation type,

elevation, and age of the activity. Although these relationships

could be defined for many land uses, only those related to

timber management are usually included. Values for each

land type and use category are then compared to values for a

clearcut to calculate the area of clearcut that would produce

the same change, and this is used to calculate the equivalent

clearcut area (ECA) coefficients for the category. The amount

of monitoring data required for full calibration of model

coefficients is usually prohibitive, so professional judgment

is often used to define ECA coefficients.

Once the model is calibrated, application to particular

sites requires measurement of the area of each land-use

activity in each elevation zone and vegetation type. Areas

are multiplied by ECA coefficients and summed to calculate

total change in water yield, and altered water yield is assumed

to be proportional to altered peak flows. Allowable thresholds

for flow modifications are specified by law in northern

Idaho, and calculated values are compared to the mandated

thresholds. Allowable increases may be modified according

to the perceived stability of channels in an area.

The ECA model is not presented as a complete CWE analysis.

Provisions are not made to evaluate the effects of other types of

land use; other mechanisms of channel destabilization or peakflow increase are not analyzed; other types of environmental

changes are not considered; and specific impacts are not addressed.

In effect, the estimated increase in water yield is used as an

index of potential impact rather than as a predictor of impacts.

Because ECAs are calculated for a particular time, they

do not account for past impacts that might interact with

conditions at the evaluation time. Thus, the persistent effects

of old landslides are not accounted for in an ECA analysis.

Potential impact is assumed to be proportional to a year’s

transgression, and the recovery period for the impacted

resources is implicitly assumed to be the same as that for

water yield on a clearcut. This means that the model does not

apply to morphological changes that are cumulative through

time. Figure 4 illustrates this problem. In this case, the

driving variable (e.g., increased runoff, sediment input, or

similar) has a relatively quick recovery period, but the

impacted feature (e.g., channel width, volume of stored

sediment, smolt mortality, or similar) takes considerably

longer to recover from the effects of the temporary alteration

in the driving variable (fig. 4a). Even though the sequence of

land-use activities is carried out in such a way that the

driving variable has ample time to recover, the disturbance

frequency is too high to allow recovery of the impacted

feature between disturbances, and the impact accumulates

through time (fig. 4b). To assess a temporally cumulative

impact, recovery periods both of impacts and driving variables

would have to be considered.

A model can be applied to new sites only if its assumptions

are valid there. The ECA model assumes that (1) channel

disruption is caused by increased peak flows, (2) increased

peak flows are proportional to increased water yield, and (3)

increased water yield is proportional to area logged. If these

are valid for a particular area, then the model may be

appropriate, but assumptions must be tested carefully if the

model is to be applied with confidence. Several studies have

compared water-yield increases predicted by ECA with

measured changes. King (1989) showed a 44 percent

underestimate by ECA in basins smaller than those the model

Figure 4—Cumulative effect of differing recovery times for a driving

variable and an impact.

A. EFFECT OF ISOLATED ACTIVITY

response of driving variable

response of impacted feature

threshold of

concern (T.O.C.)

Response

Time

B. EFFECT OF REPEATED ACTIVITY

response of driving variable

cumulative response of impacted feature

T.O.C.

Response

Time
USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993. 29 Table 5—General procedures for predicting CWEs Equivalent Klock Watershed Equivalent R-1/R-4 CDF Questionnaire WRENSS Limiting Factor Rational Approach Clearcut Area Cumulative Effects Roaded Area Sediment-Fish Analysis (Grant) Analysis Model Model type empirical association weighted indices use intensity index empirical process-base checklist physical process sum impacts; process physical process Area applied northern Idaho, Mont. east Wash. Cascades Calif. Idaho Batholith California general western Wash., Oreg. west Oreg. Cascades Can be used with calibration weighting method not with calibration only if relations with redesign to suit yes only if relations are yes elsewhere? explained measured for locale local issues measured for region Tested? Belt 1980, King 1989 no no not independently no some modules in progress no Basin size third to fifth order < 4000 ha 200 - 20,000 ha type B,C streams 3rd, 4th order streams by site coho-bearing streams alluvial reaches Result water yield, peak flow index index rates and populations qualitative various process rates smolt production critical flow Impact vegetation change sheetwash, slides impacts increase with eroded sediment in all may be considered depends on module habitat change increased bedload mechanism alters water yield and introduce sediment and increasing intensity channels impact fish alters fish survival transport alters destabilizes channels destabilize channels of use and population channel Land use logging, roads logging, roads logging, roads logging, roads logging, roads logging, roads not specified not specified Impacts not specified not specified not specified fish water quality, fish, not specified coho smolt production not specified recreation, others Threshold yes, modified by yes yes; modified by site compared to natural no not applicable yes: pct of potential yes: channel-specific identified? channel stability and impact type smolt production transport threshold Calibration water yield increase slide survey, runoff impact intensity vs. relations between user must thoroughly depends on module relations between no required by habitat, practice, relative to roads index; pct. change for sediment, habitat, and understand processes fish density and elevation, region. uses relative to roads fish and impacts of region habitat parameters Data required use history, area of use history, area of use history, area of use history, use unit may use any much data required population and habitat measured channel for use each use unit each use use units, site factors areas, habitat and fish information source for most procedures inventories characteristics surveys Reproducible? yes; some judgment yes yes; some judgment; yes; some judgment diagnosis relies yes yes; some judgment yes varies by district on judgment Expertise: calibration research or expert research or expert research or expert research not applicable research or expert research not applicable operation low trained resource specialist resource specialists Reg. Prof. Forester resource specialist fisheries biologists expert Comments does not handle does not handle intended as initial based on extensive analysis procedure collection of from point of view of addresses sensitivity temporal CWEs temporal CWEs screening research in area not specified procedures impacted resource to change
30 USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993.

was designed for, and Belt (1980; quoted in King 1989)

found a 38 percent underestimate in appropriately sized basins.

The theoretical foundation for the ECA method is weak.

Logging is known to increase water yield by reducing

transpiration, but this increase occurs primarily during the

drier seasons and rarely affects the highest peak flows.

However, peak flows may be significantly increased by

logging in areas subject to rain-on-snow events, because

snow accumulates more and melts faster in cleared areas.

This is likely to have a more significant effect on channelmodifying peaks than altered transpiration. An index of clearcut

area may fortuitously address both mechanisms of change,

but numerical predictions are likely to be unfounded because

the underlying processes are different.

Klock Watershed Cumulative Effects Analysis

(KWCEA)

Klock (1985) adapted and combined elements from other

CWE assessment methods into an equation to analyze CWE

potential from timber management in the Washington Cascades. The impact of concern is again channel destabilization, but the driving mechanism is here assumed to be increased sediment input. The KWCEA equation combines

factors for local climate (the R-factor of the Universal Soil

Loss Equation, Wischmeier and Smith 1978), susceptibility

to surface erosion (the USLE K-factor adjusted by a disturbance index for logging practice and recovery through time),

a landslide occurrence factor (a local landslide frequency

adjusted by site recovery factors), a topographic factor that

incorporates gradient and distance from a stream, and a

hydrologic sensitivity factor that indexes increased evapotranspiration after logging. Model application thus requires

information on soil characteristics, topography, landslide

frequencies associated with logged sites and roads, land-use

history, and areas of land-use types. Index values are calculated for each type of site in a watershed, normalized by

watershed area, and summed to provide an index of potential

impact. An index greater than 1.0 indicates a greater than 50

percent chance of “increased impact on the downstream

aquatic ecosystem.”

Unfortunately, no documentation was provided for the

derivation of model components, and procedures for assigning

relative weightings were not explained. Disturbance and

recovery coefficients presumably reflect measurements from

central Washington, so the equation must be recalibrated for

regions where runoff and erosion processes interact with

channels in different ways. But adjustment of component

weights for new areas is an indeterminate problem because

component indices do not represent physical quantities: there

are too many knobs to twiddle and too little information to

guide their twiddling. Klock (1985) did not indicate how

impact probabilities were related to index values, the types

of impacts considered, or if and how the model was tested.

Other references to its application were not found.

The KWCEA model assumes a relatively narrow view of

CWEs. Only the effects of timber management and roads are

considered, and the model is relevant only to those impacts

that might be generated by altered sediment input. Because

site-recovery functions are used to calculate each year’s

index, effects that accumulate through time are not addressed.

This oversight is particularly relevant to sediment-related

impacts, because introduced sediment may be stored for

long periods and cause long-term lag effects.

Equivalent Roaded Area (ERA)

USDA Forest Service Region 5 staff developed the ERA

method to index CWE potential from timber management

and roads. The original model assumed that channel destabilization is the impact of most concern in California, and that

destabilization occurs primarily from increased peak flows

due to soil compaction.

The model has changed considerably since its initial release.

The recent version (USFS 1988, Cobourn 1989) extends the

procedure to address downstream impacts generated by several

mechanisms. Impact potential is indexed by relating the impacts

expected from each activity to that expected from roads. The

sum of indices for a watershed represents the percentage of

basin in road surface that would produce the same effects as

the existing or planned distribution of management activities.

Indices for different planning options can then be compared

to rank their potential for producing impacts.

Application of the method first requires identification of

important downstream values and the criteria necessary to

protect each. A use history is developed for the watershed,

sensitive sites are identified, and ERAs are calculated for

each activity with respect to the mechanism thought to be of

most concern. Values are summed for the watershed to

calculate the total ERA, and this is compared to the allowable

threshold identified for the area. If the calculated ERA value

is high, then the area is singled out for further evaluation by

other means.

Each National Forest is expected to identify focal concerns

and mechanisms and calibrate coefficients for characteristic

site types, and Haskins (1987) outlined this procedure for

the Shasta-Trinity National Forest. Recent practices often

cause less impact than older ones, so changes in land

management procedures must be identified and accounted

for, and recovery curves for various activities and site types

also must be defined. Calibration ideally incorporates

monitoring data for the identified impact mechanism from

different land use activities in that area, but the necessary

data are rarely available, and calibration usually depends on

professional judgment. Uses other than timber management

and road use can also be evaluated if their effects are

appropriately calibrated. Thresholds of concern (TOCs) are

to be identified independently for each area and are to take

into account inherent differences in sensitivity to impacts.

TOCs are usually defined by calculating ERAs for areas

showing different levels of impact. Users are expected to

exercise judgment in modifying ERA coefficients and TOCs

for particular sites.

The ERA method is essentially an accounting procedure
USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993. 31

for assessing the instantaneous influence of past, present,

and planned activities on the potential for environmental

change. The method is designed to provide a screening tool

for identifying areas of particularly intense use rather than to

predict effects, but as an index of use intensity, ERAs are

likely to be grossly correlated with many types of impacts.

The method also provides a framework for organizing local

information on land-use impacts and mechanisms of change.

ERAs are not comparable between areas because the

method is customized to address issues relevant to each

implementation area. ERA coefficients defined for an area

where landsliding is the major impact have little relation to

those defined where the major concern is increased peakflows from rain-on-snow events.

Management activities are usually planned to maintain a

watershed’s ERA below an identified TOC. If the threshold is

approached or exceeded, then activities are reviewed to

determine whether they should be modified or delayed, or

whether existing conditions can be improved to lower the

ERA values. A basin is assumed to be healthy again as soon as

subthreshold ERA values are reattained, but the recovery times

actually required by impacted resources are not considered.

The method thus cannot be used to identify sites where impacts

persist from earlier disturbances, so the method is incapable of

addressing temporally accumulating effects: if recovery time

for the impacted resource is longer than that for the driving

variable, then impacts can continue to accumulate even though

the driving variable does not (fig. 4). Complementary effects

are also excluded, because the method requires identification

of a single impact mechanism. Monitoring programs are not

yet in place to assess the success of the most recent ERA

method in avoiding impacts, and earlier formulations of this

method were not independently validated.

The early formulations of the ERA model used the results

of a study in southern Oregon that showed peak flow increases

in a basin with 12 percent roaded area (Harr and others

1975) as the basis for identifying both the driving mechanism

for change (increased peak flows) and the TOC (12 percent

compacted area). However, these results are not transferable

to California’s geology and climate. Ziemer (1981), for

example, demonstrated no significant change in peak flows

in a California Coast Range watershed with 15 percent of its

surface area compacted. In addition, because channels respond

as readily to altered sediment inputs as to altered flows,

selection of increased peak flows as the single driving

mechanism does not fully address the problem. The most

recent implementations of the ERA model avoid these

problems by using ERA values primarily as an index of

land-use intensity, so the hydrological basis of the original

method is no longer important.

R-1/R-4 Sediment-Fish Model

USDA Forest Service researchers of the Intermountain Research Station, resource specialists and managers from Forest Service Regions 1 and 4, and university researchers

worked together to develop a method for predicting fish

survival as a function of sediment input in the Idaho Batholith.

The procedure has two parts: first, sediment yields are estimated using a method described by Cline and others (1981),

and then the effects of increased yield on fish are calculated

using relations presented by Stowell and others (1983). The

R-1/R-4 model addresses settings where deposition of fine

sediment is the major impact on fish populations, and where

sediment is eroded primarily by surface erosion on logged

sites and roads. Components of the method are continually

revised to incorporate new research results, and model coefficients are calibrated locally for each application area. Developers of the model stress that it is directly applicable only

to the Idaho Batholith, and that results should be taken as

broad estimates of trends and relative impacts rather than as

precise predictions of change.

Sediment yield is predicted using relations developed from

extensive research on erosion process rates as a function of land

use in the Idaho Batholith. Average loss rates were calculated

for roads, logged areas, and burned sites, and these are applied

to the areas of each activity in a watershed to calculate on-site

loss rates. A sediment delivery relation from WRENSS (Water

Resources Evaluation of Non-point Silvicultural Sources, see

following section; USFS 1980) is used to calculate input to

streams, and a relation defined by Roehl (1962) allows estimation

of delivery to critical stream reaches.

Gravel siltation, summer rearing habitat, and winter

carrying capacity were identified as factors limiting salmonid

survival. Results of studies in the Idaho Batholith were

compiled to provide empirical correlations between substrate

embeddedness and sediment yield, between habitat measures

and substrate embeddedness, and between fish response and

habitat quality. To apply the model, calculations are carried

out for critical reaches downstream of a project area. If

natural siltation levels are known and conditions prior to the

project are measured, then the incremental effect of a planned

project can be estimated.

As presented, the model applies only to the Idaho Batholith

because its relationships depend strongly on runoff mode,

runoff timing, climate, sediment character, erosion process,

channel geometry, and species of fish. Use of the model in other

areas requires remeasurement of each of the component relations.

This procedure is one of the few that relate land-use

activities directly to a resource response, and is unique in its

recognition that impact recovery rates cannot be indexed by

recovery rates of the driving land use. Once a channel reach

is modified, the new condition becomes the baseline for

further changes, and effects that accumulate through time

can therefore be predicted. The R-1/R-4 model is not a

complete model for CWE evaluation because it addresses

only one type of CWE from one mechanism, but it is wellfounded on research results within the area for which it was

developed.

California Department of Forestry Questionnaire

The California Department of Forestry and Fire Protection

(CDF) has developed a procedure for use by Registered Pro-
32 USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993.

fessional Foresters to assess CWE potential from timber management (CDF 1991). This procedure differs from all others

described in that it relies almost completely on the users’

qualitative observations and professional judgment, and it

provides only qualitative results. It also addresses a wider

variety of uses and impacts than other procedures, and includes many that are not related to water quality, such as

recreational, esthetic, biological, cultural, and traffic uses and

values. It was designed to be used within the time and access

constraints of timber harvest plan (THP) development, and a

nonquantitative approach was adopted to avoid the complacency that accompanies a numeric, “right” answer.

Application of the method requires four steps for each

potentially impacted non-water-quality resource. The user is

first asked to identify significant beneficial uses in the

assessment area, and is then asked whether the planned

timber operation is likely to produce significant effects on

each of those uses. The third step requires the same evaluation

for the proposed project in combination with other planned

activities in the analysis area. Finally, the user is asked

whether significant cumulative effects are likely from the

proposed operation.

Analysis follows a different procedure for water-quality

resources. The analysis area for watershed assessments is an

“area of manageable size relative to the THP (usually an

order 3 or 4 watershed)” (CDF 1991). Existing channel

conditions are first inventoried, and adverse impacts from

past projects are identified. The user is then asked to judge

the likelihood of a variety of potential effects from the

proposed project, from expected future projects, and from

combined past, present, and future projects. “Past projects”

are generally restricted to those which have occurred over

the preceding 10 years, unless there is special knowledge of

an older “open sore,” and “future projects” are those which

are planned by the THP submitter over the next 5 years, or

which have been publicly announced by other users.

Likelihood is rated as high, moderate, or low, and further

analysis by specialists is recommended if CWE potential is

identified as high or moderate.

The CDF procedure is essentially a checklist to ensure

that the important issues have been considered. Although it

provides descriptions of possible CWEs, it does not specify

how the likelihood of their occurrence is to be evaluated, and

instead relies on the user’s ability to make qualitative

predictions based on observations of earlier projects in the

area. Validity of the results rests completely on the user’s

expertise, experience, and professional judgment, so results

are not necessarily reproducible. The specified spatial and

temporal evaluation scales are based primarily on feasibility

of evaluation, rather than on the nature of the potential

CWEs. This implicitly restricts the types of CWEs that can

be evaluated, although the procedure permits a larger scope

where it is deemed by the user to be necessary.

The procedure’s major strengths lie in its flexibility. This

is the only CWE evaluation method that requires assessment

of more than one type of impact from more than one type of

mechanism. It is also one of the few procedures that allow

evaluation of temporally accumulating impacts.

Water Resources Evaluation of Non-point

Silvicultural Sources (WRENSS)

The most complete process-based approach to evaluating

timber-management impacts is the series of procedures referred to as WRENSS (USFS 1980). This collection presents

quantitative evaluation procedures for a variety of water

quality impacts, including altered flows, sediment, and temperature. Pollution by nutrients and chemicals is addressed

qualitatively, as are changes in dissolved oxygen. Although

not intended specifically to address CWEs, WRENSS methods are applicable to CWE evaluations.

Application of WRENSS to a CWE analysis would require

identification of likely environmental changes generated by

a project, likely downstream impacts, and the mechanisms

generating them. Appropriate WRENSS procedures would

be selected to estimate the magnitude of expected impacts

from a planned project, and these values would be added to

those calculated for existing projects to estimate the total

effect. Because the original focus for WRENSS was water

quality, the procedures do not address impacts on other

resources, and only the effects of timber management and

roads are considered. Evaluation procedures are independent

of one another, so modules can be replaced by improved

methods as they become available.

The WRENSS procedure for evaluating hydrological

change is based on computer simulation modeling of water

budgets. The program PROSPER (Goldstein and others 1974)

was used to develop relationships for rain dominated areas,

and WATBAL (Leaf and Brink 1973a, 1973b) for areas with

snow. Results are presented as graphs and tables that allow

users to estimate changes in evapotranspiration, flow duration,

and soil moisture for various logging plans. Stream

temperature changes are assessed using the Brown model

(Brown 1970).

Sediment modules include methods for estimating surface

erosion, ditch erosion, landsliding, earthflow activity, sediment

yield, and channel stability. Surface erosion is calculated

using a modified Universal Soil Loss Equation (Wischmeier

and Smith 1978) and sediment delivery relations, and ditch

erosion is assessed by calculating permissible velocity. The

landslide module is a step-by-step guide to performing a

landslide inventory for an area: the area is subdivided into

uniform subareas, hazard indices are calculated for each,

and slides are inventoried in representative areas to determine

characteristic volumes and delivery ratios as a function of

hazard index and land use. Sediment yields are estimated

using results of monitoring and process evaluations, and

channel stability is indexed by changes in sediment and

flow.

WRENSS is neither a CWE model nor an evaluation

procedure, but is simply a collection of tools useful for

impact evaluation. A CWE analysis using WRENSS would

be flexible enough to handle a variety of impact mechanisms,
USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993. 33

but it would need to use additional methods to assess the

effects of other land uses and to evaluate impacts on particular

resources. Implementation of most of the procedures requires

training in hydrology or geomorphology, and calculations

are often complex and time-consuming. WRENSS is the

only method described here that is capable of estimating the

magnitudes of different types of watershed changes.

Each procedure presented in WRENSS was developed by

researchers and resource specialists with relevant expertise,

but procedures vary widely in sophistication, approach, and

accuracy, and many have been superseded by more effective

methods. Some of the methods have been intensively tested,

while others have not been validated at all.

Limiting Factor Analysis (LFA)

Reeves and others (1989) developed the LFA as a procedure

for identifying factors that limit coho smolt production in

coastal Oregon and Washington, and the procedure can be

adapted for use in CWE analysis. Unlike the approaches

described above, LFA is designed from the point of view of

a particular impacted resource. Any procedure designed to

predict CWEs, rather than merely to assess their likelihood,

must incorporate a component such as this.

The LFA model is in the form of a dichotomous key that

leads users through computations to estimate smolt production

from data on physical habitat in a watershed. Model application

requires detailed surveys of fish populations and areas

occupied by various habitat types in a watershed. The

procedure is based on extensive fish and habitat surveys that

were compiled to disclose patterns of habitat use in the

Pacific Northwest. LFA can thus be applied only to sites

with population-habitat relationships characteristic of the

development area, but the model could be recalibrated for

other areas if limiting factors are the same and appropriate

relationships can be measured.

Because LFA sums smolt production from different habitat

categories, the impacts of changes in habitat distribution can

be calculated. Relations between cumulative habitat change

and land-use activities would also be required for prediction

of CWEs, and these are produced in some form by most

other models described here. A procedure capable of assessing

impacts of timber management on fish would need to couple

a trigger-based model such as ECA or ERA with an impactbased model like LFA. The model is currently being validated

by the Oregon Department of Fish and Wildlife, but results

of the study are not yet available.

Rational Approach (Grant)

Grant (1987) described a process-based procedure for establishing thresholds of concern for flow-related channel disruption. As was the case with LFA, this method is designed from

the point of view of the impact rather than the impacting use.

Grant reasoned that channels can be disrupted only if

their beds and banks are remolded, and this requires

mobilization of bed sediment. If increased flows do not

affect the frequency of sediment transport, then a river will

remain stable. Grant presented equations to calculate flow

thresholds for sediment transport as a function of particle

size and channel geometry. Projected flow increases can then

be compared to threshold discharges to evaluate their potential

for altering a channel. The Rational Approach can also provide

an index of channel stability.

The Rational Approach illustrates the potential use of

process-based information for quantifying the potential effects

of changing conditions, but it was not intended to provide a

full CWE analysis. Use of the method to evaluate CWEs

would require its coupling with models that predict flow

changes from land use, and with methods to analyze channel

destabilization by other mechanisms, such as increased

sediment input or riparian disruption.

Discussion of General CWE

Assessment Procedures

The approaches discussed above fall into several categories

and are summarized in table 6. The ERA, KWCEA, and

ECA methods produce indices that represent the potential

for CWE generation in a watershed, while the CDF questionnaire provides a qualitative assessment of CWE likelihood. The other techniques produce quantitative measures

of rates, volumes, or populations. The R-1/R-4 model quantifies a limited cause/effect relation, and can be used to

assess a particular type of effect in areas for which the

relation is defined. The Rational Approach (Grant) and the

LFA evaluate effects from the point of view of the impacted

resource without reference to the land uses triggering the

impacts. Finally, WRENSS is a collection of procedures for

evaluating several types of impacts.

ECA, ERA, KWCEA, R-1/R-4, and CDF methods were

all designed to analyze CWEs. The first four methods assume

that CWEs occur primarily by channel disruption or

sedimentation, and changes affecting other mechanisms are

not addressed. Models may be useful where this assumption

is met, but none of these methods can address all CWEs

possible in an area. None of the four can be used to evaluate

on-site or complementary-influence effects, and only the R1/R-4 method is capable of addressing effects that accumulate

through time. The CDF method requires evaluation of a

broader range of mechanisms and impacts and allows

evaluation of temporally accumulating effects, but analysis

procedures are left to the professional judgment of the user,

and the recommended spatial and temporal scales for analysis

are too small to address some types of CWEs. Because of

these limitations, none of the approaches provides a complete

evaluation of CWEs.

The scientific basis for most of the general methods is

poor. The ECA method is based on extensive data showing

that decreased vegetation cover augments water yield by

decreasing evapotranspiration losses. However, the method

then assumes that changes in peak discharges are proportional

to changes in water yield, a relation contradicted by most
34 USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993.

Table 6—Attributes of CWE evaluation procedures

ECA KWCEA ERA S/F CDF WRENSS LFA RA

Uses considered

Timber management xxxxxx . .

Roads xxxxxx . .

Mining c . c c x (x) . .

Agriculture c . c c x (x) . .

Recreation c . c c x (x) . .

Urbanization c . c c x (x) . .

Impoundments . . . .x. ..

Mechanisms:

Runoff change x . x . x x x .

Peak flow change x . x . x x x x

Sediment change . xxxxx x.

Organic debris change . . . .x . x.

Nutrient change . . . .xx . .

Input of toxics . . . .xx . .

Temperature change . . . .xx x.

Impact:

Channel stability x x x . x x . x

Fish . . . x x . x .

Recreation . . . .x. ..

Water quality . . xxxx . .

Sedimentation . . xxxx . .

Form of result:

Index . x x . . . . .

Rate or value x . . x . x x x

Qualitative . . . .xx . .

Scientific basis:

Theoretical . . . x n (x) x x

Empirical x n x x n (x) x n

Validation . n . . . (x) * n

Source:

Research . n . x . x x x

Management xnxxx . x.

x Model applies ECA Equivalent Clearcut Area

. Model does not apply KWCEA Klock Watershed Cumulative Effects Analysis

c Model could apply if calibrated ERA Equivalent Roaded Area

n Not specified or unknown S/F R-1/R-4 Sediment Fish Model

(x) Varies with procedure CDF California Department of Forestry

* In progress Questionnaire

WRENSS Water Resources Evaluation of Non-point

 Silvicultural Sources

LFA Limiting Factor Analysis

RA Rational Approach (Grant)

research. Too little information is provided about KWCEA

to judge its theoretical validity. The ERA method originally

incorporated the premise that peak flows would increase if

12 percent of a watershed were occupied by roads, as suggested

by preliminary results of an Oregon study (Harr and others

1975). These results have little relevance to other regions,

and subsequent work showed that no threshold existed even

in the study area (Harr 1987). Although the premise has

since been dropped and the conceptual background expanded,

the ERA model retains the framework imposed by this

assumption. In contrast, the R-1/R-4 model is based on

research results from the area for which it was developed. Its

assumptions are clearly explained, the data on which relations

are based are presented in the procedural guide (Stowell and

others 1983), and research continues to improve the model.

WRENSS, LFA, and the Rational Approach (Grant) were

not developed specifically as methods for CWE analysis, but

can be used as components in evaluations. All are wellfounded in basic research. The relations employed by the

LFA represent decades of field measurements, the Rational

Approach (Grant) is based on fundamental sediment transport

mechanics, and many of the methods employed by WRENSS

are founded on extensive experiments and large data bases.

Other WRENSS techniques, however, are less well supported.

Surprisingly, none of the general methods (ECA, KWCEA,

ERA, CDF, R-1/R-4) have been adequately validated by

documented independent studies. Only the ECA method has

been tested in published reports, and monitoring results

demonstrated a 38 to 44 percent underestimate in predicted

water yield (Belt 1980, King 1989). Methods that produce
USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993. 35

index values or qualitative results cannot be tested by

comparing predictions with measured values, and validation

requires an audit of past predictions and comparison of

measured impacts with calculated indices. Because most

data required by the ECA, ERA, KWCEA, and R-1/R-4

methods can be reconstructed from photographs and maps,

watersheds can be evaluated for conditions pertaining in the

past. Retroactive predictions of impact potential could thus

be compared with present evidence of impacts to permit

testing of model validity without lengthy monitoring. Standard

records of impacts associated with evaluated projects would

be extremely useful for model validation and comparisons.

Bailey and Hobbs (1990) suggested a framework for auditing

the effectiveness of Environmental Impact Assessments. They

recommended establishing a database that includes the type

of project, environmental conditions, predicted impacts,

methods used for prediction, and observed impacts.

Table 5 and table 6 reveal a pattern in model characteristics.

When methods originate from management agencies, they

tend to be simple, incomplete, theoretically unsound,

unvalidated, implementable by field personnel, and heavily

used. Methods developed by researchers are more likely to be

complex, incomplete, theoretically sound, validated, require

expert operators, and not used. Only in the cases of the R-1/R4 method and the LFA have management and research expertise

been combined to design the methods, and these rank among

the highest in defensibility, scientific basis, and utility.

Methods Under Development

Three additional general CWE evaluation methods have

taken form since 1990. All are currently under development

and are not yet tested, but prototypes of each are expected to

become operational during 1992.

Sequoia National Forest Method

In response to litigation, resource specialists of the Sequoia

National Forest are currently developing a CWE evaluation

procedure that is intended to relate the ERA sediment-production index directly to the magnitude of environmental

change. The method is innovative in its proposed goal of

addressing the full range of beneficial uses that might potentially be impacted and in directly incorporating a program for

monitoring the accuracy of predictions. However, the method,

as currently planned, shares with the standard ERA approach

the problems of: 1) not evaluating impacts of land uses

coexisting with timber management, such as mining; 2) not

permitting assessment of temporal effects because only the

recovery times of impacting uses are to be considered; and 3)

considering only a single mechanism for generating impacts.

TFW Method

The Timber/Fisheries/Wildlife (TFW) group was established

in response to intensifying conflicts over land management

and impacts in Washington State. Composed of representatives from State agencies, tribes, timber industry, and environmental organizations, the group is dedicated to resolving

timber-management-related environmental conflicts through

a process of consensus and cooperation. A primary goal is

development of a method for evaluating CWEs. Current

plans call for a hierarchical approach to evaluation: watersheds will first be screened by interdisciplinary teams of

resource specialists to qualitatively assess their sensitivity to

environmental change, and if basins are found to be sensitive, they will be scrutinized by the appropriate experts to

define more precisely the potential impacts and management

alternatives. Guidelines for evaluation are now being designed, and a research program is planned to provide new

evaluation methods where existing approaches are inadequate. As planned, the method will address primarily the

effects of timber management on fisheries resources, and so

it will not constitute a general CWE procedure. However, it

is an extremely important development in CWE evaluation

because of the close cooperation of disparate interest groups

in development of the method.

NCASI Method

The National Council of the Paper Industry for Air and

Stream Improvement (NCASI) is also developing a hierarchical approach to CWE evaluation for application on timber-industry lands throughout the five western forested states.

The initial screening procedure will encompass the full range

of impact mechanisms and beneficial uses present in the

region and is expected to be in the form of a computer-based

expert system that will lead nonspecialist users through a

diagnostic procedure. The expert system will also suggest

relevant evaluation procedures for the watershed, and these

will be carried out during the second phase by appropriate

specialists. Although similar in structure to the TFW approach, the NCASI method will address all relevant types of

downstream impacts, rather than being restricted to fisheries-related effects. The plan also calls for monitoring the

method’s effectiveness. On-site CWEs will not be assessed,

however, and it is not yet clear how analysis will proceed in

areas of multiple ownership.

Prospects for a General CWE

Evaluation Procedure

A complete general procedure for assessing the potential for

CWEs does not yet exist, although specific cumulative impacts have been routinely predicted for decades. The strengths

and weaknesses of existing assessment methods provide

useful guidance for developing a general method.
36 USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993.

Attributes Required of a General Method

Comparison of existing CWE methods reveals several

attributes that a successful general method will need to

incorporate:

1. Recognition that CWEs may be generated by complexly

interacting mechanisms. Methods that assume a single

mechanism may work in limited areas, but cannot be applied

where processes, uses, and impacts are different.

2. An ability to evaluate effects from many types of land

use. Methods that address only timber-management-related

impacts will become less useful as other types of land use

increase in wildland watersheds and as the spatial scope

required of CWE analyses increases.

3. An ability to address the range of impacts likely at all

sites downstream. The potential for CWE-based litigation

does not stop at an arbitrary radius from an activity site, and

indices that reflect use intensity or condition of a resource

cannot represent susceptibility to all potential impacts.

4. Methods flexible enough to allow site-specific

prescriptions based on local conditions that influence impact

generation.

5. Evaluation of the effects of past impacts. Recovery

curves for triggering mechanisms describe persistence of the

triggers, but they provide no information about the persistence

of impacts. A site’s impact history may influence its

susceptibility to future impacts.

6. Use of the best available technology. Court tests for

adequate CWE evaluation may increasingly rest on whether

the best available technology was used, and this question

may decide the outcome of cases when opposing conclusions

are presented. Because our understanding of natural systems

is continually improving, no method will be definitive for

long, and a general CWE method must allow for modification

as analytical techniques improve.

7. Methods to track the spatial distribution of activities,

impacts, and land-use histories at each site. Geographic

information systems currently provide the best available

method for organizing and manipulating this type of spatial

information.

8. Verification by statistically sound comparisons of

predicted and observed impacts.

Methods that fit these criteria will require that users be

capable of applying sound professional judgment and have a

strong understanding of the local conditions that affect process

rates; implementation will thus need to be carried out by

specialists. A successful model would need to include multiple

modules in order to provide appropriate tools for each of the

variety of impacts likely to be encountered, but implementation

at a site would require that only a subset of those modules be

used. Appropriate modules would be selected according to

the land uses present, the mechanisms of change they

represent, and the resources they might affect. Common

patterns of cause and effect could be characterized regionally

to decrease the work necessary for specific applications.

Development and Use of a General Method

As will be described in Chapters 5, 6, and 7, methods

already exist for evaluating the effects of most types of

environmental change on watershed processes, and the

resulting effects on many beneficial uses can also be predicted.

These procedures could be compiled into a collection of

evaluative tools, and research could focus on improving

fundamental understanding of the weakest modules. As

understanding grows, teams of researchers and users could

translate research developments into improved tools to take

the place of preliminary modules. This continually updated

set of procedures would provide a general evaluation method

for CWEs.

Application of a general method would first require its

customization to address regional issues and conditions. The

selected modules would then be calibrated and verified for

the region, and application to specific sites would then become

relatively routine. A mechanism for monitoring the model’s

validity and utility must be incorporated into each regional

customization. MacDonald and others (1991) reviewed a

variety of monitoring techniques and provided criteria to aid

in selecting appropriate monitoring methods and strategies.

Once the general procedures are compiled, different levels

of expert input and time commitment would be required at

different stages of implementation. Regional customization

and application could proceed according to the following

framework:

I. Develop a regional submodel by selecting appropriate

modules

A. Use CDF-style analysis to identify:

1. Resources and values that might be impacted

2. Types of impacts likely to be of concern

3. Types of changes in watershed processes capable

of causing these impacts

4. Land-use activities present or planned in

watersheds in the area

5. Types of changes in watershed processes that

might occur from these uses

B. Identify impact mechanisms that will need to be

included in the submodel by comparing likely changes

to those that might cause impacts

C. Select modules that address identified regional needs

 II. Develop database required for implementation (will

usually involve GIS)

A. Collect information on land-use history in the region

at whatever scale is necessary to meet the agency’s

goals

B. Collect information on environmental parameters

throughout the region (e.g., soil type, vegetation, etc.)

C. Determine tolerable impact levels for potentially

impacted resources using:

1. Published tolerance levels

2. Public input
USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993. 37

3. Local studies

D. Calibrate values required by the selected modules,

using:

1. Published values

2. Local monitoring records

E. Identify types of data that can be collected during use

to improve the model

 III. Test submodel

A. Verify each calibrated module

1. Carry out short-term monitoring studies

2. If possible, compare retroactively predicted

results

of each module with:

a. Existing long-term monitoring data

b. Surrogate time-series data (e.g., air photos)

B. If possible, compare retroactively predicted results of

entire submodel with:

1. Existing long-term monitoring data

2. Surrogate time-series data (e.g., air photos)

C. Establish monitoring sites for the types of changes

identified in IA2

1. To test individual modules

2. To test the entire submodel

3. To allow future improvement of calibrated

coefficients (as per IIE)

IV. Package submodel

Modules could include computer algorithms, questionnaires,

“cookbook” field procedures, predictive equations, and other

approaches; methods will vary with each type of mechanism

to be evaluated. Some may produce quantitative results,

some qualitative. Calibrated coefficients (IID) will need to

be incorporated into the modules, and instructions for using

the customized submodel and database will need to be

prepared. Users will need to be trained.

 V. Apply submodel to specific projects

A. Evaluate CWEs from project

1. Examine the site and downstream resources to

ensure that the assumptions on which the

regional

submodel is based are valid for this

implementation.

2. Carry out evaluation procedures specified by the

submodel

3. Assess reasonableness of predictions by:

a. Examining nearby sites where similar projects

have been carried out

b. Comparing predicted values with published

results for similar sites

c. Comparing predictions with monitoring data

for the area

B. Enter procedures used, environmental conditions,

and predictions into a database to allow future

auditing of the submodel’s effectiveness

C. Establish a monitoring program to detect likely changes

1. Use predicted changes to identify sensitive

parameters

2. Measure status of parameters at the time of

analysis

3. Periodically assess changes in parameters after

the project is carried out

VI. Review submodel

A. Periodically assess success of submodel

1. Assess condition of resources below previously

evaluated sites

2. Use auditing database to compare observed

conditions with those predicted

3. Compare observed conditions with tolerance

limits established in IIC

4. If the submodel is not successful, identify and

reconstruct weak modules

B. Periodically update coefficients using data from

ongoing monitoring

C. Incorporate updated modules from the general model

as they become available

Extensive input from resource experts and researchers

would be required during the development phase, and lesser

inputs during implementation phases. Expert input is also

required to assess features of project sites or downstream

resources that might require modification of the submodel.

A continuing research component is necessary to update and

improve modules.

This approach to CWE evaluation can be implemented

using existing information and technology. The research

described in Chapters 5, 6, and 7 provides the basis for module

construction, but teams of researchers and users will be needed

to turn this information into useful management tools.
38 USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993.

Chapter 5

Effects of Land-Use Activities on Environmental Parameters

Land-use activities can directly affect vegetation, soilproperties, and topography, and can import or remove

water, chemicals, pathogens, and fauna. Changes in these

environmental parameters, in turn, provoke changes in

watershed processes and affect other resources. CWEs are

generated when two activities influence the same

environmental parameter, transport process, or beneficial

use. On-site CWEs can result directly from altered

environmental parameters, whereas off-site CWEs reflect

changes in watershed transport processes. In both cases,

CWEs ultimately result from alteration of these few

environmental parameters.

Land use can directly influence environmental parameters,

and these changes can then provoke indirect changes in other

environmental characteristics. Conversion of vegetation to

increase runoff is a direct effect, but vegetation changes caused

by changes in runoff are indirect effects. Most direct effects

can be observed and measured, and many are predictable.

This section reviews direct and indirect effects of land use

activities on environmental parameters and describes studies

that evaluate these changes. Subsequent chapters discuss the

effects of environmental changes on watershed processes

(Chapter 6) and beneficial uses (Chapter 7). This organization

is adopted because many activities provoke similar

environmental changes, and impacts reflect the type of change

rather than the land use generating it. Thus, many activities

compact soils, but compaction from each source has the same

effect on the hydrologic system. The effect of specific landuse activities on compaction is described in this chapter, and

the effect of compaction on hydrologic response is discussed

in the next. These chapters represent a brief overview of the

many disciplines involved, and provide only an introduction

to each topic. Recent publications and reviews are cited to

allow readers to pursue topics of particular interest. The

review focuses on activities related to timber management,

but this is rarely the only land use in a watershed. The effects

of other uses are also reviewed, because CWE assessments

must take into account all types of land use in a basin.

Some features are common to many land-use activities:

roads are an integral part of most land use, and water

development accompanies many uses. These features are

considered first because of their general relevance.

Roads

Most land use requires roads. Forest road construction and

design are usually controlled by county grading ordinances

or by state and federal regulations that mandate best management practices (BMPs). However, little is known of how

closely these regulations are observed or how well they

satisfy their intended goals. Recent studies showed that 17

percent of culverts at an Oregon site were too small to pass

the design flow (Piehl and others 1988b), and even appropriately sized culverts were often constricted by debris, sediment, or structural damage (Piehl and others 1988a). Such

chronic under-design is likely to result in cumulative impacts during storms that approach or exceed design standards. Roads also strongly influence other land uses. They

provide recreational access to remote areas, and their construction requires development of quarries for aggregate and

surfacing gravel.

Roads and Vegetation

Roads cause appreciable vegetation changes. Trafficked surfaces are usually bare, and roadsides are often cleared to

increase visibility and aid maintenance. Exotic species introduced to reduce erosion on roadcuts may spread to become

pest plants. Kudzu vine, for example, was once used to

control erosion and is now a major weed species in the

southeastern United States. Some seeds can be dispersed by

vehicles, and a few plants have distribution patterns that

correspond to road and railway corridors.

Other effects on vegetation are indirect. Timber

management and minerals exploration establish many roads

that are abandoned after use, but regrowth on compacted

soils is slow. Exposed mineral soils and oversteepened

roadcuts and fills provide unstable and nutrient-poor substrates

that inhibit revegetation, and de-icing salt suppresses

vegetation in areas receiving road drainage. Fleck and others

(1988) and Hofstra and Hall (1971) measured damage from

road salt to birch and pine, respectively, and demonstrated

decreased regeneration and increased foliar damage with

increased salt exposure. Walker and Everett (1987)

documented changes caused by road dust in tundra plant

communities. Species distribution changed to reflect the

increased mineral-soil content at the soil surface and the

shortened duration of snow cover caused by quicker melting

of dusty snow.

Clearing for road construction exposes new forest margins

that are susceptible to blowdown. Canopy openings along

roads can accelerate growth in adjacent trees, although rapid

drainage above roadcuts may suppress growth upslope (Pfister

1969). Disruption of soils and physical damage to trees can

increase their susceptibility to pathogens (Hansen 1978),

and pathogens can be carried by vehicles and distributed

along roadways. Spores of the fungus responsible for Port

Orford root disease can be dispersed by wheel traffic (Zobel

and others 1985).
USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993. 39

Roads and Soils

Traffic forms dust on dry soils or throws particles into

suspension if runoff is present, and soil textures on roads are

often modified by applying gravel to control erosion and

improve trafficability. Both paved and unpaved road surfaces are usually almost impermeable. Traffic collapses soil

pores on unsurfaced roads, and this compacts soils and alters

their erosivity (Voorhees and others 1979). Surfacing gravels are often cemented by fine sediments introduced by

gravel breakdown and by “pumping” of wet substrate into

gravels. Road construction disrupts soil profiles and often

strips roadsides of topsoil. Soil and rock excavated during

road construction are exposed to erosion on spoil heaps,

sidecast, and fill emplacements.

Roads and Topography

Road construction generally requires topographic changes

that alter drainage patterns and affect slope stability. Benches

are cut and fill is emplaced to support roads; embankments

are built to cross waterways; and revetments and levees are

constructed for protection from channel erosion and flooding. To minimize the area impacted, roadcuts and fills are

often constructed at gradients near their stability limit. Excessive amounts of organic debris are occasionally incorporated into fills, which then become increasingly susceptible

to failure as the organics decompose.

Stream crossings and road drainage structures modify

channel morphology, and slope failures and gullying are

common where roads divert or restrict passage of channel

flows. Poorly designed drainage structures create knickpoints

and plunge pools along channels. The effects of these

topographic changes on water and sediment production and

transport are described in the following chapter.

Roads and Chemicals

Chemicals are introduced during road construction and use.

Oil products collect on road surfaces and are removed by

runoff. Where leaded fuel is still used, lead is concentrated in

roadside soil and vegetation (Motto and others 1970) and at

dump sites for plowed snow (Lockery and others 1983). Salt

used to de-ice roads accumulates where road drainage collects (Hofstra and Smith 1984), and McBean and Al-Nassri

(1987) described the distribution of salt splashed from roads

by traffic. Hofstra and Smith (1984) found that salt accumulation over a 20-year period decreased the availability of

calcium and magnesium to plants. Surfacing materials can

also modify the chemical composition of water draining from

roads, and Helvey and Kochenderfer (1987) demonstrated

increased pH for runoff from limestone-gravelled roads. Toxic

chemicals can be spilled during transport, and herbicides are

commonly used to control roadside shrubs. Bare roadcuts

and fill-slopes are often fertilized to promote revegetation.

Atmospheric pollutants disperse over wider areas than

those introduced on the ground. Recent decreases in

atmospheric and aquatic lead correspond to decreasing use

of leaded gasoline and demonstrate the past importance of

traffic as a source of lead (Alexander and Smith 1988,

Elsenrelch and others 1986). Traffic-related atmospheric

pollution can alter nutrient availability. Fenn and Dunn (1989)

showed that litter decomposition rates in the San Bernardino

Mountains of southern California increase with pollution

impact and may reflect an increase in litter nutrients from

premature foliage loss.

Impoundments and Water

Development

Water supplies are developed for domestic, industrial, and

agricultural use. Development usually requires transport and

storage of surficial runoff or groundwater, and hydrologic

processes may be altered to augment runoff. Clouds may be

seeded to increase precipitation, fences built to increase

snow accumulation, or vegetation reduced to decrease evapotranspiration. Reservoirs contribute not only to water supply, but also to power generation and flood control, and a

reservoir’s effects are partially determined by its purpose.

All impoundments alter immersed vegetation and that on

their shores. Seasonal fluctuations in reservoir levels prevent

establishment of typical riparian communities, and vegetation

within the range of fluctuation is usually limited to grasses

and sedges. Shallow reservoirs frequently support a high

concentration of algae, which contributes organic material

to downstream reaches.

Altered low flows and peak flows modify downstream

riparian vegetation. Increased variability of discharge usually

inhibits riparian vegetation, while more evenly distributed

flows allow riparian communities to encroach on channels

(Knighton 1988, Petts 1977). Decreased flooding promotes

colonization and establishment of mature plant communities

on floodplains. Harris and others (1987) documented changes

in riparian communities below dams, but noted that the

environmental characteristics of specific streams must be

known before response can be predicted. Watersheds from

which flow is extracted may show decreased vegetation covers,

and plant communities may change to reflect drier conditions.

In contrast, watersheds receiving transported water often

show higher cover densities and more mesic communities.

Construction of dams and diversions strips wide areas of

vegetation and topsoil, and sites used by construction and

maintenance vehicles are often heavily compacted. Stockponds

are very small impoundments, but they are common. In some

areas, stockponds provide the only surface water during the

dry season and may create riparian habitat where none

previously existed. These ponds become focal points for

livestock and wildlife use, and vegetation cover may be reduced

on adjacent land. Periodic inundation of soils on reservoir

margins alters their physical and chemical characteristics.
40 USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993.

Impoundments and diversions alter topography by creating

lakes or artificial drainage divides. Reservoirs may induce

landsliding by exposing steep hillslopes to wave action and

to increased pore pressures from raised water tables. Earthen

stockpond embankments are rarely protected from erosive

outflows, and breached dams are frequent in some areas.

Large reservoirs may increase topographical loading and

groundwater pore pressures enough to cause seismic activity,

and filling of one Indian reservoir apparently generated a

magnitude-6.5 earthquake (Haws and Reilly 1981). Most

dams are topographical barriers to aquatic animals.

Large amounts of oxygen are added to flow by whitewater

turbulence. Impoundments can remove this source, and Thene

and others (1989) constructed a model to predict the magnitude

of this impact. Oxygen content is also decreased by

decomposition of organic sediments trapped in reservoirs,

and anoxic water often collects at depth. Dominy (1973)

cited anoxic water as a hazard to fish, and also attributed

several fish kills in eastern Canada to nitrogen supersaturation

of reservoir outflows. Decomposition of immersed vegetation

can eutrophy new impoundments and modify nutrient cycling

and water chemistry.

Water yield can be increased by reducing evapotranspiration

losses in a watershed. Some forest lands have been converted

to grassland to increase runoff, and riparian vegetation is

occasionally removed for the same purpose. Mechanical

vegetation removal disrupts and compacts soil. Clearcutting

for water production produces the same responses as timbermotivated cutting, and burning can induce hydrophobic soil

conditions. Use of herbicides to reduce evapotranspiration

and enhance water supplies can influence the chemical

environment, and chemically induced water repellency has

also been proposed to increase runoff.

Water developments often import water from other watersheds.

Transbasin inputs may locally increase groundwater and soil

moisture levels, but can severely decrease flows in source basins.

Where groundwater is pumped, heavy use may lower the regional

water table and cause salt-water intrusion or subsidence.

Timber-Management Activities

Timber management encompasses all activities associated

with growing and cutting trees, including yarding, site preparation, and planting. Road densities are high in logged areas,

and impacts from timber management are often overshadowed by those of roads. Road-related impacts are described

in an earlier section.

Timber Management and Vegetation

The dynamics of natural forest communities must be understood to interpret the effects of vegetation changes induced

by timber management, but natural disturbance frequencies,

patterns, characteristics, and recovery rates have rarely been

determined. Natural frequencies and impacts of drought,

insect infestation, and blowdown are poorly understood, and

little is known of the susceptibility of different forest types

to infestations. A few studies have examined infestation

patterns within a forest community (e.g., Roe and Amman

1970). Fire frequencies have been determined at many sites

by dating scarred vegetation (e.g., Arno and Gruell 1983),

and Arno and Sneck (1977) described a procedure for such

studies, but data are still too few to reveal general patterns.

Hemstrom and Franklin (1982) compared fire disturbance

with other natural disturbances in Mt. Rainier National Park

and showed fires to be more significant than either lahars or

snow avalanches. Native Americans practiced vegetation

management by prescribed burning, but their strategies are

also poorly understood (Lewis 1973).

Ecological studies show that disturbance size, intensity,

and frequency influence species composition and community

structure (Oliver 1981). Lorimer (1989), for example,

documented differences in communities colonizing small

and large forest gaps. Parker (1986) found that lodgepole

pines dominate in several situations in the Sierra Nevada:

where disturbance is frequent, stands of this colonizing species

are even-aged and susceptible to successional replacement,

but stands are present at infrequently disturbed sites only

where site conditions are poor, and these stands are unevenaged and self-perpetuating. Response is also strongly

influenced by the types of species and communities present.

Halpern (1988) found that community resistance and resilience

in the Oregon Cascades is lowest for forest communities

with low species diversity.

The short-term effects of most timber-management

activities on vegetation are well known because of their

economic significance. Regeneration of marketable species

after logging has been widely researched, and many studies

correlate regeneration and growth rates to site indices.

Strothmann (1979) measured regeneration rates on clearcuts

in northwest California to relate stocking success to site

conditions and management practices, and showed that small,

tractor-yarded cuts planted in December, January, or May

were the most successful in the area examined. Less is

known of longer-term effects. Veblen and Lorenz (1986)

used evidence from 80-year-old logged sites in Colorado to

identify the effect of site characteristics on successional

patterns. On good sites, climax species dominated by

persevering through the successional stages, but at harsh

sites, a pioneering stage was necessary to make the

environment habitable for later-arriving climax species.

Abrams and Scott (1989) described Michigan forest

communities in which the commercial species are components

of an early successional stage, so that clearcutting accelerates

succession by releasing hardwood climax species.

Understory succession is less well understood. Everett

and Sharrow (1985) found that understory assemblages

remained after tree removal in a pinyon-juniper forest, but

species diversity declined and ground cover increased. Dyrness

(1973) documented differences in understory regeneration

rates and species composition on burned and unburned
USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993. 41

clearcuts in the H.J. Andrews Experimental Forest in the

Oregon Cascades, and Halpern (1989) compared the longterm distribution and growth patterns of residual and invading

species at that site and a nearby site.

The effects of yarding depend on the method used, with

physical disruption of ground-cover vegetation decreasing as

trafficked area decreases. High-lead and skyline methods are

less disruptive than tractors and skidders, and helicopter yarding,

yarding over a snow-pack, and balloon yarding further reduce

disturbance. Many studies document vegetation and litter

disturbance by different yarding methods (e.g., Dyrness 1965,

1972, Klock 1975, Miller and Sirois 1986, and Ruth 1967).

Sites are often manipulated after logging to promote natural

or plantation regrowth through burning, physical site

preparation, planting, fertilization, brush suppression, and

thinning. These activities affect vegetation communities by

removing vegetation, altering community composition, and

influencing growth rates. Lewis and others (1988) documented

undergrowth succession following several methods of site

preparation in combination with grazing in Florida. Most

communities are redesigned to promote a high-value species

and hardwoods are suppressed, so species diversity decreases.

Different methods of burning affect vegetation in different

ways. If slash is piled and burned, most disturbance is from

tractor work, but broadcast-burned sites are affected primarily

by the fire. Occasional managed fires escape control and

disturb large areas. Harris (1966) demonstrated that fire had

little positive effect on regeneration in southeast Alaska, but

Hooven and Black (1978) recommended burning in coastal

Oregon to control pests and suppress competing vegetation.

However, intense burning may destroy mycorrhizal fungi

necessary for effective conifer regeneration (Li and others

1986), and in some cases fire can increase brush competition

by stimulating germination of chaparral seeds.

Burning is increasingly used to reduce fire hazard in

uncut forests. Armour and others (1984) found that understory

community composition in a ponderosa pine forest in northern

Idaho reflected the amount of duff consumed by prescribed

fires, and that pre-burn vegetation communities strongly

influenced the nature of post-burn vegetation. Decreased

fire frequency encourages understory thickets of shade-tolerant

species and accumulation of ground fuels (Parsons and

DeBenedetti 1979). These changes eventually alter species

composition in the forest canopy.

The persistence of a vegetation change depends on an activity’s

duration and on the period required to reestablish pre-disturbance

communities of the original age distribution. Because most

logged sites are not intended to revert to old-growth forest,

changes in vegetation character are effectively permanent. If

species are intentionally removed or added, as by brush control

or establishment of plantations, comprehensive disturbance may

be required to reestablish original communities. In some areas,

intensive removal of a particular species has depleted the reservoir

for recolonization. High-quality cedar is becoming rare in parts of

Washington and is not usually replanted, and Mazurski (1986)

attributed wholesale changes in forest communities in Poland to

five centuries of selective logging.

Timber Management and Soils

Timber management affects soils primarily by compaction,

removal, accumulation, and disruption. Felling and yarding

locally compact soils and expose mineral horizons, with

impact severity dependent on the methods used. Most studies of soil impacts measure surface disruption or describe

soil properties before and after yarding, and compaction is

usually identified as the most severe physical impact. Extent

of compaction has been measured as a function of location

in the soil profile (Gent and Ballard 1985), soil texture

(Dickerson 1976), moisture content (Moehring and Rawls

1970), type of equipment (Green and Stuart 1985), and

traffic load (Hatchell and others 1970, Sidle and Drlica

1981). Results show that most compaction occurs during the

first few passes, and that its rate and severity are affected by

soil texture. Soil moisture can hasten compaction for some

soils and delay it for others. Hatchell and others (1970)

suggested using different yarding plans to minimize compaction on different soil types. Disruption of soil horizons is

particularly severe where site preparation includes soil scarification to expose mineral horizons, or where brush is cleared

by blading or chaining.

How long soil remains compacted after disturbance is

influenced by its texture and by the processes acting to

restore porosity, including root growth, freezing and thawing,

wetting and drying, and animal activity. Each of these

processes is inhibited by compaction, so rehabilitation may

be slow where disturbance is severe. Wert and Thomas

(1981) and Froehlich and others (1985) found that some

soils remain compacted 32 and 23 years after tractor yarding,

respectively, while soils studied by Dickerson (1976) were

estimated to recover in about 12 years. Because of the lengthy

recovery period, both Froehlich (1979) and Geist and others

(1989) considered additional compaction from subsequent

cuts to be a potential cumulative effect. Beasley and Granillo

(1988) expressed concern that the frequency of entry required

by selection cutting may lead to cumulative compaction

because soils cannot recover between entries. Some studies

have measured the effectiveness of artificial means of reducing

compaction (e.g., Gent and Ballard 1985, Voorhees 1983).

Burning can destroy organic-rich horizons and alter soil

structure. Organic loss rates have been measured as functions

of soil moisture, season, and burn intensity (Kauffman and

Martin 1989), and Beaton (1959) described the effects of

fire on soil porosity. Burning makes some soils water-repellent,

which increases runoff and decreases soil-moisture recharge

(e.g., Dyrness 1976, McNabb and others 1989), but burning

enhances infiltration in other soils (Scott 1956). Why soils

become hydrophobic has been studied (e.g., Debano 1981,

Savage 1974), but which soils will be affected cannot yet be

consistently predicted. Durgin (1985) demonstrated that

heating and ash leachates may have opposite effects on soil

dispersivity and erodibility, and that the effects are influenced

by a soil’s stage of development.

Removal of a tree canopy alters microclimates at the

ground surface, and Yin and others (1989) demonstrated
42 USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993.

severe decreases in decomposition rates where oak forest

floors were exposed by logging in Wisconsin.

Timber Management and Topography

Tractor skid trails and bulldozed fire breaks create depressions that can channel water or intercept shallow subsurface

flow. Where logs are cable-yarded, trails are engraved as

rays radiating upslope or downslope from landings, and

individual trails can generate erosive flows. Cable- or tractor-yarding across low-order and ephemeral channels breaks

down channel banks, and construction of landings usually

requires excavation and emplacement of fill.

Although timber management rarely requires intentional

hydrological changes, minor drainage disruptions can occur

where construction or skid trails divert flow from one drainage

to another. Drainage disruption is avoided when cut units are

carefully laid out, but unmaintained drainage structures may

become plugged and divert flow after a unit is closed.

Topographic effects of timber management were more

severe in the past. During the late 1800s and early 1900s

splash dams were commonly used to create floods to wash

logs downstream (e.g., Wendler and Deschamps 1955), and

channels were cleared to prevent log jams. These effects are

still evident in many west coast streams and must be considered

in CWE analyses, even though the practices were discontinued

long ago. Changes in timber management guidelines during

the 1970s further reduced the extent of topographic disruption

during subsequent logging.

Timber Management and Chemicals

Operation of machinery and chainsaws can lead to spillage of

oil products, and residues remain until washed away or altered by chemical and biological processes. Other chemicals

are added by fertilizers, herbicides, and pesticides. Bengston

(1981) evaluated fertilizer mobility and discussed the cost of

nutrient replacement using several application methods. Most

applied chemicals are quickly metabolized or weathered, and

care is taken to avoid introducing them into streams. However, even the most careful aerial application is subject to

wind drift. Markin (1982) measured insecticide drifting more

than a mile from an application site and demonstrated that

less than half the applied chemical accumulated on the intended plot. Use of the most harmful pesticides was discontinued in the 1970s, but the effects of earlier practices may

linger because residues of some compounds persist for years.

Use of sewage sludge as a fertilizer and of mill liquor as a

dust suppressant may introduce other compounds that

accumulate in the environment. Brockway (1983) documented

continued elevation of heavy metal concentrations in a pine

forest in Michigan 14 months after sludge was applied, and

Harris and Urie (1983) measured accumulation of nutrients

during 5 years of sludge application in a hardwood forest in

Michigan. Van Cleve and Moore (1978) demonstrated

cumulative changes in soil chemistry and organic matter content

over a 6-year period of fertilizer application in central Alaska.

Logging removes nutrients in wood, and in some ecosystems

this represents a high proportion of existing nutrients. Recent

studies documented the effects of tree removal on nutrient

capital (Gholz and others 1985, Mann and others 1988), and

Hornbeck and others (1986) calculated that 20 to 30 years

would be required to reestablish the nutrient balance in a

Vermont forest after logging. Nitrogen and phosphorus are

usually the most limiting nutrients for forest growth. Herbicides

increase nitrogen availability by increasing the volume of

decomposing vegetation (Vitousek and Matson 1985), but

windrowing of slash displaces nutrients to sites inaccessible

for regrowth (Morris and others 1983). Page-Dumroese and

others (1987) explored the effects of mechanical site

preparation in northern Idaho on the distribution of nutrients

and recommended that organics be mixed with topsoil and

mounded into planting beds at nutrient-poor sites.

Burning releases some nutrients to the atmosphere as

volatiles or smoke particles, and alters others into forms

more readily metabolized by plants but more easily leached

from soils. Knight (1966) used laboratory experiments to

show that a fire decreased total nitrogen in a forest floor but

increased the nitrogen available to plants. Little and Ohmann

(1988) and Feller (1988) showed nitrogen loss from burning

in Pacific Northwest forests to be correlated to the proportion

of the forest floor consumed.

Although most impacts from nutrient depletion are expected

only after multiple cutting cycles, long-term effects of timber

management on nutrients have rarely been studied. Results

will undoubtedly vary between sites: Kraemer and Hermann

(1979) showed recovery of nutrient balance 25 years after

logging and burning at 34 sites in western Oregon and

Washington, but Austin and Baisinger (1955) documented

failure of regeneration at multiply-burned sites in western

Oregon where excessive leaching depleted nutrients. The extent

and persistence of chemical changes depend on the types of

compounds involved, soil and vegetation types, and climatic

factors, and are poorly understood.

Storage and milling of logs also introduce chemicals to

the atmosphere and to aquatic systems. Tannins released by

leaching of bark are toxic to aquatic organisms (Temmink

and others 1989) and are likely to reach high concentrations

around log storage decks and in water used to debark logs.

Indirect Effects of Timber Management

on Environmental Parameters

Most impacts described above also influence other environmental characteristics. For example, vegetation changes alter input of organic matter to soils and affect soil structure,

infiltration capacity, and water-holding capacity. Canopy

vegetation moderates forest-floor microclimates, and its removal can increase frequencies and depths of soil freezing

(Pierce and others 1958) and decrease cold-season permeability. Bare soils also experience higher surface temperatures during summer, which can affect soil chemistry, regrowth, and soil biota.
USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993. 43

Vegetation changes may trigger biological succession. Many

early-stage species are capable of fixing nitrogen. However,

silvicultural practice often excludes successional species in

favor of climax conifers and so may impair a forest’s ability

to restore its nitrogen balance after disturbance. Vegetation

changes alter the chemical environment of a site and thus may

affect soil-forming processes, soil biota, and, eventually, the

nature of the soil. Vigorous regrowth induced by logging can

aid recovery of soil structure and reverse the effects of

compaction. Young growth also attracts burrowing animals

that loosen soil and form holes capable of channeling water.

Removal of mature trees alters a forest’s structure and

exposes adjacent trees to windthrow. Studies have evaluated

many factors controlling susceptibility to windthrow, including

age of the cut (Weidman 1920), cutting pattern (Boe 1965),

moisture (Day 1950), topography, and soil type. Schaetzl

and others (1989) reviewed the causes and effects of treethrow.

Uprooting disrupts, mixes, and transports soils, and decreased

windthrow frequency within clearcuts alters soil disturbance

regimes. Armson and Fessenden (1973) predicted accelerated

podzolization with decreased treethrow on clearcuts, and

decreased windfall reduces the volume of woody debris on

forest floors. Downed wood is an important component of

natural forest and aquatic habitats, and these communities

will change if wood inputs decrease.

Establishment of monoculture plantations can make wide

areas susceptible to disease outbreaks, insect infestations, and

other disturbances, but these effects are poorly understood.

Because limited seed stocks were often used in the past for

establishing plantations throughout a region, genetic diversity

of a species may decrease. Reduced variability within a species

can impair its ability to accommodate future environmental

changes and can decrease population fitness by allowing

expression of deleterious recessive genes (Ledig 1986).

Timber-management activities can alter organic matter

content in soils, compact them, expose mineral horizons,

disrupt soil profiles, and alter distribution of soil pores.

These changes, in turn, alter infiltration rates and

permeabilities and influence habitability to vegetation and

fauna. Loss of organic matter by soil displacement or burning

can greatly reduce water-holding capacities of soils. In a

study of regrowth, Duffy and McClurkin (1974) found that

soil bulk density provided the best predictor of regeneration

failure on logged sites in northern Mississippi, and Wert and

Thomas (1981) documented differences in growth rates

between compacted and less disturbed sites in the Oregon

Coast Range. Taylor and others (1966) demonstrated

decreased root penetration with increased soil compaction,

and compaction also hinders root growth by reducing soil

aeration. Moehring and Rawls (1970) found that soil

compaction also reduced growth rates of established loblolly

pines in Arkansas and noted that the only beetle-damaged

trees on their study plots were near soils gouged by machinery.

Soil disruption may have provided pathways for insects to

attack the trees, or the added stress may have lowered the

trees’ resistance. Pre-commercial thinning has been shown

both to increase (Mitchell and others 1983) and decrease

(Harrington and others 1983) the incidence of disease and

insect infestation.

Chemical changes usually affect vegetation. A decrease

in nutrient availability can change community composition

by restricting growth rates or selecting for particular species.

Most studies of fertilizer application are short-term and focus

on growth rates of commercial species, but Dunavin and

Lutrick (1983) examined the effects of sludge application on

understory vegetation for many years after application.

Displacement of nutrients, as by windrowing slash, can

accelerate growth rates near accumulations while reducing

growth at depleted sites (Morris and others 1983). Kimmins

(1977) predicted that short-cycle, whole-tree extraction for

fiber use will result in excessive nutrient depletion and outlined

the types of data needed to evaluate potential effects. Longterm effects of herbicides on community composition are

poorly understood, and McGee and Levy (1988) described

the types of information needed to understand them.

Other impacts related to timber management result from

the influence of logging and associated activities on other

land uses. Logging requires a road network, which increases

recreational use and influences wildfire incidence, and timber

production supports milling and manufacturing industries

and their attendant water developments, population centers,

and potential for degraded air and water quality.

Timber management also influences regional and globalscale changes that can contribute to CWEs. Burning affects

local climate and contributes to global warming by releasing

carbon dioxide and particulates into the atmosphere. Logging

increases rates of biomass turnover in a forest, and most of

the displaced biomass decays as slash, paper products, and

sawdust; CO2

 is released by decay. Harmon and others (1990)

modeled carbon cycling in forests and found that repeated

logging and regrowth in Washington and Oregon has

contributed an annual average of 10 to 20 million tons of

atmospheric carbon for the past 100 years, and thus may be a

significant contributor to global warming. Many parts of the

United States are undergoing net reforestation, however, and

this will counteract some excess CO2 production as long as

standing biomass continues to increase (Sedjo 1990).

Grazing

Many forest lands are grazed, and grasslands often interfinger

with forested tracts, so the effects of grazing must be considered both in forests and on grasslands. Kauffman and Krueger

(1984) reviewed the effects of grazing on riparian vegetation, soils, and topography.

Grazing and Vegetation

To understand changes caused by grazing, natural patterns

of rangeland disturbance must first be understood. Characteristic fire frequencies have been measured for some grass-
44 USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993.

lands using scarred trees (Arno and Gruell 1983) or historic

records (Higgins 1984), but data are rare because fire scars

are poorly preserved. Drought is also an important disturbance. Murphy (1970) developed a method to predict Coast

Range grass yields from fall precipitation, and Gibbens and

Beck (1988) described temporal patterns of rangeland response to drought and demonstrated differences between

annual and perennial species. Descriptions of drought effects are common (e.g., Ellison and Woolfolk 1937).

Most rangeland impacts are caused by grazing, and most

grazing alters vegetation. Each species of herbivore prefers

certain plants, and these are depleted on pastures grazed by

that species. Thus, grass density tends to decrease on cattle

allotments in Utah and to increase on sheep allotments (Shupe

and Brotherson 1985). Ruyle and Bowns (1985) suggested

that complementary grazers be pastured together to balance

forage use and preserve the original plant community.

Grazing preferences and trampling can prevent regeneration

of some species, and the sparsity of young oaks in California

savannas is thought by some to reflect grazing pressure (e.g.,

Rossi 1980) or changes in grass vegetation caused by grazing

(Danielsen and Halvorson 1991). California Coast Range

grasslands changed from predominantly perennial bunchgrasses

to European annuals with the introduction of cattle (Burcham

1982), and bunchgrasses now are dominant only at sites

protected from grazing. Annual grasses replaced annual forbs

in Central Valley prairies (Wester 1981), and shrub and tree

densities have increased in interior rangelands (Branson 1985,

Herbel 1985). The relative importance of grazing, logging,

changes in fire frequency, and loss of endemic herbivores in

causing these changes is not known, but the overall impact is

a cumulative effect of the combination of uses. Grazing gives

a competitive advantage to star thistle, artichoke, tarweed,

and other plants associated with disturbed soils.

Although grazing allotments are granted on the basis of

average stocking rates, use of the range is not uniform.

Roath and Krueger (1982) reported that 81 percent of forage

use was sustained by a riparian corridor that occupied only

1.9 percent of their Oregon study area. Senft and others

(1985), Mitchell and Rodgers (1985), and Marlow and

Pogacnik (1986) showed seasonal differences in the types of

plants and sites used by cattle, with increased use of browse

plants and riparian zones as the dry season progressed. Gillen

and others (1985) explored the influences of different grazing

strategies on riparian habitat use.

Heavy grazing decreases ground-cover density both by

consumption and by trampling, and Abdel-Magid and others

(1987b) measured these effects in a laboratory. Olson and

others (1985) examined the interaction between grazing

intensity and precipitation and showed that their effects on

ground cover density varied with the type of plant. Vegetation

can be completely absent along cattle tracks, in holding

pens, and near water sources, where use intensities are high.

Riparian vegetation is particularly heavily used and is often

noticeably sparser than in exclosures (Kauffman and others

1983b, Platts and others 1983), and Kauffman and others

(1983b) showed that grazing retarded succession on channel

bars in northeast Oregon. Schulz and Leininger (1990)

compared vegetation inside and outside a 30-year-old riparian

exclosure in Colorado to demonstrate decreased total cover,

decreased peak standing crop, increased exposure of bare soil,

and altered community composition due to grazing.

Damage to cryptogam communities is less visible. These

alliances of algae, lichen and moss reduce erosion and promote

infiltration by coating bare soil surfaces (Loope and Gifford

1972). Trampling destroys fragile cryptogam crusts, and

Anderson and others (1982) measured a 50 percent decrease in

cryptogam cover at some grazed sites in Utah. Little is known

of recovery rates after grazing, but Anderson and others (1982)

documented continued recovery after 15 years of exclosure.

Even less evident is grazing-related damage to the mycorrhizal

fungi that aid plant growth and stabilize erosive soils. These

fungi rely on certain vascular plants for nutrients, and grazing

can destroy mycorrhizal communities by suppressing their host

plants (Bethlenfalvay and Dakessian 1984).

Grazing strategy influences the types of changes caused

by grazing, although Van Poollen and Lacey (1978) compiled

published research from many sites to demonstrate that

stocking rate is a more important influence. Managers can

select grazing intensities, rotation cycles, and seasonality of

use, and much research has been devoted to quantifying the

effects of these variables on forage production, animal

nutrition, and species composition (e.g., Heady 1961, Wood

and Blackburn 1984). Platou and Tueller (1985) argued that

natural grazing patterns in plant communities should indicate

the appropriate grazing strategies for those communities,

with Great Basin communities responding best to rest-rotation

plans while environments of the Great Plains are better

suited to high-intensity, short-duration grazing.

Range-improvement plans are usually designed to alter

vegetation to more palatable species, and may include weederadication by herbicide or physical removal, fertilization,

mechanical brush removal, plowing, tree thinning, and

prescribed burning. The benefits and long-term effects of

some widely used practices remain poorly documented. Rippel

and others (1983) found that tree and shrub densities 20

years after brush removal in a pinyon-juniper community in

New Mexico were approximately the same as before, and

that grass and forb densities were lower. Johansen and others

(1982) showed that burning for range improvement in Utah

altered perennial age structures and decreased cryptogam

covers, though apparently without major changes to the

community structure. Towne and Owensby (1984) explored

the effects of seasonality of burning on vegetation

communities and used this information to design a burning

plan for a Kansas prairie.

Recovery rates for vegetation communities must be

determined if the cumulative impacts of grazing are to be

predicted. Gambel oak brushlands studied by Austin and

others (1986) in Utah had recovered from grazing impacts

by 30 years after abandonment. Short-term studies indicated

significant improvements in riparian vegetation at an eastern

Oregon site within 3 years (Kauffman and others 1983b) and

partial recovery of woody vegetation in 10 years (Rickard
USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993. 45

and Cushing 1982). Dry grasslands in Utah accomplished

only 40 percent of their recovery in ground cover density

after 10 years of exclosure (Kleiner 1983). In other cases,

recovery may not be possible. Brushlands exclosed from

grazing for 13 years in central Utah showed no tendency to

reestablish their original sagebrush-grassland community

(West and others 1984), suggesting that this grazing-induced

vegetation change is stable under present conditions.

Collins (1987) explored the combined effects of grazing

and fire on a prairie community and found that each

disturbance influenced the response to the other. This type

of synergistic effect is likely for most combinations of

ecosystem disturbance, but has rarely been studied.

Grazing and Soils

Heavy trampling can compact subsurface layers while loosening the soil surface, and many studies have examined the

influence of stocking rates, seasonality, soil moisture and

soil texture on compaction (e.g., Abdel-Magid and others

1987a; Warren and others 1986a, 1986b, 1986c). Compaction generally increases with stocking rate and is most pronounced on moist, fine-textured soils. Cattle trails are particularly heavily impacted, and Walker and Heitschmidt (1986)

described variation in trail density as a function of grazing

strategy and proximity to a water source. Abdel-Magid and

others (1987b) measured average daily travel distance for

cattle and found it to depend on grazing strategy. Persistence

of compaction has rarely been measured. Stephenson and

Veigel (1987) demonstrated a 92 percent recovery of bulk

density in a fine loam after 2 years of exclosure in southern

Idaho, and Bohn and Buckhouse (1985) showed that restrotation grazing allowed partial recovery of soil conditions

on a sandy loam soil in northeastern Oregon while deferred

rotation (grazing alternate years) did not. Trampling of wet

soils was found to be particularly detrimental.

Vegetation changes also affect soil properties. Plowing to

suppress brush increases soil surface roughness and may

improve infiltration, while burning can cause water repellency

(Brown and others 1985). Over a long period, vegetation

change by grazing or range improvement can modify soilforming processes and change chemical and physical

properties of soils.

Grazing and Topography

Grazing animals often follow hillslope contours as they

roam, and repeated grazing creates parallel trails on hillsides

that lend them a staircase-like profile. On a finer scale,

trampling on wet soils engraves footprints in the mud and

increases soil-surface roughness, while trampling on dry

surfaces breaks down irregularities. Animals avoid stepping

on grass tussocks, and channeling of traffic around tussocks

can increase elevation contrasts around the plants (Balph

and Malecheck 1985). These changes in surface roughness

affect rates and erosivity of overland flow.

Topography is often altered by construction of stockwatering ponds. These small dams decrease downstream

flows early and late in the rainy season and modify peaks

slightly during most storms. Streams are occasionally diverted

for water sources. Trampling by grazing animals strongly

influences channel morphology, and channels in grazed areas

tend to be abnormally wide and shallow (Platts and others

1983) and have a lower proportion of overhanging banks

(Kauffman and others 1983a).

Other Effects of Grazing

Use of fertilizers or herbicides for range improvement adds

chemicals to the environment, but range management of this

intensity is not common in forest lands. Grazing redistributes nutrients as manure and makes some nutrients more

accessible for plant use. Manure often increases nutrient

inputs to streams and can contribute to salinization.

Fecal coliform counts in streams usually increase with

grazing intensity, although Buckhouse and Gifford (1976)

showed no significant change in fecal coliforms with moderate

grazing intensities in Utah. Coliforms are important as indicators

of accompanying pathogens (Van Donsal and Geldreich 1971).

Coliform counts are strongly influenced by the distribution of

animal use in relation to channels, and Tiedemann and others

(1987) showed that coliform concentrations varied with grazing

strategy in eastern Oregon. Tiedemann and others (1987) also

found that coliforms could overwinter at a site in the absence

of herbivores, and Jawson and others (1982) measured elevated

bacterial levels at a site in Washington that persisted until 2

years after grazing ended. Stephenson and Rychert (1982)

measured high coliform concentrations in stream sediments in

a grazed area of southern Idaho.

Indirect Effects of Grazing on

Environmental Parameters

Grazing influences many environmental parameters indirectly. Altered vegetation affects soil porosity, texture, and

organic content by changing the rate of litter accumulation

and the distribution and longevity of roots, and suppression

of cryptogam and mycorrhizal communities changes soil

texture and surface roughness and retards nitrogen fixation.

Vegetation change eventually alters soil-forming processes

and may change the type of soil present. Vegetation changes

can provoke further successional change, which may completely alter plant communities. This is the objective of

vegetation management, and Sturges (1983) showed that

herbage production maintained a 100 percent increase 10

years after sagebrush had been removed from a site in Wyoming. Changes in bulk density, pore distribution, and organic content alter infiltration rate, soil strength, and waterholding capacity, and so modify a soil’s ability to support

particular plant species. Compaction inhibits growth in many

species, and Rickard (1985) described replacement of native

plant communities by alien annuals at trampled sites in

eastern Washington. Trampling of streambanks often causes
46 USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993.

channel widening and destroys riparian vegetation.

Most chemicals applied to rangelands are intended to

alter vegetation. Fertilizers increase biomass and cover density,

while herbicides suppress particular species, and extensive

research is directed at enabling prediction of these effects

(e.g., Wight and Godfrey 1985). Grazing animals preferentially

occupy riparian areas, and their manure can increase stream

nutrient loads and cause algal blooms and eutrophication.

Other land uses associated with grazing also influence

environmental parameters. In particular, new grazing access

roads open remote areas to recreational use; impoundments

are constructed for watering stock; and channels may be

diverted or wells drilled as water sources.

Interactions Between Grazing and

Timber Management

Grazing and timber management are frequently associated,

and interactions between these uses are likely causes for

CWEs. Multiple-use mandates often provide for coexistence

of these uses at a site, and grazing is sometimes used as a

timber-management tool. In other cases, grazing and logging are carried out in different parts of a watershed and both

can contribute to the same downstream impact.

Foresters occasionally prescribe grazing to decrease fuel

loading and reduce fire hazard. Removal of fine fuels by

grazing may decrease the effectiveness of prescribed burns

at some sites, however, and thereby increase long-term fire

danger (Zimmerman and Neuenschwander 1983, 1984).

Grazing is also prescribed to suppress brush and reduce

competition for young trees. Richmond (1983) showed

increased radial growth rates of 7 to 14 percent for Douglasfir at a site grazed by sheep in the Oregon Coast Range.

However, grazing must be managed carefully if impacts on

timber production are to be minimized. Most studies indicate

that moderate grazing does not hinder conifer regeneration

(e.g., Kosco and Bartolome 1983), but Eissenstat and others

(1982) found that 19 percent of seedlings in a cattle-grazed

Douglas-fir plantation in northern Idaho had been trampled

and suggested that grazing should be excluded from newly

planted sites. Studies of seasonal diets suggest that use patterns

may be adjusted to remove particular plant species (e.g.,

Fitzgerald and Bailey 1984, Leininger and Sharrow 1987).

Kingery (1983) discussed interactions between timber

management and grazing and indicated the need for data

showing which strategies are effective in which settings.

Mining

Underground mines now represent only a minor part of the

minerals industry in the United States, and most income and

environmental change result from oil and gas extraction,

open-pit mines, strip mines, and gravel pits. Mining of sand,

gravel, and rock for construction materials is the largest nonfuel component of the U.S. mining industry (Hoffman 1990).

In-stream gold dredging is common as a quasi-recreational

pastime in parts of California.

Mining and Vegetation

Open-pit and strip mines denude large areas of vegetation,

and oil fields require extensive road networks and cleared

pads for access to wells. Gravel mines often remove riparian

vegetation from bars, floodplains, and terraces. Accumulating deposits, such as bars, are usually stripped annually,

while terraces and other relic deposits are progressively

cleared. In-stream gold dredging disrupts stream-bed plant

communities and can increase downstream bacterial counts

by releasing bacteria associated with bottom sediments

(Grimes 1975).

Mine dumps are often difficult to revegetate, and many

studies test methods to improve growth on rock waste and

refilled mine pits. Holechek and others (1982) tested the use

of fertilizers and warned against planting aggressive exotic

plants. Stark and Redente (1985) correlated revegetation

success on simulated oil-shale waste with various site and

soil conditions in northwest Colorado, and McGinnies (1987)

measured the effects of mulching on coal mine tailings in

Colorado. Power and others (1981) tested different depths of

topsoil and subsoil to determine optimal thickness for

promoting plant growth on reclaimed mine land in North

Dakota. Exposure of mineral soil retards colonization of

higher plants by suppressing soil mycorrhizae. Lindsey and

others (1977) noted the importance of mycorrhizae to growth

and survival of native species, and attributed the effect to

increased nutrient uptake. Reeves and others (1979) found

that 99 percent of plants in undisturbed Great Basin

communities were mycorrhizal, while only 1 percent of

those at disturbed sites were. Doerr and others (1984) noted

that severity of soil disturbance influenced the type of

regrowth: highly disturbed substrates no longer had the

mycorrhizae needed to support native perennials and were

instead colonized by annuals.

Other changes to vegetation are indirect. Over- or

undercompaction of tailings piles inhibits revegetation by

decreasing infiltration, suppressing root growth, or creating

overdrained substrates. Topographic changes can retard

revegetation if slopes are steep enough to permit ravelling of

surface sediments. Unvegetated tailings piles can be abnormally

erodible because surfaces are not protected by vegetation and

because developing soils are low in organic content and are

less structured than those influenced by vegetation.

Toxic chemicals introduced into streams by mining can

suppress floodplain vegetation and promote bank erosion

(Lewin and others 1977), and atmospheric mill emissions

contributed to devegetation of 125 km2 of forestland at

Sudbury, Ontario, and damaged plants on another 500 km2

(Pearce 1976). Amiro and Courtin (1981) described the

vestigial plant communities remaining there. Even where

chemicals do not destroy vegetation, they can be assimilated

by plants and passed into the food chain. Neuman and
USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993. 47

Munshower (1984), for example, showed that high

molybdenum contents in coal mine soils led to copper

deficiencies in cattle.

Mining and Soils

Tailings piles range from overdrained to overcompacted;

rarely are permeabilities and bulk densities characteristic of

natural sites. Reconstructed topsoil density on reclaimed

mine lands must be similar to surrounding soils to avoid

runoff and landslide problems. Machinery used in pit and

strip mines compacts trafficked surfaces, and chemicals in

some spoils piles alter their hydrologic character. Gilley and

others (1977) described coal mine dumps in North Dakota

where exposure of dispersive sodic materials rapidly sealed

the soil surface and prevented infiltration.

Surface mining removes or disrupts soils. Gravel mines,

for example, prevent bar stabilization and floodplain soil

formation by repeatedly removing point-bar deposits and

disrupting incipient soils. Gold dredging of the past century

covered many floodplains with porous gravel deposits, leaving

broad areas denuded along rivers in California’s Central Valley.

Mining and Topography

Historic mining left an imprint of holes, mounds, and rhythmically ridged floodplains over much of the western United

States, and most of these topographic changes can reroute

low-order channels. Hydraulic mining in California removed

hills, filled valleys, and caused impacts that provoked the first

formal CWE analysis and are still felt today. Mine and quarry

faces and tailings dumps often have been left at steep angles

that promote rockfall, landsliding, and surface erosion.

In-channel dredging and stream-side mines alter channel

morphology, and gravel mines on floodplains excavate pits

that are later reoccupied by channels. These effects were

reviewed by Sandecki (1989), who noted their cumulative

nature. Bull and Scott (1974) described a flood on Tujunga

Wash during which the channel was rerouted by overtopping

of gravel pits and destroyed a long section of highway.

During lower flows, pits can prevent infiltration through the

channel bed by ponding water and allowing fine sediments

to settle (Bull and Scott 1974). Local stream-bed excavation

can trigger upstream incision by knickpoint retreat, and

Harvey and Schumm (1987) and Collins and Dunne (1989)

documented cases where continued extraction of bed materials

resulted in channel incision. Floodplain vegetation helps

stabilize stream banks, and its removal by gravel mining can

provoke bank erosion and channel widening. Despite the

importance of gravel mining in California, little is known of

its impacts on channel morphology.

In-channel dredging generates high turbidities from

disturbance of streambed sediments. Reynolds and others

(1989) measured five-fold increases in suspended sediment

concentrations downstream of gold dredges in Alaska. Fine

sediments are usually redeposited downstream, and Bjerklie

and LaPerriere (1985) described the resulting decrease in

bed infiltration and lowering of stream-side water tables.

Hydraulically unstable mounds of gravel left by dredging

are easily rearranged during high flows, and the increased

gravel transport can destabilize banks, widen channels, and

disrupt floodplains.

Mining and Chemicals

Mining alters the chemical environment by releasing wastes

and by exposing weatherable material. Erdman and others

(1978) measured molybdenum enrichment in vegetation growing on coal spoils piles in the Great Plains and noted that

levels were high enough to poison livestock. Tailings leachates

may contain toxic pollutants, and acid mine drainage can

severely lower pH in streams. Collier and others (1964)

found pH levels lethal to fish downstream of a Kentucky

coal mine. Streams near cobalt-arsenide mines in Idaho are

heavily contaminated by arsenic from the ore, and have also

been acidified by weathering of newly exposed sulfides

(Mok and Wai 1989). Drainage from a uranium mine introduced cadmium, strontium, and uranium into streams (Nichols

and Scholz 1989), and placer mining for gold can mobilize

heavy metals (LaPerriere and others 1985). Activities associated with mining also have chemical effects: processing

mills often release atmospheric contaminants, and oil carriers and pipelines are subject to leaks.

Interactions Between Mining and

Timber Management

The high demand for timbers to shore up early underground

mines deforested large areas in the western United States,

and some of these sites have not fully recovered. Smelter

emissions have also taken large areas out of forest production (Amiro and Courtin 1981, Pearce 1976).

Sand and gravel mining is common on California forest

lands, and is also usually present downstream of forested

watersheds. Gravel mining has even been suggested as a

mitigating measure for excessive sediment input from earlier

logging. In-stream gold dredging is common along forested

streams of northwest California and the Sierra Nevada.

Agriculture

Agriculture often occupies bottomlands in forested watersheds. Agricultural use includes soil preparation, planting,

irrigation, fertilization, pest control, weed control, and harvesting of field crops, row crops, and orchards.

Agriculture replaces natural vegetation with cultivated

species and periodically removes the crop. Ground cover

densities may be higher than natural while field crops are

growing, but are usually low after tillage, and Wischmeier

and Smith (1978) compiled measurements of seasonal cover

densities for many crops. Introduced crop species occasionally
48 USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993.

thrive in the wild and may threaten other uses. Wild artichoke

decreases range productivity, for example, and teasel, mustard,

and wild radish are considered undesirable weeds.

Tillage usually reduces compaction near the soil surface

(Voorhees 1983), but compaction may increase below the

plow layer (Blake and others 1976). Hill (1990) found lower

bulk densities throughout the soil column in conventionally

tilled plots than in no-till plots on a Maryland silt loam.

Agricultural servicing areas are usually highly compacted

and have correspondingly low infiltration rates. Soil

preparation activities are designed to make soils more

workable and often increase their organic content. Crop

residues and imported organic matter may be plowed back

into the soil, and chemicals may be added to alter soil

structure. If plants are harvested without reincorporating

organic materials, however, soil organic matter may be

depleted. Woods and Schuman (1988) reported lower organic

matter content in cropped soils than in the same soils without

crops at a site in Colorado and showed that much of the

change occurred during the first year of cultivation.

Repeated fertilization and cropping can modify soil

structure, and structure can be improved by planting forage

crops (Perfect and others 1990). Anderson and others (1990)

measured decreased bulk density and increased water

retention, organic matter, and pore size in Missouri fields

manured annually for 100 years. Soil changes may persist

over long periods and affect vegetation communities

regrowing at the site. Kalisz (1986) measured abnormally

coarse soil textures more than 60 years after eroded agricultural

land was abandoned in Kentucky. Plowing of shallow soils

underlain by calcrete can mix the calcified layer into the

topsoil and damage its texture and productivity, but deep

ripping can improve soil drainage by breaking up hardpans.

Repeated plowing around obstructions or along fence

lines can cause topographic discontinuities to grow because

most plows displace soil to one side. Terracing for erosion

control and access modifies profiles of steep slopes, but

even terrace faces are rarely steep enough to be unstable in

areas of mechanized agriculture. On a smaller scale, plowing

roughens the soil surface. Some crops are planted in furrows

or on ridges or mounds, while paddy rice requires ponded

depressions. Surface flow within fields is controlled by

drainage and irrigation ditches, which must be carefully

designed to prevent incision, aggradation, or widening.

Reclamation for agricultural development has severely

modified channel morphology in fertile lowland areas. In the

Willamette Valley, for example, the complex network of

bayous and floodplain channels that once occupied the valley

floor has been reduced by land reclamation to a single

meandering channel (Sedell and Froggatt 1984). Changes in

California’s Central Valley are as extreme. Valley-wide

seasonal marshes have been drained for farmland, and mainstem

channels are dredged and revetted to contain flows. Seasonally

flooded islands of the Sacramento River delta are now leveed

and drained, and progressive drying and oxidation of their

peat substrates has caused cumulative subsidence. Floodplains

are often the most desirable sites for farmland, and channels

are commonly engineered to reduce flooding, bank erosion,

and channel migration. Modifications include bank protection,

levees, and dredging. Many California crops are irrigated and

require a complex infrastructure of reservoirs and canals.

Chemicals are widely used in agriculture as fertilizers,

pesticides, and herbicides, and each chemical has a different

mobility and persistence in the environment. In California,

10 percent of tested wells had higher nitrate concentrations

than standards permit, and most contamination came from

agricultural fertilizers and livestock waste (Mackay and Smith

1990). Pesticide contamination was also frequent (Mackay

and Smith 1990). Many contaminants have long been out of

use but persist in groundwater and stored sediment (Gilliom

and Clifton 1990).

Harvesting removes nutrients from fields at rates that vary

with crop type, harvesting method, and the fate of crop residues.

Nutrient loss is reduced where residues are left in fields, but

where residues are burned, some nutrients are volatilized and

lost while others alter to forms both more accessible to plants

and more readily lost by leaching. Burning of residues

introduces particulates into the atmosphere and contributes to

carbon dioxide accumulation, but the practice continues as an

inexpensive control for many agricultural pests.

Irrigation alters soil chemistry by mobilizing and

precipitating soil salts, and salt may also be imported with

irrigation water. Vast tracts have been made unarable by salt

accumulation, and irrigation effluent salinizes streams in many

semiarid areas.

Irrigation developments often move water from one

watershed to another in natural channels or constructed canals.

Water imports increase soil moisture in irrigated fields, permit

higher evapotranspiration rates, and may locally raise water

tables. Areas from which water is diverted experience the

opposite effects.

Logging and agriculture are often closely linked. Woodlots

often abut fields in main-stem river valleys, and water from

most forested land eventually flows through agricultural areas.

Logging has historically accompanied agricultural clearing.

In many parts of the nation, once-tilled lands are now reverting

to forest, but nutrient depletion and soil changes from cropping

influence the regenerating communities. Deforestation for

agricultural development in western Australia has decreased

evapotranspiration and raised valley-bottom water tables,

bringing salts to the surface and preventing further agriculture.

Measures to reduce salinity now include reforestation to

lower the water tables (Bell and others 1990).

Urbanization

Growth of urban and suburban communities is rapid in the

western United States and has affected many watersheds

that historically supported only resource-based activities.
USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993. 49

Vacation and retirement homes have long occupied wildland

margins, but now rising costs of urban living have made 1-

to 2-hour commutes economically feasible, and primary homes

are increasingly common at wildland margins. In addition,

regional economic centers have grown to become commute

destinations with their own suburban fringes.

Urbanization includes construction activities as well as

industrial and residential use. Construction rates are episodic

at most sites; some population centers are now stable in size,

while others undergo exponential growth. Wolman and Schick

(1967) provided estimates of construction area per unit

population increase.

Urbanization is accompanied by power development.

Hydroelectric dams and diversions, fossil-fuel plants, and

nuclear reactors provide much of California’s power, and

wind and solar power are increasingly used in some areas.

Power generation may be on the scale of domestic or industrial supply, or may merely provide energy for an internal

combustion engine. Majumdar and others (1987) edited a

volume that examines environmental impacts caused by power

development, and El-Hinnawi (1981) reviewed a variety of

impacts from energy production.

Urbanization and Vegetation

Urban growth entails comprehensive vegetation change.

Construction removes vegetation over much of a project

area, and native species are replaced by introduced plants

that often require irrigation and pesticides. Native shrubs are

often removed to lessen fire danger in chaparral areas. Altered vegetation cover may locally be more continuous than

the original cover, but much of a developed area is covered

by asphalt and structures, and unpaved ground subjected to

traffic is usually left bare.

Where natural vegetation is preserved, it is often in

unnatural contexts. For example, plant communities in isolated

parks and natural areas change character because they no

long sustain natural grazing pressures and disturbance

frequencies, and chaparral stands preserved in the absence

of wildfire accumulate catastrophic levels of fuel. Wilson

and Ferguson (1986) found that fire intensity was the best

predictor of damage to structures in developed chaparral

areas, and implied that fire intensity was a function of fuel

loading. Even in low-density vacation settlements, vegetation

is often manipulated to reproduce a more urban flora. Clark

and Euler (1984) documented this change around a rural

vacation development in Ontario.

Introduced plants may escape from cultivation and alter

community composition in surrounding areas. Ornamental

foxglove, for example, has become a common component of

coastal forests, and introduced pampas grass inhibits conifer

regeneration on clearcuts in some areas. Increased nutrient

loads in urban streams and lakes can trigger rapid growth of

algae (Welch and others 1989), which depletes dissolved

oxygen and decreases penetration by light.

Changes in soil structure can alter vegetation communities,

and many imported species have a competitive advantage on

disturbed sites. Where slopes are steepened by excavation,

continued ravelling can prevent revegetation. Riparian

vegetation can be modified by channelization near urban areas

and by reduced or fluctuating flows below hydroelectric projects.

Urban smog is associated with damage to natural

vegetation. Omart and Williams (1979) measured decreased

growth rates in smog-damaged trees in southern California.

Treshow and Stewart (1973) exposed understory plants to

various concentrations of atmospheric pollutants for 2 hours,

and of the 70 species tested, only five emerged undamaged.

Urbanization and Chemicals

Urban and suburban land use alters the chemical environment in many ways. Oil products collect on road surfaces and

are washed into streams with runoff. Domestic and industrial

operations make heavy use of environmentally toxic substances such as polychlorinated biphenyls (PCBs), pesticides,

herbicides, and solvents, and many of these enter sewer

systems. Past disposal practices for these hazardous materials are now haunting communities with area closures, environmental illness, and costly cleanups. Between 5 and 20

million gallons of oil have been spilled on U.S. waters each

year during the 1980s (Reid and Trexler 1991), underground

fuel storage tanks are increasingly found to be leaking into

groundwater, and toxic spills have become common.

Sewage disposal often increases phosphorus and nitrogen

loads in waterways. Welch and others (1989) documented

phosphorus enrichment downstream of urban areas on the

Spokane River and related phosphorus levels to algal biomass.

Gold and others (1990) showed nitrate inputs in areas served

by septic tanks to be as high as those in agricultural areas, and

Osborne and Wiley (1988) correlated phosphorus and nitrogen

content in streams with the percentage of basin area urbanized.

The major impacts from fossil-fuel-burning power sources

are caused by the wastes they emit: smoke particles, toxic

compounds, and chemicals that react with other atmospheric

constituents. Some components of smog increase the acidity

of precipitation, and thus of soils and surface waters. Others

weaken the atmospheric ozone layer and permit more

ultraviolet radiation to reach the earth’s surface. Carbon

dioxide released by burning may increase atmospheric

insulation, raising global temperatures and altering weather

patterns. Hansen and others (1987) predicted a 3° average

temperature rise with the expected doubling of atmospheric

carbon dioxide during the next century. Global warming

may eventually destabilize polar ice and raise sea level

enough to flood coastal communities.

Other Effects of Urbanization

Construction activities compact soils and remove organic

horizons. In residential areas, natural soils may be replaced

by more workable loams or altered by addition of soil

amendments. Developed sites include impermeable surfaces

such as roofs, roads, and parking lots, and much of the land

surface is impermeable in heavily urbanized areas. Stankowski
50 USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993.

Flood Control and Navigation

Flood control programs include construction of impoundments, levees, and overflow channels to contain flows, and

dredging and straightening of channels to increase flow

capacity. Navigational modifications to improve or maintain

water courses are similar, and include dredging, meander

cutoffs, bank protection, construction of locks, and removal

of logs and obstructions.

Conveyance and navigability of natural channels are often

increased by removing drift logs, and channels modified for

flood control may be cleared of obstructing riparian vegetation.

Riparian vegetation is also destroyed when channel

morphology or banks are altered, and regrowth is prevented

by bank protection works designed to maintain the channel’s

new form. Vegetation can be altered over wide areas where

flooding is controlled by temporary filling of overflow

channels or leveed lowland basins.

Dredging disrupts in-channel vegetation (Pearson and

Jones 1975), and Grimes (1975) found that navigational

dredging increased concentrations of fecal coliforms by

resuspending the sediments with which they are associated.

Dredge spoils are commonly placed on floodplains or are

used to augment levees. Spoils piles are often difficult to

revegetate and must be carefully placed to avoid reintroducing

sediments to the channel. Other effects of dredging are

similar to those caused by in-stream mining.

Levee construction and channel engineering usually involve

replacement of near-channel soils with riprap, grout, or

revetments to protect banks. Soils in areas protected from

flooding are no longer replenished by sediment and organic

material carried in by flood waters, and channelization in

estuaries and deltas can lead to erosion of unreplenished

deposits.

Commercially trafficked waterways like the Mississippi

have undergone major morphological alteration: the planform

is straightened, the thalweg dredged, and the long-profile

broken by locks. Many of the changes described by Sedell

and Froggatt (1984) on the Willamette River in Oregon were

carried out to improve navigability. Canals are often dug to

avoid obstructions or connect waterways, and these alter

network configurations and allow passage of biota from one

system to another. Some species are readily transported on

the hulls of ships.

Flood control projects almost always involve direct

topographical changes: channels are moved and reshaped, bed

topography is altered, and levees are constructed. Each of these

affects the velocity and depth of flow through the altered reach

and the rate at which water is delivered to reaches downstream.

Desiccation of reclaimed floodplain soils can cause land to

subside and may induce even more catastrophic flooding should

levees fail and newly settled land again be immersed. Where

flood diversions return flow to a different watershed, source

channels experience reduced runoff and peak flows, while

target channels must adjust to increased flows.

(1972) presented a method for calculating percent

impermeable area on the basis of population density. Soil

compaction on bare surfaces may decrease permeabilities in

rural developments. Removal of natural vegetation at building

sites can alter soil strength and contribute to landsliding.

Urban hills are leveled for building sites, valleys filled

for road access, and slopes cut back to provide level ground.

Channel networks are often completely restructured, and

streams are replaced by sewer systems, conduits, and concrete

channels to hasten removal of peak flows. Graf (1977)

described topographic changes to stream networks in a

developing area near Iowa City and demonstrated a 50 percent

increase in drainage density due to artificial channels. Urban

river banks are usually rip-rapped or reinforced to prevent

channel migration and leveed to prevent flooding.

Walter and others (1989) showed virus concentrations in

two German rivers to be proportional to sewage inputs, and

Cowan and others (1989) found increased levels of fecal

coliforms in streams draining residential and industrial areas

in Newfoundland. Other organisms also accompany

urbanization: introduced rats, mongooses, cats, starlings,

and pigeons have all affected native biota, and urban fauna

have influenced world history as vectors for human disease.

Plant diseases and pests are also imported and spread by

introduced species. Replacement of transpiring vegetation

by impervious surfaces alters local climates, as does addition

of atmospheric particulates.

Hydroelectric dams usually release flows during times of

high power demand, and fluctuating outflows can destabilize

channels by increasing pore pressure fluctuations in

downstream channel banks. Where penstocks transport

reservoir outflow to increase power-generating head, flows

are reduced to part of the downstream channel.

Nuclear power plants create radioactive waste and excess

heat that must be disposed of. There are occasional reports

of radioactive groundwater contamination at the Hanford

Nuclear Reservation in Washington State, and other storage

sites for low-level radioactive wastes are likely to be subject

to similar leakage. Large volumes of water are used to cool

reactors, and the thermal plume introduced to the Columbia

River from Hanford is visible on infrared aerial photographs.

Recent incidents at Three Mile Island and Chernobyl have

renewed public concern over the susceptibility of nuclear

power plants to catastrophic failure and radioactive releases.

Interactions Between Urbanization

and Timber Management

The timber industry employs many people and requires infrastructure support from many others, and logging-supported

communities have grown with the timber industry. Recreation-based communities are also common in forested watersheds. Urbanization increases timber demands because of its

requirements for building material and paper products. Forest lands surrounding developing population centers are progressively cleared as the communities grow, and concerns of

residents can limit timber management on nearby lands.
USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993. 51

Boat wakes increase turbidity by eroding banks and

disrupting bottom sediment, and their turbulence can keep

sediment suspended. Garrad and Hey (1988) measured diurnal

turbidity fluctuations associated with use levels in trafficked

waterways. Increased turbidity decreases light penetration,

inhibiting growth of macrophytes in the waterway and thus

further promoting erosion (Garrad and Hay 1988).

Chemicals and nutrients can be introduced to channel

systems if flood control measures include overflow drains

on urban sewer systems, or if overflow basins are seasonally

cropped with the aid of herbicides or pesticides. Urban flood

drainage is repeatedly implicated for catastrophic fish kills

on eastern rivers. If flood waters are stored in shallow basins,

primary productivity in the temporary ponds can be high

enough to increase the nutrient content of returning water.

Boat traffic can introduce pollutants to waterways. Twocycle outboard engines release up to half of their fuel unburned

(Liddle and Scorgie 1980), and anti-fouling paints contribute

to toxic conditions around marinas and harbors. Oil slicks on

lake surfaces inhibit gas exchange to the lake waters (Barton

1969). Turbulence from wakes can increase phosphorus

activity by resuspending bottom sediments (Yousef and others

1980), and may contribute to eutrophication.

Recreation and Fishing

Recreational uses of wildlands range from hiking, fishing,

skiing, and rock-climbing to water skiing, trail biking, and

driving off-road vehicles (ORVs). Developments such as

resorts, ski areas, and campgrounds are designed to encourage recreational use, while new roads into remote areas

promote recreation as an unintended by-product of development. In other cases, lack of development is itself the recreational attraction, and wilderness areas and wild and scenic

rivers become foci for those seeking experiences in nature.

Boden and Ovington (1973) noted that areally averaged

visitor use in undeveloped areas misrepresents actual use

intensities because use is concentrated in a small proportion

of the area. Wall and Wright (1977) reviewed the impacts of

recreation on vegetation, soil, water quality, and wildlife.

Vegetation is altered both by development of focal sites

and by repeated use of dispersed sites. Hikers trample some

meadows to the point that they are no longer viable (Willard

and Marr 1971), pack-stock overgraze meadows and riparian

zones, and ORVs create bare tracks (Davidson and Fox

1974). Some species and vegetation communities are more

susceptible to trampling than others. Cole (1978) found that

vegetation in dense forests in northeast Oregon is more

sensitive to change than in meadows or open forest, and Bell

and Bliss (1973) showed impacts on alpine stone-stripe

vegetation to be more severe than on snow-bank communities.

Soil conditions also affect susceptibility. Willard and Marr

(1970) and Payne and others (1983) found vegetation on moist

soils to be most sensitive.

The persistence of these vegetation changes in different

environments is poorly understood. Willard and Marr (1971)

estimated that hundreds of years would be required for heavily

trampled alpine tundras in the Colorado Rocky Mountains to

recover. Several studies determined use levels for which

impacts remained visible the following year. Bell and Bliss

(1973) found that 300 passes by a hiker on alpine tundra in

Olympic National Park created a path recognizable a year

later, while Payne and others (1983) showed that only eight

ORV passes were required to carve a lasting track on clay

loams and silt-clay loams at a rangeland site in Montana.

Leonard and others (1985) measured the highest rate of change

in vegetation with the first 100 to 300 passes by hikers.

Recreational developments such as marinas and ski slopes

involve major redesign of vegetation. Recreational use and

impacts are often concentrated in riparian zones, and

Settergren (1977) reviewed recreational impacts on riparian

vegetation and soils. Repeated searching for firewood has

depleted some camp sites of dead wood, and even green

wood is occasionally cut. Plant collectors, wildflower pickers,

and mushroom enthusiasts alter plant communities by

selectively removing species, and recreational boats carry

aquatic weeds from one lake to another (Johnstone and

others 1985). Wheel traffic transports spores of a root fungus

lethal to Port Orford Cedar (Zobel and others 1985), and

ORV use could further spread the disease.

Hikers and horseback riders compact soils around trails

and campsites. Dotzenko and others (1967) measured a 55

percent increase in soil bulk density and a decrease in organic

matter content in coarse-textured soils at campgrounds of

Rocky Mountain National Park, and Monti and Mackintosh

(1979) described physical changes in soils due to trampling.

Weaver and Dale (1978) compared soil impacts caused by

hikers, trail motorcycles, and horses in meadows and forests

in Montana, and found that impact level increased in roughly

that order and was worst on stone-free soils on steep slopes.

ORVs compact soils over wide areas because drivers often

desire to “break new ground,” but heavily compacted trails

are also established by repeated traffic at popular ORV

locations. Where vegetation is already sparse, traffic can

destroy the protective armoring of coarse particles and

cryptogam communities. Surface soils loosened by shear are

more erodible, and mobilized sediment may plug soil pores

(Eckert and others 1979). Motorcycles and trucks impact

soils in similar ways (Eckert and others 1979).

Some work has identified characteristics of resilient sites

to allow better planning of focal use areas. Marion and Merriam

(1985) found that soil impacts in the Boundary Waters Canoe

Area were greatest on soils with low bulk density and low

organic matter content. Summer (1980) related site sensitivity

in Rocky Mountain National Park to geomorphic classification

and showed that outcrops, talus slopes, river terraces, and

moraine tops were most resistant to change.

At sites where vegetation is depleted by trampling, soil

properties change in response. Moisture-holding capacity

decreases and bulk density increases because organic matter

is no longer cycled into the soil (Dotzenko and others 1967).
52 USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993.

These changes further decrease vegetation cover and alter

plant communities to favor disturbance-resistant plants.

Foot and vehicle traffic engrave depressions in hillslopes by

repeated use, and tracks can form conduits for overland flow

where they do not follow hillslope contours. Channelized flow

is particularly pronounced along ORV tracks and where paths

cut between trail switchbacks. Improperly designed and located

trails can divert low-order channels onto their own surfaces.

Careless use of fuels and soaps by campers introduces

chemicals into the environment, and recreational vehicles

can contribute fuel, oil, combustion products, and sewage.

Littering also introduces hazardous wastes. Cryer and others

(1987) measured the litter produced by anglers at several

sites in Great Britain and found that ingestion of discarded

fishing weights and lead shot was the major cause of mortality

among British mute swans.

Angling and hunting directly alter biological communities

by targeting particular species, while management to support

these activities preferentially cultivates the target species.

Either effect can alter community structure. Cultivated strains

of game fish represent select elements of the gene pool and

may reduce genetic diversity in wild populations. Exotic

fish and game species have been widely introduced. In some

cases, substantial effort is required to protect native species

from this additional competition. Recreational activities are

also associated with increased bacteria concentrations in

surface waters (e.g., Skinner and others 1974), and may be

partially responsible for the rapid spread of Giardia.

Although most fishing within watersheds is for recreation

or subsistence, anadromous fish originating from forested

watersheds also support a large commercial fishery that strongly

affects fish populations. Hatcheries have been developed to

support both recreational and commercial fisheries. Hatchery

stock intermix with wild fish, decreasing the genetic diversity

between river systems and destroying specialized strains.

Planted fish may spread diseases to wild populations.

Table 7—Potential direct effects of selected land-use activities on watershed properties.

Auxiliary uses are also noted.

Auxiliary Vege- Topo- Chemuse tation Soil graphy icals Other

Activity BCILRV CDP DS CFMNS INR FHPW

Construction ...LR. C.P DS CFMNS I.R F...

Impoundments .C..R. CDP .. CF.N. I.. F..W

Channelization .C..R. CD. .. CFMN. ... ....

Road use and maintenance .C.L.. C.. .S CFMN. I.. ..P.

Vegetation conversion B..... C.P DS ..M.. INR F.P.

Burning ...... CDP .S ..M.. ..R FH..

Water development

Transbasin imports .CILR. .D. .. ...N. I.. F.PW

Groundwater ...... ... .. ..... I.. ...W

Timber management

Logging and yarding .C..R. CDP DS C.MN. I.R F...

Planting and regeneration B....V ... .. ..... ... ....

Pest and brush control B..... C.. .. ..... I.. F.P.

Fire control BC..R. CDP .. ..M.. ... ....

Range use - grazing B.I.RV CDP .S C.M.. .N. F.PW

Mining

Open pit mining .C..R. C.P D. CFMNS I.R F...

Underground mining .C..R. ... .F... I.. ....

Placer gold and gravel .C.LR. CDP D. CFMNS I.R F...

Tailings storage ....R. CDP DS CFMNS I.. F...

Mine reclamation ....RV ... .S CFMN. ... ....

Agriculture

Tillage and cropping ....RV .D. .S ..MN. ..R F.P.

Irrigation ..ILR. ... .. ...N. I.. ...W

Insect and weed control B..... C.. .S ..M.. I.. F.P.

Urbanization and power

Habitation .CILR. ..P .S ..M.. IN. FHPW

Industry .CILR. ..P .S ..MN. I.. .H.W

Power plants .CILR. ... .. ..... I.. .H.W

Recreation and fishing

ORVs ....R. C.. .S C.MN. I.. ....

Trails .C.... C.. .S C.MN. ... ....

Camping .C..R. C.. .S C.M.. IN. ..P.

Fishing and hunting ....R. ... .. ..... I.. F...

Auxiliary use

B Burning

C Construction

I Impoundments

L Channelization

R Road use/maintenance

V Vegetation conversion

Vegetation

C Community composition

D Disturbance frequency

P Pattern of communities

Soils

D Disruption of horizons

S Altered soil structure

Topography

C Channel/bank morphology

F Emplacement of fill

M Altered microtopography

N Altered channel network

S Oversteepening of slopes

Chemicals

I Non-nutrient chemical input

N Introduction of nutrients

R Removal of nutrients/organics

Other

F Faunal introduction/removal

H Introduction of heat

P Introduction of pathogens

W Import/removal of water
USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993. 53

Recreational use commonly overlaps with timber

management activities. Logged areas attract hunters because

they provide habitat for some game species. In addition,

access provided by logging roads increases recreational use

in adjacent undisturbed areas, and logging may be accompanied

by recreational developments such as campgrounds. Mountain

bikes and ORVs often use logging roads.

General Patterns of Change

Most land use can directly affect only vegetation, soils, topography, chemicals, fauna, and water, and it can affect these

environmental parameters in only a limited number of ways.

Table 7 lists changes resulting directly from specified activities. Only habitation, industrial activities, and fire are shown

to directly add heat, for example, although many other activities influence heat distribution by modifying channel morphology, microclimate, and shading. Similarly, only activities that import water affect the amount of water entering a

basin, although many others influence the amount of runoff a

catchment generates. Each of the activities listed is associated with other types of land use, and these associations are

indicated in the “auxiliary use” category. Thus, most activities require road use, which itself requires construction activities and channelization of road-related drainage.

Table 8—Interactions between altered environmental parameters; letters indicate effect of

parameter listed in row on parameter listed in column.

Vege- Topo- Chemtation Soil graphy icals Other

CDP DS CFMNS INR FHPW

Vegetation

C Altered community composition .DP DS ..M.. .N. F...

D Altered disturbance frequency C.. DS ..M.. .NR F...

P Altered pattern of communities .D. .. ..... ... F...

Soils

D Disruption of horizons C.. .S ..M.. ... F...

S Altered soil structure C.. .. ..... ... F...

Topography

C Altered channel or bank morphology CD. .. ..M.S ... F...

F Emplacement of fill C.. DS C.M.S ... ....

M Altered microtopography ... .. ..... ... ....

N Altered channel network .DP .. ..... ... F...

S Oversteepening of slopes .D. .. ..... ... ....

Chemicals and nutrients

I Import of non-nutrient chemicals C.. .. ..... ... F...

N Introduction of nutrients C.. .. ..... ... ....

R Removal of nutrients or organics C.. .. ..... ... ....

Other effects

F Introduction or removal of fauna CD. DS C.M.. .N. ..P.

H Introduction of heat C.. .. ..... ... F...

P Introduction of pathogens CD. .. ..... ... F...

W Import or removal of water CD. .. C.... I.. F.P.

Direct effects of land-use activities on vegetation include

changes in community composition, disturbance frequencies,

and the pattern of vegetation communities. These can result

from intentional removal of vegetation, as in the case of

logging or construction, or selective pressure from grazing

or trampling, or addition of desired species during agricultural

or silvicultural vegetation conversions. Land-use activities

can also directly alter soil characteristics. Many activities

modify soil structure through compaction or loosening, and

others disrupt soil profiles by scarification or excavation.

Common topographic changes on hillslopes include

oversteepening of slopes, accumulation of unconsolidated

materials, and alteration of soil surface roughness. Land use

also reroutes channel networks and alters channel morphology.

Changes to chemical parameters usually involve import or

export of chemicals or nutrients. This may be by removal of

watershed products or by addition of waste materials or

management aids. Other direct changes include introduction

of pathogens or heat, and input or removal of fauna or water.

One environmental parameter usually influences many

others, and the effects of changes in environmental parameters

on others are outlined in table 8. These constitute indirect

effects of land use on environmental parameters, but are

direct effects of changes in environmental parameters. For

example, compaction of soils usually excludes some plant

species from a site, and thus alters the vegetation community

present. This change, in turn, can trigger changes in nutrient

inputs, fauna, and disturbance frequency. Similarly, soil
54 USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993.

characteristics are themselves influenced by the vegetation

community present, and faunal changes can affect most

other environmental characteristics.

Cumulative Effects of Land Use

on Environmental Parameters

Cumulative impacts on particular resources and values are

considered in later chapters. However, environmental parameters can undergo cumulative change without reference to

resources or values, and impacts that result from a cumulative

change to an environmental parameter can also be considered

CWEs. Cumulative changes to environmental parameters can

be generated by several types of land-use interactions.

Effects of land use can be cumulative if changes caused

by an activity persist long enough to interact with subsequent

activities. Repeated trampling of a site over a short period

can cause cumulative compaction, because most soils require

long periods to recover from compaction. Decreased

vegetation cover or altered plant communities at compacted

sites thus are cumulative effects of multiple vehicle passes.

Progressive changes in activity characteristics can also lead

to cumulative effects, as can changes caused by a single

activity that continues for a long duration. For example,

impervious area increases through time in urban areas because

development density increases as urban centers age.

Some effects can accumulate from one activity cycle to

the next. For example, seasonally grazed pastures may show

cumulative changes in species composition, and multi-cycle

clearcutting can cumulatively deplete wood inputs to the

soil. Changes in soil fertility may also accumulate through

cutting or cropping cycles as unreplenished nutrients are

exported with the harvest, and repeated mining of gravels can

cumulatively incise a channel if extraction rates are higher

than replenishment rates.

A single type of activity may progressively alter an

expanding area, as when effects of urbanization accumulate

in a watershed as population centers grow: impervious area

increases through time, so areally averaged infiltration rates

progressively decrease. Progressive enlargement of an openpit mine can cumulatively increase the impacts on

environmental parameters, and logging has progressively

reduced the area of old-growth forests since European

settlement of North America.

Many activities are performed in sequence, and effects

may accumulate if responses to early activities are persistent.

Logging commonly precedes agriculture or grazing, and

each use produces environmental changes that can reinforce

one another. Removal of trees exports nutrients from

watersheds, as does subsequent cropping, so nutrient capital

can be cumulatively depleted if recovery is slow.

Activities occurring at the same time in different parts of

a watershed can also cumulatively influence environmental

parameters. Road use, grazing, logging, and mining all induce

soil compaction, and together they cumulatively decrease

the areally averaged infiltration rate in a watershed.

Research Needs for

Understanding Changes in

Environmental Parameters

Science is an ongoing process of information refinement, so

research rarely provides complete answers to questions. From

a management perspective, a research project is successfully

completed when it provides the detail of understanding and

information necessary to improve management procedures as

much as desired. Most of the topics described in this chapter

could profit by further work, but some will provide greater

returns in the form of practical applications, and some require

more work because they are less well understood.

Understanding Environmental

Parameters

Fundamental to all management applications is development of a basic understanding of how natural systems function. Effects caused by land use are important only if they

differ in character or magnitude from those occurring naturally, so natural patterns of disturbance must be understood.

Little is known of long-term patterns of vegetation disturbance and recovery in natural ecosystems. We need to know

more about the roles played by disturbance and by adjacent

and interfingering communities in the maintenance of vegetation communities. Qualities lending resilience to biological systems must also be better understood, as well as dependencies that make some species more sensitive to change.

The effects of interactions between disturbances are very

poorly understood.

Soil distribution is comprehensively mapped, yet little is

known of soil formation rates, or of how rapidly soil

characteristics change in response to environmental change.

Disturbance is important in maintaining soil character, yet

its role and frequency are poorly understood. Compaction is

a common soil impact and is induced by most land uses, so a

method of predicting a soil’s susceptibility to compaction and

its recovery time would be an extremely useful management

tool. Long-term effects of fire on soils are poorly understood,

and recovery mechanisms and persistence of fire effects need to

be documented.

Watershed chemistry is more poorly described than

vegetation or soils. Rates and mechanisms of natural nutrient

production need to be better understood before the nature

and duration of human-induced impacts can be predicted.

Reactions of vegetation to altered nutrient inputs require

measurement.
USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993. 55

Understanding the Effects of Land Use

The direct effects of many types of land use are known, but

their distribution and persistence have rarely been adequately

described. Instead, many studies focus on the implications

of land use for watershed processes. Clearcut area is correlated to changes in runoff, for example, without exploring

reasons for the changes. As a result, mechanisms of change

are not fully understood, so study results cannot be applied

to other sites. Long-term changes due to land use also must

be defined to predict future effects, and rates and mechanisms of recovery need to be measured.

A very basic research need for most land uses is a simple

inventory of what practices are being used in what areas,

how these uses are changing, and how closely practices

adhere to guidelines. Road construction is heavily regulated,

for example, but little is known of how closely roads actually

conform to guidelines. Only where violations result in

immediate, large-scale damage, as was the case with the

Highway 101 bypass project in Redwood National Park, are

shortcomings recognized. Other problems are more subtle.

For example, Piehl and others (1988b) found that most of the

culverts they inventoried in the Oregon Cascades were

undersized.

Many road-related changes result from chemicals

introduced by construction or traffic. The distribution and

persistence of these chemicals is poorly understood, as are

their effects on surrounding vegetation communities. Effects

have been measured, but they are not yet understood well

enough to allow prediction.

Both State and Federal timber-management guidelines

leave many decisions to land managers. The frequency,

nature, and effects of these decisions need to be determined,

and unintentional or overlooked variances from established

guidelines also require inventory. Eckerberg (1988)

demonstrated that observance of guidelines in Sweden was

biased: measures that did not restrict logging were most

carefully observed, while those designed to protect other

interests at the expense of logging efficiency were often

overlooked.

Particular aspects of timber management that require further

study include measurement of the effects of different cutting

patterns and strategies on the overall community composition

of a watershed. Resulting patterns must be compared with

natural disturbance patterns and ecosystem structures to

understand long-term changes that might result. In particular,

the effects of management and cutting strategies on disease

outbreaks, insect infestations, and genetic diversity require

more complete understanding, and long-term effects of

burning on community structure, diversity, pattern, and soil

chemistry need to be determined. Chemical inputs and exports

from forest practices are poorly defined for most areas and

require basic descriptive work. The effects of previous timbermanagement practices on modern vegetation patterns and

channel morphology must be determined. Splash dams and

log drives significantly altered channel morphology in many

small channels, and most logging today is in second-growth

forests. Many systems of concern today are no longer in

their natural state and may still be adjusting to changes of

the past century.

Not much is known of the pattern of grazing plan

implementation on public and private lands, and the

correspondence between plans and implementations needs

to be inventoried. One study of management-related

differences showed that range condition in Utah was positively

correlated with expenditure (Loring and Workman 1987).

Although the effects of grazing strategies and management

efforts on vegetation type are usually well-described, longterm implications are more poorly understood. Physical

changes caused by grazing also require better description.

The effects of trampling on channel morphology need to be

systematically determined and related to channel

characteristics, and recovery rates and mechanisms must

also be determined. Rates of compaction and recovery from

trampling need to be measured as a function of soil type and

texture. Grazing can affect water quality by introducing

bacteria to streams, but the conditions under which these

inputs might be detrimental are not well known.

The effects of early mining activity are poorly documented

even where they affect ongoing use, and the extent to which

today’s channels reflect mining disturbance is usually

unknown. Weathering of exposed mine spoils can be a

persistent source of chemical pollutants, but factors controlling

it are poorly understood. Short- and long-term effects of inchannel and floodplain mining are not known, and the extent

to which mining-related disruption destabilizes channel beds

and introduces fine sediments needs to be determined.

The effects of introduced plant species on California’s

original vegetation communities are poorly documented, and

some communities and physical systems may still be

responding to vegetation changes that occurred a century

ago. Agricultural chemicals cause many of the same problems

as chemicals introduced by roads and timber management,

and also require evaluation for their persistence and role in

the environment.

Urban effects of particular note are introductions of toxic

waste and atmospheric effluent, and altered water inputs by

transbasin imports and groundwater extraction. The extent of

each of these and their effects on vegetation need to be

determined. Associated uses, such as power development,

impoundments, water development, flood control, and

navigation, strongly affect channel morphology. The direct

influence of these uses is well understood, and their effects on

watershed processes will be considered in the following section.

Recreation is a rapidly growing aspect of land use, with

more people making use of a diminishing recreational land

base. Patterns of recreational use and factors attracting

recreationists to particular sites must be identified to understand

future interactions between recreation and other uses.
56 USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993.

Chapter 6

Effects of Land Use and Environmental Change on

Watershed Processes

Most land-use activities alter environmental param-etersin a limited number of ways: they change the character of vegetation and soils; they modify topography; they

import and remove chemicals, water, and fauna; and they

introduce pathogens and heat. As these environmental parameters change, processes associated with transport of water through watersheds change in response and alter the

production and transport of water, sediment, organic matter,

chemicals, and heat. This chapter describes how changes in

environmental parameters affect generation and transport of

these watershed products.

Relation Between Watershed

Processes and CWEs

Off-site CWEs can be generated only by changes in the transport

of watershed products, because the effects of a land-use activity

can influence a remote site only if something is transported to

that site. Changes in the production or transport of watershed

products can also cause impacts at the site of land use.

Table 9—Effects of altered environmental parameters on watershed processes.

Runoff Sediment Organics Chemicals Heat

PIEMHCY AHCBY ADHRCBY AHCBY AW

Vegetation

C Altered community composition P.E.H.. .HC.. A.H..B. A.... AW

F Altered disturbance frequency ....... .H... A....B. ..... ..

P Altered pattern of communities ....... ..... ....... ..... ..

Soils

D Disruption of horizons ....... A.... ....... A.... ..

S Altered soil structure .I..... .H... ....... ..... ..

Topography

C Altered channel/bank morphology .....C. ..C.. ....C.. ..... .W

F Emplacement of fill ....... A..B. ....... ..... ..

M Altered microtopography ....H.. .H... ..H.... ..... ..

N Altered channel network .....C. ..C.. ....C.. ..... .W

S Oversteepening of slopes ....H.. .H... ..H.... ..... ..

Chemicals and nutrients

I Import of non-nutrient chemicals ....... ..... ....... A..B. ..

N Introduction of nutrients ....... ..... ....... A..B. ..

R Removal of nutrients or organics ....... ..... A...... ..... ..

Other effects

F Introduction or removal of fauna ....... .HC.. A...... ..... ..

H Introduction of heat ....... ..... ....... ..... AW

P Introduction of pathogens ....... ..... ....... ..... ..

W Import or removal of water P....CY ..... ....... ...B. .W

Runoff Organic material Heat

P Production process A Amount and character on hills A Air temperature

I Infiltration D Decay rate on hillslopes W Water temperature

E Evapotranspiration H Transport rate on hillslopes

M Soil moisture R Decay rate in channel

H Hillslope hydrograph C Transport rate in channel

C Channel hydrograph B Amount and character in channel

Y Annual water yield Y Volume and character exported

Sediment Chemicals

A Amount and character on hills A Soil chemistry

H Hill erosion process and rate H Transport on hillslopes

C Channel erosion process and rate C Transport in channel

B Amount and character in channel B River chemistry

Y Sediment yield and character Y Volume and character exported
USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993. 57

Whenever multiple activities or activities at multiple sites

produce similar or complementary changes to environmental

parameters or watershed processes, impacts resulting from

those changes can be considered cumulative. Table 9 indicates

the direct effects that altered environmental parameters can

have on watershed products, and table 10 shows the effects

of interactions between watershed products. These tables can

be used in conjunction with table 7 and table 8 to recognize

land-use interactions that might generate CWEs. Cumulative

effects on an environmental parameter can occur when two

land-use activities affect the same parameter (table 7), or

affect other environmental parameters that influence the same

parameter (table 8). Similarly, changes can be considered

cumulative if two altered parameters affect the same watershed

product (table 9), or affect other watershed products that, in

turn, affect the same product (table 10). Chapter 7 describes

the effects of these cumulative changes on particular resources

and values.

The tables are not useful for identifying particular effects

of land-use activities, because each of the activities listed in

table 7 is directly or indirectly capable of altering each of the

product attributes or processes listed in table 10; this is

evident when the effects of a particular activity are traced

through the sequence of tables. However, the tables can be

used to make clear the pathways of influence that can result

in environmental change, and if information for a particular

site indicates that some influences are inconsequential, then

the tables can be used to narrow the field of likely effects.

Effects of Environmental Change

on Hydrology

Runoff volume, its mode and timing of production, and its

rate of transport through a channel system all affect both the

rate of water delivery to any point and its ability to transport

other watershed products. Changes in any of these characteristics affect downstream conditions. Published research that

describes the effects of logging and roads on hydrology is

listed in table 11.

Water Input

Canopy cover influences how much precipitation the ground

receives. Snow and rain trapped in foliage evaporates rapidly because it is exposed to wind, so less snow and rain

Table 10—Interactions between watershed processes. Symbols are explained in the left column.

Runoff Sediment Organics Chemicals Heat

PIEMHCY AHCBY ADHRCBY AHCBY AW

Runoff

P Production process ....H.. .H... ....... ..... .W

I Infiltration P..M... ..... ....... A.... ..

E Evapotranspiration ...M.CY ..... ....... A.... A.

M Soil moisture .IE.... .H... .D..... A.... ..

H Hillslope hydrograph .I...C. .H... ..H.... .H... ..

C Channel hydrograph ....... ..CBY ....C.. ..C.. .W

Y Annual water yield ....... ..... ....... ..... ..

Sediment

A Amount and character on hills ..E.H.. .H... ....... ..... ..

H Hill erosion process and rate ....... A.CB. ..H.... .H... ..

C Channel erosion process and rate ....... .H.BY ..H.... ..C.. ..

B Amount and character in channel .....C. ..C.. ....... ...B. .W

Y Sediment yield and character ....... ..... ....... ..... ..

Organic material

A Amount and character on hills .IE.H.. .H... .DH.... A.... ..

D Decay rate on hillslopes ....... ..... A...... A.... ..

H Transport rate on hillslopes ....... ..... A....B. .H... ..

R Decay rate in channel ....... ..... .....B. ...B. ..

C Transport rate in channel ....... ..... .....BY ..C.. ..

B Amount and character in channel .....C. ..CB. ...RC.Y ..C.. .W

Y Volume and character exported ....... ..... ....... ..... ..

Chemicals

A Soil chemistry .I..... .H... .D..... .H... ..

H Transport on hillslopes ....... ..... ....... A..B. ..

C Transport in channel ....... ..... ....... ....Y ..

B River chemistry ....... ..... ...R... ..C.Y ..

Y Volume and character exported ....... ..... ....... ..... ..

Heat

A Air temperature P.E.... ..... .D..... A.... .W

W Water temperature ....... ..... ...R... ...B. ..
58 USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993.

Table 11—Studies of the hydrologic effects of roads and timber management. Symbols are explained below.

Treatment Measurements

Reference Location Approach Scale F R T CEFILMPSTWY

Anderson and Hobba 1959 OR A B T P

Andrus and others 1988 OR D C T W

Aubertin and Patric 1974 WV T B T Y

Austin and Baisinger 1955 OR TS P F M

Beasley and Granillo 1988 AR T M T Y

Beaton 1959 BC AS P F F

Berndt and Swank 1970 OR AT C T Y

Berris and Harr 1987 OR T P T ST Y

Beschta 1990 - O - F F L P Y

Bethlahmy 1962 OR T P T M

Bethlahmy 1967 ID E P T Y

Blackburn and others 1986 TX T M T Y

Bosch and Hewlett 1982 - O - T E Y

Bren and Leitch 1985 AU TM CP R P T Y

Bryant 1980 AL T P T W

Carlson and others 1990 OR S P T W

Caspary 1990 WG T C T E Y

Chanasyk and Verschuren 1980 BC M HC T E M Y

Cheng 1989 BC T B T P Y

Cheng and others 1975 BC T B T P T

Christner and Harr 1982 OR A M T PS

Courtney 1981 - O - T E I

Cullen and others 1991 MT S P R T C M

Debano 1981 - O- F F

DeByle and Packer 1972 MT T P F T Y

Dickerson 1976 MS E P R T C F

Dietterick and Lynch 1989 PA T B T P Y

Dyrness 1965 OR S P R T C

Dyrness 1976 OR T P F F

Ffolliott and others 1989 AZ,NM O - T S Y

Froehlich 1979 OR A P R T C

Froehlich and others 1985 ID T P R T C

Geist and others 1989 OR A P R T C

Gent and Ballard 1985 NC T P R T C

Gray and Megahan 1981 ID T P F R T M

Greacen and Sands 1980 - O - R T C

Green and Stuart 1985 GA E P R T C

Harr 1980 OR T B T Y

Harr 1986 OR T MC T PS

Harr and others 1975 OR T B R T P T Y

Harr and others 1979 OR T M R T C L P Y

Harr and others 1982 OR T B T L P Y

Harr and McCorison 1979 OR T CB T PST

Harris 1977 OR T B T P Y

Hatchell and others 1970 SC,VA AES P R T C

Heede and King 1990 AZ T P T Y

Hewlett and Helvey 1970 NC T C T P T

Hicks and others 1991 OR T M F T L

Hillman and Verschuren 1988 - M H T M

Hogan 1987 BC D PB T W

Hornbeck 1973 NH T B T PST

Hornbeck and others 1970 NH T B T L S Y

Johnson and Beschta 1980 OR S P R T

Keppeler and Ziemer 1990 CA T B R T L Y

King 1989 ID ST M R T PS Y

King and Tennyson 1984 ID T M R L P T Y

Klock and Helvey 1976 WA T B F MP

Lockaby and Vidrine 1984 LA S P R T C

Mahacek-King and Shelton 1987 CA M C T P Y

Martin and Tinney 1962 OR A C T Y

McCarthy and Stone 1991 FL E M T M

McDade and others 1990 OR,WA S P T W

McNabb and others 1989 OR T P F F M

Megahan 1972 ID T P R P

Megahan 1983 ID T P F R T MPS
USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993. 59

Table 11—(Continued)

Treatment Measurements

Reference Location Approach Scale F R T CEFILMPSTWY

Moehring and Rawls 1970 AR S P R T C

Patric and Gorman 1978 WV AD P R T C F

Patric and Reinhart 1971 WV T C T F L P Y

Pierce and others 1958 NG S P T F

Potts 1984 MT A C L P T

Potts and others 1989 - MP F

Reinhart 1964 WV T CP R T F PS

Rice and Wallis 1962 CA T B T Y

Riekerk 1989 FL T B T C F MP

Rothacher 1963 OR T P T I

Rothacher 1970 OR T B R T E L Y

Rothacher 1973 OR T B R T P

Savage 1974 L EP F F

Schmidt and Troendle 1989 CO T P T

Scott 1956 CA S P F F

Scott and Burgy 1956 L EP F F

Sidle and Drlica 1981 OR S P R T C

Snider and Miller 1985 OR D P R T C F

Springer and Coltharp 1980 KY T B R P T

Steinbrenner and Gessel 1955 WA D P R T C

Stransky 1981 TX T P T C

Striffler 1959 MI ST P T F

Swank and Helvey 1970 NC T B T Y

Tarrant 1956 WA,OR S P F T F

Toews and Moore 1982 BC T B T W

Troendle 1970 WV T B T L

Troendle 1988 CO T BP T E M S Y

Ursic and Esher 1988 MS R C T Y

Van Sickle and Gregory 1990 OR M P T W

Wood and others 1989 LA E P T Y

Wright and others 1990 CA T B R T P T Y

Ziemer 1964 CA A P T E M

Ziemer 1979 - O - T E

Ziemer 1981 CA T B R T P T Y

Location:

AL Alabama FL Florida MS Mississippi PA Pennsylvania

AR Arkansas GA Georgia MT Montana SC South Carolina

Au Australia ID Idaho NC North Carolina TX Texas

AZ Arizona KY Kentucky NG New England VA Virginia

BC British Columbia L Laboratory NH New Hampshire WA Washington

CA California LA Louisiana NM New Mexico WG West Germany

CO Colorado MI Michigan OR Oregon WV West Virginia

Approach or method: Scale:

A Reconstructs past using air photos or B Paired basins or sites

records, or by sampling age distribution C Case study

D Based on descriptive measurements H Hypothetical or general case

E Experimental M Multiple basins or sites

M Modeling or calculated results P Process study

O General review of topic

R Reasoned from basic principles Treatment:

S Survey of multiple sites F Fire

T Long-term temporal monitoring study R Roads

T Timber management

Measurement:

C Compaction of soil P Peak flow

E Evapotranspiration S Snow melt or accumulation

F Infiltration into soil T Timing of hydrograph peak during storm

I Interception loss W Woody debris

L Low-flow characteristics Y Water yield

M Soil moisture
60 USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993.

reaches the ground in forested areas than in adjacent clearings. Berris and Harr (1987) showed that snow accumulation

and its water equivalent was higher in clearings than in

adjacent forest in the Oregon Cascades, and Rothacher (1963)

reported loss of 18 to 24 percent of rainfall to evaporation

from foliage in the same area. On the other hand, fog can

condense on leaf surfaces and contribute some precipitation

by fog drip. Courtney (1981) reviewed studies of interception loss and noted that merely measuring the rates is not

sufficient; studies can be interpreted only if the processes

that create the effect are understood.

Some land-use activities modify climate. Precipitation or

cloudiness may be increased locally by intentional cloud

seeding or seeding by atmospheric pollutants. Kahan (1972),

for example, quoted increases of 10 to 30 percent in local

precipitation from cloud-seeding programs. Local increases

may be balanced by decreases downwind where atmospheric

moisture content is reduced. Increased cloud or fog cover

may increase runoff by decreasing evapotranspiration rates.

Broader changes may be caused by global increases in

atmospheric CO2 and other gases. Whether atmospheric

alterations will increase or decrease precipitation at a site

depends on the site’s location and the type and extent of

atmospheric change. Climatic models have been developed

to predict the likely effects of atmospheric trends, but model

validity is difficult to assess, and spatial resolution is sufficient

only for regional projections. Gibbs and Hoffman (1987)

reviewed available climatic models and discussed their

shortcomings, strengths, and assumptions. Rathjens (1991)

described the consensus he perceives among climate change

researchers: there has already been a 0.5 °C temperature

increase over the past 100 years; there will likely be a 1.5-4.5

°C increase by 2035; and this will be accompanied by a 0.2-

to 0.5-m rise in sea level. He noted that the magnitude of the

temperature increase may be lessened due to increased albedo

from increased cloudiness, but that the increased cloudiness

might also enhance a temperature increase by altering the

atmospheric water vapor content. In any case, increased

cloudiness would in itself markedly change local and global

precipitation patterns, irrespective of temperature effects.

Changes in precipitation timing, mode, and intensity also

affect watershed hydrology. Increased dry-season rainfall is

likely to have more effect on vegetation than equivalent

increases during the wet season, and a shift from rain to

snow alters annual hydrographs. Flood peaks will respond if

rain intensities increase. Lettenmaier and Gan (1990) used

global circulation models to predict hydrologic changes in

California due to global warming, and suggested that an

increased proportion of precipitation falling as rain would be

a more important influence than an increase in the amount of

precipitation.

Irrigation and domestic use can augment flow downstream

of developments by importing water from other areas. Glancy

and Whitney (1989) described transformation of Las Vegas

Wash from an ephemeral channel to a perennial stream with an

average discharge of 4 m3

/s as the population of Las Vegas grew.

Runoff Generation

Changes in vegetation density and age structure affect rates

of evapotranspiration, so altered vegetation usually changes

runoff volume and timing. Ziemer (1979) reviewed research

on evapotranspiration processes. Decreased evapotranspiration increases average soil moisture (Bethlahmy 1962), raises

dry-season water tables, and augments dry-season baseflows.

These changes can increase storm peaks early and late in the

wet season, but mid-season peaks are rarely affected because soil moisture is usually high at this time even before

disturbance. Rothacher (1973) demonstrated this pattern in

the Oregon Cascades, but Hewlett and Helvey (1970) showed

that altered soil moisture may affect peaks more consistently

in areas where precipitation is distributed throughout the

year. Knox and others (1975) found three-fold increases of

peak flows after forests were converted to cropland in the

northern Midwest.

Vegetation may be intentionally modified to increase

water yield, but altered yields more commonly are unintended

by-products of other uses. Bosch and Hewlett (1982) reviewed

studies of the hydrologic effects of vegetation management

and showed a pattern of increased water yield after

clearcutting. Harr and others (1979), for example, measured

a 43 percent increase for 5 years after a clearcut in the

Oregon Coast Range. Martin and Tinney (1962), however,

found no change in runoff in a 55 mi2 basin nearby and

suggested that regrowth can be rapid enough to mask the

hydrologic effects of logging if the cutting rate is very low.

Swank and Helvey (1970) estimated that recovery of runoff

rates from a North Carolina clearcut will require 35 years,

while Ziemer (1964) documented recovery after about 16

years in the Sierra Nevada. Evapotranspiration rates are also

affected by forest declines from air pollution. Caspary (1990)

found increased water yield in pollution-stressed forests even

before foliar damage was evident, and attributed this change

to the physiological effects of acid soils on fine roots.

Snow melts more quickly in clearings than under forest

canopies because air circulation and solar radiation are higher.

Hornbeck and others (1970) documented an earlier snowmelt

season in clearcuts at the Hubbard Brook Experimental Forest

in New Hampshire, and Berris and Harr (1987) extended the

argument to predict more rapid runoff during particular melt

events. In combination with higher snow accumulations in

clearings, the increased melt rates can generate higher peak

flows. This effect is enhanced if snow is melted during rain

storms, and clearcutting in the transitional snow zone may

increase flood peaks during rain-on-snow events (Christner

and Harr 1982, Harr 1986). Buttle and Xu (1988) demonstrated

that urbanization also caused earlier snowmelt and augmented

discharge peaks from rain-on-snow events.

Processes of runoff generation and evapotranspiration

loss are relatively well described, so models can be constructed

to predict the effects of altered forest vegetation on basin

hydrology. Chanasyk and Verschuren (1980) developed a

mathematical model to predict the effect of clearcutting on
USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993. 61

soil moisture and runoff, and Hillman and Verschuren (1988)

used a version of this model to evaluate the influence of

cutting pattern on soil moisture, but cautioned that the model

is not capable of handling macropore flow or spatial variations

in soil properties.

Riparian vegetation may decrease baseflows by transpiring

stream water. Orme and Bailey (1971) documented increased

baseflow after removal of riparian plants and conversion of

chaparral vegetation to grassland in southern California,

although accompanying morphological changes were

catastrophic. Van Hylckama (1974) measured water use by

tamarisk along floodplains of the Gila River in Arizona and

showed evapotranspiration losses of more than 200 cm/yr in

dense thickets.

Overland flow is produced by rainfall and snowmelt to the

extent that the rate of water input exceeds infiltration into the

soil, and infiltration rates are controlled by soil properties

and vegetation. Infiltrated water is held in the soil mantle

when soils are dry, but wet soils can transmit water downward.

Compaction of soil decreases pore space and collapses conduits

between pores, reducing soil porosity and permeability and

increasing rates of overland flow. Removal of topsoil can

also cause overland flow. Sponge-like humic layers may

store enough water that through-flow is slowed to rates that

less permeable horizons can absorb. Without these surface

horizons, excess rainfall may run off before infiltrating.

Unsurfaced roads and construction sites are often highly

compacted and readily generate runoff. Even briefly used skid

roads show impaired infiltration (e.g., Dickerson 1976). Johnson

and Beschta (1980) measured reduced infiltration capacities

on skid trails, cable paths, windrows, and burned surfaces at

sites in western Oregon, but showed that infiltration on clearcuts

was unchanged in the absence of these other activities.

Trampling by animals or people also compacts the soil

and alters its hydrologic properties. Gifford and Hawkins

(1978) reviewed the impacts of grazing on infiltration and

compiled results of published studies. Heavy grazing was

generally found to reduce infiltration rates on porous soils

by about 50 percent, while light and moderate grazing

decreased rates to about 75 percent of their original values.

Blackburn and others (1980) reviewed hydrologic effects of

different grazing strategies and described studies in Texas

that indicated the site-specific nature of results. Thurow and

others (1988) found average stocking rate to be more important

than grazing strategy in influencing infiltration rates on the

Edwards Plateau in Texas, and Warren and others (1986b)

found that recovery of the range in that area was controlled

more by the length of rest period than by short-term stocking

density for a given time-averaged stocking rate. Packer (1953)

artificially trampled plots with a steel “hoof” to measure the

effects of stocking rate on infiltration.

Several studies have examined basin-scale runoff from

grazed lands. Higgins and others (1989) found no change in

runoff for several grazing intensities in eastern Oregon,

while Lusby (1970) demonstrated a 40 percent increase in

runoff from grazed land in western Colorado. Results are

expected to be dependent on climate, soil type, stocking

density, and vegetation type.

Compaction can also be severe on recreation sites. Eckert

and others (1979) measured changes in infiltration capacity

caused by ORV traffic on desert soils of southern Nevada,

and Monti and Mackintosh (1979) measured decreased

infiltration capacities at camp sites in northwest Ontario.

Dotzenko and others (1967) documented decreased soil

moisture in campgrounds in Rocky Mountains National Park.

Replacement of natural surfaces by impermeable material

in urban areas allows a high runoff ratio and increases both

peak flows and total runoff volume. Owe (1985), for example,

showed a 51 percent increase in annual runoff after

development of an area in Pennsylvania. Hollis (1975) designed

a procedure for predicting changes in flow peaks as a function

of flood recurrence interval and percent of the basin surface

that is impervious, and Stankowski (1972) developed equations

to estimate the percent of an area impervious as a function of

population density. Driver and Troutman (1989) produced

regression equations to predict runoff characteristics for urban

areas throughout the United States.

Physical disturbance by plowing or scarification may

increase soil permeability and infiltration capacity, and the

accompanying increases in soil surface roughness can slow

overland flow and allow it more time to infiltrate. Increased

infiltration decreases runoff peaks, increases low-flow

discharge, and decreases runoff volume by providing more

water for evapotranspiration. However, sediment carried by

rainsplash and surface erosion can plug surface pores of

some exposed soils, and infiltration rates at these sites can

be reduced to less than pre-disturbance levels. Reconstruction

of soils on mine spoils and reclaimed mine land can either

enhance or decrease infiltration, depending on the severity

of compaction and the type of material. Gilley and others

(1977) found decreased infiltration capacities on coal mine

spoils that contained sodic sediments, because high sodium

content increased soil dispersion and promoted formation of

a surface crust that retarded infiltration.

Burning of some plants releases volatile oils that can coat

soil particles to form a water-repellent (hydrophobic) layer

that restricts infiltration. Dyrness (1976) measured a tenfold decrease in infiltration rates after a wildfire in the

Oregon Cascades, and McNabb and others (1989) found a

15 percent decrease after a slashburn in southeastern Oregon.

Runoff rates increase if hydrophobic effects are widespread,

and flood peaks may be increased, low-flow discharges

diminished, and available moisture decreased. Debano (1981)

reviewed research on hydrophobic soils and described

characteristics of soils susceptible to water repellency.

Reservoirs alter runoff volumes by increasing the area of

water surface susceptible to evaporation. In addition,

impoundments held for consumptive use can remove an

appreciable proportion of a small catchment’s runoff. Milne

and Young (1989) found that stockponds in the Little Colorado

watershed retain up to 7 percent of the basin’s runoff during

dry years.
62 USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993.

sediment load and thus contributes to floodplain construction,

channel aggradation, and loss of flow capacity. Graf (1978)

documented a 27 percent decrease in channel width after

colonization of Colorado Plateau streambanks by introduced

tamarisk. Encroachment of riparian vegetation is common

where dams decrease downstream peak flows. Northrup (1965),

for example, found channel capacities downstream of dams

on the Republican River to have decreased by two-thirds, and

cited vegetation encroachment as a contributing cause. Encroachment can also be encouraged by heightened water tables

due to floodplain irrigation (Nadler and Schumm 1981).

Channel hydrographs are also influenced by channel form,

which is controlled, in part, by bank stability. Plants contribute

to bank stability by increasing soil cohesion and shielding

erodible sediment from high-velocity flow. Headward

expansion of channel networks speeds delivery of water

downstream and contributes to increased flood peaks and

runoff volumes. Channel network stability is influenced by

the resistance of unchannelled swales to erosion, and here,

too, resistance is often controlled by vegetation cover.

Developed areas are usually protected from flooding by

levees, and some reaches may be fully channelized to improve

flood conveyance and prevent channel migration. Flood flows

are no longer slowed by floodplain roughness at these sites,

and they are transmitted more rapidly through the channelized

reach. However, flooding may increase downstream of

channelization works: flood waves attenuate less because

flows are delivered downstream more efficiently; excess

water is no longer stored temporarily on upstream floodplains;

and the contrast in conveyance at the end of the channelized

reach allows flow to back up in this area. Brookes (1987)

described channel enlargement in British rivers in response

to increased peak discharges from channelization.

Other topographic modifications may divert tributaries

from one stream to another. Road crossings and mine tailings

can dam and reroute streams, hydroelectric developments

may divert runoff to increase head, and irrigation ditches

may return flow to other channels. In these cases, flow is

reduced in the original channel while the new channel must

accommodate higher flows. More minor diversions may

result from rerouting of road drainage or formation of ruts

on ORV tracks, cattle paths, and hiking trails.

Impoundments may decrease water yield by increasing

the proportion of runoff lost to evaporation, and they strongly

affect downstream hydrographs even if they freely pass

flows. Impoundments usually retard and decrease the

amplitude of flood waves because they provide some storage

for flood waters and because outflow rates do not respond

instantly to changes in inflow. The frequency of high,

sediment-flushing flows is usually reduced in downstream

channels. A variety of other effects depend on the purpose of

the reservoir. Where flows are controlled for power generation,

diurnal flow variation may be extreme as power demands

fluctuate. Reservoirs built to supply water for consumptive

use usually decrease downstream water yields. At the

beginning of the runoff season, peak flows are reduced

downstream of these impoundments until storage capacity is

Alteration of other watershed processes can also influence

rates, timing, and modes of runoff generation. Erosion often

leaves mineral soil exposed, which increases rates of overland

flow and, in turn, promotes erosion. Walker and Everett

(1987) described acceleration of snowmelt near a road in

arctic Alaska where road dust decreased albedo of the snow.

Water Transport on Hillslopes

Ground-cover vegetation slows surface runoff and decreases

peak discharges by roughening the flow path. Temple (1982)

and Petryk and Bosmajian (1975) described methods for estimating vegetation roughness. Where surface flows are slowed,

water has more time to infiltrate and recharge soil moisture.

Land-use activities often affect hillslope flow by altering

topography. Roadcuts and other excavated faces can increase

overland flow by intercepting subsurface flow, and Megahan

(1972) documented the importance of this mechanism in

permeable Idaho soils. Similarly, channel incision in response

to gravel mining can lower streamside water tables (Sandecki

1989), and open-pit mines and quarries can intercept aquifers.

Grazing, tillage, yarding, and ORVs engrave depressions on

hillslopes. Where depressions are along contour they roughen

the surface, slow overland flow, and promote infiltration, but

where they are parallel to gradient, flow can collect and accelerate.

Road ditches and ruts commonly act as channels, and flow

generated on impervious road surfaces or intercepted by roadcuts

rapidly enters this expanded channel system and often is

efficiently routed to natural streams. Artificial expansion of the

drainage network is extreme in urbanized areas. Each paved

street becomes part of the network and rapidly carries runoff to

sewer systems designed for efficient conveyance. Urban peak

flows are increased not only because of the vast increase in

impermeable area, but also because of the efficiency of the

engineered drainage network. Graf (1977) measured a 50 percent

increase in drainage density after suburbanization near Iowa

City and attributed changes in runoff characteristics to the

altered channel network.

Several models have been developed to predict the effects

of urbanization on hydrographs. Arnold and others (1987)

used such a model to reconstruct pre-urbanization runoff

near Dallas and compared predicted and measured yields to

show a 12 percent volume increase from urbanization. Ng

and Marsalek (1989) used an urban runoff model developed

by the U.S. Environmental Protection Agency to predict

changes in runoff volume and flood peaks from hypothesized

land-use changes in Newfoundland.

Water Transport in Channels

Streambank and channel vegetation contribute to increased

peak stages by slowing flood flows. Burkham (1976) measured this effect for floods on the Gila River and demonstrated differences in flood character for three conditions of

bank vegetation. Pasche and Rouve (1985) developed a model

to predict flow resistance imparted by floodplain vegetation.

Where flow is slowed by vegetation it deposits part of its
USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993. 63

reached, but later peaks are allowed to pass. Flood control

reservoirs, on the other hand, are intentionally drawn down

before flood season, and flood peaks are temporarily stored

and released as lower flows. Most reservoirs serve multiple

purposes, and management strategy is modified to

accommodate a variety of demands.

Impoundments that enhance downstream baseflows by

seepage or release can promote riparian vegetation and allow

banks to stabilize and encroach. In a case reported by Bergman

and Sullivan (1963), the result was decreased channel capacity

downstream and a corresponding increase in flood severity. Yost

and Naney (1975) reviewed the effects of dam seepage on channel

morphology, water tables, salinization, and other characteristics.

Woody debris modifies channel flows by forming

obstructions and by triggering the formation of scour pools,

and this increased channel roughness slows and deepens

flows. Heede (1972) found that 70 percent of pools in a

Colorado forest stream were associated with organic debris,

and MacDonald and Keller (1987) measured a 2.5-fold

increase in flow velocity after natural organic debris was

removed from a forest stream. Some organic debris armors

banks and prevents channel widening, while other pieces

deflect flow toward erodible banks. Debris jams often force

channels to take new paths.

Altered transport of watershed products such as sediment,

organic debris, and chemicals also affects hydrology. Input

of fine sediments can decrease channel bed permeability,

and Bjerklie and LaPerriere (1985) and Bull and Scott (1974)

warned that sedimentation from in-stream mining can reduce

groundwater recharge. Coarser sediments also accumulate

in channels. Collier and others (1964) described infilling of

pools by sediment eroded from coal mine spoils, and Orme

and Bailey (1971) documented valley aggradation of up to

10 m at a site in southern California after erosion rates

increased upstream. These changes in channel morphology

affect water transport rates, and aggradation can increase the

frequency of overbank flooding. Hess (1984) attributed

increased flooding in Cabin Creek, Washington, to increased

input of sediment and organic debris from logging.

Rapid rates of channel aggradation can mantle stream

beds with permeable sediment, and low flows may trickle

through the bed sediment rather than flowing over it. Surface

flow may become discontinuous or ephemeral at these sites

even though discharge has not changed. Altered sediment

input rates also change the size distribution of sediments on

the channel bed (Dietrich and others 1989), and so can alter

flow velocity by modifying channel roughness.

Water Budgets and Runoff Modeling

Changes in basin hydrology are often assessed using water

budgets. A budget quantifies water inputs, changes in storage, and export from a defined area by using the principle

that input, less change in storage, equals output. The effects

of particular land-use activities on hydrologic conditions

can be estimated if each component is defined and controlling processes are understood.

Water budgets can be constructed from field measurements

where sufficient data exist. Lewis and Burgy (1964), for

example, used measurements of precipitation, runoff, and

groundwater levels at a field site in the Sierra Nevada foothills

to back-calculate consumptive use of water by native

vegetation. Groundwater outflow removed about 5 percent

of the precipitation input to their study area. Available data

are rarely sufficient for a complete analysis, however, and

balances are usually calculated using models to quantify

particular components. Thornthwaite and Mather (1957)

outlined a method for calculating water balance, and the

method was further discussed and illustrated by Dunne and

Leopold (1978).

Precipitation is assessed using field measurements or

regional records. If nonlocal measurements are used, they

must be adjusted to reflect the characteristics of the study

watershed. Precipitation usually increases with elevation,

but rates of change are often difficult to evaluate because

high-elevation data are rare. Other inputs may include artificial

imports or contributions from regional groundwater.

Output is in the form of transpiration, evaporation, artificial

exports, basin outflow, and contributions to regional

groundwater. Transpiration and evaporation can be estimated

using methods described by Thornthwaite and Mather (1955)

and Penman (1948), and artificial exports are usually gauged.

Groundwater recharge generally presents the most intractable

unknown, and outflow is usually the component solved for.

Outflow can often be estimated using regional water yield

measurements.

Changes in storage are often small relative to input and

output, and include changes in soil moisture, changes in volume

of water in transport or in ponded storage, and changes in local

groundwater volume. Although seasonal variation of these values

is often high, year-to-year variation is low and is often ignored.

Mahacek-King and Shelton (1987) constructed a water

budget to estimate the effects of logging on peak discharges

in Redwood Creek. In this case, modelling consisted merely

of calculating Thornthwaite evapotranspiration and including

an estimated runoff delay factor calibrated using pre-logging

records. Post-logging flows were then reconstructed assuming

natural vegetation covers, and comparison of predicted and

measured flows showed an increase in peak flows.

Models for predicting hydrologic change as a function of

watershed conditions are increasingly common. Ng and

Marsalek (1989) used a hydrologic simulation program

developed by the EPA to assess likely effects of urban

development on water balance, and Simons and others (1981)

developed a runoff model specifically for use on forest roads

and calibrated it for conditions in California. Leavesley and

others (1987) described the Precipitation Runoff Modeling

System (PRMS), a general runoff simulation model currently

under development by the U.S. Geological Survey. PRMS is

being calibrated to predict the effects of timber management

on water yield and peak flows in coastal Oregon and the

Oregon Cascades. El-Kadi (1989) reviewed existing watershed

models and noted that few adequately evaluate groundwater

components.
64 USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993.

Effects of Environmental Change

on Sediment

The amount of sediment produced, its mode and timing of

production, its grain size, and its transport through a channel

network are important characteristics of a watershed’s sediment regime. Changes to any of these factors can alter

watershed function.

Many studies provide measurements of erosion from various

types of land use. Results can be attributed to particular

processes at specific sites if sources are carefully isolated, but

many studies measure the combined effects of several activities

by monitoring only a watershed’s outflow. These studies can

measure impacts in that basin and indicate what responses are

important, but the effects of specific changes can rarely be

identified and results cannot be generalized. Dunne (1984)

discussed methods of predicting erosion in forested areas,

Larson and Sidle (1980) described erosion measurements

from the Pacific Northwest, and Toy (1984) reviewed the

effects of strip and open-pit mines on sediment production.

Erosion and Sediment Transport on

Hillslopes

Erosion rates are controlled by properties of erosive agents

and by those of erodible materials, and both are influenced

by environmental parameters. Most erosion processes actively transport sediment, though the processes that initially

dislodge sediment are often not the same as those that eventually contribute it to streams. For example, a sand grain may

be eroded by rainsplash and carried downslope by sheetwash.

Plants shield erodible materials from erosion by wind,

rain, and running water, and they modify the erosivity of

those agents by decreasing air and water velocities. Plants

also impart root cohesion to soils and increase their resistance

to erosion. Erosion rates usually change when vegetation is

altered, and changes in soil properties also affect erosion.

Compaction generally reduces soil erodibility but increases

the erosivity of flows, and changes in soil texture and structure

affect erodibility. Topographical changes can alter slope

stability and flow erosivity by changing the gradient of a

surface or diverting flow to a new path.

Because water is an almost ubiquitous factor in erosion,

most hydrologic changes alter erosion rates. Each type of erosion

process reacts differently to changes in environmental parameters.

Surface Erosion

Surface erosion includes processes of rainsplash, sheetwash,

rilling, and dry ravel (the progressive shedding of particles

from materials that are losing cohesion). Extensive experiments by Federal, State, and university researchers in the

1950s and 1960s produced the Universal Soil Loss Equation

(USLE), which calculates surface erosion by sheetwash and

rilling as a function of hillslope gradient, soil type, slope

length, rainfall regime, conservation practice, and vegetation cover (Wischmeier and Smith 1978). The equation demonstrates that surface erosion rates increase with decreased

ground-cover vegetation and soil organic matter or increased

hillslope gradient and length. Although originally developed

for agricultural lands, the equation has been used on rangeland (Renard and Foster 1985, Savabi and Gifford 1987),

forest land (Dissmeyer and Foster 1981), construction sites

(Holberger and Truett 1976), and roads (Farmer and Fletcher

1977, Meyer and others 1975). The equation’s assumptions

and limitations must be understood if results are to be valid,

and Wischmeier (1976) provided guidelines for appropriate

use. Williams (1975) modified the USLE to calculate sheet

erosion for individual storms and to estimate the proportion

of sediment redeposited.

Researchers are currently developing a more processbased equation to take the place of the USLE for estimating

sheetwash and rill erosion. Known as the Water Erosion

Prediction Project (WEPP), this effort is intended to provide

an equation for application to many land uses (Lane and

others 1988, Nearing and others 1989).

Sheetwash, rill, and gully erosion rates usually increase

where soil compaction or loss of a permeable horizon increases

the volume of overland flow. Mechanical disruption of the

soil surface can inhibit surface erosion by reducing overland

flow, but it can also accelerate erosion by producing erodible

sediments, and disruption of organic soil horizons can accelerate

soil loss by exposing more erodible subsurface soils. Soil

erodibility generally increases as organic matter decreases

(Wischmeier and Smith 1978), so any activity that depletes

surface soils of organic matter is likely to promote erosion.

Vehicular traffic and livestock trampling can compact

subsurface soil horizons while producing a surficial dust

layer. The resulting sediment loss can be particularly high

because these changes increase both overland flow and

transportable sediment. Disaggregation can also promote wind

erosion, and Campbell (1972) suggested that roads in rural

subdivisions could be important sources of wind-blown dust.

Construction of long or steep slopes can increase rates of

sheetwash erosion and rilling, as is common on roadcuts.

More subtle topographic changes, such as furrows produced

by noncontour plowing or hiking trails, can also channel

enough runoff to trigger rilling. Dry ravel is common on

excavated soil faces, and Megahan (1980) measured the

combined effects of dry ravel and other surface processes on

roadcuts in Idaho.

Changes in the chemical environment generally have only

minor effects on the sediment regime of a watershed. Chemical

changes can make a soil more or less erodible by changing

its propensity toward aggregation. Some clay-rich soils, for

example, form clods that disintegrate rapidly when wetted,

while others produce clods that can be transported for tens of

meters as bedload in streams. This behavior is in part

determined by chemical properties of fluids in soil pores, and

Heede (1971) found the greatest incidence of dispersionrelated piping at a site in Colorado in soils with high

exchangeable sodium contents.
USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993. 65

The process of sheetwash erosion has been widely studied,

and erosion rates or trends can now be predicted for a variety

of conditions and for several types of hydrologic change,

although some uncertainties persist. Abrahams and others

(1988) and Govers (1987) both defined conditions under

which erosion was initiated, but with conflicting results.

Bryan (1979) examined the influence of gradient on sheetwash

and rainsplash erosion rates and found that antecedent moisture

and soil type influenced the form of the relation and that

unexplained variability was high. Govers and Rauws (1986)

evaluated the effects of surface roughness on transport, and

Lyle and Smerdon (1965) studied the influence of soil

properties on erosion rates. Kilinc and Richardson (1973)

used a rainfall simulator to examine the mechanics of

sheetwash erosion and developed a transport equation. Moore

and Burch (1986) found that sheetflow transport capacity

was well predicted by the product of flow velocity and

gradient (unit stream power). Most of these studies concerned

rain-generated flow and took into account the combined

effects of sheetwash and rainsplash. Hart and Loomis (1982),

however, examined surface erosion during snowmelt and

discussed problems of applying the USLE to snowmelt runoff.

Surface erosion selectively mobilizes fine sediment, so

delivered sediment may not reflect the size distribution present

in the eroding soil. Gilley and others (1987) found that the

grain size distribution of sediment in transport varied along a

slope profile, and Lu and others (1988) examined selective

transport and deposition of different sediment size classes.

Duncan and others (1987) measured the size classes of roadsurface sediment delivered to streams through ephemeral

channels at a site in western Washington and found that sands

tended to accumulate in the channels while clays were washed

on through. Hamlett and others (1987) demonstrated that

during large storms, differences in character decrease between

sediment available for transport and that removed by erosion.

Persistence of accelerated surface erosion often depends

on how quickly vegetation recovers. Megahan (1974) found

that erosion rates on roadcuts and fill slopes in the Idaho

Batholith decreased exponentially with time as sites recovered.

Gullies

The distinction between rills and gullies is based on the size

of the feature: incised channels that cannot be plowed over

are traditionally considered gullies while smaller ones are

called rills. Gully development represents several erosion

processes. Gully widening is accomplished by sheetwash

and ravel on walls, in-channel tractive erosion, and smallscale landsliding, and incision is a form of channel erosion.

Melton (1965) described the potential importance of

vegetation in controlling gully initiation and hypothesized

that arroyo formation in the American Southwest may have

been promoted by decreased swale vegetation. Heede (1971)

examined processes of gully development on two soil types

in Colorado and found that pipe collapse formed gullies

more readily on soil with high concentrations of exchangeable

sodium. Soil compaction is often a factor in gully development

because it promotes surface runoff.

Topographic modification by road construction or skidtrail use is the most common cause for gullying in logged

areas. Gullies often form where drainage is diverted onto

unprotected slopes by roadside ditches and culverts, where

culverts block and divert flow over roadbeds and fill slopes,

or where ruts concentrate flow.

Most changes affecting gully erosion involve altered

hydrology. Either runoff is increased and increases erosion

power, or channel networks are modified or expanded and

expose susceptible sites to erosion.

Landslides

Mass-wasting processes attack the entire soil profile. Plant

roots inhibit shallow landsliding and soil creep by increasing

soil cohesion. Swanston (1970) calculated failure conditions

for a debris avalanche in the Maybeso Valley of southeastern Alaska and evaluated importance of root cohesion, and

Swanston (1969) showed that root decay rates may explain

the delay observed by Bishop and Stevens (1964) in landslide initiation after clearcutting in the same area. Hawley

and Dymond (1988) found that failure frequency increased

with distance from trees at a site in New Zealand. Transpiration by vegetation can reduce activity levels on deep-seated

failures by decreasing soil moisture.

Construction and mining activities modify topography

and often create cut or fill slopes that are steeper and more

unstable than those originally present. Sites where

oversteepened materials are newly emplaced or where drainage

has been disrupted are at particular risk, and these conditions

are common along roads and on tailings piles. Failure generally

occurs as a debris avalanche or slump in these cases. Hagans

and Weaver (1987) estimated that 40 percent of erosion in

the Redwood Creek watershed resulted from road-related

diversions and channel crossings. Construction can also

accelerate earthflows by loading the head of the flow or

undercutting the toe, and roadcuts in earthflow terrain often

gradually encroach on roads.

Undercutting of rock slopes occasionally destabilizes

bedrock and causes slides that range in scale from single

dislodged blocks to rock avalanches of several thousand

cubic meters. Mining activity on Turtle Mountain in Canada

may have helped trigger a slide that brought down 30 to 90

million metric tons of mountainside (Selby 1982).

Much research has been devoted to developing methods

of recognizing unstable sites and evaluating site stability.

Rice and Lewis (1986) and Rice (1985) evaluated landslide

risk by statistically analyzing the settings of slides in northwest

California, and Duncan and others (1987) used a similar

method in Oregon and Washington. Rollerson and others

(1986) measured landslide frequency as a function of terrain

class to develop a risk rating for sliding in coastal British

Columbia. Landslide risk varies with time after an activity

has been completed. If organic debris is incorporated in fills,

road-related slides may occur after it decays, and failures on

clearcuts may be delayed until root networks have rotted.

Ziemer and Swanston (1977) measured decreases in hemlock

and spruce root strength as a function of time after clearcutting.
66 USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993.

The National Council of the Paper Industry for Air and

Stream Improvement (NCASI 1985) reviewed processes of

landsliding in forested sites, discussed its relation to timber

management, described methods of assessing landslide risk,

and summarized results from a variety of landslide inventories

in the Pacific Northwest.

Landslide scars are susceptible to surface erosion as long as

they remain bare, and landslide frequencies at a site depend in

part on how rapidly landslide scars heal. Several studies have

examined rates of site recovery after landsliding by describing

vegetation changes (Smith and others 1984) and changes in soil

character (Trustrum and DeRose 1988) through time.

Because pore-water pressure is so important in triggering

landslides, hydrologic changes that increase pore pressures

often cause slides. Swanston (1967) measured piezometric

head as a function of precipitation at several Alaskan sites

and found that head increases were higher in drainage

depressions, thus providing an explanation for high rates of

landsliding in swales. Caine (1980) compiled rainfall

intensities at 73 sites throughout the world as a function of

duration for storms that triggered debris flows and identified

threshold values for landslide initiation on undisturbed slopes.

Soil Creep

Soil-creep rates are likely to increase with devegetation and

regrowth on hillslopes. Tunnels left by decaying roots are

filled by particles falling vertically rather than perpendicularly to the hillslope surface, and this displaces the particles

slightly downslope. In addition, creep may be promoted by

loss of root cohesion, and soil moisture increases from decreased transpiration are likely to prolong the season for

plastic creep. Barr and Swanston (1970) and Dedkov and

others (1978) found that rates tended to be higher at wetter

sites or during wetter periods. Accelerated creep may increase soil accumulations along channel margins and increase bank erosion rates.

Saunders and Young (1983) compiled creep measurements

and indicated the range of rates that might be expected, but

measurements are difficult to carry out and so are rare. Accurate

measurements require long monitoring periods, and most measurement techniques either are not sensitive to vertical

discontinuities (e.g., inclinometer tubes) or can be used only

once because they require disruption of the profile to record a

measurement (e.g., columns of marked particles). Most

measurements of creep rates after a change in land use either have

been of too short a duration to provide significant results or are in

rapidly creeping earthflows rather than typical hillslope soils.

Treethrow

Uprooting of trees disturbs large volumes of soil, and rootwads

are usually displaced downslope. This contributes to

downslope soil transport and exposes soil to further erosion

by sheetwash and rainsplash, but transport rates by treethrow

have rarely been measured. Treethrow is particularly common in riparian zones, where trees tend to be short-lived,

roots are shallow, and banks are susceptible to undercutting

(McDade and others 1990).

Sediment Delivery to Streams

Much of the sediment eroded on hillslopes is redeposited

elsewhere on the slope. For example, Vice and others (1968)

estimated that over half the sediment mobilized during freeway construction at a site in Virginia remained within the

watershed. Although surface erosion rates are widely measured, much less is known of sediment delivery rates to

channel networks. Williams (1975) incorporated a sediment

delivery term into the USLE, and Tollner and others (1976)

developed an equation for estimating deposition from sheetflow

as a function of flow character, vegetation character, and

transport distance through vegetation. Heede and others (1988)

examined the role of vegetation recovery after a chaparral

fire in controlling the timing and rates of sediment delivery

to streams, and thus in controlling the timing and location of

channel adjustments. Haupt (1959) identified factors that

control the redeposition of sediment downslope of logging

roads, and Cook and King (1983) measured the volume of

sediment filtered out by windrows on fill slopes below roads

at a site in Idaho. Khanbilvardi and Rogowski (1984) and

Novotny and Chesters (1989) reviewed methods of estimating delivery ratios on the scale of plots and hillslopes.

Sediment delivery in large watersheds has been correlated

with morphological factors. Roehl (1962), for example, found

that the proportion of sediment eroded on hillslopes that

arrives at a watershed’s mouth decreases with increasing

watershed area and channel length, and increases with

increasing relief ratio. This relation implies that some sediment

is lost in transport and may reflect lowland aggradation or

chemical dissolution during transport and storage.

Erosion, Transport, and Deposition

in Channels

Changes in a channel’s ability to erode and transport sediment alter the channel’s morphology, and altered morphology affects transport. Although some mechanisms and interactions between erosion, transport, deposition, and morphological change are well known for low-gradient alluvial

streams, their expression in the small, high-gradient channels characteristic of forested watersheds is poorly understood (see Lisle 1987 for discussion).

The extent of major channel changes is usually measured

using sequential aerial photographs. Grant (1988) described

techniques for assessing channel changes using aerial

photographs and hypothesized relationships between the

pattern of a channel alteration and the type of process

generating it. Diagnostic features include the presence or

absence of landslide scars, the length and continuity of the

change, and whether the magnitude of the change increases

or decreases downstream.

Bank and Channel Erosion

Processes active on stream banks include surface erosion,

small-scale landsliding, and tractive erosion from channel

flows. Rates of bank erosion in alluvial soils have often been
USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993. 67

measured (e.g., Hooke 1980), but measurements from bedrock or colluvial banks are less common. Hooke (1979)

described factors controlling rates of surface erosion,

landsliding, and tractive erosion on banks. Pizzuto and

Meckelnburg (1989) related bank erosion rates along

Brandywine Creek in Pennsylvania to near-bank flow velocities and also noted the importance of bank vegetation.

Roots enhance bank stability by increasing soil cohesion;

ground cover shields erodible sediment from flow; and stems

and leaves slow flows and decrease their erosivity. Smith

(1976) found that removal of bank vegetation in the Alexandra

Valley, Alberta, increased erosion rates by a factor of 20,000.

Unstable banks allow channels to widen and become

shallower. Zimmerman and others (1967) showed that channel

morphology in a small stream that drains forest and grassland

varies with bank vegetation: forested reaches tend to be

wider than those with grass on their banks. Similarly,

Murgatroyd and Ternan (1983) found that streamside canopies

in a Dartmoor forest plantation were dense enough to inhibit

ground cover on banks, and channels widened in response.

Graf (1979) showed an association between the location of

incised channels and sites of low riparian biomass, though it

is unclear whether lack of vegetation promoted incision or

incision inhibited vegetation. Some Welsh streams have

widened because toxic chemicals from mining have stripped

vegetation from streambanks (Lewin and others 1977). Bray

(1987) measured vegetation-related changes in channel

morphology in a channelized reach in New Brunswick, where

excavation of a new channel without riparian vegetation

triggered rapid widening and formation of midstream bars.

As riparian communities redeveloped, the channel began to

reassume its original form. Kondolf and Curry (1986)

attributed widening of the Carmel River in part to decreased

bank vegetation, and Graf (1978) attributed narrowing of

Colorado Plateau rivers to increased bank vegetation.

Vegetation also affects channel morphology after it dies:

logs are important structural elements in forest streams.

Woody debris decreases flow velocity by roughening

channels; it forms obstacles that induce pool formation; and

it deflects flow into or away from banks. Fallen trees also

divert flow into new paths, and Keller and Swanson (1978)

found that meander cutoffs on moderate-sized, low-gradient

streams in the Midwest are often associated with fallen trees.

Logs no longer enter channels where riparian forests are

removed, and morphology must adjust to a decreasing

frequency of obstructions as the remaining logs decay. Hogan

(1987) associated simplification of channel morphology with

decreased input of organic debris after logging in the Queen

Charlotte Islands. Sedell and Froggatt (1984) described

extreme decreases in wetted margin along the Willamette

River that were due, in part, to removal of organic debris for

navigation. Organic debris had helped to maintain the

Willamette’s sloughs and backwaters, and many channels became

choked with sediment when logs and rootwads were removed.

Woody debris was cleared from channels to “improve”

habitat and decrease debris flow risk in many areas logged

10 to 30 years ago, and the resulting disruption of channel

morphology was extreme. Many of these channels now

resemble gravel flumes: they are without significant pools

and have mobile, unstable beds. Debris removal also

contributed to sediment loads by liberating the sediment

stored behind logs (Klein and others 1987).

Structures designed to armor banks or deflect flow are

often emplaced to protect floodplain developments from

channel migration. Deflected flows may impinge on a

downstream bank and accelerate erosion there. Direct

disturbance of stream beds by gravel mining, road and bridge

construction, trampling, ORV use, dredging, and other uses

mobilizes fine sediments and elevates turbidity. Burns (1970)

measured sedimentation during logging in northern California

and found that the highest rates accompanied debris removal

or use of heavy machinery in or near streams. Dredging or

gravel mining can provoke channel incision as erosion

compensates for loss of bedload (Collins and Dunne 1989).

Sediment previously deposited by a river and held in

storage in floodplains or terraces can be remobilized by

channel erosion. Kelsey and others (1987) used transitional

probability matrices to evaluate inputs from storage elements

with different characteristic stabilities in Redwood Creek.

Altered channel morphology has often been associated

with particular types of land use such as grazing (e.g.,

Kauffman and others 1983a) and dam construction (e.g.,

Petts 1979). Rango (1970) predicted the effects of cloud

seeding on channel morphology by examining channel

geometry as a function of rainfall at 673 sites along a climatic

gradient. Richards and Greenhalgh (1984) described problems

with evaluating nonequilibrium channels to understand the

impacts of land use, and warned that most channels are not

completely in equilibrium.

Flow leaving impoundments is depleted of sediment

relative to its carrying capacity and can therefore entrain

fine sediments. Pemberton (1976) used sediment transport

equations to predict the approximate volume of sediment

lost from the bed of the Colorado River after closure of Glen

Canyon Dam. Trapping of sediment in Flaming Gorge

Reservoir has caused erosion of fine sediments downstream,

but decreased peak flows have prevented removal of coarse

sediments introduced by tributaries, and riffles have grown

at confluences (Graf 1980). Where channels downstream of

dams have erodible beds, the sediment imbalance may trigger

entrenchment that can propagate up tributaries (Germanoski

and Ritter 1987). Moglen and McCuen (1988) suggested

that downstream scour is important even on the scale of

sediment detention basins.

Channel Sediment Transport

Sediment transport rates and the mode of sediment transport

depend on discharge. Bed material is not mobilized until a

particular discharge is attained, so changes in peak discharge

can strongly influence both the bedload transport rates and the

proportion of the total load that is conveyed as bedload. In most

cases, however, bedload comprises less than a third of the total

load, and few non-sand-bedded channels convey more than 15

percent of their load as bedload (Vanoni 1975, p. 348).
68 USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993.

Considerable research has been devoted to developing

methods of predicting transport rates as a function of flow

characteristics and topography. Equations developed by

Einstein, Meyer-Peter and Müller, and Yalin are most

common, and Gomez and Church (1989) and White and

others (1978) reviewed the use of these and other equations.

Transport predictions have been consistently poor for

mountain streams, so Smart (1984) developed an equation

for use in high-gradient channels. Nouh (1988) modified an

existing equation to account for transport by nonsteady flow

in ephemeral streams subject to flash floods. However,

transport equations have rarely been adequately validated in

field settings. Results vary widely in accuracy at particular

locations, and extrapolation of any method beyond the

conditions for which it was developed usually invalidates

the application.

Suspended sediment is widely measured and has been

monitored at U.S. Geological Survey gaging stations for

decades, but bedload measurements are less common. The

temporal and spatial variability of bedload transport and the

logistical problems of sampling moving rocks during floods

make direct measurement difficult and complicate the

interpretation of results. Bedload is often estimated by

measuring suspended load upstream and downstream of a

settling pond, measuring the volume of sediment deposited

in the pond, and using these data to calculate the proportion

of infill that represents bedload.

Direct hydrologic modifications by climate change or

interbasin transfers alter the frequency of sediment-transporting

flows. If peak flows or runoff increase, streams scour their

bed and banks more effectively, erosion and transport rates

increase, and channels tend to incise. Abbott (1976)

documented widening and entrenchment when water imports

increased flow in a channel in Colorado, and Dzurisin (1975)

demonstrated entrenchment, armoring, and knickpoint retreat

where flow was augmented by stream capture at a site in

Death Valley. Where the frequency or magnitude of sedimenttransporting flows is decreased, aggradation is likely. Gregory

and Park (1974) observed a reduction in channel capacity

through aggradation and bar stabilization after a British

reservoir decreased flood peaks. Jackson and Van Haveren

(1987) used measured regional relations between discharge

and channel geometry to predict the effects of decreased

flows on channel morphology in Beaver Creek, Alaska.

Urbanization commonly increases runoff and peak flows,

and Neller (1988) measured bank erosion rates in an Australian

urban area to be 3.6 times higher than in a similar rural

stream, and rates of knickpoint retreat to be 2.4 times greater.

Graf (1975) found that streams draining a developing suburb

near Denver initially aggraded their floodplains, but then

entrenched when postconstruction sediment loads decreased.

Wolman and Schick (1967) described channel widening to

accommodate bar deposition from increased sediment input

during urban construction in Maryland. Nanson and Young

(1981) attributed a two- to three-fold increase in channel

cross-sectional area in urban areas near the Illawarra Escarpment

of Australia to the combined effects of increased peak flows

from urban runoff and decreased area of floodplain storage

from levee construction.

Sediment Deposition in Channels

Many land-use impacts involve deposition in channels and

resulting changes in channel morphology. Rates and locations of channel erosion and deposition are controlled by

spatial and temporal variations in both a channel’s ability to

transport sediment and the contribution of sediment from

hillslopes. High rates of sediment input from hillslopes can

cause localized aggradation in channels. Short (1987) documented extreme widening and aggradation after landslidetriggering storms at Cuneo Creek, California, and Anthony

and Harvey (1987) described a channel’s change from meandering to braided after an extreme increase in bedload

transport in Colorado. Perkins (1989) measured erosion rates

of landslide deposits in western Washington streams and

found that 20 to 80 percent of the volume was removed

within 7 years. Hansen and Alexander (1976) introduced

sand into a Michigan channel and monitored the resulting

decreases in pool volumes, increases in channel width, and

decreases in channel roughness.

Long-term changes from chronic sediment inputs are also

important, but are harder to evaluate. Madej (1982) compared

geometries of disturbed and undisturbed channels in western

Washington to describe a change in form that reflects the

cumulative influence of a century of land use. Miller (1990)

recorded the distribution of contrasting channel responses to

sediment input and found that low-order Appalachian streams

tended to scour during floods while higher-order channels

aggraded or reworked deposits. Mosley (1981) found that

much deposition of coarse sediment in New Zealand streams

was localized behind woody debris, and that sediment transport

rates were strongly influenced by the collapse of debris dams.

Fine sediment is transported in suspension. Suspended

loads are often influenced more by sediment input rates than

by transport capacity. However, debris flows can overwhelm

channels with slurry-like concentrations of sediment, and a

portion of this sediment is likely be deposited near the input

site, at obstructions, and where channel gradient decreases.

Benda (1985) examined patterns of debris flow transport

along channels in the Oregon Coast Range and identified

slide locations likely to produce the most mobile flows.

Fine sediments can be deposited from suspension if flow

velocities decrease, as may occur on floodplains, in ponds or

reservoirs, or in gravel interstices of the channel bed. A

vegetative filter equation developed by Tollner and others

(1976) can be used to estimate deposition rates among plants.

Shapley and others (1965) found an exponential decrease in

suspended sediment with distance downstream of an input

point and inferred substantial deposition within the channel.

Duncan and others (1987) showed that over 55 percent of

fine sediment introduced into ephemeral channels during an

experiment in western Washington was trapped in the

channels, and Megahan and Nowlin (1976) found that 40

percent of hillslope sediment input from sandy Idaho batholith

soils was stored in channels.
USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993. 69

Processes of fine sediment deposition among channel gravels

are poorly understood. Cooper (1965) noted that intergravel

deposition can occur even from high-velocity flows because

sediment-laden flow percolates into low-velocity pockets in

the gravel bed. Bed compositions are widely measured and

are occasionally correlated to land-use intensities or activities.

Cederholm and Reid (1987) and Adams and Beschta (1980)

related percent intergravel fines to extent of roads and logging

in watersheds. Platts and others (1989) documented changes

in percent fines associated with increasing and decreasing

sediment inputs from timber management over a 35-year

period in Idaho. Beschta and Jackson (1979) examined intrusion

of fine sediment in gravel-bedded laboratory flumes, and

Shapley and others (1965) described field observations after

introduction of sediment into a stream in southeast Alaska.

Lisle (1989) measured intergravel sedimentation as a function

of sediment transport rates in northwestern California channels.

Work on intergravel deposition was reviewed by Jobson and

Carey (1989) to identify physical factors that influence

sedimentation. Sedimentation was found to be most profound

in less active parts of the stream bed, and location of the

deposits within the bed was influenced by the grain-size

distribution of bed sediments.

Impoundments usually form efficient sediment traps. Coarse

sediment is deposited in a delta as it encounters ponded

water, and only suspendible sediments are carried on. Brune

(1953) described a method for calculating the proportion of

sediment trapped by a reservoir; Moglen and McCuen (1988)

evaluated the ability of small impoundments to trap sediment;

and Mahmood (1987) reviewed the extent and significance

of reservoir sedimentation. Formation of a delta at the head

of an impoundment decreases the gradient upstream and

decelerates flow, causing some upstream aggradation.

Dams affect deposition downstream by changing the

distribution of channel-modifying flows. Decreased peaks

from flood-control dams contribute to aggradation downstream

of tributary sediment inputs, and this effect can propagate

upstream (King 1961). Petts (1984) described aggradation

due to the cumulative effects of damming and drainage ditch

excavation in a forest plantation in England. A dam on one

tributary decreased sediment transport capacity at the

confluence just as excavation on the other fork increased

sediment input, and the confluence aggraded rapidly. Petts

(1979) and Williams and Wolman (1984) reviewed the effects

of dams on downstream channels.

Land Use and Sediment Yield

Many studies describe the effects of particular types of landuse activities on sediment yields without differentiating between sediment sources. Sediment yields from logging and

roads are widely documented (table 12), and studies generally show a 2- to 50-fold increase over background levels,

with most of the increase associated with roads. Increases in

sediment input can be much larger at sites where landsliding

is particularly important. Sediment yields decrease rapidly

after road use is discontinued and logged areas regenerate,

so yields measured more than 5 years after logging are

usually less than five times higher than background rates.

Urbanization also provokes increased sediment yields.

Walling and Gregory (1970) reported 2- to 100-fold increases

in suspended sediment concentration below construction sites,

and Wolman and Schick (1967) estimated sediment yields of

700-1800 t per 1000-person population increase due to

construction near Washington, D.C.

Although sediment yields are an important indicator of

land-use impact, they are difficult to interpret without

information on how the sediment is produced. Van Sickle

(1981) compiled long-term annual sediment yields from

many Oregon watersheds and showed that year-to-year

variation in yield was large. Basin sediment yields must

therefore be measured for a long period if they are to accurately

represent a long-term average. Roels (1985) noted that many

local measurements and results of plot-sized experiments

cannot be used to estimate rates over larger areas because of

defects in the study design, and emphasized that all

measurement programs should be carried out within the

framework of a statistically valid sampling plan.

Sediment Budgeting and Modeling

In 1917, Gilbert introduced a new approach to evaluating

land-use impacts. He inventoried sources of sediment in

watersheds affected by hydraulic mining in the Sierra Nevada foothills and estimated sediment production and delivery from each source. Gilbert used this information to estimate total sediment input to the Sacramento River system,

and from this he could predict trends in sediment transport

and evaluate impact persistence. The approach was largely

forgotten until 1966, when Leopold and others used a similar method to evaluate sediment inputs in a New Mexico

watershed. Since then, sediment budgets have been constructed for many other sites (table 13).

A sediment budget is a quantitative account of sediment

input, transport, and deposition in a watershed (Dietrich and

others 1982). Component processes are identified, and process

rates are usually evaluated independently of one another. If

the effects of particular land-use activities on each process

are known, the overall influence of a suite of existing or

planned land-use activities can be estimated. Sediment

budgeting is particularly effective for evaluating

nonequilibrium situations, where channel loads do not

necessarily represent hillslope erosion rates. Trimble (1977)

used a sediment budget to show that modern sediment yields

in the southeastern United States reflect recent remobilization

of deposits left by long-abandoned erosive land-use practices.

Church and Slaymaker (1989) used a similar approach to

demonstrate the influence of Pleistocene landforms on presentday measurements in British Columbia. In both of these cases,

sediment yield measurements alone would not have provided

enough information to identify the influence of land use.

Storage is often the most difficult component of a sediment

budget to define. Madej (1987) evaluated sediment storage in

Redwood Creek and used results to predict long-term residual
70 USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993.

Table 12—Studies of sediment production from roads and timber management. Symbols are explained below.

Treatment Measurements

Location Approach Scale F R T C L O S Y

Amaranthus and others 1985 OR A P R T L

Anderson 1954 OR SA M R T Y

Anderson and Potts 1987 MT T C R T Y

Aubertin and Patric 1974 WV T B T Y

Beasley 1979 MS T B T Y

Beasley and Granillo 1988 AR T M T Y

Beschta 1978 OR T B R T L

Beschta 1990 - O - F S Y

Bethlahmy 1967 ID E P T S

Bethlahmy and Kidd 1966 ID T P R S

Bilby and others 1989 WA T P R S

Bishop and Stevens 1964 AK D P T L

Biswell and Schultz 1957 CA D P F R S

Blackburn and others 1986 TX T M T S Y

Brown and Krygier 1971 OR T B R T L Y

Burroughs and King 1989 - O - R S

Burroughs and others 1984 ID E P R S

Carr and Ballard 1980 BC T P R S

Cook and King 1983 ID T P R S

Day and Megahan 1975 ID D P R T L

DeByle and Packer 1972 MT T P F T S

Diseker and Richardson 1962 GA T P R S

Duck 1985 SL D C R Y

Duncan and others 1987b WA,OR S P R L

Durgin 1985 L E P F S

Dyrness 1967 OR D P R T L

Dyrness 1970 OR T P R S

Dyrness 1975 OR T P R S

Fowler and others 1988 WA T M R T Y

Fredriksen 1970 OR T B R T L Y

Froehlich 1991 Po T P R S

Furbish and Rice 1983 CA S P R T L

Gonsior and Gardner 1971 ID S P R L

Grant and Wolff 1991 OR T B R T L Y

Gray 1970 - O - T L

Gray and Megahan 1981 ID TMS P F R T L S

Gresswell and others 1979 OR D P R T L Y

Hagans and Weaver 1987 CA D P R T C L S

Harden and others 1978 CA A P R T L

Harr and Fredriksen 1988 OR T B F R T Y

Haupt 1959 ID S P R S

Hawley and Dymond 1988 NZ D P T L

Heede 1984 AZ T B T S Y

Heede and King 1990 AZ T P T S

Helvey and Fowler 1979 OR T P T S

Helvey and others 1985 WA T B F L Y

Hess 1984 WA D C T C L

Hornbeck and Reinhart 1964 WV T P R S

Johnson and Beschta 1980 OR S P R T S

Johnson and Smith 1983 ID T P R S

Johnson 1988 SL T C R T Y

Kidd 1963 ID T P R T S

Klock and Helvey 1976 WA T B F L Y

Kochenderfer and Helvey 1987 WV T P R S

Krammes and Burns 1973 CA T B R L S Y

LaHusen 1984 CA S P R T L

Leaf 1974 CO T P R S

Lewis and Rice 1989 CA S P R T L

Lewis and Rice 1990 CA S P R T L

McCashion and Rice 1983 CA S P R L

McClurkin and others 1985 TN T B T Y

McNabb and Swanson 1990 - O - F L S Y

Megahan 1974 ID T P R S

Megahan 1975 ID T M R T L S Y

Megahan 1978 ID T P R S

Megahan and others 1983 ID A P R S
USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993. 71

Table 12—(Continued)

Treatment Measurements

Location Approach Scale F R T C L O S Y

Megahan and others 1986 ID T P R S Y

Megahan and others 1991 ID T P R S

Megahan and Bohn 1989 ID T P R T L

Megahan and Kidd 1972a ID T P R T S

Megahan and Kidd 1972b ID T M R L S Y

Mersereau and Dyrness 1972 OR T P F T S

Meyer and others 1975 WI MT P R S

Mosley 1980 NZ D P R L S

NCASI 1985 - O - R T L

Nolan and Janda 1981 CA T M R T Y

O’Loughlin 1972 BC S P R T L

Patric 1976 - O - R T Y

Reid and Dunne 1984 WA T P R S

Reid and others 1981 WA AT P R L S

Rice 1985 CA S P T L

Rice and Datzman 1981 CA S P R T C L

Rice and Gradek 1984 CA S P R T C L S

Rice and Lewis 1986 CA S P R T L

Rice and McCashion 1985 CA S P R L

Rice and Pillsbury 1982 CA D P T L

Rice and Wallis 1962 CA T B T Y

Rice and others 1979 CA T M R T C L S Y

Rice and others 1985 OR S P T L

Riekerk 1985 FL T B T Y

Roberts and Church 1986 BC DA P T C L O S Y

Ryan and Grant 1991 OR D M R T C L

Sartz 1953 OR T P F S

Schroeder and Brown 1984 OR S P R T L

Sullivan 1985 OR T C R T L Y

Swanson and Dyrness 1975 OR S P R T L

Swanston 1969 AK S P R T L

Swanston and Marion 1991 AK AS P T L

Swanston and Swanson 1976 - O - R T L

Swift 1984a NC T P R S

Swift 1984b NC T P R S

Trimble and Weitzman 1953 WV T P R S

Vice and others 1968 VA T C R S Y

Watts and others 1986 ID E P R S

Wolfe and Williams 1986 CA A P R T L

Wood and others 1989 LA E P T S

Wu and Swanston 1980 AK M P T L

Ziemer and Swanston 1977 AK A P T L

Ziemer and others 1991 CA M P R T C L

Location:

AK Alaska FL Florida NC North Carolina TX Texas

AR Arkansas GA Georgia NZ New Zealand VA Virginia

AZ Arizona ID Idaho OR Oregon WA Washington

BC British Columbia L Laboratory Po Poland WI Wisconsin

CA California MS Mississippi SL Scotland WV West Virginia

CO Colorado MT Montana TN Tennessee

Approach or method: Scale:

A Reconstructs past using air photos or B Paired basins or sites

records, or by sampling age distribution C Case study

D Based on descriptive measurements M Multiple basins or sites

E Experimental P Process study

M Modeling or calculated results

O General review of topic Treatment:

S Survey of multiple sites F Fire

T Long-term temporal monitoring study R Roads

T Timber management

Measurements:

C Channel-related erosion and gullying

L Landsliding

O Other erosion processes

S Surface erosion

Y Sediment yield
72 USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993.

Table 13—Sediment budgeting studies. Symbols are explained below.

Site Land use Cover Process Type

Reference ADGILMRU CFGRVW ABCDEFGILPQRSTW OHRSW

Caine and Swanson 1989 CO ...I.... ..G... ..CD......QRS.. .H.SW

Caine and Swanson 1989 OR ...I.... .F.... ..CD.F....QRST. .H.SW

Collins and Dunne 1989 WA .D...... ...R.. A...E..I..Q.S.. ..RS.

Dietrich and Dunne 1978 OR ...I.... .F.... A.CDEF....Q.S.. ...SW

Dietrich and others 1982 - ........ ...... ..C.....L..R... OH.S.

Duijsings 1987 Lu ...I.... .F.... .BCD....L.QRS.. .H..W

Gilbert 1917 CA ADGI.MR. CFG..W A.....GI.P.RS.. .H.SW

Kelsey 1980 CA ..G.L.R. .FG..W AB...F.IL...S.. .H.SW

Kelsey and others 1987 CA ........ ...R.. A......I..Q.... ..RS.

Lehre 1982 CA ...I.... .FG... ABCD..GIL.QRS.. .H.SW

Lehre and others 1983 WA ...I.... ....V. AB....GI..QRS.. .H.SW

Leopold and others 1966 NM ..G..... .....W ABC...GI.P.R... .H.SW

Madej 1982 WA ...IL.RU .F.... A.C.E.GIL.QRST. .H.SW

Madej 1987 CA ........ ...R.. A......I....... ..RS.

McLean and Tassone 1991 BC .D...... ...R.. A......I..Q.S.. ..R..

Megahan and others 1986 ID ......R. ...... A......I.P..S.. ...SW

Parker 1988 NE ........ ...R.. ..........Q.S.. ..R..

Phillips 1990 NC A.G.L.RU .FG... .B....G..P.RS.. .H.SW

Prestegaard 1988 PA A.G..... .FG... .B....G.....S.. .H...

Reid 1990 Tz A.GI..R. C.G..W .B....G....R..W .H...

Reid and others 1981 WA ......R. .F.... .....FG.L..R... .H...

Roberts and Church 1986 BC ...IL.R. .F.... ABC..F..L..R.T. .H.SW

Sing 1986 CA ........ ...R.. AB........Q.S.. ..RS.

Smith and Swanson 1987 WA ...I.... ....V. A.......L..R... .H.SW

Stott and others 1986 Sl ..GI.... .FG... .B........Q.S.. ..R..

Sutherland and Bryan 1991 Ke ..G..... .....W ABCD...I..QRS.. .H.SW

Swanson and others 1982 OR ...I.... .F.... ..CD.F..L.QRST. .H..W

Trimble 1983 WI A.G..... .FG... AB....GI.P.RS.. .H.SW

Location: Land use: Processes evaluated: Budget type:

BC British Columbia A Agriculture A Channel aggradation O Overview

CA California D Gravel dredging B Bank erosion H Hillslopes

CO Colorado G Grazing C Creep R River reach

ID Idaho I Inactive D Dissolution S Storage

Ke Kenya L Logging E Transport abrasion W Watershed

Lu Luxembourg M Mining F Debris flows

NC North Carolina R Roads G Gully erosion

NE Nebraska U Urbanized I Channel incision

NM New Mexico L Landslide

OR Oregon Cover type: P Pond aggradation

PA Pennsylvania C Chaparral Q Bedload

Sl Scotland F Forest R Surface erosion

Tz Tanzania G Grassland S Suspended load

WA Washington R River T Treethrow

WI Wisconsin V Volcanic blast W Wind

W Woodland

plot studies, and long-term sediment yields in the Idaho

Batholith to characterize average erosion rates for timber

management activities in different land classes, and can be

used to estimate erosion rates for different planning options.

Effects of Environmental Change

on Organic Material

Land-use activities that affect input, transport, character, or

decay rate of organic material alter erosion, nutrient cycling,

and hydrologic processes both on hillslopes and in streams.

Organic debris on hillslopes alters rates of surface erosion,

provides a moisture reservoir, contributes nutrients, and supeffects of floods and land use. Swanson and Fredriksen (1982)

demonstrated the application of sediment budgeting to evaluate

impacts of timber management, and Reid (1989) used sediment

budgets to predict the response of channel morphology in a

small California grassland watershed to altered stocking rates,

suburban development, and climatic change.

Models that predict sediment export from watersheds are

in increasing demand. Most existing models couple a USLEbased sheetwash erosion equation with a flow-routing model,

and cannot be applied to sites with other erosion processes.

Those that incorporate sediment budgeting, however, are

more flexible. Watson and others (1986) constructed a model

for channel erosion in north central Mississippi by coupling

bank stability, sediment routing, and flow routing models. A

procedure developed by the USDA Forest Service (Cline and

others 1981) combines information from sediment budgeting,
USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993. 73

ports microbial action important in nutrient cycling. In channels, woody debris forms obstructions that at some sites

have been shown to slow flow by 60 percent (MacDonald

and Keller 1987), trap 15 years’ yield of sediment (Megahan

1979), provide 60 percent of total elevation drop along

streams (Keller and Tally 1979), localize scour to form

pools, and deflect flow into or away from banks. Leaves and

other small organic pieces supply nutrients to stream ecosystems. Maser and others (1988) reviewed the role of woody

debris on forest floors, in streams, and along coasts.

Production of Organic Material from

Streamsides

Vegetation change alters the production of organic material,

and changes in riparian vegetation are particularly effective

in modifying organic inputs to streams. Logging can decrease input of stable woody debris to streams by removing

trees that would eventually become large snags under natural conditions. The largest, most geomorphically effective

boles are often removed even from streamside management

zones, and when the smaller hardwoods left to provide shade

eventually enter channels, they are quickly removed by decay or carried off by high flows. McDade and others (1990)

measured distance travelled by incoming woody debris along

39 streams in western Washington and Oregon and used

results to predict future debris loading as a function of buffer

strip width, and Van Sickle and Gregory (1990) constructed

a model to predict inputs of woody debris on the basis of

measurements of tree frequency, size, and mortality in riparian zones. Windthrow often provides a burst of woody debris from buffer strips soon after logging, but this depletes

the source of future wood. Toews and Moore (1982) measured sizes of wood pieces in streams before and after logging at Carnation Creek on Vancouver Island and found that

average size decreased within two years by removal of stable

debris, breakup of large pieces, and introduction of logging

slash. Bryant (1980) found that an initial increase in debris

loading after logging in Alaska was followed by a decrease

as the logging debris destabilized natural debris accumulations and washed them out. Swanson and Lienkaemper (1978)

discussed the effects of timber management on debris inputs

and suggested management strategies to lessen impacts. Logging can also increase input of hardwood leaf litter by allowing more light to reach riparian hardwoods.

Grazing is particularly intense in riparian corridors because

of their proximity to water and their support of green vegetation

during dry periods. Grazing and trampling can prevent

regeneration of riparian trees, and heavy use can disrupt

banks, force channels to widen, and trigger removal of riparian

stands by undercutting (Kauffman and Krueger 1984).

Riparian vegetation is occasionally removed to increase

water yield or to reduce flood hazard by improving channel

conveyance. Riparian vegetation is often sparse in developed

areas. Urban input of organic materials is usually through

storm drains and sewers, and contribution of woody debris is

rare. Agricultural areas also rarely preserve riparian trees, and

crop plants provide only fine organic materials. Roads in steep

terrain are often built on floodplains or along bedrock-walled

channels, where construction may require replacement of natural

channel banks by unvegetated riprap or grouted rock.

Gravel mining can greatly reduce inputs of stable organic

debris by widening rivers and suppressing bank and bar

vegetation. Widened channels may begin to braid, and woody

plants rarely survive long on braided river bars because

channels rework the deposits frequently. Where vegetation

is allowed to regrow, natural stands of mixed hardwoods and

conifers may be replaced by disturbance-tolerant species

such as alder. Inputs of large, stable boles then decrease

while deciduous leaf inputs increase.

Soil alterations can affect organic matter input if they

influence vegetation. For example, compaction can reduce

growth rates and decrease the size of in-falling trees, and

soil disruption can prevent regrowth of trees.

Topographical changes alter debris inputs by changing

the accessibility of waterways. Roads built on benches along

river canyons create topographic breaks that prevent transport

of organic materials from hillslopes to channels, and

urbanization replaces riparian habitats with concrete-lined

channels or subterranean conduits. Agricultural stream banks

are often leveed or protected by unvegetated riprap and

concrete linings to prevent channel migration, and

channelization for flood control or navigation has similar

effects. Flood-control works also weaken interactions between

channel and floodplain: the area capable of contributing

organic debris to the channel is reduced, and fewer trees are

uprooted by floods.

Vegetation can be killed and regrowth inhibited where

toxic chemicals accumulate. Eroded mining waste is often

redeposited on downstream floodplains and may suppress

riparian vegetation for decades (e.g., Lewin and others 1977).

Atmospheric pollutants can have similar effects. Mill effluents

have deforested wide areas around Sudbury, Ontario (Amiro

and Courtin 1981); acidification from industrial pollutants is

increasing tree mortality in many areas; and smog is injuring

vegetation in some parts of California (Omart and Williams

1979). In each case, an initial increase in input of organic

material to streams is expected as trees die, but future inputs

will decrease as woody plants are replaced by more tolerant

grasses and herbs.

Erosion processes often contribute organic debris to

channels. Bank erosion undermines riparian trees; landslides

deposit trees in channels along with their sediment loads; and

overland flow rafts fine organics into channels. Land use that

accelerates these processes can increase debris input to streams.

Hydrologic changes also affect organic matter inputs.

Channels may be left too dry to support riparian trees where

water is diverted, but riparian vegetation may be promoted

by reservoirs that reduce peak flows and heighten base flows

(e.g., Petts 1977). Seepage from dams and stockponds can

support riparian communities where seasonal desiccation

formerly prevented their survival (Yost and Naney 1975). In

general, changes that decrease diurnal or seasonal flow

fluctuation promote riparian vegetation, while those that
74 USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993.

increase variability reduce vegetation by destabilizing

substrates and altering water tables.

Organic material can also be contributed by fauna. Insect

drift is an important food source for in-stream biota; beavers

import woody debris while building dams; and cattle

contribute manure. Some land-use activities intentionally

add woody debris to channels. Logs are placed to stabilize

banks, improve fish habitat, prevent channel incision, and

dissipate energy. In-channel structures such as docks, bridges,

and pilings may fail during floods and contribute wood.

Systemic changes such as acidification, air pollution, and

global warming can provoke replacement of sensitive species

by more tolerant ones, and inputs of dead wood are likely to

increase while plant communities are reacting to these changes.

In-Channel Production of Organic

Material

Much organic material originates within channel systems.

Fine organics are produced by in-situ biological activity and

are an important component of aquatic food chains, and

carcasses of anadromous fish once were important sources

of nutrients in streams and riparian areas of western forests.

Richey and others (1975) found that decomposition of returning sockeye salmon in Tahoe basin streams still provides a significant nutrient subsidy to the channel ecosystems. Management activities change in-situ production by

altering physical environments and nutrient inputs.

Activities that reduce riparian canopy cover promote insitu primary production by increasing water temperatures

and the amount of light reaching streams. Logging and grazing

can have this effect, as can floodplain development for

agriculture or urbanization. In each case, a long-term decrease

in woody debris input is accompanied by increased production

of fine organics. Where dense second-growth is reestablished,

productivity may drop below original levels as the young

canopy closes over the channel.

Input of nitrates, phosphates, and other nutrients associated

with urbanization and agriculture can increase aquatic primary

production. If increases are too high, biological activity depletes

dissolved oxygen and makes streams uninhabitable to respiring

organisms. Welch and others (1989) related increased algal

biomass downstream of urban areas in the Spokane River to

phosphorus inputs from urban sources, and Larsson and others

(1985) suggested that eutrophication of the Baltic Sea is

promoted by the cumulative contribution of nutrients from

municipal and industrial sewage delivered by rivers.

Inputs of fine sediment can reduce in-stream production

by coating substrates, thereby making surfaces unstable and

blocking light (Power 1990). Similarly, increased turbidity

reduces light penetration through the water column. Dredging

and other in-stream uses can also reduce substrate stability

and increase turbidity. Decay of silt-sized organic particles

in the channel substrate decreases survival of fish eggs,

alevins, and invertebrates by depleting oxygen from

intergravel environments. Sharpley (1986) measured high

nutrient concentrations associated with suspended sediment

and inferred that high sediment loads might accelerate

eutrophication.

Fluctuating flows downstream of water and power

developments usually restrict most in-stream production to

low-flow zones, with only those organisms tolerant of periodic

desiccation inhabiting channel margins. If reservoirs are

shallow and algae-rich, they may contribute to organic loads

downstream.

Modification of Organic Material

Many land-use activities modify decay rates of organic

material by altering vegetation communities, moisture distribution, and temperature. Edmonds (1980) measured litter

decomposition rates on forest floors in Washington, and

Harmon and others (1987) measured rates of wood decomposition in the Sierra Nevada. Fogel and Cromack (1977)

showed that Douglas-fir litter in a western Oregon forest

decays more rapidly under wetter conditions, and Fenn and

Dunn (1989) found that litter decomposition rates increased

with ozone pollution in the San Bernardino Mountains.

Most research on decay has been carried out in terrestrial

environments, and little is known of decay rates for instream logs. Organic material is also altered by being eaten

and metabolized or excreted.

In-Channel Transport of Organic

Material

How long a log remains in a channel depends on how well it

is anchored, how large it is relative to flows, and how long it

takes to decay. Different species provide wood of different

structural roles, and different parts of a channel system

interact differently with logs. In general, logs are progressively less stable and of less structural importance with

increasing stream size. Swanson and Lienkaemper (1978)

found that logs were randomly distributed in first- and second-order channels where streams are not large enough to

mobilize them, but most logs were transported into debris

jams in third- through fifth-order channels. Activities that

affect the size of peak flows or incoming woody debris can

alter transport of organic matter. Bilby and Ward (1989)

measured pools and sediment accumulations formed by logs

as functions of channel size and documented their decreasing role as channel width increases, and Bisson and others

(1987) described debris mobility as a function of debris size

and channel width.

Some land-use activities suppress transport of woody

debris. Low-order channels along roads are often routed

through culverts that are too small to pass organic debris,

and debris may accumulate at the intakes. Not only does this

deprive downstream reaches of woody debris, but

accumulations can plug culverts and cause flows to overtop

and wash out road fills.

Where debris flows occur, large woody debris can be

mobilized even in low-order channels. Swanson and

Lienkaemper (1978) found 35 percent of the length of first-
USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993. 75

order channels in a coastal Oregon basin to be scoured to

bedrock by debris flows. They warned that recovery of these

channels was unlikely because timber-managment-related

erosion rates remained high and logging had removed the

source of future large wood.

Some land-use activities remove woody debris once it is

in transport. Logs may be cleared from channels to aid

navigation, improve fish passage, reduce flood hazard, or

provide material for wood products. Impoundments decrease

inputs to downstream reaches by trapping woody debris.

Loss of woody debris can accelerate the transport of other

organic materials through the stream system. Cederholm

and Peterson (1985), for example, found that retention of

salmon carcasses in small channels on the Olympic Peninsula

was correlated with the frequency of woody debris.

Organic Material Budgets

There is growing interest in the construction of woody debris and organic material budgets for watersheds, although

few have yet been measured. Hogan (1987) evaluated the

effects of logging on large organic debris by constructing

budgets for pre- and postlogging conditions, and Caine and

Swanson (1989) contrasted sediment and organic material

budgets for small watersheds in the Cascades and in the

Rocky Mountains. Cummins and others (1983) reviewed

components of organic-material budgets, described their construction and philosophy, and compared budgets for five to

seven stream orders in four parts of North America.

Effects of Environmental Change

on Chemicals

Chemical inputs and transport are of concern primarily because of toxic effects and changes to nutrient cycles. Moody

(1990) provided a general review of the extent of groundwater contamination by chemicals in the United States; Smith

and others (1987) reviewed contamination of surface waters;

and Gilliom and others (1985) surveyed pesticide concentrations in rivers. Nutrients are chemicals required by plants

and animals, and include nitrates, phosphates, and many

others. Although nutrients can be toxic and degrade water

quality in high concentrations, their major significance to

forest management is their role in plant growth.

Input and Removal of Chemicals

Chemical inputs from land use must be considered in the

context of the natural chemical balance. Many compounds

are naturally present in solution due to bedrock weathering.

Reynolds (1986), for example, measured solution load in a

catchment in Wales and found that 78 percent of the material

lost from the basin was dissolved.

Vegetation is a major component of nutrient and chemical

cycles, and many nutrients are stored in plant tissues until

released by decay. Long-term nutrient budgets in stable

ecosystems are balanced, and amounts returned to the

environment are sufficient to support regrowth. Edmonds

(1980) evaluated nutrient inputs from litter decomposition in

several forest habitats of western Washington, and Graham

and Cromack (1982) assessed nutrient content of decaying

wood in the Olympic rainforest. Little and Waddell (1987)

noted the importance of the forest floor and downed wood as

nutrient reservoirs in forests of the eastern Olympic Peninsula.

Waide and Swank (1976) described nutrient storage and

transfer through components of a plant community in the

southern Appalachians and illustrated the concept that

communities storing large amounts of nutrients tend to be

resistant to change under natural conditions, whereas those

that transfer nutrients rapidly between storage elements tend

to be resilient rather than resistant. Communities with stored

nutrients thus respond to isolated disasters with little change

in composition, while rapid-turnover communities may initially

change in character but quickly reassume their initial state.

Under natural conditions, sites depleted of nutrients by

catastrophes like fires are recolonized by species that require

low nutrient levels or can extract nutrients from sources

inaccessible to other plants. Less tolerant species return

when nutrient levels are reestablished. If land use repeatedly

devegetates a site and prevents regrowth of colonizing species,

the site may eventually be depleted of some nutrients. Forests

and croplands are particularly susceptible to loss of nitrogen

and phosphorus, and farmers and foresters avoid this problem

by adding fertilizers. Little and Waddell (1987) assessed the

chemical composition of undisturbed forests and soils and

tested seedling growth rates to ascertain that logging would

not cause nutrient deficiencies at a site on the Olympic

Peninsula.

Nitrogen can be depleted if large amounts of organic

carbon are introduced to soils. High carbon levels temporarily

lock up available nitrogen by promoting increased microbial

activity as materials decompose. Cochran (1968) used

measured nutrient concentrations and ratios to evaluate the

potential for carbon-induced nutrient deficiencies in a nutrientpoor Oregon soil. In other cases, increased decay rates or

volumes of decaying material can increase nutrient availability

at a site. Burger and Pritchett (1988), for example, found

high nutrient levels where logging slash was chopped and

mixed into soils in Florida.

Many land-use activities introduce chemicals to watersheds,

but often their amount and location are poorly documented.

Markin (1982) found that less than half the insecticide applied

as an aerial spray to a forested area on the east slope of the

Washington Cascades landed on the intended plot, and Miller

and Bace (1980) found that measured herbicide exports in

streamflow corresponded to the estimated volume of aerially

applied herbicide pellets that fell directly onto surface water.

Brockway (1983) measured accumulation of nutrients and

heavy metals in a Michigan forest floor after sludge application,

and Harris and Urie (1983) measured these effects over a
76 USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993.

longer period at another site in Michigan. Nitrate contamination

due to heavy fertilizer applications has polluted groundwater

in many agricultural areas. Staver and Brinsfield (1990)

examined seasonal changes in nitrate availability in corn fields

and used this information to suggest a fertilization schedule

that reduces contamination of groundwater by excess nutrients.

Other activities expose existing chemicals to the environment.

For example, LaPerriere and others (1985) found high levels of

heavy metals downstream of gold dredges. Mining also exposes

unweathered rock to leaching in tailings piles.

Transport of Chemicals

Chemicals can be transported as particles, in solution, or

adsorbed onto sediment and organic matter. Recent concern

over water quality and the disposal and leakage of hazardous

wastes has generated considerable research on chemical transport in the environment. Much of this work is relevant to

chemicals used in forest management.

The properties of introduced chemicals must be known if

their impacts are to be understood. Chemicals that are insoluble

and nonreactive may accumulate where applied, while more

mobile species can contribute to off-site effects. Gilliom and

others (1985) noted that persistent, soluble pesticides are

likely to show up in river water, while persistent, hydrophobic

species are more common in riverbed sediments. Modes of

migration are also extremely important. Sophocleous and

others (1990) investigated movement of agricultural herbicides

and their breakdown during transport. Results demonstrated

the importance of cracks and permeability variations in

governing subsoil transport times and paths, and indicated

that diffusive models are insufficient for predicting transport.

Soil structure influences chemical leach rates by affecting

the infiltration and residence time of soil moisture, and Price

and Watters (1989) evaluated the role of soil horizons in

controlling chemical fluxes in an Ontario forest. Altered soil

structure and compaction also affect root penetration and

change the distribution of organic materials and nutrient

availability within soils. Organic compounds strongly influence

the mobility of chemicals, so land-use activities that alter the

organic content of soils modify their capacity to transport

chemicals and nutrients.

Because chemical transport is dependent on water

movement, any activity that alters runoff volumes or timing

also affects rates of chemical and nutrient transport. The

mode of runoff generation can also influence chemical

transport. Ford and Naiman (1989) examined the role of

groundwater in supplying dissolved nutrients to streams at a

forested site in Quebec, and Deverel and Gallanthine (1989)

associated the occurrence of saline groundwater with patterns

of irrigation, geomorphic features, and hydrologic conditions in

the San Joaquin Valley.

Many compounds are adsorbed by and transported with

silt or clay particles, so land-use activities and soil changes

that increase erosion also increase transport of chemicals

and nutrients. Alberts and Moldenhauer (1981) demonstrated

that cultivation methods that produce lower-density aggregates

lead to higher concentrations of nitrogen and phosphorus in

eroded sediment. Suspended sediment carries high

concentrations of bioactive phosphorus, a nutrient associated

with eutrophication of waterways (Sharpley 1986). Duffy

and others (1986) found that 40 percent of the nitrogen and

70 percent of the phosphorus exported from southern coastalplain watersheds was associated with sediment. Cerco (1989)

evaluated chemical and temperature factors that control adsorption

of nutrients on sediment particles.

Surface erosion preferentially removes the most nutrientrich part of the soil and leaves soils impoverished, and

erosion of chemically contaminated deposits can spread the

impact to wider areas. Roberts and others (1982) found that

the area of contamination rapidly increased as sediments

contaminated first by subsurface flows were dispersed by

sheetwash erosion.

Some chemicals have been evaluated for mobility in field

settings by applying the chemical and monitoring its

concentration in runoff. Davis and Ingebo (1973) found

picloram concentrations in runoff below a chaparral

conversion site that were high enough to damage downstream

crops, and they detected residues of the herbicide in soil 14

months after application. Michael and others (1989) and

Neary and others (1985) also monitored export of picloram

and found that detectable residues remained in soil solution

for at least 40 weeks at sites in the southeastern United

States. Similar studies have examined other commonly used

herbicides and pesticides. Miller and Bace (1980) measured

concentrations of forest herbicides in streams and found that

herbicide applied in pelletized form reached streams more

readily than that applied as a liquid spray. Pelletized chemicals

fell through the canopy to enter surface waters, while some

of the sprayed herbicide was trapped on foliage.

Monitoring studies indicate that logging usually causes a

short-term increase of nitrogen concentrations in soil solutions

and streams (e.g., Mann and others 1988, Sollins and

McCorison 1981, Tiedemann and others 1988). Increases

may result from reduced nutrient uptake, increased subsurface

flow, increased alteration of nitrogen compounds to leachable

forms, and increased volume of decaying organics. Vitousek

and Melillo (1979) reviewed these and other mechanisms of

nutrient loss. Baker and others (1989) measured release rates

of nine nutrients from logging slash during decomposition

and found that nitrogen was released most slowly. Other

activities that contribute to these mechanisms, such as

herbicide application (Feller 1989, Vitousek and Matson

1985) and urbanization (Gold and others 1990), may have

similar effects.

Urban land use contributes a variety of chemicals to

streams. Osborne and Wiley (1988) found stream nutrient

levels to be correlated to the proportion of a watershed

urbanized in Illinois, while Scott (1981) evaluated the

topographic, design, and management factors that affect

delivery of road salt to streams in Toronto. McBean and AlNassri (1987) related distance travelled by road salt to average
USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993. 77

vehicle velocity and stressed the importance of splash in

spreading the chemical impact. Lead from gasoline is also

transported in streams (Alexander and Smith 1988). Roberts

and others (1982) documented the migration of PCBs from a

highly contaminated spill site.

Streams in mining districts tend to have high chemical

loads, and Platts and others (1979) described the distribution

in an Idaho stream of toxic chemicals from an abandoned

mine. Most contaminants at this site emanated from mine

drainage and from leaching of spoils piles. Mok and others

(1988) and Mok and Wai (1989) measured concentrations in

groundwater and surface water, respectively, of arsenic species

near a mine in Idaho.

Many studies have explored the effects of land-use activities

on stream chemistry without examining how the contaminants

were transported to the streams. Tiedemann and others (1989)

found that grazing did not appreciably affect stream chemistry

at a site in eastern Oregon, and Johnson and others (1978)

found little chemical change due to grazing at a Colorado

site but noted a significant increase in bacterial counts.

Truhlar and Reed (1976) related pesticide concentrations in

streams to different types of land use in Pennsylvania, and

Mackay and Smith (1990) surveyed groundwater

contamination by agricultural pesticides in California.

Lawrence and Driscoll (1988) detected increased acidity

downstream of a New Hampshire clearcut and attributed the

change to increased soil nitrification. Increased aluminum

transport after logging was also noted during the study, and

concentrations lethal to fish were measured downstream of

the clearcut.

Alteration of Chemicals

Many chemicals are toxic when first introduced but break

down to harmless byproducts through biological activity or

weathering. The persistence of introduced species must therefore be understood if long-term or off-site effects are to be

predicted. Sophocleous and others (1990) examined the persistence of atrazine, a crop herbicide, in soils and groundwater, and Sharom and others (1980) measured the persistence

of 12 pesticides in natural, distilled, and sterilized water to

assess the relative importance of biological and chemical

degradation. Norris and others (1977) measured breakdown

rates of the herbicide 2,4,5-T, and Neary and Michael (1989)

measured residues of a site preparation herbicide

(sulfometuron methyl) over time at a site in the southeastern

United States. Although data exist for particular chemical

species at particular sites, information has not yet been

generalized to the point that chemical persistence can be

predicted for the range of site conditions encountered.

Some practices alter existing chemicals and cause them

to interact with the environment in different ways. For

example, Helvey and Kochenderfer (1987) found that

limestone road gravels locally moderate the pH of runoff

from acid precipitation, and Birchall and others (1989)

demonstrated that aluminum released by acid precipitation

is less toxic to fish if streamwaters are enriched in silica.

Bolton and others (1990) found that vegetation conversion

alters the spatial distribution of nitrogen mineralization

because plants lend soils different mineralization potentials.

Introduced chemicals can modify the mobility and chemical

composition of chemical species already present. Acid rain,

for example, increases mobility of nutrients and may contribute

to nutrient deficiencies (Johnson and others 1982). Chemical

changes may also alter the ability of plants to make use of

available nutrients.

Burning increases nitrogen loss by transforming stored

nitrogen into more soluble compounds that are susceptible

to leaching (DeByle and Packer 1972, Grier 1975), and

increased leaching can raise nitrate levels downstream.

Burning also removes nutrients by volatilization, and Feller

(1988) related nutrient loss in British Columbia forests to

the proportion of forest floor consumed by fire.

Accumulation of Chemicals

Chemicals introduced to channels may be precipitated or

redeposited downstream. Many chemicals are adsorbed onto

fine sediment particles and accumulate where sediment is

deposited. Miles (1976) measured insecticide residues in

stream sediments in Ontario, and Gilliom and Clifton (1990)

detected high levels of discontinued pesticides, including

DDT, in sediments of the San Joaquin River. Imhoff and

others (1980) found that 31 percent of heavy metals introduced from urban sources in the Ruhr watershed was retained in stream sediments, and Marron (1989) showed accumulation of mining arsenic in floodplain deposits in Colorado. Nutrients also accumulate at deposition sites. Johnston

and others (1984) calculated rates of nutrient trapping in a

Wisconsin wetland using measurements of sediment nutrient contents and aggradation rates.

Aggrading impoundments can accumulate chemicals

adsorbed onto sediment, and reservoirs can also concentrate

dissolved pollutants if evaporation rates are high. Reservoirs

accepting irrigation drainage in California’s San Joaquin

Valley have become progressively enriched in selenium salts

and other toxic substances. Log jams also trap sediment, and

Bilby (1981) measured their effectiveness by monitoring

chemical fluxes in a New Hampshire stream before and after

removal of log jams. Triska and others (1989) injected nitrate

into a forested tributary of Redwood Creek and measured

outflow and changes in storage along the stream. Results

showed that 29 percent of the injected nitrate was held in

mineral storage and 19 percent in biotic uptake.

Repeated addition of a chemical to a site can cause a

cumulative increase in concentration. Lockery and others

(1983) measured particularly high levels of lead contamination

at dump sites for road snow, and Hofstra and Smith (1984)

found that salt concentration in soils decreased as a function

of distance from a road.

Evapotranspiration of chemical-rich irrigation water

precipitates salts in soil. Salinization is severe enough in the
78 USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993.

San Joaquin and Imperial Valleys to prevent further

agricultural use of some tracts, and the effect is spreading.

Runoff from these areas adds to salt loads downstream, and

the impact is compounded where this water is used again for

irrigation. The Colorado River eventually becomes too saline

for further use because of heavy irrigation use.

Biota can accumulate and concentrate introduced

chemicals. Erdman and others (1978) and Neuman and

Munshower (1984) found high levels of molybdenum in

forage plants growing on coal mine dumps, and Nichols and

Scholz (1989) measured increased levels of radioactive

elements in trout downstream of a uranium mine in

northeastern Washington.

Nutrient Cycling, Chemical Budgets,

and Modeling

Chemical budgets can be constructed in much the same way

as sediment budgets. In this case, however, alterations to the

species present become an extremely important aspect of the

budget. Where the chemicals of concern are nutrients, entire

budgets can describe cycles of alterations at a single site as

nutrients are repeatedly incorporated into plant tissues and

released by decay. Mahendrappa and others (1986) reviewed

methods used to analyze nutrient cycling and described problems in budget construction. Hornbeck and others (1986)

constructed a nutrient budget to examine the effects of

clearcutting on a New Hampshire forest, and Gholz and

others (1985) assessed the influence of early stages of regrowth on nutrient budgets after clearcutting in the H.J.

Andrews Experimental Forest of western Oregon.

Some budgets focus on particular chemicals. Waide and

Swank (1976) used nitrogen budgeting to predict forest

management impacts in the southeastern United States and

evaluated inaccuracies caused by lumping storage components

together. Mitchell and others (1989) studied the effects of

clearcutting on sulfur cycling at Hubbard Brook Experimental

Forest, New Hampshire. Ryan and others (1989) examined

inputs of sulfur to soils from atmospheric pollution in the

southeastern United States and found that soil adsorption rates

for sulfur are high but finite. They noted that rates of sulfur

loss to streams are already increasing and inferred that the

soil’s capacity is approaching saturation. Budget calculations

indicated that a steady state between atmospheric inputs and

runoff outputs would be reached in about 100 years.

Berndtsson (1990) constructed a chemical budget for sources

and sinks of pollutants downstream of Lund, Sweden, and

Imhoff and others (1980) used a similar approach to examine

heavy metal pollution in the Ruhr River. Larsson and others

(1985) constructed a nutrient budget for the Baltic Sea and

used it to demonstrate the cumulative importance of municipal

and industrial sewage delivered by rivers throughout the basin.

Researchers now often attempt to predict flow paths and

transport rates for chemical contaminants. Padilla and others

(1988) constructed a model to predict subsurface transport

of chemicals and found that transport is strongly affected by

the dependence of chemical breakdown rates on temperature.

The CREAMS model (Chemicals, Runoff, and Erosion in

Agricultural Management Systems; Knisel 1980) is one of

the most widely known methods for predicting chemical

transport from croplands. This model takes into account the

affinity of chemicals for sediment particles and also predicts

transport of sediment, but it does not calculate subsurface

transport and it requires calibration. Lorber and Mulkey

(1982) compared CREAMS with two other models for

predicting pesticides in runoff and found that all provided

valid estimates. Nutter and others (1984) found good

agreement between CREAMS predictions and measured

herbicide concentrations in forested basins for initial storms,

but the lack of a subsurface component caused underestimates

during later storms. Kenimer and others (1989) developed a

similar model that is claimed to have stronger hydrologic

and sediment transport components and can also take into

account the transfer of chemicals between adsorbed and

dissolved phases. Pennell and others (1990) compared the

predictions of five pesticide transport models to measured

distributions, and provided guidelines for selecting an

appropriate model. Emmerich and others (1989) compared

models for predicting transport by surface runoff and stressed

the importance of addressing nonhomogeneous conditions if

they are present.

Models have been constructed to predict chemical transport

rates in alluvial channels of lowland streams, but little work

has been done on mountain channels. Bencala and Walters

(1983) demonstrated that ordinary models for stream transport

do not work for pool-riffle sequences characteristic of steep

streams and developed a model that does apply to these

settings.

Chemical transport models must account for changes in

chemical state during transport. MacQuarrie and others (1990)

constructed a model that incorporates chemical and biological

breakdown of pollutants during groundwater transport, and

Tim and Mostaghimi (1989) and Padilla and others (1988)

modeled breakdown of pesticides during transport above the

water table. Jury and Gruber (1989) used a model to examine

the effects of soil variability on pesticide transport and found

that variability contributes to more rapid removal of pesticide

residues from the biologically and chemically active surface

soil layers, where most breakdown takes place. Results showed

that contamination by degradable chemicals may be more

widespread than expected because some toxic compounds

can persist for long periods once they leave the active zone.

Effects of Environmental Change

on Heat

Heat is transported through a watershed by changes in streamwater temperature. Stream temperature is determined by the

temperature of inflowing water, radiant inputs and exports,

conductive inputs and exports, convective inputs and ex-
USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993. 79

ports, and heat extraction from evaporation. Radiant inputs

are highest where infrared radiation impinges on the water

surface, and losses are highest where infrared radiation emitted

from the stream is not reflected back to the water. Conductive heat exchange is rapid where water is in contact with air

or substrate of contrasting temperature, and is accelerated

where the contrast is maintained by convective removal of

partially equilibrated air or water from the interface. Evaporating water extracts heat from air and water to accomplish

the phase change. Beschta and others (1987) reviewed research on stream temperatures and discussed changes caused

by timber management.

Vegetation influences all types of heat exchange in streams.

Tree canopies over streams decrease temperature fluctuations

by deflecting incoming infrared radiation and reflecting back

much of that emitted from the water surface. Because of

evapotranspiration and shading, daytime air temperatures

are generally lower under tree canopies than in clearings.

Differences between water and air temperature are thus less

under the canopy, and rates of conductive exchange during

warm parts of the day are reduced. There is also less convection

where temperature differences are small and where streams

are shielded from wind. Land-use activities that reduce riparian

cover usually increase daytime stream temperature and the

range of temperature fluctuation. Beschta and Taylor (1988)

and Holtby (1988) measured increased stream temperatures

associated with logging in the Oregon Cascades and on

Vancouver Island, respectively, although mechanisms

generating the change were not identified.

Altered soils can affect stream temperature both by causing

vegetation changes and by altering the relative importance

of overland and subsurface flow. Cold-season precipitation

is likely to be colder than groundwater contributions to

streams, but quickflow may be warmer than groundwater if

precipitation is warm or falls onto a warm surface.

Channel morphology influences rates of heat exchange in

streams by controlling the water surface area exposed to the

atmosphere. Channels often widen and aggrade to

accommodate increased erosion rates or altered peak flows,

and heat exchange is accelerated in wide, shallow channels.

Daytime temperatures and diurnal fluctuations are expected

to increase at such sites, especially if the widening has

removed riparian vegetation. Beschta and Taylor (1988)

found that stream temperatures remained elevated for several

years after major storms while riparian zones and bars

remained partially devegetated.

Whether a reservoir increases or decreases downstream

water temperature depends on the size of the impoundment

and the source of outflow. Broad reservoirs absorb more

infrared radiation than a channel and are less shaded by

vegetation, and downstream flows are warmer than usual if

outflow is removed from the reservoir surface. Cold water

collects at the bottom of deep lakes, however, so if outflow

is removed from the base of a dam, downstream flows are

colder than normal and have low organic nutrient contents.

Hubbs (1972) discussed the effects of dams on heat input.

Water transports heat from one site to another, so models

of temperature change in a watershed must address transport

and routing of water. Theurer and others (1985) used a

stream temperature model to compare the effects of potential

rehabilitation plans on stream temperatures in the Tucannon

River, Washington. The Timber/Fish/Wildlife Temperature

Work Group compared the sensitivity and accuracy of seven

predictive models for channel reaches and networks in areas

of timber management in Washington (Sullivan and others

1990), and recommended use of stream-reach models

described by Adams and Sullivan (1990) and Beschta and

Weatherred (1984; model up-dated 1986).

Research Needed to Understand

Watershed Processes

Land use alters environmental parameters, which then influence watershed processes. A watershed process is considered adequately understood for management purposes if

successful predictive models have been developed for it.

The development of such models requires an understanding

of the interactions between environmental parameters and

watershed processes.

Because water is the transport vehicle for watershed

products, an understanding of water transport through

watersheds is essential for understanding and predicting

transport of sediment, organic matter, chemicals, and heat.

Runoff generation processes are relatively well understood,

but models for predicting rain-on-snow runoff are needed to

evaluate the influence of logging pattern on flow generation.

Generation of surface flow on shallow saturated soils during

snowmelt is more common than previously thought, and the

runoff mode must therefore be reevaluated in some areas.

The effects of vegetation management on water yield are

widely measured, and these studies provide a data set for

developing and testing process-based predictive models.

Surface water transport on hillslopes can be approximately

modeled using existing methods. Subsurface flow, however,

requires further work to determine the significance and

influence of inhomogeneities in soil and bedrock. Flow

through subsurface soil pipes is common but rarely recognized,

and the influence of land-use activities on this flow path is

virtually unknown. Flow in alluvial channels is well

understood, and flow-routing models are capable of predicting

runoff timing and volume at points along a channel. Analogous

transport equations and models for high-gradient, low-order

streams are inadequate, yet these are often the channels of

most concern to foresters.

Most erosion processes have been widely studied, but

erosion in soil pipes and stream-bank erosion remain difficult

to evaluate. Landsliding has often been measured as a function

of land-use activity, and slide mechanics are fairly well

understood. However, predictions of long-term landsliding
80 USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993.

rates are tenuous, and work is needed to evaluate how

occurrence of a landslide affects future erosion processes in

its vicinity.

Transport rates due to soil creep are inadequately measured,

and the effects of land-use activities on creep rates are

unknown. A change in creep rate can affect future sediment

input and process distribution, so definition of land-userelated changes is important. The influence of altered creep

rate on landsliding and stream-bank erosion requires

evaluation.

The Universal Soil Loss Equation is effective for evaluating

sheetwash erosion on uncompacted lands. It is also useful

for identifying factors influencing erosion rates on compacted

surfaces and for estimating relative rates between sites, but

its numerical predictions are often not accurate for compacted

surfaces, and predictive equations for this application need

to be developed. Work is now being done to incorporate

road-surface erosion into the WEPP model, so appropriate

tools might be available in the near future.

Sediment transport in stream channels has been widely

studied and is relatively well-understood for low-gradient

alluvial channels. However, transport through high-gradient

channels with riffle-pool sequences and large organic debris

is not well described, and no predictive models are yet

adequate for this application. More detailed measurements

are needed to develop transport relations at such sites.

New types of sediment budgets are being explored that

allow prediction of landform evolution and spatially explicit

responses to environmental change (e.g., Reid 1989). Methods

for constructing these spatially distributed sediment budgets

need to be refined.

Input rates of organic debris into streams are easily

measured, and data exist for many areas. However, rates of

debris decay and breakdown are more poorly known, and

residence times must be determined as a function of species,

stream order, microhabitat, and hydrologic regime.

Measurement of organic material budgets for a variety of

site types would facilitate recognition of patterns of input

and transport and promote understanding of the factors

controlling them.

A variety of models now exist to predict transport and

accumulation of introduced chemicals, but these generally

apply to low-gradient, lowland settings. More work is needed

on movement of forest herbicides and pesticides so that

models may be adjusted for these species and environments.

Transport through inhomogeneous soils and steep, rough

waterways needs further research, and basic descriptive work

must still be done to determine distribution and residence

times of applied chemicals in various microenvironments.

Methods of evaluating nutrient cycles also need to be improved

so that sites at risk of nutrient depletion can be identified.

Models for predicting temperature change due to land use

and vegetation character in discrete stream reaches now

exist, but those that attempt to evaluate entire watersheds are

not as well developed. Most research on heat transport in

streams has considered summertime warming, and relatively

little is known of cold-season effects.
USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993. 81

Chapter 7

Impacts on Beneficial Uses and Values

Flow Characteristics

Land use can change peak discharge, discharge variance,

low-flow discharge, and seasonal runoff distribution. Each

of these changes affects fish and the organisms they rely on

for food. Every species of fish has particular environmental

requirements for spawning, rearing, and adult stages. Salmonids spawn in gravels by excavating a hole, depositing eggs,

and replacing gravel over the redd, so high flows during

incubation and intergravel life stages can scour redds and

wash away eggs or young fish. Natural floods periodically

destroy redds, and Thorne and Ames (1987) demonstrated a

negative correlation between sockeye smolt production per

spawner in Washington’s Cedar River and peak daily discharge during incubation. Salmon compensate for these disasters by having multiple runs in a single system, by spreading

a single run over several months, and by excavating redds to

depths unlikely to scour. In addition, some anadromous fish

return during off-years, so if one year’s production is destroyed by a storm, pioneers from other years will return to

reestablish that run. These coping mechanisms are no longer

adequate if peak flows are consistently raised due to landuse activities. Many other types of fish spawn on particular

substrates or aquatic plants, and these, too, can be disturbed

by unseasonal or chronically high flows.

Erman and others (1988) found that rain-on-snow floods

cause abnormally severe scour because they are contained

between snowbanks; a particular discharge is thus deeper

than usual and exerts more force on the channel bed. Snow

accumulation depths increase where riparian vegetation has

been cleared, so preservation of buffer strips in snow country

may help to prevent excessive scour.

Decreased flow can also affect spawning. Anadromous

adults require sufficient discharge during runs to allow

upstream migration, and parts of a stream system may become

inaccessible if these flows are delayed or prevented by

diversion or impoundment. Channels may also dry out if

accumulations of coarse sediment increase channel-bed

permeability and allow more flow to percolate through the

substrate. Seasonal or periodic desiccation can reduce the

spawning habitat available for both anadromous and resident

fish populations. Discharge fluctuations below hydroelectric

dams can increase in-gravel mortality by luring some fish to

spawn in gravels submerged during power generation but

exposed during off-hours. Chapman and others (1986) found

that 18 percent of chinook redds on a bar downstream of a

dam on the Columbia River were located in areas subject to

subaerial exposure, although little damage appears to have

occurred because redds were deep.

High peak flows can displace fish and destroy their food

resources. Ottaway and Clarke (1981) measured susceptibility

A change in environmental parameters inducescompensating changes in watershed processes, and a

combination of changes may produce responses that interact.

A CWE occurs when interacting responses are strong enough

to disturb something of philosophical or economic value,

and the significance of the impact is determined by the

magnitude of disturbance to the value. The potential effects

of watershed alterations on specific resources and values

must be understood if CWEs are to be evaluated, and this

chapter examines those effects. Many land-use activities

alter watershed function in the same way, so resource

responses -the impacts -are here related to the watershed

processes generating them rather than to the underlying

land-use activity. Almost all beneficial uses and values can

be affected by changes occurring in watersheds, and those of

particular concern in California are considered here.

Impacts on Fisheries Resources

and Aquatic Communities

Impacts on anadromous fish are often the focus of CWE

analyses in the western United States, and concerns over

impacts on resident fish are growing. Resident fish are affected by changes in flow, physical habitat, temperature,

accessibility of food, predation, and pathogens within a watershed. Anadromous fish are influenced, in addition, by

changes occurring in habitats they use beyond their natal

watershed.

An impact on fisheries resources is usually recognized by

a change either in community structure or in abundance of a

target species. However, natural abundance is often difficult

to establish. Estimates of undisturbed populations have been

made on the basis of carrying capacities in streams and from

historical evidence (e.g., Chapman 1986), but large natural

population fluctuations can make evaluation of change difficult

(Platts and Nelson 1988).

Several publications reviewed potential impacts on

fisheries of particular land-use activities, such as hydropower

development (Russell and St. Pierre 1987), recreation (Clark

and others 1985), range use (Platts 1981), and timber

management (Chamberlin 1982, Chapman 1962, Meehan

and others 1969). Gibbons and Salo (1973) and MacDonald

and others (1988) provided comprehensive bibliographies

of the effects of timber management on fish, and Cordone

and Kelley (1961) reviewed impacts caused by increased

sediment load.
82 USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993.

to displacement by high flows as a function of the ages of

brown trout and Atlantic salmon in artificial channels, and

Elwood and Waters (1969) attributed decreased brook trout

growth after flooding in central Minnesota to decreased

invertebrate populations. Low or fluctuating flows restrict

living space, strand fish, and expose them to predation and

high temperatures, and complete desiccation either kills or

displaces fish. Michael (1989) correlated survival of juvenile

salmonids with summer low-flow discharges in a Washington

stream, and Gibson and Myers (1988) demonstrated the

same effect in a Newfoundland stream where low flows were

caused by freezing during winter. In contrast, increased low

flows can benefit species by increasing available habitat

(Johnson and Adams 1988).

Changes in flow regime affect some species more than

others and so are likely to alter the composition of in-stream

communities. Bain and others (1988) compared fish

communities in natural flow regimes with those subjected to

artificially fluctuating flows and found that community

complexity was reduced by fluctuating flow.

Channel Morphology

Different species and life stages of fish have different habitat requirements and react differently to changes. Gravel

spawners require a stable substrate while eggs and young

fish are resident in gravels, so morphological changes that

decrease substrate stability can reduce spawning success.

Land-use activities that rearrange stream gravels create a

bottom topography that is not adjusted to high flows. Rearranged substrates are thus abnormally mobile during floods

as bed topography readjusts, and the increased scour can

destroy redds. Piles of gravel left by gold dredges provide

attractive sites for redds but are easily rearranged by high

flows. If few adult salmon escape to spawn and those that do

are lured to these unstable sites, the cumulative effect may

be a severe decrease in salmon populations.

Other activities affect substrate stability by altering the

size and distribution of stream gravels. Dietrich and others

(1989) found that the mean diameter of surface gravels

decreases as sediment supply increases, so increased sediment

input may make beds more mobile and increase the incidence

of redd scour. Debris flows destroy spawning and rearing

habitat in small tributaries by completely removing gravels,

so activities that destabilize slopes can strongly influence

spawning habitat.

Spawning gravel must be highly permeable so that

intergravel flow can deliver enough oxygen to support eggs

and young, and intergravel pore spaces must be large enough

that emerging fish can escape to the surface. Coble (1961)

and McNeil and Ahnell (1964) demonstrated decreased

survival to emergence for steelhead trout and pink salmon,

respectively, with increasing percent fines in gravels, and

Phillips (1970) reviewed evidence that suggests there is a

cumulative interaction between mortality due to decreased

intergravel flow and that due to blocking. Shaw and Maga

(1943) found that silver salmon emergence dates were delayed

when silt was added to gravels in a hatchery and suggested

that the delay might decrease fitness of fish during rearing

stages. Chapman (1988) reviewed research on the effects of

intergravel fines on salmonids.

The proportion of intergravel pore space occupied by fine

sediment increases with increased transport of fine sediment

(Lisle 1989). Meehan and Swanston (1977) found that angular

gravel accumulates more fines than rounded gravel, and

angular gravels contributed by landsliding may thus be more

susceptible to sedimentation than fluvially rounded gravels.

Increased sediment inputs from mining (Shaw and Maga

1943), roads (Sheridan and McNeil 1968), and logging

(Scrivener and Brownlee 1989) have been shown to increase

sedimentation in spawning gravels.

Rearing habitat must be stable during storms, give shelter

from predators, and provide appropriate flow velocities and

food resources. In particular, many species of fish require

pools for refuge, overhanging banks for cover and temperature

control, and organic debris for cover and food production

(Moore and Gregory 1988a). These qualities are largely

controlled by channel morphology, and fish move to more

suitable habitats if morphology changes. Salmonid species

often do not compete with each other because they prefer

different microhabitats. For example, steelhead fingerlings

are relatively common in riffles, while coho prefer pools

(Bisson and others 1988). Environmental changes that promote

one type of habitat at the expense of others are likely to alter

species composition in streams.

Many studies attempt to correlate fish populations to habitat

variables, and these were reviewed by Marcus and others

(1990). Kozel and others (1989) and Kozel and Hubert (1989),

for example, correlated the standing crop of brook trout and

brown trout to a variety of topographic and cover variables

for channels in meadows and forest lands in Wyoming. Moore

and Gregory (1988b) constructed complex banks in natural

reaches and found that cutthroat trout populations increased

with increasing area of channel margin habitat. Riparian

vegetation is a strong control on channel margin habitat and

food availability, and Dolloff (1987) demonstrated that

different densities of juvenile coho salmon were associated

with different riparian communities in southeast Alaska.

Other studies have isolated particular habitat elements

and have examined their influence on fish abundance. Morantz

and others (1987) showed that water velocity was a consistent

predictor of microhabitat selection for juvenile Atlantic salmon

and demonstrated that fish preferred higher water velocities

as they grew. Large gravel with open interstices provides

cover for juvenile trout (Heggenes 1988), but increased

sediment input destroys this habitat element by embedding

gravels in fine sediment. Some land-use activities reduce

cover by reducing inputs of organic debris to streams. Dolloff

(1986) and Elliott (1986) measured decreases in salmonid

size and population of juvenile Dolly Varden trout and coho

salmon when woody debris was removed from sites in

southeast Alaska, and Murphy and others (1986) found that
USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993. 83

winter survival of juvenile salmonids in southeast Alaska

was correlated with the amount of woody debris left in

streams after logging. Bisson and others (1987) reviewed the

effects of woody debris on fish.

Habitat requirements vary with season. Site stability is

particularly important during winter, and some fish migrate

into floodplain channels, ponds, and backwater channels for

winter rearing. Young fish are protected from scouring flows

at these sites, and food is plentiful. Peterson (1982b) recorded

migration of coho into ponds from 33 km upstream, and

Peterson and Reid (1984) estimated that 20 to 25 percent of

smolt from the 375 km2

 Clearwater drainage on the Olympic

Peninsula overwinters in ponds and tributaries that support

no spawning. Hartman and Brown (1987), at Carnation Creek

on Vancouver Island, and Swales and Levings (1989), in

interior British Columbia, examined differences in refuge

habitat use between species, and Brown and Hartman (1988)

found that 15 to 25 percent of the juvenile coho at Carnation

Creek migrated into off-channel refuges during the winter.

Peterson (1982a) measured survival and growth of juvenile

coho in two refuge ponds on the Olympic Peninsula and

attributed differences to contrasting pond morphology. Landuse activities that degrade or make these habitats inaccessible

can disproportionately impact winter survival. Because these

sites are often on floodplains or low terraces, they are

commonly disturbed by agriculture, roads, urbanization, and

industrial development. Sedell and others (1990) discussed

the importance of refuge habitats of all scales for imparting

resistance and resilience to fish populations.

Fish also move to more protected sites within a reach

during winter, and their populations decrease if suitable

habitat is absent. Cunjak (1988) showed that Atlantic salmon

heavily used cobble substrates for winter refuge in Nova

Scotia, and Hillman and others (1987) increased overwinter

chinook population densities by introducing cobbles into

sedimented streams in Idaho. Tschaplinski and Hartman

(1983) examined winter population densities of coho in

Carnation Creek and found survival to be high in suitable

microhabitats both before and after logging. However, the

abundance of these microhabitats decreased after logging,

and more fish were displaced to off-channel refuges. Chapman

and Knudsen (1980) showed severely depressed winter

populations of coho in channels with morphology altered by

grazing or channelization in the Puget Sound area.

Some land-use activities alter morphology to improve

habitat for particular species, and others fortuitously improve

habitat. Cooper and Knight (1987) found that fish biomass

in plungepools created by erosion control structures was

comparable to that in natural pools in midwestern streams,

and that the growth rate, proportion of marketable fish, and

population stabilities were higher.

Channel obstructions, such as culverts, dams, and debris

jams, can eliminate fish migrations beyond those points, and

migrating fish can also be diverted by inlets to irrigation

channels, diversions, and turbines. In other cases, natural log

jams and waterfalls are blasted out to make new reaches

accessible, but such changes can impact recreational use and

esthetics and can increase sediment loads. Unique upstream

communities may be destroyed when removal of an

obstruction ends the isolation preserving them.

Sedimentation in streams is usually accompanied by

sedimentation and habitat loss in downstream estuaries.

Estuaries are extremely important as spawning grounds and

nurseries for coastal fish and invertebrates, and many

anadromous salmonids use estuaries during part of their life

cycle. Munro and others (1967, quoted in Gregory 1977)

documented destruction of an Australian oyster fishery by

sedimentation.

Water Temperature

Most fish thrive in a relatively narrow range of temperatures, so temperature affects their choice of microhabitat.

Baltz and others (1987) found temperature, height above

bed, and flow depth to be the most important variables in

predicting the fish species present at sites along the Pit River

in California. Temperature affects fish behavior by altering

metabolic rates and demands, and juvenile chinook and coho

in a laboratory flume tended to stay in slower water at low

temperatures (Taylor 1988). Reeves and others (1987) compared community composition in western Oregon streams of

different temperatures and found that different species dominated at different temperatures, so temperature change is

likely to affect community composition. Beschta and others

(1987) reviewed the effects of temperature on stream biota.

Water temperature is moderated by shade, and prevention

of high temperatures is a primary reason for leaving buffer

strips in logged areas. Theurer and others (1985) used models

of fish population response to temperature change to predict

the effect of riparian revegetation on fish production in the

Tucannon River basin, Washington. Holtby (1988) related

changes in stream temperatures after logging in the Carnation

Creek watershed to fish production, and similar studies have

been carried out in other areas. Beschta and Taylor (1988)

demonstrated a cumulative increase in average maximum

temperature with increased logging disturbance in the Salmon

Creek watershed in the Oregon Cascades. Morphological

changes can also increase stream temperatures. Wide streams

are more quickly warmed than narrow streams with

comparable flow volumes because they have a large surface

area for temperature exchange and because marginal

vegetation shades less of the water volume.

Some management activities increase shading by providing

artificial structures or introducing debris. Meehan and others

(1987) found that biomass and abundance of juvenile chinook

salmon increased with the amount of shade added to sites

along a side channel of the South Fork Salmon River in

Idaho. Erman and others (1977) measured invertebrate

populations as a function of buffer strip width on 62 streams

in northwest California and found that 30-m buffers preserved

community structure.

Loss of canopy cover increases temperature fluctuations
84 USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993.

by accelerating heat exchange, and fluctuations cause stress

in fish (Thomas and others 1986). Fluctuating discharges

from power generation also increase temperature variation,

and Hubbs (1972) reviewed the effects of impoundments on

water temperature and fish.

If streams are initially colder than optimal, artificial

warming can increase in-stream production, increase fish

populations, and accelerate growth. Bilby and Bisson (1987)

found greater coho production in clearcut channels than

under old-growth in the Deschutes River basin of western

Washington. Holtby (1988) showed a similar increase in

coho populations and attributed it primarily to earlier

emergence caused by higher early-season temperatures.

However, lowered ocean survival due to earlier out-migration

nearly compensated for the increased numbers. Power plants

add heat directly to streams when they return coolant water

to channels. Steen and Schubel (1986) warned of a tradeoff

between fish mortality due to warming and that due to

diversion: efforts to reduce thermal pollution require more

cooling water, so mortality from diversion increases.

Winter temperatures can also constrain survival. Fish

must have access to nonfreezing sites when ice forms.

Chisholm and others (1987) found that brook trout in highaltitude streams in Wyoming migrated to low-gradient,

deep, low-velocity sites as streams started to freeze. Some

warming during cold months may increase growth rates,

but it can also increase food requirements during seasons of

low production.

Water Quality

Land-use activities affect water quality by altering sediment

and chemical loads, and both changes affect in-stream biota.

Norris and others (1983) reviewed the effects of forest pesticides, fertilizers, and fire retardants on fish and suggested

that problems are most likely to arise when chemicals are

applied directly to surface waters. They stressed the importance of understanding the role and efficacy of buffer strips,

the dependence of toxicity on dose and duration, the instream

fate of chemicals, interactions between multiple chemicals,

and latent effects on biota.

High suspended sediment concentrations can harm animals

directly by damaging gills or influence them indirectly by

decreasing light penetration and visibility. Animals that hunt

by sight are less successful at gathering food in turbid water.

Bisson and Bilby (1982) found that juvenile coho attempted

to emigrate from water turbid enough to inhibit feeding, but

also showed that fish became somewhat habituated to elevated

turbidity. McLeay and others (1987) measured physiological

stress responses in grayling exposed to placer mining sediment

and observed direct mortality at high loadings. Chronic

exposure impaired feeding, decreased growth rates, and

reduced resistance to toxins. Reynolds and others (1989)

found that grayling that could not escape from sediment

plumes either died or sustained gill damage and arrested

growth. Redding and others (1987) observed physical stress

responses of coho salmon and steelhead trout to sublethal

sediment concentrations and documented a corresponding

reduction in tolerance to bacterial infection.

Fish may vary seasonally in their sensitivity to suspended

sediment as their metabolic demands and physiology change.

Noggle (1978, quoted in Cederholm and Reid 1987) found

that young Olympic Peninsula coho were most sensitive to

turbidity in summer and that tolerance increased during fall.

Servizi and Martens (1991) showed that temperature and

fish size are major controls on the sensitivity of coho salmon

to suspended sediments. Timing of sediment input may

therefore influence impacts, and high turbidities produced

by winter storms may be less damaging than equivalent

turbidities caused by dredging and road-building during the

summer. Fish from river systems with high natural suspended

sediment loads, such as the Eel River, may have evolved

physiological adaptations or coping behaviors that allow

them to tolerate higher sediment loads than fish inhabiting

less turbid streams.

Land-use activities can introduce many chemicals to

streams. Toxic spills often kill fish (e.g., Saunders 1969),

and coastal oil spills are relatively frequent. Siewert and

others (1989) measured changes in aquatic communities

after a toxic waste cleanup and found that species diversity

increased significantly when water quality improved. Toxic

chemicals introduced by mine drainage and leaching of spoils

can make channels uninhabitable (Platts and others 1979),

and Temmink and others (1989) described physiological

damage to fish from leaching of bark tannins around paper

mills. Hughs and Gammon (1987) circumstantially related

changes in fish assemblages along the Willamette River to

variations in water quality by demonstrating that an index of

biotic integrity decreased with decreasing water quality.

Toxic residues from marine anti-fouling paints have

accumulated around marinas, and Bushong and others (1988)

measured their effects on biota. Other chemicals are introduced

to streams to control vegetation or pests.

Chemical toxicities are usually determined in laboratories

rather than in the field, and results are often related only to

the lethal dose. Woodward (1978), however, found that

cutthroat trout exhibited a range of effects from reduced

growth to mortality as exposure to the herbicide picloram

increased, and Lorz and others (1979) measured some

physiological effects of atrazine on coho salmon even at

very low doses. Davis (1976) reviewed sublethal effects of

pollutants on salmonids and cited evidence for altered growth,

behavior, metabolism, respiration, and circulation, and also

noted that pollutants reduced tolerance of fish to temperature

extremes and made some game fish inedible. Kleerekoper

(1976) examined behavioral responses of fish to pollutants.

Heavy metal contamination and pollution in urban and

industrial areas have eliminated some local fisheries where

fish accumulate pollutants by eating contaminated food.

Ocean fisheries can also be affected by pollutants, as was the

case with mercury-tainted tuna.

Changes that reduce dissolved oxygen levels decrease

habitability for gilled creatures. Increased nutrient loads

deplete dissolved oxygen by stimulating algal blooms, and
USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993. 85

detrital organic materials consume oxygen as they decay.

Dominy (1973) attributed fish kills below a Canadian dam

to supersaturation of nitrogen, which induced gas-bubble

disease in fish.

Some chemical changes interact to modify their effects

on fish. Aluminum toxicity decreases with silica content in

acidified streams (Birchall and others 1989), while the

combined effects of acidity and aluminum are worse than

either alone (Cleveland and others 1986). Increased silt load

decreases toxicity of some chemicals by adsorbing compounds

or altering their chemical environment (Hall and others 1986).

Woodward (1982) examined interactions between range

herbicides to assess combined toxicity to fish.

Food Resources

Most fish rely on plants, invertebrates, or other fish for food.

Foods can be produced within a channel reach, fall in from

banks, or drift in from upstream. Salmonids depend on both

instream and riparian food sources, so land use affecting

either infall or instream production alters food resources and

can affect salmonid populations. Infall depends primarily on

the nature of riparian vegetation, while instream production

is influenced by temperature, sedimentation, turbidity, nutrient

load, and channel morphology. Huryn and Wallace (1987)

found that invertebrate community composition could be

predicted by channel morphology in the southern Appalachians.

Wilzbach and others (1986) documented an increase in

insect drift in logged areas in the Oregon Cascades, and also

showed that decreased riparian shading and blocking of

interclast crevices aided cutthroat trout in catching their

prey. Jones and Clark (1987) showed that insect communities

decreased in complexity and species richness as intensity of

urbanization increased at sites in Virginia. Radford and

Hartland-Rowe (1972) found that in-stream invertebrate

biomass decreased with flow fluctuations below a reservoir

in Alberta, and Scrimgeour and Winterbourn (1989)

documented decreased algae and invertebrate populations

after flooding and gravel transport in a New Zealand river.

Increased turbidity and deposition of fine sediment on

streambeds also affect food resources. Sedimentation on

channel substrates reduces primary production by providing

unstable substrates and by decreasing light penetration. Barko

and Smart (1986) found that macrophyte growth is limited

on sandy or organic-rich sediments. Van Nieuwenhuyse and

LaPerriere (1986) demonstrated 50 percent decreases in

primary production downstream of moderate in-stream placer

mining in Alaska, and absence of primary production

downstream of heavily mined sites. Other studies have

measured the effects of siltation on aquatic invertebrate

communities (e.g., Luedtke and Brusven 1976).

Predation and Fishing Pressure

Fish are subject to natural predation, predation by introduced species, and capture by sport, subsistence, and commercial fisheries, but little information on predation rates

exists. Wood (1987b) measured predation rates of 24 to 65

percent on juvenile salmonids by mergansers in Vancouver

Island streams, but juvenile losses to other fish, aquatic

garter snakes, otters, kingfishers, and other birds were not

known. Wood (1987a) also showed that only 10 percent of

out-migrating smolt were consumed by mergansers in the

same system. Peterson (1982a) suggested that most overwinter mortality in two off-channel ponds of the Olympic

Peninsula is by avian predation.

Natural predation is affected by altered vegetation and

channel morphology. Clearing of riparian vegetation can

both destroy predator habitat and remove cover used by fish

to escape predation. Shallowed channels, in-filled pools,

sedimented gravels and decreased flows can also reduce

cover and increase susceptibility to predation, and the effects

of these influences are often combined. Murphy and Hall

(1981) examined the effects of timber management on

predatory insects and vertebrates in streams of various sizes

in the Oregon Cascades and found that abundance increased

soon after logging but decreased when second-growth shades

channels. Russell and St. Pierre (1987) noted that

impoundments can increase predation on juvenile fish by

creating habitat for piscivorous fish.

Introduced predators can change the entire structure of a

community. Introduction of lamprey to the Great Lakes

destroyed productive fisheries, for example, and there is a

concerted effort to prevent introduction of piranhas to

California streams for fear of their effect on native fish.

Sport fisheries are closely regulated, and creel surveys

provide estimates of catch rates. Subsistence fisheries rarely

have catch records, however, and commercial ocean fisheries

involve so many nations that record-keeping is difficult.

Most information on ocean survival of salmonids is based on

the proportion of marked smolt recaptured as returning adults,

and these data combine the effects of natural predation and

fishing. Starr and Hilborn (1988) used information on migration

path, travel speed, and catch and escapement to estimate

catches by various fisheries for several stocks in the Pacific

Northwest. Fishing pressure is increasing as ocean fishing

technology becomes more sophisticated and recreational

fisheries grow. Current efforts to maintain sufficient stocking

levels depend primarily on regulation of coastal and sport

fisheries by establishing season dates and closed areas, while

the larger international fishery is largely unregulated.

River systems originally contained genetically distinct

stocks of fish adapted to local stream conditions (e.g., Bartley

and Gall 1990), and different strains could often be recognized

by size, color, timing of runs, and behavioral differences

(e.g., Rosenau and McPhail 1987). As native stocks declined,

they were often replaced by hatchery-raised fish derived

from only a few strains, and mixing of hatchery stock with

native stock further decreases regional genetic diversities

(Gausen and Moen 1991, Wehrhahn and Powell 1987).

With large hatchery releases, fishing pressure grows. If

hatchery and native fish are caught at the same rate and

hatchery stock outnumber native fish, lower absolute numbers

of native fish escape. Even though the total population may
86 USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993.

survive a high catch-to-escapement ratio (C-to-E), this may

leave too few fish of a particular genetic stock to be viable.

In other words, a native stock may be able to support a 4:1

C-to-E. With a 3-fold population increase by addition of

hatchery stock, fishing pressure may rise to a 12:1 C-to-E

without changing the number of escaping fish. However, the

native stock also sustains the 12:1 C-to-E, and this represents

a 3-fold decrease in the number of native fish that escape.

Such pressure may extinguish genetically distinct strains

that evolved to suit particular conditions.

Michael (1989) found decreasing populations of sea-run

cutthroat in Olympic Peninsula creeks and suggested that

fishing pressure is contributing to their replacement by

nonmigratory cutthroat. Continuation of this trend will decrease

genetic diversity within the channel systems and is likely to

decrease the species’ ability to cope with changing

environmental conditions.

Other Effects

Nonnative game fish have been introduced in many areas,

and the effects of their competition with native species are not

well understood (Fausch 1988). Planting of hatchery stock

introduces pathogens and parasites contracted under crowded

hatchery conditions. Hatchery fish have introduced Bacterial

Kidney Disease (BKD) to some anadromous fish populations

in California, and the disease is now endemic in those

populations because it is transferable through eggs. Although

BKD does not kill fish directly, it severely reduces their

physiological resistance to stress. Infected fish thus become

more susceptible to environmental impacts, and resulting

mortality represents a cumulative effect of hatchery policy

and environmental change (J. Nielsen, USDA Forest Service

Pacific Southwest Research Station, personal communication).

Similarly, Servizi and Martens (1991) showed that a viral

kidney infection reduces tolerance of coho salmon to suspended

sediment in southern British Columbia.

Many anadromous fish depend on estuaries during part of

their life cycle, and estuaries are being severely altered by

land use. Reclamation projects fill in wetlands for building

sites, and accelerated upland erosion enhances sedimentation.

Reid and Trexler (1991) cited the cumulative effects of

dams, dredging, oil extraction, soil conservation, and

channelization on loss of wetland habitat and potential damage

to fisheries in Louisiana, and noted that a sea-level change

due to global warming would add an additional stress to this

environment. Wissmar and Simenstad (1988) evaluated

metabolic energy costs and intake rates to develop a model

of salmonid growth rates in estuaries and suggested that this

type of energy budget may be useful for assessing carrying

capacities for estuaries.

Barton and others (1986) demonstrated that the stress response

of salmonids to multiple handling disturbances is cumulative. If

this is a general response to environmental stress, then multiple

disturbances of any kind might produce cumulative physiological

effects and decrease fitness and survival.

Impacts on Timber Resources

Timber resources are adversely affected by activities that

restrict the area suitable for growing trees, decrease productivity of timberlands, or increase mortality of desired trees.

Because trees grow slowly, even very gradual changes can

be quite important.

Loss of Land Base

Competing land uses adversely affect timber resources by

decreasing the area available for logging and by restricting

the types and intensity of timber management activities.

Expansion of agriculture and urbanization reduce forested

area on private lands, while set-asides for reservoirs, wilderness areas, and other uses incompatible with timber production decrease the timber base on public land. Past timber

management has reduced forestland in some areas by ineffectual reforestation; some once-productive land now supports only successional hardwoods and chaparral.

Productive land base is also decreased by particular

activities. Logging roads and landings no longer produce

trees, and in some areas these account for 10 percent of the

surface (e.g., Miller and Sirois 1986, Ruth 1967). Elsewhere,

soil is completely removed by landslides and gullies and will

require decades to recover. Trustrum and DeRose (1988)

measured soil formation rates averaging 2.5 mm/yr on landslide

scars on deforested New Zealand slopes. Cumulative loss of

land base by combinations of these effects can be large.

Forest Productivity and Mortality

“Productivity” reflects the growth rates of desired species.

Compaction reduces growth rates, and as much as 27 percent

of a tractor-yarded site may be compacted (Dyrness 1965).

Duffy and McClurkin (1974) found that regeneration failure

on logged sites in northern Mississippi is best predicted by

soil bulk density, and Wert and Thomas (1981) showed that

compacted sites produced only 26 percent of the wood volume of noncompacted sites 32 years after logging in the

Oregon Coast Range.

Growth may also be slowed by loss of organic horizons,

erosion of mineral soils, and cumulative loss of nutrients.

Klock (1979) estimated changes in forest productivity from

logging-related soil erosion in the Washington Cascades.

Waide and Swank (1976) modeled nutrient balances over

three cutting cycles and predicted changes in forest productivity

for different cutting strategies in the southern Appalachians.

Routledge (1987) used economic models to demonstrate the

potential effects of such changes on timber values.

Factors that affect productivity may also influence tree

mortality. Thus, partial loss of a soil profile may increase

drought stress on seedlings and decrease survival rates. Misuse

of herbicides occasionally results in widespread seedling mortality
USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993. 87

and foliar damage on treated slopes. Logging often provokes

increased populations of burrowing mammals that feed on

seedlings, and cattle may browse or trample young conifers if

grazing is improperly managed. Where channel sedimentation

rates are high, aggradation can raise water tables in riparian

zones and kill bottomland forests (Duda 1986).

Increased fire frequency and size can destroy wide areas

of forest, and such increases often accompany intensification

of land use. Some of the most destructive fires in California,

for example, have been caused by arson in urbanizing areas

and by recreational use. Disease and insects can also provoke

widespread mortality, and forest management practices may

increase susceptibility to these impacts. Logging equipment

can transfer pathogens to new sites (Zobel and others 1985),

and damage caused by thinning has been found to encourage

some insect pests (Mitchell and others 1983). Replacement

of mixed forests by monocultures may aid the spread of

some pests and diseases. Plantation forestry can reduce genetic

variation within a species in a region, and this may decrease

its resistance to some diseases, pests, and environmental

perturbations. Replacement of native strains by nurserybred stock may decrease fitness of a stock in particular

settings. Campbell (1979) found genetic differences between

native stands of trees growing in microhabitats a few tens of

meters from one another, and Fryer and Ledig (1972) showed

that genetically controlled photosynthetic temperature optima

for balsam fir closely followed the adiabatic lapse rate in the

White Mountains of New Hampshire. This type of genetic

specificity cannot be reproduced from nursery stock.

Air pollution has killed or damaged forests in many areas,

and acid rain is implicated in forest damage in central Europe

(e.g., Nihlgard 1985) and parts of North America. Pitelka

and Raynal (1989) reviewed the state of knowledge on acid

rain effects and discussed possible mechanisms for observed

forest changes. Toxic emissions have deforested wide areas

around Sudbury, Ontario (Amiro and Courtin 1981), and

have damaged forests around other smelter sites (e.g., Bruce

1989). Increased atmospheric ozone and other smog

constituents have damaged conifers in the Sierra Nevada

(Peterson and others 1987a) and decreased growth rates in

southern California (Omart and Williams 1979).

Large-scale climatic change from atmospheric pollution

has yet to be demonstrated, although many researchers expect

changes to become apparent within a few decades. If

atmospheric circulation patterns change, then temperature,

precipitation, and seasonality will be altered over broad areas.

Major vegetation changes are expected in areas like California,

where ecosystems intricately interfinger and are strongly

influenced by slight variations in climatic conditions. Ecotones

and forest types would be displaced along elevational gradients.

Predictions of climate change using global circulation models

have been used to estimate economic impacts of global warming

to timber income (e.g. Van Kooten and Arthur 1989).

Many forests in the eastern United States exhibit signs of

major decline, but reasons for the change have not been

determined. Hinrichsen (1986), Klein and Perkins (1987),

and Mazurski (1986) attributed forest decline in North America

and Europe to multiple and synergistic effects. They

hypothesized that combinations of air pollution and other

environmental impacts introduced multiple stresses into the

forest and made it more susceptible to mortality by drought,

insects, and disease. Hamburg and Cogbill (1988) suggested

that natural climatic warming is one cause of present forest

decline in the northeastern United States. Klein and Perkins

(1987) described a variety of effects that would be triggered

by forest decline and would further influence forest health.

Impacts on Recreation and

Esthetic Values

Recreational use and esthetic values strongly influence forest land-use planning because of the size of the recreational

constituency and because of its political importance. Timber-producing lands are often sites of heavy recreational

use, and the two interests are often in conflict; recreationists

are thus an important source of complaints about on-site

timberland conditions and forest practices. Sierra Club, Friends

of the River, Ducks Unlimited, Trout Unlimited, the Wilderness Society, and many other environmental and conservation organizations are founded on both recreational and philosophical values.

Recreational Needs

Different recreational pursuits require sites of different physical character, and recreationists also have less-well-defined

expectations that make some sites particularly attractive.

Satisfaction with a recreational experience depends strongly

on how well those expectations are met. Backpackers, for

example, are dissatisfied if areas are not pristine or if campsites are overcrowded. If expectations in an area are consistently unmet, then recreationists will shift their activities to

more desirable sites, their expectations will change, or they

will discontinue their activity. With the exception of watching television, participation levels for activities are directly

correlated to the level of satisfaction received (Ragheb 1980).

The attributes valued by particular types of recreationists

must be identified if the potential for impacts is to be

understood. Several studies explore recreationists’ motives

for participation and identify site characteristics that support

those motivations. Crandall (1980) identified general

motivations for use of leisure time, while Knopp (1972)

contrasted the needs of rural and urban groups and emphasized

that different populations have different recreational needs.

McDonald and Hammitt (1983) surveyed river rafters and

tubers and found that affiliation, experiencing nature, and

excitement were primary motives for their participation. On

this basis, the authors suggested that group-use sites be

developed, and that an element of risk be allowed. Searle
88 USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993.

and Jackson (1985) identified barriers to recreational

participation and discussed their implications for recreation

management. Pfister (1977) evaluated reasons for selection

of particular campsites.

Within each recreational activity there are usually

subgroups with distinct motivations and values. Heywood

(1987) defined expectations of different types of river runners

and noted that “pick-up” groups are active primarily for

adventure and social contact, while parties of acquaintances

tend to participate for quiet escape. In addition, expectations

and standards change as a participant’s experience level

grows. Schreyer and others (1984) found that experienced

river runners are more critical of environmental degradation

and user conflicts than novice paddlers are.

Cumulative impacts on recreational use generally take

the form of decreased accessibility to appropriate sites, and

may involve changes in physical site characteristics, crowding,

and ambiance.

Physical Site Characteristics

Every recreational wildland pursuit requires particular site

conditions, and sites appropriate for some uses are rare. Few

sites are suitable for activities like whitewater rafting or

drift-boat fishing, and land use can further limit their

availability. Access to river recreation, for example, is

restricted by dam construction, hydropower diversions,

irrigation projects, and stream-side developments. A landuse activity can affect a single recreational use in different

ways. Although dam construction destroys river recreation

sites, flow regulation may lengthen the recreation season

downstream: dam releases make some whitewater runs

boatable well into the dry season. Meanwhile, lake fishing

opportunities are created, while fluctuating flows may harm

downstream river fisheries. Recreational activities also have

basic environmental requirements for health and safety, and

physical changes brought about by land-use activities can

affect recreational risk.

Land use that alters flow affects recreational use of rivers.

Activities that decrease dry-season flows usually reduce

recreational opportunities, while those that enhance low flows

tend to promote recreation (Daubert and Young 1981). In

contrast, increases in higher discharges may increase hazards

to swimming and stream-side uses, and increased peak flows

destabilize channels and decrease their esthetic attraction.

Fluctuating flows usually decrease the overall recreational

potential (Radford and Hartland-Rowe 1972).

Altered channel morphology also affects river recreation.

Landslides often modify adjacent rapids and usually make

them more challenging for whitewater enthusiasts. Other

increases in sediment input usually broaden and aggrade

rivers, and this shortens the season for river recreation,

degrades the quality of rapids, and fills in pools used by

game fish. Removal of channel obstructions to promote fish

migration may destroy attractive rapids, while introduction

of logs to enhance fish habitat can make whitewater runs too

dangerous for recreational use. Fluctuating flow levels and

decreased sediment transport due to closure of Glen Canyon

Dam have eroded beaches used as campsites by Grand Canyon

river trips, and flow fluctuations degrade the esthetic

experience of the trips. Decreased peak flows below Flaming

Gorge Reservoir have allowed accumulation of coarse

sediment from tributaries, and this has increased the severity

of rapids in Dinosaur National Monument (Graf 1980).

Woody debris presents a safety hazard for river recreation

because it can trap swimmers underwater, and obstacles such as

bridge footings and scrap metal also contribute to drownings

and injuries. In particular, weirs and low-head dams are notorious

for their record of drowning swimmers in inescapable “holes”

of recirculating water created by their overfalls. Recreational

risk is increased by land uses that contribute these features, and

by uses that are in direct conflict with recreation. During the

summer of 1991 the most dangerous hazard on the popular

Pigeon Point whitewater run of the Trinity River was a gold

dredge located at the base of a rapid, and fear of assault by illicit

agriculturalists has effectively closed parts of several National

Forest districts to recreational use.

Opportunities for lake-based recreation have generally

been enhanced by land-use changes: there are many more

lakes now than there were before water and power

development. But artificial control of water levels in natural

lakes introduces fluctuations that destroy lakeshore plant

communities, and these changes decrease their attraction to

campers and bird watchers and can also affect fish populations.

Many activities require site developments like trail building,

campsite construction, and clearing of ski runs. Increased

urbanization usually creates an increased demand for such

facilities and increases the number available to choose from,

but increased use often more than compensates for new

facilities. Many recreational activities benefit from increased

road access. Mountain bikers, ORV drivers, rock collectors,

and anglers are quick to use new logging roads, and land

managers often must use extraordinary measures to exclude

recreational use from areas with road access. Cross-country

skiers frequent snow-covered roads, and new roads open rivers

to whitewater sports.

Land use can affect the incidence of disease contracted

during recreational use. Recreational use apparently has

contributed to the spread of Giardia, which is now a ubiquitous

threat to users of natural water supplies. Fecal coliform

counts are high in areas of heavy recreational use, suburban

development, and grazing, and high counts imply the presence

of pathogens. Introduction of environmental pollutants has

also increased health risks for recreational users. Fish taken

from contaminated water can pass on toxicants to those

eating them, and swimmers contract skin ailments and

infections from polluted waters.

Crowding and Ambiance

Crowding reduces the recreational value of many activities,

and is a cumulative effect of the decreasing number of
USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993. 89

appropriate wildland sites and the increasing number of

wildland recreationists. Recognition of this conflict has led

to regulation of recreational use at the most desirable sites.

Permits for private whitewater trips on the Grand Canyon

now require several years’ wait, and campsites in popular

national parks are booked well in advance. Many wildland

recreationists avoid physically appropriate sites simply because too many people use them (Anderson and Foster 1985).

Manning (1985) reviewed factors contributing to perceived

crowding in wildlands and noted that most are subjective;

perceptions of crowding seem to have less to do with use

intensity than with users’ motivations, preferences, and expectations. Backpackers’ desires for solitude increase with

experience level (Stewart and Carpenter 1989), but even tent

campers rate opportunity for solitude as a more important

factor in their satisfaction with a camping experience than

facility condition, social interactions, and opportunity for

challenge and achievement (Connelly 1987).

However, social affiliation is also an important dimension

of some recreational activities. Recreational vehicle (RV)

campers often are attracted to crowded sites because they

enjoy a sense of community with other RV campers. The

margin of Highway 101 south of Redwood National Park,

for example, collects several hundred summer RV residents.

In other cases, crowding is simply not a consideration. Anglers

regularly stand shoulder-to-shoulder during steelhead runs

on Pacific northwest streams. Each would prefer to be the

only one present, but company is tolerated because the fish

are the major attraction. Downhill skiers complain of crowding

on slopes, but the social milieu of a ski resort is part of its

advertised attraction.

Because recreationists who value solitude usually choose

sites that are relatively free of other land uses, most conflicts

over solitude involve competing recreationists. Although users

indulging in the same activity can contribute to crowding, the

perception of solitude often is based mostly on the absence of

conflicting activities. Competing recreationists are tolerant

of others insofar as they perceive them to be similar to

themselves: operators of powerboats tend to view canoeists

as fellow boaters, while canoeists see people in powerboats

as motorists (Adelman and others 1982). Recreational activities

also conflict if they increase the risk for other users, and

ORV use is rarely compatible with horseback riding. Some

permit programs are designed to avert such conflicts.

Powerboats are restricted to certain seasons on the Grand

Canyon, for example, and those for whom peace is an important

recreational goal schedule trips for other seasons. Jacob and

Schreyer (1980) reviewed the basis for recreational conflicts.

Many wildland recreationists value a pristine setting. They

desire a wilderness experience, and any intrusion of “civilization”

decreases their enjoyment. Introduction of roads or logging

usually excludes backpackers from an area even if its physical

attributes are appropriate and solitude is ensured, and popular

backpacking destinations may be abandoned by backpackers

and adopted by tent campers when roads reach them.

Each additional development in a watershed contributes

to the cumulative loss of land available for recreational

activities that require pristine settings. Even recreational

developments are considered intrusive by many wildland

recreationists (Benthem 1973), and recreational activities

can conflict if they affect the ambiance of a site. Backpackers

are even repelled by evidence of previous hikers, although

standards are modified according to how close an impacted

site is to a trailhead (Shelby and others 1988). Recreational

use is usually concentrated in a few areas even if the potential

recreational land base is large, and preferred sites are

disproportionately degraded (Frissel and Duncan 1965). This

concentration of activities heightens the perceived loss of

pristine ambiance.

The value of many recreational experiences is degraded

in proportion to the severity of visual impacts caused by

other land uses, so recreational enjoyment can often be

maintained if coexisting land-use activities are designed to

minimize visual impacts. A high proportion of hikers at a

site in Maryland did not realize that the area had been

clearcut in the past and reported enjoying their experience

there even though they disapproved of clearcutting (Becker

1983). Esthetic values are an important component of

recreational use, and several methods have been developed

for assessing the esthetic attraction of a landscape. Leopold

(1969) presented a method for evaluating rivers based on

their physical characteristics, and Hamill (1976, 1986)

critiqued evaluation methods and suggested other elements

to be included. Sanderson and others (1986) related the

esthetic attraction of rangelands to management practices

and showed that perceived attraction depends on the

backgrounds of the individuals viewing a landscape. Work

has also been done to assess the esthetic impact of different

forest management options (e.g., Benson 1981).

Economic modeling studies have been carried out to

attempt to reconcile timber and recreational interests by

designing logging rotations that maximize the sum of timber

and recreation values (e.g., Englin 1990). However, this

approach may undervalue recreational use by not accounting

for changing recreational demands during future generations,

when recreational opportunities will be constrained by the

cumulative effects of today’s management decisions.

Impacts on Water Supply and

Power Generation

How suitable a water source is for domestic, industrial, and

agricultural supply depends on its quality, its yield, its timing, and the cost of developing the resource and transporting

the water. Suitability for power generation is affected by the

same factors, and by the topographic position of the source.
90 USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993.

Water Quality

Water quality is degraded by sediment, chemicals, heat, and

bacterial content. Duda (1986) cites sediment as the major

impact on water quality in the southeastern United States.

Turbidity increases treatment cost and wear on power-generating

turbines, and is increased by activities that introduce fine

sediment to streams. Most land uses increase sediment input,

so most combinations of activities contribute cumulatively to

decreased water quality. Forster and others (1987) calculated

that a 10 percent reduction in cropland soil erosion in Ohio

would reduce water treatment costs by 4 percent.

Water quality is also degraded by chemical inputs. Many

land uses introduce herbicides, pesticides, and oil products,

and concentrations of these pollutants are sometimes high

enough to prevent domestic use. Atmospheric pollutants can

degrade water quality when they are deposited by precipitation,

and toxic wastes have contaminated aquifers. Of 45,000

wells tested in California, 5500 showed elevated

concentrations of potentially harmful agricultural chemicals

(Bouwer 1990). Contamination of water supplies by herbicides

has long been an issue, but its significance has not yet been

resolved. Kimmins (1975) reviewed the effects of silvicultural

herbicides. Hoar and others (1986) presented evidence for

an association between agricultural pesticide use and nonHodgkin’s lymphoma, and Colton (1986) reviewed studies

on this association. In the cases reviewed, affected individuals

had physical contact with the chemicals, and water supplies

were not a primary source of contamination.

Water Quantity and Runoff Timing

Water supplies are strongly affected by changes in the amount

and timing of surface runoff. Land-use activities that alter

infiltration rates or vegetation cover can change the proportion of precipitation that runs off, and most activities affect

both of these properties. Deforestation increases water yield

by decreasing evapotranspiration rates, and vegetation has

often been manipulated to augment water supply. Urbanization also usually increases runoff because of decreased evapotranspiration, decreased permeability, and water imports.

Changes in water yield are usually proportional to the area

of watershed altered, but the magnitude of the change depends on the time-distribution of precipitation, type of precipitation, soil type, vegetation, and channel characteristics.

Enough water yield experiments have been carried out (e.g.,

Bosch and Hewlett 1982, Ffolliott and others 1989) that

general patterns of increase and the factors controlling their

magnitudes should be discernible.

Changes in runoff timing can alter the suitability of a

water source for particular uses. Power generation and most

consumptive water uses require relatively even water input

through the year, though irrigation demands are highest

during the growing season. Increased seasonality of runoff

increases dependence on water storage to balance supply

and demand, and reservoirs receiving most their inflow over

a short period must be capable of storing larger volumes

than those for which inflow is distributed through the year.

A snowpack represents a large volume of stored water. If the

proportion of precipitation falling as snow changes, or if

snowmelt is accelerated, snowpack storage must be replaced

by reservoir storage to maintain storage capacity.

Altered runoff changes the requirements for water

development. Climatic change can either increase or decrease

the demand for irrigation water, for example, and a change

in runoff seasonality can prolong or shorten the period during

which irrigation is required. Seasonality may change because

of climate alterations or because altered vegetation affects

the proportion of runoff occurring as rapid snowmelt.

Water Storage

Increased sedimentation in reservoirs can severely abridge

their usable lifespan. For example, China’s Sanmexia

Reservoir had silted in to the point that it was unusable in 4

years; California’s Black Butte Reservoir is losing storage

volume to sedimentation at a rate of 8 percent a decade; and

capacity behind Dam Number 3 on the Ocoee River in North

Carolina has decreased 73 percent in 30 years (Reisner

1986). Storage in most of the Nation’s reservoirs will decrease

noticeably within a few centuries and many will become

unusable. New water sources will need to be developed, but

the most suitable dam locations have already been used.

Regional sedimentation rates can be estimated using

compendia of reservoir infill data (e.g., Dendy and Champion

1978). Brune (1953) outlined a method for calculating

reservoir trap efficiencies if storage volume, inflow rates,

and river sediment loads are known.

Impacts on Floodplain and

Channel Use

Fertile floodplain soils often support intensive agriculture

and livestock production, and the sites provide relatively

level settings for roads, railways, and towns. Floodplain use

is particularly intensive in lowland areas distant from logging and other wildland activities, but even these lowland

sites can be affected by forest practices upstream.

A variety of in-stream structures and modifications are

associated with floodplain use. Roads and railways require

bridges, and agricultural developments commonly include

irrigation diversions, levees, and bank-protection works.

Riverside urban centers are often serviced by river traffic,

which requires construction of ports, docks, and locks. These

uses and structures can all be affected by changes in the

morphology and flow of the rivers they depend on, and

occasionally by changes in aquatic communities.

Stream Flow

Peak flows can be altered chronically, as by a change in

basin hydrology or vegetation, or catastrophically, as by a
USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993. 91

dam failure or major landslide. Chronic changes can increase flood frequencies and cause property damage, loss of

life, increased maintenance costs for transportation and communication systems, and damage to levees and bank protection structures. Occasional anthropogenic catastrophic events,

such as the Teton and St. Francis dam failures, have resulted

in one-time floods of extremely large size. Baldewicz (1984)

reviewed the incidence of catastrophic dam failures. Increased debris loading can cause log jams that back up flow

and increase effective stages for a given discharge, and

channel aggradation can have the same effect.

Low-flow discharges can be decreased by consumptive

water use, impoundments, and diversions, and lowered flows

may impede river navigation and floodplain irrigation.

Decreased low flows usually cause draw-down of riparian

water tables and increase irrigation demands. Aggradation

of coarse sediment in channels can reduce surface flows as

water percolates through permeable gravels, and this also

restricts the use of surface water.

Channel Morphology

Bank erosion undermines and destroys channel-bank structures and roads and increases maintenance and construction

costs. Channels can either move incrementally by bank erosion or shift catastrophically, as when temporary damming

by a log jam diverts flow into an erodible overflow channel.

Resulting channel shifts can remove valuable lands from

production and replace them with infertile gravel bars or

widened channels. Major channel shifts can destroy structures far from the original channel. Channel changes provoked by gravel mining along Tujunga Wash in southern

California destroyed a length of freeway and several bridges

(Bull and Scott 1974).

Increased sediment loads can aggrade lowland channels,

increase the frequency of overbank flows, and force channels

to migrate more rapidly. As bed elevation increases, channelbed gravels may be deposited at sites previously reached

only by silts, and unworkable gravels may cover productive

floodplain soils. Aggradation can also raise local water tables

so that drains must be constructed to maintain agriculture.

Sedimentation at domestic and irrigation water intakes can

increase maintenance and water treatment costs, and

aggradation or incision around bridges can endanger bridge

footings and occasionally lead to collapse and fatalities (Duda

1986). Increased upland erosion also contributes to accelerated

sedimentation in estuaries and may force more frequent dredging

of navigation channels.

In contrast, channel incision lowers adjacent water tables

and can make gravity-feed irrigation impossible. Ancient

gully erosion episodes in the American Southwest may have

destroyed the economic base of Native American civilizations

dependent on floodplain irrigation. Reduction of sediment

load downstream of a dam caused the Carmel River to

incise, and the resulting lowering of streamside aquifers

killed riparian vegetation (Matthews and Kondolf 1987).

Increased debris loading and log jams are associated with

altered patterns of bank erosion, channel migration, and

flooding. Anecdotal reports of saw-log jams tearing out

bridges during floods are common in forested areas of the

western United States, and choking of culverts by logging

debris is a common mechanism for logging road failures.

Other Changes

Imported water weeds can restrict navigation. For example,

navigation in Lake Washington is seasonally hampered by

milfoil growth, and expensive clearing operations are necessary. Dams obstruct river navigation, and riverside communities may lose a source of transportation if locks are not

constructed. Floodplain soils are easily contaminated by

pollutants, because many chemicals accumulate on silt particles that preferentially settle out there. Lewin and others

(1977) and Marron (1989) measured elevated concentrations of toxic chemicals in floodplain deposits downstream

of mines.

Impacts on Other Resources and

Values

Most resources and values are influenced by basic watershed properties either as habitat elements or as physical

influences on processes. Human health is affected by changes

in water quality and distribution of toxic materials and pathogens. Changes in vegetation, erosion rates, and hydrology

affect range resources and wildlife, and cattle health has

been impaired by mining-related chemical changes (Erdman

and others 1978).

Progressive soil erosion alters soil structure and depletes

nutrients on croplands. Schertz and others (1989) measured

decreased soybean yields of 24 percent at sites with severe

cumulative soil erosion in Indiana. Water development in

combination with repeated use for irrigation increases salinity

in many rivers of the western United States, and may

eventually raise concentrations enough that further use is

impossible. Brown and others (1990) calculated the cost of

maintaining tolerable salt levels in the Colorado River.

Colacicco and others (1989) reviewed a variety of impacts

resulting from agricultural soil erosion and estimated costs

associated with them.

The western United States is rich with the cultural heritage

of Native Americans. Many natural areas and topographic

features hold spiritual significance, and the cumulative cultural

loss grows as incompatible uses encroach on an increasing

number of these sites. In addition, many sites of spiritual and

cultural importance are located along rivers and are susceptible

to destruction by altered flows and channel morphology.

Economists have defined several types of noneconomic

values that are affected by environmental changes. These

address the observation that many people value the existence
92 USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993.

of healthy forests, wild rivers, undisturbed areas, and other

qualities of wilderness. In some cases, importance is attached

to preserving a quality for future generations, while in others

people want to know that the opportunity to experience a

setting is available for them if they should decide to visit it in

the future. People also recognize the importance of preserving

genetic resources and biodiversity in case future needs arise.

Methods of assessing these values are not yet satisfactory.

Pope and Jones (1990) attempted to quantify the value of

wilderness preservation in Utah by asking residents how

much they would pay to preserve wilderness. Walsh and

others (1990) and Sanders and others (1990) used similar

methods to measure the value of forest quality and river

protection, respectively. Results of such studies can be difficult

to interpret, however, because a significant proportion of the

respondents often refuse to consider the concept of paying

for something that they believe is a fundamental mission of

public land management. In the case of the study by Sanders

and others (1990), these respondents were simply excluded

from the analysis. Increasingly, environmental organizations

such as Nature Conservancy and Save-the-Redwoods are

demonstrating the economic value of land preservation by buying

land to exclude incompatible uses.

Research Needed to Understand

Impacts

Much basic research is needed to understand and predict

responses of aquatic communities to changing conditions.

Little is known of survival strategies during harsh winter

conditions or of mechanisms of recovery from disturbance.

Many impacts involve slight changes in timing of events,

and the effects of these changes on aquatic biota are only

now beginning to be studied. Interactions between competing organisms are poorly understood, and these have a strong

influence on the response of organisms and communities to

change. Habitat variables that control fish use need to be

better defined to improve the usefulness of habitat assessments and to predict the effects of altered habitat. For example, Baltz and others (1987) demonstrated that the three

habitat variables most often used for in-stream flow studies

were not optimal for predicting species use of microhabitats

in a northern California river.

Vegetation changes brought about by suspected systemic

changes in atmospheric composition and global climate may

be of increasing importance to timber resources but are very

poorly understood. All other land-use related changes to

forest productivity will be overlaid on this backdrop, and

effects will be difficult to isolate. Work is needed to quantify

the effects of ongoing, well-understood changes on secondand third-cycle logging. In particular, impacts of altered

nutrient regimes are poorly understood, and the influence of

management strategies on the resilience and stability of

forest ecosystems must be determined.

Further work on recreational use patterns and needs is

necessary to determine the effects of altered access and

physical and biological setting on recreational values.

Motivations and needs must be determined for various activity

groups, and patterns of recreational response to changing

conditions need to be understood. Impacts of altered conditions

on recreational safety are not well known. Esthetic and other

noneconomic values must be far better understood if impacts

on these resources are to be evaluated adequately, and methods

to assess the magnitude of such impacts need to be developed.

Many of these problems require sociological and psychological

research.
USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993. 93

Chapter 8

Summary of Research and Cumulative Watershed Effects

One researcher has described cumulative effects as “theeffects of everything on anything.” Although intended

to be flippant, this description underscores the inherent

complexity of the subject. Prediction of CWEs would be

essentially impossible without a framework for understanding

their nature and causes. A strictly empirical approach to

prediction cannot succeed because too many land-use activities

can combine in too many ways and affect too many potential

resources and values.

A Framework for Understanding

CWEs

The problem of understanding CWEs is simplified when

potential mechanisms for triggering impacts are considered.

Land-use activities can influence only a limited number of

environmental parameters (path A in Figure 5). Changes in

vegetation, soil characteristics, topography, chemicals, water,

pathogens, and fauna can induce compensatory changes in

one another (path B) and can influence watershed processes

(path C). Watershed processes arise from an area’s role as a

concentrator of runoff, and include production and transport

of runoff, sediment, chemicals, organic material, and heat.

These processes can influence environmental parameters (path

D), and they can also interact (path E). Changes in either

environmental parameters or watershed processes can generate

on-site CWEs (paths F and G), but only changes in watershed

processes can produce off-site CWEs (paths H and I).

A cumulative effect is a change influenced by multiple,

progressive, or repeated activities. Effects can accumulate

through time or grow by contributions from multiple sources.

Accumulations through time require that either the triggering

mechanism be persistent, or the recovery time be greater

than the period between disturbances. In contrast, a spatially

accumulating effect may occur through instantaneous changes.

Cumulative effects are complicated by properties of

watersheds and ecosystems that obscure the relation between

cause and effect. Effects may be delayed until long after the

triggering activity has occurred, and frequently the location

of impact is far removed from the original land-use disturbance.

Local conditions may modify the form of an impact, and a

single impact may have many contributing causes.

Some changes may accumulate until a threshold is reached

and then trigger a catastrophic change that is not evident until

it occurs. Thus, slow channel incision may progress until

banks are high enough to fail. Other changes may accumulate

benignly until the occurrence of an external triggering event

like a large storm. These effects also may not be evident until

after preventative measures are no longer possible. Thresholdtype changes are particularly insidious from a management

perspective, because precedents for their occurrence may not

exist. Such effects can be predicted only from a basic

understanding of the processes contributing to them.

State of Present Knowledge

The ability to predict CWEs depends on understanding how

and why the effects occur. Complete understanding would

require knowledge of all interactions represented in figure 5.

Many of these have been extensively studied, and some

interactions are now qualitatively or even quantitatively predictable. Most fields of natural science have contributed to

our present understanding of CWEs, and work in hydrology,

geomorphology, and ecology is particularly relevant.

Direct influences of specific land-use activities on

environmental parameters are easily observed and measured,

and abundant data exist. In particular, effects of most activities

Figure 5—Influences that generate on-site and off-site CWEs. Interaction paths are described in the text.

Land

use

activities

on-site

impact

off-site

impact

off-site

environmental

parameter

on-site

environmental

parameter

A C

B E

I

F GD H

watershed

process
94 USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993.

on vegetation, soil properties, topography, water import, and

faunal import are well-defined. Influences on chemical import

and pathogen distribution are more poorly understood, but

some measurements exist. The nature of interactions between

altered environmental parameters can often be predicted, but

quantitative information is rare. Land-use activities having

similar influences on an environmental parameter will have

similar influences on watershed processes, so once the nature

of the direct environmental change is defined, the type of

land-use generating it is no longer of importance to evaluation

of impacts.

Rates, distributions, and modes of watershed processes

are controlled by environmental parameters. The influence

of environmental parameters on production and transport of

water and sediment have been quantified in many areas, and

predictive equations exist for many processes and conditions.

Credible water and sediment budgets can now be developed

for most areas using available methods, published data, and

a few local measurements, and effects of land-use activities

on sediment production and runoff can thus be estimated.

Results are usually only approximate, but are adequate for

most analyses. Methods of flow-routing in channels are

well-established and allow estimation of changes in peak

flow due to changes in runoff distribution, but available

techniques are not well suited for rough, high-gradient

channels. Sediment routing is more poorly understood because

methods to evaluate sediment delivery are poorly developed.

Future input rates for organic debris often can be estimated

for planned land-use changes using local measurements of

mortality and edge effects. Decay and breakdown rates of

woody debris in channels are rarely known, however, and

too few measurements have been made to develop general

relationships. Qualitative patterns are evident and may be

predicted, but quantitative predictions of altered debris

transport and accumulation are not yet possible.

Components of nutrient cycles have been measured at

many sites, but nutrient production and transport processes

are not well understood. Both nutrient loss at a site and

nutrient accumulation downstream may cause impacts and

need to be predicted. Qualitative predictions of loss and

accumulation can often be made on the basis of local

experience and general patterns, but quantitative predictions

are not yet possible. Because of current concerns over toxic

waste disposal, methods are being developed for tracking

the movement of introduced chemicals through watersheds.

Results of these studies are applicable to the problem of

nutrient routing and downstream accumulation. Production

and transport of heat has been more widely modeled, and

several methods are now being used to predict the effects of

altered land use on stream temperatures.

Few predictive models for watershed processes can assess

the results of interactions between processes. For example, a

temperature model may accurately predict changes occurring

from increased flow, but the thermal impact of secondary

effects, such as that due to the loss of riparian vegetation

from destabilized streambanks, are not predicted by a flowbased model. Similarly, changes in sediment input can alter

channel morphology, which may then modify rates of sediment

input from bank erosion. Predictive methods should not be

applied without evaluating the potential for secondary changes

that might affect results.

Our current inability to accurately calculate rates of

sediment accumulation and scour in channels as a function

of sediment input and flow is of particular concern. These

mechanisms are the driving force for altering channel

morphology, and morphological change affects other

watershed processes and frequently impacts resources. Patterns

and trends can often be predicted qualitatively, but quantitative

predictions are rarely possible.

The final steps in the framework illustrated by figure 5

(paths F, G, H, and I) are the links between altered parameters

or processes and resource impacts. In many cases, these

links are well known and predictable. For example, loss of

reservoir capacity is directly proportional to sedimentation,

and rates can be calculated from measured or predicted

sediment yields and trap-efficiency equations. In general,

impacts on physical resources can often be predicted if the

nature of changes in parameters or processes is known.

However, responses of biological systems are more difficult

to predict because of the complex interdependencies within

biological communities. Correlative studies have identified

some general patterns of response among communities, so a

population’s response to change can often be predicted

qualitatively. Quantitative predictions are possible only in

systems where the nature of biological interactions and

dependencies on physical habitat are understood.

Many studies have provided information on the knowledge

gaps identified above, and results of existing studies need to

be compiled and examined for patterns of response. This

approach is often useful for identifying variables that control

the expression of a response and for focusing future studies.

Methods for Evaluating CWEs

Land managers need usable and accurate procedures for

evaluating the combined effects of multiple land-use activities on other resources and values. Although several procedures have been developed and are in use, none can be

applied generally. Current models were developed to apply

to particular issues in particular areas, and do not address the

variety of land-use activities, mechanisms of watershed response, and types of impacts that occur at other sites.

Although a general predictive technology does not yet

exist, the ability to predict many types of effects clearly does,

and many studies have evaluated and predicted cumulative

effects of particular land-use activities on particular resources.

Successful studies usually evaluated impacts by examining

the processes that are altered by land use, or by comparing

multiple sites to infer patterns and magnitudes of occurrence.
USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993. 95

Three general approaches to predictive CWE assessment

have been developed:

1. Models that produce a predicted value for a physical or

biological parameter or an index value that indicates the

potential for triggering a particular type of impact.

2. A collection of procedures for use in evaluating a

variety of types of impacts, where a relevant subset is selected

for a particular application.

3. A checklist of items to consider during assessment, but

with specific approaches left to be selected by the operator.

The first approach may work well in the area for which a

model was developed, but the model cannot be applied to

sites with different characteristics, processes, or types of

impact. Most of these models assess the potential for an

impact at a specific time and do not take into account the

extent to which the impact has occurred in the past. Because

of this, potential impacts that accumulate because of their

slow recovery rate cannot be assessed. The second approach

allows the flexibility of determining the impacts and

mechanisms relevant to a particular setting, but provides no

guidance for deciding which aspects must be evaluated. The

final approach provides the guidance but not the methods.

A Framework for CWE Evaluation

A workable general procedure for CWE evaluation might

contain elements of all three approaches. A checklist, key, or

expert system could guide users through a decision tree to

identify the types of impacts likely to be of concern in a

management region. Appropriate procedures could then be

selected from a compilation of state-of-the-art methods to

evaluate identified concerns. These procedures might include existing methods from categories 1 and 2.

This combined approach has the advantage of flexibility in

addressing unique concerns and characteristics of particular

sites while still providing standardized evaluation procedures.

In addition, it allows improved predictive methods to be

incorporated as they become available, and thus supports use of

the best available technology for each aspect evaluated. This is

a critical requirement both for responsible land management

and for accountability in the face of litigation, and it recognizes

a fundamental characteristic of technology: no product is

definitive for long. Approaches that incorporate a capacity to

evolve will be useful far longer than those that do not.

In practice, a particular management region would be

characterized by a relatively small subset of likely impacts

and corresponding evaluations. This subset could be

incorporated into a streamlined “cookbook” for use in the

region. Implementation would be relatively straightforward

after measurement of the coefficients and relationships

required by the selected procedures. Any implementation of

a CWE assessment method must include a mechanism for

evaluating its validity.

The Role of Research in

Assessing CWEs

Research is essential in all phases of development and

implementation of a CWE evaluation procedure. Model

development depends on research that defines relationships

between change and response or explains the mechanisms by

which response occurs. This requires continued basic research

to increase understanding of how watersheds and biological

communities function. It also requires increased emphasis on

the knowledge gaps discussed above, and interdisciplinary

work is necessary if interactions between physical and

biological systems are to be clearly understood and predicted.

Once an impact mechanism is identified and understood,

that knowledge must be transformed into an ability to predict

response. At this point, interaction between researchers,

resource specialists, and land managers is essential to ensure

that the procedures developed are useful and practical. Models

that are useful to managers provide results at scales relevant

to management problems, are not overly vague or overly

detailed in their predictions, represent the range of conditions

likely to occur in the management setting, and do not require

excessive data for calibration and use.

Each impact mechanism requires a model or evaluation

procedure, and each model or procedure will be outdated as

further research reveals additional information. Research to

understand and predict CWEs will thus continue as long as

CWEs remain an issue.

Research may be necessary to provide data for model

calibration. Most models require an initial calibration for

application to particular settings. This may simply involve

measurement of topographic attributes of a region, or it may

require construction of a sediment budget to assess sediment

inputs for various types of land use in the region.

Verification is essential for the confident application of

any model or procedure. In the case of CWE assessments,

verification is likely to include two independent aspects.

First, the validity of individual modules must be tested by

comparing predicted and measured values. This must be

done both during model development and under field

conditions. Verification usually requires monitoring of

selected sites.

The second type of verification considers the success of

the CWE assessment as a whole, rather than that of its

components. To accomplish this, all impacts generated at a

selection of evaluated sites must be surveyed and compared

to the risks predicted by the original CWE assessments.

This, too, will require input from researchers on experimental

design and monitoring techniques.

Methods are currently available for qualitatively or

quantitatively predicting many types of CWEs from many

types of land use, so initial development of a protocol for

CWE assessment need not wait for new research results.
96 USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993.

Appendix—People Interviewed

During Report Preparation

Name Affiliation Location

Neil Berg USDA Forest Service, Pacific Southwest Research Station Albany, Calif.

C. Jeffery Cederholm Washington State Department of Natural Resources Olympia, Wash.

John Chatoian USDA Forest Service, Region 5 San Francisco, Calif.

Glenn Chen USDA Forest Service, Siskiyou National Forest Powers, Oregon

Lynn Decker USDA Forest Service, Region 5 San Francisco, Calif.

William Dietrich University of California,Department of Geology and Geophysics Berkeley, Calif.

Thomas Dunne University of Washington, Department of Geological Sciences Seattle, Wash.

Frederick Euphrat University of California, Department of Forestry Berkeley, Calif.

Fred Everest USDA Forest Service, Pacific Northwest Research Station Juneau, Alaska

Michael Furniss USDA Forest Service, Six Rivers National Forest Eureka, Calif.

Gordon Grant USDA Forest Service, Pacific Northwest Research Station Corvallis, Oregon

Russell Henly University of California, Department of Forestry Berkeley, Calif.

Dennis Harr USDA Forest Service, Pacific Northwest Research Station Seattle, Wash.

Eugene Hetherington Forestry Canada Victoria, British

James Hornbeck USDA Forest Service - Northeastern Research Station Durham, N.H.

George Ice National Council of the Paper Industry for Air and Stream Improvement Corvallis, Oreg.

Terry Kaplan-Henry USDA Forest Service, Sequoia National Forest Porterville, Calif.

Harvey Kelsey Western Washington University, Department of Geology Bellingham, Wash.

G. Mathias Kondolf University of California, Department of Landscape Architecture Berkeley, Calif.

Antonius Laenen US Geological Survey Portland, Oregon

George Leavesly US Geological Survey Denver, Colo.

Donna Lindquist Pacific Gas and Electric Company San Ramon, Calif.

Thomas Lisle USDA Forest Service, Pacific Southwest Research Station Arcata, Calif.

Keith Loague University of California, Soil Science Department Berkeley, Calif.

Lee MacDonald Colorado State University, Department of Earth Resources Fort Collins, Colo.

Mary Ann Madej Redwood National Park Arcata, Calif.

Bruce McGurk Southwest Research Station Albany, Calif.

Walter Megahan National Council of the Paper Industry for Air and Stream Improvement Port Townsend, Wash.

David Montgomery University of Washington, Department of Geological Sciences Seattle, Wash.

Thomas Nickelson Oregon Department of Fish and Wildlife Corvallis, Oreg.

Jennifer Nielsen USDA Forest Service, Pacific Southwest Research Station Arcata, Calif.

K. Michael Nolan US Geological Survey Menlo Park, Calif.

N. Phil Peterson University of Washington Center for Streamside Studies Seattle, Wash.

Gordon Reeves USDA Forest Service, Pacific Northwest Research Station Corvallis, Oreg.

Ray Rice USDA Forest Service, Pacific Southwest Research Station Arcata, Calif.

Roy Sidle USDA Forest Service, Intermountain Research Station Logan, Utah

 John Stednik Colorado State University Department of Earth Resources Fort Collins, Colo.

Kathleen Sullivan Weyerhaeuser Company Tacoma, Wash.

Fred Swanson USDA Forest Service, Pacific Northwest Research Station Corvallis, Oreg.

William Trush Humboldt State University Fisheries Department Arcata, Calif.

Clyde Wahrhaftig US Geological Survey Menlo Park, Calif.

Robert Wissmar University of Washington, Center for Streamside Studies Seattle, Wash.

Peter Wohlgemuth USDA Forest Service, Pacific Southwest Research Station Riverside, Calif.

Robert Ziemer USDA Forest Service, Pacific Southwest Research Station Arcata, Calif.
USDA Forest Service Gen. Tech. Rep. PSW-GTR-141-WWW. 1993. 97

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