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The State of the Colorado River 

Ecosystem in Grand Canyon

A Report of the Grand Canyon

Monitoring and Research Center

1991–2004

Edited by Steven P. Gloss, Jeffrey E. Lovich, and Theodore S. Melis

USGS Circular 1282

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

Patrick Leahy, Acting Director

U.S. Geological Survey, Reston, Virginia: 2005

For product and ordering information:

World Wide Web: http://www.usgs.gov/pubprod

Telephone: 1-888-ASK-USGS

For more information on the USGS--the Federal source for science about the Earth, its natural and living resources, 

natural hazards, and the environment:

World Wide Web: http://www.usgs.gov

Telephone: 1-888-ASK-USGS

Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the 

U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Suggested citation:

Gloss, S.P., Lovich, J.E., and Melis, T.S., eds., 2005, The state of the Colorado River ecosystem in Grand Canyon: U.S. 

Geological Survey Circular 1282, 220 p.

Library of Congress Cataloging-in-Publication Data

The state of the Colorado River ecosystem in Grand Canyon : a report of the Grand Canyon Monitoring and Research 

Center / Steven P. Gloss, Jeffrey E. Lovich, Theodore S. Melis, editors.

 p. cm.

 Includes bibliographical references and index.

 1. Stream ecology--Arizona--Grand Canyon. 2. Stream ecology--Colorado River (Colo.-Mexico) I. Gloss, Steven. II. 

Lovich, Jeffrey E. III. Melis, Theodore S. IV. Grand Canyon Monitoring and Research Center. 

 QH105.A65S73 2005

 577.6’4’0979132--dc22

 2005018333

This report is a scientific product of the U.S. Geological Survey. As such, it will be an important element in informing the policy dialogue for decisionmakers and stakeholders involved with or interested 

in operations of Glen Canyon Dam and the protection of downstream resources of Grand Canyon 

National Park. Like all scientific documents, however, it will be only one element of the policy dialogue. 

Ultimately, many other factors will also be considered by decisionmakers when they formulate official 

policy governing the operation of Glen Canyon Dam.
iii

As a “larger than life” Director of the 

U.S. Geological Survey and the first 

person known to have successfully rafted 

the Colorado River through Grand 

Canyon, John Wesley Powell sent out a 

call to raise science aloft, a call that has 

particular resonance for the Glen 

Canyon Dam Adaptive 

Management Program. In 

Grand Canyon, science 

offers a means of understanding and predicting the relationships 

between the operations of Glen Canyon 

Dam and downstream 

resources of concern. 

This fact was recognized by both the Grand 

Canyon Protection Act of 

1992 and the final environmental impact statement that proposed the 

Adaptive Management Program. Monitoring and research were selected as the 

tools to allow scientists to unravel the 

many uncertainties that existed, and continue to exist, about downstream impacts 

from dam operations.

Significantly, science within the context 

of adaptive management is intended to 

serve management and policy. Scientists 

are responsible for developing relevant 

information, and river managers are 

responsible for making resource decisions 

by using the best information available. 

When scientists and managers work 

together, science can be the olive branch 

of peace and emblem of hope needed to 

mitigate the adverse effects of dam operations and improve the values for which 

Glen Canyon National Recreation Area 

and Grand Canyon National Park were 

established. These are the wishes of the 

American people as expressed in the 

Grand Canyon Protection Act of 1992. 

The following chapters summarize a decade of monitoring 

and research activities for 

many key resources in 

the Colorado River 

corridor below Glen 

Canyon Dam. Where 

possible, scientists assess 

the effects of dam operations, particularly the 

modified low fluctuating flow alternative, 

on given resources and 

highlight the linkages 

among system features that 

managers identified as important. 

The role that John Wesley Powell envisioned for science in 1882 reflects the 

highest goals of the scientists and other 

professionals of the U.S. Geological 

Survey today. In keeping with this vision, 

The State of the Colorado River Ecosystem in 

Grand Canyon is emblematic of the high 

quality science that the U.S. Geological 

Survey is committed to providing to 

its customers. Science of the type 

reported here, which can be used to 

make informed decisions, is the return 

on investment that American taxpayers 

deserve and appreciate. 

P. Patrick Leahy, Ph.D.

Acting Director

U.S. Geological Survey

Foreword

“Let us not 

gird science to our 

loins as the warrior 

buckles on his sword. Let 

us raise science aloft as the 

olive branch of peace and the 

emblem of hope.”

—John Wesley Powell, 

 1882, p. 70

Powell, J.W., 1882, Darwin’s contribution to philosophy, in Addresses delivered on the occasion of the

Darwin memorial meeting, May 12, 1882: Washington, D.C., Biological Society of Washington, p. 60–70.
iv
v

Project Staff

Editors

Steven P. Gloss, Ph.D.

U.S. Geological Survey

Southwest Biological Science Center

Tucson, Arizona

Jeffrey E. Lovich, Ph.D.

U.S. Geological Survey

Southwest Biological Science Center

Flagstaff, Arizona

Theodore S. Melis, Ph.D.

U.S. Geological Survey

Southwest Biological Science Center

Flagstaff, Arizona

Project Coordinator

Lara M. Schmit

Northern Arizona University

Center for Sustainable Environments

Flagstaff, Arizona

Research Assistant

Christopher N. Updike

Northern Arizona University

Center for Sustainable Environments

Flagstaff, Arizona

USGS National Wetlands 

Research Center 

Production Staff

Tammy Charron, Victoria Chachere Jenkins, 

Natalie Gormanous Trahan, Ann Gaygan: 

IAP World Services;

Beth Vairin: USGS

Additional production support

Gaye Farris and Rhonda Davis: USGS; 

Connie Herndon and Jarita Davis: 

IAP World Services

Jeff Sorensen, Arizona Game and Fish Department

Special Acknowledgment

The assistance of Stephanie Wyse Mietz, 

librarian, U.S. Geological Survey, Grand 

Canyon Monitoring and Research Center; 

Dale Blank, network administrator, 

U.S. Geological Survey, Grand Canyon 

Monitoring and Research Center; David 

J. Topping, research hydrologist, U.S. 

Geological Survey, National Research 

Program, Water Resources Discipline; Scott 

A. Wright, hydrologist, U.S. Geological 

Survey, Grand Canyon Monitoring and 

Research Center; and Thomas M. Gushue, 

GIS coordinator, U.S. Geological Survey, 

Grand Canyon Monitoring and Research 

Center, is gratefully acknowledged.

Peer and Technical 

Reviewers

The content of this report was enhanced by 

the wisdom and research of many colleagues 

and cooperators. In particular, we gratefully 

recognize the indepth and timely technical 

reviews by the Glen Canyon Dam Adaptive 

Management Program Science Advisors and 

individual peer reviewers.
vi

Ted Angradi, Ph.D.

U.S. Environmental Protection Agency

Denver, Colorado

Jill Baron, Ph.D. (Science Advisor)

U.S. Geological Survey

Fort Collins, Colorado

Bryan Brown, Ph.D.

SWCA Environmental Consultants, Inc.

Salt Lake City, Utah

David E. Busch, Ph.D.

U.S. Geological Survey

Portland, Oregon 

Steven W. Carothers, Ph.D.

SWCA Environmental Consultants, Inc.

Flagstaff, Arizona 

Bonnie G. Colby, Ph.D.

University of Arizona

Tucson, Arizona 

David Cole, Ph.D.

Aldo Leopold Wilderness Research Institute

Missoula, Montana 

Chip Colwell-Chanthaphonh, Ph.D.

Center for Desert Archaeology

Tucson, Arizona 

Kenton R. Corum, Ph.D.

Northwest Power and Conservation Council

Portland, Oregon 

Virginia Dale, Ph.D. (Science Advisor)

Oak Ridge National Laboratory

Oak Ridge, Tennessee

Gregg Garfin, Ph.D.

University of Arizona

Tucson, Arizona

L.D. Garrett, Ph.D.

(Executive Director of Science Advisors)

M3 Research

Olathe, Colorado 

Randy Gimblett, Ph.D.

University of Arizona

Tucson, Arizona 

Brian Graeb

South Dakota State University

Brookings, South Dakota 

Gordon E. Grant, Ph.D.

U.S. Department of Agriculture

Forest Service

Corvallis, Oregon 

Al Groeger, Ph.D.

Texas State University

San Marcos, Texas 

Lance Gunderson, Ph.D. 

(Science Advisor)

Emory University

Atlanta, Georgia

Steve Gutreuter, Ph.D.

U.S. Geological Survey

La Crosse, Wisconsin

Joel R. Hamilton, Ph.D.

University of Idaho (emeritus)

Moscow, Idaho

Judson W. Harvey, Ph.D.

U.S. Geological Survey

Reston, Virginia 

Alan Howard, Ph.D. (Science Advisor)

University of Virginia

Charlottesville, Virginia

W. Carter Johnson, Ph.D.

South Dakota State University

Brookings, South Dakota 

Pierre Y. Julien, Ph.D.

Colorado State University

Fort Collins, Colorado 

James Kitchell, Ph.D. (Science Advisor)

University of Wisconsin

Madison, Wisconsin

G. Richard Marzolf, Ph.D.

U.S. Geological Survey (retired)

Berryville, Virginia 

Barbara J. Mills, Ph.D.

University of Arizona

Tucson, Arizona 
vii

Craig J. Palmer, Ph.D.

University of Nevada, Las Vegas

Las Vegas, Nevada

Margaret Palmer, Ph.D. (Science Advisor)

University of Maryland

College Park, Maryland

Craig Paukert, Ph.D.

U.S. Geological Survey

Manhattan, Kansas 

Bruce Peacock, Ph.D.

National Park Service

Fort Collins, Colorado 

Roger Pulwarty, Ph.D.

National Oceanic and Atmospheric 

Administration–Cooperative Institute for 

Research in Environmental Sciences

Climate Diagnostics Center

Boulder, Colorado

J. Jefferson Reid, Ph.D.

University of Arizona

Tucson, Arizona

Bruce L. Rhoads, Ph.D.

University of Illinois

Urbana, Illinois

Catherine A. Roberts, Ph.D.

College of the Holy Cross

Worcester, Massachusetts 

Dale Robertson, Ph.D. (Science Advisor)

U.S. Geological Survey

Middleton, Wisconsin

Douglas Schwartz, Ph.D. (Science Advisor)

School of American Research

Santa Fe, New Mexico

Michael L. Scott, Ph.D.

U.S. Geological Survey

Fort Collins, Colorado

Francisco J.M. Simões, Ph.D.

U.S. Geological Survey

Denver, Colorado 

Kristin E. Skrabis, Ph.D.

U.S. Department of the Interior

Washington, D.C.

Charles R. Smith, Ph.D.

Cornell University

Ithaca, New York 

Alan P. Sullivan, Ph.D.

University of Cincinnati

Cincinnati, Ohio 

David Tarboton, Ph.D.

Utah State University

Logan, Utah

Todd Tietjen, Ph.D.

Mississippi State University

Mississippi State, Mississippi 

Mark R. Vinson, Ph.D.

Utah State University

Logan, Utah 

Joe Watkins, Ph.D. (Science Advisor)

University of New Mexico

Albuquerque, New Mexico

Michael Welsh, Ph.D.

Monona, Wisconsin 

Ellen Wohl, Ph.D.

Colorado State University

Fort Collins, Colorado
viii

Contents

Overview......................................................................................................................................................... 1

Lara M. Schmit, Steven P. Gloss, and Christopher N. Updike

Chapter 1. Influence of Glen Canyon Dam Operations on

Downstream Sand Resources of the Colorado River in Grand Canyon ........................................... 17

Scott A. Wright, Theodore S. Melis, David J. Topping,

and David M. Rubin 

Chapter 2. Fishes of Grand Canyon ...................................................................................................... 33

Steven P. Gloss and Lewis G. Coggins

With text box by Jeffrey E. Lovich ....................................................................................................50

Chapter 3. Climatic Fluctuations, Drought, and Flow

in the Colorado River .................................................................................................................................. 57

Robert H. Webb, Richard Hereford, and Gregory J. McCabe

Chapter 4. Water Quality in Lake Powell and the Colorado River ................................................... 69

William S. Vernieu, Susan J. Hueftle, and Steven P. Gloss

Chapter 5. Aquatic Ecology: the Role of Organic Matter

and Invertebrates........................................................................................................................................ 87

Theodore A. Kennedy and Steven P. Gloss

Chapter 6. Riparian Vegetation and Associated Wildlife ................................................................ 103

Barbara E. Ralston

With text box by Charles Drost .......................................................................................................116

Chapter 7. Birds of the Colorado River in Grand Canyon:

a Synthesis of Status, Trends, and Dam Operation Effects ............................................................... 123

Jennifer A. Holmes, John R. Spence, and Mark K. Sogge 

Chapter 8. Debris Flows in Grand Canyon and the Rapids

of the Colorado River ................................................................................................................................ 139

Robert H. Webb, Peter G. Griffiths, Christopher S. Magirl,

and Thomas C. Hanks

Chapter 9. Recreation Use Values and Nonuse Values

of Glen and Grand Canyons..................................................................................................................... 153

John Loomis, Aaron J. Douglas, and David A. Harpman 
ix

Chapter 10. Status and Trends of Hydropower Production

at Glen Canyon Dam ................................................................................................................................. 165

David A. Harpman and Aaron J. Douglas

Chapter 11. Cultural Resources in the Colorado River Corridor .................................................... 177

Helen C. Fairley 

Chapter 12. Recreational Values and Campsites

in the Colorado River Ecosystem ........................................................................................................... 193

Matt Kaplinski, Jeff Behan, Joseph E. Hazel, Jr.,

Roderic A. Parnell, and Helen C. Fairley

Chapter 13. Lessons from 10 Years of

Adaptive Management in Grand Canyon .............................................................................................. 207

Jeffrey E. Lovich and Theodore S. Melis

Jeff Sorensen, Arizona Game and Fish Department

Overview

Lara M. Schmit

Steven P. Gloss

Christopher N. Updike

Introduction

This report is an important milestone in the effort 

by the Secretary of the Interior to implement the Grand 

Canyon Protection Act of 1992 (GCPA; title XVIII, secs. 

1801–1809, of Public Law 102-575), the most recent 

authorizing legislation for Federal efforts to protect 

resources downstream from Glen Canyon Dam. The 

chapters that follow are intended to provide decision 

makers and the American public with relevant scientific 

information about the status and recent trends of the 

natural, cultural, and recreational resources of those 

portions of Grand Canyon National Park and Glen 

Canyon National Recreation Area affected by Glen 

Canyon Dam operations. Glen Canyon Dam is one of 

the last major dams that was built on the Colorado River 

and is located just south of the Arizona-Utah border 

in the lower reaches of Glen Canyon National Recreation Area, approximately 15 mi (24 km) upriver from 

Grand Canyon National Park (fig. 1). The information 

presented here is a product of the Glen Canyon Dam 

Adaptive Management Program (GCDAMP), a federally 

authorized initiative to ensure that the primary mandate 

of the GCPA is met through advances in information 

and resource management. The U.S. Geological Survey’s 

(USGS) Grand Canyon Monitoring and Research 

Center (GCMRC) has responsibility for the scientific 

monitoring and research efforts for the program, including the preparation of reports such as this one. 

The Study Area

Carved from the Earth by the Colorado River, 

Grand Canyon is a natural wonder that is “absolutely 

unparalleled throughout the rest of the world,” as 

President Theodore Roosevelt said upon seeing it for the 

first time in 1903 (Roosevelt, ca. 1905, p. 369). Considered one of the world’s most spectacular gorges, Grand 

Canyon exhibits a depth of more than 6,720 ft (2,048 

m) at its most extreme in Granite Gorge (Annerino, 

2000). The colorful strata of the canyon’s walls also 

reveal an invaluable record of the Earth’s geologic history dating back to the 1.84-billion-yr-old rock formations found at Elves Chasm, which are the oldest rocks 

known in the Southwestern United States (Beus and 

Morales, 2003). President Woodrow Wilson signed the 
2 The State of the Colorado River Ecosystem in Grand Canyon

bill that established Grand Canyon as a national park 

on February 26, 1919, in recognition of its exceptional 

natural beauty and geologic wonders. Grand Canyon 

National Park is also of cultural and spiritual significance 

to many of the region’s Native Americans and contains 

more than 2,600 documented prehistoric ruins, which 

span thousands of years and provide an important record 

of human adaptation to an arid environment. In addition to its geologic and cultural significance, the Grand 

Canyon ecosystem is home to a diverse array of plants 

and animals such as the humpback chub (Gila cypha) and 

the southwestern willow flycatcher (Empidonax traillii extimus), both of which are species that are federally listed as 

endangered. Because of its global significance as a natural 

and cultural treasure, Grand Canyon National Park was 

inscribed by the United Nations Educational, Scientific and Cultural Organization (UNESCO) as a World 

Heritage Site in 1979.

The GCPA (see timeline) directs the Secretary of the 

Interior to operate Glen Canyon Dam and exercise other 

authorities “in such a manner as to protect, mitigate 

adverse impacts to, and improve the values for which 

Grand Canyon National Park and Glen Canyon National 

Recreation Area were established, including, but not 

limited to natural and cultural resources and visitor use” 

(GCPA, sec. 1802(a)). As a result, the Glen Canyon Dam 

Adaptive Management Program, created by the 1996 

Record of Decision (ROD) for the operation of Glen 

Canyon Dam, focuses on a study area that encompasses 

the Colorado River corridor from Glen Canyon Dam to 

the western boundary of Grand Canyon National Park. 

The study area includes the approximately 15 river miles 

(RM) of river from the dam to Lees Ferry within Glen 

Canyon National Recreation Area and the entire 277-

RM river corridor below Lees Ferry and within Grand 

Canyon National Park. In total, the study area includes 

some 293 RM of the Colorado River (fig. 1). 

Administrative History

The Colorado River is the most important water 

resource in the American West, serving as the main 

source of drinking water for more than 25 million people 

(Water Education Foundation, 2001). The Colorado 

River has been extensively engineered to meet the 

demands placed upon it (see timeline). There are 22 

major storage reservoirs in the Colorado River Basin 

and 8 major out-of-basin diversions (Pontius, 1997). 

The two largest storage projects—Hoover and Glen 

Canyon Dams—are located on either end of Grand 

Canyon National Park. Glen Canyon Dam is located 

just north of the Grand Canyon National Park boundary, where it creates Lake Powell. At full capacity, Lake 

Powell was designed to hold 27 million acre-feet (maf) 

(>33,000 million m3

) of water and is the key storage unit 

within the Colorado River Storage Project (CRSP) (U.S. 

Department of the Interior, 1970). 

Signed into law by President Dwight D. Eisenhower 

in 1956, the Colorado River Storage Project Act 

authorized four mainstem water-storage units, including Glen Canyon Dam. Construction of Glen Canyon 

Dam began on September 29, 1956, and the last bucket 

of concrete was poured on September 13, 1963 (U.S. 

Department of the Interior, 1970). The regulation 

of the Colorado River by Glen Canyon Dam began 

with the closure of the dam in 1963 and when Lake 

Powell began filling. The CRSP reservoirs allow the 

upper basin States—Utah, Colorado, Wyoming, and 

New Mexico—to store water in wet years and release 

water in times of shortages, thereby enabling the upper 

basin to meet its obligations under the 1922 Colorado 

River Compact while also maximizing future water uses 

(Ingram and others, 1991). To repay Federal expenditures for the water-storage units and supplement the costs 

of related irrigation units, CRSP dams were equipped 

with hydroelectric generators to produce salable power. 

Glen Canyon Dam operates eight electric generators, 

which produce 78% of the total power generated by the 

CRSP (Hughes, 1991). In 2004, Glen Canyon Dam generated approximately 3.3 million megawatthours (MWh). 

The power is sold to approximately 200 wholesale 

customers—municipal and county utilities, rural electric 

cooperatives, U.S. Government installations, and other 

nonprofit organizations—located primarily in six States: 

Arizona, Colorado, Utah, Wyoming, New Mexico, and 

Nevada (National Research Council, 1996).

Natural History

Before the dam, the Colorado River was a sedimentrich river that when swelled with snowmelt from the 

Rocky Mountains transported large quantities of sediment during spring and early summer and commonly 

produced flood events. Peak discharge typically reached 

85,000 cubic feet per second (cfs) at 2-yr intervals and 

120,000 cfs at 6-yr intervals during these seasonal flood 

events (Topping and others, 2003). By contrast, flows of 

less than 3,000 cfs were typical during late summer, fall, 

and winter. Prior to the dam, water temperature also 
Overview 3Figure 1. Study area.
4 The State of the Colorado River Ecosystem in Grand Canyon

fluctuated seasonally from 32°F to 80°F (0–29°C) (U.S. 

Department of the Interior, 1995).

Glen Canyon Dam has changed the seasonal flow, 

sediment-carrying capacity, and temperature of the 

Colorado River. Operation of the dam has altered the 

frequency of floods on the Colorado River and increased 

median discharge rates at Lees Ferry, whereas managing 

for hydroelectric power generation has introduced wideranging daily fluctuations (Topping and others, 2003). 

For example, from 1963 to 1991 (the no action period 

or historical operations), when the dam was managed 

primarily to maximize hydroelectric power revenue, it 

was not uncommon for daily flows to vary from 5,000 

to 30,000 cfs (U.S. Department of the Interior, 1988). 

Release patterns of this type caused the river level below 

the dam to change 7–13 ft (2–4 m) per day, creating public concerns about the quality and safety of fishing and 

boating and about adverse impacts to natural resources 

(U.S. Department of the Interior, 1988). Because the 

sediment load of the Colorado River is deposited in 

Lake Powell, water released from Glen Canyon Dam is 

essentially clear. Furthermore, because the penstocks of 

the dam are well below the surface of Lake Powell, the 

water released from the dam is cold, with an average 

temperature of 46°F (8°C) (Webb and others, 1999).

The construction of Glen Canyon Dam also 

affected a number of aquatic and terrestrial resources 

downstream in lower Glen and Grand Canyons. Daminduced changes in the Colorado River’s flow, temperature, and sediment-carrying capacity are blamed for 

narrowing rapids, beach erosion, invasion of nonnative 

riparian vegetation, and losses of native fishes (Webb and 

others, 1999). These same changes are also associated 

with an increase in total species richness within Grand 

Canyon National Park; however, the increases are primarily for species not originally found in Grand Canyon. 

Some changes to the ecosystem of the Colorado River, 

such as the introduction of nonnative fish, were already 

taking place before the construction of Glen Canyon 

Dam (Wieringa and Morton, 1996). 

It is important to note that Glen Canyon Dam 

was completed before the enactment of the National 

Environmental Policy Act of 1969 and the Endangered 

Species Act of 1973 (see timeline). At the time of Glen 

Canyon Dam’s construction (1956–63), little consideration was given to how dam operations might affect the 

downstream environment in Grand Canyon National 

Park (Babbitt, 1990). Nevertheless, public values were 

undergoing a shift: at the same time that Congress authorized Glen Canyon Dam in 1956, authorization of Echo 

Park Dam on the Green River was defeated because of 

environmental reasons (Ingram and others, 1991).

Federal Efforts to Protect 

Grand Canyon

The international prominence of Grand Canyon 

National Park and public concern about the impacts of 

Glen Canyon Dam caused the Bureau of Reclamation 

in 1982 to undertake a science program, Glen Canyon 

11,000 BP Paleo-Indian peoples 

occupy Grand Canyon region

1869 Major John Wesley 

Powell leads first recorded 

expedition to traverse 

Grand Canyon 

1893 President Benjamin 

Harrison creates Grand 

Canyon Forest Reserve

1908 President

Theodore Roosevelt 

creates Grand Canyon 

National Monument

1916 National Park 

Service Organic Act passed

1902 Reclamation Act creates 

the Bureau of Reclamation
Overview 5

Environmental Studies, to examine the effects of dam 

operations on downstream resources. Glen Canyon 

Environmental Studies, the USGS Grand Canyon 

Monitoring and Research Center’s predecessor, issued 

a final report in 1988 concluding that changes in dam 

operations “could reduce the resource losses occurring under current operations and, in some cases, even 

improve the status of the resources” (U.S. Department 

of the Interior, 1988, p. xvi). In 1989, in response to 

these findings, Secretary of the Interior Manuel Lujan, 

Jr., ordered the Bureau of Reclamation to complete an 

environmental impact statement on the operation of 

Glen Canyon Dam. To further ensure the protection of 

downstream resources, Secretary Lujan adopted interim 

operating criteria for the dam in 1991, which restricted 

dam operations and remained in effect until the end of 

the environmental impact statement process.

Congress passed the Grand Canyon Protection Act 

of 1992 to provide guidance and legal support to the 

Secretary of the Interior in his efforts to protect Grand 

Canyon. In addition to directing the Secretary to operate 

Glen Canyon Dam to protect and improve downstream 

resources, the act also validated the interim operating 

criteria, provided a deadline for the completion of the 

environmental impact statement, required the creation 

of a long-term monitoring and research program, and 

allocated program costs. The act clearly stated that it 

was to be implemented in accordance with existing laws, 

treaties, and institutional agreements that govern allocation, appropriation, development, and exportation of the 

waters of the Colorado River Basin (GCPA, sec. 1802(b)).

The Operation of Glen Canyon Dam Final Environmental Impact Statement (hereafter EIS) was filed in 

March 1995, and the Record of Decision was signed by 

Bruce Babbitt, Secretary of the Interior, in October 1996. 

The Record of Decision noted that the goal “was not to 

maximize benefits for the most resources, but rather to 

find an alternative dam operating plan that would permit 

recovery and long-term sustainability of downstream 

resources while limiting hydropower capacity and flexibility only to the extent necessary to achieve recovery 

and long-term sustainability” (U.S. Department of the 

Interior, 1996, p. G-11). Having established this goal, the 

Secretary’s decision was to implement the modified low 

fluctuating flow (MLFF) alternative (the preferred alternative in the EIS) as described in the EIS but with minor 

changes in the upramp rate, maximum release rate, and 

the timing of beach/habitat-building flows (BHBF; see 

below). The document also formally established the Glen 

Canyon Dam Adaptive Management Program.

Glen Canyon Dam Adaptive 

Management Program

The creation of an adaptive management program 

was a common element for all alternatives considered 

in the EIS, and its implementation was subsequently 

mandated by the Record of Decision. Adaptive management was selected to create a process whereby “the 

effects of dam operations on downstream resources 

1919 Grand Canyon 

National Park created

1921-23 U.S. Geological Survey’s 

Birdseye Expedition surveys possible 

dam sites along the Colorado River 

1922 Colorado River Compact signed allocating the water of the 

Colorado River between the upper and lower basins. Upper basin 

States have the right to use 7.5 maf/yr only if that quantity is available 

after meeting delivery requirements of 7.5 maf/yr to the lower basin 

plus the amount required to satisfy anticipated claims by Mexico

1928 Boulder Canyon Project Act 

passed authorizing Hoover Dam
6 The State of the Colorado River Ecosystem in Grand Canyon

would be assessed and the results of those assessments 

would form the basis of future modifications of dam 

operations” (U.S. Department of the Interior, 1995, p. 

34). The selection of adaptive management and the 

focus on the effects of dam operations on downstream 

resources have significant implications. First, the prominence of Grand Canyon National Park elevates adaptive 

management and the GCDAMP to national significance. 

Second, the program’s focus on the effects of dam operations on downstream resources constrains the range of 

management options and creates a relatively well-defined 

geographic area within which to operate.

Envisioned as a new paradigm for addressing complex environmental management problems through a 

dynamic interplay of ecosystem science, management, 

and policy, adaptive management has gained attention 

and has been tested in various contexts in the last several 

decades (National Research Council, 1999). Although 

concepts and methods continue to evolve, adaptive 

management is generally understood to be a systematic 

process for continually improving management practices 

by emphasizing learning through experimentation. Also, 

adaptive management incorporates collaboration among 

stakeholders, managers, and scientists as a means of social 

learning that can prevent policy gridlock. In Downstream,

the National Research Council (1999, p. 53) noted that 

the key components of adaptive management include

(1) commitment to ongoing management adjustments based, in part, upon scientific experimentation, (2) shift from “trial and error” to formal 

experimentation with management actions and 

alternatives, (3) shift from fragmented scientific 

investigations to integrated ecosystem science, 

(4) explicit attention to scientific uncertainties in 

ecosystem processes and effects of management 

alternatives, (5) formal experimental design and 

hypothesis testing to reduce those uncertainties 

and help guide management adjustments, (6) 

careful monitoring of ecological and social effects 

and of responses to management operations, (7) 

analysis of experimental outcomes in ways that 

guide future management decisions, and (8) close 

collaboration among stakeholders, managers, 

and scientists in all phases of these processes.

The Role of Science 

The Colorado River provides many benefits to 

society including numerous natural processes; habitat 

for unique organisms such as native fishes; water for 

humans, agriculture, and recreational purposes; and 

hydroelectric power generation. Science-based status 

and trends information is increasingly valuable as society attempts to balance the competing uses of natural 

resources. The need for credible scientific information 

that can serve as a feedback loop between management 

actions and the effects of those actions is of critical 

importance in adaptive management.

The role of science in the GCDAMP is fourfold: 

(1) to provide the aforementioned credible scientific 

information about management actions deemed appro1935 Hoover

Dam completed

1944 Treaty with Mexico 

obligating the United States 

to provide 1.5 maf of Colorado 

River water to Mexico annually

1946 Robert R. Miller 

describes humpback chub 

(Gila cypha) from specimens 

taken in Grand Canyon

1948 Upper Colorado River 

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8 The State of the Colorado River Ecosystem in Grand Canyon

was concern for the effects on sensitive resources such as 

sediment or endangered species.

On the basis of significant scientific research since 

1995, some of the assumptions about how Colorado 

River resources would respond to ROD operations have 

been modified or rejected. As a result, several additional 

experimental flows that temporarily modified Glen 

Canyon Dam ROD operations have been implemented 

since 2000. Additional experimental flows discussed elsewhere in this report include the 2000 low summer steady 

flow (LSSF) test, the 2003–05 experimental fluctuating 

nonnative fish suppression flows, and the November 

2004 experimental high flow. The LSSF test included 

1966 National Historic 

Preservation Act passed

1967 Humpback chub and Colorado 

pikeminnow (Ptychocheilus lucius)

federally listed as endangered 

1968 Colorado River 

Basin Project Act passed

1969 National Environmental Policy Act of 1969 

passed requiring Federal agencies to consider the 

environmental impacts of their proposed actions 

and reasonable alternatives to those actions

1970 Long-range Operating 

Criteria developed for Glen 

Canyon Dam operations

Table 1. Glen Canyon Dam release prescriptions under the modified low fluctuating flow alternative (cfs = cubic feet 

per second).

Monthly release 

volume

(acre-feet)

Minimum

release (cfs)1

Maximum

release (cfs)

Allowable daily 

fluctuation (cfs)

Upramp/

downramp (cfs/hr)

<600,000 8,000/5,000 25,000 5,000 4,000/1,500

600,000–800,000 8,000/5,000 25,000 6,000 4,000/1,500

>800,000 8,000/5,000 25,000 8,000 4,000/1,500

1

 8,000 cfs between 7 a.m. and 7 p.m. and 5,000 cfs at night; releases each weekday during the recreation season (Easter to Labor Day) would 

average not less than 8,000 cfs for the period from 8 a.m. to midnight.

two habitat maintenance flows (31,000 cfs for 4 d) in 

spring and late summer, with June through August flows 

held constant at 8,000 cfs. Fluctuating nonnative fish 

suppression releases allowed the flow of the river to fluctuate daily between 5,000 cfs and 20,000 cfs with relaxed 

hourly upramp and downramp rates of 5,000 and 2,500 

cfs/h, respectively, from January to March. In summer 

and fall 2004, fine-sediment inputs from the Paria River 

(15 mi below the dam) reached the agreed-upon levels for 

triggering an experimental high flow of 41,000 cfs for 2.5 

d (see chapter 1, this report).

Experimentation has largely focused on experimental 

flows of the type described above to achieve downstream 
Overview 9

benefits, with a particular focus on improving finesediment resources and conditions for endangered native 

fish. Another experimental effort underway is the manual 

removal of nonnative fishes in order to protect native fish, 

particularly humpback chub (see chapter 2, this report).

Collaboration

As for collaboration, the EIS outlined an innovative 

organizational structure for pursuing the GCDAMP. The 

program is administered by a senior Department of the 

Interior official (designee) and facilitated by the Adaptive 

Management Work Group (AMWG), which is organized 

as a Federal Advisory Committee. The AMWG makes 

recommendations to the Secretary of the Interior on 

how to best alter the operating criteria at Glen Canyon 

Dam or other management actions to protect downstream resources in order to fulfill the Department of the 

Interior’s obligations under the GCPA (U.S. Department 

of the Interior, 1995). The Secretary of the Interior 

appoints the group’s 25 members, who include representatives from Federal and State resource management 

agencies, the seven Colorado River Basin States, Native 

American tribes, environmental groups, recreation 

interests, and contractors of Federal power from Glen 

Canyon Dam (fig. 2). The GCDAMP also includes a 

monitoring and research center (USGS Grand Canyon 

Monitoring and Research Center), the Technical Work 

Group, and independent scientific review panels. 

As directed thus far by the AMWG, monitoring and 

research on sediment dynamics, cultural resources, native 

1972 Last verified record of 

Colorado pikeminnow caught in 

Grand Canyon at Havasu Creek

1973 Endangered Species Act of 1973 passed to protect and promote 

the recovery of animals and plants that are in danger of becoming extinct 

because of the activities of people. The act is administered by the U.S. Fish 

and Wildlife Service (terrestrial and freshwater species) and the National 

Oceanic and Atmospheric Administration–Fisheries (marine species)

1974 First lawsuit filed over Glen Canyon Dam 

operations by commercial raft operators contending 

that the disruption of normal flows was interfering 

with their ability to conduct river trips

and nonnative fish, and endangered species have been 

emphasized. Monitoring and research of these resources 

have resulted in better understanding of their condition 

and behavior. 

For example, recent studies suggest that, contrary 

to expectations under current dam operations, sand 

contributed from Colorado River tributaries is rapidly 

exported downstream and does not remain available 

over multiyear timescales for restoration floods implemented between January and July, which is the current 

implementation schedule. Restoration floods are likely 

to be more effective if they are carried out in the same 

year that sand deliveries occur, before the new sand is 

lost downstream. Progress has also been made in understanding the dynamics of fish populations and the value 

of mechanical removal of nonnative fish for enhancing 

native fish populations.

Report Organization

The chapters that follow provide status and trend 

data for the natural, cultural, and recreational resources 

of the Colorado River ecosystem in Grand Canyon. The 

report deals first with the aspects of the natural environment that have been most emphasized in monitoring 

and research—sediment and native fishes—followed by 

other important environmental factors including climate 

and drought, water quality, aquatic ecology, debris flows, 

birds, and shoreline ecology and its associated wildlife. 

The report then shifts emphasis to various human uses 
10 The State of the Colorado River Ecosystem in Grand Canyon

1975 Grand Canyon National 

Park Enlargement Act passed

1978 U.S. Fish and Wildlife 

Service files jeopardy opinion 

on the effects of Glen Canyon 

Dam on endangered fishes

1979 Grand Canyon National Park 

designated a UNESCO World Heritage 

Site; Bureau of Reclamation proposes an 

upgrade of Glen Canyon Dam’s generators 

1980 Lake Powell reaches full pool 

(3,700 ft); bonytail chub (Gila elegans)

federally listed as endangered

Figure 2. Adaptive Management Work Group committee members.

Interior Secretary’s Designee

Tribes

Hopi Tribe 

Hualapai Tribe

Navajo Nation

Pueblo of Zuni

San Juan Southern Paiute Tribe

Southern Paiute Consortium

State and Federal Cooperating Agencies 

Arizona Game and Fish Department

Bureau of Indian Affairs

Bureau of Reclamation

National Park Service 

U.S. Department of Energy, Western Area Power 

Administration

U.S. Fish and Wildlife Service

Colorado River Basin States 

Arizona: Arizona Department of Water Resources

California: Colorado River Board of California

Colorado: Colorado Water Conservation Board

Nevada: Colorado River Commission of Nevada

New Mexico: New Mexico Office of the State Engineer

Utah: Water Resources Agency 

Wyoming: State Engineer’s Office

Nongovernmental Groups

Environmental:

Grand Canyon Trust

Grand Canyon Wildlands Council 

Recreation:

Federation of Fly Fishers/Northern Arizona Flycasters 

Grand Canyon River Guides

Contractors for Federal Power from Glen Canyon Dam:

Colorado River Energy Distributors Association

Utah Associated Municipal Power Systems
Overview 11

of the ecosystem, including the economic importance of 

the ecosystem, hydroelectric power generation, cultural 

resources, and camping beaches. In each case, the information is then used to discuss the management options 

available to decision makers and the public based on the 

best scientific information available. In large measure, 

this report represents the first comprehensive assessment 

of how effectively the MLFF alternative is allowing the 

Secretary of the Interior to meet the resource management goals of the Grand Canyon Protection Act of 1992.

Place Names and Units

Throughout the report, “Grand Canyon” is used 

broadly to refer to the Colorado River corridor between 

Glen Canyon Dam and the western boundary of Grand 

Canyon National Park, including Glen, Marble, and 

Grand Canyons. The study area is referred to as the 

“Grand Canyon ecosystem.” The Colorado River is 

discussed in terms of four distinct sections: Lees Ferry 

1982 Glen Canyon Environmental 

Studies created to study effects 

of Glen Canyon Dam operations

1983 Glen Canyon Dam releases more 

than 92,000 cfs to stop Lake Powell 

from overtopping Glen Canyon Dam

1984 One of the last razorback 

suckers (Xyrauchen texanus)

seen in Grand Canyon is caught 

and released at Bass Rapids

1987 National Research Council completes 

review of Glen Canyon Environmental Studies, 

publishing River and Dam Management: a 

Review of the Bureau of Reclamation’s Glen 

Canyon Environmental Studies

reach, Marble Canyon, upper Grand Canyon, and lower 

Grand Canyon. The “Lees Ferry reach” extends from the 

downstream end of Glen Canyon Dam to Lees Ferry, and 

“Marble Canyon” extends from Lees Ferry to the mouth 

of the Little Colorado River. For this report, “upper 

Grand Canyon” refers to the river corridor that extends 

from the mouth of the Little Colorado River to the Grand 

Canyon gaging station (Topping and others, 2003), while 

“lower Grand Canyon” extends from the Grand Canyon 

gaging station to the western boundary of the park.

In this report, U.S. customary units are used for all 

measurements to facilitate understanding by the general 

reader. Metric equivalents are provided in parentheses 

after the U.S. customary units for all measurements except 

for river flow, the standard measure of which is cubic 

feet per second, and river mile, which is used to describe 

distances along the Colorado River in Grand Canyon 

(Stevens, 1990). The use of the river mile has a historical precedent and provides a reproducible method for 

describing location: Lees Ferry is the starting point, as 
12 The State of the Colorado River Ecosystem in Grand Canyon

1988 Glen Canyon Environmental Studies 

issues Glen Canyon Environmental Studies 

Final Report, completing Phase I and starting 

Phase II, which would be accelerated to support 

environmental impact statement process

1989 Secretary of the Interior Lujan orders 

an environmental impact statement on dam 

operations, and National Research Council 

sponsors symposium that reviews existing 

knowledge on Colorado River ecosystem

1990-91 Research flows 

used to evaluate a variety 

of discharge patterns 

1991 Interim operating criteria for 

Glen Canyon Dam implemented; 

razorback sucker and Kanab ambersnail 

(Oxyloma haydeni ssp. kanabensis)

federally listed as endangered

One challenge following completion of the 1995 Operation of Glen Canyon Dam Final Environmental Impact 

Statement (EIS) was to identify and implement monitoring efforts that would produce scientific data suitable for 

evaluating the new operating policy at Glen Canyon Dam. At that time, there was also a sense among managers and 

scientists that additional, comprehensive syntheses of available data needed to be undertaken with respect to major 

resource categories, such as sediment and fisheries. In addition, the need for development of a conceptual model 

for the Colorado River ecosystem, consistent with the adaptive environmental assessment and management process 

(now popularly called “adaptive management”), was also identified by the USGS Grand Canyon Monitoring and 

Research Center (GCMRC) and its cooperators. This modeling effort began in 1998 and was continued concurrently with the establishment of the stakeholder-based, Federal Advisory Committee—the Adaptive Management 

Work Group—and the development of the group’s strategic goals for the Colorado River ecosystem (1998–2002). 

Key objectives for the conceptual modeling exercise were to (1) conduct an exhaustive knowledge assessment of 

the various elements of the ecosystem on the basis of existing data and hypotheses posed in the EIS and within 

the context of workshops that supported stakeholder and scientist interactions; (2) identify, through this process 

of modeling and simulation, key areas where data or knowledge did not exist and therefore were impediments to 

developing realistic simulations (by using historical data as a means of verification); and (3) identify future research 

directives (both experimental or otherwise) that would effectively fill knowledge gaps in the program related to 

management needs.

Development of the physical elements of the conceptual model (the Grand Canyon Model or GCM) proceeded 

relatively quickly, mostly because there were abundant data in some key areas (hydrology, sediment, and river flow) 

and an operational model for the Colorado River Basin (RiverWareTM) had already been developed by the Bureau 

of Reclamation. Other critical areas of the model development, however, were limited by the paucity of available 

data related to biology and sociocultural resource areas (Walters and others, 2000). By 2000, it became clearer that 

The Role of Conceptual Modeling in Support of Adaptive 

Management in Grand Canyon
Overview 13

1992 Grand Canyon 

Protection Act of 

1992 passed

1994 Programmatic Agreement on Cultural Resources signed between the State of Arizona, 

Department of the Interior agencies, and six tribes over protection of cultural resources in the river 

corridor below Glen Canyon Dam; U.S. Fish and Wildlife Service designates critical habitat for four 

species of endangered Colorado River fish and completes Biological Opinion outlining reasonable 

and prudent alternatives that must be evaluated for dam operation

1995 Operation of Glen Canyon Dam Final Environmental Impact Statement 

completed; Transition Work Group and Grand Canyon Monitoring and 

Research Center begin formulating strategic plan; southwestern willow 

flycatcher (Empidonax traillii extimus) federally listed as endangered; 

Department of the Interior constitutes the Grand Canyon Monitoring and 

Research Center and locates it in Flagstaff, Arizona

certain critical modules of the model could not even reliably predict the general direction of ecosystem response, 

such as response of native fishes to warmer water conditions through implementation of a proposed temperature 

control device. While water could be routed through the ecosystem with confidence, there was considerably less 

confidence about the longer term relationship of flows to fine-sediment flux and beaches on the basis of remaining downstream sand supplies alone. Although the inability of the GCM to accurately simulate higher level trophic 

(e.g., fishes) responses in critical areas was cause for concern among managers, the goal of systematically identifying 

gaps in data and knowledge so that future research (including experimentation) and monitoring could be designed 

and implemented to fill the gaps was an acknowledged objective of the modeling effort.

In a sense, the largest contribution made by the conceptual modeling project was the identification of various experimental flow and nonflow treatments that would need to be tested (presumably, within some longer term 

design) to provide managers with scientifically based options for most effectively meeting the proposed management 

goals. Experimentation has long been identified as a sign of “active” adaptive management and has been shown to 

be an efficient means of resolving the uncertainty associated with various alternative management policies (Walters 

and Holling, 1990). Simultaneously, the modeling project helped identify additional monitoring data that would be 

required to more fully evaluate the influence of the modified low fluctuating flow policy on downstream resources 

of concern. Although evaluation of all the resources outlined in the EIS has not been possible because of program funding limitations, the GCM identified the general linkages between the varied resources as related to dam 

operation. The experimental designs proposed and implemented in the Glen Canyon Dam Adaptive Management 

Program have been a direct and logical outcome of conceptual modeling activities. Though still not complete, to 

date, the experimental results have greatly advanced ecosystem understanding. Ultimately, the knowledge gained 

through these scientific activities in the Colorado River ecosystem should lead to improved management options for 

Glen Canyon Dam that will benefit society.
14 The State of the Colorado River Ecosystem in Grand Canyon

1996 Experimental controlled flood of 45,000 

cfs conducted at Glen Canyon Dam; Record of 

Decision for the operation of Glen Canyon Dam 

signed by Secretary of the Interior Bruce Babbitt

1997 Interior Secretary Bruce Babbitt signed a Notice 

of Establishment of the Adaptive Management Work 

Group, a Federal Advisory Committee with first meeting 

of the group in September; first test of the concept of 

the habitat maintenance flows conducted in November

2000 Test of low summer steady flows for the 

possible benefit of endangered species of fish, 

second and third tests of the habitat maintenance 

flows concept conducted in spring and summer

2001 Draft strategic plan for the Glen 

Canyon Dam Adaptive Management Program 

developed by program members

RM 0, with mileage measured for both upstream and 

downstream directions.

References

Annerino, J., 2000, Canyons of the Southwest: a tour of 

the great canyon country from Colorado to northern 

Mexico: Tucson, University of Arizona Press, 144 p.

Babbitt, B., 1990, Introduction: down the imperiled 

Colorado: Land and Water Law Review, v. 25, no. 1, 

p. 1–9.

Beus, S.S., and Morales, M., eds., 2003, Grand 

Canyon geology (2d ed.): New York, Oxford 

University Press, 432 p.

Hughes, T.C., 1991, Reservoir operations, in Committee to Review the Glen Canyon Environmental Studies, Commission on Geosciences, Environment, and 

Resources, eds., Colorado River ecology and dam management: proceedings of a symposium: Washington, 

D.C., National Academy Press, p. 207–225.

Ingram, H., Tarlock, D.A., and Oggins, C.R., 1991, 

The law and politics of the operation of Glen Canyon 

Dam, in Committee to Review the Glen Canyon Environmental Studies, Commission on Geosciences, Environment, and Resources, eds., Colorado River ecology 

and dam management: proceedings of a symposium: 

Washington, D.C., National Academy Press, p. 10–27.

National Research Council, 1996, River resource management in Grand Canyon: Committee to Review 

the Glen Canyon Environmental Studies, Commission on Geosciences, Environment, and Resources: 

Washington, D.C., National Academy Press, 226 p.

National Research Council, 1999, Downstream: adaptive 

management of Glen Canyon Dam and the Colorado 

River ecosystem: Committee on Grand Canyon 

Monitoring and Research, Water Science and Technology Board, Commission on Geosciences, Environment, and Resources: Washington, D.C., National 

Academy Press, 230 p.

Pontius, D., 1997, Colorado River Basin study: Report 

to the Western Water Policy Review Advisory 

Commission, 126 p.

Roosevelt, T., ca. 1905, The works of Theodore 

Roosevelt, presidential addresses and state papers, 

pt. 1: New York, P.F. Collier and Sons, Publishers, p. 

369–370.

Stevens, L., 1990, The Colorado River in Grand Canyon: 

a guide: Flagstaff, Ariz., Red Lake Books, 115 p.

Topping, D.J., Schmidt, J.C., and Vierra, L.E., Jr., 2003, 

Computation and analysis of the instantaneous-discharge record for the Colorado River at Lees Ferry, 

Arizona—May 8, 1921, through September 30, 2000: 

Reston, Va., U.S. Geological Survey Professional Paper 

1677, 118 p.
Overview 15

2004 Drought conditions cause water level at Lake 

Powell to drop to lowest level since the dam began 

filling; triggering thresholds based on sand inputs from 

the Paria River and lesser Marble Canyon tributaries met; 

and high flow experiment initiated on Sunday, Nov. 21

2002 U.S. Fish and Wildlife Service announces recovery goals 

for endangered fishes of the Colorado River Basin; Adaptive 

Management Work Group recommends implementation of 

the first 2 yr of an experimental design proposed by the Grand 

Canyon Monitoring and Research Center

2003 Experiment begun 

to remove nonnative fish 

from the Colorado River 

in Grand Canyon

2003-05 Fluctuating nonnative fish 

suppression releases from January 

through March implemented and 

continued through 2005

 U.S. Department of the Interior, 1970, Glen 

Canyon Dam and powerplant: technical record of the 

design and construction: Denver, Colo., Bureau of 

Reclamation, Technical Center, 658 p.

U.S. Department of the Interior, 1988, Glen Canyon 

Environmental Studies final report: Salt Lake City, 

Utah, Bureau of Reclamation, 84 p.

U.S. Department of the Interior, 1995, Operation of 

Glen Canyon Dam Final Environmental Impact 

Statement, Colorado River Storage Project, Coconino 

County, Arizona: Salt Lake City, Utah, Bureau of 

Reclamation, Upper Colorado Regional Office, 337 p.

U.S. Department of the Interior, 1996, Record of Decision, Operation of Glen Canyon Dam Final Environmental Impact Statement: Washington, D.C., Office 

of the Secretary of the Interior, 15 p. 

Walters, C., Korman, J., Stevens, L.E., and Gold, 

B., 2000, Ecosystem modeling for evaluation 

of adaptive management policies in the Grand 

Canyon: Journal of Conservation Ecology, v. 4, no. 

2, http://www.consecol.org/vol4/iss2/art1, accessed 

July 14, 2005.

Walters, C.J., and Holling, C.S., 1990, Large-scale management experiments and learning by doing: Ecology, 

v. 71, no. 6, p. 2060–2068.

Water Education Foundation, 2001, Layperson’s guide to 

the Colorado River: Sacramento, Calif., 28 p.

Webb, R.H., Wegner, D.L., Andrews, E.D., Valdez, R.A., 

and Patten, D.T., 1999, Downstream effects of Glen 

Canyon Dam in Grand Canyon: a review, in Webb, 

R.H., Schmidt, J.C., Marzolf, G.R., and Valdez, R.A., 

eds., The controlled flood in Grand Canyon: scientific 

experiment and management demonstration: Washington, D.C., American Geophysical Union, Geophysical Monograph Series, v. 110, p. 1–21.

Wieringa, M.J., and Morton, A.G., 1996, Hydropower, 

adaptive management, and biodiversity: Environmental Management, v. 20, no. 6, p. 831–840. 
16 The State of the Colorado River Ecosystem in Grand Canyon

Timeline photograph credits:

Page 4, Major John Wesley Powell: U.S. Geological Survey Photo Library Archive

Page 5, Colorado River in Grand Canyon: U.S. Geological Survey Photo Library Archive

Page 6, Hoover Dam: Bureau of Reclamation

Page 7, Glen Canyon Dam: Bureau of Reclamation

Page 8, Humpback chub art: Randall D. Babb, Arizona Game and Fish Department

Page 9, Raft and rafters: Jeff Sorensen, Arizona Game and Fish Department

Page 10, Glen Canyon Dam: © 2005 Christopher Taesali; used with permission

Page 11, Benthic sampling fieldwork: Jeff Sorensen, Arizona Game and Fish Department

Page 12, Kanab ambersnail: Roy Averill-Murray, Arizona Game and Fish Department

Page 13, Willow flycatcher: Suzanne Langridge, U.S. Geological Survey

Page 14, Glen Canyon: © 2005 Christopher Taesali; used with permission

Page 15, USGS staff seining fish: © 2005 Dawn Kish; used with permission

First page photograph credit: see front matter for credits

Contact Information:

Lara M. Schmit

Associate Editor

Northern Arizona University

Center for Sustainable Environments

Flagstaff, AZ

lschmit@usgs.gov

Steven P. Gloss

Ecologist

U.S. Department of the Interior

U.S. Geological Survey

Southwest Biological Science Center

Tucson, AZ

sgloss@usgs.gov

Christopher N. Updike

Research Assistant

Northern Arizona University

Center for Sustainable Environments

Flagstaff, AZ

Chris_Updike@nau.edu
Chapter 1

Influence

of Glen

Canyon Dam 

Operations on 

Downstream 

Sand Resources 

of the Colorado 

River in

Grand Canyon

Scott A. Wright

Theodore S. Melis

David J. Topping

David M. Rubin

Introduction

The closure of Glen Canyon Dam and the beginning of flow regulation of the Colorado River through 

Grand Canyon in 1963 all but eliminated the mainstem 

sand supply to Grand Canyon and substantially altered 

the seasonal pattern of flows in the Colorado River. 

Dam-induced changes in both sand supply and flow have 

altered the sedimentary processes that create and maintain sandbars and related habitats, resulting in smaller 

and coarser grained deposits throughout the ecosystem.

From the perspective of river management, the 

ecological implications associated with such changes 

are not well understood and are the focus of ongoing 

integrated science studies. The effects of Glen Canyon 

Dam operations on fine-sediment resources (i.e., sand 

and finer material), particularly the erosion and restoration of sandbars, are of interest because sandbars are a 

fundamental element of the Colorado River’s geomorphic framework and the landscape of Grand Canyon 

(see Webb, 1996; Webb and others, 2002). Sandbars 

are also of interest in terms of the essential role finesediment resources play in other ecosystem processes 

(U.S. Department of the Interior, 1995). For example, 

emergent sandbars create terrestrial habitats for riparian vegetation and associated fauna. Similarly, sandbars 

create areas of stagnant or low-velocity flow that may 

be used as rearing habitat by the endangered humpback 

chub (Gila cypha) and other native fish. Recreational river 

runners and other backcountry visitors frequently use 

sandbars as campsites. Finally, abundant sand and silt 

deposits near and above the elevation of typical predam 

floods contain archeological resources and protect those 

resources from weathering and erosion.

Conservation of Grand Canyon’s fine-sediment 

resources is a primary environmental goal of the Glen 

Canyon Dam Adaptive Management Program. Despite 

this fact, the dam’s hydroelectric powerplant operation 

under the Record of Decision (U.S. Department of 

the Interior, 1996) continues to erode the limited finesediment deposits that exist downstream. Changes in 

the abundance, distribution, size, and composition of 

sandbars began to occur under the no action period (historical operations) of dam operation from 1963 through 

1991. Sandbar erosion continued despite changes in 

the operation of the dam that resulted from the implementation of the interim operating criteria in 1991 and 

the modified low fluctuating flow (MLFF) alternative in 
18 The State of the Colorado River Ecosystem in Grand Canyon

1996. The MLFF was the preferred alternative identified in the 1995 Operation of Glen Canyon Dam Final 

Environmental Impact Statement (EIS) and was selected 

in the Record of Decision (U.S. Department of the 

Interior, 1996). 

The U.S. Geological Survey’s (USGS) Grand 

Canyon Monitoring and Research Center and its 

cooperators have conducted extensive monitoring and 

research on fine-sediment transport and sandbar evolution in Grand Canyon. This chapter presents a summary of the results of studies since the 1970s, as well as 

conclusions derived from recent syntheses of streamflow, 

sediment transport, and geomorphic data from 1921 to 

2004, including recent sediment budgets. The effects of 

the MLFF operating alternative at Glen Canyon Dam 

(1996–2004) on fine-sediment transport and sandbars are 

examined in the context of these historical data. Finally, 

options identified by sediment scientists for testing alternative operations aimed at more effective conservation 

of fine-sediment resources are discussed.

Background

Predam Sedimenttransport Processes

As described by Rubin and others (2002), sandbars 

below Glen Canyon Dam in Marble and Grand Canyons 

are maintained by fine sediment that is transported by the 

Colorado River through the ecosystem. As sand is carried through these bedrock canyons by the river, some of 

it is deposited along channel margins and along shorelines within hundreds of eddies, thus building sandbars. 

The eddy areas, which are typically located immediately 

downstream from channel constrictions created by tributary debris fans, are susceptible to fine-sediment deposition because the flow tends to recirculate and be of lower 

velocity than the flow in the main channel. Using historical sediment-transport records from the Lees Ferry (RM 

0) and Grand Canyon (RM 87) gages, Laursen and others 

(1976) and later Topping and others (2000b) identified 

that before closure of Glen Canyon Dam, sand would 

accumulate in the Colorado River channel during late 

summer, fall, and winter. Annual accumulation of sand 

in the channel during predam years apparently resulted 

from large sediment inputs from tributaries that occurred 

during periods of seasonal low flows in the main channel 

of the Colorado River. Following these periods of sand 

enrichment in the main channel, spring snowmelt floods 

would erode the accumulated sand from the channel and 

transport it out of the canyon, along the way depositing 

some of the sand in the low-energy eddy areas and thus 

leading to the building of the high-elevation sandbars. 

Following the spring replenishment of sandbars, some of 

this sand would in turn be redistributed to even higher 

elevations by winds (Topping and others, 2000b). On 

an annual basis, the inputs of sand to the system would 

approximately balance the export, maintaining equilibrium in background sand storage in the eddies. 

Effects of Lake Powell on

Sand Transport

Before the closure of Glen Canyon Dam in 1963, 

approximately 25 million tons (23 million Mg) of sand 

passed the Lees Ferry stream gage annually. With the 

addition of 1.7 million tons (1.5 million Mg) of sand 

from the Paria River, which joins the Colorado River just 

downstream from Lees Ferry, the total predam annual 

sand supply to Marble Canyon reached about 27 million 

tons (24 million Mg). At the end of Marble Canyon, the 

Little Colorado River joins the Colorado River and contributed, on average, about 1.9 million tons (1.7 million 

Mg) to the annual sand supply. Thus, the total predam 

sand supply to Grand Canyon, from the Colorado River 

upstream from Lees Ferry and with the Paria and Little 

Colorado Rivers combined, was approximately 29 million 

tons (26 million Mg).

Today, because Lake Powell traps all of the sediment 

upstream from Glen Canyon Dam, the Paria River is the 

primary source of sand to Marble Canyon, supplying 

approximately 6% of predam sand levels. In the case 

of Grand Canyon, Glen Canyon Dam has reduced its 

sand supply to primarily the contributions of the Paria 

and Little Colorado Rivers. Other lesser tributaries also 

contribute a small amount of sand to Grand Canyon, 

with an estimated cumulative supply that is approximately 10% to 20% of the mean annual load provided 

by the Paria River. Taken together, the contributions of 

sand from various sources provide Grand Canyon with 

approximately 16% of its predam sand levels. The findings presented here are drawn from Topping and others 

(2000b) and Webb and others (2000); readers interested 

in more details on the predam and postdam sediment 

budgets for Marble and Grand Canyons should consult 

these reports. 
Influence of Glen Canyon Dam Operations on Downstream Sand Resources 19

Effects of Dam Operations on 

Flow Frequency and Duration

Changes in the flow regime of the Colorado River 

since construction of Glen Canyon Dam have also been 

dramatic in terms of seasonal variability, as well as in 

terms of daily fluctuations that occur because of “peaking” hydroelectric power generation. Dam operations 

have altered seasonal variability by eliminating longduration flood flows that occurred during the spring 

snowmelt and short-duration flood flows that occurred 

during the late summer and early fall thunderstorm 

season, as well as the very low flows that occurred during summer, fall, and winter. With regard to the highest 

flows, dam operations have reduced the 2-yr recurrence 

interval flood (i.e., the flood that occurs every other 

year on average) from 85,000 cubic feet per second 

(cfs) during the predam period to 31,500 cfs during the 

postdam period. In the predam era, discharge exceeded 

9,000 cfs only 44.3% of the time, while in the postdam 

era this percentage has gradually increased by decade, 

from 52.7% in the 1960s to 82.6% in the 1990s. This 

decrease in the duration of low flows has important 

implications for sediment transport because Topping and 

others (2000b) showed that flows less than about 9,000 

cfs result in accumulation of tributary sand inputs in 

the Marble Canyon and Grand Canyon reaches of the 

river, whereas flows above this generally lead to transport 

of new sand inputs through these reaches or erosion of 

sand from these reaches.

Dam operations have introduced large daily variations in discharge to generate hydroelectric power that 

tracks daily peaks in demand throughout the Western 

United States. Also, because peak energy demand varies 

seasonally in the West, with peak demand occurring in 

midsummer and winter, the month-to-month flow pattern 

related to dam operation is substantially different from 

natural, predam, seasonal patterns. Highest discharges in 

the river now occur during the two seasons when predam 

discharge had typically been the lowest, midsummer and 

winter. Furthermore, daily patterns of flow in the river 

have been altered by dam operations. For example, during the predam period the median daily range in discharge was only 524 cfs, whereas in the postdam era the 

median daily range increased to 8,580 cfs, a value greater 

than the predam median discharge. Before dam operation, the daily range in discharge exceeded 10,000 cfs 

only about 1% of all days; postdam, the daily discharge 

range exceeded 10,000 cfs on 43% of all days. 

Initially, operation of the dam’s powerplant was 

characterized mostly by unconstrained daily fluctuations that were designed to optimize electrical generation 

around peak daily demand, which had patterns that also 

varied on a monthly timescale related to seasonal changes 

in energy demand. From 1963 through 1991, these operations typically caused the Colorado River’s discharge to 

fluctuate on a daily basis from less than 5,000 cfs to near 

powerplant capacity of about 31,000 cfs. These so-called 

“no action” daily operations (because they were considered the no action alternative in the EIS) were first altered 

in 1990 to facilitate experimental release patterns implemented through July 1991 as part of field investigations 

associated with the EIS on dam operations. The experimental flows of 1990–91 were then followed by “interim 

operating criteria” from August 1991 until October 

1996, when Secretary of the Interior Bruce Babbitt 

implemented current Record of Decision dam operations. 

Implementation of the interim operating criteria in 1991, 

as well as the MLFF in 1996, constrained the change in 

discharge over any 24-h period to 5,000; 6,000; or 8,000 

cfs, depending on the monthly volume-release schedule 

specified in the annual operating plan for the Colorado 

River Storage Project. The flow history of the Colorado 

River into Grand Canyon as measured at the Lees Ferry 

gaging station is shown in figure 1. These flow data 

illustrate a transformation of the Colorado River from a 

fluvial ecosystem with significant seasonal variability in 

the predam era to a postdam river ecosystem with little 

seasonal variability and substantial daily fluctuations. 

Another important aspect of the MLFF operation 

is the schedule of monthly release volumes in relation to 

the seasonality of sediment inputs. Because of energy 

demand and hydropower economics, monthly release 

volumes are highest during months with high demand, 

including those in late summer. Historically, however, the 

late summer months were characterized by low mainstem 

flows and the highest tributary inputs, leading to sediment 

accumulation during the predam era. Postdam, high 

summer releases coincide with tributary inputs, leading 

to rapid export instead of accumulation. Therefore, not 

only has the sand supply been drastically reduced through 

the impoundment of Lake Powell, but the seasonal timing 

of low and high flows has also been both highly compressed and significantly shifted to later periods of the 

year that coincide with tributary sand inputs. The information in this section was taken from Topping and others 

(2003); readers with further interest in the Colorado 

River’s hydrology, both before and after the dam was 

closed, should consult this report. 
20 The State of the Colorado River Ecosystem in Grand Canyon

Figure 1. Instantaneous discharge (A) and daily range in discharge (B) in cubic feet per second of the Colorado River at Lees Ferry 

(RM 0) between 1921 and 2004 (modified from Topping and others, 2003). Before construction of Glen Canyon Dam, the annual peak

flow routinely exceeded 100,000 cfs. Dam operations during the period from 1963 through 1990 were characterized by daily fluctuations

from typically less than 5,000 cfs to near powerplant capacity, or about 31,000 cfs, and included the record wet period of the mid-1980s,

which resulted in the use of the spillways in 1983 for emergency releases exceeding about 90,000 cfs. Interim operating criteria, which 

constrained daily release fluctuations, began in 1991 and were followed by the modified low fluctuating flow operating alternative that 

was implemented as part of the Secretary of the Interior’s Record of Decision (ROD) in 1996 (BHBF = beach/habitat-building flow).

Status and Trends of 

Fine Sediment Below 

Glen Canyon Dam

Changes in sand supply and flow regime downstream from a dam affect the geomorphology of the 

downstream channel. When a dam traps sand and 

releases clear water, this clear water is often termed 

“hungry” because it still has the capacity to transport an 

amount of sand and gravel proportional to the flow and 

will erode the downstream channel and banks in order 

to satisfy its appetite with respect to sediment transport. 

On the basis of resurveys of historical cross-sections 

upstream from Lees Ferry, approximately 20 million tons 

(18 million Mg) of material—gravel and fine sediment, 

including sand—have been eroded from the first 15 mi 

(24 km) of the Colorado River downstream from the 

dam, an area referred to in this report as the Lees Ferry 

reach (Grams and others, 2004). The amount of material removed is equivalent to a 6 to 10 ft (2–3 m) drop in 

channel elevation averaged over the entire reach. Most 

of this sediment was removed by daily, high-release 

dam operations designed to scour the channel of the 

Colorado River below the powerplant during April–June 

A. B.

1965 (fig. 1). Daily suspended-sediment measurements 

made by the USGS at the Lees Ferry and Grand Canyon 

gaging stations indicated that these high flows in 1965 

eroded 4.4 million tons (4.0 million Mg) of fine sediment 

(mostly sand) from the Lees Ferry reach and 18 million 

tons (16 million Mg) of fine sediment (mostly sand) from 

Marble and upper Grand Canyons. Channel scour was 

anticipated below the dam during its design and was 

later needed to optimize energy generation within the 

operating range of the hydroelectric powerplant (Grams 

and others, 2004). Typical dam releases today do not 

result in much erosion from the Lees Ferry reach, and 

as a result very little fine sediment is transported downstream to Marble and upper Grand Canyons.

Despite the fact that its contributing drainage area 

is approximately 18 times smaller than that of the Little 

Colorado River, the single largest sand supplier to the 

reaches below Glen Canyon from 1990 through 2004 

was the Paria River. Farther downstream in Marble and 

upper Grand Canyons, the fate of fine-sediment deposits is dependent upon the long-term balance between 

inputs to the system (i.e., tributary supply) and exports 

from the system (i.e., mainstem sediment-transport rates). 

Although sand inputs have been greatly reduced by the 

closure and operation of Glen Canyon Dam, the annual 
Influence of Glen Canyon Dam Operations on Downstream Sand Resources 21

mainstem transport—and thus export—has also most 

likely been reduced because of the elimination of the 

highest flood flows. As a result, two possibilities exist for 

the postdam fine-sediment balance downstream from 

the Paria River. First, if the supply from the Paria River 

and other lesser Marble Canyon tributaries exceeds the 

postdam transport rate on an annual basis, then new 

sand inputs would accumulate in the channel and in lowelevation portions of eddies over multiple years. Such 

accumulated sand supplies would then be available at 

any time for redistribution to higher elevation sandbars 

through release of periodic controlled floods (i.e., beach/

habitat-building flows in the EIS; hereafter BHBF) from 

Glen Canyon Dam. This scenario was the conclusion 

reached by Howard and Dolan (1981), Andrews (1990, 

1991), Smillie and others (1993), and the EIS study 

team (U.S. Department of the Interior, 1995) for the 

MLFF alternative, leading to its implementation in 1996. 

Howard and Dolan (1981) reached their conclusion by 

using an estimate for the sand contribution from the lesser 

tributaries that is now regarded to be about a factor of 

four too high (Topping and others, 2000b; Webb and 

others, 2000). Andrews (1990, 1991) and Smillie and 

others (1993) reached their conclusions by using stable 

sand-transport relationships, also called “rating curves.” 

A stable sand-transport rating curve exists where there is 

a unique value for sand concentration for any given flow. 

This approach invokes the assumption that the upstream 

sand supply is in equilibrium with transport capacity. 

The methods and data used to reach the conclusion in 

the EIS are discussed further in the following section.

Alternatively, if the annual mainstem transport rate 

(export) exceeds tributary supply (input), then systematic 

long-term erosion of fine sediment from the channel 

would be expected. In fact, this second scenario was 

originally predicted by Dolan and others (1974) and 

Laursen and others (1976) on the basis of their early 

sediment-transport studies related to effects of Glen 

Canyon Dam on downstream resources. In order for 

high-flow releases to be effective at restoring and maintaining sandbars under this second scenario, controlled 

floods would need to be strategically timed to coincide 

with or immediately follow tributary sand inputs. These 

early studies predated the concept of using controlled 

floods to restore eroded sandbars; hence, their estimates 

of sand transport in the postdam era could only result 

in net export of new sand inputs and continued erosion 

of existing sandbars of predam origin. More recent evidence presented in the following section further supports 

the conclusion that this second scenario prevails under

the current reoperating strategy and that this situation is 

leading to systematic, long-term erosion of fine sediment 

from the channel bed and eddies of Marble and Grand 

Canyons. On the basis of existing data, it is still uncertain whether or not strategically timed managed floods 

can restore and maintain eroded sandbars by using only 

the limited and infrequent tributary-derived sand that 

enters the river below the dam.

Recent Findings

The Paradigm of Sand Transport and 

Storage Used in the 1995 Environmental 

Impact Statement

The EIS concluded that sand would accumulate 

over multiyear timescales in the channel of the Colorado 

River in Marble and upper Grand Canyons during MLFF 

powerplant releases in all but the highest release years 

(U.S. Department of the Interior, 1995). The basis for 

this conclusion was the assumption that the relationship 

between the water discharge and sand transport in the 

Colorado River did not change substantially over time. 

This assumption was used because sediment-transport 

data collected in the postdam Colorado River were sparse. 

Prior to the early 1970s, suspended-sediment concentration was measured on a daily basis at the three 

USGS gaging stations that are critical to constructing 

a sand budget for Marble and Grand Canyons: the 

Paria River at Lees Ferry, the Little Colorado River at 

Cameron, and the Colorado River near Grand Canyon. 

The sediment sampling program at the Colorado River 

near Grand Canyon gaging station began in October 

1925; the daily sediment sampling programs at the Paria 

and Little Colorado Rivers began in October 1947. The 

Little Colorado River sediment record was discontinued 

on September 30, 1970; the Colorado River sediment 

record at the Grand Canyon gaging station was discontinued on September 30, 1972; and the Paria River sediment record was discontinued on September 30, 1976. 

Thus, the only postdam period of overlap between these 

stations that could be used to construct a sand budget 

was the period from closure of the dam in March 1963 

through September 30, 1970. Furthermore, no postdam sand-transport data were collected within Marble 

Canyon during this early period. 

To fill this data gap, the USGS began a program of 

quasi-daily sediment sampling on the major tributaries 

to the Colorado River (that is, the Paria River, the Little 

Colorado River, and Kanab Creek) and at five locations 

on the mainstem Colorado River in Marble and Grand 

Canyons (Garrett and others, 1993). On the tributar-
22 The State of the Colorado River Ecosystem in Grand Canyon

Figure 2. Reproduction of figure III-15 from the final 

environmental impact statement (EIS) (U.S. Department of the 

Interior, 1995), which shows the sand budget as computed 

by Randle and Pemberton (1987). Recent studies refute the 

conclusion of the EIS that sand accumulates on the bed of the 

Colorado River over multiple years under normal dam operations.

(Phantom Ranch is the location of the Grand Canyon gage.)

ies, this program extended from July through December 

1983. On the mainstem, this program included the 

periods from July through December 1983 and October 

1985 through January 1986. All suspended-sediment 

samples collected under this program were analyzed for 

grain size to allow use in constructing sand budgets. 

The sand budget for the Colorado River in Marble 

and Grand Canyons used in the EIS was constructed by 

Randle and Pemberton (1987) and Pemberton (1987). 

For tributary sand input, they constructed stable sandrating curves by using all of the historical and 1983 

data from the Paria River, the Little Colorado River, 

and Kanab Creek. They also included an estimate for 

the sand supply from the lesser tributaries. Pemberton 

(1987) developed stable sand-transport rating curves at 

the five mainstem locations based on the USGS 1983–86 

data, and the EIS states, “The sand transport equations 

of Randle and Pemberton (1987) and Pemberton (1987) 

were used for these computations” (U.S. Department of 

the Interior, 1995, p. 95) in reference to the sediment 

budget presented in figure III-15 of the EIS (and reproduced here as fig. 2). Therefore, the EIS sediment budget was based on the assumption of stable sand-transport 

rating curves. Results of recent studies presented in the 

following section suggest that this assumption is incorrect 

for the Colorado River below Glen Canyon Dam.

Studies Since 1996 That Refute 

the Environmental Impact 

Statement Findings

Research and monitoring conducted during and 

after the 1996 BHBF experiment, also known as the 

1996 controlled flood, have led to several findings that 

refute the EIS predictions for sand conservation and 

suggest that the implementation of this strategy has 

not led to sustainable restoration and maintenance of 

sandbars in either Marble or Grand Canyon. Instead, 

the canyons’ sandbars continue to erode (figs. 3–6). The 

primary results of several of these studies are briefly 

summarized below:

Rubin and others (1998) and Topping and others (1999) showed that the sand supply during 

the 1996 BHBF was not as great as was assumed 

before the experiment and that the sand on the 

bed of the river and in suspension coarsened 

dramatically as the upstream supply of sand 

decreased over time during this flood. This process led to flood deposits that coarsened vertically 

upward (i.e., inversely graded deposits).

Topping and others (2000a) demonstrated that 

the grain size of sand on the bed of the Colorado 

River can change by over a factor of four as functions of tributary resupply of finer sand and higher 

dam releases that winnow the bed and that this 

factor-of-four change in bed-sand grain size corresponds to a change of two orders of magnitude 

in the concentration of sand in suspension (for the 

same discharge of water). Identification of this 

dynamic process precludes the use of stable sandtransport relationships in the Colorado River, 

thus invalidating the approach used to construct 

the sand budget in the EIS. Topping and others 

(2000a) also showed that Randle and Pemberton 

(1987) incorrectly predicted sand accumulation 

in the Colorado River because the data they used 

to verify their modeled stable sand-export relationships were from periods in the mid-1980s, 

when sand in the river was anomalously coarse 

and sand-transport rates were anomalously low 

following prolonged releases above powerplant 

capacity between 1983 and 1986.

Rubin and Topping (2001) showed that sand 

transport in the postdam Colorado River in 

Grand Canyon is regulated by both the discharge 

of water and the grain size of the sand available 

for transport in suspension. This information also 

•

•

•
Influence of Glen Canyon Dam Operations on Downstream Sand Resources 23

Figure 3. Repeat photographs of Tapeats Creek at the Colorado River, Grand Canyon (RM 133.8, right shore). A. (July 1952) This view 

downstream from below the mouth of Tapeats Creek shows a large sandbar with few rocks or boulders exposed. This sandbar was 

frequently used for layovers during river trips in the 1950s (Kent Frost, courtesy of the photographer). B. (March 27, 2003) Large rocks 

and boulders are now exposed because of severe beach erosion. New sand was deposited here during the 1996 beach/habitat-building

flow but was quickly removed. This camp is no longer used, which creates a problem for river runners who want to visit Tapeats Creek

(J. Janssen, stake 2676, courtesy of the Desert Laboratory Collection of Repeat Photography). (Figure after Webb and others, 2002.)

B.

Figure 4. Time series of repeat photographs of sandbars along the left shore of the Colorado River near RM 44.5 (Eminence Break) 

illustrating deposition on the sandbar during the 1996 beach/habitat-building flow (March 26–April 2; high flow occurred between

photographs B and C) and subsequent erosion since April 1996. Images provided by Northern Arizona University, Department of 

Geology in cooperation with the U.S. Geological Survey.

A.

A.

B.

C.

D.

E.

F.

March 13, 1994

March 25, 1996

April 4, 1996

April 19, 1998

June 17, 2000

September 11, 2000
24 The State of the Colorado River Ecosystem in Grand Canyon

Figure 5. A decrease in elevation of the sandbar surface is 

seen at Jackass Creek camp located along the left shore of the 

Colorado River, 23 mi (37 km) downstream of Glen Canyon Dam. 

Elevations were determined by examining oblique and aerial 

photographs of the site and by field survey of the elevation and 

the former sand surface at its contact with large talus blocks. This 

graph shows the elevations near one prominent talus block that 

was inundated by predam mean annual floods, but since the dam 

was completed, the talus block has been inundated infrequently 

(modified from Rubin and others, 2002). 

Figure 6. Changes in sandbar size (total surface area) are shown 

for 14 long-term sandbar study sites between the Lees Ferry and 

Grand Canyon gages (RM 0 to RM 87). Area of bars exposed 

above water discharges of 8,000 cfs decreased by 22% from 1991 

to 2004. The 1996 beach/habitat-building flow resulted in a net 

transfer of sand from mid elevations to high elevations (modified 

from Rubin and others, 2002).

contradicts the approach of the EIS, where it was 

assumed that sand transport was regulated only by 

the discharge of water.

Topping and others (2000b) showed through 

their analysis of the 1965–70 daily sedimenttransport data collected by USGS that, under 

normal powerplant flows, newly input tributary 

sand is exported past the Grand Canyon gaging 

station within several months. Their analysis of 

predam data indicated that, prior to closure of 

Glen Canyon Dam, sand would accumulate in 

Marble and upper Grand Canyons only during the 

9 mo of the year when discharges were typically 

lower than about 9,000 cfs.

Measurements of the channel bed indicate that 

tributary sand, which is typically much finer than 

the sand on the bed of the Colorado River, accumulates on the bed for only a short time before 

being eroded and transported out of the canyon 

under normal MLFF dam operations (Topping 

and others, 2000a).

Since August 1999, detailed suspended-sediment 

transport measurements have been collected at 

the Paria and Little Colorado Rivers to document 

•

•

•

inputs and at the USGS gaging stations above 

the mouth of the Little Colorado River and near 

Grand Canyon to document export. Initially, 

these quasi-daily measurements were made by 

using only conventional USGS methodologies 

to obtain cross-sectionally integrated samples 

of suspended-sediment concentration and grain 

size (methods described in Edwards and Glysson, 

1999). Because substantial and rapid (within a 

day) changes that are due to tributary inputs can 

occur in suspended-sediment concentration and 

grain size, emerging technologies for continuous 

monitoring of suspended-sediment concentration and grain size were tested and implemented 

beginning in 2001. These technologies include 

acoustic backscatter and laser-diffraction methods 

and are described in detail in Melis and others (2004) and Topping and others (2004). The 

detailed sediment-transport measurements allow 

for the ability to construct sediment budgets 

based on continuous data instead of on rating 

curves, a very important distinction from the 

EIS approach of using a limited data set. These 

data show that the overall mass balance of sand 

(input minus export) continues to be negative 

(fig. 7), as originally predicted by Laursen and 
Influence of Glen Canyon Dam Operations on Downstream Sand Resources 25

Figure 7. Mass balance of sand between Lees Ferry and 

Grand Canyon gages from August 1999 through July 2004 (A) and 

separately for sediment years (July–June) 2003 (B) and 2004 (C). 

Mass balance is computed by subtracting measured, mainstem 

suspended-sand export (10% uncertainty) from estimated and 

measured sand inputs from the Paria River (20% uncertainty) and 

Little Colorado River (30% uncertainty), as well as from estimated 

inputs from numerous lesser tributaries (50% uncertainty). The 

measurements illustrate the rapid export of tributary inputs by high 

dam releases and the continued overall loss of sand from Grand 

Canyon under the modified low fluctuating flow (MLFF) alternative, 

even during the drought-hydrology, minimum-volume release years 

of 2003 and 2004 (modified and updated from Rubin and others, 2002).

others (1976). Most significantly, the sand mass 

balance remained negative during water years 

2000 through 2004, despite 5 consecutive years 

in which minimal release volumes (8.23 million 

acre-feet (10,148 million m3)) from Lake Powell 

occurred during prolonged drought in the upper 

Colorado River Basin. These measurements 

and calculations of sand transport also show 

that tributary inputs are typically transported 

downstream and out of the canyon within a few 

months under typical Record of Decision operations (Rubin and others, 2002).

Repeat topographic mapping of sandbars (Hazel 

and others, 1999) showed that the 1996 BHBF 

did increase the surface area of high-elevation

sandbars, but more than half of the sand deposited at higher elevations was taken from the lower 

portions of the sandbars (Schmidt, 1999) rather 

than being derived from tributary sand supplies 

accumulated on the channel bed, as originally 

hypothesized in the 1995 EIS.

Repeated surveys of channel cross-sections (Flynn 

and Hornewer, 2003) revealed erosion at 55 of the 

57 locations between 1991 and 1999, even though 

daily operations were constrained during the time 

series of repeat measurements.

Schmidt and others (2004) conducted geomorphic 

mapping from air photos and land surveys for the 

predam and postdam periods. They estimated the 

loss of sand to be about 25% of the area typically 

exposed at base flow in predam photographs, but 

estimates range from 0% to 55% depending on 

study reach and method of analysis. Their studies 

further suggested that loss of the sandbar area 

continued at a relatively steady rate between 1983 

and 2002, despite constraints on daily operations 

imposed after 1991.

Importance of Continuous Long-term 

Sediment-transport Data

Because of a lack of continuous data on sediment 

inputs and export that would have allowed for a sediment budget based on measured data, the EIS study 

team used stable sand-transport rating curves. Stable 

rating curves assume that for any given flow there is a 

single value for the corresponding sand concentration 

and, therefore, a predictable sand-transport rate related 

to flows released from Glen Canyon Dam. The recent 

•

•

•

A.

C.

B.
26 The State of the Colorado River Ecosystem in Grand Canyon

Figure 8. Looking upstream into Glen Canyon from the 

Paria River confluence with the main channel Colorado River 

during a Paria River flood. Tributary inputs of sand, such as 

the one pictured, now encounter clear Colorado River water 

because Lake Powell traps incoming fine sediment. The 

Paria River is the primary source of sand to Marble Canyon 

but is only about 6% of the predam sand supply (photograph 

by Scott A. Wright, U.S. Geological Survey).

studies reported above, however, have demonstrated that 

in the postdam Colorado River the relationship between 

flow and sand transport is not stable but instead shifts 

quickly and substantially relative to the grain size of sand 

on the bed of the river (which is controlled by tributary 

inputs and mainstem flows). Rubin and Topping (2001) 

and Rubin and others (2002) showed that the grain size 

of the sand in the regulated Colorado River ecosystem 

depends greatly on the recent history of tributary activity. For example, during low tributary flow periods the 

only source of sand to the mainstem Colorado River 

is that on the channel bed and in eddies, and that sand 

tends to be much coarser than tributary-delivered sand 

because of the winnowing of the finer sizes. When 

tributaries are flooding and delivering large quantities of 

fine sand (fig. 8), however, the supply is no longer limited to the coarser channel bed sand, resulting in much 

higher mainstem sand concentrations and, hence, greatly 

increased suspended-sediment export for any given flow 

released from the dam.

Because sand transport cannot be predicted based 

on discharge alone, sediment budgets for the Colorado 

River in Grand Canyon can only be constructed based 

on measurements of sand transport at a frequency great 

enough to capture changes in concentration and grain 

size resulting from tributary inputs. Fundamentally, 

the conclusions drawn by the EIS team, which are not 

supported by the more recent data, resulted from a lack 

of continuous data in the postdam era; that is, if daily 

records had been continued beyond 1972 and into the 

EIS period, then the fine-sediment budget would have 

been constructed based on these data rather than on 

stable rating curves. Recent sediment budgets suggest 

that under this scenario the conclusions of the EIS would 

have been different and possibly would have led to a 

different strategy for operation of Glen Canyon Dam in 

1996. Though it is somewhat costly to collect long-term, 

high-frequency sediment-transport records, in this case it 

may have prevented 13 yr of dam operations that have 

continued to erode sandbars from Grand Canyon.

Current Experimental 

Plan for Fine Sediment

Because recent research has shown that sand does 

not accumulate on the river bed in Marble and Grand 

Canyons under normal Record of Decision dam operations, scientists have recently proposed two possible field 

tests of dam operating options that might more effectively conserve limited, downstream sand resources. One 

approach is to implement floods immediately following 

large tributary inputs that commonly occur in late summer and early fall. A second approach is to follow tributary sand-input events with low flows, in order to limit 

export and retain most of the sand input, until flooding 

can be implemented. This approach would require a 

change in the pattern of monthly release volumes and 

associated dam operations because July and August 

releases of recent drought years still resulted in half of 

the sand introduced by a tributary flood being exported 

within days or weeks (Rubin and others, 2002).

In September 2002, the U.S. Department of the 

Interior (2002) approved implementation of the second 

approach described above. Under this plan, changes 

in dam operations and restoration floods are linked to 

triggering thresholds based on sand inputs from the Paria 

River and lesser Marble Canyon tributaries and retention 

of sand in Marble and Grand Canyons. For example, 

the “autumn sediment input” scenario described in the 

2002 environmental assessment (EA) (U.S. Department of 
Influence of Glen Canyon Dam Operations on Downstream Sand Resources 27

Figure 9. Sequence of events established in the autumn sediment 

input scenario in an environmental assessment by U.S. Department 

of the Interior (2002) related to fine-sediment inputs and retention 

to trigger a 2-d, 42,000–45,000-cfs experimental high flow in 

January. If fine-sediment inputs do not reach specified levels, then 

modified low fluctuating flow (MLFF) operations, as specified in the 

Record of Decision (ROD) (U.S. Department of the Interior, 1996), 

are continued.

the Interior, 2002) defined a sequence of events related 

to sand inputs and retention that would trigger a 2-d, 

42,000–45,000-cfs experimental high flow in the following January (fig. 9). Significant sand inputs to Marble 

Canyon that exceeded the triggering threshold for an 

experimental high flow occurred during September–

November 2004. Instead of constraining operations 

through December (a winter, peak-demand month) in 

order to retain sand in Marble Canyon as laid out in the 

2002 EA, a supplemental EA was prepared that allowed 

for a hybrid of the first and second approaches to be 

tested and evaluated. Approval of the supplemental EA 

paved the way for the experimental high flow that began 

on Sunday, November 21, 2004, when the Bureau of 

Reclamation opened the bypass tubes of Glen Canyon 

Dam for 90 h. The peak high flows ran for 2.5 d (60 h) 

at about 41,000 cfs. Scientists will evaluate data collected during and after the high-flow event to determine 

whether or not this strategy succeeded in enlarging existing beaches and sandbars.

Other dam operation scenarios may be more effective at retaining tributary inputs, such as Record of 

Decision operations modified such that equal volumes 

of water are released from the dam each month. Alternatively, a scenario of seasonally adjusted steady flows, 

which was an alternative in the EIS process, may be 

effective. Because of the severely reduced sand supply, however, even during periods of minimum release 

requirements of 8.23 million acre-feet (10,148 million 

m3) per year the possibility exists that no operational 

scenario will result in management objectives being 

achieved for restoring sandbars, simply because of the 

volume of water that must be released on an annual 

basis. If so, other, more effective alternatives for restoring and maintaining sandbars and related habitats may 

need to be evaluated. 

Sediment augmentation, one possible alternative, was eliminated during the development of the 

EIS, partly because of the belief that sandbars could 

be restored and maintained by constraining the hourly 

ramping rates and range of daily dam operations and 

partly because of concerns about contamination of sediment upstream in Lake Powell (Graf, 1985). Addition of 

sediment—continuously, seasonally, or perhaps only during floods—may offer greater powerplant operating flexibility and therefore may cost less than further restrictions 

on annual dam operations. To this end, the feasibility 

of mechanically transporting fine sediment around Glen 

Canyon Dam and introducing it into the Colorado River 

below the dam is currently being investigated.
28 The State of the Colorado River Ecosystem in Grand Canyon

Discussion and Future 

Research Needs

Extensive research and monitoring of fine-sediment 

transport and sandbars since the completion of the EIS 

have resulted in a better understanding of the geomorphology of the Colorado River in Marble and Grand 

Canyons and of the effects of the operations of Glen 

Canyon Dam on the river’s downstream resources. Probably the single most important finding of this research 

and monitoring is that postdam mainstem sand transport 

exceeds the postdam supply of sand from tributaries on 

a seasonal to annual basis, such that the postdam river 

is in an annual fine-sediment deficit (i.e., export exceeds 

input). This sediment deficit has resulted in a consistent 

downstream pattern of erosion of channel and sandbar 

deposits from Marble and Grand Canyons despite restrictions on daily powerplant fluctuations required by the 

implementation of the MLFF alternative.

The finding of an annual sediment deficit directly 

contradicts the critical EIS assumption that sand will 

accumulate on the bed of the Colorado River over multiple years under the MLFF operating alternative (and 

minimum annual volume releases) and has important 

implications for the potential success of managing tributary sediment inputs. It is also worth noting that the 

EIS conclusion resulted fundamentally from a lack of 

long-term records for tributary sand supply and mainstem sand-transport rates, illustrating the importance of 

long-term data sets in river management. A continuous sediment budget for the Colorado River in Grand 

Canyon since construction of Glen Canyon Dam, 

based on high-frequency measurements, likely would 

have resulted in a different EIS conclusion about finesediment dynamics below the dam, one that may have 

prevented the continued erosion of sandbars between 

1991 and 2004.

A second important finding of recent research 

and monitoring efforts is that during the 1996 BHBF 

the primary source of sand for building high-elevation

sandbars was the low-elevation portion of the sandbars 

instead of the channel bed as hypothesized in the EIS. 

This scenario of building high-elevation sandbars at 

the expense of the low-elevation portions was repeated 

during the powerplant capacity flow in September 2000 

(Hazel and others, in press). This process of sandbar 

building is supported by the finding of an absence 

of multiyear accumulation on the channel bed: sand 

cannot be transported from the bed to high-elevation 

sandbars because there is typically little sand available 

on the channel bed.

Neither of these two findings supports the EIS 

hypotheses, but they have led scientists and managers 

to reassess the management strategy for sand resources 

within Grand Canyon. An emerging paradigm is the 

need to strategically time high-flow releases in order to 

take advantage of sporadic tributary sediment inputs, 

a scenario that requires greater flexibility in the annual 

operating plan for the dam with respect to both hydroelectric power generation and economic cost. Only 

immediately after these inputs is significant sand available on the channel bed for transfer to high-elevation 

sandbars through high-flow releases. Alternatively, dam 

releases may be constrained following inputs for a period 

of time until a high flow can be released from the dam; 

however, during extended periods of above-average 

upper Colorado River Basin hydrology and high storage 

in Lake Powell, constraining daily operations may not be 

possible (see fig. 1, 1995 through 1998). In the absence 

of high-flow releases strategically timed to redistribute 

tributary inputs to high-elevation sandbars, the inputs 

are exported from Grand Canyon in a period of weeks 

or months under normal dam operations, leading to 

continued long-term erosion of sandbars.

In November 2004, this paradigm of strategically 

timed, high-flow releases was tested for the first time 

on the Colorado River. Scientists are in the process of 

evaluating the results of this experiment. The findings 

will be critical for the long-term management of finesediment resources and sandbars in Grand Canyon. If a 

management approach of strategically timed, high-flow 

releases, triggered by tributary inputs, is to be followed, 

then further research will be required to define the 

appropriate triggering criteria and to develop high-flow 

hydrographs (peaks and durations) that may optimize 

deposition of tributary sand inputs within eddies while 

minimizing export during controlled flood peaks.

If strategically timed, high-flow releases are deemed 

inadequate for meeting the management objectives for 

Grand Canyon sandbars, then alternative approaches 

must be considered, such as further restraints on daily 

powerplant operations, changes in monthly volume 

release patterns, or sediment augmentation. 
Influence of Glen Canyon Dam Operations on Downstream Sand Resources 29

References

Andrews, E.D., 1990, The Colorado River: a perspective from Lees Ferry, Arizona, in Wolman, M.G., 

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Andrews, E.D., Lisle, T.E., Buchanan, J.P., Schumm, 

S.A., and Winkley, B.R., The riverscape, in Wolman, 

M.G., and Riggs, H.C., eds., Surface water hydrology, The geology of North America, v. O-1: Boulder, 

Colo., Geological Society of America, p. 281–328.

Andrews, E.D., 1991, Sediment transport in the Colorado 

River basin, in Committee to Review the Glen Canyon 

Environmental Studies, Water Science and Technology Board, Commission on Geosciences, Environment, 

and Resources, eds., Colorado River ecology and dam 

management: Washington, D.C., National Academy 

Press, p. 54–74.

Dolan, R., Howard, A., and Gallenson, A., 1974, Man’s 

impact on the Colorado River in the Grand Canyon: 

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Edwards, T.K., and Glysson, G.D., 1999, Field methods 

for measurement of fluvial sediment: U.S. Geological 

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book 3, chap. C2, 89 p.

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sand storage measured at monumented cross sections 

in the Colorado River between Glen Canyon Dam 

and Lava Falls Rapid, northern Arizona, 1992–99: 

U.S. Geological Survey Water Resources Investigations 

Report 03-4104, 39 p.

Garrett, W.B., Van De Vanter, E.K., and Graf, J.B., 1993, 

Streamflow and sediment-transport data, Colorado 

River and three tributaries in Grand Canyon, Arizona, 

1983 and 1985–86: U.S. Geological Survey Open-File 

Report 93-174, 624 p.

Graf, W.L., 1985, Mercury transport in stream sediments 

of the Colorado Plateau: Annals of the Association of 

American Geography, v. 75, p. 552–565.

Grams, P.E., Schmidt, J.C., Topping, D.J., and Goeking, 

S., 2004, The degraded reach: rate and pattern of bed 

and bank adjustment of the Colorado River in the 25 

km immediately downstream from Glen Canyon Dam: 

final report to the Grand Canyon Monitoring and 

Research Center: Logan, Utah State University, 111 p.

Hazel, J.E., Topping, D.J., Schmidt, J.C., Kaplinski, M., 

and Melis, T.S., in press, Downstream effects of a dam 

on sediment storage in a bedrock canyon: the relative roles of eddy and channel storage in the sediment 

budget for the Colorado River in Marble Canyon, AZ: 

Journal of Geophysical Research, Earth Surface.

Hazel, J.E., Jr., Kaplinski, M., Parnell, R., Manone, M., 

and Dale, A., 1999, Topographic and bathymetric 

changes at thirty-three long-term study sites, in Webb, 

R.H., Schmidt, J.C., Marzolf, G.R., and Valdez, 

R.A., eds., The controlled flood in Grand Canyon: 

Washington, D.C., American Geophysical Union, 

Geophysical Monograph Series, v. 110, p. 161–184.

Howard, A., and Dolan, R., 1981, Geomorphology of 

the Colorado River in the Grand Canyon: Journal of 

Geology, v. 89, p. 269–298.

Laursen, E., Ince, S., and Pollack, J., 1976, On sediment 

transport through the Grand Canyon, in Proceedings 

of the Seventh Federal Interagency Sediment Conference, Denver, Colo., p. 4-76–4-87.

Melis, T.S., Topping, D.J., and Rubin, D.M., 2004, Testing laser-based sensors for continuous in situ monitoring of suspended sediment in the Colorado River, 

Arizona, in Bogen, J., Fergus, T., and Walling, D.E., 

eds., Erosion and sediment transport measurement in 

rivers: technological and methodological advances: 

International Association of Hydrological Sciences 

Publication 283, p. 21–27.

Pemberton, E.L., 1987, Sediment data collection and 

analysis for five stations on the Colorado River from 

Lees Ferry to Diamond Creek: Glen Canyon Environmental Studies, Salt Lake City, Utah, Bureau of 

Reclamation, Upper Colorado Region, 156 p.

Randle, T.J., and Pemberton, E.L., 1987, Results and 

analysis of STARS modeling efforts of the Colorado 

River in Grand Canyon: Glen Canyon Environmental 

Studies, Salt Lake City, Utah, Bureau of Reclamation, 

Upper Colorado Region, 190 p.

Rubin, D.M., Nelson, J.M., and Topping, D.J., 1998, 

Relation of inversely graded deposits to suspendedsediment grain-size evolution during the 1996 flood 

experiment in Grand Canyon: Geology, v. 26, p. 

99–102.
30 The State of the Colorado River Ecosystem in Grand Canyon

Rubin, D.M., and Topping, D.J., 2001, Quantifying the 

relative importance of flow regulation and grain-size 

regulation of suspended-sediment transport (α), and 

tracking changes in bed-sediment grain size (β): Water 

Resources Research, v. 37, p. 133–146.

Rubin, D.M., Topping, D.J., Schmidt, J.C., Hazel, J., 

Kaplinski, M., and Melis, T.S., 2002, Recent sediment 

studies refute Glen Canyon Dam hypothesis: Eos, 

Transactions, American Geophysical Union, v. 83, no. 

25, p. 273, 277–278.

Schmidt, J.C., 1999, Summary and synthesis of geomorphic studies conducted during the 1996 controlled 

flood in Grand Canyon, in Webb, R.H., Schmidt, J.C., 

Marzolf, G.R., and Valdez, R.A., eds., The controlled 

flood in Grand Canyon: Washington, D.C., American 

Geophysical Union, Geophysical Monograph Series, v. 

110, p. 329–341.

Schmidt, J.C., Topping, D.J., and Grams, P.E., 2004, System-wide changes in the distribution of fine-grained 

alluvium in the Colorado River corridor between Glen 

Canyon Dam and Bright Angel Creek, Arizona: final 

report to the Grand Canyon Monitoring and Research 

Center: Logan, Utah State University, 117 p.

Smillie, G.M., Jackson, W.L., and Tucker, D., 1993, 

Colorado River sand budget: Lees Ferry to Little 

Colorado River: National Park Service Technical 

Report NPS/NRWRD/NRTR-92/12, 11 p.

Topping, D.J., Melis, T.S., Rubin, D.M., and Wright, 

S.A., 2004, High-resolution monitoring of suspendedsediment concentration and grain size in the Colorado 

River in Grand Canyon using a laser-acoustic system, 

in Chunhong Hu, Ying Tan, and Cheng Liu, eds., 

Proceedings of the Ninth International Symposium on 

River Sedimentation, October 18–21, 2004, Yichang, 

China, v. 4, p. 2507–2514.

Topping, D.J., Rubin, D.M., Nelson, J.M., Kinzel, P.J., 

III, and Bennett, J.P., 1999, Linkage between grainsize evolution and sediment depletion during Colorado 

River floods, in Webb, R.H., Schmidt, J.C., Marzolf, 

G.R., and Valdez, R.A., eds., The 1996 controlled 

flood in Grand Canyon: Washington, D.C., American 

Geophysical Union, Geophysical Monograph Series, v. 

110, p. 71–98.

Topping, D.J., Rubin, D.M., Nelson, J.M., Kinzel, P.J., 

III, and Corson, I.C., 2000a, Colorado River sediment transport: pt. 2: systematic bed-elevation and

grain-size effects of supply limitation: Water Resources 

Research, v. 36, p. 543–570. 

Topping, D.J., Rubin, D.M., and Vierra, L.E., Jr., 2000b, 

Colorado River sediment transport: pt. 1: natural 

sediment supply limitation and the influence of Glen 

Canyon Dam: Water Resources Research, v. 36, p. 

515–542.

Topping, D.J., Schmidt, J.C., and Vierra, L.E., Jr., 2003, 

Computation and analysis of the instantaneous-discharge record for the Colorado River at Lees Ferry, 

Arizona—May 8, 1921, through September 30, 2000: 

U.S. Geological Survey Professional Paper 1677, 118 p.

U.S. Department of the Interior, 1995, Operation of 

Glen Canyon Dam Final Environmental Impact Statement: Salt Lake City, Utah, Bureau of Reclamation, 

Upper Colorado Region, 337 p., appendices.

U.S. Department of the Interior, 1996, Record of Decision, operation of Glen Canyon Dam: Washington, 

D.C., Office of the Secretary of the Interior, 13 p.

U.S. Department of the Interior, 2002, Proposed experimental releases from Glen Canyon Dam and removal 

of nonnative fish: environmental assessment: Salt Lake 

City, Utah, Bureau of Reclamation, Upper Colorado 

Region, 112 p., appendices.

Webb, R.H., 1996, Grand Canyon, a century of change: 

Tucson, University of Arizona Press, 290 p.

 Webb, R.H., Griffiths, P.G., Melis, T.S., and Hartley, 

D.R., 2000, Sediment delivery by ungaged tributaries of the Colorado River in Grand Canyon, Arizona: 

U.S. Geological Survey Water Resources Investigation 

Report 00-4055, 67 p.

Webb, R.H., Melis, T.S., and Valdez, R.A., 2002, Observations of environmental change in Grand Canyon: 

U.S. Geological Survey Water Resources Investigation 

Report 02-4080, 33 p.
Influence of Glen Canyon Dam Operations on Downstream Sand Resources 31

Contact Information:

Scott A. Wright

Hydrologist

U.S. Department of the Interior

U.S. Geological Survey

Southwest Biological Science Center

Flagstaff, AZ

sawright@usgs.gov

Theodore S. Melis

Physical Scientist

U.S. Department of the Interior

U.S. Geological Survey

Southwest Biological Science Center

Flagstaff, AZ

tmelis@usgs.gov

David J. Topping

Research Hydrologist

U.S. Department of the Interior

U.S. Geological Survey

Water Resources Discipline, National Research Program 

Denver, CO

dtopping@usgs.gov

David M. Rubin

Geologist

U.S. Department of the Interior

U.S. Geological Survey

Pacific Science Center

Santa Cruz, CA

drubin@usgs.gov

Jeff Sorensen, Arizona Game and Fish Department

First page photograph credit: Jeff Sorensen, Arizona Game and Fish Department

Chapter 2

Fishes of 

Grand Canyon

Steven P. Gloss

Lewis G. Coggins

Introduction

Fishes of the Colorado River vary from coldwater 

trout species found in the river’s mountainous headwaters to uniquely adapted desert river species found at 

lower elevations. Within the study area, the Colorado 

River corridor between Glen Canyon Dam and the western boundary of Grand Canyon National Park (hereafter 

Grand Canyon), the Colorado River was a seasonally 

warm and turbid river characterized by large seasonal 

variations in flow before it was altered by the closure of 

Glen Canyon Dam in 1963 (Topping and others, 2003). 

Although water temperatures fluctuated between 32°F 

(0°C) during winter to a high approaching 86°F (30°C) 

during late summer, several warmwater native fish species successfully inhabited this stretch of the river (Cole 

and Kubly, 1976). Because of the harsh environment 

created by dramatic seasonal fluctuations in the river’s 

predam flow and temperature, only 8 of the 32 species 

of native fish historically found in the Colorado River 

were common in the Grand Canyon reach of the river. 

Other native fishes within the study area were restricted 

to small tributary streams or occurred only in transient 

or seasonal numbers. Of the eight fish species that were 

originally common to the study area, only four species 

are known to persist today.

The number of species that made up the original 

fish community of the Colorado River was altered well 

before the construction of mainstem dams because 

of the introduction of nonnative fishes by early European settlers. Nonnative fishes, from sport fishes to 

escapees from aquaria, have been intentionally and 

inadvertently stocked in the Colorado River for more 

than 100 yr (Mueller and Marsh, 2002). Today, nonnative fishes originating in many parts of the world 

are found in the Colorado River. Table 1 contains a 

list of the native and nonnative fishes of the Colorado 

River in Grand Canyon. 

This chapter examines the status, trends, and recent 

condition of Grand Canyon fishes, focusing particular 

attention on the endangered humpback chub (Gila cypha)

because of its prominence within the Glen Canyon 

Dam Adaptive Management Program (see Overview, 

this report). The chapter begins with a discussion of the 

conditions that led to the development of the Grand 

Canyon’s unique native fish populations and then moves 

on to the reasons for their decline. The effects of the 

modified low fluctuating flow (MLFF) alternative on fish 
34 The State of the Colorado River Ecosystem in Grand Canyon

Table 1. Historical and present relative abundance of fish species in the Colorado River from Glen Canyon to Separation 

Canyon. P = present, abundance unknown; A = abundant; C = common; LC = locally common; R = rare; and - = not encountered. 

[Modified from Valdez and Ryel, 1995. Species that are federally listed as endangered are indicated by an asterisk (*). Species that are endemic 

to the lower basin of the Colorado River but occurred almost exclusively in smaller streams or rivers tributary to the mainstem Colorado River 

are indicated by a plus sign (+)]

 Species Pre-1850 1958–59 1970–73 1984–86 1990–93

Family: Clupeidae, shads (introduced)

 Threadfin shad - - R - C 

Family: Cyprinidae, minnows 

 Native 

 *Humpback chub P - R R LC

 *Bonytail chub P - - - -

 Roundtail chub P R - - -

 *Colorado pikeminnow P R - - -

 Speckled dace P A A A C

 Virgin spinedace+ P - R - -

 Woundfin+ P - - - -

 Introduced 

 Red shiner - - R - A

 Common carp - C A A A

 Utah chub - R - R -

 Golden shiner - - R R R

 Fathead minnow - A C A LC

Family: Catostomidae, suckers (all native) 

 Bluehead sucker P C C C C

 Flannelmouth sucker P C C C C

 *Razorback sucker P R - R -

Family: Ictaluridae, bullhead catfishes (all introduced) 

 Black bullhead - C - R R

 Yellow bullhead - - - R -

 Channel catfish - A C R LC

Family: Salmonidae, salmon and trout (all introduced) 

 Cutthroat trout - - - R -

 Coho salmon - - R - -

 Rainbow trout - - C A A

 Brown trout - - - C C

 Brook trout - - - C R

Family: Cyprinodontidae, killifishes (introduced) 

 Plains killifish - R C R LC

Family: Poeciliidae, livebearers (introduced) 

 Mosquitofish - R R - LC

Family: Percichthyidae, temperate basses (introduced) 

 Striped bass - - - R R

Family: Centrarchidae, sunfishes (all introduced) 

 Green sunfish - C R R R

 Bluegill - R R - R

 Largemouth bass - R R R R

 Black crappie - - - - R

Family: Percidae, perch (all introduced)

 Yellow perch - R - - -

 Walleye - - - - R

Total number of species 10 17 18 20 22
Fishes of Grand Canyon 35

populations are also examined. The chapter concludes 

with a discussion of possible management options to 

slow or reverse the decline of humpback chub numbers.

Background

The Colorado River was one of the last areas of the 

continental United States to be explored by Europeans; it 

was first traversed during the expedition headed by John 

Wesley Powell in 1869. For this reason, it is not surprising that scientific descriptions of many of the organisms 

in the Colorado River corridor, especially the fishes, 

did not begin until the 1930s and 1940s; earlier expeditions collected and described fishes generally rather 

than specifically. Emery and Ellsworth Kolb, explorers 

and photographers of the Colorado River in the early 

1900s, reported that fishes were very abundant (Kolb 

and Kolb, 1914). The humpback chub was the last of 

the native fishes in Grand Canyon to be described in 

1946 by Robert R. Miller from specimens taken from the 

Colorado River in Grand Canyon (Miller, 1946).

Scientific description of the native fishes of Grand 

Canyon showed that these species were unique in at least 

two ways. Most noticeably, several of the species share 

unusual body shapes, including large adult body size, 

small depressed skulls, large predorsal humps or keels, 

and small eyes, which presumably developed as adaptations to life in a large, turbid, and seasonably variable 

riverine environment. These features are perhaps best 

observed in the razorback sucker (Xyrauchen texanus) and 

the humpback chub (see accompanying text box, p. 51). 

A second, and perhaps more important, measure 

of the uniqueness of Grand Canyon native fishes is that 

most of these species are not found elsewhere in the 

world. Organisms that are native to a certain location 

and do not occur anywhere else are called endemic species. Of the eight native species common to the Grand 

Canyon, six are species endemic to the Colorado River 

Basin. As early as 1895, scientists recognized the special 

nature of Colorado River fishes and the high rates of 

endemism (Minckley, 1991). Later research did not alter 

this conclusion, and despite a relatively low number 

of species compared to other drainages in the United 

States, the Colorado River Basin has a recognized endemism at the species level of approximately 75% and supports the most distinctive ichthyofauna in North America 

(Minckley, 1991).

Before European settlement, the native fishes found 

in the Grand Canyon portion of the Colorado River 

were exclusively minnows and suckers. The biggest of 

these fish was the Colorado pikeminnow (Ptychocheilus 

lucius), which is also the largest of all native minnow 

(cyprinid) species in North America and was found 

only in the Colorado River Basin (fig. 1). Called a white 

salmon by early settlers, the Colorado pikeminnow 

reached up to 6 ft (2 m) in length and had a weight of up 

to 80 lb (36 kg) (Mueller and Marsh, 2002). 

Today, three of the eight native fish species have 

been eliminated from the Colorado River in Glen and 

Grand Canyons (roundtail chub (Gila robusta), bonytail 

chub (Gila elegans), and Colorado pikeminnow), and two 

are federally listed as endangered (humpback chub and 

razorback sucker) under the Endangered Species Act. 

Although listed as an endangered species with designated 

critical habitat in Grand Canyon, the razorback sucker 

has rarely been collected (Minckley, 1991; Valdez and 

Carothers, 1998) and is widely thought to no longer be 

found in Grand Canyon. The status of the flannelmouth 

sucker (Catostomus latipinnis) is common, and the species 

persists in the study area and throughout much of the 

upper Colorado River Basin. The remaining two fish 

(bluehead sucker (Catostomus discobolus) and speckled dace 

(Rhinichthys osculus)) are relatively common. Brief descriptions of the life histories of all the Grand Canyon native 

fishes can be found in Minckley (1991); this chapter 

provides text boxes (see p. 50) with summary information 

for the four native fishes that continue to inhabit Grand 

Figure 1. Historical photograph (date unknown) of someone 

identified as James Fagen holding a large Colorado pikeminnow in 

lower Granite Gorge (courtesy of the Kolb Collection, Cline Library, 

Northern Arizona University, NAU.PH.568.5737).
36 The State of the Colorado River Ecosystem in Grand Canyon

Canyon, as well as for the two most common nonnative 

species, rainbow trout (Oncorhynchus mykiss) and brown 

trout (Salmo trutta).

Decline of Native Fish

Introductions of Nonnative Fishes

There are a number of reasons for the decline of 

native fishes, including the potential effects of nonnative fish species. Nonnative fish have been found in the 

Colorado River since the 1800s (Minckley, 1991). These 

species are potential predators of and competitors with 

native fish and include common carp (Cyprinus carpio),

fathead minnow (Pimephales promelas), plains killifish 

(Fundulus zebrinus), rainbow trout, brown trout, red shiner 

(Cyprinella lutrensis), and channel catfish (Ictalurus punctatus).

Nonnative species may share rearing habitats used by 

native fish, habitats which include complex shorelines, 

tributaries, backwater areas, and eddies. The presence 

of warmwater, coolwater, and coldwater nonnative fish 

species in the Colorado River is an issue of considerable importance (U.S. Department of the Interior, 

1995) because there are now nonnative fishes that may 

negatively interact with native fishes under virtually any 

temperature regime and in any habitat of the river. 

Today, the Colorado River has nearly twice as many 

nonnative species (60) as native species (32); in the Grand 

Canyon reach of the river the situation is even more 

extreme, where the ratio of native to nonnative species is more than 4 to 1 (Valdez and Carothers, 1998). 

The introduction of nonnative species to the Colorado 

River, both intentionally and unintentionally, was well 

underway before 1900. As such, the ratio of nonnative 

to native fishes was high in Grand Canyon before the 

construction of Glen Canyon Dam. For example, the 

National Park Service introduced both brown trout and 

rainbow trout to tributaries like Bright Angel Creek in 

the 1920s to provide sport fishing opportunities (Valdez 

and Carothers, 1998). Because of the continuous nature 

of the river and its tributaries before dam building, species introduced almost anywhere in the basin had the 

potential to find their way to the Grand Canyon portion of the river, and many did. Before Glen Canyon 

Dam, the Grand Canyon reach was dominated by a 

single introduced species, the channel catfish (Valdez 

and Carothers, 1998). Following construction of the dam 

in 1963, Federal and State agencies again introduced 

rainbow trout below Glen Canyon Dam to establish and 

maintain a sport fishery in the 15-RM reach between the 

dam and Lees Ferry. This stocking continued for more 

than 30 yr, until the mid-1990s. Numerous other species of nonnative fishes were also introduced into Lake 

Powell and Lake Mead to create or enhance recreational 

fishing (Mueller and Marsh, 2002). 

The effects of nonnative fish on native species, 

including predation and competition, are important 

considerations when evaluating any management action 

intended to benefit native fishes. These considerations 

are particularly important given the proximity of Lake 

Powell and Lake Mead, reservoirs with diverse nonnative fish populations, to Grand Canyon. Any management action intended to improve habitat conditions for 

native warmwater fishes also runs the risk of providing 

additional habitat that is suitable for nonnative predators 

and competitors. Nonnative fish predators currently in 

the Grand Canyon reach of the Colorado River include 

striped bass (Morone saxatilis), channel catfish, largemouth 

bass (Micropterus salmoides), green sunfish (Lepomis cyanellus),

brown trout, and rainbow trout. Currently, nonnative 

coldwater species (trout) are abundant, while the nonnative warmwater species exist in relatively low numbers.

Glen Canyon Dam Effects

The predam success of nonnative species was, in 

part, due to the fact that the river was generally what 

fishery biologists term a “warmwater habitat.” The 

annual temperature cycle of the Colorado River through 

Grand Canyon was similar to temperate lakes and 

streams at lower elevations, where temperatures ranged 

from cold or cool in winter to warm in summer. Native 

species require warmer temperatures to spawn and 

reproduce successfully. This seasonal pattern also allowed 

many of the introduced species to complete their life 

cycle. One of the major impacts of Glen Canyon Dam 

on the Colorado River was the change in water temperature to a relatively cold, steady temperature that favored 

coldwater species like trout over native fishes and introduced, warmwater species. While most of the warmwater species can survive in these colder waters, they cannot 

reproduce and do not grow well, having been adapted to 

at least seasonally warmer temperatures. 

Other possible effects of dam operations on the 

riverine environment that may affect fishes include 

increased water clarity, altered flow patterns, and 

reduced sediment. All species that are native to Grand 

Canyon evolved in highly turbid environments, so the 

clear water released from the dam may favor nonnative 
Fishes of Grand Canyon 37

predators like trout, which are adapted to hunting in 

clear water (Valdez and Ryel, 1995). Similarly, the postdam river hydrology is different from the predam river 

with respect to daily flow variation, flood frequency, and 

seasonal pattern and magnitude of maximum and minimum flows (Topping and others, 2003). These alterations 

in flow patterns potentially affect the spawning cues, 

habitat use, and distribution of native fish, as well as the 

suitability of mainstem Colorado River rearing habitat, 

in ways that are largely unknown and potentially complex (Korman and others, 2004). Finally, as Glen Canyon 

Dam blocks the majority of sediment transported by the 

Colorado River to the upstream portions of Lake Powell, 

the nearshore physical habitat available to native fish is 

fundamentally different from the predam river (Goeking 

and others, 2003; also see chapter 1, this report). Except 

for temperature, the other potential effects of the dam 

that are mentioned here are based on inferences about 

what is known regarding fishes from other river systems. 

Little direct scientific evidence from the Colorado River 

itself exists regarding these effects, and there remains 

considerable uncertainty regarding the potential effects 

of management actions associated with these factors 

(Walters and others, 2000). 

Other Factors 

New fish parasites in the system, changes in tributary hydrology, and alterations in the food base that 

support fish populations are additional environmental 

factors that may be affecting native and nonnative fish 

species in Grand Canyon. Asian tapeworm (Bothriocephalus acheilognathi), a parasitic cestode, is a prominent example of a recently introduced parasite. Introduced into 

the United States in the 1970s with imported grass carp 

(Ctenopharyngodon idella) from China, the Asian tapeworm 

was discovered in 1990 in the Little Colorado River, 

which is an important spawning area for humpback chub 

(Choudhury and others, 2004). The tapeworm can cause 

mortality, but most often it is responsible for reduced 

growth and poor condition of infected fish. This parasite is currently restricted to the Little Colorado River 

because cold mainstem temperatures preclude completion of its life cycle. The Little Colorado River is also 

an example of a tributary system in which upstream 

water use and development have changed the amount 

and timing of flows reaching the Colorado River. 

Such changes could affect fishes in the Little Colorado 

River and throughout Grand Canyon, especially below 

the tributary.

Status and Trends 

Until the 1990s, there were few attempts to monitor 

the status and trends of fishes in Grand Canyon. Information before the mid- to late-1980s was anecdotal and 

was provided by explorers, river runners, and occasional 

scientific expeditions. As a result, few data are available 

for the first 20 yr after Glen Canyon Dam was closed. 

Early fish collection efforts were reviewed by Valdez and 

Carothers (1998), and where appropriate these earlier 

data are used in comparison to current data for fishes in 

Grand Canyon. 

Efforts to estimate population size or relative abundance of fishes in Grand Canyon began under Glen 

Canyon Environmental Studies Phase II when private 

consulting firms, university researchers, the U.S. Fish 

and Wildlife Service (USFWS), and the Arizona Game 

and Fish Department conducted surveys and undertook population estimates in the mainstem Colorado 

River and in the Little Colorado River. Beginning in 

1997, these efforts became the responsibility of the U.S. 

Geological Survey’s (USGS) Grand Canyon Monitoring 

and Research Center, which has worked cooperatively 

on monitoring activities with the U.S. Fish and Wildlife 

Service, the Arizona Game and Fish Department, and 

consulting firms (SWCA Environmental Consultants, 

Inc., and Ecometric Research). For the purposes of 

monitoring, the study area is divided into three segments: the Lees Ferry reach (15 RM of Colorado River 

corridor from Glen Canyon Dam to Lees Ferry); the 

mainstem Colorado River (downstream of Lees Ferry, 

RM 0, and the Paria River to RM 226 at the confluence of Diamond Creek); and the Little Colorado River 

(the 8.7 mi (14 km) of the tributary upstream from the 

mainstem). The status and trends of fish found in each 

of these reaches will be discussed separately. Humpback 

chub are discussed in a separate section. 

Lees Ferry

The Lees Ferry reach of the river is managed primarily as a rainbow trout sport fishery. The Lees Ferry 

reach is known as a tailwater trout fishery because it 

occurs downstream from a large dam where deepwater 

discharges afford cooler water temperatures that allow 

coldwater species like trout to survive. In fact, trout not 

only survived in the Lees Ferry reach following their 

initial stocking in 1964 but also flourished in the new 

habitat created by Glen Canyon Dam. The Lees Ferry 
38 The State of the Colorado River Ecosystem in Grand Canyon

rainbow trout fishery gained a reputation by the mid1970s as a world class, blue ribbon fishery famous for its 

scenic grandeur and large, trophy-sized trout. Monitoring in this reach is primarily done through electrofishing and surveys of anglers by the Arizona Game and 

Fish Department in cooperation with the USGS Grand 

Canyon Monitoring and Research Center. The fishery 

was initiated with stocking efforts and was maintained 

primarily by stocking until the late 1990s.1

 Since closure 

of Glen Canyon Dam in 1963, however, this fishery has 

experienced variable success rates by anglers, and the 

trout populations have changed in response to stocking, 

dam releases, and food availability (McKinney and others, 1999, 2001).

Recently, more stable river flows, which are the 

result of the interim flows in 1991 and subsequent 

implementation of the MLFF alternative in 1996, have 

encouraged natural reproduction and made stocking 

unnecessary. Stable flows and increased natural reproduction resulted in an expanding number of fish (fig. 

2), but the larger number of fish was offset by smaller 

average size and decreasing condition (plumpness) of the 

fish (fig. 3). Because the overall carrying capacity of the 

river remains relatively constant, the Lees Ferry reach 

is able to produce a smaller number of large fish or a 

greater number of small fish, a principle that is known as 

conservation of biomass. As early as 1996, the Arizona 

Game and Fish Department recognized the declining 

size of trout in this fishery and recommended changes in 

angling regulations to increase the size of fish; however, 

anglers appeared unwilling to accept lower catch rates 

of larger fish (Niccum and others, 1998). Average fish 

condition continued to decline for several more years but 

finally rebounded in 2002 (fig. 3). 

As part of the Glen Canyon Dam Adaptive 

Management Program, fluctuating nonnative fish suppression flows were initiated beginning in 2003 and 

continued through 2005 in an effort to reduce the 

number of trout and increase their average size. The 

experimental flow treatment involved increased diurnal flow fluctuations of 5,000 to 20,000 cubic feet per 

second (cfs) from January through March of each year. 

Overall, these fluctuating flows were intended to disrupt 

spawning activity, to reduce egg survival, and to disadvantage young-of-year (YOY) trout that did survive. Early 

indications suggest that these experimental flows have 

had only minimal effects on the recruitment dynamics of 

rainbow trout. The total egg deposition loss because of 

Glen Canyon Dam operations in 2003 ranged from 30% 

to 40% in the Lees Ferry reach, with about half of this 

mortality being a direct consequence of the enhanced 

fluctuating flows in January through March (Korman 

and others, 2005); however, electrofishing catch rates 

began to increase in 2003 (fig. 2). There also appears to 

be a corresponding increase in angler use associated with 

1

 Stocking of fingerling rainbow trout was reduced in the mid1990s to about 20,000 fish per year and ended completely in 1999 

when it was apparent that natural reproduction under the modified 

low fluctuating flow alternative was producing more than enough 

recruitment to sustain the fishery (William R. Persons, Arizona Game 

and Fish Department, oral commun., 2005).

Figure 2. The average number of rainbow trout caught by using 

electrofishing at several fixed sampling locations in the Lees Ferry 

reach of the Colorado River from 1991 to 2003. Increasing catchper-unit effort is thought to be indicative of an increasing number 

of fish in the population (Arizona Game and Fish Department and 

U.S. Geological Survey, unpub. data, 2005).

Figure 3. Condition factor, or relative weight, of Lees Ferry 

trout from 1991 to 2003. Condition factor expresses the lengthto-weight relationship and is an attribute that reflects the health 

of individual fish as well as affects angler satisfaction. Relative 

weight declined with the increase in fish density in the late 1990s 

but increased in 2002–03. Present condition seems acceptable to 

anglers (Arizona Game and Fish Department and U.S. Geological 

Survey, unpub. data, 2005).
Fishes of Grand Canyon 39

the increased electrofishing catch rate and the implementation of fluctuating flows (see chapter 9, this report).

Otoliths (minute boney structures found in the inner 

ear) of young rainbow trout (fig. 4) were examined in 

2003 and 2004 to infer growth rate patterns during the 

late spring and summer months following the end of 

fluctuating nonnative fish suppression flows. Microscopic 

examination of these bony structures allows researchers to determine daily growth patterns. Results of these 

examinations suggest that YOY rainbow trout experienced more growth on Sundays than on other days of 

the week in 2003; however, otoliths collected in 2004 

do not display increased growth on Sundays. Korman 

and others (2005) hypothesized that this difference was 

related to less severe flow fluctuations on Sundays during 

2003 as compared to 2004. 

Mainstem Colorado River

Management objectives of the Glen Canyon Dam 

Adaptive Management Program call for managing the 

mainstem Colorado River and its tributaries below the 

Paria River for the benefit of native fishes (GCDAMP, 

2001, http://www.usbr.gov/uc/rm/amp/amwg/mtgs/

02jan17/Attach_06.pdf, accessed July 14, 2005). Fish 

monitoring in the mainstem Colorado River is primarily 

conducted by electrofishing or with trammel nets, hoop 

nets, and beach seines. Each of these methods is “selective,” or has higher efficiency for particular species or 

fish sizes. For instance, electrofishing is very effective in 

catching rainbow and brown trout and common carp 

but is inefficient in capturing adult humpback chub. 

Alternatively, trammel and hoop nets are more efficient 

than electrofishing in capturing humpback chub. These 

differences in sampling gear efficiency, coupled with 

differences in abundance, influence the ability of the 

monitoring program to detect differences in abundance 

over time and space.

The current monitoring program, which uses electrofishing for rainbow trout, brown trout, and common 

carp, is able to show trends in the abundance of these 

species over time and space (fig. 5 a, b, c). The abundance of rainbow trout declines as a function of distance 

Figure 4. Photomicrograph of an otolith cross-section of young-of-year rainbow trout sampled from Glen Canyon in April 2003. Otoliths 

are minute boney structures found in the inner ear that show daily growth patterns in many fishes. The image shows the weekly striping

pattern (identified by white arrows and shown at magnifications of 16x (A) and 400x (B)) caused by increased growth during lower peak 

Sunday flows (8,000 cfs) during April 2003 when normal weekday operations ranged from 7,000–13,000 cfs on a 24-h cycle (photographs

courtesy of Steven Campana, Bedford Institute of Oceanography, Canada).

A. B.
40 The State of the Colorado River Ecosystem in Grand Canyon

downstream of Glen Canyon Dam, but common carp 

increase downstream. Brown trout abundance is centered near RM 88 and declines with distance upstream 

or downstream of this location. This pattern is explained 

most readily by the occurrence of several tributaries in 

this reach that are suitable for spawning by this species.

Monitoring efforts in the mainstem Colorado River 

for both native and nonnative species have generally 

resulted in an adequate description of species distribution. In general, humpback chub distribution is centered 

near the Little Colorado River where successful spawning and rearing is known to occur (Douglas and Marsh, 

1996; Gorman and Stone, 1999). Also, humpback chub 

occur in several other smaller aggregations throughout 

the river corridor (see below). Flannelmouth sucker, 

bluehead sucker, and speckled dace abundance typically increases with distance downstream of the Little 

Colorado River and is generally high near major tributary confluences (e.g., Little Colorado River, Paria River, 

Kanab Creek, and Bright Angel Creek) (Gorman and 

Coggins, 2000; Johnstone and others, 2003; Johnstone 

and Lauretta, 2004). Warmwater nonnative species such 

as channel catfish and striped bass increase in abundance 

with distance from Glen Canyon Dam, particularly 

below RM 160. Small-bodied, nonnative fish such as 

fathead minnow, red shiner, and plains killifish are found 

almost exclusively downstream of the Little Colorado 

River confluence, and all evidence suggests that this 

tributary is the dominant source of these fishes in the 

Colorado River ecosystem (Johnstone and others, 2003; 

Johnstone and Lauretta, 2004).

Although the current monitoring program is sufficient to describe these general patterns in distribution of 

native and certain nonnative fishes, it cannot provide a 

specific measure of trends in relative abundance. Despite 

sampling efforts that are randomly distributed over the 

226 mi (364 km) of river from Lees Ferry to Diamond 

Creek, the monitoring program is unable to measure 

with any certainty the spatial or temporal trends in the 

relative abundance of native or nonnative fishes in the 

mainstem Colorado River. An exception is the abundance and distribution of rainbow trout, brown trout, 

common carp, and the Little Colorado River population 

of humpback chub previously discussed. Low abundance 

of these fishes coupled with the very poor sampling 

efficiency of current sampling gear make measuring 

trends in relative abundance difficult. Typically, monitoring efforts include over 600 trammel net sets each year 

and between 100 and 200 seining sites. Several examples 

of the low and highly variable catch rate experienced 

with trammel nets are illustrated for select species and 

sites in figure 6.

Figure 5. Relative abundance (mean catch-per-unit efforts, or 

fish/hour) of rainbow trout (A), brown trout (B), and common carp 

(C) as indicated by electrofishing catch rates from Lees Ferry (RM 

0) to Diamond Creek (RM 226) (Arizona Game and Fish Department 

and U.S. Geological Survey, unpub. data, 2005). Note inverse 

abundance of coldwater trout to warmwater carp as distance 

from Lees Ferry and Diamond Creek increases. Increase in brown 

trout abundance in the middle of Grand Canyon is thought to 

be caused by spawning, which occurs in Bright Angel Creek, a 

tributary at RM 88. The National Park Service is trying to reduce 

spawning in Bright Angel Creek. 

A.

B.

C.
Fishes of Grand Canyon 41 Figure 6. Trends in the relative abundance (trammel net catch rate, fish/hour) of selected species near the confluences of several tributaries where native fishes, particularly the suckers, attempt to reproduce. These figures illustrate the inability of the current monitoring program to detect all but extremely large changes in the relative abundance of key native and nonnative species in most areas of the Colorado River. Error bars depict 95% confidence intervals for mean catch rate. Note that catch rate estimates with overlapping confidence intervals are statistically insignificant and represent years of no statistically apparent difference in relative abundance (U.S. Geological Survey, unpub. data, 2005). 
42 The State of the Colorado River Ecosystem in Grand Canyon

The presence of many nonnative fish in the system 

has created a substantial management challenge. It is 

known that some of these nonnative species, particularly brown trout, prey upon native fishes (Valdez and 

Carothers, 1998). Furthermore, nonnative species may 

compete for habitat and food with native species in ways 

that are difficult to document. Monitoring the relative 

abundance of nonnative fish in this part of the river 

provides some insight into the potential severity of the 

problem. Both coldwater nonnative species such as trout 

and warmwater fishes such as carp inhabit the river. 

Coldwater species dominate the upstream reaches of 

Grand Canyon, whereas warmwater species are more 

prominent further downstream because the temperature of the river water gradually increases after leaving 

the dam. 

Little Colorado River

The Little Colorado River, which flows into the 

Colorado River at RM 61, represents perhaps the best 

remaining native fish habitat in Grand Canyon under 

the current temperature and flow management regimes 

in the Colorado River. Because native fish are abundant 

and the sampling gear is efficient in the Little Colorado 

River, relative abundance of native fish and some nonnative fish can be well determined in this tributary. 

Two kinds of fish sampling are conducted in the Little 

Colorado River: spring and fall hoop netting aimed primarily at collecting humpback chub to estimate population size and hoop netting conducted in April and May 

at fixed sites in the lower 0.75 mi (1,200 m) of the river. 

The humpback chub data are discussed separately below. 

Despite the presence of several nonnative fishes in the 

Little Colorado River, the catch in hoop nets suggests 

that native fish (>80%) dominate the fish community 

in most years (fig. 7). The data from the lower 0.75 mi 

(1,200 m) sampling depict trend information for the relative abundance of three native species: humpback chub 

(fig. 8), bluehead sucker (fig. 9), and flannelmouth sucker 

(fig. 10). These data represent the best time series regarding status and trends of flannelmouth and bluehead 

suckers in the Little Colorado River. 

Humpback Chub

The life history and ecology of humpback chub in 

Grand Canyon have been intensively studied (Suttkus 

and Clemmer, 1979; Carothers and Minckley, 1981; 

Kaeding and Zimmerman, 1983; Maddux and others, 

1987; Gorman, 1994; Valdez and Ryel, 1995; Valdez 

and Carothers, 1998). The humpback chub population 

in Grand Canyon is centered near the confluence of 

the Colorado and Little Colorado Rivers (Kaeding and 

Zimmerman, 1983; Douglas and Marsh, 1996; Gorman 

and Stone, 1999). Valdez and Ryel (1995) defined the 

humpback chub distribution as occurring in nine aggregations throughout Glen and Grand Canyons. Only the 

aggregation near the confluence of the Little Colorado 

and Colorado Rivers (hereafter referred to as the LCR 

population) is known to successfully reproduce. The 

other eight aggregations are much smaller in abundance, 

averaging from a few dozen to a few hundred fish. Most 

likely these eight aggregations are not supported from 

local reproduction but primarily from the emigration 

of juveniles and limited numbers of subadult and adult 

fish from the LCR population (Valdez and Ryel, 1995). 

Additionally, because of abiotic and biotic changes in 

the Colorado River following the construction of Glen 

Canyon Dam, the LCR population relies on the Little 

Colorado River as the primary spawning and juvenilerearing habitat (Gorman and Stone, 1999). 

Reproduction and Early Life History

Adult fish in the LCR population initially stage for 

spawning runs in large eddies near the confluence of 

the Little Colorado River in February and March and 

make spawning runs into the tributary that average 17 d 

from March through May. As the Little Colorado River’s 

spring flows decrease and the water warms and clears, 

reproduction increases and larval fish appear (Valdez and 

Ryel, 1995). Spawning has not been observed, primarily 

because of the turbid water, but ripe males have been 

seen gathering in areas of complex habitat structure 

(boulders and travertine masses near gravel deposits); it 

is thought that ripe females move to these areas to spawn 

(Gorman and Stone, 1999). After spawning, some adult 

chub return to specific locations in the mainstem, while 

others remain in the Little Colorado River for unknown 

periods of time.

Humpback chub require warm water to reproduce 

successfully. Perennially cold mainstem water temperatures are thought to be the reason for unsuccessful mainstem reproduction. The minimum water temperature for 

successful reproduction is 61oF (16oC) (Hamman, 1982; 

Marsh, 1985), which is well above the summer mainstem 

temperatures commonly observed of 50°F–54oF (10°C–

12oC). Mortality of larval and postlarval humpback chub 

emerging from the warm waters of the Little Colorado 

River has been attributed to thermal shock and their 

enhanced susceptibility to predation caused by the more 

protracted debilitating effects of cold water on swim-
Fishes of Grand Canyon 43

Figure 7. Observed species composition of all fish captured in hoop nets in the Little Colorado River, 1988–2004 (U.S. Fish and Wildlife

Service, Arizona Game and Fish Department, Arizona State University, and U.S. Geological Survey, unpub. data, 2005). The top panel

(A) includes species composition of the four native species and a pooled nonnative category. The bottom panel (B) displays the annual

species composition of the nonnative catch. Dominant species of minnows include fathead minnow, red shiner, and common carp. 

Dominant species of catfishes include channel catfish and black and yellow bullheads.

A.

B.
44 The State of the Colorado River Ecosystem in Grand Canyon

ming ability and growth (Lupher and Clarkson, 1994; 

Clarkson and Childs, 2000; Robinson and Childs, 2001; 

Ward and others, 2002).

A key issue associated with humpback chub is lack 

of recruitment to the adult population because of the 

low survivorship of young fish (Valdez and Ryel, 1995). 

Young humpback chub remain in the Little Colorado 

River or drift and swim into the mainstem (Robinson 

and others, 1998). The lack of recruitment and documented predation indicate that mortality is extremely 

high in the mainstem (Lupher and Clarkson, 1994; 

Valdez and Ryel, 1995; Marsh and Douglas, 1997; 

Clarkson and Childs, 2000; Robinson and Childs, 2001). 

During summer, the young humpback chub that survive 

in the mainstem occupy low-velocity, talus, and vegetated 

shoreline habitats, including backwaters; however, low 

survivorship over the year virtually eliminates the YOY 

humpback chub in the mainstem. As a result, few if any 

humpback chub spawned during the previous year are 

present in the mainstem in March. Those YOY humpback chub that do survive, and ultimately recruit to the 

adult population, are fish that remain resident in the 

Little Colorado River during their early life history.

Limited breeding of humpback chub occurs among 

other subpopulations in the Colorado River. Valdez 

and Ryel (1995) documented limited spawning success at a warm underwater spring near RM 30, known 

locally as 30-Mile Spring, in upper Marble Canyon. 

YOY humpback chub in the size range of 0.4–1.2 

inches (10–30 mm) have been sporadically collected at 

considerable distances below the Little Colorado River, 

usually beginning in June (Kubly, 1990; Arizona Game 

and Fish Department, 1996; Brouder and others, 1997). 

Some limited reproduction may occur in other smaller 

tributaries. Young humpback chub have been collected 

in or near Bright Angel Creek, Shinumo Creek, Kanab 

Creek, and Havasu Creek, but spawning success has 

not been well documented (Maddux and others, 1987; 

Kubly, 1990; Arizona Game and Fish Department, 1996; 

Brouder and others, 1997). These limited observations of 

spawning success among subpopulations outside of the 

Little Colorado have not been shown to lead to successful 

recruitment, likely because of the factors mentioned above.

Food Habits and Diseases

Dietary analyses reveal humpback chub to be 

opportunistic feeders, selectively feeding on algae, 

aquatic and terrestrial invertebrates, and small fish 

(Kaeding and Zimmerman, 1983; Kubly, 1990; Valdez 

and Ryel, 1995; Stone, 2004). Humpback chub diet 

changes over the course of the year in response to food 

Figure 8. Humpback chub catch-per-unit effort (fish/hour) with 

95% confidence intervals in the lower 0.75 mi (1,200 m) of the 

Little Colorado River using hoop nets, 1987–2003 (no sampling 

conducted 2000–01). Solid squares are for fish between 5.9 and 

7.8 inches (151–199 mm) total length (TL) and open diamonds are 

for fish more than 7.9 inches (200 mm) total length (modified from 

Coggins and others, in press).

Figure 9. Hoop net catch (fish/hour) of adult bluehead sucker 

more than 7.5 inches (190 mm) in total length in the lower 0.75 

mi (1,200 m) of the Little Colorado River (Arizona Game and Fish 

Department and U.S. Geological Survey, unpub. data, 2005).

Figure 10. Hoop net catch (fish/hour) of adult flannelmouth 

sucker more than 13.8 inches (350 mm) in total length in the lower 

0.75 mi (1,200 m) of the Little Colorado River, 1987–2004 (Arizona 

Game and Fish Department and U.S. Geological Survey, unpub. 

data, 2005).
Fishes of Grand Canyon 45

availability and turbidity-related decreases in benthicstanding biomass over distance downstream from Glen 

Canyon Dam (Blinn and others, 1992). Nonnative scuds 

(Gammarus lacustris) and simuliid (black fly) larvae occasionally make up a large proportion of humpback chub 

diet. Gammarus lacustris selectively feeds on epiphytes (i.e., 

diatoms) associated with Cladophora glomerata, the dominant algae species in the upper reaches where clear water 

conditions often prevail. Chironomid (midge fly) larvae 

are also important in all areas of the river. As the river 

becomes more turbid downstream, simuliids become the 

dominant food source (see chapter 5, this report).

Kaeding and Zimmerman (1983) identified 13 

species of bacteria, 6 protozoans, and 1 fungus from 

humpback chub in Grand Canyon. The role of these 

organisms in the life history of humpback chub is not 

known. In 1990, the Asian tapeworm was first identified from humpback chub in the Little Colorado River 

(Clarkson and others, 1997; Choudhury and others, 

2004). This cestode is particularly worrisome because 

it infects humpback chub at a high rate and has been 

reported to be pathogenic and potentially fatal in a 

variety of other fish (Hoffman and Schubert, 1984; 

Hoffnagle and others, 2000). 

Population Dynamics

Very large numbers of humpback chub, as well as 

of flannelmouth sucker and bluehead sucker, have been 

tagged in Grand Canyon since 1989. As a result, today 

most of the older humpback chub have been tagged. 

Previous analyses of the recapture data of tagged fish 

indicate that there is likely strong age-dependence in 

survival rates and that recruitment of humpback chub 

has likely declined considerably since the early 1990s 

(Coggins and others, in press). The USGS Grand 

Canyon Monitoring and Research Center uses an 

analysis method for the mark and recapture data that 

reinforces these results and allows recovery of information about likely recruitment changes that date back 

to the early 1980s. The mark and recapture data are 

analyzed by assigning each marked fish an age at first 

capture based on its size at that time and then performing mark-recapture analysis on the resulting age-structured data on first captures and later recaptures (Coggins 

and others, in press). Results of this open population 

mark-recapture model, known as age-structured mark 

recapture (ASMR), show decreases in the recruitment 

of young humpback chub into the adult population and 

as a consequence an overall decline in numbers of adult 

humpback chub (figs. 11 and 12).

Figure 11. Age-2 humpback chub recruitment estimated by 

using the three formulations of the annual age-structured mark 

recapture (ASMR) model (from Coggins and others, in press). 

Figure 12. Adult (age-4+) humpback chub population estimates 

for 1989–2001 made by using the age-structured Jolly-Seber 

model and the three formulations of the annual age-structured 

mark recapture (ASMR) model (from Coggins and others, in press).

Overall, about 15%–20% of the adult humpback 

chub are dying each year. If this mortality rate and the 

dramatically reduced recruitment rate of young chub 

experienced since the early 1990s remain unchanged, 

there will be a decline in the adult population of 

humpback chub from the present 3,000–5,000 fish to a 

level of 1,500–2,000 adult fish over the next 10–15 yr.

Cause and Effect Relationships

The Glen Canyon Dam Adaptive Management 

Program has a goal of maintaining a self-sustaining population of humpback chub in Grand Canyon 

(GCDAMP, 2001, http://www.usbr.gov/uc/rm/amp/

amwg/mtgs/02jan17/Attach_06.pdf, accessed July 

14, 2005); however, this goal is qualitative and has no 
46 The State of the Colorado River Ecosystem in Grand Canyon

defined target population abundance levels. The U.S. 

Fish and Wildlife Service, which has jurisdiction over 

the humpback chub as a federally endangered species, 

promulgated recovery goals based on the known distribution of the species (U.S. Fish and Wildlife Service, 2002). 

These goals recognize the Grand Canyon population of 

humpback chub as the only potentially viable population 

in the lower Colorado River Basin and include it, along 

with at least one population from the upper Colorado 

River Basin, as having to attain certain population numbers before the species can be considered for downlisting 

or delisting under the Endangered Species Act. Briefly, 

these goals require that a viable population be attained 

and maintained for a period of at least 5 yr, with a minimum of 2,100 sexually mature individuals in each population. Furthermore, the recruitment of new individuals into the population must meet or exceed the adult 

mortality rate, thereby providing a stable or increasing 

adult abundance trend. In the case of the Grand Canyon 

population, sexually mature fish are assumed to be 4 yr 

old or older.

Of paramount importance in conserving the 

population of the federally endangered humpback chub 

is determining the factors contributing to population 

decline and implementing management actions designed 

to minimize or eliminate the effect of those factors. 

Not all of the factors that may be responsible for the 

recruitment decline of humpback chub beginning in 

the early 1990s are clear, but a list of likely factors that 

could be acting either singly or in combination include 

(1) Colorado River and Little Colorado River hydrology 

(discharge and temperature), (2) infestation of juvenile 

humpback chub by Asian tapeworm, (3) predation by 

or competition with warmwater native cyprinids and 

catostomids and nonnative cyprinids and ictalurids 

within the Little Colorado River, and (4) predation by or 

competition with coldwater nonnative salmonids within 

the Colorado River.

The body of evidence available to evaluate specific questions varies among these postulated factors. 

For instance, beginning in 1990 the operation of Glen 

Canyon Dam was changed through the implementation 

of research flows (a series of discharges and data collection programs conducted from June 1990 through July 

1991) and the interim operating criteria. This hydrology, 

and the subsequent MLFF alternative that continues to 

present, can generally be characterized as having less 

severe daily flow fluctuations than the previous 28 yr 

of the no action period when the dam was managed 

primarily to maximize hydroelectric power revenue. 

This major change in Colorado River hydrology correlates closely to the decline in humpback chub recruitment. Also, it is possible that the decline in humpback 

chub recruitment in the early 1990s was caused by the 

nearly continuous flooding in the Little Colorado River 

that occurred during the summer of 1992, particularly 

during the early summer when larval humpback chub 

emerge (Robinson and others, 1998). It is also possible 

that the high infestation rate of juvenile humpback chub 

by the Asian tapeworm is a factor. Humpback chub 

infected with Asian tapeworm were first found in 1990, 

and infestation rates in 2001 exceeded 90% (Choudhury 

and others, 2004). Finally, predation and competition 

by nonnative fishes either in the Little Colorado River 

or in the Colorado River may be driving the humpback 

chub recruitment trend. Although robust relative abundance data do not exist for common carp and channel 

catfish within the Little Colorado River, there was a large 

increase in the abundance of nonnative salmonids in 

the Colorado River documented near the confluence of 

the Little Colorado River (RM 56.6–68.3) (Gorman and 

Coggins, 2000). 

Recent Management Actions 

Undertaken or Proposed

While it is difficult to determine the factor most 

responsible for the humpback chub recruitment decline, 

a likely primary factor is negative interactions (predation 

and competition) with nonnative fish. Interaction with 

nonnative fish is implicated in the decline and extinction 

of native fishes throughout the Colorado River Basin. In 

response to the need to address this factor, a program of 

selective removal of nonnative fishes (known as mechanical removal) was implemented in 2003 near the confluence of the Little Colorado River and in other tributaries (work in Bright Angel Creek and other tributaries 

has been undertaken by the National Park Service). To 

complement these efforts, the work group also approved 

initiation of a multiobjective study to evaluate the potential effect of rainbow trout and brown trout predation on 

humpback chub recruitment and the efficacy of mechanical removal of nonnative fishes from the Colorado River 

near the confluence of the Little Colorado River.

In early 2003, a major effort was begun by the 

Glen Canyon Dam Adaptive Management Program to 

remove nonnative fish from the area of the river near the 

confluence of the Little Colorado River (RM 61), which 

is considered important habitat for native fish, especially 

humpback chub. A total of 16,045 rainbow trout and 

many other nonnative fish (fig. 13a) were removed from 

this river reach during 2003–04. While native fish contributed only approximately 5% of the overall catch in 

January 2003, native fish contributed greater than 35% 
Fishes of Grand Canyon 47

in September 2004, generally reflecting a reduction in 

nonnative fish abundance. Also, the overall abundance 

of rainbow trout has been reduced by more than 60% 

in this river reach (fig. 13b). Whether this reduction in 

nonnative fish density will benefit native fish is unknown 

at this time.

Moreover, an experimental program to move YOY 

humpback chub upstream of an impassable barrier 

(where few nonnative fish live) in the Little Colorado 

River was initiated and has shown some early signs of 

success (Stone and Sponholtz, 2005). Future introductions of humpback chub into other Grand Canyon 

tributary streams may help augment the population in 

Grand Canyon. Additional management options include 

potential hatchery rearing of humpback chub as a refugium population or for stocking in the river. 

Other management options include the installation 

of a multilevel water intake structure(s) for Glen Canyon 

Dam to warm the water in Grand Canyon. The Bureau 

of Reclamation has developed preliminary plans and 

is scoping out the possible installation of a temperature 

control device, which would provide flexibility to release 

warmer water into the river. Warmer water could create more favorable habitat conditions for native fishes 

in general; however, its operation could also improve 

habitat conditions for nonnative, warmwater species 

and degrade habitat quality for trout inhabiting the Lees 

Ferry reach. Obviously the operation of such a device, 

if built, will need to be carefully considered and implemented experimentally.

Discussion and Future 

Research Needs

The salient findings of the research and monitoring programs undertaken by the Glen Canyon Dam 

Adaptive Management Program regarding fishes are 

twofold. First, there has been a dramatic and continuing 

decline in the number of adult humpback chub in the 

Grand Canyon ecosystem since at least the late 1980s. 

This decrease in adult fish is due to a steady decline in 

the recruitment of young fish into the population beginning in the early 1980s, with an additional reduction in 

the early 1990s. This decline in recruitment results in a 

dwindling population of adults as older age fish die off 

and are not replaced. It is currently estimated that if 

recruitment remains stable at this level, the adult population of humpback chub in the Grand Canyon ecosystem 

will stabilize at approximately 1,500–2,000 fish over 

the next decade or so. The current population decline 

combined with the low recruitment in this population 

relative to adult mortality indicates that this population 

will attain neither positive trends nor sufficient numbers 

of fish to meet USFWS recovery goals in the foreseeable 

future.

The second major result regarding fishes is the 

proliferation of rainbow trout in both the Lees Ferry 

reach and downstream as far as RM 75. Numbers of 

brown trout have also increased dramatically in the area 

around Bright Angel Creek and upstream to above the 

Little Colorado River confluence. Both of these species 

are known to prey on native fishes, and their substantial 

increase in abundance near the principal remaining 

native-fish habitat in Grand Canyon remains a concern. 

It has yet to be determined whether the experimental 

management action to reduce the numbers of nonnative 

fish in the area around the Little Colorado River confluence has resulted in any increase in survival and recruitment of the federally endangered humpback chub. 

Dam Operations

It is not possible to say conclusively that the decline 

in humpback chub recruitment that began to occur in 

the early 1990s is because of the implementation of the 

MLFF regime; however, the flow regime has not reversed 

the decline in recruitment and adult abundance either. 

Approximately 15%–20% of the adult humpback chub 

population is dying each year. These fish are most likely 

being replaced, albeit at a lower rate, predominately by 

young humpback chub that have spent the first 3 to 4 

yr of their lives in the Little Colorado River. In other 

words, the MLFF alternative had either a negative effect 

or no effect at all, but it has not had a measurable beneficial effect on humpback chub. 

The MLFF alternative has not improved conditions for other native fishes as indicated by their stable or 

declining numbers. Different daily, seasonal, or annual 

changes in river flows could be considered on an experimental basis. Such flow options could include reduced 

daily fluctuations and equalized monthly volumes to provide a more stable environment for young native fishes. 

There is a good chance that juvenile humpback chub dispersing into the mainstem in summer and fall would be 

able to grow, survive, and return to the Little Colorado 

River for extended rearing if they were to encounter (1) 

reduced predation and competition by nonnative trout 

(trout would have to be removed by mechanical removal 

treatments) and (2) relatively warm refuges in nearshore 

locations (these locations could be created by steady flow 

conditions in late summer and fall). The low summer 
48 The State of the Colorado River Ecosystem in Grand Canyon

Figure 13. A. Total catch and percent contribution by species and month during mechanical removal efforts in the Little Colorado River 

removal reach, 2003–04. B. Estimated abundance of rainbow trout in the Little Colorado River removal reach before and after each

mechanical removal effort, 2003–04 (U.S. Geological Survey, unpub. data, 2005).

A.

B.
Fishes of Grand Canyon 49

steady flow (LSSF) experiment demonstrated that such 

lateral warming of backwater areas can be quite dramatic. A summer-fall steady flow experiment would need 

to maintain conditions for backwater warming from the 

time of the first summer high flow that disperses juveniles into the mainstem until around November 1, when 

the equilibrium temperature in standing backwaters 

decreases (because of nighttime cooling) to about the 

same as the mainstem temperature.

Three additional flow possibilities for Glen Canyon 

Dam could be made based on recommendations from 

the 2003 YOY rainbow trout surveys and analyses of 

otoliths: (1) fluctuating flows targeting YOY rainbow 

trout could be implemented from April through July to 

coincide with the timing of hatch, (2) summer steady 

flows could likely improve the growth of YOY rainbow trout, and (3) sudden reductions in the minimum 

daily flow could have the potential to strand or displace 

many YOY rainbow trout in the Lees Ferry reach. 

The latter recommendation was based on an almost 

complete absence of fry from low-angle shorelines 

after the reduction in the minimum flow from 10,000 

cfs to 5,000 cfs in early September 2003 and a similar 

but less dramatic reduction in September 2004 (Korman and others, 2005). An event-based approach—in 

which flows are increased to approximately 20,000 cfs 

for 2 d, followed by a reduction to 5,000 cfs for 1 d, and 

implemented on a monthly basis from May through 

September—would almost certainly be much more effective at reducing recruitment in the Lees Ferry reach than 

the January–March fluctuating nonnative fish suppression flows implemented beginning in 2003. Steady flows 

could be conducted between events to increase water 

temperatures for native fish downstream and would not 

have beneficial effects for YOY rainbow trout, as their 

densities would be controlled through the temporary 

reductions in minimum flow. 

Researchers have been unable to identify or implement an effective mainstem monitoring program for 

native fishes or most nonnative species (the exceptions 

are rainbow trout, brown trout, and carp). Because of 

this situation, the USGS Grand Canyon Monitoring 

and Research Center has called for a research initiative to investigate the utility of alternative sampling 

methods such as acoustic devices that may assist in 

providing better measures of relative abundance and 

change detection.

The most important research task associated with 

humpback chub conservation is determining the factors controlling the recruitment dynamics of this species. These factors can only be determined through an 

appropriately designed experiment that addresses the 

multiple important biotic and abiotic factors likely influencing humpback chub. As stated by Korman and others 

(2004, p. 395–396) in summary of an intensive modeling effort aimed at characterizing changes in nearshore 

humpback chub habitat with changes in Glen Canyon 

Dam operation,

The interaction between habitat and ecosystem 

processes like competition and predation remain 

highly uncertain. Ultimately, questions regarding 

the effects of dam operations on juvenile humpback chub must be addressed by monitoring the 

response of critical population parameters to 

flow manipulations conducted within a sound 

experimental design.
50 The State of the Colorado River Ecosystem in Grand Canyon

Speckled dace 

(native)

Size–

rarely exceeds 3 inches (7.6 cm).

Distribution–

extensively distributed 

throughout Western 

United States.

Status–

abundant in some areas and widely distributed. This 

species is represented by several subspecies.

Natural history–

The speckled dace (Rhinichthys osculus) is the only 

native dace in Arizona, although the genus is widely 

Bluehead sucker 

(native)

Size–

maximum of about 20 inches 

(51 cm).

Distribution–

found in fast-flowing river 

systems in Arizona, Colorado, 

New Mexico, Utah, and Wyoming. 

distributed elsewhere. Dace are widely distributed 

in the Colorado River, with many inhabiting 

backwaters in western Grand Canyon. Diet includes 

algae, insect larvae, small crustaceans, and small 

snails. Spawning occurs in spring and late summer. 

Large schools of dace congregate over gravel 

bottoms to spawn. Populations appear to be stable 

in Grand Canyon.

Randall D. Babb, Arizona Game and Fish Department Randall D. Babb, Arizona Game and Fish Department

Profiles of Selected Fish Species Found 

in the Grand Canyon Ecosystem

Information compiled by Jeffrey E. Lovich
Fishes of Grand Canyon 51

Status–

not uncommon in some areas.

Natural history–

This species (Catostomus discobolus) occurs in the 

Colorado River upstream from Lake Mead. Diet 

includes algae, diatoms, insects, amphipods, and 

organic debris that it scrapes from rocks with 

Randall D. Babb, Arizona Game and Fish Department

specialized cartilage lips. In Grand Canyon, 

spawning occurs over gravel, sand, and cobbles in 

April and May, when water temperatures exceed 

61o

F (16oC). Young inhabit backwaters in Grand 

Canyon. Bluehead suckers are known to hybridize 

with other sucker species. Populations appear to be 

stable in Grand Canyon. Individuals can live for 

more than 20 yr.

Humpback chub 

(native)

Size–

maximum of about 20 inches 

(51 cm).

Distribution–

found only in the Colorado 

River system.

Status–

federally endangered.

Natural history–

The humpback chub (Gila cypha) formerly 

ranged downstream to the area now occupied by 

Lake Mohave, but it is now confined to several 

aggregations in Grand Canyon and isolated 

populations in various deep canyon stretches of 

the Colorado River and its major tributaries above 

Lake Powell. Most humpback chub in Grand 

Canyon are found in the vicinity of the Little 

Colorado River (LCR) and its confluence with 

the mainstem. Humpback chubs are omnivorous, 

and their diet includes a diversity of aquatic and 

terrestrial invertebrates, small fish, algae, and 

other plant material. In Grand Canyon the diet 

of the nonnative rainbow trout is almost identical, 

setting the stage for possible resource competition 

between the species. Spawning occurs in spring in 

the LCR, and young enter the mainstem during 

floods associated with storm events, most commonly 

in spring and late summer/fall. Aggregations of 

humpback chub, well upstream and downstream of 

the LCR population, may result from (1) emigration 

of juveniles, subadults, or adults from the LCR; 

(2) survival of relict fish from before the dam; or 

(3) mainstem spawning. The latter has not been 

documented in the postdam era, so additional 

research is needed to resolve this issue. The 

estimated adult population in Grand Canyon has 

declined sharply from about 10,000 a decade ago to 

about 3,000–5,000 today.
52 The State of the Colorado River Ecosystem in Grand Canyon

Flannelmouth

sucker (native)

Size–

can exceed about 20 inches 

(51 cm).

Distribution–

Colorado River Basin in 

Arizona, California, Colorado, 

New Mexico, Nevada, Utah, 

and Wyoming. Extirpated from the Gila River

Basin of Arizona.

Status–

not uncommon in some areas.

Natural history–

This species (Catostomus latipinnis) occurs in the 

Colorado River upstream from Lake Mead. 

Flannelmouth suckers below Lake Mead exist 

because of the success of reintroduction from the 

Paria River in the mid-1970s. Diet varies with age 

class and size but includes algae, insects, plankton, 

ostracod, crustaceans, plant materials, and detritus. 

This species likely makes spawning runs in most 

of the major tributaries in Grand Canyon before 

returning to the mainstem. Spawning occurs 

from March to July, when water temperatures are 

between 43°F and 68o

F (6°C and 20oC). Spawning 

occurs in shallow water over sand and gravel 

bottoms. Females lay from 4,000 to 40,000 eggs. 

Juveniles are frequently captured in the mainstem 

from lower Marble Canyon downstream to Lake 

Mead. Juveniles are also frequently captured in 

the Little Colorado River and other tributaries 

downstream. Known to hybridize with the 

razorback sucker, a species that is presumed to be 

gone from the Grand Canyon region. Populations 

appear to be stable in Grand Canyon.

Rainbow trout 

(nonnative)

Size–

up to 47 inches (120 cm). 

Arizona State record was 32.25 

inches (81.9 cm).

Distribution–

extensively distributed 

throughout Western North 

America in river systems 

Randall D. Babb, Arizona Game and Fish Department Randall D. Babb, Arizona Game and Fish Department
Fishes of Grand Canyon 53

draining into the Pacific Ocean. Widely introduced 

worldwide, including into the Colorado River.

Status–

common.

Natural history–

Rainbow trout (Oncorhynchus mykiss) were 

introduced into the Grand Canyon area in the 

1920s for sport fishing. Originally confined to 

clear tributary streams, the construction of Glen 

Canyon Dam created cold, clear conditions that 

allowed trout to colonize the mainstem. Trout 

were also stocked in the tailwaters of the dam by 

the State of Arizona shortly after construction 

Brown trout 

(nonnative)

Size–

Arizona State record is 36 

inches (91.4 cm). The world 

record is a 40 lb, 4 oz (18.3 kg) 

specimen caught in Arkansas.

Distribution–

widely introduced worldwide, 

including into the Colorado River.

Status–

common.

Natural history–

Native to Europe and Asia, brown trout (Salmo

trutta) were introduced into the Grand Canyon area 

in the 1920s for sport fishing. Originally confined 

to clear tributary streams, brown trout were able 

to colonize the mainstem of the Colorado River 

when the construction of Glen Canyon Dam 

created cold, clear conditions. Brown trout eat 

a variety of aquatic and terrestrial insects and 

other invertebrates. Large specimens are highly 

predaceous on other fish, including smaller trout. 

Reproduction is as in other species of trout (see text 

box for rainbow trout). Bright Angel Creek is an 

important spawning stream for mainstem trout that 

move into the smaller tributary for this purpose in 

winter and early spring. Brown trout are capable 

of tolerating slightly higher water temperatures 

than most other trout. Most brown trout in Grand 

Canyon today occur near the confluence with 

Bright Angel Creek.

was completed in the 1960s. The diet consists 

mainly of both aquatic and terrestrial insects and 

other aquatic invertebrates including amphipods. 

Spawning in Grand Canyon occurs in winter 

and early spring. After fertilization by males, 

females excavate a depression, or redd, in gravelly 

bottoms, and the eggs are buried in the substrate to 

hatch unattended. Rainbow trout like cold water 

temperatures and rarely live in water above about 

77o

F (25oC). The Lees Ferry reach of the Colorado 

River is where most spawning occurs in the Grand 

Canyon area and is managed as a “blue ribbon” 

trout fishery. Trout numbers have been increasing 

in recent years, possibly to the detriment of the 

endangered humpback chub.

Randall D. Babb, Arizona Game and Fish Department
54 The State of the Colorado River Ecosystem in Grand Canyon

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56 The State of the Colorado River Ecosystem in Grand Canyon

Contact Information:

Steven P. Gloss

Ecologist

U.S. Department of the Interior

U.S. Geological Survey

Southwest Biological Science Center

Tucson, AZ

sgloss@usgs.gov

Lewis G. Coggins

Fishery Biologist

U.S. Department of the Interior

U.S. Geological Survey

Southwest Biological Science Center

Flagstaff, AZ 

lcoggins@usgs.gov

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Valdez, R.A., and Carothers, S.W., 1998, The aquatic 

ecosystem of the Colorado River in Grand Canyon. 

Report to Bureau of Reclamation, Salt Lake City, 

Utah: Flagstaff, Ariz., SWCA, Inc., Environmental 

Consultants, 250 p.

Valdez, R.A., and Ryel, R.J., 1995, Life history and ecology of the humpback chub (Gila cypha) in the Colorado 

River, Grand Canyon, Arizona. Final report to the 

Bureau of Reclamation, Salt Lake City, Utah, contract 

no. 0-CS-40-09110: Logan, Utah, BIO/WEST, Inc.

Walters, C., Korman, J., Stevens, L.E., and Gold, B., 

2000, Ecosystem modeling for evaluation of adaptive 

management policies in the Grand Canyon: Conservation Ecology, v. 4, no. 2, p. 1, http://www.consecol.

org/vol4/iss2/art1/, accessed July 14, 2005.

Ward, D.L., Maughan, O.E., Bonar, S.A., and Matter, 

W.J., 2002, Effects of temperature, fish length, and 

exercise on swimming performance of age-0 flannelmouth sucker: Transactions of the American Fisheries 

Society, v. 131, p. 492–497.

First page photograph credit: Matt Lauretta, SWCA Environmental Consultants, 

Inc., for the U.S. Geological Survey
Chapter 3

Climatic 

Fluctuations, 

Drought,

and Flow

in the 

Colorado River

Robert H. Webb

Richard Hereford

Gregory J. McCabe

Introduction

Climate is the cumulative pattern of daily atmospheric conditions in a particular geographic area, and 

weather is the daily and seasonal expression of these 

conditions. Climate varies over periods of years, decades, 

or centuries, altering weather conditions in a region, particularly precipitation amounts and temperatures. In the 

arid and semiarid Southwest, climatic fluctuations affect 

many hydrologic characteristics of watersheds, including the quantity of base flow, the occurrence of large 

floods, and the timing of snowmelt runoff (Dettinger and 

Cayan, 1995; Cayan and others, 1999; Stewart and others, 2004, 2005; McCabe and Clark, in press). 

Reservoirs in the Western United States, particularly those in the Colorado River Basin, were built to 

reduce, if not eliminate, annual variations in water 

supply that occurred historically because of periods 

of above- or below-average precipitation. A persistent 

drought beginning in 2000 raised concern that decreases 

in runoff entering Lake Powell could follow and releases 

from Glen Canyon Dam could be severely reduced or 

constrained. Inflows to Lake Powell on the Colorado 

River were below average from 2000 through 2004, 

leading to drawdown of both Lake Powell (figs. 1 and 2) 

and Lake Mead, the primary flow-regulation structures 

on the river. On January 27, 2005, the level of Lake 

Powell was at 3,562.5 ft (1,085.9 m) (full pool capacity 

is 3,700 ft (1,128 m)), and the reservoir contained 8.5 

million acre-feet (maf) (10,481 million m

3

) of water (fig. 

1), which is only 35% of the reservoir’s capacity and a 

little more than 1 yr of annual flow releases. Reduction 

in annual flow releases can reduce the water available for 

prescribed flow releases—particularly flood releases—

designed to benefit riverine habitat within Grand 

Canyon. By 2004, it was speculated that Glen Canyon 

Dam would be unable to produce hydroelectric power 

by 2006 or 2007 if drought conditions persisted and the 

lake level continued to decline. 

Conditions changed in fall and winter 2004–05 as 

a series of storms led to greatly above-average precipitation in the southern portion of the watershed. The 

high precipitation may have been associated with the 

onset of El Niño conditions in the Pacific Ocean, which 

presumably could have enhanced fall and early winter 

storms. On February 1, 2005, inflows to Lake Powell 

were forecast to be 125% of normal, the first aboveaverage forecast since 1999. This reversal of conditions 
58 The State of the Colorado River Ecosystem in Grand Canyon

from the previous 5 yr could suggest that the drought is 

over, although some long-term records suggest that this 

may not be the case since average years have occurred 

within periods of extended dryness. To date, it is unclear 

whether the early 21st century drought is over or not, 

and the possible persistence and magnitude of the 

drought are of great concern for the Glen Canyon Dam 

Adaptive Management Program.

Unfortunately, the factors that caused and sustained 

the early 21st century drought have not been positively 

identified. Although conditions in the tropical Pacific 

Ocean were considered to be ideal for drought conditions in the continental United States (Hoerling and 

Kumar, 2003), new studies suggest that the Atlantic 

Ocean may also influence drought (Gray and others, 

2004; McCabe and others, 2004). In the case of the 

Colorado River, it is possible to examine the drought 

in a broader historical and climatic context, which can 

be developed through historical records and statistical 

models. First, a historical record exists of actual observations and estimates of annual flows in the river at 

various places, including Lees Ferry. Second, scientists 

have gained an understanding of precipitation patterns 

by using annual growth rings in trees to reconstruct the 

hydrologic conditions in a basin several hundred years 

before the historical record began. Third, climatologists 

and other scientists have recently developed statistical 

techniques and dynamical models that improve understanding of the relations between various ocean temperature patterns and observed precipitation patterns. 

This chapter makes clear that the drought beginning 

in 2000 probably had its origins in several hemisphericscale atmospheric and oceanic processes that affect 

moisture delivery to the Colorado River Basin (fig. 3). In 

this context, the chapter describes the general causes of 

drought in the Southwest, the long-term perspective on 

drought duration in the basin based on tree-ring reconstructions, the use of global climate indices to explain 

Colorado River flows, and scenarios of future climate 

and runoff in the Colorado River Basin.

Figure 1. Fluctuations in the level of Lake Powell following 

closure of Glen Canyon Dam in 1963 (from www.summittech.com/

LakePowell/LP_waterDB.php, accessed February 20, 2005).

Figure 2. Lake Powell at Glen Canyon Dam (photograph by Dale 

Blank, U.S. Geological Survey).

Figure 3. Moisture sources for the Colorado River Basin 

(outlined in red). Lees Ferry is the separation point between 

the upper and lower Colorado River Basins as specified in the 

Colorado River Compact of 1922.
Climatic Fluctuations, Drought, and Flow in the Colorado River 59

Background

Drought is caused by persistent low precipitation 

over a region. As such, the severity of a drought is a 

function of spatial extent, duration, and magnitude of 

the precipitation deficit. Moreover, the area affected by 

a drought may shift in space and time. This combination of variable factors makes drought prediction and 

estimation of drought frequency extremely difficult. 

The causes of persistent drought over a large drainage 

basin, such as the Colorado River Basin, are particularly 

difficult to determine because the basin spans a large 

latitudinal range. 

Sources of Moisture

The most important sources of water to the 

Colorado River Basin are frontal systems that originate 

in the North Pacific Ocean and occur in winter and 

spring. These systems tend to carry moisture at high levels 

in the atmosphere, and precipitation is orographically controlled, meaning that it typically increases with elevation 

in the mountains. Cold frontal systems drop substantial 

amounts of snow at high elevations and rain at low elevations in the Rocky, Uinta, and Wind River Mountains, 

which in turn become the headwaters of the Colorado 

River and its principal tributary, the Green River (fig. 3). 

The frequency and moisture content of frontal systems are 

strongly affected by the strength of atmospheric circulation patterns and sea-surface temperatures in the Pacific 

Ocean.

There are two basic types of winter storms that 

affect flow in the Colorado River. Cold winter storms 

deliver moisture in the form of snow at most elevation ranges in Utah, Colorado, New Mexico, northern 

Arizona, and Wyoming. These storms build snowpacks 

that melt in the spring, providing runoff to the Colorado 

River. Warm winter storms, originating in the tropical 

Pacific Ocean, may produce rain on snowpacks, resulting in large runoff events and floods on major rivers, 

which tend to overwhelm reservoir systems, particularly 

in Arizona.

Moisture delivered to the Colorado River during 

summer months typically originates from a combination 

of the Gulf of Mexico, the Gulf of California, and the 

eastern North Pacific Ocean. Known variously as the 

“Arizona monsoon,” the “Southwestern United States 

monsoon,” the “summer monsoon,” or even the “North 

American monsoon,” this moisture arrives in July and 

August at low atmospheric levels. The moist air rises rapidly over the desert landscape, spawning high-intensity 

thunderstorms that produce runoff mostly at elevations 

of less than 7,000 ft (2,134 m). The thunderstorms tend 

to be of small spatial extent, and, although they spawn 

severe flash flooding locally, few floods are generated on 

larger rivers in the region.

Status and Trends

Flow at Lees Ferry

Flow in the Colorado River measured at Lees Ferry 

(fig. 3), the political boundary between the upper and 

lower Colorado River Basins, varied substantially during 

the 20th century. Calendar-year flow volumes (fig. 4a) 

were combined from three data sets that were measured 

or estimated by using different techniques. From 1895 

through 1922, annual flow volumes at Lees Ferry were 

estimated by LaRue (1925, p. 108); from 1922 through 

1962, unregulated flow was measured at the Lees Ferry 

gaging station; and from 1963 through 2004, flow was 

estimated as the sum of tributary flows entering Lake 

Powell (Webb and others, 2004). Consumptive water 

use in the basin upstream of the gage at Lees Ferry is 

not accounted for in these data. As a result, flow values 

measured at Lees Ferry are due to climatic fluctuations 

and changes in consumptive water use in the upper basin 

States of Wyoming, Colorado, Utah, and New Mexico. 

The average annual flow volume shown in figure 

4a was 12.3 maf (15,166 million m3

) from 1895 through 

2004. This volume is less than the more-commonly 

quoted annual volume of 15.0 maf (18,495 million m3

)

because the time series in figure 4a was not adjusted for 

water consumed in the upper basin States. The period 

from 1905 to 1922, which was used to estimate water 

production allocated under the Colorado River Compact, 

had the highest long-term annual flow volume in the 

20th century, averaging 16.1 maf (19,851 million m3

)

at Lees Ferry; however, the highest annual flow volume 

occurred in 1984 (22.2 maf (27,373 million m3

)), and the 

highest 3-yr average is 20.3 maf (25,030 million m3

) for 

1983 through 1985. The lowest annual flow volume is 

3.8 maf (4,685 million m3

) in 2002, followed by 3.9 maf 

(4,809 million m3

) in 1934 and 4.8 maf (5,918 million 

m3

) in 1977. The trend in annual flow volume, which 

decreased by about 0.5 maf (617 million m3

) per decade 

from 1895 through 2003, is due in part to upstream 

water consumption.

These data show that flow in the early 21st century 

is the lowest in more than a century. The current drought 
60 The State of the Colorado River Ecosystem in Grand Canyon

has contributed to the lowest flow period on record, 

producing an average of only 5.1 maf (6,288 million m3

)

for the 3-yr period from 2002 through 2004. In contrast, 

other low 3-yr averages include 6.2 maf (7,645 million 

m3

) for 1989 through 1991, 6.3 maf (7,768 million m3)

for 1988 through 1990, 7.3 maf (9,001 million m3

) for 

1954 through 1956, and 8.0 maf (9,864 million m3

) for 

1933 through 1935. The 5-yr average of 5.9 maf (7,275 

million m3

) centered on 2002 is the lowest in the 110-yr 

record. By any measure, the early 21st century drought is 

the most severe in the unadjusted gaging record.

The Bureau of Reclamation (BOR) adjusted the 

flow record at Lees Ferry to account for consumptive 

uses in the upper basin (fig. 4b). In the BOR record, flow 

volumes are available by water year (October 1 through 

September 30) for the period of 1905 through 2004, a 

99-yr record. The adjusted average annual flow volume 

at Lees Ferry is 15.0 maf (18,495 million m3

), and the 

decrease in flow is 350,000 acre-feet (431,550,000 m3

)

per decade (fig. 4b). Using this adjusted data, the lowest 

flow year was 1977 with 5.6 maf (6,905 million m3

), followed by 2002 with 6.4 maf (7,891 million m3

). The 3-yr 

averages for 2002 through 2004 (9.2 maf (11,344 million 

m3

)), 2000 through 2002 (9.45 maf (11,652 million m3)),

and 2001 through 2003 (9.51 maf (11,726 million m3

))

are the lowest in the period of record. Similarly, the lowest 5-yr average is 9.9 maf (12,207 million m3

) for 2000 

through 2004, which is 1 maf (1,233 million m3

) less than 

the average flow of the second lowest 5-yr period (1988 

through 1992). Using either the actual or adjusted flow 

values, the early 21st century drought produced the lowest flows of the past century.

Figure 4. Colorado River flow volume at Lees Ferry (before 1963) and inflows to Lake Powell (after 1963). A. Actual calendar-year flow 

volumes derived from three sources. From 1895 through 1922, annual flow volumes at Lees Ferry were estimated by LaRue (1925). From

1922 through 1962, flow volumes were measured at Lees Ferry, Arizona. From 1963 to 2004, inflow to Lake Powell was estimated from

gaging records on the Colorado River and its major tributaries. B. Water-year flow volumes for Lees Ferry adjusted for consumptive use 

in the upper basin (Bureau of Reclamation, unpub. data, 2005).
Climatic Fluctuations, Drought, and Flow in the Colorado River 61

Tree-ring Reconstructions 

of Drought

Considerable research has addressed the question 

of the magnitude, frequency, and duration of droughts 

affecting the Colorado River Basin, including studies 

examining the effects of the most severe known droughts 

on record at Lees Ferry (Tarboton, 1995). Many of these 

studies are based on the seminal work of Stockton and 

Jacoby (1976), who used dendrochronology to reconstruct long-term river flows using the actual flow record 

at Lees Ferry for calibration. Recent large-scale work 

(e.g., Cook and others, 2004), as well as efforts within 

the drainage basin (Woodhouse, 2003; Gray and others, 2003, 2004), while suggestive, remains insufficient to 

resolve the basic magnitude-frequency questions concerning the early 21st century drought and its effects on 

the Colorado River Basin. 

What is clear from the Stockton and Jacoby (1976) 

work and other studies (Salzer, 2000; Woodhouse, 2003; 

Cook and others, 2004) is how unusual the high precipitation of the early 20th century was in terms of runoff 

in the Colorado River. The unusually wet period of the 

20th century accentuates the severity of the dry conditions experienced during the early 21st century drought. 

The difference between extreme wet and extreme dry 

conditions is accentuated because observational records 

of climate and hydrologic conditions in the Colorado 

River Basin generally span 100 yr or less, limiting our 

ability to quantitatively understand the current drought 

in a long-term context. It is possible, however, to qualitatively view this drought in a long-term context from 

analysis of tree rings, which provide an indication of 

moisture conditions going back several centuries.

Using dendrochronological reconstructions from 

tree rings from the Western United States, Cook and 

others (2004) analyzed long-term changes in the area 

affected by drought from A.D. 800 to 2003. Although the 

region they considered is far larger than the Colorado 

River Basin and subject to a larger array of climatic 

influences, their reconstruction provides some perspective on the 2000 through 2004 drought in the Colorado 

River Basin. Cook and others (2004) concluded that 

the present drought is not comparable to the so-called 

“megadroughts” of A.D. 936, 1034, 1150, and 1253, 

primarily because of its short duration; however, the 

early 21st century drought may not yet be over. At the 

very least, their drought-area reconstruction (Cook and 

others, 2004) suggests that the present drought may surpass other 20th century droughts in the Western United 

States, including the droughts of the midcentury and the 

1930s, and be comparable to droughts of the mid-19th 

or late 16th centuries.

Several researchers (Tarboton, 1995; Cook and 

others, 2004; Gray and others, 2004) have noted that 

decadal-scale persistence of below-average precipitation 

is of paramount importance when considering drought 

frequency. Tarboton (1995) and Meko and others (1995) 

provided data based on the Stockton and Jacoby (1976) 

reconstructions that, when compared to conditions of 

2001–04, suggest that the low-flow conditions of the 

early 21st century may be the lowest since the drought of 

A.D. 1579 to 1600. 

Recent Findings

Several indices of atmospheric and oceanic processes 

are used to explain climate variability in the Western 

United States, including the Southern Oscillation Index 

(SOI), the Pacific Decadal Oscillation (PDO), and the 

Atlantic Multidecadal Oscillation (AMO). These indices 

reflect short- to long-term conditions that can affect the 

discharge of the Colorado River.

Southern Oscillation Index

Perhaps the most well known of the climatic indices 

is the Southern Oscillation Index (SOI), which is often 

used to indicate the status of the El Niño-Southern 

Oscillation (ENSO) phenomenon in the Pacific Ocean. 

The SOI is the measure of the strength of tropical 

Pacific atmospheric circulation based on the sea-level 

pressure difference between Tahiti, French Polynesia, 

and Darwin, Australia (fig. 5a). Negative values, implying 

weakened trade winds, are mainly the result of higherthan-normal surface pressures at Darwin and are associated with El Niño conditions. The impacts of ENSO are 

felt worldwide through disruption of the general circulation of the atmosphere and associated global weather 

patterns. In terms of the Colorado River Basin, ENSO 

affects interannual variation of climate and precipitation in Arizona (Andrade and Sellers, 1988) and helps 

to explain the occurrence of floods and droughts in the 

Western United States (Cayan and others, 1998, 1999). 

The ENSO is a change between three basic states 

of the ocean. The warm phase, called El Niño, involves 

warming of the eastern Pacific Ocean off Peru and the 

northward spread of warm surface water to the west 

coast of the United States. Because warming of sea-surface temperatures (SSTs) is a hallmark of El Niño conditions (Knutson and others, 1999), several indices based 
62 The State of the Colorado River Ecosystem in Grand Canyon

on SSTs have been developed, including the NINO3 

index (fig. 5b). Reduced sea-level pressure over the eastern tropical Pacific Ocean combined with increased sealevel pressure over Indonesia (negative SOI) leads to a 

weakening in the trade winds, enabling warm water from 

the central equatorial Pacific Ocean to move toward and 

along the west coast of South America (positive NINO3 

index). The cold phase, called La Niña, is the opposite 

of the warm phase. Thus, El Niño and La Niña are the 

warm and cold phases of the ENSO system, which also 

includes a neutral condition that can persist for several 

years between the two polar phases. ENSO phases typically last 6–18 mo and are the single most important factor affecting interannual climatic variability on a global 

scale (Diaz and Markgraf, 1992). 

The ENSO also affects atmospheric circulation and 

SSTs in the eastern Pacific Ocean, which in turn affect 

the transport of moisture across the Western United 

States. During El Niño conditions, the warmer-thanaverage water in the eastern tropical Pacific Ocean and 

a shift in storm tracks tend to produce above-average 

precipitation (Redmond and Koch, 1991), above-average 

runoff (Cayan and Webb, 1992), and, potentially, floods 

in the Southwest. Not all El Niño events lead to increased 

runoff, however; during the 2003 El Niño, runoff was 

below average. 

During La Niña events, cooler-than-average SSTs 

in the eastern tropical Pacific Ocean tend to cause less 

moisture to flow over the continent, typically causing 

below-average flow in the Colorado River and predictable below-average precipitation in the Southwestern 

United States. This below-average precipitation occurs 

despite a tendency for above-average precipitation in the 

headwaters of the Colorado River Basin, although precipitation gained is negated by most of the basin having 

below-average precipitation.

Figure 5. A. The Southern Oscillation Index (SOI) varies with a 4- to 7-yr periodicity between negative (El Niño) and positive (La Niña)

states. B. The NINO3 index is a standardized anomaly index of sea-surface temperatures (SSTs) in an area of the equatorial Pacific

Ocean from 150oW to 90oW longitude and ±5o latitude centered on the equator. Comparison of these diagrams shows that when SOI is

negative, the NINO3 index generally is positive.
Climatic Fluctuations, Drought, and Flow in the Colorado River 63

Pacific Decadal Oscillation

The Pacific Decadal Oscillation (PDO) index (fig. 

6) was developed from SSTs in the Pacific Ocean north 

of 20oN latitude (Mantua and Hare, 2002). Two main 

characteristics distinguish the PDO from ENSO: (1) the 

PDO state (positive or negative) persists for decades, 

while typical ENSO events persist for 6 to 18 mo; and 

(2) the climatic signal of the PDO is most visible in the 

North Pacific Ocean instead of the tropics. The PDO 

index is commonly used to explain long-term periods 

of above- or below-average precipitation in the Western United States. When the PDO is positive, there is 

colder water in the central and western Pacific Ocean 

and warmer waters in the eastern Pacific Ocean; under 

negative PDO, the reverse is true. Positive PDO values 

are usually associated with wetter conditions in the 

Southwestern United States, while negative PDO values 

are suggestive of persistent drought in the Southwest. 

Long-term changes in the PDO may also influence 

snowmelt runoff in the Western United States, which is 

occurring earlier in the year, particularly in the Pacific 

Northwest and in the Sierra Nevada Range of California (Stewart and others, 2005).

Shifts in the phase of the PDO occurred in about 

1944 and 1977 (Hereford and others, 2002; McCabe 

and others, 2004); from 1999 through 2004, PDO values 

have varied from negative (1999–2001) to positive (2002–

04). While this might be viewed as an inconsistency with 

the persistent drought conditions during that period, the 

geographic center of drought conditions shifted towards 

the Pacific Northwest in a manner consistent with a positive (warm) PDO. At present, neither the causes of the 

variations in PDO values nor their predictability are well 

known; although, recent studies indicate that the PDO 

may be associated with decadal-length periods of aboveand below-average precipitation and streamflow in the 

Colorado River Basin (Hidalgo and Dracup, 2004).

Atlantic Multidecadal Oscillation

The Atlantic Multidecadal Oscillation (AMO) 

(Kerr, 2000) reflects conditions in the Atlantic Ocean 

that may affect climate in the continental United States 

(fig. 7). The AMO is discussed only to point out that it 

is an interesting and possibly significant index; much 

additional research is needed to demonstrate its usefulness. As its name implies, AMO events have a persistence 

of 20 to 35 yr. Warm conditions indicated by positive 

AMO values are thought to be associated with drought 

conditions (Enfield and others, 2001), such as the Dust 

Bowl on the Great Plains (Schubert and others, 2004) 

and other periods of drought during the last century 

(McCabe and others, 2004). 

Cool phases in the Atlantic Ocean occurred from 

1902 to 1925 and from 1970 to 1994; these periods 

coincide with generally above-average precipitation 

and runoff in the Colorado River Basin. A warm phase 

occurred almost continuously from 1926 to 1963, which 

coincides with persistent average or below-average 

rainfall and runoff in the Colorado River Basin between 

the early 1930s and 1960s. More recently, the Atlantic 

Ocean warmed in 1996 and remained so through 2004. 

Fluctuations in the AMO combined with those of the 

PDO may help explain long-term drought frequency 

(Gray and others, 2003, 2004) and, therefore, fluctuation 

in runoff in the Colorado River Basin. 

Climate Indices and Drought

As knowledge increases about the influence of the 

oceans on the climate of the United States, so too does 

the awareness of the enormous complexity of the oceanatmosphere system, particularly its variation over time. 

After intense scrutiny, scientists have learned that no 

single index of the system can explain all climate variations. It is increasingly evident that the various factors 

occur together in a complicated fashion. As a result, 

Figure 6. The Pacific Decadal Oscillation (PDO) is typically 

associated with long-term climatic variation in the Western 

United States. Positive PDO values suggest wetter periods (e.g., 

1976 through 1995) for the Southwest and drier periods for the 

Northwestern United States. In contrast, negative values suggest 

persistent drier-than-average conditions in the Southwest (e.g., 

mid-1940s through mid-1970s).
64 The State of the Colorado River Ecosystem in Grand Canyon

researchers attempt to use a combination of indices to 

explain the occurrence and spatial extent of droughts 

(e.g., McCabe and others, 2004). 

In terms of the Colorado River Basin, the river’s 

flow is related to the indices of global climate change in 

a complex way (Hidalgo and Dracup, 2004). From an 

interannual perspective, large floods and high runoff 

volumes typically occur during strong El Niño conditions 

(e.g., 1916–17, 1983–84), whereas La Niña conditions 

typically cause low-flow conditions (e.g., 1934, 1996). 

Above-average precipitation during El Niño, however, 

tends to occur in the southern part of the watershed 

while the northern part remains dry, a situation that 

tends to reverse during La Niña conditions. 

Furthermore, the watershed of the Colorado River 

spans more than 10o of latitude, and precipitation patterns 

over that range do not necessarily respond in concert to 

regional climatic fluctuations. For example, above-average 

runoff in part of the watershed (e.g., the northern half) 

may overcome low runoff in other parts (e.g., the southern half) during some low-flow periods. As a result, much 

of the variability in the annual flow record is not easily 

explained by climate indices. For example, the midcentury drought, which was severe on the Colorado Plateau 

(Hereford and others, 2002), caused only slightly belowaverage runoff in the entire basin; the average runoff volume during this period was 11.1 maf (13,686 million m3

)

Figure 7. The detrended Atlantic Multidecadal Oscillation (AMO) 

is related to persistent sea-surface temperature (SST) conditions 

in the Atlantic Ocean. Positive values are associated with higherthan-average drought frequencies in the United States.

for the period from 1948 to 1963. The response of Colorado River flow to the interaction of these climate indices 

is complicated, underscoring the concept that hydrologic 

drought results from an integrated set of climatological 

factors that are not easily predicted or explained.

The predicted effects of future climatic change suggest overall warming conditions and decreased average 

annual runoff in the basin (Christiensen and others, 

2004), although a simple hydrologic response to this 

complex climatic framework seems unlikely. Predicted 

temperature increases suggest that snowmelt runoff may 

be less prevalent and may occur earlier in spring (Stewart 

and others, 2004, 2005). These analyses raise the possibility that legally mandated flow releases from Glen 

Canyon Dam may be possible in only 80% of future 

years owing to climatic change. 

Drought Persistence and Relation 

with Indices of Global Climate

Dendrochronological analyses show that since A.D. 

1226, nine droughts have occurred in the Colorado 

River Basin lasting 15–20 yr and four droughts have 

occurred lasting more than 20 yr (Gray and others, 

2003). Several of these droughts were punctuated by 

above-average precipitation related to discrete El Niño 

events, which could be analogous to the effect of current 

El Niño conditions on the Colorado River Basin. Moreover, tree-ring records indicate that some past droughts 

in the Colorado River Basin persisted for several decades 

(Meko and others, 1995), leaving open the possibility that 

the present drought could resume after the ongoing El 

Niño ends and continue for many more years. 

By using tree-ring records spanning 700 yr, Gray 

and others (2003, 2004) found 30- to 70-yr multidecadal 

oscillations in drought frequency in the area that includes 

the headwaters of the Colorado River Basin. They also 

found a strong relation between drought occurrence and 

SSTs in the North Atlantic Ocean as manifested particularly in the AMO index (fig. 7) but which also included 

the PDO index (fig. 6). While neither index has a strong 

statistical relation to annual Colorado River flow (fig. 8), 

the combination may provide a context for the potential 

duration of the early 21st century drought. The broad 

relation between the PDO index and drought suggests 

that the present drought could persist for several decades 

after the end of the present El Niño period.
Climatic Fluctuations, Drought, and Flow in the Colorado River 65

Figure 8. Time series showing the complex interrelations among indices of global climate and annual flow volumes of the Colorado 

River from 1895 through 2003. Colored vertical bars delineate dry (tan) and wet (light blue) climate periods. A. Southern Oscillation Index 

(SOI, dimensionless). B. Pacific Decadal Oscillation (PDO, dimensionless). C. Atlantic Multidecadal Oscillation (AMO, deviation in oC), not 

detrended as in figure 7. D. Actual annual flow volume (in millions of acre-feet (maf)) passing Lees Ferry or entering Lake Powell (fig. 4).
66 The State of the Colorado River Ecosystem in Grand Canyon

Discussion and Management 

Implications

From 2000 through 2004, the early 21st century 

drought caused abnormally low flows in the Colorado 

River and its tributaries upstream from Lake Powell. 

By using either actual annual flow data or annual flow 

records adjusted for consumptive uses in the upper basin, 

it was found that runoff from 2000 through 2004 was the 

lowest in the period of record (99–110 yr). This low flow 

has caused considerable concern about the ability of the 

reservoirs on the Colorado River to deliver water from 

upper basin States to lower basin States. Water managers 

increasingly want to know the predictability of climate 

and its effects on water resources over annual, decadal, 

and longer term spans.

Climate, drought, and streamflow in the Colorado 

River are linked in poorly understood ways. Initial 

understanding of flows in the system was based on a 

relatively short historical record that is now believed to 

be a period of above-average precipitation. Examination 

of long-term records based on tree-ring analyses suggests that drought magnitude and persistence patterns 

are associated with much broader hemispheric climate 

patterns; however, these correlations are imperfect and 

do not provide a clear understanding of long-term precipitation patterns. 

Currently, there is no reliable way to predict how long 

the early 21st century drought will last in the Colorado 

River Basin. Components of the climate system, such 

as sea-surface temperature of the Atlantic and Pacific 

Oceans, provide some context for understanding past 

variations in precipitation and streamflow, but they are 

insufficient for predicting the fate of the ongoing drought. 

Time series of the relevant climate indices indicate a 

large amount of year-to-year variability and relatively 

rapid changes from one regime to another. Above-average 

precipitation for winter 2004–05 and forecasts for aboveaverage runoff may signal the end of the drought, or the 

drought conditions may resume after the present El Niño 

ends. Both outcomes underscore the unpredictability of 

climatic shifts affecting the Colorado River Basin. 

References

Andrade, E.R., Jr., and Sellers, W.D., 1988, El Niño and 

its effect on precipitation in Arizona and western New 

Mexico: Journal of Climatology, v. 8, p. 403–410.

Cayan, D.R., Dettinger, M.D., Diaz, H.F., and Graham, 

N.E., 1998, Decadal variability of precipitation over 

western North America: Journal of Climate, v. 11, p. 

3148–3166.

Cayan, D.R., Redmond, K.T., Riddle, L.G., 1999, 

ENSO and hydrologic extremes in the western United 

States: Journal of Climate, v. 12, p. 2881–2893.

Cayan, D.R., and Webb, R.H., 1992, El Niño/Southern Oscillation and streamflow in the western United 

States, in Diaz, H.F., and Markgraf, V., eds., El Niño, 

historical and paleoclimatic aspects of the Southern 

Oscillation: Cambridge, England, Cambridge University Press, p. 29–68. 

Christiensen, N.S., Wood, A.W., Voisin, N., Lettenmaier, 

D.P., and Palmer, R.H., 2004, The effects of climate 

change on the hydrology and water resources of the 

Colorado River basin: Climatic Change, v. 62, p. 

337–363.

Cook, E.R., Woodhouse, C.A., Eakin, C.A., Meko, 

D.M., and Stahle, D.W., 2004, Long-term aridity 

changes in the western United States: Science, v. 306, 

p. 1015–1018.

Dettinger, M.D., and Cayan, D.R., 1995, Large-scale 

atmospheric forcing of recent trends toward early 

snowmelt runoff in California: Journal of Climate, v. 

8, p. 606–623.

Diaz, H.F., and Markgraf, V., eds., 1992, El Niño, 

historical and paleoclimatic aspects of the Southern 

Oscillation: Cambridge, England, Cambridge University Press, 476 p. 

Enfield, D.B., Mestas-Nuñez, A.M., and Trimble, P.J., 

2001, The Atlantic multidecadal oscillation and its 

relation to rainfall and river flows in the continental 

U.S.: Geophysical Research Letters, v. 28, p. 2077–2080.

Gray, S.T., Betancourt, J.L., Fastie, C.L., and Jackson, 

S.T., 2003, Patterns and sources of multidecadal oscillations in drought-sensitive tree-ring records from the 

central and southern Rocky Mountains: Geophysical 

Research Letters, v. 30, doi:10.1029/2002GL01154, p. 

1:49-1:49-4.

Gray, S.T., Graumlich, L.J., Betancourt, J.L., and 

Pederson, G.T., 2004, A tree-ring based reconstruction 

of the Atlantic Multidecadal Oscillation since 1567 

A.D.: Geophysical Research Letters, v. 31, L12205, 

doi:10.1029/2004GL019932, 4 p.
Climatic Fluctuations, Drought, and Flow in the Colorado River 67

Hereford, R., Webb, R.H., and Graham, S., 2002, 

Precipitation history of the Colorado Plateau region, 

1900–2000: U.S. Geological Survey Fact Sheet 119-

02, 4 p.

Hidalgo, H.G., and Dracup, J.A., 2004, Evidence of the 

signature of North Pacific multidecadal processes on 

precipitation and streamflow variations in the upper 

Colorado River basin, in Van Riper, C., III, and Cole, 

K.L., eds., The Colorado Plateau, cultural, biological 

and physical research: Tucson, University of Arizona 

Press, p. 257–265.

Hoerling, M., and Kumar, A., 2003, The perfect ocean 

for drought: Science, v. 299, p. 691–694.

Kerr, R.A., 2000, A North Atlantic climate pacemaker 

for the centuries: Science, v. 288, p. 1984–1986.

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note on 20th century equatorial Pacific sea surface 

temperatures: http://www.gfdl.noaa.gov/~tk/Note_

on_Eq_Pac_SSTs.html, accessed February 21, 2005.

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Colorado River below Green River, Utah: U.S. Geological Survey Water Supply Paper 556, 176 p.

Mantua, N.J., and Hare, S.R., 2002, The Pacific decadal 

oscillation: Journal of Oceanography, v. 58, p. 35–42.

McCabe, G.J., and Clark, M.P., in press, Trends and 

variability in snowmelt runoff in the western United 

States: Journal of Hydrometeorology.

McCabe, G.J., Palecki, M.A., and Betancourt, J.L., 2004, 

Pacific and Atlantic Ocean influences on multidecadal 

drought frequency in the United States: Proceedings of the National Academy of Science, v. 101, p. 

4136–4141.

Meko, D.M., Stockton, C.W., and Boggess, W.R., 1995, 

The tree-ring record of severe sustained drought: 

American Water Resources Association, Water 

Resources Bulletin, v. 31, p. 789–801.

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and streamflow variability in the western United States 

and their relationship to large-scale circulation indices: 

Water Resources Research, v. 27, p. 2381–2399.

Salzer, M.W., 2000, Dendroclimatology in the San Francisco Peaks region of northern Arizona, USA: Tucson, 

University of Arizona, unpublished Ph.D. dissertation, 211 p.

Schubert, S.D., Suzrez, M.J., Pegion, P.J., Koster, R.D., 

and Bacmeister, J.T., 2004, On the cause of the 1930s 

Dust Bowl: Science, v. 303, p. 1855–1859.

Stewart, I.T., Cayan, D.R., and Dettinger, M.D., 2004, 

Changes in snowmelt runoff timing in western North 

America under a ‘business as usual’ climate change 

scenario: Climatic Change, v. 62, p. 217–232.

Stewart, I.T., Cayan, D.R., and Dettinger, M.D., 2005, 

Changes toward earlier streamflow timing across 

western North America: Journal of Climate, v. 18, p. 

1136–1155.

Stockton, C.W., and Jacoby, G.C., Jr., 1976, Long-term 

surface-water supply and streamflow trends in the 

upper Colorado River basin based on tree-ring analyses: Lake Powell Research Project Bulletin No. 18, 70 p.

Tarboton, D.G., 1995, Hydrologic scenarios for severe 

sustained drought in the southwestern United States: 

American Water Resources Association, Water 

Resources Bulletin, v. 31, p. 803–813.

Webb, R.H., McCabe, G.J., Hereford, R., and 

Wilkowske, C., 2004, Climatic fluctuations, drought, 

and flow of the Colorado River: U.S. Geological Survey Fact Sheet 2004-3062, 4 p.

Woodhouse, C.A., 2003, A 431-yr reconstruction of 

western Colorado snowpack from tree rings: Journal 

of Climate, v. 16, p. 1551–1561.
68 The State of the Colorado River Ecosystem in Grand Canyon

Contact Information:

Robert H. Webb

Research Hydrologist

U.S. Department of the Interior

U.S. Geological Survey

Water Resources Discipline

Tucson, AZ 

rhwebb@usgs.gov

Richard Hereford

Research Geologist (emeritus)

U.S. Department of the Interior

U.S. Geological Survey

Western Earth Surface Processes Team

Flagstaff, AZ

rhereford@usgs.gov

Gregory J. McCabe

Physical Scientist

U.S. Department of the Interior

U.S. Geological Survey

Water Resources Discipline

Denver, CO

gmccabe@usgs.gov

First page photograph credit: Andrew Pernick, Bureau of Reclamation Andrew Pernick, Bureau of Reclamation
Chapter 4

Water Quality 

in Lake Powell 

and the 

Colorado River

William S. Vernieu

Susan J. Hueftle

Steven P. Gloss

Introduction

Water temperature, nutrient concentrations, turbidity, and other water-quality parameters are of interest 

to managers and scientists because these parameters 

influence a range of ecosystem components, from support of aquatic microorganisms and invertebrates to the 

behavior of native and nonnative fishes. For example, 

declines of Colorado River Basin native fishes and 

changes in their condition have been attributed, in part, 

to low water temperatures downstream from dams, such 

as Glen Canyon Dam, that release water from deeper 

portions of the reservoir (Clarkson and Childs, 2000). 

Similarly, water quality is an important determinant of 

food-web structure in aquatic habitats and abundance of 

consumers like fish in those food webs (Carpenter and 

Kitchell, 1996; Wetzel, 2001). 

Any investigation of the dynamics of the Colorado 

River ecosystem in Grand Canyon must not only 

document and understand the water quality in Grand 

Canyon itself but also the water quality in Lake Powell, 

the reservoir created by Glen Canyon Dam. The 

impoundment of a river system in a reservoir alters 

downstream water quality in many ways (Nilsson and 

others, 2005). The formation of Lake Powell in 1963 was 

accompanied by reductions in suspended-sediment and 

nutrient transport and by changes in seasonal temperatures, discharge levels, and benthic community structure of the Colorado River (Paulson and Baker, 1981; 

Stevens and others, 1997; Topping and others, 2000 

a, b). More recently, reservoir and downstream water 

quality has been affected by reservoir drawdown from 

a 5-yr basinwide drought in the Western United States. 

Water released from Glen Canyon Dam in 2003 and 

2004 was the warmest recorded since August 1971, when 

Lake Powell was in its initial filling period (initial filling 

of the reservoir began in 1963 with the closure of Glen 

Canyon Dam, and it reached full pool of 3,700 ft for the 

first time in 1980). Changes in stratification and the fate 

of inflow currents in Lake Powell under various storage 

conditions, as well as various operational scenarios such 

as experimental releases and a proposed temperature 

control device, could have significant effects on the quality of water released from Glen Canyon Dam.

This chapter provides an overview of water-quality 

trends and conditions in Lake Powell and the Grand 

Canyon ecosystem. Because Lake Powell and Glen 

Canyon Dam operations have a strong influence on 
70 The State of the Colorado River Ecosystem in Grand Canyon

downstream water quality, the water quality of the 

reservoir is discussed in some detail. The chapter also 

addresses recent drought-induced changes and the effects 

of the modified low fluctuating flow (MLFF) alternative. The monitoring of water quality in Lake Powell 

is conducted by the U.S. Geological Survey’s (USGS) 

Grand Canyon Monitoring and Research Center under 

separate funding from the Bureau of Reclamation 

and is not funded by the Glen Canyon Dam Adaptive 

Management Program.

Background

Glen Canyon Dam has a structural height of 710 ft 

(216 m). This high, concrete-arch dam backs up water 

for 186 mi (299 km) to form Lake Powell, the second 

largest reservoir in the United States. Lake Powell had 

an original capacity of 27.1 million acre-feet (maf) 

(33,414 million m3

) and a surface area of 161,390 acres 

(65,315 ha) at full pool elevation of 3,700 ft (1,128 m). 

By 1986, this capacity had been reduced to 26.2 maf 

(32,305 million m3

) because of an estimated loss of 

capacity of 30,000 acre-feet (af) (36,990,000 m3

) per 

year resulting from sedimentation (Ferrari, 1988). Water 

can be released from Glen Canyon Dam through three 

separate structures (spillways, penstocks, and river outlet 

works). The majority of water is routed through eight 

penstocks, which feed the powerplant turbines. The 

penstock inlets are at an elevation of 3,470 ft (1,058 m) 

and have a maximum combined discharge capacity of 

approximately 33,200 cubic feet per second (cfs) when 

the reservoir is full. Water can also be released from (1) 

the river outlet works at an elevation of 3,374 ft (1,028 

m) and (2) two spillways at an elevation of 3,648 ft (1,112 

m), both of which bypass the powerplant turbines and 

have discharge capacities of 15,000 cfs and 208,000 cfs, 

respectively (Bureau of Reclamation, 1981) (fig. 1). 

Lake Powell 

Glen Canyon Dam began storing water on March 

13, 1963, and full pool elevation was reached on June 

22, 1980. Ninety-six percent of the reservoir’s inflow is 

received from the Colorado and San Juan Rivers; the 

majority of this inflow is received from May to July as 

the result of snowmelt in the Rocky Mountains (Stanford 

and Ward, 1991). The impoundment of the Colorado 

River by Glen Canyon Dam altered the quality, seasonal 

release volumes, and the amount of daily fluctuations 

for the Colorado River ecosystem downstream of the 

dam. Colorado River water is now transformed by an 

approximate 2-yr residence time in Lake Powell and 

by the structure and operation of Glen Canyon Dam. 

These factors influence the temperature, suspended and 

dissolved solids, nutrients, and organisms that pass downstream as well as the volume of water released and the 

magnitude of fluctuations. 

Lake Powell has a maximum depth immediately 

upstream of Glen Canyon Dam of approximately 515 ft 

(157 m) at full pool elevation; the lake is vertically stratified into density layers and differs longitudinally as the 

currents move through the reservoir. Vertical stratification varies seasonally and is determined by the relative 

density of the different layers of the reservoir. Density 

is determined by water temperature and the amount of 

dissolved minerals and suspended solids. The surface 

layer of the reservoir, or epilimnion, warms through 

summer and is eventually mixed with deeper water by 

the wind and convective currents during the winter cooling period, which extends from October to early March. 

The epilimnion exhibits the highest level of biological 

activity because of warm temperatures and light availability. Water temperature decreases with depth in the 

metalimnion, the layer that separates the epilimnion 

from the bottom layer of the reservoir, or hypolimnion. 

The hypolimnion consistently exhibits lower temperatures, lower dissolved oxygen levels, and higher salinity 

concentrations than the other layers of the reservoir. 

Because of the subsurface position of the penstocks, 

water may be withdrawn from the epilimnion, metalimnion, or hypolimnion depending on reservoir level, reservoir hydrodynamics, timing and strength of stratification, 

and magnitude of withdrawals.

Longitudinal variation in water quality is the result 

of currents moving through the reservoir. The portions of the reservoir farthest from the dam exhibit 

characteristics similar to those of the river entering the 

reservoir, with more variable temperature and salinity 

patterns and higher sediment and nutrient concentrations. Primary productivity from photosynthesis is limited 

by light availability in this more turbid riverine zone. 

The deeper portions of the reservoir closest to the dam, 

or the lacustrine zone, exhibit characteristics similar to 

those of a lake system, with more stable temperature and 

salinity patterns, low suspended-sediment concentration, 

and lower nutrient concentrations. Primary productivity 

in this zone is limited by nutrient availability. A transition 

zone of intermediate characteristics separates the riverine and lacustrine zones (Kimmel and Groeger, 1984; 

Department of the Army, Corps of Engineers, 1987; 

Ford, 1990). The relative location of these zones depends 

on reservoir levels and the magnitude of inflows. In the 
Water Quality in Lake Powell and the Colorado River 71

main channel of Lake Powell, the riverine zone extends 

from the Colorado River inflow to Hite Bay, the transition zone extends from Hite Bay to the Bullfrog Bay 

area, and the lacustrine zone extends from Bullfrog Bay 

to Glen Canyon Dam (figs. 1 and 2).

 The depth at which river water enters the reservoir 

is dictated by its density relative to the density of the 

water already in the reservoir. Spring and early summer 

snowmelt runoff entering the reservoir tends to be dilute, 

has warmed during its passage through the canyonlands, 

and represents the lowest density water entering the 

reservoir during the year. Consequently, this water travels 

through the reservoir as an overflow density current. 

During the winter months, inflows are colder and more 

saline and represent the highest density water entering 

the reservoir. Depending on the relative density of the 

hypolimnion, winter inflows will either flow along the 

bottom of the reservoir, routing fresh water to the hypolimnion and displacing older water upward, or flow into 

intermediate layers, leaving deeper waters stagnant. 

Convective mixing takes place in the epilimnion as 

the reservoir cools during the fall and winter months. 

By the end of the calendar year, convective mixing in 

the upper layers progresses to the point that penstock 

withdrawals begin to exhibit characteristics of the 

epilimnion, which contains the warmest water in the 

reservoir at that time of year, despite the cooler weather 

conditions. This convective mixing results in the warmest 

release temperatures of the year occurring in late fall or 

early winter. 

Downstream of Glen Canyon Dam

Changes to the chemical and physical quality of the 

water of the Colorado River after its release from Glen 

Canyon Dam are affected by ambient meteorological 

conditions, primary production and respiration from the 

aquatic environment, aeration from rapids, inputs from 

other tributary sources and overland flow, and various 

aspects of the operation of Glen Canyon Dam.

Figure 1. Profile of Lake Powell from Glen Canyon Dam to the inflow of the Colorado River, illustrating the vertical stratification and 

horizontal zonation of the reservoir at or near full pool elevation, September 1999. Also shows the elevations of each of the three

release structures and their capacities as well as an approximation of the wedge of deltaic sediments. Y axis on left is measurement of 

elevation above mean sea level and on right is actual depth.
72 The State of the Colorado River Ecosystem in Grand Canyon

Water released from Glen Canyon Dam is usually 

colder than the surrounding environment and warms as 

it flows downstream with exposure to solar radiation and 

warmer ambient air temperatures. The exception to this 

pattern is during portions of the winter months when 

dam releases are slightly warmer than the surrounding 

environment and cool as they flow downstream before 

warming again in lower elevation reaches.

The aquatic environment affects dissolved oxygen 

concentrations and pH in the tailwater (referred to as the 

Lees Ferry reach elsewhere in this report), which is the 

15 mi (24 km) of the river that extends downstream from 

Glen Canyon Dam to Lees Ferry. This area is free of 

significant tributary sediment inputs that limit light availability for primary production (Yard and others, 2005). 

As a result of photosynthetic activity, therefore, dissolved 

oxygen concentrations and pH in the tailwater display 

daily oscillations at Lees Ferry. During daylight hours dissolved oxygen concentrations and pH increase because 

of the addition of oxygen and removal of carbon dioxide 

during photosynthesis. The opposite occurs at night 

when respiratory processes become dominant (Marzolf 

and others, 1999; U.S. Geological Survey, 2001).

Under normal powerplant discharges, limited aeration of the river occurs in the tailwater reach of the river 

compared to downstream reaches. Generally, released 

water that may be lower in dissolved oxygen does not 

reach full saturation until the first rapids in Marble 

Canyon, where the water is aerated by turbulence; 

however, during periods when the river outlet works are 

Figure 2. Lake Powell water-quality sampling sites.
Water Quality in Lake Powell and the Colorado River 73

operated, such as during the 1996 beach/habitat-building flow or the 2004 experimental high flow, turbulence 

immediately below the dam is sufficient to bring release 

water up to full oxygen saturation (Hueftle and Stevens, 

2001).

Various tributaries that enter Grand Canyon 

can significantly affect water quality of the Colorado 

River below Glen Canyon Dam. The Paria and Little 

Colorado Rivers can carry large amounts of fine sediment that limit light availability for primary production 

and may enhance conditions for native fish that use turbid water for cover from predation (Shannon and others, 

1994; Topping and others, 2000 a, b). Some tributaries, 

such as the Little Colorado River, are significant sources 

of salinity to the mainstem Colorado River (Cole and 

Kubly, 1976). 

Water-quality Monitoring

Lake Powell

The purpose of water-quality monitoring in Lake 

Powell is to document and understand the water-quality 

changes that occur during the residence time of the 

water in the reservoir and how those changes may affect 

the quality of water being released from Glen Canyon 

Dam under various conditions.

Water-quality monitoring of Lake Powell currently 

has two main components. Monthly surveys of the forebay, the pool of water in front of the dam, take place at 

the mouth of Wahweap Bay, approximately 1.5 mi (2.4 

km) upstream of Glen Canyon Dam, to document the 

quality of water in dam releases. Reservoir-wide surveys 

are conducted quarterly to describe seasonal changes in 

the stratification and hydrodynamics of the reservoir and 

to better understand the reason for observed changes in 

downstream releases.

Water-quality sampling in Lake Powell was initiated 

by the Bureau of Reclamation in 1964 and continued 

through 1990, including several phases of differing 

sampling frequencies for the reservoir and forebay. Glen 

Canyon Environmental Studies conducted the monitoring from 1990 to 1996. The USGS Water Resources 

Discipline conducted monitoring in Lake Powell on 

several dates in 1992, 1994, and 1995 (U.S. Geological 

Survey, 1998). Since 1997, monitoring has been conducted by the USGS Grand Canyon Monitoring and 

Research Center.

 Monthly monitoring of the forebay allows for the 

observation of conditions immediately upstream of Glen 

Canyon Dam and for the description of the dynamics 

of the water column that is the immediate source for 

downstream releases. Quarterly reservoir-wide sampling 

describes seasonal conditions at 20–25 stations throughout the reservoir during the maximum extent of winter 

convective mixing, spring runoff, post runoff/late summer stratification, and early winter conditions during the 

early phases of convective mixing (fig. 2, table 1). 

At each station, data on basic water-quality parameters—temperature, specific conductance, dissolved 

oxygen, pH, oxidation-reduction potential, and turbidity—are collected through the water column. At selected 

depths, chemical (major ions and nutrients) and biological (chlorophyll and plankton) sampling is performed to 

characterize the major strata in the water column. Major 

ions are the common negative (e.g., chloride) and positive 

(e.g., calcium) ions that constitute the majority of minerals dissolved in water. Nutrients represent the total and 

dissolved fractions of compounds of phosphorus and 

nitrogen, which are essential for the production of plant 

life (algae or phytoplankton).

Glen Canyon Dam Tailwater

Water-quality monitoring activities in the dam’s 

tailwater assess the initial quality of water leaving the reservoir and entering Grand Canyon. These baseline measurements are important for detecting changes occurring 

in Grand Canyon and for understanding the relationship 

between the quality of water leaving the reservoir and its 

relationship to the downstream aquatic ecosystem (fig. 3). 

The USGS recorded daily instantaneous water 

temperatures at Lees Ferry from 1949 to 1977 (U.S. 

Geological Survey, 2004). Since then, temperatures 

recorded at Lees Ferry reflect mean daily values of multiple observations (U.S. Geological Survey, 1985–2004). 

Glen Canyon Environmental Studies began monitoring 

the temperature and conductivity of dam releases in 

1988 by using remotely deployed, continuously logging monitors. In 1991, this program was expanded to 

include continuous monitoring at Lees Ferry. Dissolved 

oxygen and pH measurements were added to the monitoring protocol shortly afterwards. 

Tailwater monitoring activities currently include the 

continuous measurement of temperature, salinity, dissolved oxygen, and pH and monthly sampling for phosphorus, nitrogen, major-ion chemistry composition, and 

biological indicators such as chlorophyll and plankton.
74 The State of the Colorado River Ecosystem in Grand Canyon

Table 1. Lake Powell and tailwater sampling sites.

Site name Distance in Chemical and

 miles (kilometers) biological sampling

 from Glen Canyon Dam

Tailwater

 Colorado River below Glen Canyon Dam 0 X

 Colorado River at Lees Ferry -15.5 (-24.9) X

Colorado River main channel

 Wahweap 1.5 (2.4) X

 Crossing of the Fathers 28.1 (45.2) X

 Oak Canyon 56.2 (90.5) X

 San Juan River confluence 62.2 (100.1) 

 Escalante 72.6 (116.9) X

 Iceberg 86.7 (139.5) 

 Lake Canyon 98.6 (158.7) 

 Bullfrog Bay 104.3 (167.9) X

 Moki Canyon 111.8 (179.9) 

 Knowles Canyon 120.1 (193.3) 

 Lower Good Hope Bay 129.6 (208.5) X

 Scorup Canyon 140.1 (225.5) 

 Hite Basin 148.3 (238.7) X

 Colorado River inflow 149.1–185.8 (240.0–299.0) X

San Juan River arm

 Cha Canyon 12.0 (19.3) X

 Lower Piute Bay 20.4 (32.9) 

 Upper Piute Bay 26.8 (43.1) X

 Lower Zahn Bay 38.8 (62.5) 

 Mid Zahn Bay 42.6 (68.6) 

 San Juan inflow 32.3–54.1 (52.0–87.0) X

Escalante River arm

 Escalante at Clear Creek 4.5 (7.2) 

 Escalante at Davis Gulch 7.4 (11.9) X

 Escalante at Willow Creek 12.4 (20.0) 

 Escalante inflow 13.7–24.8 (22.0–40.0) X

Downstream Thermal Monitoring 

in Grand Canyon

Downstream thermal monitoring provides an 

indication of status and trends in water temperature and 

how warming is affected by river reach, seasonality, and 

dam operations. Concerns about the effects of the thermal regime on both native and nonnative fish resulted 

in the development of a continuous thermal monitoring 

program in Grand Canyon beginning in 1990. Thermal 

monitoring was conducted at 10 mainstem stations at 

intervals of roughly 30 mi (48 km) and at 8 additional 

sites on major tributaries. Tributary sites have been 

monitored since 1994, providing thermal baseline data 

for streams that may act as warmwater refugia for many 

aquatic species, particularly native fish. In 2005, thermal 

monitoring in tributaries was reduced to four sites, the 

Paria River, the Little Colorado River, Kanab Creek, 

and Havasu Creek.

In 2002, the thermal monitoring program in the 

mainstem Colorado River was expanded to include 

multiparameter monitoring stations throughout Grand 

Canyon to collect time-series measurements of water 

temperature, specific conductance, dissolved oxygen, 

and pH at five sites where suspended-sediment transport 

is also monitored. In 2005, mainstem monitoring was 

reduced to temperature and specific conductance measurements (fig. 3). 
Water Quality in Lake Powell and the Colorado River 75

Trends and Current 

Conditions

Hydrology

Because of a prolonged drought between 2000 and 

2005, Lake Powell water storage was reduced by approximately 60%. Water year (WY) 2004, which ended on 

September 30, 2004, was the fifth consecutive year of 

below-normal inflows to Lake Powell; inflows were at 

51% of average in WY 2004 (table 2). Inflow in WY 

2002 was the lowest observed since the completion of 

Glen Canyon Dam in 1963. This drought period resulted 

in a 130 ft (40 m) drop in reservoir elevation and a 13 

maf (16,029 million m3

) decline in storage in Lake Powell 

by the end of WY 2004 (fig. 4). While precipitation in 

the upper Colorado River Basin increased substantially 

during the first part of WY 2005, storage in Lake Powell 

continued to decline until the reservoir reached an elevation of 3,555 ft (1,084 m) on April 8, 2005, after which 

snowmelt runoff and reduced dam releases increased the 

reservoir elevation. Average unregulated inflow to Lake 

Powell is 12.056 maf (14,865 million m3

), as determined 

from the 30-yr record that spans WY 1971 through WY 

2000 (Tom Ryan, Bureau of Reclamation, oral commun., 2005); however, the average inflow for the water 

years from 2000 to 2004 was 5.962 maf (7,351 million 

m3

) (table 2).

Figure 3. Grand Canyon water-quality sampling sites.
76 The State of the Colorado River Ecosystem in Grand Canyon

Salinity

Salinity levels are of concern throughout the entire 

Colorado River Basin because high salinity can be 

damaging to soils and crops. Furthermore, treaty obligations with Mexico limit the salinity of water that can be 

delivered to that country. As the Colorado River flows 

to the Gulf of California, it leaches salts from soils and 

other geologic substrates through and over which it flows. 

Salinity levels are also increased by irrigation returns, 

by evaporation in storage facilities, and by rate of flow 

(slow-flowing water picks up higher levels of dissolved 

solids than do high flows during runoff). 

Table 2. Recent inflows and releases at Glen Canyon Dam (maf = million acre-feet).

 Water April–July Percent WY Percent Glen End of End of

 year unregulated of unregulated of Canyon year year

 (WY) inflow average inflow average Dam release storage elevation

 (maf) (maf) (maf) (maf) (ft)

 1998 8.625 112 13.661 116 13.511 22.403 3687.7

 1999 7.621 99 12.71 108 11.202 22.997 3691.6

 2000 4.352 56 7.310 62 9.380 20.939 3677.8

 2001 4.301 56 6.955 59 8.238 19.135 3664.8

 2002 1.115 14 3.058 25 8.230 14.468 3626.5

 2003 3.918 51 6.358 53 8.228 12.110 3603.8

 2004 3.640 46 6.128 51 8.231 9.169 3570.8

Figure 4. Lake Powell surface elevation, 1963–2005.

Periodically the salinity of water released from the 

dam increases as a result of drought. This increase is 

due to a combination of factors, including increases in 

the salinity of base flows into the reservoir, lack of large 

volumes of dilute snowmelt runoff, and reduced reservoir 

volume to dilute the effects of reservoir inflows. At the 

end of WY 2004, releases from Glen Canyon Dam had a 

specific conductance of approximately 850 microsiemens 

per centimeter (S/cm) at 25°C, corresponding to a total 

dissolved solids concentration of 575 mg/L (fig. 5).

Water Temperature

Impounding water in Lake Powell significantly 

affected the water temperature of dam releases to the 

Colorado River ecosystem because of reservoir stratification and the location of the penstock release structures 

(fig. 1). During the summer months, the epilimnion of 

Lake Powell warms considerably from inflows, ambient 

air temperature, and solar radiation, reaching temperatures as high as 86°F (30°C); however, the hypolimnion is 

isolated from these processes, maintaining temperatures 

between 43°F and 48°F (6°C and 9°C). 

Before closure of the dam, mean water temperature for what is now the tailwater was approximately 

57°F (14°C), ranging from 32°F to 80°F (0°C to 27°C) 

over the course of a year (U.S. Geological Survey, 2004). 

Before 1973, during the reservoir’s initial filling stage, 

release temperatures were affected by surface or epilimnetic withdrawals because of the proximity of the 

reservoir’s surface to the penstock withdrawal zone. Max-
Water Quality in Lake Powell and the Colorado River 77

imum release temperature during that period occurred 

during the months of August and September, reflecting 

the surface warming of the reservoir. 

Trends in water temperature of the tailwater stabilized from 1973 to 2003, when the reservoir surface 

elevations were above 3,600 ft (1,097 m) and the epilimnion was situated above the penstock withdrawal zone. 

During this period, release temperatures as measured at 

Lees Ferry averaged 48.7oF (9.3°C). Temperatures fluctuated between 44°F and 54°F (7°C and 12°C), with minor 

excursions beyond this range during periods of spillway 

releases (fig. 6). Under these conditions, there was some 

seasonality to Glen Canyon Dam release temperatures, 

with slight warming beginning in May and June and 

increasing through the year. The highest temperatures 

occurred at the end of December as a result of the influence of the relatively warm, convectively mixed epilimnion on penstock releases. Peak temperatures under 

these conditions appeared to be affected by the volume 

of the previous year’s snowmelt runoff, which affects the 

thickness of the warm epilimnion near the dam during the latter months of the year. Although seasonality 

in temperature patterns exists in the postdam era, the 

annual variation has been reduced to approximately 9oF 

(5°C) from approximately 48oF (27°C) in the predam 

era. Also, the highest river temperatures immediately 

below the dam now occur in late fall or winter instead of 

in summer, which is when they occurred in the predam, 

unregulated river. 

The water level of the reservoir dropped more than 

140 ft (42 m) between 1999 and 2005 as a result of a 

basinwide drought that began in 2000 (fig. 4). This drop 

placed the warmer epilimnetic water much closer to 

the penstock withdrawal zone and resulted in reservoir 

releases being drawn from this epilimnetic layer. Substantially warmer release temperatures have occurred in the 

fall and early winter months since 2003. An annual maximum mean daily release temperature of 55°F (12.9°C) 

was observed on November 14, 2003; on November 6, 

2004, the annual maximum mean daily temperature 

reached 59°F (15°C) (fig. 6). These values represent the 

highest release temperatures from Glen Canyon Dam 

since August 1971, when the reservoir was filling. As of 

July 11, 2005, the mean daily release temperatures had 

reached 56.4°F (13.6°C), showing earlier warming and 

higher temperatures than had occurred in the past 2 yr. 

Seasonal and longitudinal water temperature patterns in Grand Canyon have been measured from 1994 

to 2005 from Glen Canyon Dam to Diamond Creek, 

241 mi (388 km) below Glen Canyon Dam (fig. 7). During summer months, gradual downstream warming 

occurs because of the transfer of heat from the warmer 

surrounding air mass, heat stored in the canyon walls 

adjacent to the river, and solar radiation. 

A comparison of weekly average increase in water 

temperature between Glen Canyon Dam and Diamond 

Creek to average weekly discharge during mid-June from 

1994 to 2004 demonstrates the effect of Glen Canyon 

Dam releases on warming patterns in the Colorado 

River in Grand Canyon (fig. 8). High steady flows of 

approximately 26,000 cfs in 1997 resulted in 9°F (5°C) 

warming at Diamond Creek, while low steady flows of 

Figure 5. Mean daily specific conductance (in microsiemens per 

centimeter (μS/cm) at 25°C), an indicator of salinity, below Glen 

Canyon Dam, 1995–2005.

Figure 6. Daily water temperature (red line) at Lees Ferry as 

affected by changes in Lake Powell’s elevation (green line).
78 The State of the Colorado River Ecosystem in Grand Canyon

8,000 cfs in 2000 exhibited 18°F (10°C) warming. This 

difference is because large volumes of water have greater 

mass and a lower surface area to volume ratio as well as 

less exposure time for atmospheric heat exchange that is 

due to higher velocity, reducing the amount of warming 

from ambient temperatures and solar radiation. The 

warming occurring at low discharges affects water temperatures in lower Grand Canyon to a greater degree 

than the elevated release temperatures observed in the 

past 2 yr.

Lateral variation in river temperature also occurs 

throughout Grand Canyon. Substantial warming occurs 

in various nearshore environments, ranging from shallow, 

open-water areas to enclosed backwaters. Water in certain nearshore environments becomes isolated from mixing with the main channel current and warms with solar 

radiation and equilibration with ambient temperatures. 

These environments may be important to the survival, 

growth, and eventual recruitment of the larval life stages 

of native fish (see chapter 2, this report).

Figure 7. Water temperatures along the Colorado River from Glen Canyon Dam to Diamond Creek, 1994–2005. Black dots represent 

monitoring locations.

Figure 8. Mid-June warming above release temperatures 

at Diamond Creek, 1994–2004, as a function of mean weekly 

discharge (in cubic feet per second). Warming at Diamond Creek 

= 0.000532 * Q + 21.01.
Water Quality in Lake Powell and the Colorado River 79

Turbidity and

Suspended Sediment

Construction of Glen Canyon Dam dramatically 

altered the sediment-transport processes of the Colorado 

River. Before the completion of Glen Canyon Dam, the 

total sand supply to Grand Canyon, from the Colorado 

River upstream from Lees Ferry, with the Paria and Little 

Colorado Rivers combined, was approximately 29 million tons (26 million Mg). Today, because Lake Powell 

traps all of the sediment upstream from Glen Canyon 

Dam, the Paria River is the primary source of sand to 

Marble Canyon, supplying approximately 6% of predam 

sand levels (see chapter 1, this report). Only a small portion of the suspended sediment entering Lake Powell is 

transported for any distance because most of it is deposited near the inflows of major tributaries.

Turbidity and suspended-sediment concentrations 

are of interest in the downstream environment because 

water clarity affects the amount of light available for 

photosynthesis for downstream algal communities, which 

are an important part of the overall food base for native 

and nonnative fishes. Turbidity also affects the behavior 

and distribution of various native and nonnative fishes 

in providing cover from various predators or by affecting sight-feeding abilities. Turbidity is measured in both 

Lake Powell and downstream. Turbidity measurements 

in Lake Powell indicate the location of advective tributary inflows and also can be used as an indicator of 

primary productivity in the reservoir because increased 

turbidity indicates the presence of phytoplankton. The 

rather abrupt decrease in filamentous green alga below 

Lees Ferry most probably results from inputs of sediment 

from major tributaries, including the Paria and Little 

Colorado Rivers, which reduce light penetration (Cole 

and Kubly, 1976; Stevens and others, 1997).

Nutrients

Nutrients such as phosphorus, nitrogen, and silica 

are essential for microbial production and algal growth. 

Most phosphorus entering Lake Powell is associated 

with suspended clays in the inflows of river water. 

The reservoir acts as a nutrient sink, especially for 

phosphorus. More than 95% of phosphorus reaching 

Lake Powell is in particulate form or is associated with 

suspended sediment particles. A large fraction of this 

phosphorus load is deposited within the reservoir by 

sedimentation (Gloss, 1977). Most of the remaining dissolved phosphorus is removed from the water by uptake 

from biological activity. 

Bioproduction in Lake Powell apparently is directly 

related to the intensity and duration of enriched spring 

inflow events that are responsible for delivering the bulk 

of nutrient capital to the reservoir (Gloss and others, 

1980). Surface concentrations of dissolved phosphorus 

generally decline from the upper end of the reservoir to 

the dam because of the uptake from primary production, 

to the point that dissolved phosphorus is usually below 

detection limits within 30–60 mi (48–97 km) upstream 

from the dam in the upper water column. Phosphorus 

is the limiting factor for primary production near the 

dam, while low light availability is the limiting factor to 

productivity in the upper portion of Lake Powell because 

of turbidity from inflow currents, especially during early 

summer months (Gloss and others, 1980) (fig. 9a).

Nitrate-nitrogen concentrations from the surface of 

the reservoir forebay fluctuate in a manner that reflects 

the utilization of the nitrogen by algae and begin to 

increase in fall as primary production slows. Nitratenitrogen concentrations show a peak in winter when 

temperatures are coolest and productivity is relatively 

low. During the summer months, when primary productivity is at a maximum, nitrate-nitrogen concentrations reach a minimum because of uptake by primary 

producers. From 1999 to 2004, surface nitrate-nitrogen 

concentrations in the forebay above Glen Canyon Dam 

averaged 0.09 parts per million (ppm). Nitrate-nitrogen 

concentrations in the deepest part of the hypolimnion 

averaged 0.39 ppm, about four times higher than surface 

concentrations. For the same period, nitrate-nitrogen 

concentrations in Glen Canyon Dam releases averaged 

0.29 ppm (fig. 9b). 

The highest productivity in Lake Powell is seen in 

surface waters of the reservoir and results from a combination of temperature, light availability, and nutrient 

concentrations. Because primary productivity processes 

consume nutrients, nutrient concentrations are eventually depleted in the surface waters of Lake Powell and 

remain at elevated concentrations in the hypolimnion, 

where there is little primary production taking place. 

Consequently, hypolimnetic releases from Glen Canyon 

Dam are relatively nutrient rich. Periods of epilimnetic 

releases from Glen Canyon Dam may cause a reduction in the amount of nutrients available to the downstream ecosystem.

Dissolved Oxygen

Dissolved oxygen concentrations in Lake Powell are 

affected by inflow, seasonal water-circulation patterns, 

and biological processes (Johnson and Merritt, 1979). 
80 The State of the Colorado River Ecosystem in Grand Canyon

The spring snowmelt runoff enters Lake Powell as an 

overflow density current, representing the lowest density 

water entering the reservoir during the year. This inflow 

current then travels through the reservoir slightly below 

the surface, eventually reaching Glen Canyon Dam by 

late summer or early fall. A large amount of suspended 

sediment, nutrients, and organic material may be associated with this inflow current. As the inflow current travels through the reservoir, the organic material undergoes 

bacterial decomposition, removing large amounts of 

oxygen from this water. This situation, combined with 

decomposition of plankton from the epilimnion, results 

in a marked reduction of dissolved oxygen in the metalimnion of the reservoir by late summer.

During periods of reservoir drawdown, as in the 

past several years, tributary inflows cause the resuspension of exposed deltaic sediments in the upstream 

portions of the reservoir. This resuspension entrains 

large amounts of suspended sediment and decomposing organic material in the advective inflow currents 

that move through the reservoir. In September 2003, the 

inflow plume extended downstream to Padre Bay, about 

28 mi (45 km) above Glen Canyon Dam, at depths of 

50–80 ft (15–25 m) (fig. 10). Dissolved oxygen concentrations at or near zero were observed throughout its extent. 

This plume reached Glen Canyon Dam by October and 

was eventually dissipated by convective mixing in the 

following months. The pattern was observed to a lesser 

extent in 2004 because runoff volumes were smaller.

The hypolimnion of Lake Powell is isolated from 

photosynthetic oxygenation and reaeration caused 

by surface mixing and acts as a sink for organic matter falling through the water column. As a result, the 

water at the bottom of Lake Powell is often oxygen poor 

throughout the year. Depending on the density of cold 

winter inflows relative to that of the receiving reservoir, 

winter inflows will either form an interflow between 

layers of higher and lower density or may underflow the 

entire reservoir, displacing hypolimnetic water upwards 

for withdrawal by dam releases. In the former case, the 

density of the hypolimnetic water is too great to allow 

an underflow current, and as a result, the hypolimnion 

remains stagnant and oxygen concentrations continue to 

decline. In the latter case, the hypolimnion is refreshed 

with oxygenated water. 

There have been three distinct periods of hypolimnetic stagnation during Lake Powell’s history, usually 

lasting several years and characterized by a buildup of 

relatively saline water, followed by decreasing dissolved 

oxygen concentrations. These periods of hypolimnetic 

stagnation appear to have been dissipated by a series of 

above-average inflows that flushed the reservoir. Abovenormal inflows to Lake Powell during the mid-1990s left 

the reservoir fairly dilute, and distinct winter underflows 

have occurred since 1999 (fig. 11). 

Dissolved oxygen concentrations in the tailwater are 

usually slightly below saturation but have not dropped to 

concentrations low enough to affect the aquatic ecosystem in Grand Canyon. As the reservoir ages or there are 

periods of extended drought, however, it is likely that the 

chances of water low in dissolved oxygen being released 

from Glen Canyon Dam will increase. 

Plankton

The epilimnion of Lake Powell is fairly warm and 

receives abundant sunlight for photosynthesis through 

much of the year. As a result, the majority of phytoFigure 9. A. Total phosphorus concentrations measured (in parts 

per million) in the forebay of Lake Powell and Glen Canyon Dam 

releases, 1999–2004. B. Dissolved nitrate-nitrogen concentrations 

measured (in parts per million) in the forebay of Lake Powell and 

Glen Canyon Dam releases, 1999–2004.

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.10

1/1/1999 1/1/2000 1/1/2001 1/1/2002 1/1/2003 1/1/2004 1/1/2005

P (ppm)

Forebay surface

Forebay bottom

Below dam

0

0.1

0.2

0.3

0.4

0.5

0.6

1/1/1999 1/1/2000 1/1/2001 1/1/2002 1/1/2003 1/1/2004 1/1/2005

N (ppm)

Forebay surface

Forebay bottom

Below dam

A.

B.
Water Quality in Lake Powell and the Colorado River 81

plankton and zooplankton reside in this layer, supporting a recreational fishery in Lake Powell. Under periods of epilimnetic withdrawals—for example during 

winter mixing or during the recent late-summer events 

of 2003, 2004, and 2005, when warmer epilimnetic 

waters are released—plankton from the epilimnion 

can be released downstream, potentially providing an 

alternate food source for the downstream ecosystem (fig. 

12). Little is known about the downstream importance 

of plankton and other organic matter released from 

Glen Canyon Dam.

Effects of Dam Operations

Operation of Glen Canyon Dam affects the water 

quality of Lake Powell and downstream releases. High 

sustained penstock releases during 1973, through the 

mid-1980s, and under modified low fluctuating flow 

(MLFF) operations in 1997 acted to route increased 

volumes of water through the reservoir. The operation 

of different release structures can affect the downstream 

environment by withdrawing water of different quality 

than that at the penstock elevation. Since the implementation of the 1996 Record of Decision and the MLFF 

alternative, the river outlet works have been used during the 1996 beach/habitat-building flow (Hueftle and 

Stevens, 2001) and the 2004 experimental high flow. 

During these events, water of cooler temperature and 

higher salinity and nutrient content was released, and 

the turbulence created downstream of the dam increased 

oxygen concentrations.

During periods of normal operations under the 

MLFF, daily fluctuations in temperature, specific conductance, and dissolved oxygen can be observed in 

Glen Canyon Dam releases because of the effects of 

various discharge volumes and fluctuation patterns 

on the dimension of the withdrawal zone in the reservoir. Large or fluctuating releases draw water from a 

thicker withdrawal zone than do small or steady releases 

(Monismith and others, 1988; Ford, 1990; Casamitjana 

and others, 2003).

Figure 10. Dissolved oxygen concentrations (in parts per million) in the main channel of Lake Powell, from Glen Canyon Dam to the 

Colorado River inflow, September 2003.
82 The State of the Colorado River Ecosystem in Grand Canyon

Figure 11. Forty-year temperature (degrees Fahrenheit), specific conductance (microsiemens per centimeter (μS/cm) at 25°C), and dissolved oxygen (parts per million) patterns 

in the forebay of Lake Powell, December 1964 to March 2005. Elevations of spillways, penstocks, and river outlet works are indicated by horizontal lines in each plot. Warmer colors represent higher values for each parameter. Shaded areas represent areas of low temporal resolution or poor data quality. Note three periods of hypolimnetic stagnation (red areas in conductance section), followed by periods of hypolimnetic hypoxia (blue areas in dissolved oxygen section). Dissolved oxygen section shows recent pattern of winter underflows since 1999.
Water Quality in Lake Powell and the Colorado River 83

In terms of downstream water quality, the magnitude of discharge and the volume of water in the 

river determine how much water is exposed to ambient 

air temperatures and solar radiation, which, in turn, 

determines the amount of in-stream warming in Grand 

Canyon. The magnitude of dam discharges also influences the amount of sediment in suspension, which 

can limit light availability to the downstream aquatic 

environment.

There are many interactions at work, and the use 

of a hydrodynamic water-quality model will help understand the changes in Lake Powell and their effects on 

downstream water quality.

Conclusion and 

Recommendations

The current drought demonstrates how long-term 

climatic trends influence inflows to Lake Powell and 

affect the quality of dam releases. As the elevation of 

Lake Powell drops, the warmer surface water is drawn 

downstream through the penstocks, seasonally increasing 

downstream temperatures. Furthermore, salinity concentrations increase, more epilimnetic biota are exported 

downstream, and dissolved oxygen levels may decrease 

seasonally. Current dissolved-oxygen minimums are 

within the tolerance limits of tailwater organisms, including rainbow trout (Oncorhynchus mykiss). Entrainment of 

an oxygen-depleted inflow plume or hypolimnion, along 

with the aging of the reservoir, however, could affect the 

downstream ecosystem.

If the drought continues, these trends will be exacerbated as summer and fall releases draw increasingly 

from the warmer surface waters of the reservoir. Release 

patterns during an extended drought may replicate those 

of the 1960s when the reservoir began filling, including 

wider fluctuations in water quality that may begin to 

mimic predam conditions. These conditions create an 

opportunity to observe some of the possible effects in 

the Colorado River ecosystem of a temperature control device, which would route warmer surface waters 

through the Glen Canyon Dam powerplant. 

Future Monitoring and Modeling

Current understanding of water quality downstream 

of Glen Canyon Dam in Grand Canyon is limited 

because more extensive sampling has only recently been 

initiated and because there has been a lack of modeling 

Figure 12. Plankton concentrations and biomass in Lake 

Powell forebay.

for river-water quality. Exceptions are water temperature and sediment concentrations (see chapter 1, this 

report), which have been more extensively monitored 

and analyzed. Future water-quality programs should 

emphasize a model-based approach and close linkages 

with other sampling programs like aquatic ecology and 

fine-sediment monitoring.

The extensive 40-yr database available for Lake 

Powell presents a very clear opportunity to model the 

hydrodynamic properties of the reservoir under a variety 
84 The State of the Colorado River Ecosystem in Grand Canyon

of inflow and operational conditions. Such modeling not 

only would increase the understanding and characterization of reservoir limnology but also would serve as an 

important predictive tool to anticipate what the quality of water released to the Colorado River ecosystem 

in Grand Canyon will be. A number of river-flow and 

water-quality models are available and could be used or 

modified to better understand and predict downstream 

changes in water quality and their effects on various 

components of the ecosystem.

References

Bureau of Reclamation, 1981, Water and power 

resources: project data: Denver, Colo., Bureau of 

Reclamation.

Carpenter, S.R., and Kitchell, J.F., eds. 1996, The 

trophic cascade in lakes: Cambridge, England, 

Cambridge University Press, 385 p.

Casamitjana, X., Serra, T., Colomer, J., Baserba, C., and 

Pérez-Losada, J., 2003, Effects of the water withdrawal 

in the stratification patterns of a reservoir: Hydrobiologia, v. 504, p. 21–28.

Clarkson R.W., and Childs, M.R., 2000, Temperature 

effects of hypolimnial-release dams on early life stages 

of Colorado River basin big-river fishes: Copeia, v. 2, 

p. 402–412.

Cole, G., and Kubly, D.M., 1976, Limnologic studies 

in the Colorado River from Lees Ferry to Diamond 

Creek: Colorado River Research Program Technical 

Report No. 8, Colorado River Research Series Contribution No. 37, Grand Canyon National Park.

Department of the Army, Corps of Engineers, 1987, 

EM 1110-2-1201, Engineering and design, reservoir 

water quality analysis: Washington, D.C., p. 2–11.

Ferrari, R.L., 1988, 1986 Lake Powell survey: Denver, 

Colo., Bureau of Reclamation.

Ford, D.E., 1990, Reservoir transport processes, in

Thornton, K.W., Kimmel, B.L., and Payne, F.E., eds., 

Reservoir limnology: ecological perspectives: New 

York, Wiley, p. 15–41.

Gloss, S.P., 1977, Application of the nutrient loading concept to Lake Powell, the effects of nutrient 

perturbations on phytoplankton productivity, and 

levels of nitrogen and phosphorus in the reservoir: 

Albuquerque, University of New Mexico, Ph.D. thesis, 

225 p. 

Gloss, S.P., Mayer, L.M., and Kidd, D.E., 1980, Advective control of nutrient dynamics in the epilimnion of 

a large reservoir: Limnology and Oceanography, v. 25, 

p. 219–228.

Hueftle, S.J., and Stevens, L.E., 2001, Experimental 

flood effects on the limnology of Lake Powell reservoir, 

southwestern USA: Ecological Applications, v. 11, p. 

644–656.

Johnson, N.M., and Merritt, D.H., 1979, Convective and advective circulation of Lake Powell, Utah 

and Arizona, during 1972–1975: Water Resources 

Research, v. 15, p. 873–884. 

Kimmel, B.L., and Groeger, A.W., 1984, Factors controlling phytoplankton production in lakes and reservoirs: 

a perspective, in Lake and reservoir management: 

Washington, D.C., U.S. Environmental Protection 

Agency, EPA 440/5/84-001, p. 277–281.

Marzolf, G.R., Bowser, C.J., Hart, R.J., Stephens, D.W., 

and Vernieu, W.S., 1999, Photosynthetic and respiratory processes: an open-stream approach, in Webb, 

R.H., Schmidt, J.C., Marzolf, G.R., and Valdez, R.A., 

eds., The 1996 controlled flood in Grand Canyon: 

Washington, D.C., American Geophysical Union, 

Geophysical Monograph Series, v. 110, p. 205–216.

Monismith, S., Imberger, J., and Billi, G., 1988, Shear 

waves and unsteady selective withdrawal: Journal of 

Hydraulic Engineering, v. 114, no. 9, p. 1134–1152.

Nilsson, C., Reidy, C.A., Dynesius, M., Revenga, C., 

2005, Fragmentation and flow regulation of the 

world’s large river systems: Science, v. 308, p. 405–408.

Paulson, L.J., and Baker, J.R., 1981, Nutrient interactions 

among reservoirs on the Colorado River, in Stephan, 

H.G., ed., Proceedings of the Symposium on Surface 

Water Impoundments: New York, American Society 

of Civil Engineers, p. 1648–1656.

Shannon, J.P., Blinn, D.W., and Stevens, L.E., 1994, 

Trophic interactions and benthic animal community 

structure in the Colorado River, Arizona, U.S.A.: 

Freshwater Biology, v. 31, p. 213–220.
Water Quality in Lake Powell and the Colorado River 85

Stanford, J.A., and Ward, J.V., 1991, Limnology of 

Lake Powell and the chemistry of the Colorado 

River, in Colorado River ecology and dam management: Washington, D.C., National Academy Press, p. 

75–101.

Stevens, L.E., Shannon, J.P., Blinn, D.W., 1997, 

Colorado River benthic ecology in Grand Canyon, 

Arizona, USA: dam, tributary, and geomorphological 

influences: Regulated Rivers: Research & Management, v. 13, p. 129–149.

Topping, D.J., Rubin, D.M., and Vierra, L.E., Jr., 2000a, 

Colorado River sediment transport: pt. 1: natural 

sediment supply limitations and the influence of the 

Glen Canyon Dam: Water Resources Research, v. 36, 

p. 515–542. 

Topping, D.J., Rubin, D.M., and Vierra, L.E., Jr., 2000b, 

Colorado River sediment transport: pt. 2: systematic 

bed-elevation and grain-size effect of sand supply limitation: Water Resources Research, v. 36, p. 543–570. 

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for Arizona, water year 1985–1996: U.S. Geological 

Survey Water Data Reports AZ-85-1 through 04-1.

U.S. Geological Survey, 1998, Depth profiles of temperature, specific conductance, and oxygen concentrations in Lake Powell, Arizona-Utah, 1992–1995: U.S. 

Geological Survey Open-File Report 97-835.

U.S. Geological Survey, 2001, Daily and seasonal variability of pH, dissolved oxygen, temperature, and 

specific conductance in the Colorado River between 

the forebay of Glen Canyon Dam and Lees Ferry, 

northeastern Arizona, 1998–1999: U.S. Geological 

Survey Open-File Report 01-222.

 U.S. Geological Survey, 2004, National water information system, water resource data for Arizona online 

data retrieval, water years 1949–2004: U.S. Geological 

Survey Water Data Report, http://waterdata.usgs.

gov/nwis/uv?09380000, accessed December 2, 2004.

Wetzel, R.G., 2001, Limnology, lake and river ecosystems (3d ed.): San Diego, Calif., Academic Press.

Yard, M.D., Bennett, G.E., Mietz, S.N., Coggins, L.G., 

Stevens, L.E., Hueftle, S., and Blinn, D.W., 2005, 

Influence of topographic complexity on solar insolation estimates for the Colorado River, Grand Canyon, 

AZ: Ecological Modelling, v. 183, p. 157–172.

Contact Information:

William S. Vernieu 

Hydrologist

U.S. Department of the Interior

U.S. Geological Survey

Southwest Biological Science Center

Flagstaff, AZ

bvernieu@usgs.gov

Susan J. Hueftle

Limnologist

U.S. Department of the Interior

U.S. Geological Survey

Southwest Biological Science Center

Flagstaff, AZ

shueftle@usgs.gov

Steven P. Gloss

Ecologist

U.S. Department of the Interior

U.S. Geological Survey

Southwest Biological Science Center

Tucson, AZ

sgloss@usgs.gov

First page photograph credit: Andrew Pernick, Bureau of Reclamation

Chapter 5

Aquatic 

Ecology: 

the Role 

of Organic 

Matter and 

Invertebrates

Theodore A. Kennedy

Steven P. Gloss

Introduction

Closure of Glen Canyon Dam and the beginning of 

flow regulation of the Colorado River through Grand 

Canyon in 1963 changed the river through the canyon 

considerably. The river changed from having highly 

variable discharge rates and temperatures and high 

suspended-sediment loads to having a relatively constant 

flow regime (Topping and others, 2003), cold and constant water temperatures, and suspended-sediment loads 

that are dramatically reduced relative to predam levels 

(see chapter 1, this report). These changes in the physical 

environment, coupled with changes in the quantity and 

types of organic matter present in the Colorado River 

and intentional introductions of aquatic invertebrates 

that occurred shortly after Glen Canyon Dam was closed 

(Blinn and Cole, 1991), have led to substantial changes in 

the kinds of aquatic invertebrates present in the Grand 

Canyon ecosystem. Since the closure of the dam, considerable effort has been directed toward understanding 

the aquatic ecology of this altered ecosystem (Blinn and 

Cole, 1991).

This chapter describes the results of the research 

and monitoring activities that have investigated the 

kinds of organic matter and invertebrate communities 

in the Colorado River below Glen Canyon Dam. Collectively, organic matter and the aquatic invertebrates 

that consume it largely constitute the food base for fish in 

the Colorado River ecosystem. This chapter focuses on 

patterns, trends, and important controls on the amount 

and sources of organic matter and invertebrates that are 

primary food resources for humpback chub (Gila cypha)

and rainbow trout (Oncorhynchus mykiss) in an effort to 

understand the role that food plays in determining the 

distribution, population density, and growth of these fish 

in this ecosystem. Furthermore, most of the research and 

monitoring that have been conducted on organic matter and invertebrates in this ecosystem have centered on 

the food items that are important for these two species. 

This chapter also addresses how organic matter and 

invertebrates are affected by the timing and magnitude 

of water releases from Glen Canyon Dam, including the 

modified low fluctuating flow (MLFF) alternative, which 

was implemented in 1996 and continues as the operating 

regime for Glen Canyon Dam today. Finally, this chapter concludes with a brief discussion of recommended 

research directions and management actions.
88 The State of the Colorado River Ecosystem in Grand Canyon

Background

Virtually all food webs, including those in rivers, 

are fueled by energy that comes from autotrophs (also 

known as primary producers), which are organisms that 

can convert sunlight into chemical energy. Examples of 

autotrophs include vascular plants and algae. Without 

autotrophs there would be no food energy available to 

other organisms that lack the capability to fix light energy. 

In rivers, this autotrophic material can come from two 

places: the terrestrial environment, such as leaves falling into a river from trees lining the river’s banks, or the 

aquatic environment, such as algae growing on river 

rocks. Terrestrially derived material is an extremely abundant source of energy in many streams and rivers (Bayley, 

1989; Meyer and Edwards, 1990). Although algae often 

represent just a small fraction of the available energy in 

river ecosystems, they are frequently an important energy 

source that contributes to secondary production (Bilby 

and Bisson, 1992; Lewis and others, 2001; Thorp and 

Delong, 2002) because they are far more nutritious than 

terrestrial material (Anderson and Sedell, 1979). Aquatic 

or terrestrial autotrophic material is also called “organic 

matter” and provides the energy that supports consumers 

at higher trophic levels. Trophic levels are groups of organisms that occupy the same position in a food web (fig. 1). 

The importance of understanding patterns, trends, 

and controls of organic matter and invertebrates is 

reflected in the goals of the Glen Canyon Dam Adaptive 

Management Program. For example, the program’s first 

goal is to “protect or improve the aquatic foodbase so 

that it will support viable populations of desired species 

at higher trophic levels” (Glen Canyon Dam Adaptive 

Management Program, 2001, p. 11). Two additional 

goals are to maintain populations of rainbow trout in the 

Lees Ferry reach and maintain, and ultimately increase, 

populations of native fish, particularly endangered 

humpback chub, in sections of the river downstream of 

the Lees Ferry reach. Recent trends for important fish 

in the Grand Canyon ecosystem may be partly due to 

changes in food resources or to an increase in the severity 

of competition between humpback chub and other fish 

and highlight the need for continued research on organic 

matter and invertebrates. The number of rainbow trout 

in the Lees Ferry reach has generally been high since 

intensive population measurements began in 1991, but 

the condition of fish as determined by weight relative to 

length declined during the late 1990s (McKinney and 

others, 2001). Also, the average condition (Meretsky and 

others, 2000) and size of humpback chub populations 

have declined considerably since intensive measurements 

began in 1986 (see chapter 2, this report).

Figure 1. Idealized and simplified food-web diagrams for two different sections of the Colorado River ecosystem, the Lees Ferry reach 

and the Colorado River at the Little Colorado River (LCR) confluence. The sizes of the boxes reflect qualitative differences in the standing 

mass within trophic levels, and the size of the arrows reflects qualitative differences in the amount of food energy moving between

trophic levels. Green arrows linking algae to fish are used to highlight the fact that fish consume algae but do not actually derive

significant nutrients or energy from them. Rainbow trout also consume New Zealand mudsnails, but the snails often survive passage

through the gut alive and intact, which is noted with the X. Humpback chub may be capable of actually digesting mudsnails because 

they are capable of crushing their shells; however, it is unclear whether humpback chub ingest snails. Areas of greatest uncertainty are 

noted with dashed lines. 
Aquatic Ecology 89

Several of the hypothesized causes of humpback 

chub decline are, in part, based on the assumption that 

there is a limited amount of food available in the river 

to support populations of humpback chub and other 

fish. For example, Gloss and Coggins (see chapter 2, 

this report) list competition with nonnative and native 

fish as one of several possible reasons for the decline 

of the endangered humpback chub populations. Thus, 

documenting the food resources that humpback chub 

and rainbow trout are dependent on and whether these 

resources vary over space and time will help determine 

the validity of the food-limitation hypothesis. This 

information will also clarify the role that food availability 

plays in determining the population density and condition of both native and nonnative fishes and may prove 

useful as an indicator of ecosystem health.

Status and Trends

Identifying the Food Items of Fish

A great deal is known about the types of food items 

consumed by humpback chub and rainbow trout. In general, both fish appear to consume mostly the introduced 

invertebrate Gammarus lacustris (small crustaceans, also 

called scuds or side-swimmers, hereafter Gammarus) (fig. 

2), larval chironomids (midges, also called bloodworms), 

larval simuliids (black flies), terrestrial invertebrates, 

and the filamentous algae Cladophora glomerata (hereafter Cladophora) (fig. 3). The small aquatic invertebrates 

mentioned range from about 0.25 to 1 inch (6 to 25 mm) 

in length, and individual Cladophora filaments can attain 

nearly 20 ft (about 6 m) in length. 

Gut pumping was used to nondestructively investigate the food items consumed by humpback chub 

collected from two known chub aggregations, the Little 

Colorado River confluence aggregation (at about RM 

61) and the Middle Granite Gorge aggregation (at about 

RM 127) (Valdez and Ryel, 1995). Cladophora represented 

24% of gut contents by volume for chub at the Little 

Colorado River confluence site, with invertebrates representing the remaining 76%. In contrast, humpback chub 

at the Middle Granite Gorge site consumed exclusively 

invertebrates. Of the invertebrates, simuliids, Gammarus,

and terrestrial invertebrates were the most common 

items consumed. Valdez and Ryel (1995) also quantified 

the density of algae and specific invertebrates in the drift 

(fig. 4) and then compared chub diets with the availability of food items in the drift to determine whether 

chub were “selectively” feeding. These analyses indicated 

that chub selectively avoided consuming Cladophora, even 

though it represented 88%–93% of potential food in the 

drift, and chub generally consumed simuliids, chironomids, and Gammarus in “approximate proportion to their 

availability in the drift” (Valdez and Ryel, 1995, p. 9–13).

The feeding habitats of rainbow trout in the Lees 

Ferry reach were determined by McKinney and Speas 

(2001). They analyzed the stomach contents of 658 

rainbow trout caught in the Lees Ferry tailwater from 

1991–97 and found that Cladophora, Gammarus, and chironomids accounted for more than 90% of the stomach 

contents by volume. Of the invertebrates consumed by 

these fish, Gammarus and chironomids together accounted 

for more than 90% of the total by volume. 

For a complete understanding of the energy sources 

that are driving a food web, it is important to know 

not only what the fish are consuming but also what the 

invertebrates themselves are consuming. To this end, the 

diets of some aquatic invertebrates that are commonly 

consumed by fish have also been investigated by using 

gut-content analysis and habitat-choice experiments. 

Pinney (1991) found that diatoms, a class (Bacillariophyceae) of microscopic algae common in aquatic environments, made up more than 93% by volume of gut 

contents for Gammarus in the Lees Ferry reach. Shannon 

and others (1994) used field and lab-based habitat-choice 

Figure 2. A preserved Gammarus lacustris (also known as a 

scud or side-swimmer), which is consumed by humpback chub 

and rainbow trout. Live animals are more translucent (photograph 

by Michael Booth).
90 The State of the Colorado River Ecosystem in Grand Canyon

experiments to determine that Gammarus preferred 

Cladophora as habitat over all other choices, including 

Oscillatoria spp. (another species of filamentous algae), 

gravel, and detritus. These same researchers determined 

that Gammarus were only relying on Cladophora as habitat and were actually eating diatoms that were attached 

to Cladophora. Stevens and others (1997) determined the 

diet composition of chironomids by using gut-content 

analysis and found that the relative importance of algae 

in chironomid diets declined with distance downstream 

from the dam: algae represented 61.4% of chironomid 

diets at Lees Ferry, 30.7% at RM 32, and only 7.5% at 

RM 224. This trend is consistent with observed downstream declines in algae biomass (discussed below) and 

indicates that algae may not be the most common form 

of organic matter consumed by invertebrates at downstream locations. 

Identifying the food items consumed by fish and 

invertebrates by using gut-content analysis provides an 

indication of the food resources that are most important to fish; however, relying solely on this approach 

also has weaknesses. First, gut contents only reflect the 

items consumed by fish or invertebrates within about an 

hour of their capture, providing only a “snapshot” of 

the food items consumed. Even in this short timeframe, 

however, labile food items may be more readily digested 

than others, leaving behind the more resistant items and 

the appearance that these items are the most important 

food sources. Furthermore, if, for example, rainbow 

trout consume other fish very infrequently, the snapshot 

taken through gut-content analysis is unlikely to detect 

this relatively rare event. Yet these infrequent events of 

predation may be a significant source of calories and 

nutrients for the fish and may represent an important 

type of food that might be overlooked when using only 

gut-content analysis. Second, just because an item is 

consumed by a fish does not mean that it is actually an 

important source of energy or nutrients. For example, 

both humpback chub and rainbow trout regularly 

consume the filamentous algae Cladophora, but energetic 

and stable-isotope analyses (discussed below) indicate 

that this material is not actually assimilated because it 

is difficult to digest and is low in essential nutrients such 

as fatty acids, nitrogen, and phosphorus (Angradi, 1994; 

McKinney and Speas, 2001). It has been suggested that 

the humpback chub and rainbow trout that consume 

Cladophora may actually be after the more nutritious 

invertebrates that are imbedded in Cladophora (Valdez and 

Ryel, 1995). Finally, the relative contribution of terrestrial and aquatic organic matter to invertebrate growth 

Permanently submerged zone

Varial zone

Figure 3. Cobble bar in Glen Canyon 

showing the varial zone (shoreline 

habitat that is both inundated and 

exposed to air for long periods 

each day) and the presence of the 

filamentous algae Cladophora in

the permanently submerged zone. 

Cladophora is unable to grow in the 

varial zone because dam operations 

result in discharge rates that 

regularly expose the varial zone to air 

(photograph by Theodore Kennedy, 

U.S. Geological Survey). 
Aquatic Ecology 91

and production, and hence fish growth and production, 

is often unclear when only gut-content analysis is used. 

To determine what is being consumed and actually 

assimilated by fish and small invertebrates and to quantify the relative importance of terrestrial and aquatic 

organic matter in fueling fish growth require a combination of gut-content analysis and a specialized technique 

known as stable-isotope analysis. Terrestrial and aquatic 

organic matter often has distinct stable-isotope signatures (Fry and Sherr, 1984) that are largely conserved up 

the food chain, and these signatures provide information about the source of energy at the base of the food 

web (Peterson and Fry, 1987). For example, if Gammarus 

consume exclusively algae, then they will have a carbon 

stable isotope signature identical to that of algae, as will 

a fish that consumes exclusively Gammarus. In contrast, 

nitrogen stable isotope values change predictably with 

each link in a food chain, increasing at a rate of 3.4 parts 

per thousand (‰) with each link, and therefore provide 

an indication of trophic position (Minagawa and Wada, 

1984). Thus, herbivores typically have nitrogen stable 

isotope signatures that are 3.4‰ higher than plants, and 

primary carnivores in turn have nitrogen stable isotope 

signatures that are 3.4‰ higher than herbivores and 

6.8‰ higher than plants. Also, stable-isotope analyses 

provide time-integrated measures of diet. That is, the 

carbon and nitrogen stable isotope signatures for a large 

fish will usually reflect the food it has consumed and 

assimilated over the past several months, which provides 

a contrast to the snapshot picture of diets obtained with 

gut-content analysis.

A combination of gut-content analysis of rainbow 

trout and stable-isotope analysis of the entire food web 

provided a clear picture of the aquatic food web in the 

Lees Ferry reach of the Colorado River (Angradi, 1994). 

Rainbow trout gut-content data collected by Angradi 

(1994) were remarkably similar to those reported by 

McKinney and Speas (2001): both studies showed 

Gammarus, chironomids, and Cladophora to be the dominant food items. Analysis of stable-isotope ratios showed 

that rainbow trout were assimilating nutrients from 

only the Gammarus and chironomids and not from the 

Cladophora they consumed (Angradi, 1994). Furthermore, 

Gammarus and chironomids were feeding almost exclusively on benthic algae, which are algae attached to the 

bottom of the riverbed. Thus, the entire food web of the 

Colorado River in the Lees Ferry reach, from aquatic 

invertebrates to fish, was based on algae.

Our understanding of the aquatic food web at 

sites downstream of Lees Ferry is much more limited. 

Figure 4. Volume of specific invertebrates in humpback chub stomachs from the Little Colorado River confluence aggregation (about 

RM 61) and composition of these same items in the drift. Data were collected during 1992–93 and exclude algae (Cladophora) that were 

consumed by chub. Modified from Valdez and Ryel (1995).
92 The State of the Colorado River Ecosystem in Grand Canyon

Although gut-content analysis for humpback chub 

and rainbow trout collected at downstream locations 

indicated that both species consume mostly aquatic 

invertebrates and algae, the relative importance of 

terrestrial and aquatic energy sources remains unclear. 

Using stable-isotope analysis, Haden and others (1999) 

investigated the diet of humpback chub and other fish 

in the Little Colorado River, the largest tributary of the 

Colorado River in Grand Canyon and the single most 

important breeding habitat for humpback chub. They 

found a complex food web in the Little Colorado River 

with small chub, less than 6 inches (<150 mm) in length, 

relying heavily on invertebrates, especially chironomids, 

mayflies (Ephemeroptera), and caddisflies (Trichoptera), 

while large chub, greater than 6 inches (>150 mm) in 

length, were found to be relying on invertebrates and 

small fish. They also found evidence that terrestrial 

and aquatic organic matter was fueling the food web, 

although they were unable to determine the proportional 

contribution of each energy source. Angradi (1994) 

investigated food-web structure in tributary streams of 

the Colorado River and found that the food web in some 

tributaries was supported by leaf litter from streamside 

vegetation while others were supported by leaf litter 

from upland plants. Shannon and others (2001a) collected samples of algae, aquatic invertebrates, and eight 

species of fish (rainbow trout and humpback chub 

were collected, but the identity of the other six species 

was not specified) from seven sites that span the entire 

Grand Canyon ecosystem and found that carbon isotope 

values for algae increased consistently with downstream 

distance. Further, they found that invertebrate and fish 

isotope values roughly tracked the downstream shift in 

algae isotope values, providing evidence that algae is 

contributing to invertebrate and fish growth along the 

entire length of the Grand Canyon ecosystem. 

Spatial and Temporal Patterns in 

Organic Matter and Invertebrates

There are very few data on the relative abundance 

of terrestrial or aquatic organic matter, or the density 

or kinds of invertebrates present, in the Grand Canyon ecosystem before the construction of Glen Canyon Dam (Blinn and Cole, 1991). In general, aquatic 

invertebrate diversity (the number of different species) 

has declined following closure of Glen Canyon Dam, 

while invertebrate density and biomass have probably, 

perhaps even dramatically, increased (Blinn and Cole, 

1991, and references therein). Comparison of invertebrate diversity in tributaries relative to the mainstem 

provides an indication of changes in the invertebrate 

fauna that have occurred following closure of Glen 

Canyon Dam; Hofknecht (1981) found 52 insect families 

in tributaries of the Grand Canyon ecosystem, compared to just 5 insect families for the mainstem Colorado 

River. Haden and others (2003) studied relatively pristine and free-flowing sections of the lower Green River 

and the Colorado River in Canyonlands National Park 

in Utah to determine what the food web in the predam 

Colorado River in Grand Canyon might have looked 

like. They found that terrestrial organic matter was the 

primary energy source for aquatic invertebrates in this 

free-flowing reach because high levels of suspended sediment prohibited algae growth. They also found an invertebrate community that was markedly different from 

that in the Grand Canyon ecosystem; the invertebrate 

community in the free-flowing reach was dominated by 

filter feeders (simuliids and caddisflies) and collectors 

(mayflies and chironomids), reflecting the importance 

of terrestrial organic matter to this system. Prior to the 

closure of Glen Canyon Dam, the Colorado River contained large quantities of coarse woody debris (i.e., whole 

trees and branches) and other terrestrial plant material 

that were transported from upstream sources (Valdez and 

Carothers, 1998). This material accumulated along river 

banks and in eddies and supported a high diversity and 

abundance of terrestrial invertebrates. When this material was entrained by the river during spring floods, the 

terrestrial invertebrates probably served as an important 

food resource for fish in the Colorado River (Valdez and 

Carothers, 1998). 

Because Glen Canyon Dam has created clear water 

conditions that allow sunlight to reach the river bottom, 

algal standing mass is extremely high in the Lees Ferry 

reach of the Grand Canyon ecosystem. Much of the 

terrestrial organic matter that formerly moved through 

the Colorado River system is now trapped behind Glen 

Canyon Dam. Stevens and others (1997) quantified 

river-bottom algae and invertebrate standing mass at 11 

stations between Lees Ferry and Diamond Creek on a 

bimonthly basis during 1991. They found that Cladophora 

was the dominant algae throughout the Lees Ferry reach, 

exhibiting an average of 0.5 oz carbon (C)/yd2 (15.5 

g C/m2

). Downstream of the Paria River confluence, 

Cladophora standing mass abruptly decreased to 0.01 oz 

C/yd2 (0.5 g C/m2

), and it remained low at the remaining downstream sampling stations (fig. 5). Oscillatoria spp., 

mat-forming algae, tended to dominate aquatic habitats 

at sites downstream of Lees Ferry with average biomass 

of 0.02 oz C/yd2 (0.6 g C/m2

) at the site immediately 

downstream of the Paria River. 
Aquatic Ecology 93

Figure 5. Downstream patterns of sediment concentration and biomass of Cladophora and macroinvertebrates along the Colorado 

River ecosystem. Modified from The Colorado River Through Grand Canyon by Steven W. Carothers and Bryan T. Brown. © 1991 The 

Arizona Board of Regents. Reprint by permission of the University of Arizona Press.

The species composition and biomass of aquatic 

invertebrates also vary with distance downstream. 

Stevens and others (1997) reported that Gammarus and

chironomids were the dominant aquatic invertebrates 

in the Lees Ferry reach, while simuliids were the dominant invertebrates at sites downstream from the Paria 

River. Sublette and others (1998) identified 38 species 

of chironomids in the Grand Canyon ecosystem, and 

Stevens and others (1998) studied the factors that influence chironomid distribution in the Grand Canyon 

ecosystem. Stevens and others (1998) found that turbidity 

strongly influenced chironomid diversity, with 11 species 

present in the clear water of the Lees Ferry reach, 18 

species present in what they termed the “variably turbid” 

segment of the Colorado River (Lees Ferry to Little 

Colorado River confluence), and 24 species in the “usually turbid” segment (Little Colorado River confluence 

to Diamond Creek). In contrast, the biomass of invertebrates declined downstream (Stevens and others, 1997), 

with mean biomass of 0.09 oz C/yd2 (2.9 g C/m2

) in the 

Lees Ferry reach and much lower values, less than 0.003 

oz C/yd2 (<0.1 g C/m2

), at downstream locations. 

Even though the Lees Ferry reach accounted for 

only 6.9% of the aquatic habitat in the 242 mi (390 km) 

of river studied by Stevens and others (1997), it supported 63.5% of the primary producer biomass and 
94 The State of the Colorado River Ecosystem in Grand Canyon

87% of the invertebrate biomass in the entire study area. 

Stevens and others (1997) attributed the downstream 

decline in Cladophora biomass, and hence the invertebrates 

which are dependent on Cladophora and its attached 

diatoms (i.e., especially Gammarus and chironomids), to 

episodic inputs of suspended sediments from tributaries such as the Paria and the Little Colorado Rivers that 

reduce water clarity and light penetration enough to limit 

algal production (fig. 5).

In contrast to the patterns described above, riverbottom detritus (nonliving organic matter), which can 

be derived from both terrestrial and aquatic sources, 

peaked at approximately RM 124. This peak could 

have occurred here because detritus is transported from 

upstream locations and accumulates in this region of the 

river. Detritus is an important component of the aquatic 

food web because fish at downstream locations regularly 

consume simuliids that feed on detritus via filter feeding. 

In general, the overall quantity of drifting organic matter 

increases and the composition changes from predominantly aquatic to terrestrial material with distance downstream (Shannon and others, 1996; Benenati and others, 

2001). Shannon and others (1996) noted that tributary 

inputs of organic matter constituted less than 0.1% of 

the total organic drift of the Colorado River. We suspect, 

however, that 0.1% is a gross underestimate of tributary 

organic inputs because it does not appear that sampling 

of tributary organic inputs was carried out during periods of flooding. Determining whether tributary inputs of 

organic matter and energy are important to the food web 

within the Colorado River will be a major focus of future 

aquatic ecology efforts, as outlined in the Discussion and 

Future Research Needs section of this chapter.

The density of many important components of the 

aquatic food web appears to vary with season. The densities of Cladophora, the dominant algae in the Lees Ferry 

reach, and Oscillatoria, the dominant algae at downstream 

locations, vary over time with peak density occurring 

during summer (Stevens and others, 1997). The highest density of aquatic invertebrates also occurred during 

the summer (Stevens and others, 1997). In contrast, the 

concentration of river-bottom detritus was greatest during autumn (Stevens and others, 1997), perhaps because 

this is when trees adjacent to the river were dropping 

their leaves or because lower river flows during this time 

of year allowed the material to settle out of the water 

column and accumulate on the river bottom. 

It should be noted that because of logistical challenges, samples from only about 10 locations along the 

entire 241 mi (386 km) of the Grand Canyon ecosystem 

have been used to characterize spatial and temporal 

variability of organic matter and invertebrates. Further, 

much of the sampling (e.g., Shannon and others, 1994, 

2001b; Blinn and others, 1995, 1998; Stevens and others, 

1997; Benenati and others, 1998; McKinney and others, 

1999) focused heavily on quantifying organic matter and 

invertebrate dynamics at cobble bar habitats; yet, cobble 

bars make up less than 10% of the aquatic habitat downstream from Glen Canyon Dam (Mietz, 2003). Thus, 

these results may not accurately characterize spatial and 

temporal variability of organic matter and invertebrates 

within the Grand Canyon ecosystem.

Mathematical modeling is a powerful tool that can 

be used to predict or estimate variables of interest (e.g., 

algae productivity) across large areas, such as the Grand 

Canyon ecosystem, where logistics prevent intensive 

field sampling. Yard and others (2005) measured and 

modeled the influence of canyon orientation and topographic complexity on solar inputs to the Grand Canyon 

ecosystem. Light is the resource that most often limits 

the growth of algae and plants in aquatic environments 

(Wetzel, 2001), so these results provide an indication of 

potential algae growth across the entire Grand Canyon 

ecosystem. One of the most striking results of these 

modeling efforts is that river reaches that are oriented 

east-west receive far less solar radiation during the winter 

months relative to north-south reaches because the sun 

is lower on the horizon at this time of year. That is, the 

river has a clear view of the sun as it traces a path across 

the horizon during the winter months along north-south 

reaches, but the sun never gets high enough on the horizon to shine on the river along east-west reaches. Thus, 

Yard and others (2005) forecast that algae production 

should vary predictably with canyon orientation and season because of differences in solar radiation and because 

of the general downstream decline in algae production 

that is associated with tributary sediment inputs that 

reduce water clarity and light penetration.

The Influence of Dam Releases on 

Organic Matter and Invertebrates

Understanding how Glen Canyon Dam discharge 

regimes influence Cladophora and associated invertebrates has been a major focus of recent research efforts 

(Angradi, 1994; Shannon and others, 1994, 1996, 2001b; 

Valdez and Ryel, 1995; McKinney and Speas, 2001). 

With discharge from Glen Canyon Dam fluctuating as 

much as 8,000 cubic feet per second (cfs) daily, there 

exists a large varial zone of shoreline habitat that is both 

inundated and exposed to air for long periods each day 

(fig. 3). Several studies have determined that the varial 

zone supports a relatively low density of algae, which is 
Aquatic Ecology 95

often dominated by Oscillatoria spp. because Cladophora

and associated invertebrates cannot thrive in the varial 

zone (Blinn and others, 1995; Shaver and others, 1997; 

Benenati and others, 1998). Specifically, Blinn and others 

(1995) found fourfold higher invertebrate mass in permanently submerged zones compared to the varial zone. 

Using a series of in situ experiments, they determined 

that snails readily recolonized cobbles that were resubmerged after initially being subjected to long-term desiccation; however, the density of Cladophora, Gammarus, and 

chironomids on resubmerged cobbles was still less than 

30% of control sites after 4 mo. 

Fluctuations in river flows also have an impact on 

drifting organic matter because periodic desiccation can 

weaken algae and invertebrates, making them more susceptible to fragmentation and entrainment by the river. 

Moreover, higher river flows lead to more turbulent and 

faster water that is more likely to entrain organic matter 

and invertebrates. Shannon and others (1996) found that 

the quantity of drifting organic matter increased with 

discharge.

Historical data are insufficient to quantitatively 

determine what impact the MLFF alternative has had 

on organic matter and invertebrates; however, it is possible to qualitatively describe the likely impacts of this 

flow regime based on the research described above. By 

restricting daily fluctuations in discharge to less than 

8,000 cfs and limiting minimum discharge to 5,000 

cfs, MLFF flows have reduced the size of the varial 

zone and increased the amount of river bottom that is 

permanently submerged. Both of these changes probably increased the productivity and standing mass of 

important components of the aquatic food web, including Cladophora and Gammarus; however, abrupt changes 

in monthly release volumes that are permitted under 

the Record of Decision and MLFF may be detrimental 

to algae and aquatic invertebrates. For example, when 

monthly release volumes are decreased, the amount of 

river-bottom habitat that is permanently submerged 

also decreases. It seems likely that there is a subsequent, 

abrupt decrease in the quantity of food available for fish 

in the Grand Canyon ecosystem. Further, when monthly 

release volumes are abruptly increased, algae and invertebrates that were in shallow, nearshore habitats may 

find themselves under several yards of water. While this 

increase in monthly volume may eventually lead to an 

increase in the quantity of food available (by increasing 

available habitat), it seems likely that there is a shortterm decrease in food as algae and invertebrates adjust to 

the new conditions. 

Experimental high flows, or controlled floods, have 

been used principally as a tool to restore sandbars in the 

Grand Canyon ecosystem, but these floods also impact 

organic matter and invertebrates. Blinn and others (1999) 

found that the 1996 beach/habitat-building flow scoured 

more than 90% of the primary producer biomass (i.e., 

algae and submerged aquatic plants) and about 50% 

of the river-bottom invertebrates from a site at Lees 

Ferry; primary producers (1 mo) and invertebrates (2 

mo) quickly recovered to preflood levels. In contrast, 

McKinney and others (1999) found that the 1996 beach/

habitat-building flow caused short-term reductions in the 

standing mass of primary producers and invertebrates 

only in depositional habitats (i.e., areas of sand/silt) 

and not in more resistant habitats like cobble bars (fig. 

6). Brock and others (1999) found that the 1996 beach/

habitat-building flow actually led to significant increases 

in algae production rates (the rate at which photosynthesis is occurring within the algae); they hypothesized that 

algae production increased because the flood removed 

senescent, or old, material and detritus from the algae. 

Marzolf and others (1999) measured oxygen production, 

a byproduct of photosynthesis, along several river segments within the Lees Ferry reach and found that those 

segments produced less oxygen after the flood relative to 

preflood values. These data suggest that the 1996 beach/

habitat-building flow did in fact scour large quantities 

of algae and aquatic macrophytes from the Lees Ferry 

reach, resulting in a systemwide reduction in primary 

production. Even though the flood may have reduced the 

standing mass of invertebrates from some areas in the 

Lees Ferry reach, the quantity of food items in rainbow 

trout stomachs was actually greater immediately after the 

flood relative to before the flood (McKinney and others, 

1999). Blinn and others (1999) used stable-isotope analyses to determine that riparian vegetation and upland 

vegetation were the dominant types of drifting organic 

matter during the flood, while river-bottom algae were 

the dominant drifting organic matter during normal dam 

operations. Thus, experimental high flows can scour benthic algae and invertebrates and capture large quantities 

of terrestrial organic matter, which may temporarily 

increase the amount of food available for fish.

Recent Findings

New Zealand Mudsnail Invasion

Biological invasions represent a significant threat to 

the persistence of resident species because invaders are 

capable of altering food-web structure, rates of disease 
96 The State of the Colorado River Ecosystem in Grand Canyon

or parasitism, and the amount or type of energy at the 

base of a food web (Vitousek, 1990; Wilcove and others, 

1998; Kennedy and Hobbie, 2004). Although the food 

web in the Colorado River below Glen Canyon Dam 

has already been dramatically changed because of the 

installation of the dam and intentional introductions of 

nonnative sport fish, it is still susceptible to the impacts 

of biological invasions. 

The New Zealand mudsnail (Potamopyrgus antipodarum), a species that is rapidly spreading throughout 

North American rivers and lakes (Hall and others, 2003, 

and references therein), has recently invaded the Grand 

Canyon ecosystem. The presence of the mudsnail in 

Grand Canyon was first identified from samples collected 

in March 2002; however, the analysis of archived collections revealed that mudsnails were actually present as 

early as May 1995 (Benenati and others, 2002). Presently, 

this small snail (fig. 7), which measures approximately 0.2 

inch (about 5 mm), occurs in high densities throughout 

the Grand Canyon ecosystem, particularly in the Lees 

Ferry reach where densities of more than 32,800/yd2

(40,000 snails/m2) have been found (Benenati and others, 

2002). Before the New Zealand mudsnail invasion, snails 

represented a minor component of total invertebrate 

mass in the Lees Ferry reach (less than 6% of the total 

in 1992). Snails are now the dominant category of 

invertebrate in the Lees Ferry reach, representing more 

than 66% of invertebrate mass in 2001 (Benenati and 

others, 2002).

The potential impacts of the New Zealand mudsnail 

invasion on the aquatic food web of the Grand Canyon 

ecosystem are significant because nonnative rainbow 

trout apparently cannot easily digest the snails (fig. 1); 

snails eaten by trout often survive intact after passage 

through the gut (Mike Yard, oral commun., 2004), 

perhaps because of the snails’ protective operculum. 

Humpback chub have pharyngeal gills that are capable 

of crushing snail shells and therefore might be able to 

more completely digest them; however, the only detailed 

diet analysis for chub was conducted before the mudsnail invasion of the Colorado River ecosystem. Because Figure 6. Algae (A) and invertebrate (B) response to the 

1996 beach/habitat-building flow. The flood occurred between 

the March and April sampling dates. Bars represent average 

standing mass (algae) or density (Gammarus) at cobble bars 

in the Lees Ferry reach, while the thin lines on top of each bar 

represent 1 standard error (an indication of the uncertainty 

associated with each estimate). Data from McKinney and others 

(1999), table 1, p. 251.

Figure 7. New Zealand mudsnails on a concrete anchor in Glen 

Canyon. The density of snails shown here is typical for the Glen 

Canyon ecosystem (photograph by Michael Booth).

A.

B.
Aquatic Ecology 97

rainbow trout cannot easily digest the mudsnail, the 

mudsnail may have a competitive advantage over other 

herbivores such as Gammarus, potentially allowing it to 

displace these other organisms as it continues to spread. 

If the density of Gammarus and other invertebrates that 

are regularly consumed by fish declines because of competition with the New Zealand mudsnail, it seems likely 

that these changes will affect fish density and condition. 

Even if mudsnails do not compete with or displace herbivores such as Gammarus, which seems unlikely given their 

extremely high density, they dramatically alter important 

ecosystem functions, including rates of nutrient cycling 

and primary production (Hall and others, 2003). Unfortunately, historical data are inadequate to determine 

whether the density or biomass of Gammarus and other 

invertebrates that are important food items for fish in the 

Colorado River ecosystem has been affected by the New 

Zealand mudsnail invasion.

Discussion and Future 

Research Needs

With recent declines in the size of native fish populations and the condition of rainbow trout (see chapter 2, 

this report), understanding what food resources drive fish 

growth and production, as well as what sources of energy 

drive production of these food resources, becomes 

increasingly important. Previous research on the aquatic 

food web has produced a relatively clear picture of the 

food habits of fish in the Grand Canyon ecosystem. 

Aquatic invertebrates—particularly Gammarus, simuliids, 

and chironomids—appear to be the most important food 

items for both rainbow trout and humpback chub. In the 

upper reaches of the Colorado River near Lees Ferry, 

it appears that invertebrates, and therefore the fish that 

consume them, are fueled almost exclusively by algae, 

particularly diatoms attached to Cladophora. Although 

there is evidence that the invertebrates consumed by fish 

at downstream locations are relying on both aquatic and 

terrestrial organic matter, the relative importance and 

sources of this material remain unclear. This uncertainty 

prevents a complete understanding of the role that food 

availability plays in determining the condition and population size of native and nonnative fish.

Food-web analysis provides a framework for quantifying the movement of terrestrial and aquatic material into higher trophic levels, the trophic positions of 

consumers, and the importance of interactions such as 

competition and predation. Applied research efforts have 

repeatedly benefited from studying an ecosystem from 

a food-web perspective (Winemiller and Polis, 1996). 

Fisheries management, in particular, can benefit from a 

food-web perspective because it is critical for accurately 

predicting the responses of both predators and prey to 

management actions (Parsons, 1992). Although many 

food-web studies of terrestrial and aquatic systems have 

focused on trophic pathways based exclusively on aquatic 

production, it is increasingly recognized that leaf litter 

and other types of terrestrial organic matter play a major 

role in determining ecosystem structure and function 

(Winemiller and Polis, 1996).

In the case of the Grand Canyon ecosystem, previous research efforts on the aquatic food web have focused 

almost exclusively on trophic pathways associated with 

aquatic organic matter, namely the filamentous algae 

Cladophora, the diatoms attached to this algae, and the 

invertebrate consumers of these attached diatoms (Usher 

and Blinn, 1990; Hardwick and others, 1992; Shannon 

and others, 1994). Focusing on these aquatic sources 

of organic matter seems appropriate for the tailwater 

section of the Colorado River, that is, from the dam 

to Lees Ferry. Yet, the tailwater is not representative of 

downstream portions of the river and accounts for less 

than 7% of the total wetted area of the Grand Canyon 

ecosystem. A thorough analysis of the trophic significance of terrestrially derived material has never been 

conducted, although there are some correlative data 

that support the contention that the downstream decline 

in algae production limits secondary production at 

downstream sites (Shaver and others, 1997; Stevens and 

others, 1997). Importantly, the one detailed food-web 

analysis that has been conducted in the Grand Canyon 

ecosystem indicates that terrestrially derived carbon is 

contributing to invertebrate and fish production at downstream tributaries (Angradi, 1994). 

Given these considerations, it seems clear that future 

research and monitoring efforts should take a broader 

view of the food web and attempt to document the 

relative importance of aquatic and terrestrial organic 

matter to invertebrate and fish production. For example, 

if invertebrates, and by extension humpback chub and 

rainbow trout, are dependent on algae throughout the 

ecosystem, a systemwide reduction in algae production 

would likely have strong negative consequences for fish, 

especially because algae biomass is already very limited 

at downstream sites. Alternatively, if terrestrial organic 

matter is fueling production of invertebrates at downstream sites, then findings from the proposed food-web 

research would provide managers with some of the 

information necessary to assess the effects of proposed 

management actions, including sediment augmentation 

and thermal modifications, on fish production.
98 The State of the Colorado River Ecosystem in Grand Canyon

Food-web analysis should continue to focus on the 

two most ecologically and economically important fish 

species, humpback chub and rainbow trout, and use a 

combination of gut-content and stable-isotope analysis. 

To determine whether the resource base of the food web 

shifts downstream and is affected by tributary inputs of 

organic matter, a food-web analysis should encompass 

the entire study area including the major tributaries 

of the Colorado River, including the Paria and Little 

Colorado Rivers. Shannon and others (2001a) found 

downstream shifts in the isotopic composition of algae, 

invertebrates, and fish. This downstream shift may complicate interpretation of stable-isotope data, but another 

technique for determining trophic linkages, known 

as quantitative fatty acid analysis (Iverson and others, 

2004), may allow investigators to resolve any uncertainties associated with stable-isotope analysis. Manipulative experiments involving New Zealand mudsnails also 

represent an important research direction because these 

experiments could help scientists determine whether or 

not this invasive species is having a negative impact on 

important food items for fish and identify the factors that 

control New Zealand mudsnail density. Collectively, these 

activities will determine the short- and long-term feeding 

habits of humpback chub and rainbow trout, the energy 

resources at the base of the food web, whether humpback chub are consuming and digesting New Zealand 

mudsnails, and the impact of the mudsnail invasion on 

the aquatic food web.

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100 The State of the Colorado River Ecosystem in Grand Canyon

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Aquatic Ecology 101

Contact Information:

Theodore A. Kennedy

Aquatic Ecologist

U.S. Department of the Interior

U.S. Geological Survey

Southwest Biological Science Center

Flagstaff, AZ

tkennedy@usgs.gov

Steven P. Gloss

Ecologist

U.S. Department of the Interior

U.S. Geological Survey

Southwest Biological Science Center

Tucson, AZ

sgloss@usgs.gov

Jeff Sorensen, Arizona Game and Fish Department

First page photograph credit: Jeff Sorensen, Arizona Game and Fish Department

Chapter 6

Riparian 

Vegetation 

and Associated 

Wildlife

Barbara E. Ralston

Introduction

In the Western United States, riparian areas are 

conspicuous as narrow belts of dense, green vegetation 

along streams and rivers. Fluvial marshes—areas of 

wetland characterized by emergent herbaceous plants 

like sedges (Carex sp.), grasses (Poaceae family), and cattails (Typha sp.)—can be part of the riparian areas that 

are associated with erosion and sediment deposition 

patterns of the adjoining stream or river. The riparian 

and wetland plant community is dependent on surface 

water and groundwater flows (Busch and Smith, 1995; 

Stromberg and others, 1996; Stromberg, 2001) and is 

transitional between aquatic and upland systems. In 

Grand Canyon, the upland system is characterized by 

limited moisture and includes Great Basin desertscrub, 

Mohave desertscrub, and Sonoran desertscrub plant 

constituents (classifications per Brown, 1982), such as 

mormon tea (Ephedra nevadensis), sagebrush (Artemisia

spp.), white brittle bush (Encelia farinosa), and barrel 

cactus (Ferocactus cylindraceus). The dry uplands are a stark 

contrast to the lush plant community along the Colorado 

River in Grand Canyon (figs. 1a–d).

Riparian areas are a junction between aquatic and 

terrestrial habitat types. In the West, they tend to exhibit 

higher levels of species diversity, richness, and population 

densities than either adjacent habitat. Because of these 

characteristics, riparian areas are of high value to managers, scientists, and the public, particularly to Native 

American communities (see chapter 11, this report). The 

importance of riparian areas in the maintenance of biodiversity is well documented (Nilsson and others, 1989; 

Naiman, 1992; Nilsson, 1992; Decamps, 1993; Lock and 

Naiman, 1998; Saab, 1999; National Research Council, 

2002). Riparian areas are especially important in the 

Southwestern United States, where more than 50% of 

166 species of breeding birds in the lowlands are completely dependent on water-related habitat (Johnson and 

others, 1977; Farley and others, 1994). Riparian areas 

also buffer the movement of materials, such as nitrogen 

and carbon, between aquatic and terrestrial environments and help retain nutrients along the river or stream 

channel (Schlosser and Karr, 1981; Jacobs and Gilliam, 

1985; Naiman and Decamps, 1990). Vegetation along a 

water course also provides cover and food such as insects 

and seeds for animal life. In Grand Canyon, whitewater 

recreationists and hikers also use the shade of riparian 

shrubs and trees in the hot summer months. Understand-
104 The State of the Colorado River Ecosystem in Grand Canyon

Figure 1. A. Riparian zone along the Colorado River in Grand Canyon. The predam high-water zone is noticeable as a line of vegetation 

well above the shoreline. Postdam riparian vegetation has progressed downslope and become thicker (photograph © 2005 Geoff 

Gourley; used with permission). B. Tamarisk (Tamarix ramosissima), which is one of the species that was found along the predam highwater zone. Tamarisk was introduced to the Colorado River Basin in the 1880s and was present in Grand Canyon in 1938 (photograph

by Jeff Sorensen, Arizona Game and Fish Department). C. Seep willow (Baccharis sp.), a new high-water zone constituent. This 

species was also present before the dam, but in lower densities (photograph by Jeff Sorensen, Arizona Game and Fish Department).

D. Examples of fluvial marsh and postdam species. The foreground is composed of common reed (Phragmites australis) and water 

sedge (Carex aquatilis), but the background shows tamarisk and arrowweed (Pluchea sericea) (photograph by Jeff Sorensen, Arizona 

Game and Fish Department).

A. B.

C. D.
Riparian Vegetation and Associated Wildlife 105

ing how this community has changed over time is key to 

developing appropriate management strategies for this 

important resource.

This chapter describes changes in the riparian and 

fluvial marsh communities along the Colorado River in 

Grand Canyon from the closure of the Glen Canyon 

Dam and the beginning of the regulation of the river in 

1963 to the present. To provide a better understanding 

of how dam operations have affected riparian vegetation, 

we examine changes in Grand Canyon riparian vegetation during three periods of time (1963–80; 1981–91; 

1991–present) that correspond to major operational 

changes at Glen Canyon Dam. The effects on riparian 

vegetation of both the modified low fluctuating flow 

(MLFF) alternative, which was implemented beginning in 1996, and the recent drought are discussed. The 

chapter concludes with a summary of the findings with 

respect to riparian vegetation as habitat and its relationship to other resources and with a discussion of monitoring priorities within the context of the Glen Canyon 

Dam Adaptive Management Program.

Background

Predam Vegetation

Much of what is known about predam vegetation 

comes from the 1938 investigation of the Colorado River 

corridor by Clover and Jotter (1944). Predam vegetation in the high-elevation benches (fig. 2), the areas well 

above the river that are less frequently scoured by floods, 

was dominated by mesquite (Prosopis glandulosa), catclaw 

acacia (Acacia greggii), Apache plume (Fallugia paradoxa),

and tamarisk (Tamarix ramosissima). Tamarisk was introduced to the Colorado River Basin in the 1800s and was 

present in Grand Canyon in 1938 (Clover and Jotter, 

1944). These same investigators noted the presence of 

coyote willow (Salix exigua), rabbitbrush (Chrysothamnus 

nauseosus), four-wing salt bush (Atriplex canescens), and 

Goodding’s willow (Salix gooddingii) along or close to the 

shoreline (moist sand) at Lees Ferry. On higher benches, 

they found arrowweed (Pluchea sericea) and four-wing salt 

bush. Mormon tea and rabbitbrush were found in the 

talus above the flood plain. The presence of plant cover 

from the river up to the talus at Lees Ferry (RM 0) in 

1938 illustrated the degree of community development. 

Greater vegetation cover may have occurred previously 

because the 1930s was the period of the lowest discharges in the record (Topping and others, 2003). 

Elsewhere in the river corridor, Clover and Jotter 

(1944) documented patchy riparian development, including the absence of vegetation in the moist sand zone 

at President Harding Rapids (approximately RM 43.8) 

(Stevens, 1990) associated with a recent sand deposit. 

They also noted the existence of marsh emergent species, including cattails and rushes (Juncaceae family). 

The predam riparian zone of Grand Canyon was 

periodically disturbed with floods of variable frequency 

and magnitude that redistributed sediment of varying grain size and influenced what plants colonized the 

bare area. Plants like acacia and mesquite located in the 

high-water zone were disturbed less frequently than were 

herbaceous and marsh plants located near the lower 

benches. The riparian assemblage likely represented 

different stages of maturity and succession as it progressed either landward or downstream from a tributary 

source of disturbance. At the higher benches, the vegetation would be the most mature and stable, while at the 

shoreline, the vegetation would be composed of more 

flood-tolerant pioneering species. Areas from the channel 

upslope to the former high-water zone (also known as the 

old high-water zone) were composed of a mix of perennial and annual plants that corresponded with tolerances 

to moisture and disturbance. (Hereafter, refer to fig. 2 for 

references that relate discharge to riparian elevations.)

Variables Affecting 

Vegetation Change

The abundance, distribution, and composition of 

riparian and fluvial marsh vegetation along the river 

corridor in Grand Canyon are influenced by many 

variables including yearly discharge, soils, sedimentation, 

time since disturbance, and temperature (Turner and 

Karpiscak, 1980; Baker, 1989; Stromberg and Patten, 

1991; Busch and Smith, 1995; Stevens and others, 1995; 

Stromberg, 2001). A conceptual model illustrates some 

of the linkages between physical processes and riparian 

habitat (fig. 3); however, the model shown in figure 3 is 

not comprehensive with respect to all variables that affect 

riparian habitat. 

Following closure of Glen Canyon Dam in 1963, 

changes in the hydrologic and sediment regime occurred 

that affected vegetation in Grand Canyon. The operations reduced annual peak discharge and duration and 

increased the yearly base flow (Topping and others, 

2003). The yearly hydrograph was replaced by monthly 

volume releases that followed energy demands (high 

releases in December–February and in July–September) 

rather than seasonal patterns (high flows occurring 
106 The State of the Colorado River Ecosystem in Grand Canyon

in June and July). Postdam median daily discharge 

increased to 12,600 cubic feet per second (cfs), which 

was 58% greater than the predam volumes of 7,980 cfs 

(Topping and others, 2003). Larger average discharges 

increased sediment-export rates (Topping and others, 

2000) and reduced sand storage along channel margins 

and reattachment bars associated with debris fan-eddy 

complexes (Schmidt and Rubin, 1995). Both channel 

margins and reattachment bars are substrates for plant 

colonization. Higher sediment-export rates were most 

noticeable in Marble Canyon (Topping and others, 

2000). Also, the dam reduced the amount of upstream 

sediment coming into the river by 99.9% (Topping and 

others, 2000); tributaries below the dam, including the 

Paria and Little Colorado Rivers, now provide the bulk 

of sand and smaller sediment fractions (see chapter 1, 

this report). 

Disturbance frequency along a river or stream is 

one of several variables that affect riparian community 

development (Stromberg and Patten, 1991; Bendix, 

1994; Toner and Keddy, 1997). Scour and sediment 

reworking within depositional environments like debris 

fans, channel margins, and return channels (Schmidt, 

1990; Schmidt and Rubin, 1995) provided sites for 

colonization by marsh and riparian plant species (Clover 

and Jotter, 1944; Turner and Karpiscak, 1980; Gecy and 

Wilson, 1990; Decamps, 1993). The pioneer assemblage 

may be from an introduced seed source or from vegetative regrowth following scour (Gecy and Wilson, 1990). 

Before regulation of the Colorado River through Glen 

and Grand Canyons, large portions of the river’s flood 

plain were periodically scoured. Predam 1-yr return 

flood discharge reached approximately 50,000 cfs, with 

larger discharges of 120,000 cfs occurring every 6 yr on 

average (fig. 2) (Topping and others, 2003). Yearly flooding reduced vegetation below the 50,000-cfs water-surface elevation, while larger, less frequent floods affected 

vegetation communities on higher benches. Lower peak 

flows caused by Glen Canyon Dam allowed species, 

including nonnative plants, to occupy lower flood-plain 

Figure 2. Habitat zones according to inundation frequency and flow magnitude (cubic feet per second). Regulation of the Colorado 

River by Glen Canyon Dam has resulted in reduced flood frequencies and magnitudes. In the postdam setting, the area below 50,000-

cfs water-surface elevation is the active riparian zone. (In the postdam period, the active riparian zone remained at 50,000-cfs watersurface elevation during the 1980s, dropping to 45,000 cfs after 1991, when new restrictions constrained dam operations to minimize

resource impacts.) Above the active riparian zone are the higher elevation benches, areas well above the river that are less frequently

scoured by floods and were subjected to a predam 6-yr return flood frequency. Within the active riparian zone are bands of vegetation

that follow a moisture gradient from water-tolerant plants located near shoreline to species that tolerate drier upslope conditions.

Figure modified from Carothers and Aitchison (1976) with data from Topping and others (2003).
Riparian Vegetation and Associated Wildlife 107

benches. Riparian areas are particularly prone to invasion by nonnative plants because they are frequently disturbed by flood events, which create favorable conditions 

for the seeds of nonnative plants that can be dispersed 

long distances by a variety of processes (Decamps, 1993). 

Status and Trends

Riparian community changes following the closure of 

Glen Canyon Dam in 1963 occurred over three periods.

Period I: Initial Vegetation 

Expansion (1963–80)

Immediately following the closure of Glen Canyon 

Dam, operations focused on filling Lake Powell, delivering water to Lake Mead, and producing peak power. 

Discharges were reduced to between 1,000 and 20,000 

cfs during this period (median discharge was 9,490 cfs in 

the 1960s). A series of discharges of 50,000 cfs conducted in 1965 cleaned the channel below the dam and 

raised the elevation of Lake Mead (Topping and others, 

2003). Daily fluctuations in the 1970s were large, varying 

between 4,000 and 25,000 cfs, with a median discharge 

of 11,600 cfs (Topping and others, 2003). The result 

of dam operations during this period was to encourage 

plant colonization along the channel in the low-elevation

benches. 

Vegetation expansion below the 50,000-cfs watersurface elevation was documented by Turner and 

Karpiscak (1980), who used repeat photography from 

historical expeditionary trips through Grand Canyon 

such as J.W. Powell’s second trip in 1872 (Darrah, 1948), 

the Robert Stanton expedition in 1889 (Stanton, 1965), 

and the U.S. Geological Survey expedition in 1921 

(LaRue, 1925). Although Turner and Karpiscak (1980) 

did not quantify vegetation change, they did qualitatively 

demonstrate an increase in vegetation in the postdam 

Figure 3. A conceptual model of physical factors that affect riparian vegetation development along the river corridor. Thicker arrows 

indicate a greater degree of effect on riparian vegetation. The closure of Glen Canyon Dam and the beginning of flow regulation of the 

Colorado River through Grand Canyon in 1963 all but eliminated the mainstem sand supply to Grand Canyon. Also, dam operations are

now independent of tributary inputs of sediment. Taken together, dam-induced changes in both sand supply and flow have altered the

sedimentary processes that provide substrate for riparian vegetation.
108 The State of the Colorado River Ecosystem in Grand Canyon

fluvial sediment zone (up to 30,000 cfs) and in the 

predam fluvial sediment zone (30,000–85,000 cfs; 2-yr 

return period) (Topping and others, 2003). They noted 

dense stands of tamarisk, coyote willow, and arrowweed 

throughout the corridor, with desert broom (Baccharis 

sarothroides), Bermudagrass (Cynodon dactylon), Emory seep 

willow (Baccharis emoryi), and some cottonwoods (Populus 

fremontii) along the postdam fluvial zone. The expansion 

of coyote willow, tamarisk, and arrowweed within the 

predam and postdam fluvial zones was predictable given 

the creation of a stable water source and exposed land 

area (figs. 4a and b). The expansion of emergents such 

as cattails along the channel was also noted by Turner 

and Karpiscak (1980). The higher bench (land above 

>50,000 cfs) was found to be changing at a slower rate 

and composed of predam high-bench species like acacia, 

mesquite, sand dropseed (Sporobolus cryptandrus), and 

Apache plume. 

The qualitative findings of Turner and Karpiscak 

(1980) are consistent with a land area change study 

of selected sites in Marble Canyon and upper Grand 

Canyon through the use of geographic information 

systems (Waring, 1995). Waring estimated a 100% 

increase in vegetation in the postdam and predam fluvial 

sediment zones between 1965 and 1973 (56.5 acres vs. 

108.6 acres (228,503 m2

 vs. 439,420 m2)). Anderson and 

Ruffner (1988) examined the predam high-bench terrace 

vegetation and determined that this zone showed little 

recruitment of new acacia or mesquite individuals. In 

other words, this zone was not showing signs of replacement of similar species. They hypothesized that the 

vegetation was becoming more mature, with individuals 

becoming larger and more closely spaced. Over time, 

the vegetation in this zone would become less dense as 

mature individuals died and were not replaced. Species 

found in this predam bench would “move shoreward” 

over time.

Period II: Inundation and 

Habitat Reworking (1981–90) 

In 1980, Lake Powell reached full pool elevation, 

and operations over the next decade focused primarily 

on water delivery and power generation. Because the 

early 1980s was a wet period, causing a high-release 

spill of 97,000 cfs in June 1983 (Martin, 1989), however, 

Glen Canyon Dam was also operated to manage spring 

inflows and protect the integrity of the dam. As a result, 

the 1980s produced several years of releases greater 

than 20,000 cfs for portions of the year (U.S. Geological 

Survey, 2005), with higher releases occurring in spring 

to reduce the frequency of spills. Median discharge for 

the decade was 15,900 cfs (Topping and others, 2003), 

approximately 32% greater than the median releases 

of the 1970s. Fine-sediment erosion and export from 

the Marble Canyon and upper Grand Canyon reaches 

occurred in this decade (Topping and others, 2000; 

Schmidt and others, 2004). Sediment export exceeded 

inputs during these events, resulting in an overall loss of 

sediment in the system. 

The higher peak and median discharge presented 

situations of sustained inundation of riparian vegetation 

along the channel and increased water-table elevations

that promoted expansion of woody vegetation in this 

decade (figs. 4b and c). The peak flow likely redistributed 

seeds from the predam flood-plain surfaces, which may 

have promoted establishment and growth of acacia and 

mesquite within the lower elevation benches. Waring 

(1995) detected a 13% decrease in vegetation occupying area below 50,000-cfs discharge. Waring showed 

an increase in vegetated area in the higher elevation 

benches for 1984 compared with 1975; measurements of 

mesquite in the predam flood plain by other researchers, 

however, did not indicate a growth response to the flood 

events of the 1980s (Anderson and Ruffner, 1988). 

Period III: Low Fluctuating 

Flows and Experimentation 

(1991–present)

River Flows

The operation of Glen Canyon Dam since 1991 has 

focused on meeting water allocation requirements, producing power, and complying with environmental constraints designed to minimize the effects of Glen Canyon 

Dam on the erosion of recreational and archaeological 

sites and on the deterioration of habitats for endangered 

species, particularly humpback chub (Gila cypha) (U.S. 

Department of the Interior, 1995). During this period, 

flows have been further stabilized, not varying more 

than 8,000 cfs daily, though median annual flows have 

decreased only 15% when compared with those of the 

1980s (13,500 cfs vs. 15,900 cfs) (Topping and others, 

2003). The frequency of high peak flows has diminished 

with two experimental high-flow events of 41,000 cfs 

and 45,000 cfs occurring in November 2004 for 2.5 d 

and in March 1996 for 7 d (Webb and others, 1999). As 

a result, since 1991 the active riparian zone has been 

reduced to 45,000-cfs water-surface elevation. Smaller, 

shorter duration spikes of up to 31,000 cfs occurred in 
Riparian Vegetation and Associated Wildlife 109

Figure 4. A. Aerial photograph from 1965 at RM 55.4 showing sparse vegetation occupying the sandbar and beginning to expand into 

the postdam fluvial zone. B. Aerial photograph from 1984 of RM 55.4 showing effects of flooding, which scoured low-lying riparian

habitats. Vegetation occupies larger expanses along the shoreline and upslope from the river. C. Aerial photograph from 1994 of RM 

55.4 showing expansion of riparian and marsh vegetation since 1984 on the sandbar and along the shoreline (source: U.S. Geological

Survey file photographs).

A.

B.

C.
110 The State of the Colorado River Ecosystem in Grand Canyon

1997 and 2000 (Schmidt and others, 2004). Peak flows 

(flows >120,000 cfs) have also been reduced by more 

than 50% of mean long-term high flows experienced 

before 1963 (Topping and others, 2003). As a result, the 

active riparian zone in Grand Canyon has contracted in 

width to that area below approximately 50,000-cfs waterelevation stage, with greatest change occurring below 

30,000 cfs. The higher elevation bench (>50,000 cfs) 

persists through yearly rainfall events and is little affected 

by operations compared to predam hydrology. 

Response in the Plant Community

 This 14-yr period of stable but fluctuating flows 

and experimentation resulted in the expansion of vegetation into open areas, including into campsites and channel margins within the active riparian zone (Kearsley 

and others, 1994; Webb and others, 2002); the reduction 

of marsh habitat associated with eddy return channels 

(i.e., backwaters) (Stevens and others, 1995; Goeking and 

others, 2003); and most recently (since 2002), a reduction 

of vegetative cover in low channel positions (Kearsley, 

2004b). The community as a whole has become more 

mature, providing complex habitat for riparian breeding 

birds. With the onset of the drought in 2000 there has 

also been a reduction in cover of annual and perennial 

grasses in areas located above flows of 35,000-cfs elevation (Kearsley, 2004b). 

Forty-one percent of camping sites surveyed 

between 1983 and 1991 were determined to be unusable 

because of vegetation overgrowth (Kearsley and others, 

1994). Vegetation expansion into campsites occurred 

in reaches that are classified as wide reaches (Schmidt, 

1990) or in areas where more sediment is available for 

plant establishment. Kaplinski and others (2005) discussed trends in vegetation expansion into campsites 

since 1991; their findings are summarized in chapter 

12 of this report. Vegetation expansion between the 

high-water periods of 1984 and 1992 was supported 

by Waring (1995), who showed expansion during this 

period at a percentage of change similar to that which 

occurred with initial dam closure through 1973. Waring 

(1995) speculated that the rate of vegetation expansion 

increased during the early 1990s with the implementation of interim operating criteria. Expansion within 

the zone between shoreline and up to the 50,000-cfs 

water-surface elevation included the establishment by 

nonnative plants and pioneer species like camel thorn 

(Alhagi maurorum) and clonal growth by woody vegetation, 

including arrowweed and coyote willow. This vegetation 

expansion resulted in an increase in riparian bird habitat 

(see chapter 7, this report). Vegetation expansion was 

greatest in channel margin habitats used primarily by 

wildlife and was least in sites adjacent to rapids associated with debris fans where disturbance was more likely 

to occur (Melis and others, 1995; Waring, 1995; Webb 

and others, 2002). 

Debris fan-eddy complexes (Schmidt and Rubin, 

1995) are geomorphic features that support fluvial marsh 

habitat, primarily because they are low-velocity habitats 

that accumulate silt and clay fractions (Schmidt and 

Rubin, 1995; Stevens and others, 1995). Daily inundation frequency, soil texture, and distance from the dam 

influence marsh locations and assemblages (Stevens 

and others, 1995). Wet-marsh constituents like cattails, 

sedges, and common reed (Phragmites australis) are found 

in sites with increased inundation frequency, while drier 

marsh-associated species like tamarisk, arrowweed, 

horsetails (Equisetum sp.), and willows (Salix sp.) are associated with lower inundation frequencies. Interim operating criteria, initiated in 1991 and followed by the MLFF 

alternative in 1996, reduced inundation frequency. This 

change is coincident with a reduction in wet-marsh habitat since 1991 (Stevens and others, 1995; Kearsley and 

Ayers, 1996). 

Geomorphic Effects

The effect of geomorphology on plant assemblages 

is illustrated in marsh plots that were surveyed in the 

mid-1990s. Narrow reaches of the river such as that of 

Marble Canyon experienced losses of marsh patches, 

which correspond to a reported loss of sediment in this 

reach during the same time (Kearsley and Ayers, 1996; 

Schmidt and others, 2004). Wider reaches found near 

the Little Colorado River and in western Grand Canyon 

that have greater sediment-storage capacities showed 

gains and losses of marsh patches during these same 

years in the mid-1990s. The variability in the number 

of patches within these reaches may reflect local sediment inputs from ungaged tributaries as well as inputs 

from the Little Colorado River. For these same years, a 

drying trend (i.e., plants encountered were associated 

with lower moisture gradients) was noted for riparian 

plants (Kearsley and Ayers, 1996). Reduced numbers of 

marshes support a hypothesis that interim flows reduced 

inundation frequency and that species encountered were 

more likely to be associated with a lower moisture gradient. Alternatively, the change may reflect infilling and 

riparian community succession. Marsh census numbers 

since 1995 are not available, but geomorphic studies of 

debris fan-eddy complexes detected reduction in backwaters from 1984 to 2000 (Goeking and others, 2003). A 

reduction in these sites may be an indication that fine-
Riparian Vegetation and Associated Wildlife 111

sediment habitats were also declining during this period. 

Sediment export, particularly silt and clay, and reduced 

fluctuations likely reduced the area of cover represented 

by wet-marsh species in the 1990s through 2001. 

Hydrology and Climatic Influences

Reservoir levels, yearly operations, and local precipitation affect riparian vegetation growth and development within all vegetation zones along the river corridor. 

A persistent, basinwide drought was identified in July 

2000 by the National Drought Mitigation Center (www.

drought.unl.edu/dm/archive/2000/drmon0725.htm, 

accessed February 8, 2005). Since then, inflows to Lake 

Powell have been below average, leading to drawdown 

of both Lake Powell and Lake Mead (see chapter 4, this 

report). As a result, Glen Canyon Dam has released the 

minimum amount of water needed to meet delivery 

requirements, which is 8.23 million acre-feet (10,148 

million m3

). While overall volume of delivery has been 

reduced, monthly median flows have not changed appreciably. Beginning in 2002, the months from January 

to March and from June to August are dominated by 

power-generation flows in summer months and fish suppression measures in winter months (U.S. Department 

of the Interior, 2004a). Fluctuations vary from 5,000 to 

20,000 cfs daily in the winter months and by 8,000 cfs 

daily in summer, with base flow being approximately 

10,000–12,000 cfs. Other months have lower volumes 

allocated with corresponding lower base flow and 

reduced daily range (e.g., 5,000–10,000 cfs in April). The 

abrupt shifts in monthly volumes in April and September expose areas in spring for plant establishment but 

are soon followed by high summer fluctuations in June. 

By September, the area inundated to 17,000-cfs watersurface elevation is often sparsely vegetated and reduced 

in sediment (Kearsley, 2004b). Precipitation variability 

and operational shifts of Glen Canyon Dam resulted in 

continued changes in the vegetation cover, abundance, 

and density along the river corridor.

Recent Monitoring Results

Between 2001 and 2003, riparian vegetation was 

affected both by changes in dam operations and by a 

persistent drought. The summers of 2002 and 2003 had 

higher daily minimum flows than 2001, and these years 

also had winter (January to March) discharges that varied 

from 5,000 cfs to 20,000 cfs. Vegetation volume (a surrogate for structure) in the active riparian zone (5,000 cfs 

to 45,000 cfs) responded markedly each year. Between 

2001 and 2002 volume decreased by 15% but had 

recovered by approximately the same amount between 

2002 and 2003 (fig. 5) (Kearsley, 2004a). The recovery 

was attributable to the operational change that took 

place in January 2003. How these volumes may influence 

riparian bird density or abundances between years is not 

known. In contrast, vegetation at higher water-surface 

elevations (>45,000 cfs) changed little between years (fig. 

5) (Kearsley, 2004a). Vegetation at higher water-surface 

elevations may respond more to localized precipitation 

events than to dam operations (Kearsley, 2004a). In 

general, operations had the greatest effect on vegetation 

located below the 35,000-cfs water-surface elevation. 

Measures of plant abundance, species richness, 

diversity, and distribution all showed a decline since 

2001 (Kearsley, 2004b). Operations and local precipitation differentially affected plants along the elevational 

gradient. Herbaceous annuals and perennials like cheat 

grass (Bromus tectorum), sand dropseed, and spiny aster 

(Chloracantha spinosa) located above 35,000-cfs watersurface elevation were affected by yearly precipitation 

and showed the greatest decline in cover (fig. 6a). The 

effect of the drought on higher elevation plants was also 

evident when species composition was examined. Species richness changed significantly at sites at 45,000 cfs 

and 60,000 cfs (fig. 6b) (Kearsley, 2004b); the change 

was associated mostly with a loss of annual and rarely 

encountered plant species. Increased summer precipitation in 2003 was responsible for increases in species 

richness in both of these elevations (fig. 6b). Annuals 

appearing in wetter years likely contributed to these 

increases (Kearsley, 2004b). Compositional shifts did not 

Figure 5. Change in vegetation volume (a surrogate measure 

of structure) from 2001 to 2003 in the riparian zone along the 

Colorado River in Grand Canyon at surface-water elevations 

above and below 50,000-cfs surface elevation. Figure modified 

from Kearsley (2004a).
112 The State of the Colorado River Ecosystem in Grand Canyon

occur for zones below 35,000 cfs, and these data, in combination with vegetation volume measurements, suggest 

that changes in this zone were associated with increased 

growth of established woody species rather than with 

recruitment or mortality. Moreover, reductions in cover, 

richness, and diversity in lower water-surface elevation

locations (<25,000 cfs) were caused by sediment loss, 

inundation, or scour that coincided with operational 

changes in January 2003. Dam operations influenced 

changes in vegetation beyond recorded discharge levels, possibly up to approximately 15,000 cfs, while local 

precipitation appeared to have a greater influence, in the 

short term, on vegetation above the 35,000-cfs water-surface elevation. 

Since the 1990s, reservoir levels, yearly operations, 

and local precipitation have affected riparian vegetation growth and development along the river corridor in 

Grand Canyon (figs. 3, 5, and 6). Though other factors 

do affect riparian vegetation dynamics, these variables 

appear to be significant drivers in riparian vegetation 

development and change. The riparian zone in Grand 

Canyon has contracted shoreward as flows have stabilized. Riparian vegetation at water-surface elevations

up to about 15,000 cfs above daily maximum discharge 

responds to operational changes. Vegetation below a 

water-surface elevation of 45,000 cfs has become denser 

and has expanded into open sites, including campsites. 

Vegetation cover and richness at low water-surface 

elevation locations (below 20,000 cfs) are most directly 

affected by dam operations. The fluvial marsh community is the most responsive of vegetative communities within Grand Canyon to changes in hydrology and 

sediment supply. Not surprisingly, cover and richness 

decline during flow fluctuations that promote scour and 

sediment export (Kearsley, 2004b). At the same time 

these flows can increase inundation frequency at higher 

elevations and can shift plant composition to more watertolerant species like cattails and rushes as a result of 

annual changes in operations (Stevens and others, 1995). 

Essentially, the riparian area has declined quantitatively in 

some aspects (less spatial coverage, fewer numbers of species) and has changed qualitatively (denser, more mature).

Riparian Vegetation as 

Terrestrial Habitat

As stated in the Introduction of this chapter, riparian communities in the Southwestern United States play 

an important role as wildlife habitat (Carothers and 

Brown, 1991; Farley and others, 1994; Skagen and othFigure 6. A. Percent change in vegetation covers at five watersurface elevations (cubic feet per second) between 2001 and 2003 

in the riparian zone along the Colorado River in Grand Canyon. 

Cover has declined in all zones since 2001. Discharge had the 

greatest effect on species richness at water-surface elevations 

from 35,000 cfs to 15,000 cfs. Overall vegetation cover within the 

riparian zone is not dense although cover is greatest at the 25,000-

cfs water-surface elevation, which corresponds with areas used by 

campers and breeding birds. Figure modified from Kearsley (2004b). 

B. Species richness in five water-surface elevations (cubic feet per 

second) from 2001 to 2003 and change between years in the riparian 

zone along the Colorado River in Grand Canyon. Species richness 

is a measure of the total number of species found at each watersurface level and is an indication of biodiversity. Species richness 

in this case is measured from a scale of 0 to 10 with 10 being the 

highest amount of richness. Overall species richness within the 

riparian zone is greatest in those zones that are above the 25,000-

cfs water-surface elevation where vegetation is not directly 

scoured by flow. Species richness in the upper water-surface 

elevations is more affected by yearly precipitation. Species at 

upper surface water elevations have declined since 2001 because 

of drought conditions and include annual cheat grass (Bromus 

tectorum) and perennial sand dropseed (Sporobolus cryptandrus). 

Figure modified from Kearsley (2004b).

A.

B.
Riparian Vegetation and Associated Wildlife 113

ers, 1998; Stevens and Ayers, 2002). Along the Colorado 

River, riparian birds have had a greater emphasis placed 

on them than other types of wildlife in terms of monitoring and are treated in a separate chapter of this report 

(chapter 7). Wildlife other than endangered species 

and birds has not been emphasized to date in the Glen 

Canyon Dam Adaptive Management Program. For these 

reasons, the following section addresses threatened and 

endangered species that occupy riparian habitat marshes 

and springs within the Colorado River ecosystem. Other 

faunal constituents are briefly mentioned. 

Threatened and 

Endangered Species

Kanab Ambersnail

The Kanab ambersnail (Oxyloma haydeni ssp. kanabensis) is a terrestrial succineid snail (fig. 7) associated with 

wetland and spring vegetation on the Colorado Plateau. 

The snail was listed as endangered in 1992 (England, 

1992). Presently, the species is found at three locations: 

Vaseys Paradise and Elves Chasm (a translocated population), in Grand Canyon National Park, and private land 

in southern Utah. Data presented here pertain to the 

snails located at Vaseys Paradise. 

Vaseys Paradise (figs. 8a–c) is a small patch of 

spring-fed riparian vegetation at RM 31.8 (Stevens, 

1990). Ambersnails are found in the vegetation, usually 

associated with cardinal monkeyflower (Mimulus cardinalis) (fig. 8b), watercress (Rorippa nasturtium-aquaticum) (fig. 

8c), and water sedge (Carex aquatilis). Ambersnail adults 

overwinter and reproduce in spring. Recruitment into 

the adult population takes place during summer and fall 

(Stevens and others, 1998; Nelson, 2001). 

Habitat

The greatest gains in habitat area, measured by 

traditional land-survey methods, occurred between fall 

2001 and fall 2002 when snail habitat at Vaseys Paradise 

increased 23% in area (~2,374 ft2 vs. ~3,103 ft2 (220.6 

m2 vs. 288.4 m2

)) (fig. 9a) (U.S. Geological Survey, unpub. 

data, 1998–2004). Increases in measured habitat may 

be attributable to low minimum flows in 2001 that 

increased area for colonization by watercress, monkeyflower, or other plants; however, watercress, which is a 

species adapted to disturbance and that requires sustained moisture, has decreased since 1998 because spring 

discharges declined in association with the drought. At 

the same time, monkeyflower increased (U.S. Geological 

Survey, unpub. data, 1998–2004) and accounted for most 

of the habitat increase measured between fall 2001 and 

fall 2002. Monkeyflower, while still requiring moisture, 

appears to be more tolerant of drier habitats. Water 

sedge is patchily distributed in Kanab ambersnail habitat 

and is a source of forage for bighorn sheep (Ovis canadensis). As a site that provides a reliable source of vegetation 

in a drought, the springs are now habitually visited by 

bighorn sheep, resulting in vegetation used by the snails 

being regularly trampled.

Snail Abundances

The number of snails has not changed significantly 

since 1998. Fall numbers generally exceed spring numbers as would be expected with seasonal recruitment (fig. 

9b; U.S. Geological Survey, unpub. data, 1998–2004). 

Curiously, while habitat has increased, snail numbers 

have not had a correlated increase. The lack of increase 

in snail numbers may be associated with soil moisture, 

shifts in plant-species composition, and mortality associated with trampling by bighorn sheep rather than with 

the amount of habitat available.

Figure 7. Kanab ambersnail (Oxyloma haydeni ssp.

kanabensis), which is monitored at Vaseys Paradise 

(photograph by Roy Averill-Murray, Arizona Game and 

Fish Department).
114 The State of the Colorado River Ecosystem in Grand Canyon

Figure 8. A. Vaseys Paradise along the Colorado River, which is one of three locations known to support the endangered 

Kanab ambersnail (Oxyloma haydeni ssp. kanabensis). Cardinal monkeyflower (Mimulus cardinalis) (B) and watercress (Rorippa

nasturtium-aquaticum) (C) are primary plant species associated with Kanab ambersnail (photographs by Jeff Sorensen, Arizona Game 

and Fish Department).

Other Wildlife

Invertebrates

Invertebrate species, as in other ecosystems, account 

for the greatest number of species found along the river. 

There are several thousand invertebrate species from 

over 200 families (Stevens and Ayers, 2002; Lightfoot 

and others, 2004), including scorpions, spiders, flies, 

ants, moths, and butterflies. Surveys for invertebrates 

conducted over the past several years identified either 

range extensions for species (e.g., butterflies: Arizona 

powdered-skipper (Systacea xampa), piute agave skipper 

(Agathymus alliea piute), desert marble (Euchloe lotta), and 

desert elfin (Callophrys fotis)) or species not previously 

known to exist (e.g., moth, Schinia immaculate) (Stevens 

and Ayers, 2002; Pogue, 2004). Invertebrate composition 

associated with higher elevation riparian vegetation consists largely of native invertebrate taxa, while the lower 

elevation riparian vegetation includes a mix of native 

and nonnative invertebrate species (Lightfoot and others, 2004). The mix of species is not unexpected because 

vegetation in this area consists of a more pronounced 

mix of native and nonnative plants. 

Amphibians, Mammals, 

and Reptiles

Surveys for mammals, reptiles, and amphibians 

have been sporadic (Carothers and Aitchison, 1976; 

Warren and Schwalbe, 1986; Frey, 2003). Past surveys 

found 14 mammal species, 16 reptile species, and 4 

amphibian species along the corridor. Amphibians 

of special concern are detailed in the accompanying 

text box. Only the deer mouse (Peromyscus maniculatus) is restricted to the riparian zone (Frey, 2003; U.S. 

Department of the Interior, 2004b). Larger mammals 

include beavers (Castor canadensis), coyotes (Canis latrans),

bighorn sheep, mule deer (Odocoileus rafinesque), mountain lions (Puma concolor), and bobcats (Lynx rufus) (U.S. 

Department of the Interior, 2004b). Mountain lions 

and bobcats are seen infrequently. Of these mammals, 

beavers appear to have expanded their numbers since 

the 1960s (Carothers and Brown, 1991) in association 

with riparian vegetation expansion. Beavers appear to 

be relatively evenly distributed throughout the river corridor (U.S. Department of the Interior, 2004b).

Discussion and Future 

Research Needs

A long-term data set associated with changes in 

riparian vegetation is lacking for the Colorado River 

ecosystem. The data that are provided here are results 

associated with specific research questions of 2- to 3-yr 

duration rather than monitoring to detect trends. Trend 

detection associated with riparian vegetation requires 

local and regional scale monitoring because local and 

regional hydrology and geomorphology affect the riparian zone. The long-term goal for monitoring in the 

A. B. C.
Riparian Vegetation and Associated Wildlife 115

Figure 9. A. Change in the area of Kanab ambersnail (Oxyloma haydeni ssp. kanabensis) habitat (ft2) at Vaseys Paradise from 1998 

through 2004 from spring and fall surveys. B. Estimated snail abundances at Vaseys Paradise from 1998 through spring 2004 from spring 

and fall surveys at Vaseys Paradise (U.S. Geological Survey, unpub. data).

A. B.

Colorado River should be to use yearly data on cover, 

species richness, and diversity in concert with semidecadal vegetation mapping data to discern operational 

versus climatic effects on the riparian vegetation. Largescale trend detection at the reach or regional scale will be 

able to provide information about changes in vegetated 

area, increases and decreases in vegetation patches, and 

the vegetation classes that have changed the most. The 

mapping data can also be integrated with bird monitoring, and other faunal resource monitoring that may be 

developed, to produce information about habitat quality. 

Habitat quality affects invertebrate communities that are 

food resources for both riparian breeding birds and fish 

communities along the river corridor. 

Linkages among vegetation, faunal assemblages, 

and habitat quality are needed before comprehensive 

assessments of the riparian zone for the Colorado River 

can be made. One step toward a comprehensive assessment has been initiated by an inventory of invertebrates 

along the corridor. The intent of the inventory is to 

determine what types of invertebrates exist along the 

corridor, whether or not any species can be used to indicate environmental conditions, and in what quantities 

these potential indicators occur. Both quantity and type 

of invertebrates encountered are affected by vegetation 

assemblages. The challenge associated with riparian vegetation is to determine how dam operations affect plant 

species assemblages and densities, which, in turn, influence habitat quality and food resources for vertebrates 

found along the river corridor.

In the long term, continued loss of sediment along 

the shoreline and changes in the size fraction of the 

substrate will reduce available colonizing substrate and 

affect subsequent species establishment (i.e., marsh communities may shift to constituents that persist in coarser 

substrates) (Stevens and others, 1995). At elevations

above the 20,000-cfs flow but still in the active riparian 

zone, woody vegetation is becoming more mature and 

less diverse in association with the reduction in high 

flow frequency. Under current operations (modified low 

fluctuating flows), precipitation affects vegetation above 

the 35,000-cfs water-surface elevation more than operations do (Kearsley, 2004 a, b). Remnants of the predam 

high-water riparian zone have remained relatively 

unchanged, depending on seasonal precipitation rather 

than on yearly hydrology for its maintenance. Implications for these changes within the river corridor include 

the potential reduction in numbers of some riparian 

bird species that depend on wetter marsh-plant species 

for nesting or food resources, though other riparian bird 

species may benefit from the more mature, dense habitat 

(Anderson and Ohmart, 1984; Farley and others, 1994). 

Furthermore, continued loss of campsite area associated 

with vegetation expansion may occur. The trade-offs 

between recreation and wildlife habitats are value-based 

management decisions that the adaptive management 

program will have to address. Some of these trends could 

be ameliorated through higher frequency disturbances 

up to and above 45,000-cfs discharges, but these events 

should be timed to coincide with sufficient sediment 

inputs and existing system supply. 
116 The State of the Colorado River Ecosystem in Grand Canyon

Declining Riparian Species: 

Leopard Frogs in Grand 

Canyon and Glen Canyon

Charles Drost

Amphibians have been relatively neglected in studies of plants and animals and of the effects of dams in 

Grand and Glen Canyons. Amphibians were surveyed 

along the Colorado River in Glen Canyon before the 

construction of Glen Canyon Dam (Woodbury, 1959), 

but extensive surveys were not conducted in Grand 

Canyon until well after the completion of the dam (e.g., 

Aitchison and others, 1974; Suttkus and others, 1976). 

Over the last 15 yr there has been increasing recognition 

and concern about declines in amphibian populations 

in areas throughout the world (Wyman, 1990; Wake, 

1991; Vial and Saylor, 1993). The cause of many of 

these declines is unknown, but they have even occurred 

in national parks and other protected areas, suggesting that causes are widespread regional ones (Blaustein 

and Wake, 1990). We describe here the current status of 

amphibians in the Colorado River corridor of Grand 

Canyon National Park and Glen Canyon National 

Recreation Area based on extensive surveys conducted 

over the last 10 yr throughout Glen and Grand Canyons, 

from the uppermost end of Lake Powell to the upper end 

of Lake Mead.

Surveys found healthy, widespread populations 

of two species of toads (Woodhouse’s (Bufo woodhousii) and red-spotted (B. punctatus)); the canyon treefrog 

(Hyla arenicolor); and the tiger salamander (Ambystoma 

tigrinum). Northern leopard frogs (Rana pipiens) (fig. 1), 

on the other hand, have declined substantially, paralleling losses reported in other areas of western North 

America. Leopard frog populations have disappeared 

from 70% of sites where they were formerly found and 

have declined in numbers at some sites where they still 

occur (fig. 2). Some of the losses are of riverside populations in Glen Canyon, which were inundated by Lake 

Powell (Drost and Sogge, 1993); however, other populations have been lost from side canyons off of the lake 

that are not impacted directly by inundation. Currently, 

seven leopard frog populations are known to occur in 

side canyons of Lake Powell, concentrated around the 

Escalante River area of the lake. The status of the species at some sites is uncertain. Earlier surveys found frogs 

as far upstream as Dark Canyon, near Hite, but none 

have been seen there recently. A small population in 

Wilson Creek off the San Juan River area of the lake has 

not been seen since 1994.

The predam distribution of northern leopard frogs 

in the Grand Canyon reach of the Colorado River is 

unknown because of the lack of early surveys. Compilation of reports from more recent surveys shows that 

northern leopard frogs occurred at least as far downstream as Cardenas Creek (RM 71) along the river corridor and in side canyons as far as Bright Angel Creek and 

Kanab Creek. In spite of intensive searches of potential 

habitat along the river and in side canyons with perennial streams, the only known remaining population is at 

a spring-fed marsh between Glen Canyon Dam and Lees 

Ferry. This population was discovered in 1992 (Drost and 

Sogge, 1993) and has experienced wide year-to-year fluctuations in numbers. The most recent surveys indicate 

a sharp decline in population size, with only two adult 

individuals found in 2004. Marsh vegetation at the site 

has become very dense, reducing areas of open water, 

and this reduction may be an important factor in the 

decline of this population. 

Although survey work is continuing, it is clear, based 

on the historical record, that there has been a severe 

contraction of the northern leopard frog’s range in both 

Glen Canyon and Grand Canyon. A major concern 

for remaining populations of frogs is that most or all of 

them are now effectively isolated from each other. No 

other extant populations have been found along the river 

below Glen Canyon Dam, so the population below Glen 

Figure 1. Northern leopard frog (Rana pipiens) (photograph by 

Charles Drost, U.S. Geological Survey).
Riparian Vegetation and Associated Wildlife 117

Figure 2. Northern leopard frog (Rana pipiens) sites in Glen Canyon above Glen Canyon Dam (A) and the corridor of the 

Colorado River and its tributaries below Glen Canyon Dam (B). Northern leopard frog numbers have declined substantially, 

paralleling losses reported in other areas of Western North America. Leopard frog populations have disappeared from 

70% of sites where they were formerly found and have declined in numbers at some sites where they still occur.

A.

B.
118 The State of the Colorado River Ecosystem in Grand Canyon

Canyon Dam is completely isolated. In addition, no 

frogs have been found along the shores of Lake Powell. 

The combination of deep lake waters, lack of vegetation 

cover, and large numbers of predatory fish in the lake probably prevents any movement of frogs among side canyons.

One unexpected, positive finding of the surveys was 

the discovery of a previously unknown population of a 

second leopard frog species in western Grand Canyon. 

In spring 2004, small numbers of leopard frogs were 

found in a pool in Surprise Canyon (RM 248) (Gelczis 

and Drost, 2004). The frogs are clearly different from 

the northern leopard frogs found farther upstream. 

Genetic studies of the population are still in progress, but 

the frogs are apparently the lowland leopard frog (Rana

yavapaiensis). This location represents a significant northward extension in range for this relatively rare species. 

There are potential threats at the site in the form of nonnative predatory fish and crayfish, but this new population appears to be healthy and thriving. 

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Contact Information:

Charles A. Drost

Research Zoologist 

U.S. Department of the Interior

U.S. Geological Survey

Southwest Biological Science Center

Flagstaff, AZ

charles_drost@usgs.gov
Riparian Vegetation and Associated Wildlife 119

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Contact Information:

Barbara E. Ralston

General Biologist

U.S. Department of the Interior

U.S. Geological Survey

Southwest Biological Science Center

Flagstaff, AZ

bralston@usgs.gov

Jeff Sorensen, Arizona

Game and Fish Department

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First page photograph credit: Jeff Sorensen, Arizona Game and Fish Department
Introduction

Riparian habitats, or vegetated areas along streams 

and rivers, in the Western United States typically support 

a disproportionately large number of birds compared 

to adjacent nonriparian habitats, both in terms of bird 

abundance and the number of species present (also 

known as species richness). The Grand Canyon ecosystem is no exception and provides important habitat 

to wintering, migrant, and breeding birds (Brown and 

others, 1987; Carothers and Brown, 1991; Sogge and 

others, 1998; Kearsley and others, 2004; Spence, 2004). 

Importantly, the ecosystem also provides habitat for 

several bird species of special concern, including the 

southwestern willow flycatcher (Empidonax traillii extimus),

California condor (Gymnogyps californianus), bald eagle 

(Haliaeetus leucocephalus), and American peregrine falcon 

(Falco peregrinus anatum).

This chapter summarizes the considerable information available from recent studies on the ecology of 

Grand Canyon bird species and communities. Because 

changes in riparian habitat undoubtedly influence the 

abundance and distribution of Grand Canyon birds, 

the chapter starts by briefly examining dam-induced 

habitat alterations that may affect birds. The direct and 

indirect effects of Glen Canyon Dam operations, including the modified low fluctuating flow (MLFF) alternative 

that was implemented starting in 1996, are considered 

for how they influence specific bird species and communities. Particular attention is given to the species of 

special concern listed above. The chapter concludes 

with a summary and a discussion of research priorities 

within the context of the Glen Canyon Dam Adaptive 

Management Program. 

Background

The riparian vegetation of the Grand Canyon 

ecoregion is complex and dynamic, changing in response 

to flooding, the invasion of new nonnative species, longterm successional patterns, and climate (Turner and 

Karpiscak, 1980; Webb and others, 1999). The primary 

driving variables in the terrestrial riparian ecosystem in 

Grand Canyon are the flow characteristics and hydrograph of the Colorado River (Carothers and Aitchison, 

1976; Stevens and others, 1995; Kearsley and Ayers, 

2001). The distribution of the riparian vegetation in the 

Chapter 7

Birds of the

Colorado River

in Grand Canyon: 

a Synthesis of 

Status, Trends, 

and Dam 

Operation

Effects

Jennifer A. Holmes

John R. Spence

Mark K. Sogge
124 The State of the Colorado River Ecosystem in Grand Canyon

Grand Canyon is also strongly influenced by the geography and geology of the region. The river corridor follows 

a 1,772-ft (540-m) elevation gradient through changing 

vegetation communities ranging from Great Basin 

desertscrub (classification per D.E. Brown, 1994) found 

at Lees Ferry (RM 0) with an elevation of 3,117 ft (950 

m) to Sonoran desertscrub at Diamond Creek (RM 226) 

with an elevation of 1,345 ft (410 m). Type of bedrock 

geology present and the presence of major side canyons 

are perhaps the most important geological factors relevant to the distribution of riparian vegetation along the 

river (Turner and Karpiscak, 1980; Stevens and others, 

1995; Spence, 2004). For example, where the bedrock 

consists of Precambrian schist and granite, which are 

hard and slow to erode, the river corridor is narrow and 

tends not to support much riparian vegetation except at 

the mouths of tributaries. 

Before the construction of Glen Canyon Dam, the 

hydrograph of the Colorado River in Grand Canyon 

was driven by spring snowmelt floods and occasional 

large tributary inflows produced by monsoonal latesummer rains (Dolan and others, 1974; Carothers and 

Aitchison, 1976; Topping and others, 2003). Spring 

flooding controlled the abundance and distribution of 

riparian vegetation, producing a distinct trim line at 

about the 125,000 cfs level. Water-surface elevation, or 

stage, is typically given in terms of rate of flow (cubic 

feet per second (cfs)) because elevation varies over the 

length of the river corridor depending on local channel 

morphology. Above this line an extensive community 

of old high-water zone (OHWZ) vegetation occurred, 

including species such as Apache plume (Fallugia paradoxa), net-leaf hackberry (Celtis laevigata var. reticulata),

mesquite (Prosopis glandulosa), and catclaw acacia (Acacia

greggii). Because the lower zone, below 50,000 cfs, was 

flooded and scoured most years (Topping and others, 2003), sparse vegetation was present below the 

trim line and consisted of coyote willow (Salix exigua),

tamarisk (Tamarix ramosissima), rushes (Juncus sp.), and 

grasses (Poaceace family) (Clover and Jotter, 1944). Most 

of the plant species found today in lower vegetation 

zones were present before the construction of the dam 

(Clover and Jotter, 1944; Kearsley and Ayers, 2001). 

Predam conditions resulted in varied riparian vegetation, 

producing dense riparian stands more than 164 ft (50 m) 

wide at some tributary mouths, but elsewhere riparian 

stands were patchily distributed and generally between 

10 and 66 ft (3–20 m) wide (Flowers, 1959; Kearsley and 

Ayers, 2001). 

 The hydrograph of the Colorado River changed 

dramatically with the completion of Glen Canyon Dam 

(Webb and others, 1999; Topping and others, 2003). Of 

the many changes wrought by the dam and discussed 

elsewhere in this report, the most influential one in terms 

of riparian vegetation is the reduction of peak annual 

flows. In the absence of historical floods that removed 

lower zone vegetation, perennial plant species were 

able to move into and colonize these areas. These new 

areas of riparian vegetation are referred to as the new 

high-water zone (NHWZ) to distinguish them from the 

higher predam riparian habitats. The amount of NHWZ 

riparian vegetation greatly increased between 1963 

and 1983 (Pucherelli, 1986), and much of the colonization of the NHWZ was by nonnative species, especially 

tamarisk (Turner and Karpiscak, 1980). Areas of marsh 

also developed in return channel-eddy complexes (Cluer, 

1997), covering approximately 1% of the NHWZ of the 

river corridor by 1991 (Stevens and others, 1995). 

The floods and subsequent high flows of 1983–85 

produced considerable scour and an estimated 13% 

(Waring, 1995) to 39% (Pucherelli, 1986) reduction in 

area of the NHWZ. Following the floods, the NHWZ 

gradually recovered. In 1991, interim flows were established that caused further changes, primarily the stabilization of marshes and riparian colonization of the 

lower portion of the NHWZ between 25,000–33,000 

cfs. The 1996 beach/habitat-building flow through the 

river corridor was designed to scour tamarisk vegetation 

in the lower portions of the NHWZ but had only shortterm burial impacts on the vegetation, which recovered 

rapidly (Kearsley and Ayers, 1999; Stevens and others, 

2001). Flows under the MLFF alternative have not 

altered the areal extent of riparian vegetation patches 

from that established during the period of interim flows 

in the early 1990s (Kearsley and Ayers, 1996, 1999). 

Dam-induced changes to riparian habitat undoubtedly affected the abundance and distribution of riparian 

birds. These changes are not directly measurable because 

Mark K. Sogge, U.S. Geological Survey
Birds of the Colorado River in Grand Canyon 125

the first surveys of breeding birds along the Colorado 

River (Carothers and Sharber, 1976) were not conducted 

until after the dam was completed. Since then, surveys 

and studies have examined many aspects of the ecology 

of Grand Canyon birds, including the direct and indirect effects of the dam on specific bird species and bird 

communities. Riparian breeding bird studies have been 

conducted along the river corridor since the initiation of 

the Glen Canyon Environmental Studies in 1982 (Brown 

and others, 1987; Brown, 1987, 1989; Brown and 

Trosset, 1989; Spence and Pinnock, 1993; Grahame and 

Pinnock, 1995; Hualapai Tribe and SWCA, Inc., 1995; 

Petterson and Spence, 1997; Spence, 1997, 2004; Sogge 

and others, 1998; Kearsley and others, 2004; Yard and 

others, 2004). These studies collected data on the composition of bird communities; patterns of species abundance, richness, and diversity; and habitat distribution. 

They also provided information on habitat associations 

and identified riparian-dependent bird species.

Species-specific bird studies have also been conducted along the river corridor. Focus was given to bird 

species that were or are federally listed as endangered 

or threatened, including bald eagle (Brown and others, 

1989, 1998; Brown and Stevens, 1992, 1997; Brown, 

1993; Leibfried and Montgomery, 1993; Spence and 

others, 2002; van Riper and Sogge, 2004), peregrine 

falcon (Ellis and Monson, 1989; Brown, 1991a; Ward, 

2000), and southwestern willow flycatcher (Brown, 1988; 

Sogge and others, 1997; Johnson, 2000; Yard, 2004a). 

A number of riparian bird species were also the subject of research, including the Bell’s vireo (Vireo bellii)

(Brown and others, 1983), black-chinned hummingbird 

(Archilochus alexandri) (Brown, 1992), and brown-headed 

cowbird (Molothrus ater) (Brown, B.T., 1994). Studies of 

the winter riparian bird community (Sogge and others, 

1998; Spence, 2004; Yard, 2004b) and the aquatic bird 

community (Stevens and others, 1997a; Spence, 2004; 

Yard, 2004b) have also been conducted. These studies provide considerable information on the ecology of 

Grand Canyon bird communities and the direct and 

indirect effects of the dam on specific bird species and 

bird communities.

Status and Trends

Breeding Riparian Birds

The breeding bird community associated with the 

riparian habitat along the Colorado River is made up 

of bird species generally restricted to riparian habitats 

and species that can also be found in adjacent upland, 

nonriparian habitats. More than 30 species have been 

recorded breeding in the riparian patches along the 

river within the study area. Most of these are songbirds 

including warblers, wrens, finches, orioles, and sparrows 

that nest and forage for insects within the NHWZ and 

OHWZ vegetation. Of the 15 most common riparian breeding bird species (table 1), 10 are Neotropical 

migrants that breed in the study area but winter primarily south of the United States-Mexico border. The rest 

of the breeding birds that use the canyon are year-round 

residents or short-distance migrants that primarily winter 

in the region or in nearby southern Arizona. 

Repeated research since the mid-1970s has shown 

that Glen Canyon Dam and its operation have few 

direct flow-related effects upon the riparian breeding 

bird community. The primary change influencing these 

birds has been increased habitat availability caused by 

the establishment of the NHWZ riparian vegetation in 

areas that had relatively sparse vegetation before the 

dam (Brown and others, 1987; Carothers and Brown, 

1991; Sogge and others, 1998; Spence, 2004). Brown 

and Johnson (1985, 1987) also found that flows directly 

affected some birds that occupied this new habitat during 

periods of high daily change in the river level or during enormous seasonal fluctuations that occurred before 

1991 and the establishment of interim flow operating 

criteria. For example, they found that flows as high as 

31,000 cfs, approximate powerplant capacity, flooded 

only a few nests, including some common yellowthroat 

(Geothlypis trichas) nests; however, flows of more than 

40,000 cfs began flooding nests and nest plants of some 

riparian breeding species, specifically the Bell’s vireo 

U.S. Geological Survey
126 The State of the Colorado River Ecosystem in Grand Canyon

and yellow-breasted chat (Icteria virens) (Brown and 

Johnson, 1987). 

The interim operating criteria, which were in effect 

from 1991–96, limited maximum releases to 20,000 cfs 

and set minimum flows at 8,000 cfs during the day and 

5,000 cfs at night. Daily fluctuations were also limited 

to a maximum of 8,000 cfs. This change in river flows 

promoted the establishment of a narrow band of vegetation near the edge of the river (Stevens and Ayers, 

1994; Sogge and others, 1998). Sogge and others (1998) 

examined the direct impact of interim operating criteria on breeding birds in the hydrologically active zone 

(HAZ), the area potentially inundated by flows between 

5,000 and 20,000 cfs. They found that few species 

nested either close to the ground or close to the river; 

only one common yellowthroat nest was placed low 

enough to be inundated at 20,000 cfs. The black phoebe 

(Sayornis nigricans), however, places its nests just 3–6 ft (1–2 

m) above the water, and some phoebe nests would be 

inundated by any flows that raised the water level by as 

little as 3 ft (1 m). For other breeding bird species, Sogge 

and others (1998) found that nests were placed well away 

from the HAZ and avoided inundation under the interim 

operating criteria. Because the MLFF alternative is 

similar to interim operating criteria (Kearsley and Ayers, 

1999), most nests (other than some black phoebe nests) 

are unlikely to be affected by changing water levels. 

Sogge and others (1998) examined the potential longterm effects of dam operations on the Grand Canyon 

ecosystem bird community. They developed models and 

identified habitat features that predicted bird abundance, 

species richness, and diversity. They found that riparian 

location along the river corridor, patch size, and volume 

of larger woody species, especially of tamarisk, within a 

riparian vegetation patch were positively correlated with 

bird abundance, species richness, and diversity. Specifically, these models predicted the following: 

Flow patterns that result in smaller, more isolated 

habitat patches would decrease bird numbers, species richness, and diversity.

Flow patterns that create larger and more contiguous 

habitat patches would increase bird abundance and 

richness within the constraints of local topography 

and geomorphology.

Loss of mesquite vegetation would decrease bird 

abundance. 

Increases in the number of habitat patches would 

increase overall number of birds and bird species. 

Changes from tamarisk shrub/tree to willow shrub/

tree are not likely to greatly affect bird abundance 

and species richness of the Grand Canyon ecosystem 

bird community.

A subsequent study by Spence (2004) modeled 

riparian bird habitat relationships but did not examine 

the relationship between riparian patch size and characteristics such as total bird abundance, species richness, 

and species diversity. Despite the fact that data were 

being derived from different riparian patches, this study 

obtained results similar to Sogge and others (1998), concluding that higher woody-species volume and river location were the best predictors of breeding bird abundance 

and richness. 

These two studies (Sogge and others, 1998; Spence, 

2004) demonstrate that riparian patch size, the volume 

of woody species within a habitat patch, and the loca1.

2.

3.

4.

5.

Table 1. The 15 generally most common terrestrial breeding 

bird species (in alphabetical order) found in riparian habitats 

along the Colorado River in Grand Canyon.

Common Name Scientific Name

Ash-throated flycatcher Myiarchus cinerascens

Bell’s vireo Vireo bellii

Bewick’s wren Thryomanes bewickii

Black-chinned hummingbird Archilochus alexandri

Blue grosbeak Passerina caerulea

Blue-gray gnatcatcher Polioptila caerulea

Bullock’s oriole Icterus bullockii

Common yellowthroat Geothlypis trichas

House finch Carpodacus mexicanus

Lesser goldfinch Carduelis psaltria

Lucy’s warbler Vermivora luciae

Mourning dove Zenaida macroura

Song sparrow Melospiza melodia

Yellow warbler Dendroica petechia

Yellow-breasted chat Icteria virens
Birds of the Colorado River in Grand Canyon 127

tion of the patch along the river corridor are primary 

factors that affect the abundance and species richness of 

birds within a riparian patch. Also, the mix of NHWZ 

and OHWZ vegetation within a patch probably affects 

the distribution and abundance of specific bird species 

because certain species have ecological preferences in 

nesting and foraging in one or the other vegetation type 

(Sogge and others, 1998; Spence, 2004). Collectively, the 

body of research indicates that dam operations with the 

greatest potential to impact breeding bird species within 

the Grand Canyon ecoregion are those that would affect 

the extent and amount of riparian vegetation along the 

river, such as large-magnitude planned or unplanned 

floods. 

Overall, there has been relatively little change in 

the distribution of riparian habitat since the initiation 

of canyon bird studies in the mid-1970s. Likewise, the 

riparian breeding bird community within the study area 

appears not to have changed appreciably in species 

composition during that 25-yr period (Spence, 2004). 

For the most part, the bird species that were most common in the 1980s are the most common today (Kearsley 

and others, 2004; Spence, 2004). Two exceptions are 

the Bell’s vireo and the song sparrow (Melospiza melodia),

which have apparently expanded their breeding ranges 

within the Grand Canyon ecoregion (Brown and others, 

1983; Spence, 2004; Yard and Blake, 2004). 

To track trends in riparian breeding bird populations, one long-term monitoring program was initiated 

in 1996 and continued through 2000 (Spence, 2004) and 

another from 2001 through 2004 (Kearsley and others, 

2004). These studies included baseline monitoring of the 

breeding riparian birds, southwestern willow flycatcher, 

and riparian habitat in selected patches along the river 

corridor. Several species, mostly Neotropical migrants, 

showed consistent detection rates during the 1996–2000 

time period (Spence, 2004). The blue-gray gnatcatcher 

(Polioptila caerulea) showed a steady decline, with detection 

rates dropping about 30%–50%. Two species, Bullock’s 

oriole (Icterus bullockii) in Glen Canyon and yellow warbler 

(Dendroica petechia) throughout the study area, showed statistically significant increases in detection rates (Spence, 

2004).

Most birds found in the study area are not yearround residents; therefore, other factors acting outside 

the Grand Canyon ecosystem influence bird populations, 

and this influence is especially true for migratory breeding birds. Outside factors—changes in winter and migratory habitat, winter weather events, and climate outside 

the region—can affect bird survivorship and are independent of the effects of adaptive management (Spence, 

2004; Holmes and others, 2005).

Overwintering Aquatic Birds 

Increases in abundance and species richness of the 

aquatic bird community—loons, grebes, cormorants, 

herons, ducks, rails, and sandpipers—in the Grand 

Canyon ecoregion correspond with the increased river 

clarity and productivity associated with the presence of 

Glen Canyon Dam (Stevens and others, 1997a; Spence, 

2004). These aquatic bird species use the Grand Canyon 

ecoregion almost exclusively in the winter, nonbreeding 

season. Two primary foraging guilds are represented: (1) 

diving species that consume mostly fish and invertebrates 

within the water column or on the river bed and (2) 

dabbling species that forage in cobble bars and shallower areas where they can reach aquatic vegetation and 

associated invertebrates (table 2). These aquatic birds can 

be directly affected by dam operations that change the 

distribution of prey species in the water column of the 

river or, in the case of dabbling species, cover or expose 

foraging beds. Also, higher discharge rates increase river 

velocity and potentially increase foraging costs for species 

in both guilds (Spence, 2004). 

Aquatic bird species are distributed fairly predictably within the study area. The upper reaches of the 

river, from Glen Canyon Dam to about 25 mi (40 km) 

downstream, tend to be relatively clear and support 

habitat and food, including an abundance of introduced 

Table 2. The 10 generally most common overwintering aquatic 

bird species (in alphabetical order) encountered during surveys 

along the Colorado River below Glen Canyon Dam. 

Common Name Scientific Name

American coot Fulica americana

American wigeon Anas americana

Bufflehead Bucephala albeola

Common goldeneye Bucephala clangula

Common merganser Mergus merganser

Gadwall Anas strepera

Green-winged teal Anas crecca

Lesser scaup Aythya affinis

Mallard Anas platyrhynchos

Ring-necked duck Aythya collaris
128 The State of the Colorado River Ecosystem in Grand Canyon

© 2005 Bob Steele; used with permission

rainbow trout (Oncorhynchus mykiss) for diving species. 

Dabblers can forage only in wider reaches with extensive 

shallow, low-turbidity water. Accordingly, dabblers are 

concentrated in wider reaches above the Little Colorado 

River (Stevens and others, 1997a; Spence, 2004).

Spence (2004) found that species composition and 

abundance of the aquatic bird communities within the 

study area show considerable fluctuations among years. 

Given similar flows, however, the resources available to 

waterfowl in the Grand Canyon ecoregion are relatively 

similar among years. Primary productivity is greatest in 

the clear water below the dam to approximately 25 mi 

(40 km) downstream and then drops rapidly as the river 

becomes more turbid as sediment and organic matter 

enter the river from tributaries. Hence, it is likely that the 

large year-to-year fluctuations in aquatic birds noted by 

Spence (2004) are due to factors outside the region. Outside factors may be numerous and potentially include 

conditions on the breeding grounds, recreation activities, 

changes in habitat availability, climate conditions, and 

hunting. All these factors can interact in complex ways 

in determining the composition and abundance of the 

winter aquatic community in the Grand Canyon ecoregion (Spence, 2004). 

Because of the high variability in abundance for 

many species, the power to detect trends in overwintering aquatic birds is low. Comparing results of surveys 

conducted between 1973 and 1994 by Stevens and others (1997a) to data from surveys between 1998 and 2000, 

Spence (2004) found strong similarities in the aquatic 

bird communities, and the most common birds detected 

during both periods were similar (Spence, 2004). 

Species of Concern

Southwestern Willow 

Flycatcher

The willow flycatcher 

(Empidonax traillii) is a small Neotropical migratory bird that breeds 

across much of North America 

and winters in portions of Central America and northern South 

America. The southwestern 

subspecies (E. t. extimus) breeds only 

in dense riparian habitats in the 

Western United States, including portions of Utah, Nevada, Colorado, New 

Mexico, Arizona, and southern California. 

Breeding generally occurs from late May through early 

August (Sogge, 2000). When the southwestern willow 

flycatcher was federally listed as an endangered species in 

1995, fewer than 400 breeding territories for the subspecies were known throughout the Southwest; however, by 

2001, that number had increased to approximately 1,000 

territories distributed among more than 200 breeding sites (Sogge and others, 2003). By 2003, because 

of increased survey effort and a population increase in 

central Arizona, there were an estimated 410 territories 

in Arizona alone (Smith and others, 2004).

Historically, southwestern willow flycatchers were 

probably found within most major drainages in Arizona 

(Paradzick and Woodward, 2003) but were uncommon 

within the Grand Canyon ecoregion primarily because 

periodic high flows limited dense riparian habitat. The 

first record of a willow flycatcher in the ecoregion is from 

Lees Ferry in 1909, but it is not known whether it was a 

migrant or a breeding bird. The first nest was found in 

1935. The next record was of a probable breeder collected in 1953 (summarized in Sogge and others, 1997). 

Flycatchers have consistently nested along the river corridor in recent years, as new riparian habitat, primarily 

tamarisk, has developed in response to altered river flow 

regimes. This expansion of riparian vegetation may have 

provided additional habitat for the flycatcher. Migrant 

willow flycatchers also occur along the river corridor, 

typically in late May and early June, and most of these 

migrants are probably of the nonendangered northern 

subspecies (E. t. adastus).

There are no direct flow-related impacts to southwestern willow flycatchers because they nest high in tamarisk vegetation, which is well above the level of normal 

fluctuating river flows. Indirect effects may occur as the 

result of flow-related changes to riparian patch size, vegetation density, and invertebrate populations that form 

the flycatcher prey base. The 1996 beach/habitat-building flow did not adversely affect 

southwestern willow flycatchers or their 

breeding habitat structure (Stevens 

and others, 2001). If future flood 

flows enhance riparian habitat and 

patch size, flycatchers may benefit. 

Conversely, if they substantially 

reduce riparian habitat at current 

breeding sites, the flycatcher may 

be impacted.

 Wetland/marsh vegetation 

has been proposed as important 

flycatcher foraging habitat in the study 

area (Stevens and others, 2001). The 
Birds of the Colorado River in Grand Canyon 129

necessity of wetlands to flycatchers is difficult to evaluate 

because the species often breeds at sites in the Southwest where extensive wetlands are absent (Sogge and 

Marshall, 2000).

Other potential impacts to southwestern willow 

flycatchers include human-related disturbance. Southwestern willow flycatchers are not apparently sensitive to 

disturbances such as rafts or boats floating past breeding sites; however, people moving through occupied 

flycatcher habitat can damage habitat, disturb the birds, 

or impact a nest. During the mid-1990s, visitor closures 

were instituted at known flycatcher breeding sites in 

Grand Canyon. To date, there is no evidence of direct, 

human-related impact to flycatchers along the river 

corridor. Potential and indirect human-related impacts 

include the eradication of tamarisk, which the flycatchers use for nesting. Its removal, particularly from known 

breeding sites, would adversely affect flycatchers.

Brown-headed cowbirds are nest parasites and lay 

their eggs in the nests of other birds, which then incubate the cowbird eggs and raise the young cowbirds as 

if they were their own young. B.T. Brown (1994) and 

Sogge and others (1997) reported that flycatchers in the 

Grand Canyon ecoregion experienced high rates (>25% 

of nests) of brown-headed cowbird parasitism, which 

reduced flycatcher nest success and productivity. There is 

no evidence, however, that dam operation or river flows 

affect cowbird populations or nest parasitism rates.

Because southwestern willow flycatchers migrate 

southward each winter, they are affected by many factors 

during the migration and wintering periods. Furthermore, flycatchers will regularly disperse long distances 

and move to different sites between years; however, the 

reasons behind these movements are not well known 

(U.S. Fish and Wildlife Service, 2002). These realities complicate interpretation of population trends in 

the study area and the evaluation of potential impacts of the operations of Glen 

Canyon Dam. 

Over the last 30 yr, the population of breeding southwestern 

willow flycatchers in upper Grand 

Canyon has been very small 

and limited to riparian patches 

between approximately RM 28 

and 71. From 1982 to 1991, 2 to 

11 male flycatchers were detected 

annually, with a maximum of 4 

nests in any 1 yr (Brown, 1988, 

1991b). Between 1992 and 2003, only 

1 to 5 territories were found in any year 

(Sogge and others, 1997; Johnson, 2000; 

Paradzick and Woodward, 2003; Yard, 2004a). Flycatchers bred only in the relatively larger patches, and breeding patch size ranged from 1.5 to 2.2 acres (0.6–0.9 ha). 

Breeding patches were dominated by tamarisk, and all 

nests had been placed in tamarisk (Sogge and others, 

1997; Yard, 2004a). Overall, the southwestern willow flycatcher population in the upper river corridor continues 

to persist at a very low level, at only one or two sites. 

In 1995, breeding flycatchers using one territory 

were first noted in newly developed native riparian 

habitat in the Lake Mead delta area, immediately downstream of the Grand Canyon-Lake Mead boundary. The 

following year this population reached 10 territories, 

but the delta was flooded during the next 2 yr by rising 

reservoir levels, and flycatchers were no longer present 

by 1998. Beginning in 1998, breeding southwestern willow flycatchers were discovered at a variety of upstream 

sites within lower Grand Canyon between RM 246 and 

273 (Paradzick and Woodward, 2003). It is possible that 

birds found before 1998 breeding downstream, in what 

is now inundated delta habitat, moved upstream to the 

lower Grand Canyon reach. Between 1998 and 2001, 

7–12 flycatcher territories were recorded in lower Grand 

Canyon; however, recent surveys in 2002 and 2003 found 

no breeding flycatchers in lower Grand Canyon (Smith 

and others, 2003, 2004) and only 2 territories in 2004 

(McLeod and others, 2005; Munzer and others, 2005).

Nesting success in the upper Grand Canyon flycatcher population is generally low, and the population 

is probably not self-sustaining (Sogge and others, 1997). 

Breeding success in lower Grand Canyon is not well 

documented, but the lack of detections in 2002 and 

2003 suggests that productivity from 1998 to 2001 was 

probably too low to provide for continued population 

persistence. Recent habitat changes along the lower 

river corridor caused by the changing reservoir levels in 

Lake Mead, however, have probably affected 

flycatcher site occupancy.

California Condor

The California condor is one 

of the rarest birds in the world 

and was federally listed as endangered in the United States in 

1967. In Arizona, reintroduction 

was conducted beginning in 1996 

under a special provision of the 

Endangered Species Act of 1973.

Condors are opportunistic scavengers that feed primarily on large, dead 

©

mammals such as deer, elk, bighorn sheep 2005 Jeff Coleman; used with permission
130 The State of the Colorado River Ecosystem in Grand Canyon

(Ovis canadensis), range cattle, sheep, and horses. Condors 

can soar and glide up to 50 mi/h (80 km/h) and travel 

100 mi (161 km) or more per day in search of food. They 

are long lived, living up to 60 yr, with low reproductive 

rates. Most nest sites have been found in caves, on rock 

ledges, or in tree cavities (Snyder and Schmitt, 2002; 

Arizona Game and Fish Department, 2004). 

In prehistoric times, condors ranged from Canada to 

Mexico, across the Southern United States to Florida, 

and to the east coast in New York. Based on 

evidence from bones, feathers, and eggshells found in caves, condors were a 

resident of Grand Canyon. A dramatic 

range reduction occurred about 

10,000 yr ago, coinciding with the 

late Pleistocene extinction of large 

mammals that condors depended 

on for food (Arizona Game and 

Fish Department, 2004). Settlement 

of the Western United States, shooting, poisoning from lead and DDT, 

egg collecting, and general habitat 

degradation resulted in further dramatic 

population reductions (Snyder and Schmitt, 

2002). Between the mid-1880s and 1920s, 

there were scattered reports of condors in Arizona, 

with the last sighting near Williams, Ariz., in 1924. By 

the late 1930s, all remaining condors were found only in 

California, and by 1982 the total population had dwindled to just 22 birds (Snyder and Schmitt, 2002; Arizona 

Game and Fish Department, 2004). 

The U.S. Fish and Wildlife Service, The Peregrine 

Fund, Arizona Game and Fish Department, National 

Park Service, and other collaborators established 

a condor captive-breeding and release program in 

Arizona. Vermilion Cliffs National Monument is the 

main reintroduction site, and birds released at this site 

frequent Grand Canyon. Since December 1996, the 

Arizona restoration project has released approximately 

6–8 birds per year. There are now over 30 condors flying 

free in Arizona, and natural reproduction is occurring in 

the Grand Canyon region: in late 2004, 2 wild-hatched 

chicks stretched their wings and successfully fledged. As 

part of the continuing reintroduction project, individual 

condors will continue to be monitored daily (Arizona 

Game and Fish Department, 2004).

Many of the reintroduced condors have been 

observed within the Grand Canyon ecoregion. Although 

typically seen soaring overhead, condors regularly 

bathe and sun themselves along the banks of the 

Colorado River (Andi Rogers, Arizona Game and Fish 

Department, oral commun., 2005). It is likely that the 

population of condors in the region will continue to 

increase because of continued reintroduction efforts and 

natural increase. Since condors make little use of riparian habitat and are not typically found along the edge of 

the river, there are no likely effects of MLFF operations. 

The only dam management actions likely to affect this 

species would be those that resulted in available carrion 

such as dead fish along the river corridor, which could 

attract concentrations of feeding condors. 

Bald Eagle 

Bald eagles are common breeders in Alaska and parts of Canada 

but are far less numerous in the 

contiguous United States, where 

they were once critically endangered. Because of extensive 

and successful recovery efforts 

since the 1960s, many bald eagle 

populations have increased, and 

in 1995 the U.S. Fish and Wildlife 

Service downlisted the species from 

endangered to threatened in the lower 

48 States. 

Although still somewhat rare as a breeder 

in Arizona, hundreds of bald eagles migrate into the 

State each winter; eagle numbers in Arizona increased 

from 225 in 1992 to 440 in 2001 (Beatty, 2001). Wintering eagles typically concentrate along rivers, lakes, and 

reservoirs where preferred prey, including fish, waterfowl, 

and carrion, is readily available (Grubb and Kennedy, 

1982; Brown, 1993). The Grand Canyon ecoregion is 

one such concentration area, and eagles are generally 

present from November through March, which coincides 

with trout spawning and an abundance of waterfowl 

within the corridor. Within the study area, bald eagles 

are found primarily from Lees Ferry downstream to the 

confluence of the Little Colorado River. From 1991 to 

1995, the maximum daily number of eagles detected 

during helicopter surveys of this reach ranged from 

11 to 24 individuals (van Riper and Sogge, 2004). The 

Colorado River corridor in Grand Canyon hosted only 

5%–10% of the wintering eagles present in Arizona on 

any given day during this time, but the total number of 

eagles using the corridor over an entire season may be 

substantially more (van Riper and Sogge, 2004). Systematic corridor-wide surveys were not conducted before or 

after the 1991–95 period.

Bald eagles often congregate at Nankoweap Creek 

(RM 52), a small tributary to the Colorado River in 

which rainbow trout sometimes spawn in large numbers. 

George Andrejko, Arizona Game and Fish Department
Birds of the Colorado River in Grand Canyon 131

Eagles have concentrated here since the early 1980s 

(Brown and others, 1989; Leibfried and Montgomery, 

1993). From 1986 to 1995, maximum daily eagle counts 

ranged from 4 to 26, with the number of eagles varying directly with the abundance and availability of 

trout in the creek (Brown and Stevens, 1992; Leibfried 

and Montgomery, 1993; van Riper and Sogge, 2004). 

Neither the size of trout spawn nor eagle abundance at 

Nankoweap Creek was related to dam release levels 

(van Riper and Sogge, 2004). There is also no 

evidence that eagle abundance throughout 

the river corridor is affected by river 

flow, although it is likely that river turbidity affects the ability of eagles to 

forage for fish along the mainstem. 

Human disturbance can affect 

bald eagles. Brown and Stevens 

(1997) and van Riper and Sogge 

(2004) documented disturbance of 

wintering bald eagles by humans, 

including flushing of eagles by 

hikers, rafters, anglers, and research 

activity. Hikers in the Nankoweap 

Creek delta area caused the greatest 

disturbance to the eagles there, but such 

disturbances were reduced in years when a visitor-use 

closure was instituted.

Evaluating the effects of the operations of Glen 

Canyon Dam and other management activities on bald 

eagles is complicated by the fact that eagle abundance 

in Grand Canyon is influenced by both local conditions 

and regional factors. Furthermore, bald eagles will travel 

long distances in search of abundant, easily available 

prey (Stalmaster, 1987) and can move readily between 

food concentrations at Grand Canyon, Lake Powell, and 

other regional lakes and rivers. Nevertheless, habitat use 

by foraging eagles is strongly influenced by fluctuating 

river flows; high flows reduce eagle foraging habitat diversity, lower foraging success in river habitat, and restrict 

foraging opportunities (Brown and others, 1998).

Management changes that alter prey availability 

could alter eagle abundance and distribution within the 

Grand Canyon ecosystem. For example, if the selective 

withdrawal of warmer water from Lake Powell increases 

the numbers of carp, catfish, and suckers (Hunt and 

others, 1992), more food resources may be available to 

eagles. Eagles may have more difficulty foraging along the 

river, however, if trout numbers decrease and/or spawning is reduced, either through water temperature/turbidity changes or through nonnative fish removal efforts.

Peregrine Falcon

Dramatic declines in peregrine falcon populations 

led to the addition of the peregrine to the Federal List of 

Endangered and Threatened Wildlife in 1970, where it 

was listed as endangered. Following successful recovery 

efforts, the peregrine falcon was delisted in 1999. The 

Endangered Species Act requires a minimum 5 yr of 

post-delisting monitoring in cooperation with State 

agencies to confirm recovery. 

The peregrine commonly breeds 

in cliffs and uses open landscapes for 

foraging. Nest sites are usually associated with water (White and others, 

2002). In winter, some breeders 

stay in their nesting areas, and 

others may migrate.

Diet of the peregrine consists 

mostly of birds, from songbirds to 

small geese. They also occasionally 

eat mammals, especially bats (White 

and others, 2002). During the breeding season, peregrine falcons in the 

Grand Canyon ecoregion feed on whitethroated swifts (Aeronautes saxatalis), swallows, 

and bats (Brown, 1991a). In winter, they feed mainly on 

waterfowl. Many of their prey items feed on invertebrate 

species, especially flies (Diptera), that emerge out of the 

Colorado River (Stevens and others, 1997b).

Given these life-history traits, any impacts to peregrine falcons from dam operations are likely to be indirect, possibly through influences on the distribution and 

abundance of aquatic macroinvertebrate populations, 

which in turn would influence the availability of the 

peregrine’s prey items such as swifts, bats, and ducks. 

The Grand Canyon peregrine population was 

thought to be low in the mid-1970s and apparently 

increased dramatically in the 1980s (U.S. Fish and 

Wildlife Service, 1984; Ellis and Monson, 1989; Ward, 

2000). In 1981 and 1982, two nests or “eyries” were 

found during surveys between the Tanner and Bright 

Angel Trails. In 1998 and 1999, 12 eyries were found in 

these same areas (R.V. Ward, Grand Canyon National 

Park, oral commun., 2005). During the same period, the 

National Park Service conducted surveys throughout 

appropriate habitat within Grand Canyon National Park, 

including along the river corridor, and concluded that 

the peregrine population in Grand Canyon appeared 

stable since 1988 (Ward, 2000). 

George Andrejko, Arizona Game and Fish Department
132 The State of the Colorado River Ecosystem in Grand Canyon

Recent Findings

Kearsley and others (2004) examined the interrelationships between vegetation and animal life, including 

birds, as part of a monitoring project for terrestrial riparian resources that took place from May 2001 to May 

2003. Preliminary findings regarding terrestrial breeding birds showed patterns similar to those of previous 

studies. More breeding pairs and higher species diversity 

were detected at larger sites (Yard and Blake, 2004). 

Vegetation density was found to be an important component of habitat quality for riparian breeding birds in 

the Grand Canyon ecoregion, with the densities of most 

bird species positively correlated with the abundance of 

mesquite and acacia (Kearsley and Lightfoot, 2004). No 

difference was found in the abundance of birds over the 

3 yr of the study although sample sizes were too low to 

analyze trends (Yard and Blake, 2004). The most commonly detected breeding species were the same as those 

in previous studies (Brown and others, 1987; Sogge and 

others, 1998; Spence, 2004). 

Discussion and Future 

Research Needs

The construction of Glen Canyon Dam and the 

subsequent changes in the hydrograph of the Colorado 

River resulted in dramatic changes in the amount of 

available habitat for both the riparian breeding and 

the overwintering aquatic bird communities within the 

Colorado River ecosystem. Perennial plant species, especially tamarisk, colonized areas previously scoured by 

floods, creating new riparian patches in the high-water 

zone. These areas provide habitat for over 30 species of 

breeding birds, including many Neotropical migrants 

and the endangered southwestern willow flycatcher. 

Increased river clarity and productivity below the dam 

provide suitable habitat for many aquatic bird species 

such as ducks, loons, grebes, and cormorants.

Patterns of abundance and distribution of riparian breeding birds and overwintering riparian and 

aquatic birds within the study area are now well known. 

Less well known are the long-term effects of adaptive 

management and the management activities needed to 

ensure the continued conservation of riparian resources, 

their associated avian communities, and bird species of 

conservation concern. Continued monitoring would be 

required to address these information needs. 

Monitoring riparian breeding birds to detect population changes requires considerable commitments of both 

time and effort to obtain sufficient data for biological 

and statistical significance. Data from the 1996 to 2000 

breeding bird monitoring program were used by Spence 

(2004) to determine the adequacy of the monitoring 

program to detect changes in bird populations. He found 

that trends could not be detected for 24 of 32 (75%) 

riparian breeding species and that 5 to 30 yr of sampling 

were required to detect a 10% change in species abundance. Half of the 16 most common species included in 

the analysis would require over 10 yr of monitoring to 

detect a 10% population change, while 5 rarely detected 

species cannot be monitored by using the sampling protocols tested in the analyses (Spence, 2004).

An alternative approach to continued monitoring 

of riparian birds would be to use aerial photography, 

remote sensing, and geographic information systems 

(GIS) in order to measure habitat variables within the 

study areas that have been shown to predict bird numbers, richness, and diversity (Sogge and others, 1998). 

Key variables for monitoring would include the size and 

distribution of riparian patches, area of NHWZ and 

OHWZ woody species, and measures of total vegetaMark K. Sogge, U.S. Geological Survey
Birds of the Colorado River in Grand Canyon 133

tion volume; however, simply monitoring habitat quality 

and extent may miss potential changes in selected bird 

species caused by factors within the study area other than 

riparian vegetation dynamics and may miss potentially 

profound changes in some bird species (Spence, 2004).

Existing data, collected in previous studies, could be 

used to better model and predict how future changes in 

the riparian vegetation will affect changes in terrestrial 

avifauna populations within the Grand Canyon ecoregion. In particular, it would be valuable to extend the 

current models of bird community and patch-level habitat variables (Sogge and others, 1998; Spence, 2004) to 

the level of individual bird species. This extension would 

allow the development of more useful conceptual models 

and more detailed predictions regarding avian resources 

in the Grand Canyon ecoregion. For example, models 

developed by Sogge and others (1998) predicted that 

changes from tamarisk shrub/tree to willow shrub/tree 

are not likely to greatly affect overall bird abundance and 

species richness within riparian patches in Grand Canyon, yet individual species have specific behavior, physiology, and ecology, and some may decline in response to 

such habitat changes. Extending models to individual 

species would allow identification of species that may be 

sensitive to future changes in the riparian vegetation.

Riparian woodlands, such as those within the Grand 

Canyon ecoregion, provide vital habitat for bird species 

of conservation concern and support the highest diversity of landbird species of all habitats in the Southwest 

(Rich and others, 2004). Dam operations affect birds 

within the ecoregion primarily through effects on breeding habitat. Under the MLFF alternative, these impacts 

are likely to be fairly minor compared with climate and 

habitat changes outside the Colorado River corridor. 

Thus, the Grand Canyon ecoregion is likely to continue 

to be an important resource for riparian birds. A welldesigned monitoring program that takes into account 

sampling design and statistical power can be used to 

establish baseline values regarding the distribution and 

abundance of specific species from which future comparisons can be made over time. If monitoring data are 

linked to information regarding ecological resources and 

habitat requirements for specific species and the monitoring is conducted in conjunction with more regional, 

large-scale monitoring, insight into the causes of population changes and the effects of management actions may 

result (Holmes and others, 2005). 

Dam operations have been shown to be directly 

linked to overwintering aquatic birds through effects on 

primary and secondary productivity; thus, they may be a 

useful resource to monitor. Overwintering aquatic birds 

can be monitored relatively easily because more than 

50% of the aquatic birds occur at or above Lees Ferry 

in a typical winter. Further study is necessary, however, 

to determine how to structure any future aquatic bird 

monitoring program (Spence, 2004).

Continued monitoring of species of special concern 

would require continuation or development of monitoring protocols specific to each species. In particular, the 

southwestern willow flycatcher population in Grand 

Canyon is extremely small relative to the current rangewide population, which encompasses approximately 

1,400 territories (U.S. Geological Survey, unpub. data, 

2004). Because flycatcher monitoring must follow a 

standard, multivisit protocol, conducting such surveys 

within the study area requires substantial resources. 

Overlaying this protocol is the challenge of relating river 

flows to any direct or indirect impacts to the flycatcher 

and its habitats. The potential impacts of tamarisk 

removal associated with riparian restoration projects 

should also be considered. Therefore, the nature and 

extent of future flycatcher monitoring, and the ability to 

interpret its results, may be worthy of discussion within 

the Glen Canyon Dam Adaptive Management Program. 

Bald eagles have not been systematically or intensively 

monitored along the Colorado River since 1995; nevertheless, eagles are still noted during some winter research 

raft trips (Yard, 2004b). van Riper and Sogge (2004) 

evaluated various monitoring techniques and noted that 

helicopter-based surveys would be the most effective 

method in terms of coverage and ability to detect the 

eagles. On the other hand, aerial surveys would have to 

be considered in light of potential recreation issues and 

current and future Grand Canyon National Park policies. 

Logistical difficulties associated with access make surveying for peregrine falcons in Grand Canyon National 

Park extremely difficult, and a thorough sample using 

unbiased or random methodologies has been impossible U.S. Geological Survey
134 The State of the Colorado River Ecosystem in Grand Canyon

(Ward, 2000). Despite these difficulties, the National 

Park Service will monitor at least five territories within 

the park (R.V. Ward, Grand Canyon National Park, oral 

commun., 2005), and Glen Canyon National Recreation 

Area will monitor one or more territories above Lees 

Ferry starting in 2005.

Many factors and processes apart from dam operations affect the structure and functioning of the Grand 

Canyon ecosystem, such as changes in regional climatic 

and atmospheric conditions, natural disturbances, adjacent land uses, the spread of invasive species, and fire 

suppression. These natural and human-caused events, 

along with adaptive management actions, have affected 

and will continue to affect the abundance, distribution, 

and composition of the Grand Canyon bird communities and their habitats. 

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Contact Information:

Jennifer A. Holmes

Wildlife Biologist, Senior 

Northern Arizona University

Colorado Plateau Research Station

Flagstaff, AZ

Jennifer.Holmes@nau.edu

John R. Spence

Ecologist

U.S. Department of the Interior

National Park Service

Glen Canyon National Recreation Area

Page, AZ

John_Spence@nps.gov

Mark K. Sogge

Ecologist

U.S. Department of the Interior

U.S. Geological Survey

Southwest Biological Science Center

Flagstaff, AZ

mark_sogge@usgs.gov

First page photograph credit: George Andrejko, 

Arizona Game and Fish Department
Chapter 8

Debris Flows 

in Grand 

Canyon and 

the Rapids

of the 

Colorado River

Robert H. Webb

Peter G. Griffiths

Christopher S. Magirl

Thomas C. Hanks

Introduction

Coarse sediment—gravel, cobbles, and boulders—is 

transported to the Colorado River almost exclusively 

by debris flows, which are irregularly occurring types of 

flash flood events. By supplying boulders that exceed the 

capacity of the river to move them at most discharges, 

debris flows create and maintain the hundreds of debris 

fans and associated rapids that control the lengthwise 

or longitudinal profile of the Colorado River in Grand 

Canyon. Debris flows occur in 740 tributaries of the 

Colorado River in Grand Canyon between Lees Ferry (at 

RM 0) and the Grand Wash Cliffs (at RM 277), the physical feature that marks the western boundary of Grand 

Canyon National Park and the end of Grand Canyon.

Coarse sediment is of interest within the Glen 

Canyon Dam Adaptive Management Program because 

of its relation to key components of the Colorado River 

ecosystem. The deposition of coarse-grained sediment at 

tributary junctures builds large debris fans that constrict 

the river and form rapids. Debris fans and debris bars, 

which develop below rapids, create the fan-eddy complex 

that is the cornerstone of the physical framework of the 

river in Grand Canyon (fig. 1). In addition, the pools 

upstream and downstream of debris fans slow sediment movement or trap it for temporary storage. The 

pool-drop system created by debris fans is prime habitat 

for the endangered humpback chub (Gila cypha), while 

coarse sediment injected into the river during debris 

flows is used by other aquatic organisms, notably the alga 

Cladophora glomerata. The navigation of the river by whitewater boaters also can be affected by debris-flow events. 

Monitoring the input of coarse sediment into the 

Colorado River ecosystem and its long-term redistribution by the river is critical to understanding how dam 

operations affect coarse sediment deposition and, indirectly, other ecosystem components. Scientists are able to 

model debris-flow magnitude and frequency from extensive data sets developed through long-term monitoring. 

Also, this chapter estimates the amount of sediment 

contributed by debris flows and models its deposition 

at tributary junctures to evaluate the effects of debris 

flows over several temporal and spatial scales, including the recent period of operations of Glen Canyon 

Dam. Data are combined with modeling to evaluate 

long-term changes in rapids and to explain large-scale 

features. The chapter also summarizes data from debrisfan monitoring activities by the U.S. Geological Survey’s 
140 The State of the Colorado River Ecosystem in Grand Canyon

(USGS) Grand Canyon Monitoring and Research 

Center (GCMRC) and research by Water Resources and 

Geology Discipline scientists. Finally, the chapter considers the role of experimental high flows and the modified low fluctuating flow (MLFF) alternative on coarsesediment reworking.

Background

Distributed along 277 mi (446 km) of river between 

the Paria River and Grand Wash Cliffs, the 740 tributaries that produce debris flows drain 4,600 mi2

 (12,000 

km2) of steep terrain between the north and south rims 

of Grand Canyon (Webb and others, 2000). Debris 

flows, which are typically more than 80% sediment by 

weight, are slurries of clay to boulder-sized sediment 

mobilized during periods of intense or sustained precipitation. The exposed bedrock landscape of Grand 

Canyon National Park provides an ideal setting for the 

initiation of debris flows: high relief combines with 

differential rock strength to create a high potential for 

slope failure (Griffiths and others, 2004). Most slope 

failures that become debris flows (75%) occur in the 

Hermit Formation and Esplanade Sandstone of the 

Supai Group and in the Muav Limestone and Bright 

Angel Shale of the Tonto Group (fig. 2a). Other prominent sources include the Dox Sandstone, Cardenas Lava, 

Vishnu Schist, and Quaternary Basalts in western Grand 

Canyon. Tributaries are documented to have produced 

debris flows throughout the Holocene (Melis and others, 

1994; Hereford and others, 1998). 

In Grand Canyon, debris flows are initiated by a 

combination of intense precipitation and subsequent 

Figure 1. Diagram showing a fan-eddy complex in Grand Canyon. Debris flows from tributary canyons carry coarse sediment that 

is deposited at the juncture with the Colorado River, forming deposits called debris fans. Debris fans constrict the Colorado River

and raise its bed elevation, creating rapids. Especially during floods, the river entrains the sediment on the debris fan and transports

it downstream through the pool, where the larger particles become lodged on debris bars that form secondary rapids. Between the

constrictions of the primary and secondary rapids, pools and eddies form, creating a depositional setting for sandbars. 
Debris Flows in Grand Canyon and the Rapids of the Colorado River 141

slope failure (Cooley and others, 1977; Webb and 

others, 1988, 1989, 1999b, 2000, 2004; Griffiths and 

others, 2004). The most common type of slope failure 

is termed the “firehose effect” (Melis and others, 1994) 

(fig. 2b), where streamflow falling over cliffs, typically 

in the Redwall Limestone, strikes bedrock and accumulated colluvium and causes slope failure and mixes 

these materials and water to form a slurry. Debris flows 

that reach the Colorado River deposit their material 

on debris fans. These enlarged or aggraded debris fans 

constrict the river and raise the riverbed elevation until

mainstem flows rework coarse-grained deposits (Webb 

and others, 1989). “Reworking” is a term describing river 

entrainment and transport of particles from debris fans, 

including the winnowing of fine-grained particles (clay 

to cobble size) and the movement of boulders, either 

on the fan surface or into the river. The large boulders 

that remain after this reworking form the core of rapids that modify the longitudinal profile of the river and 

locally control the physical framework of the presentday Colorado River in Grand Canyon (Webb, 1996). 

Debris-fan reworking was extensive before construction of Glen Canyon Dam. Reduced peak flow on the 

regulated river represents a fourfold decrease in its sediment-transport potential compared to predam conditions 

(Howard and Dolan, 1981). As a result, ability of the 

river to erode newly deposited sediment from debris fans 

has been reduced. Reworking still occurs on a limited 

basis, typically during maximum powerplant releases or 

intentional flood releases from Glen Canyon Dam (Webb 

and others, 1999a). Today, because the reworking by the 

Colorado River is limited, debris flows from unregulated 

tributaries are now an effective agent of change in the 

river corridor (Howard and Dolan, 1981), affecting the 

water-surface profile, hydraulics through rapids, and the 

associated pools and eddies downstream.

Status and Trends

Debris-flow Frequency

Debris flows in Grand Canyon were relatively well 

documented in the 20th century (Webb, 1996; Webb 

and others, 2000; Griffiths and others, 2004). We use 

the term Grand Canyon loosely to collectively refer to 

the river corridor from Lees Ferry to the Grand Wash 

Cliffs, merging Marble and Grand Canyons and their 

respective subreaches. Direct observations provided a 

complete record of debris flows from 1984 to 2004 (fig. 

3), which was augmented with repeat photography, such 

as that shown in figure 4, that provides a separate record 

(1890 through 1983) of debris flows from 147 tributaries. In this analysis, we only documented debris flows 

that reached debris fans and/or the Colorado River; we 

did not include debris flows that occurred upstream in 

tributaries but did not reach the river corridor.

Direct Observations (1984–2004)

Debris flows, rockfalls, and significant streamflow 

floods were directly observed or compiled from the 

accounts of river runners along the river in Grand 

Canyon from 1984 through 2004. These data provide 

a complete record of debris flows that reached the 

Colorado River from all Grand Canyon tributaries for 

more than 21 yr (fig. 3). During this period, a total of 

104 events occurred in 88 tributaries for an average of 

4.95 debris flows per year. A total of 14 debris flows 

occurred in 2001 and again in 2002, the most prolific 

period in the record. Webb and others (2000) analyzed 

Figure 2. A. Relative frequency of the location at which slope 

failures in bedrock or colluvium resulting in debris flows that 

reach the river (n = 101, 1939 through 2003) have occurred in 

Grand Canyon. B. Relative frequency of initiation mechanisms for 

selected debris flows from 1939 through 2003 in Grand Canyon (n = 

68) (from Griffiths and others, 2004).
142 The State of the Colorado River Ecosystem in Grand Canyon

precipitation records around Grand Canyon and found 

that the annual number of debris flows is not related to 

total summer precipitation (as illustrated by the precipitation record shown in fig. 3 in conjunction with annual 

debris flows). This suggests that antecedent moisture 

has little effect on debris-flow occurrence (Griffiths and 

others, 2004).

Most debris flows occurred in Marble Canyon or 

eastern Grand Canyon, with notable exceptions at Lava 

Falls Rapid (RM 179) in 1995 (Webb and others, 1999b) 

and between RM 189 and RM 209 from 1999 through 

2001. Several tributaries delivered more than one debris 

flow to the river between 1984 and 2004. For example, 

Seventyfive Mile Creek had four debris flows, and 

Monument Creek (RM 93.9) had three. Multiple debris 

flows within a drainage basin suggest that slope and channel destabilization caused by the initial event may lead 

to repeated events until either the loosened sediment is 

removed or sufficient time elapses between severe storms 

to allow healing of hillslopes and channel margins. 

Repeat Photography and 

Debris Flows (1890–1983)

Repeat photography (fig. 4) has been used in numerous studies in Grand Canyon to document long-term 

changes in both terrestrial ecology and geomorphology 

(Turner and Karpiscak, 1980; Stephens and Shoemaker, 

1987; Webb and others, 1989, 1999a; Melis and others, 1994; Webb, 1996; Griffiths and others, 2004). This 

type of scientific photography is particularly useful for 

Figure 3. Observational record of debris flows in Grand Canyon, 

from 1984 to 2004, compared with total summer rainfall (July 

through September) at Grand Canyon National Park airport. No 

data are available for summer precipitation in 1997.

A.

B.

Figure 4. Repeat photographs of Crystal Rapid. A. (February 

9, 1890) This downstream view from the right scout point at 

Crystal Rapid (RM 98) shows a wide, gentle rapid during the 

second expedition through Grand Canyon. This expedition, 

led by Robert Brewster Stanton, occurred in winter 1890. The 

deepest water in the rapid is on river right, and emergent rocks 

are on the left side (R.B. Stanton, courtesy of the National 

Archives and Records Administration, College Park, Maryland). 

B. (February 1, 1990) A debris flow in 1966 constricted the river 

by more than 80%, creating what was considered the most 

formidable rapid in Grand Canyon. Floods between 1966 and 

1986 widened out the constriction, reducing the navigational 

hazard this rapid posed. Although Crystal Rapid has lost some 

of its ferocious reputation, it remains one of the largest in Grand 

Canyon (T. Brownold, stake 1471).
Debris Flows in Grand Canyon and the Rapids of the Colorado River 143

evaluating the types of landscape changes associated 

with debris flows. Most of the information for historical 

debris flows was obtained through comparison of repeat 

photography and historical photographs taken between 

1871 and 1964. Between 1989 and 2002, 1,365 historical 

photographs of the river corridor were matched to determine significant changes to tributary channels, debris 

fans, and rapids throughout the canyon. The year with 

the most abundant and widespread coverage is 1890, 

when the well-documented Stanton expedition occurred 

(Webb, 1996). Several sets of low-altitude aerial photographs taken between 1935 and 1984 were also analyzed 

for evidence of debris flows at the river. 

To determine the frequency of debris flows at the 

river from 1890 through 1983, the 1890 photographs 

and their matches were interpreted for evidence of 

debris-flow occurrences at 147 debris fans. This process 

revealed that debris flows occurred at 84 of 147 tributaries (Griffiths and others, 2004), indicating that 57% of 

the tributaries generated one or more events from 1890 

through 1983. Because any of these 84 tributaries could 

have delivered more than one debris flow, additional 

data, such as written accounts, were used to identify a 

total of 93 debris flows from the 84 tributaries over a 

period of a century. From 1890 through 1983, 6% of 

tributaries produced two or more debris flows, including five at Lava Falls Rapid (RM 179) (Webb and others, 1999b). 

Analysis of aerial photograpy identified an additional 23 debris flows for a total of 107 debris flows that 

occurred between 1890 and 1983 at the mouths of 167 

tributaries from Glen Canyon Dam to Separation Rapid 

(RM 240), the head of Lake Mead. Using this data set 

as an unbiased sample of the entire population of 740 

tributaries, the rate of debris-flow occurrence at the 

river is estimated at 5.0/yr for all tributaries from 1890 

through 1983. This rate is essentially identical to the 

4.95/yr frequency observed between 1984 and 2004. 

If the results for both records are combined, 211 debris 

flows are known to have occurred along the Colorado 

River in Grand Canyon between 1984 and 2003.

Net Observed Effects of Debris Flows

Of the documented 211 historical debris flows in 

172 tributaries, 55 significantly affected the Colorado 

River by creating rapids or increasing constrictions 

during the past century (Webb, 1996; Webb and others, 

2000; Griffiths and others, 2004). From 1984 through 

2004, 8 rapids were created, and 15 were constricted 

by debris flows. The observational evidence indicates 

that the occurrence of debris flows is not spatially 

random in Grand Canyon. Debris flow activity is particularly concentrated in Marble Canyon and other 

reaches where the river trends towards the southwest or 

south-southwest. The findings indicate that about 10% 

of tributaries had two or more debris flows in the last 

century, with a maximum of six debris flows at Lava Falls 

Rapid (Webb and others, 1999b) and five debris flows at 

Seventyfive Mile Creek during the 20th century.

Modeling Debris-flow Frequency

Griffiths and others (1996) developed a model of 

debris-flow frequency (1890–1990) in Grand Canyon 

between Lees Ferry and Diamond Creek (RM 0 to RM 

226), and Webb and others (2000) extended that model 

to the Grand Wash Cliffs (RM 277). The model identified several parameters that are significantly related 

to the occurrence of debris flows that reach the river, 

including the presence and location of shale in the basin, 

drainage-basin area, mean drainage-basin gradient, and 

the aspect of the river corridor. Drainage-basin variables 

that are the most significant in influencing the occurrence of debris flows are suggested in a map showing the 

distribution of debris-flow probabilities (fig. 5). One tendency is for debris-flow frequency to decrease when the 

river corridor trends away from a southwesterly course; 

Griffiths and others (2004) attributed this to the regional 

trajectory of summer storms, which tend to move from 

the southwest. The effect of drainage-basin area is 

evident in Marble Canyon, where the largest tributaries have a higher probability of debris-flow occurrence. 

The height of the Hermit Formation and the gradient from this unit to the Colorado River appear to be 

especially important in Marble Canyon and reflect the 

dominant contribution of shale units to debris flows in 

Grand Canyon. 

In eastern Grand Canyon, a greater variety of 

source materials, combined with structural variability, 

resulted in a mosaic of probabilities (fig. 5). The presence 

of and gradient below shales strongly affect debris-flow 

probability. River aspect and drainage-basin area are significant but less influential in this reach. Certain sections 

of the river corridor that trend northwesterly generally 

have tributaries with low probabilities. In western Grand 

Canyon, the presence of three source formations and 

the overall gradient of each tributary from headwaters 

to river strongly influenced debris-flow probability. The 

height of the Hermit Formation is less influential in 

western than eastern Grand Canyon or Marble Canyon 
144 The State of the Colorado River Ecosystem in Grand Canyon

because that formation is farther from the river. Debrisflow probability is lowest downstream from Diamond 

Creek, where the river trends northwesterly, except in 

the reach immediately upstream from the Grand Wash 

Cliffs, where debris-flow probabilities are high because 

of the proximity of shales to the river corridor. 

Our observations and statistical analyses show that 

(1) all 740 Grand Canyon tributaries produce debris 

flows, albeit some at a low frequency; (2) about 60% of 

tributaries produce one or more debris flows per century; 

(3) about 10% of tributaries produce two or more debris 

flows per century; and (4) no tributary has produced 

more than six debris flows in the last century. 

Debris-flow Sediment Yield 

Data on debris-flow frequency, volume, and particlesize distributions were combined to create a model of 

debris-flow sediment yield in Grand Canyon (Webb and 

others, 2000). Using this model, it is estimated that debris 

flows contribute between 155,000 and 325,000 tons/yr 

(141,000 and 295,000 Mg/yr) of sediment to debris 

fans in Grand Canyon. Marble Canyon contributes the 

greatest amount of debris-flow sediment, which is consistent with both empirical observations and the modeled 

distribution of debris-flow occurrence in Grand Canyon 

(Griffiths and others, 2004).

Modeling debris-flow sediment yield requires a 

number of important assumptions. In this case, it was 

assumed that all debris flows from a given tributary were 

the same size, which means the model does not realistically depict a magnitude-frequency relation. Furthermore, the sediment-yield model does not account for 

extreme events not included in the historical record and 

small events that are inadequately represented. Some of 

these problems could be resolved by using a fully stochastic model of debris-flow frequency, but objectively 

determining model constraints based on the limited data 

would be difficult.

Figure 5. Debris-flow probabilities of 740 tributaries of the Colorado River in Grand Canyon (Griffiths and others, 2004). This map 

depicts group probabilities of the occurrence of one or more debris flows in a tributary during the century between 1890 and 1990.
Debris Flows in Grand Canyon and the Rapids of the Colorado River 145

Incorporating an average boulder content of 14% 

of debris-flow volumes (Melis and others, 1994; Webb 

and others, 1999b, 2000), the total boulder delivery from 

all 740 tributaries is 1.1 billion ft3

 (31 million m3) per 

thousand years. Distributing these boulders evenly along 

the river corridor without removal, dissolution, or erosion raises the bed by 2.4 ft (0.7 m) per thousand years, 

which we consider to be a reasonable order of magnitude. To distribute boulders more realistically, deposition 

was limited to the areas of recent debris fans, calculating 

the area of deposition at each tributary confluence as a 

rectangle defined by the length and average width of the 

rapid. For each confluence, local bed rise was calculated 

by dividing the total volume of sediment delivered by 

debris flow by the estimated area of deposition; these 

results were reported in Webb and others (2000, 2004).

River Reworking of 

Aggraded Debris Fans

In the years immediately following a debris flow, 

Grand Canyon rapids are known to be unstable because 

of reworking by the Colorado River (Howard and 

Dolan, 1981; Kieffer, 1985; Webb and others, 1989, 

1999b; Melis and others, 1994). Before closure of Glen 

Canyon Dam in 1963, the Colorado River removed most 

debris-flow deposits during the early summer floods, 

which averaged 82,000 cubic feet per second (cfs) and 

were as large as 220,000 cfs. Those flood events swept 

all but the largest particles downstream and redeposited cobbles and small boulders on debris bars that 

constrained the extent of eddies and controlled secondary rapids. The interaction between the frequency and 

magnitude of tributary debris flows and mainstem floods 

resulted in debris fans and rapids that were relatively 

stable in the intervening time periods between debris 

flows. 

Lava Falls Rapid offers one of the best documented 

cases of debris-fan reworking, which occurred during 

the 1996 beach/habitat-building flow (fig. 6) and during other floods in the predam and postdam periods 

(Webb and others, 1999b). Most of the reworking that 

occurred during the 1996 event happened as the discharge increased to its peak; reworking slowed markedly 

during the first day of peak discharge. Nine radio-tagged 

boulders traveled an average distance of 262 yards (240 

m) from their initial positions on the Prospect Canyon 

debris fan during the 1996 event (Pizzuto and others, 

1999; Webb and others, 1999b). Debris-flow deposits in 

1939, 1954, 1955, 1963, and 1966 were also reworked 

by subsequent floods; some of the aggraded debris fans 

(1954, 1963, 1966) were completely removed, while some 

of the deposition (1939, 1955) remained to cause persistent changes in Lava Falls Rapid.

While Kieffer (1985) stated that exceptionally 

large floods (>400,000 cfs) are required to completely 

rework some aggraded debris fans, Magirl and others 

(2005) found several examples of debris flows that were 

effectively removed by modest floods. For example, an 

8 to 10 ft (2.4 to 3.0 m) drop at Doris Rapid was nearly 

completely removed by a 220,000-cfs flood in 1921. Also, 

a 3.0 ft (0.91 m) riffle at To Hajisho Wash (RM 28.5) in 

1923 was completely removed by the 127,000-cfs flood 

in 1927. While we do not dispute that the amount of 

reworking increases with the magnitude of floods, effective reworking and redistribution of coarse sediment can 

occur at a variety of flood discharges and are heavily 

dependent on particle size of the aggraded debris fan, 

the elapsed time between debris flow and reworking 

flood, and the stream power available to transport sediment from a specific debris fan.

Webb and others (1999a) reported that reworking 

decreases with the time elapsed between the debris flow 

and the flood event because average releases may interlock particles into an overlapping network, significantly 

increasing the force necessary to dislodge and carry particles from debris fans. This process that leads to interlocked particles, which is the net result of physical rearrangement, abrasion of particle-particle contact points, 

and differential dissolution at contact points, is termed 

“suturing.” Suturing is common on debris fans that have 

not had debris-flow aggradation historically, and we have 

observed some suturing on recently aggraded debris 

fans, such as 18 Mile Wash (1987 debris flow) and 127.6 

Mile Wash (1989 debris flow), on the distal margin where 

submergence occurs frequently. The documented occurrence of suturing provides compelling reason to decrease 

the elapsed time between controlled flood releases, if 

reworking of aggraded debris fans is a priority. 

To understand how rapids have changed over time, 

water-surface profiles from 1923 and 2000 were compared to detect geomorphic change (Magirl and others, 

2005). Magirl and others (2005) compared the longitudinal profile surveyed in 1923 (U.S. Geological Survey, 

1924) with a profile constructed from lidar data taken in 

2000. Ninety-one tributary junctures along the Colorado River in Grand Canyon between Lees Ferry and 

Diamond Creek were evaluated for change. These sites 

represent 39% of all rapids and 67% of named rapids. 

At these 91 locations, 11 rapids were known to have not 

changed between 1923 and 2000, 6 rapids exhibited a 

rise in the elevation at the head of the rapid of 4.6 ft (1.4 

m) or more, and the elevation at the head of 2 rapids 
146 The State of the Colorado River Ecosystem in Grand Canyon

decreased more than 4.6 ft (1.4 m). The net change at 91 

rapids is shown in figure 7. More rapids aggraded (18) 

than degraded (7) for elevation changes greater than or 

equal to 2.3 ft (0.7 m), which is the threshold of detectable elevation change. Moreover, elevation increases 

were consistently larger than elevation decreases. Of 

the 10 debris fans associated with the largest elevation 

increases, 8 were aggraded by one or more known debris 

flows since 1923 (table 2 in Magirl and others, 2005). Of 

the five debris fans associated with the largest elevation 

decreases, only two had debris flows since 1923 (Magirl 

and others, 2005). Finally, the average elevation of pools 

at the heads of rapids was 0.85 ft (0.26 m) higher in 2000 

than in 1923.

Comparison of the 1923 and 2000 profiles also 

reveals the interaction between rapids as a result of 

debris-flow deposition (fig. 8). The 1966 debris flow 

at Crystal Rapid (RM 98) caused a rise in river level 

several miles upstream, which drowned out the tailwaves 

of Boucher Rapid to create what river runners refer to 

as “Lake Crystal.” The 1951 debris flow at Boucher 

Rapid had the opposite effect on the tailwaves of 

Hermit Rapid. The result was one of the larger hydraulic features in Grand Canyon, the notorious fifth wave in 

A. B.

Figure 6. Changes in Lava Falls Rapid (RM 179) during the 1996 beach/habitat-building flow. The river flows from bottom to top in 

these views. A. (March 24, 1996) The 1995 debris flow from Prospect Canyon (left side of the views) constricted the river by about 60%. 

This view shows the freshly deposited sediments with no vegetation on river left (left side of the view). B. (April 9, 1996) Reworking by 

the 1996 beach/habitat-building flow, which had a peak discharge of about 47,500 cfs at Lava Falls Rapid, removed 208,000 ft3 (5,900

m3

) of the aggraded debris fan, increasing the width of the rapid by an average of 16 ft (5 m) (photographs courtesy of Grand Canyon

Monitoring and Research Center, U.S. Geological Survey).
Debris Flows in Grand Canyon and the Rapids of the Colorado River 147

to transport particles more than 3 ft (0.91 m) in diameter, 

except during maximum powerplant releases or intentional flood releases and even then only at the largest 

rapids. Cobbles and boulders carried from debris fans 

by the regulated Colorado River appear to be redeposited in the pool immediately downstream of the debris 

fans instead of on the debris bar farther downstream 

(Pizzuto and others, 1999; Webb and others, 1999b). 

This altered pattern of redeposition reflects a change in 

the geomorphic framework of the Colorado River; in the 

case of Granite Rapid, repeated debris flows and modest reworking from 1984 through 2003 have resulted in a 

lengthening of the rapid tailwaves through the pool and 

into the secondary riffle. 

Many rapids in the Colorado River system have 

become larger during the last 30 yr because debris-fan 

constrictions and individual boulders cannot be totally 

removed by typical dam releases (Graf, 1979; Howard 

and Dolan, 1981; Melis and others, 1994; Webb, 1996). 

Continued deposition of coarse sediment into the river 

channel by debris flows will likely fill deeper pools above 

and below rapids while also enhancing the size of eddies; 

however, with the notable exception of the Crystal Rapid 

debris flow of 1966, most rapids affected by recent 

debris-flow aggradation are less hazardous to navigation. 

Exposed or shallowly submerged rocks are significant 

navigational obstacles, and debris-fan aggradation tends 

to narrow the channel, increasing flow depths and either 

removing or submerging existing rocks. At the same time, 

Figure 7. Net elevation change in 91 rapids of the Colorado River 

in Grand Canyon between 1923 and 2000. The locations of rapids 

that did not change between 1923 and 2000 and were therefore 

used to anchor the change in longitudinal profiles are shown as 

inverted triangles.

Figure 8. Comparison of 1923 and 2000 water-surface elevation 

profiles in upper Granite Gorge in Grand Canyon. Despite three 

debris flows, the head at Granite Rapid has changed little in 77 yr, 

but its drop has steepened. In contrast, aggradation from the 1966 

debris flow at Crystal Rapid is clearly visible in the comparison, and 

this aggradation affects Boucher Rapid upstream. Likewise, a 1951 

debris flow at Boucher Creek has affected Hermit Rapid upstream.

Hermit Rapid, which is a compressional wave associated 

with the jet of the rapid entering a backwater controlled 

by an aggraded Boucher Creek debris fan.

Leopold’s (1969) analysis of the pool-and-rapid 

morphology of the Colorado River, which is one of the 

more widely referenced figures, presents the cumulative vertical drop of the river as a function of distance 

as measured in 1923 for the first 150 mi (241 km) below 

Lees Ferry. Leopold concluded that 50% of the total 

drop occurred in only 9% of the length of the river. In 

2000, 66% of the total drop in river occurred in 9% of 

the length over the 227 mi (365 km) below Lees Ferry. 

When only the first 150 mi (241 km) of river is considered for direct comparison with Leopold (1969), 71% 

of the total rapid occurs in 9% of the length (fig. 9), 

reflecting the greater amount of aggradation in Marble 

Canyon and eastern Grand Canyon compared to western Grand Canyon. 

Impacts of Dam Operations 

on Aggradation

Operations of Glen Canyon Dam have long been 

hypothesized to increase aggradation of the riverbed 

by limiting the reworking of debris fans (Howard and 

Dolan, 1981; Kieffer, 1985; Webb and others, 1989; 

Melis and others, 1994; Griffiths and others, 2004). 

Glen Canyon Dam has reduced peak discharges on the 

Colorado River, which now has insufficient stream power 
148 The State of the Colorado River Ecosystem in Grand Canyon

the drops through these rapids are steeper, and sizes of 

waves are typically larger; these changes could lead to 

increased incidence of boat flips.

The Longitudinal Profile 

of the River

Hanks and Webb (in press) interpreted the longitudinal profile of the Colorado River through Grand 

Canyon in relation to debris-flow sedimentation at 

rapids. Rapids represent short-wavelength (about 0.6 

mi (1 km)), small-amplitude (less than about 16 ft (5 m)) 

convexities (areas that round outward from the riverbed) 

in the longitudinal profile of the river, arising from the 

shallow gradient in the upstream pool and the steep 

gradient through the rapid itself. The kinds of changes 

detected in the comparison of the 1923 and 2000 profiles (Magirl and others, 2005) discussed previously cannot be as easily detected when the longitudinal profile of 

the Colorado River is displayed for the length of Grand 

Canyon (fig. 10). 

Analysis of the entire longitudinal profile through 

Grand Canyon reveals two river-profile convexities 

that are long-wavelength (about 62 mi (about 100 km)), 

large-amplitude (49 to 98 ft (15 to 30 m)) river-profile 

convexities (Hanks and Blair, 2003; Hanks and Webb, in 

press): the eastern canyon convexity between RM 30 and 

RM 80 and the western canyon convexity between RM 

160 and RM 250. Both of these convexities are easily 

discernable in figures 11 and 12. These large-amplitude 

convexities have strong spatial correlations with high 

probabilities of debris-flow occurrence, high densities of 

debris fans, and the largest debris fans along the river. 

Convexities of intermediate scale are also identified in 

the longitudinal profile. River-profile convexities require 

an active and powerful geologic process to maintain 

them, in this case the abundant, frequent, and voluminous debris-flow activity in Grand Canyon. Presumably 

for all of the Holocene and at least some of the late 

Pleistocene, the Colorado River has been expending its 

energy transporting sediment within Grand Canyon, 

integrating short-wavelength convexities into long-waveFigure 9. Comparison between the cumulative vertical drop 

of the river in 1923 (data from Leopold, 1969) and the cumulative 

vertical drop in 2000 (Magirl and others, 2005). 

Figure 10. The longitudinal profile of the Colorado River through 

Marble and Grand Canyons (Lees Ferry to Diamond Creek) as 

surveyed in 1923 (U.S. Geological Survey, 1924) and as measured by 

using lidar in 2000 (from Magirl and others, 2005; Hanks and Webb, 

in press). At this scale, differences in the 1923 and 2000 profiles (see 

fig. 8) are not apparent. The straight line represents the average 

river gradient and illustrates profile convexities that appear to be 

related to debris-flow deposition in the Colorado River.
Debris Flows in Grand Canyon and the Rapids of the Colorado River 149

Hanks and Webb (in press) compared debris-flow 

probabilities (calculated from fig. 5) averaged over 12.4-

mi (20-km) intervals with this difference profile and 

found that the highest debris-flow probabilities have a 

strong association with intermediate and long-wavelength convexities. The falling limbs of the Eastern and 

Western Canyon convexities are both associated with 

decreasing debris-flow probabilities as they enter the 

upper and lower Granite Gorge, respectively, reflecting 

the decrease in the rate of tributary sediment delivery 

in these reaches. As shown in figure 12, the reach-averaged sediment yield from the debris-flow sediment-yield 

model is also associated with profile convexities, although 

the association is less than for the probabilities alone.

Several overall characteristics of the river corridor appear to be associated with the characteristics of 

long-wavelength convexities. The presence of abundant 

emergent islands in the river is associated with the tops 

of the convexities where overall slopes are relatively low. 

Cultural sites on fine-grained sediment deposits appear 

to be most common on the tops of large-scale convexities 

as well. The largest rapids on the river appear to be associated with the falling limbs of convexities (Hanks and 

Webb, in press). This latter characteristic may result from 

both the increased reach-scale gradient on the falling 

limbs of the convexities and a greater spacing between 

rapids, which minimizes interaction between the drops.

Summary and Management 

Implications

Debris flows transport poorly sorted sediment onto 

debris fans in the Colorado River at a frequency that 

varies through Grand Canyon. Historically, an average of 5.0 debris flows per year has occurred in Grand 

Canyon. The occurrence of these debris flows does not 

appear to be related to seasonal precipitation amounts. 

Modeling debris-flow frequency in Grand Canyon based 

on the interpretation of 1,365 photographs of the river 

corridor yielded frequency information in 167 of 740 

tributaries (23%). Of the 167 tributaries, 98 (59%) had 

debris flows during the last 100 yr. 

Frequency estimates indicate that 57% of the 

tributaries had at least one debris flow per century, while 

about 10% of the tributaries had a frequency of more 

than two debris flows per century. Estimates of sediment 

yield to the Colorado River in Grand Canyon by debris 

flow are as high as 3.3 million tons (3.0 million Mg) of 

sediment per decade, of which 452,000 tons (410,000 

Mg) are boulders larger than 10 inches (>256 mm) in 

Figure 11. A profile of the difference in elevation (ft) between the 

longitudinal profile of the Colorado River between Lees Ferry and 

the Grand Wash Cliffs and its average gradient (both are shown in 

fig. 10). The Western Canyon offset shows that this convexity likely 

has a larger magnitude if the true bedrock profile were known. 

Convexities discussed in this chapter are shown on the profile.

Figure 12. A profile of the difference in elevation (ft) between the 

Colorado River and an average gradient (fig. 11) compared with a 

reach-averaged sediment yield for debris flows as calculated by 

using a stochastic model (Webb and others, 2000).

length convexities. This suggests that little or no bedrock 

incision has occurred during about the last 11,000 yr.

Detrending of the longitudinal profile (fig. 11) 

reveals the magnitude of convexities and allows comparison with other features along the river corridor. For 

example, the largest convexities coincide with reaches 

that contain 13 of the 14 largest debris fans (Hanks and 

Webb, in press). In addition, the locations and areas of 

the 444 Holocene debris fans between Lees Ferry and 

Diamond Creek (Melis, 1997) coincide with fluctuations 

in the difference profile (Hanks and Webb, in press).
150 The State of the Colorado River Ecosystem in Grand Canyon

diameter. Distributed evenly throughout the river corridor, these boulders would raise the bed in Grand Canyon 

by 2.3 ft per thousand years (0.72 m per thousand years). 

If deposition is limited to existing debris fans at tributary mouths, these boulders would raise the bed by an 

average of 8.66 ft (2.64 m) per thousand years at each 

confluence.

 By combining the frequency model with relations 

for debris-flow volume and particle-size distribution, 

debris-flow sediment yields were calculated for several 

time periods. On average, debris flows deliver between 

0.15.

106 and 0.33.106 tons/yr (0.14.106 and 0.30.106 Mg/

yr) of sediment to the main channel. Although debris 

flows deliver only 23,142 to 48,488 tons/yr (21,000 to 

44,000 Mg/yr) of boulders to the river, these boulders 

control the longitudinal profile and geomorphic framework of the river, defining debris fans, rapids, and related 

sandbars, and are unlikely to be removed by regulated 

flows. Moreover, the effects of debris flows are shown to 

affect the river on several length and temporal scales.

Comparison of the two water-surface profiles (one 

surveyed in 1923 and one in 2000) showed a change in 

80 rapids. The average elevation of pools at the heads 

of rapids was 0.85 ft (0.26 m) higher between 1923 and 

2000, indicating net aggradation of the coarse-grained 

sediment forming the rapids throughout Grand Canyon. 

Furthermore, comparison of the two water-surface profiles showed enhanced pool-and-rapid structure; while 

50% of the total drop of the river occurred in just 9% 

of the river distance in 1923, that figure increased to 

66% by 2000. Reconstruction of water-surface profiles 

showed that debris-flow deposition can also have large 

upstream effects, particularly in the cases of reducing 

gradients between rapids and reducing the fall in the 

upstream rapid.

Analysis of the entire longitudinal profile through 

Grand Canyon reveals convexities that reflect sustained 

debris-flow deposition. Specifically, there are two longwavelength (about 62 mi (about 100 km)), large-amplitude (49 to 98 ft (15 to 30 m)) river-profile convexities: 

the eastern canyon convexity between RM 30 and RM 

80 and the western canyon convexity between RM 160 

and RM 250. These large-amplitude convexities have 

strong spatial correlations with high probabilities of 

debris-flow occurrence, high densities of debris fans, and 

the largest debris fans along the river. These convexities appear to be maintained by debris-flow activity in 

Grand Canyon, presumably for all of the Holocene. In 

this period, the Colorado River has been expending its 

energy transporting sediment within Grand Canyon and 

integrating short-wavelength convexities into long-wavelength convexities, with little or no bedrock incision. 

Coarse-sediment fill in the channel of the Colorado 

River at any wavelength has its origins in the rapids, 

which result from the tributary debris flows and fans that 

feed them. In contrast, the amount of fill in the rapids, either individually or collectively, is a small volume 

compared to the fill that creates the longer wavelength 

convexities. Periodic channel maintenance floods are 

not likely to perform the transport necessary to create 

the long-wavelength convexities, although reworking of 

locally aggraded debris fans is clearly feasible. The riverreworking processes by which point-source contributions of debris-flow sediment are aggregated into longer 

wavelength convexities are as yet unknown and certainly 

involve a more detailed understanding of the flood 

dynamics of the predam river. Nevertheless, the close 

spatial associations of the longer wavelength convexities with the locations and sizes of debris fans and with 

the frequency of debris flows lead to the conclusion that 

unusual accumulations of debris fill in the channel are 

their principal cause, just as they are for the rapids.

In terms of adaptive management and operations of 

Glen Canyon Dam, reworking of aggraded debris fans 

has been shown to be feasible. Reworking has been documented during modified low fluctuating flow releases, 

maximum powerplant releases, and flood releases up 

to 47,500 cfs. Both reworking and transport capacity increase with increasing discharge, which suggests 

that flood releases larger than powerplant capacity of 

about 33,500 cfs are more efficient than smaller events. 

Because reworking mostly occurred in the rising limb 

of flood hydrographs, large-magnitude floods designed 

for debris-fan reworking do not have to have significant duration. As shown in figure 13, a flood designed 

Figure 13. Hypothetical Glen Canyon Dam release and flow 

hydrographs designed to create a peak discharge of 100,000 

cfs for 1 min at Diamond Creek, followed by a beach-building 

discharge of 45,000 cfs for a duration of 2 d.
Debris Flows in Grand Canyon and the Rapids of the Colorado River 151

to rework debris fans could have a peak discharge of 

100,000 cfs for only 1 min at Diamond Creek then drop 

rapidly to a beach-building discharge for several days. 

This type of management prescription may be only used 

every 5 to 10 yr, with smaller intervening releases.

References Cited

Cooley, M.E., Aldridge, B.N., and Euler, R.C., 1977, 

Effects of the catastrophic flood of December, 1966, 

North Rim area, eastern Grand Canyon, Arizona: 

U.S. Geological Survey Professional Paper 980, 43 p.

Graf, W.L., 1979, Rapids in canyon rivers: Journal of 

Geology, v. 87, p. 533–551.

Griffiths, P.G., Webb, R.H., and Melis, T.S., 1996, Initiation and frequency of debris flows in Grand Canyon, 

Arizona: U.S. Geological Survey Open-File Report 

96-491, 35 p.

Griffiths, P.G., Webb, R.H., and Melis, T.S., 2004, Initiation and frequency of debris flows in Grand Canyon, 

Arizona: Journal of Geophysical Research, Surface 

Processes, v. 109, F04002, doi:10.1029/2003JF000077, 

14 p.

Hanks, T.C., and Blair, J.L., 2003, Comment (on differential incision of the Grand Canyon related 

to Quaternary faulting–constraints from U-series 

and Ar/Ar dating by J. Pederson, K. Karlstrom, 

W. Sharp, and W. McIntosh): GEOLOGY Online 

Forum, www.gsajournals.org, DOI 10.1130/0091-

7613(2003)31 <e16:DIOTGC> 2.0.CO; 2.

Hanks, T.C., and Webb, R.H., in press, Effects of tributary debris on the longitudinal profile and incision 

of the Colorado River in Grand Canyon: Journal of 

Geophysical Research, Surface Processes.

Hereford, R., Thompson, K.S., and Burke, K.J., 1998, 

Numerical ages of Holocene tributary debris fans 

inferred from dissolution pitting on carbonate boulders in the Grand Canyon of Arizona: Quaternary 

Research, v. 50, p. 139–147.

Howard, A., and Dolan, R., 1981, Geomorphology of 

the Colorado River in Grand Canyon: Journal of 

Geology, v. 89, p. 269–297.

Kieffer, S.W., 1985, The 1983 hydraulic jump in Crystal 

Rapid: implications for river-running and geomorphic 

evolution in the Grand Canyon: Journal of Geology, v. 

93, p. 385–406.

Leopold, L.B., 1969, The rapids and the pools, in The 

Colorado River region and John Wesley Powell: U.S. 

Geological Survey Professional Paper 669, p. 131–145.

Magirl, C.S., Webb, R.H., and Griffiths, P.G., 2005, 

Long-term change in the water-surface profile of the 

Colorado River in Grand Canyon, Arizona: Water 

Resources Research, v. 41, Wo5021, doi:10.1029/

2003WR002519, 10 p.

Melis, T.S., 1997, Geomorphology of debris flows and 

alluvial fans in Grand Canyon National Park and their 

influence on the Colorado River below Glen Canyon 

Dam, Arizona: Tucson, University of Arizona, unpublished Ph.D. dissertation, 495 p.

Melis, T.S., Webb, R.H., Griffiths, P.G., and Wise, 

T.J., 1994, Magnitude and frequency data for historic debris flows in Grand Canyon National Park 

and vicinity, Arizona: U.S. Geological Survey Water 

Resources Investigations Report 94-4214, 285 p.

Pizzuto, J.E., Webb, R.H., Griffiths, P.G., Elliott, J.G, 

and Melis, T.S., 1999, Entrainment and transport of 

cobbles and boulders from debris fans, in Webb, R.H., 

Schmidt, J.C., Marzolf, G.R., and Valdez, R.A., eds., 

The controlled flood in Grand Canyon: scientific 

experiment and management demonstration: Washington, D.C., American Geophysical Union, Geophysical Monograph Series, v. 110, p. 53–70.

Stephens, H.G., and Shoemaker, E.M., 1987, In the footsteps of John Wesley Powell: Boulder, Colo., Johnson 

Books, 286 p. 

Turner, R.M., and Karpiscak, M.M., 1980, Recent 

vegetation changes along the Colorado River between 

Glen Canyon and Lake Mead, Arizona: U.S. Geological Survey Professional Paper 1132, 125 p.

U.S. Geological Survey, 1924, Plan and profile of Colorado River from Lees Ferry, Ariz., to Black Canyon, 

Ariz.-Nev. and Virgin River, Nev.: U.S. Geological 

Survey, 1924, 21 p.

Webb, R.H., 1996, Grand Canyon, a century of change: 

Tucson, University of Arizona Press, 290 p.
152 The State of the Colorado River Ecosystem in Grand Canyon

Webb, R.H., Griffiths, P.G., and Melis, T.S., 2004, 

Debris flows and rapids in Grand Canyon: implications for erosion rates and evacuation of sediment 

from tributary canyons, in Young, R.A., and Spamer, 

E.E., eds., The Colorado River: origin and evolution: 

Grand Canyon, Ariz., Grand Canyon Association 

Monograph 12, p. 183–188.

Webb, R.H., Griffiths, P.G., Melis, T.S., Elliott, J.G., 

and Pizzuto, J.E., 1999a, Reworking of debris fans 

by the 1996 controlled flood in Grand Canyon, in

Webb, R.H., Schmidt, J.C., Valdez, R.A., and Marzolf, 

G.R., eds., The 1996 flood in Grand Canyon: scientific experiment and management demonstration: 

Washington, D.C., American Geophysical Union, 

Geophysical Monograph, p. 37–51.

Webb, R.H., Griffiths, P.G., Melis, T.S., and Hartley, 

D.H., 2000, Sediment delivery by ungaged tributaries of the Colorado River in Grand Canyon, Arizona: 

U.S. Geological Survey Water Resources Investigations 

Report 00-4055, 67 p.

Webb, R.H., Melis, T.S., Griffiths, P.G., Elliott, J.G., 

Cerling, T.E., Poreda, R.J., Wise, T.W., and Pizzuto, 

J.E., 1999b, Lava Falls Rapid in Grand Canyon, effects 

of late Holocene debris flows on the Colorado River: 

U.S. Geological Survey Professional Paper 1591, 90 p.

Webb, R.H., Pringle, P.T., Reneau, S.L., and Rink, G.R., 

1988, Monument Creek debris flow, 1984: implications for formation of rapids on the Colorado River 

in Grand Canyon National Park: Geology, v. 16, p. 

50–54.

Webb, R.H., Pringle, P.T., and Rink, G.R., 1989, Debris 

flows in tributaries of the Colorado River in Grand 

Canyon National Park, Arizona: U.S. Geological 

Survey Professional Paper 1492, 39 p.

Contact Information:

Robert H. Webb

Research Hydrologist

U.S. Department of the Interior

U.S. Geological Survey

Water Resources Discipline

Tucson, AZ 

rhwebb@usgs.gov

Peter G. Griffiths

Research Hydrologist

U.S. Department of the Interior

U.S. Geological Survey

Water Resources Discipline

Tucson, AZ

pggriffi@usgs.gov

Christopher S. Magirl

Hydrologist

U.S. Department of the Interior

U.S. Geological Survey

Water Resources Discipline

Tucson, AZ

magirl@usgs.gov

Thomas C. Hanks

Geophysicist

U.S. Department of the Interior

U.S. Geological Survey

Geology

Menlo Park, CA

thanks@usgs.gov

First page photograph credit: R.B. Stanton, courtesy of the National Archives 

and Records Administration, College Park, Maryland
Chapter 9

Recreation 

Use Values and 

Nonuse Values 

of Glen and 

Grand Canyons

John Loomis

Aaron J. Douglas

David A. Harpman

Introduction

Grand Canyon and Glen Canyon are unique natural environments valued worldwide, providing worldclass recreational activities and habitat for wildlife and 

endangered fish. This chapter discusses the various economic values of the Grand Canyon and Glen Canyon 

ecosystems, how these values are measured, and what we 

know about them. The economic value of the services 

provided by these ecosystems is also discussed. Other 

chapters in this report address related topics, including 

the potential effects of Grand Canyon campsite availability on recreation (chapter 12) and the hydropower values 

of Glen Canyon Dam (chapter 10).

This chapter focuses on how recreation use and 

economic values are influenced by alternative river flow 

regimes and Glen Canyon Dam operations. Recreation 

is of interest within the Glen Canyon Dam Adaptive 

Management Program for several reasons. First, the 

Grand Canyon Protection Act (GCPA) of 1992 expressly 

mentioned recreation as one of the three elements to be 

monitored. Specifically, the act stated that “long-term 

monitoring . . . shall include any necessary research and 

studies to determine the effect of the Secretary’s actions 

. . . on the natural, recreational and cultural resources 

of Grand Canyon National Park and Glen Canyon 

National Recreation Area” (GCPA, sec. 1805(b)). Second, because Glen Canyon is a national recreation area 

and recreation is one of the dual mandates of national 

parks (Loomis, 2002), the effects of alternative dam operations on recreation should be expected to receive significant attention. Third, recreation use and value are of 

interest in an integrated scientific research effort because 

of potentially important linkages between various ecosystem components. For example, it becomes important 

to understand how changes in river flows influence trout 

size and abundance and how changes in fish population 

in turn influence recreation use and value. 

As noted previously by the National Research 

Council (1999, p. 13), however, there has not been 

regular, systematic, or comprehensive monitoring of the 

effects of alternative flow regimes and other operational 

changes on recreation use and public values. Therefore, 

this chapter draws from research conducted over the last 

two decades to summarize the available information on 

recreation use, benefits, and public values of Grand and 

Glen Canyons. This partial information is the best available at the present time to inform adaptive management 
154 The State of the Colorado River Ecosystem in Grand Canyon

(see Overview, this report) of Grand and Glen Canyons 

about the consequences of operational changes on recreation use and public values. 

Background

Types of Economic Values 

Provided by Glen and 

Grand Canyons

Economic values can be divided into three main categories: (1) visitor use values such as recreation, (2) local 

economic effects associated with visitor spending that supports commercial outfitters, hotels, restaurants, and towns 

such as Page, Arizona, and (3) public nonuse values.

Use Values to Visitors

Most people are familiar with the local economic 

effects of visitor spending and the positive economic 

impacts of recreation and tourism in the form of jobs 

and local income. In addition, the visitors themselves 

also receive economic benefits from the actual recreation experience in the form of what is termed “consumer surplus.” This consumer surplus represents the 

monetary measure of the increase in economic well 

being that a visitor receives, and would be willing to 

pay, over and above the existing cost of the recreational 

trip. Consumer surplus, or net willingness to pay, is the 

federally recommended measure of benefits for use 

by agencies such as the Bureau of Reclamation when 

conducting benefit-cost analyses (U.S. Water Resources 

Council, 1983).

As an example of consumer surplus, consider the 

case of private whitewater boating on the Colorado 

River through Grand Canyon. A private boater experiencing a long awaited and possibly once-in-a-lifetime 

trip would likely pay more for the trip than just the 

travel costs to Grand Canyon in order to enjoy a 2-week 

float trip. This extra value or consumer surplus realized 

on these trips may be influenced by the river flows, as 

minimal flows will reduce the size of some rapids, as 

well as the time available for stopping at side canyons 

and visiting cultural sites. 

Local Economic Effects 

of Visitor Spending

While visitor expenditures are a cost to the visitor, 

they create positive, direct economic effects in the local 

area in the form of additional income and employment 

in the retail sector. The ripple, or multiplier effects, of 

such spending reverberates to the wholesale and distribution sectors that supply the retail sector. This second 

round of economic stimulation is known as indirect 

effects. Further, a portion of the money received by 

employees as wages and profits to business owners gets 

respent in the local economy on other consumer goods 

and services. These respent portions are often referred to 

as induced effects. The regional multiplier is the sum of 

these direct, indirect, and induced effects divided by the 

direct effect (Douglas and Harpman, 1995; Loomis and 

Walsh, 1997; Minnesota Implan Group, 1997). Thus, the 

local economic effects of recreation include employment 

and income that originate from the economic sectors 

directly catering to tourists but also include many other 

sectors indirectly affected by recreation spending.

 Nonuse Values

Nonuse values include the benefits that people 

derive from simply knowing that a unique and irreplaceable natural environment or species exists even if the 

individual does not visit it or see it. This component of 

nonuse value is often called “existence value.” Nonuse 

values also include a bequest value from knowing that 

protection today provides the unique natural resources 

to future generations. The unique natural environment 

of Grand Canyon was used as an example in the seminal article on existence value by Krutilla (1967, p. 778). 

Krutilla also noted that continued existence of a threatened and endangered species (e.g., humpback chub (Gila

cypha)) would also generate existence values. 

The U.S. Department of the Interior officially 

recognized existence values in 1986 when it included 

these values in procedures to calculate natural resource 

damages from hazardous substances (U.S. Department 

of the Interior, 1986). The inclusion of nonuse values 

was broadened by the U.S. District Court of Appeals 

decision (1989), which referred to nonuse values as passive-use values, and indicated that both use and passive-use values must be considered when performing a 

natural resource damage assessment. The concept of 
Recreation Use Values and Nonuse Values of Glen and Grand Canyons 155

nonuse values and its empirical measurement by using 

surveys were given a qualified endorsement by a blue 

ribbon panel commissioned by the National Oceanic 

and Atmospheric Administration and chaired by two 

Nobel laureates (Arrow and others, 1993). The potential role of nonuse values in policy analysis for the Glen 

Canyon Environmental Studies program is discussed in 

Harpman and others (1993). The growing acceptance 

of including nonuse values in valuation of ecosystems is 

evident in the latest National Research Council (2005, p. 

6) report, which states, “economic valuation of changes 

in ecosystem services should be based on . . . both use 

and nonuse values.”

Of course, what is relevant for this chapter is how 

the nonuse values of Grand Canyon change with alternative dam operations that affect the native species in 

Grand Canyon for current and future generations. We 

will discuss the one study, performed in 1994, that measured nonuse values for changes in natural resources in 

Grand Canyon (Welsh and others, 1995). Unfortunately, 

there have been no follow-up studies to measure how 

nonuse values have changed as a result of stabilization of 

river flows, beach building, and recovery efforts for the 

humpback chub. 

Empirical Measurement of Use 

and Nonuse Values

Recreation Use Values

To estimate visitor benefits or consumer surplus, 

there are two broad categories of methods: (1) those 

that rely upon actual behavior or what are called 

“revealed preference methods” and (2) those that rely 

upon intended behavior or what are called “stated 

preference methods.” The travel cost method (TCM) 

for estimating recreation demand is an example of a 

commonly used revealed preference method. The basic 

TCM uses variations in visitors’ travel costs as a proxy 

for the price of a trip and the number of trips taken 

as a measure of quantity to trace out a demand curve 

for recreation at the particular site. From the demand 

curve, the consumer surplus or net willingness to pay 

(WTP) can be measured (Loomis and Walsh, 1997). 

More recent TCM models involve a random utility 

model of site choice to reveal the net WTP of visitors 

for access to a recreation site. 

Within stated preference methods is the frequently 

used contingent valuation method (CVM), as well as 

newer conjoint and choice experiment techniques. These 

stated preference methods use a survey to construct a 

simulated or hypothetical market in which an individual’s 

consumer surplus is elicited. For example, the analyst 

often asks a visitor what is the maximum increase they 

would pay to visit a particular site. In most of the CVM 

studies reviewed below, a popular willingness-to-pay 

question format, called “dichotomous choice,” is used. 

In this question format, individuals are asked if they 

would pay a higher trip cost of $X. The amount of $X 

varies across the sample. At higher dollar amounts, a 

lower percentage of visitors would pay that amount, and 

conversely at lower dollar amounts, a higher percentage 

would pay that amount. By plotting the percentage of 

people who would pay each dollar amount, a demandlike relationship is traced out from which consumer 

surplus can be calculated. Conjoint and choice experiments also use a survey to present alternative trips with 

different levels of trip characteristics (e.g., flow levels, size 

of beaches) to directly estimate how consumer surplus 

changes with changes in trip characteristics. 

Both methods, the TCM and CVM, are recommended for use by Federal agencies such as the Bureau 

of Reclamation for valuing recreation (U.S. Water 

Resources Council, 1983). There have been hundreds 

of applications of both methods worldwide. Extensive 

comparison of the consumer surplus estimated by using 

the TCM and CVM suggests that the two have convergent validity, as estimates of consumer surplus by the two 

approaches are nearly identical and are highly correlated 

with each other (Carson and others, 1996). 

Measurement of Regional Economic 

Effects of Recreation

To calculate the local economic impacts associated with visitor expenditures, an input-output model 

is used to calculate the multiplier effects (Douglas and 

Harpman, 1995; Loomis, 2002). The general economic approach of combining expenditure data with 

an input-output model is called “regional economic 

analysis” because it usually measures the effect on a 

local economy (county) or a region (group of counties). 

The input-output model translates the visitor expenditures into total local income and employment. The 

regional economic analyses reported in this chapter use 

IMPLAN® software (Minnesota Implan Group, 1997) 
156 The State of the Colorado River Ecosystem in Grand Canyon

to construct an input-output model of the local region. 

The input-output model captures the ripple, or multiplier effects, of the direct spending on indirect spending 

of related industries and induced spending of workers 

receiving additional wages. 

Measurement of Nonuse Values

Stated preference methods such as the CVM or conjoint/choice experiments are the only methods currently 

available for measuring nonuse values. Much like the 

recreation application of the CVM, the application of 

these methods to measure the willingness of a consumer 

to pay for nonuse values involves using a survey to construct a simulated or hypothetical market or referendum. 

In this case, however, a random sample of households is 

interviewed or mailed a survey. They are asked whether 

they would vote in favor of or against a particular management action or program involving protection of the 

resource at a specific cost to their household in the form 

of higher taxes or prices. 

Status, Trends,

and Recent Findings

Recreation Use Values

of Glen Canyon

Fishing Use 

Because of the clear, cold water released from Glen 

Canyon Dam, the 15-mi (24-km) stretch of the Colorado 

River below the dam provides a significant trout fishery 

in Arizona. As shown in figure 1, use has fluctuated over 

the past 10 yr. In part, fluctuating use may be due to 

increasing populations of trout reaching carrying capacity of the stream, which results in a reduction of the size 

of fish from their earlier trophy size. Changes in fishing 

regulations (including greater restrictions in permissible 

fishing gear and reductions in catch limits) may also be 

partially responsible for fluctuating use patterns. Shore 

anglers are less affected than boat anglers because there 

are threshold minimum flows for floating the boats over 

the rocks in critical reaches. The downturn in Lees 

Ferry angler use may be due to a variety of factors and, 

therefore, suggests the need for an integrated multivariate statistical analysis of the relationship between angler 

use, catch-per-unit effort, and fish size. Chapter 2 of 

this report presents a catch-per-unit effort that suggests 

that the decline in angler days may be due to decreasing 

angler catch rate, which fell from a peak of nearly 1.5 

fish per hour in the 1998–99 time period to 0.5 fish per 

hour in 2002. At the same time, the size of fish continued to decrease through 2001. 

Fishing Values

There are only two economic valuation studies of 

recreational fishing in the Lees Ferry reach, both of 

which are fairly old. The first study was by Richards 

and others (1985) and used the TCM to estimate the 

consumer surplus of trophy trout anglers and nontrophy 

trout anglers. The authors calculated net willingness to 

pay over and above their trip cost of $381 for trophy 

trout anglers and $272 for nontrophy trout anglers. 

Using the percentages of trophy and nontrophy anglers 

that existed at the time of the study (now more than 20 

yr ago, and before the change in fishing regulations), 

the weighted average value was $304 in 1983 dollars. In 

2004 dollars, the consumer surplus is $580 per trip. 

The second study, and the only one we are aware 

of that related angler value to river flow levels, was 

conducted by Bishop and others (1987). During 1985, 

anglers were sampled at Lees Ferry. The investigators 

used a dichotomous choice, willingness-to-pay format 

in their CVM survey of anglers. The net willingness to 

pay for the anglers’ actual trips ranged between $130 

for those experiencing constant flows and $104 for those 

experiencing fluctuating flows. In 2004 dollars this is 

Figure 1. Angler use at Lees Ferry, 1995–2004 (developed from 

data found at http://www2.nature.nps.gov/mpur/ for Glen Canyon 

National Recreation Area).
Recreation Use Values and Nonuse Values of Glen and Grand Canyons 157

about $250 per trip for those experiencing constant flows 

and nearly $200 for those experiencing fluctuating flows. 

While the 20%–25% difference in value per trip 

with constant versus fluctuating flows gives some insight 

into the effect of Glen Canyon Dam operations on the 

value of fishing, the study by Bishop and others (1987) 

also asked anglers their net willingness to pay for a wide 

range of flow scenarios. The results indicated that angler 

benefits (WTP) peaked at a flow of 10,000 cubic feet per 

second (cfs) with constant flows at $126 per trip ($240 in 

2004 dollars). This finding is in the same range of benefits as the actual trip experience, where 75% of anglers 

experienced a flow between 5,000 and 15,000 cfs on 

their most recent trip (Bishop and others, 1987, p. 121). 

At low flows, such as 3,000 cfs, the values per trip fell to 

half at $60 per trip ($114 in 2004 dollars). At high flows 

of 25,000 cfs, the value per trip was $94 ($178 in 2004 

dollars). Thus, there is a nonlinear relationship between 

angler benefits and flows. 

Combining the values per angler trip and the number of anglers yields estimates of annual recreational 

fishing benefits that range from $2.4 to $4.8 million 

(2004 dollars) at optimum flow levels, corresponding to 

the range of past use of 10,000 to 20,000 angler days. 

At low flows these values drop by more than half to $1.1 

million to $2.3 million annually. 

While these values suggest that the recent flow 

experiments such as the low summer steady flows (LSSF) 

in 2000, which held flows steady at 8,000 cfs, probably 

did not result in a large reduction in angler benefits from 

optimum flows, it is not possible to know for sure without 

conducting surveys of anglers during this period. Judging 

solely by angler use levels, the LSSF was associated with 

angler use equivalent to the previous year (1999). This is 

corroborated by the findings of Hjerpe and Kim (2003), 

who interviewed fishing guides and found that the 2000 

season had slightly higher angler use but in line with the 

growth in angler use in the past 5 yr. 

Day-use Rafting

The 15 RM of Glen Canyon below the dam provide an opportunity for scenic day-use rafting. At low to 

moderate river flows, the half-day raft trips launch below 

Glen Canyon Dam and float downstream to Lees Ferry 

(Douglas and Harpman, 1995). At very high flows or at 

times when the national security level reaches red and 

visitors are not allowed to launch from the dam, they 

must launch from Lees Ferry and motor most of the way 

upstream and then float down. As is shown in figure 2, 

this is a popular float trip, averaging around 40,000 visitors each year. 

Economic Values of Day-use Rafting

The only study of the economic value of day-use 

rafting was performed nearly 20 yr ago in 1986 by 

Bishop and others (1987). About half of their sample 

were rafters starting their trips below Glen Canyon 

Dam and then floating down, and the other half were 

motoring up and then floating down. Dichotomous 

choice CVM was used to estimate the consumer surplus 

that day-use rafters received. Statistical tests indicated 

that the WTP functions were not statistically different 

between the rafters starting their float trip below the 

dam and those who motored up and then floated down 

(Bishop and others, 1987, p. 145). Overall, the net WTP 

or consumer surplus was $26 per day, or nearly $50 

in 2004 dollars. These figures translate into an annual 

value of $2 million. 

Based on the attribute survey and the lack of difference in WTP values for the two departure locations, 

Bishop and others (1987, p. 145–146) concluded that 

day-use rafting trip values were not sensitive to river 

flow. This insensitivity to flows may be because visitors lacked prior knowledge regarding river flows and 

because of the lack of rapids in this section of the river. 

Obviously, it would be desirable to update these values, 

especially with the new flow regimes since 1986, and 

revisit the Bishop and others (1987) finding of no apparent relationship between flow and recreation benefits for 

the day-use rafting. 

Figure 2. User days per year at Glen Canyon National Recreation 

Area for day-use rafting, 1995–2004 (source: fax from Jacki Blais, 

Glen Canyon National Recreation Area, to Lara Schmit, U.S. 

Geological Survey, January 12, 2005). 
158 The State of the Colorado River Ecosystem in Grand Canyon

Recreational Rafting Use

of Grand Canyon

Grand Canyon National Park is world renowned 

for its whitewater rafting opportunities (Behan, 2000). 

Typically, commercial motorized rafting trips navigate 

the entire 226 RM from Lees Ferry to Diamond Creek 

(one of the takeout points) in 1 week, although there are 

also 2-week commercial oar trips. Private boaters typically take about 18 d for this same trip (Hjerpe and Kim, 

2003). Passengers of commercial trips and some private 

oar trips navigate the river either from Lees Ferry to 

Phantom Ranch (88 RM) or meet their trip at Phantom 

Ranch and end at Diamond Creek.

The popularity of rafting in Grand Canyon skyrocketed during the late 1960s and 1970s, prompting 

the National Park Service to place limits on the number 

of commercial and private boaters. During the 1970s 

to 1990s, 21 commercial rafting companies took visitors 

down Grand Canyon. Today, the limits are 115,000 user 

days for commercial rafting companies and about 55,000 

for noncommercial, or private, users. The binding nature 

of these limits on use can be seen in figure 3. 

The number of user days, however, masks the large 

difference in the number of passengers permitted for the 

two groups. Since the private trips are often much longer 

than commercial trips, there are nearly six times the 

number of commercial passengers (18,500–19,600) as 

there are noncommercial passengers (3,400–3,600). 

Over time the demand for private trips has increased 

substantially relative to commercial trips. This increase 

has resulted in a substantial and growing wait to obtain 

a private permit. The wait has grown from about 5 yr 

in 1991 to 12 yr in 2003, resulting in the National Park 

Service revisiting its allocation by issuing in October 2004 

the Draft Environmental Impact Statement Colorado 

River Management Plan. The preferred alternative in 

the management plan nearly doubles the noncommercial 

user days and the number of private passengers permitted (Grand Canyon National Park, 2004).

Because of excess demand and user limits, one 

cannot cite visitor use as an indicator of visitor response 

to alternative flow regimes (Hjerpe and Kim, 2003). 

Rather, one must conduct surveys to ascertain how visitor experience and economic benefits (i.e., consumer 

surplus) change with alternative flow regimes and natural 

resource conditions. 

There are two surveys that provide some insights 

into how visitor satisfaction in nonmonetary terms varies 

with flow levels. The first survey was performed as part 

of the original Glen Canyon Environmental Studies 

program during 1984 and 1985 by Bishop and others (1987). Surveys of river guides, private trip leaders, 

commercial passengers, and private rafters found that 

constant flows in the range of 20,000–25,000 cfs yielded 

the highest satisfaction ratings. In essence, flows in this 

range allowed for larger rapids, more time for stopping at 

side canyons and attraction sites, and less time motoring 

or rowing. Low flow levels frequently resulted in visitors 

having to walk around certain rapids, which decreased 

trip satisfaction. 

The second survey was by Stewart and others 

(2000), who during 1998 and 1999 replicated the portion of the Bishop and others (1987) study that dealt 

with whitewater rafting trips. This more recent study 

found the same pattern of visitor satisfaction ratings with 

regard to flow. In particular, the study found that whitewater rafting satisfaction was highest at constant flows 

of 20,000–25,000 cfs and that several flow-related trip 

characteristics such as large rapids and time to stop and 

hike the side canyons were highly important (Stewart and 

others, 2000). This study also found that large beaches 

with shade from trees for stopping and camping was 

rated as moderately important. 

Related to these satisfaction surveys are trip diary 

data collected by Roberts and others (2002) and Roberts 

and Bieri (2001) to develop a Grand Canyon river trip 

simulator. In their simulation model, river flows are a key 

determinant of boat speed and, hence, the amount of 

time for trip-related activities such as swimming, visiting 

cultural sites, hiking, campsite selection, and decisions to 

Figure 3. Number of private and commercial boater user days, 

Grand Canyon National Park, 1998–2003 (source: www.nps.gov/

grca/crmp/documents/stats/1998-2003ByMonth.pdf).
Recreation Use Values and Nonuse Values of Glen and Grand Canyons 159

layover part of or an entire day. Roberts and Bieri (2001) 

also used trip diaries to study the LSSF of 2000. They 

found that the low flows reduced the amount of time 

visitors had for swimming, hiking, and visiting cultural 

sites from 7 h/d on a typical trip (flow averaging 19,000 

cfs) to 3.5 h with the 8,000 cfs of the LSSF. Thus, with 

the 8,000-cfs flows, an additional 3.5 h/d were spent 

motoring or rowing to make up for lost time because of 

the slower flows (Roberts and Bieri, 2001, p. 13). 

Economic Values of Private and 

Commercial Rafting in Grand Canyon

The price of a commercial Grand Canyon rafting 

trip is substantial, averaging around $215 per person 

per day (Hjerpe and Kim, 2003). For private trips, the 

National Park Service requires a $100 payment to get on 

the waiting list, coupled with a payment of $100 for every 

person taking the trip (Hjerpe and Kim, 2003). One way 

to think of the $100 payment to get on the waiting list is 

as an “option value,” a willingness to pay to maintain the 

opportunity to raft Grand Canyon in the future. 

The payment of commercial fees and related visitor 

expenditures of commercial and private rafters creates 

significant positive economic effects in the region in 

the form of income and employment. These effects are 

detailed in the regional economic effects section of this 

chapter. 

Our interest here is the amount of benefits received 

by the whitewater rafters and kayakers themselves. This 

is their net economic value, or consumer surplus. The 

first and only primary data study on whitewater rafting 

economic values and how they change with flow was 

conducted by Bishop and others in 1987 (see also Boyle 

and others, 1993). These authors used a dichotomous 

choice CVM survey of commercial and private boaters. They found that commercial passengers’ willingness 

to pay for their actual trip experience rose from $127 

($228 in 2004 dollars) at 5,000 cfs to a maximum value 

of $888 ($1,598 in 2004 dollars) at higher flows and then 

declined only slightly at 40,000 cfs (Boyle and others, 

1993). For private boaters the value of their actual trips 

rose from $111 ($200 in 2004 dollars) at 5,000 cfs to a 

maximum of $637 ($1,147 in 2004 dollars) at 28,000 cfs. 

This value falls to $455 ($819 in 2004 dollars) at 40,000 

cfs (Boyle and others, 1993). The larger decline in value 

at the highest flows is likely because of less experienced 

private trip leaders compared with professional guides 

on commercial trips. Nonetheless, these are very high 

recreation trip values, although less dramatic on a perday basis (about 7 d for a commercial trip). The relatively 

high value is commensurate with the high-quality experience and high satisfaction received by the majority of 

visitors to Grand Canyon. 

Summing these updated 2004 dollar values over the 

nearly 19,000 commercial passengers and 3,500 private 

boaters, these values at optimum flows are $30 million 

and $4 million annually in economic value to commercial passengers and private boaters, respectively. Based 

on Bishop and others (1987), there would be a drop in 

total whitewater rafting benefits from $34 million to $5 

million, or a loss of $29 million, at low flows such as 

5,000 cfs (a flow level close to the LSSF of 2000). Unfortunately, the opportunity to evaluate the actual loss in 

recreation benefits with the LSSF was missed. 

There is only one more recent estimate of whitewater river recreation in Grand Canyon that has been 

made. Hammer (2001) used the TCM with travel costs 

calculated by using the U.S. Postal Service ZIP CodeTM 

data from the 1998 recreation preference surveys of Hall 

and Shelby (2000) and Stewart and others (2000). Since 

these surveys were not originally designed for economic 

analysis, however, they do not allow for analysis of how 

the economic value of the rafting trips changes with flow 

levels. Nonetheless, Hammer (2001) did a very careful 

job of using ZIP Codes to calculate travel costs of visitors 

and using this information to estimate a TCM demand 

curve. This calculation is somewhat challenging because 

each visitor usually takes just one trip. But by using a 

zonal or visits-per-capita type demand model, Hammer 

(2001) was able to calculate an estimate of the value in 

1998–99. The value per trip for private boaters was $134 

or ($148 in 2004 dollars) and $314 ($351 in 2004 dollars) 

for commercial trip passengers (Hammer, 2001). 

The inability to link recreation benefits from 

Hammer’s (2001) study to flows is unfortunate. If there is 

an insufficient budget to fund separate recreation eco-

© 2005 Geoff Gourley; used with permission
160 The State of the Colorado River Ecosystem in Grand Canyon

nomic surveys, then an integrated social science survey 

that would allow for data to provide both preference and 

valuation in relation to flows would be desirable. Such 

recreation valuation monitoring should be planned into 

future flow experiments. 

Regional Economic Effects of 

Water-based Recreation in Glen 

and Grand Canyons

As discussed previously, recreational fishing and 

rafting involve tens of thousands of visitors each year to 

the Glen Canyon and Grand Canyon areas. Commercial 

rafting on Grand Canyon currently involves 16 different companies. One of these companies also provides 

day-use rafting in Glen Canyon, and there are support 

companies providing visitor and shuttle transportation 

to day-use rafters. Many anglers in Glen Canyon also 

employ professional fishing guides, who often use boats 

for guiding. Even the anglers and private rafters who do 

not use commercial outfitters spend a significant amount 

of money in the local areas on hotels, restaurants, groceries, ice, gasoline, and supplies. 

The local economic effects of recreation on the 

retail, supporting wholesale, and distribution sectors and 

on induced spending elsewhere in the economy from 

river-based recreation expenditures have been calculated in two studies. The most comprehensive regional 

economics study was by Douglas and Harpman (1995), 

who used data collected in 1985. Table 1 presents the 

average trip expenditures, total trip expenditures, the 

amount spent in the local region (defined by Douglas and 

Harpman as Coconino County, where Page, Arizona, 

is located, and Mohave County), and the resulting total 

employment. The employment effects reflect only visitor 

spending in the region, and the multiplier effects are calculated by using IMPLAN® (see Douglas and Harpman, 

1995). The monetary amounts used in Douglas and 

Harpman (1995) have been updated to 2004 dollars, but 

the employment estimates are calculated directly from 

their data (see Douglas and Harpman, 1995, tables 3 and 

6 using 1990 relationships) and are consistent with calculations developed by Douglas (2005) of 438 total jobs in 

whitewater boating in Grand Canyon National Park. 

As indicated in table 1, the total of nearly 600 jobs 

provided by river-based recreation is a substantial number of jobs. Commercial rafting, particularly the Grand 

Canyon National Park segment, contributes the majority 

of the jobs, 438. 

Unfortunately, there has been little systematic 

comprehensive research that links changes in flows to 

angler use or other visitor use and, hence, to associated changes in economic impacts. There has been one 

study on the effect of the lower summer steady flows on 

economic impacts to the rafting and angling outfitters 

by Hjerpe and Kim (2003). They found that these low 

flows had minor effects, mostly related to damage to 

equipment in Grand Canyon and angler boats in Glen 

Canyon. Day-use rafters in the Glen Canyon reach were 

apparently not affected (Hjerpe and Kim, 2003). The 

direct economic consequences of higher outfitter costs 

because of commercial boat damage were in the range 

of $25,000, and the loss in fishing guide services during 

the spike flows that were part of LSSF was in the range 

of $33,000 (Hjerpe and Kim, 2003). As the result of 

low flows stranding commercial whitewater trips, there 

were also three rescue operations, which cost $30,000. 

Hjerpe and Kim (2003) aggregated these as losses to 

Coconino County for the regional economic analysis. 

The IMPLAN® estimated total effects on Coconino 

County were $117,705 in losses. This estimated total is 

relatively minor to the local economy; however, Hjerpe 

and Kim (2003) acknowledged that they may have mixed 

local economic losses from reduced fishing guide income 

with increases in economic activity from boat repair 

and helicopter rescue. There is a problem with using 

regional economic impact analysis as an indicator of the 

economic effects of changes in flows: higher outfitter 

expenses, which are costs, have the potential to actually 

increase economic activity. Thus, a benefit-cost analysis 

that would treat higher costs to outfitters as the actual 

losses that they are to society would probably be more 

appropriate for monitoring the economic consequences 

of changes in the flow regime. 

© 2005 Geoff Gourley; used with permission
Recreation Use Values and Nonuse Values of Glen and Grand Canyons 161

The comparison of these relatively minor local economic effects from the LSSF illustrates the limitation of 

relying on regional economic analyses: if visitor use does 

not change much because of the National Park Service 

limits on passenger days, then the effects of river flow 

will not be manifested in changes in regional economic 

effects (Hjerpe and Kim, 2003). Rather, any effects 

of river flow changes are more likely to be manifested 

as changes in user satisfaction and use value. Without 

conducting a valuation survey, the most likely economic 

effects of changes in river flow regimes in Grand Canyon 

may be missed. 

Nonuse Values

As noted above, Grand Canyon is also a source 

of nonuse or existence/bequest values to people who 

may never or no longer visit Grand Canyon (Harpman 

and others, 1993). The same is true with the continued 

existence of endangered species, such as the humpback 

chub, in their natural habitat. A given household’s nonuse values for improving river flow management in Glen 

Canyon and Grand Canyon can be obtained by using the 

CVM. To date, there has been only one nonuse CVM 

survey that asked willingness to pay to improve native 

vegetation (and associated birds and other wildlife), 

native fishes, game fish (such as trout), river recreation, 

and cultural sites in Glen Canyon National Recreation 

Area downstream of Glen Canyon Dam and in Grand 

Canyon National Park. This study was conducted in late 

1994 and early 1995 by Welsh and others (1995). 

There were two main sampling strata in the Welsh 

and others (1995) study: a sample called market area, 

reflecting Western U.S. households receiving hydropower 

from Glen Canyon Dam, and then a national sample, 

reflecting the rest of the Nation. These sampling strata 

received different payment vehicles: the market area 

would pay for protection via higher utility bills, while 

the national sample would pay via higher taxes. There 

were three main river-flow scenarios and accompanying 

changes in five main environmental attributes (beaches, 

risk of erosion to cultural sites, vegetation/bird habitat, native fish, and trout). At the time the survey was 

conducted, the main flow regimes under consideration 

involved degrees by which the fluctuating flows previously associated with peaking power production would 

be moderated. The three main flow scenarios were (1) 

moderate fluctuating flows, (2) low fluctuating flows, and 

(3) seasonally adjusted steady flows. While all scenarios 

would maintain beaches at current conditions, decrease 

the erosion risk to cultural sites substantially, and increase 

streamside vegetation by 10%, the steady flow scenario 

was described as providing improvements for native 

fish and trout. 

The response rates in the two sample strata were 

quite high by comparison to most CVM studies in the 

literature. The national sample had an overall response 

rate of 74%, and the marketing area sample had a 

response rate of 83% (Welsh and others, 1995). 

To provide a conservative estimate of nonuse 

values, only those responses that indicated “definitely 

yes” as yes responses to the dichotomous choice CVM 

responses were counted and yielded the values per 

household shown in table 2. Counting only “definitely 

yes” responses has been shown in actual cash validity 

test comparisons to yield a valid measure of actual cash 

willingness to pay (Champ and others, 1997; Ethier 

and others, 2000). 

Table 1. Local economic effects of river recreation in northern Arizona (2004 dollars).

[Source: Douglas and Harpman (1995). The monetary amounts used in Douglas and Harpman (1995) have been 

updated to 2004 dollars, but the employment estimates are calculated directly from their data]

 Glen Canyon National Grand Canyon Total

 Recreation Area National Park

 Day -use Anglers Commercial Private 

 rafting rafting rafting 

Average trip expenditures $115.90 $355.91 $2,711.30 $981.84

Nonresident total expenditures $3,803,374 $3,655,174 $36,542,901 $2,872,876 $46,874,326

Nonresident total expenditures in region $3,803,374 $1,810,320 $15,351,554 $1,195,997 $22,161,245

Total jobs 100 48 406 32 586
162 The State of the Colorado River Ecosystem in Grand Canyon

The steady flow scenario, which was supposed to 

be the most beneficial for fish, had the highest willingness to pay, especially by households living closest to 

Grand Canyon (i.e., the market area sample strata). The 

annual per-household willingness-to-pay values are quite 

reasonable and, yet, when aggregated up to the number 

of households in the population, produce estimates in 

the $3 billion to $4 billion range. Thus, the nonexcludable and nonrival nature of protection of the Grand 

Canyon environment is evident in the fact that modest 

willingness to pay per household across the country adds 

up to a substantial total. In some sense this response is 

not unusual; after all, Grand Canyon is a national park 

and Glen Canyon a national recreation area. In fact, 

Grand Canyon was designated by the United Nations 

Educational, Scientific and Cultural Organization 

(UNESCO) as a World Heritage Site in 1979, suggesting that limiting the aggregation of benefits to just U.S. 

households probably significantly understates the total 

economic value received from Grand Canyon by people 

around the world.

This survey is now nearly 10 yr old, and much 

has been learned in that decade about the response 

of the Grand Canyon environment to changes in flow 

regime, including the decreasing trends in humpback 

chub populations. Several of the ecological responses to 

moderate or steady flows described in the survey, while 

based on the best available science at the time, have 

not been fully realized. As the name “contingent valuation method survey” suggests, the values obtained from 

such a survey are contingent upon the description of the 

environmental effects. Since the actual and anticipated 

environmental effects associated with different flow 

regimes are somewhat different from those envisioned at 

the time of survey, it may be appropriate to update the 

nonuse value survey to more current ecological estimates 

that are associated with different flow regimes. The facts 

that the alternative flow regimes currently being implemented are somewhat different from those described in 

the survey and that quite different management actions, 

such as temperature control devices, are being considered suggest that to keep nonuse valuations current, it 

would be worthwhile to update this survey. The National 

Park Service Organic Act of 1916 has been interpreted 

as a dual mandate of recreation and preservation of 

the national parks (Loomis, 2002). This dual mandate 

suggests that it is insufficient to only measure economic 

values of a national park by recreation use value. 

More recent legislation such as the Redwoods Act—as 

amended in 1978 to the General Authorities Act of 

1970 (16.U.S.C. 1a-1), which governs the National Park 

Service—provides that, when there is a conflict between 

recreation use and preserving the parks unimpaired, 

preservation is the primary objective. The preservation 

value of a national park, therefore, is also reflected in its 

nonuse values. As suggested previously by the National 

Research Council (1999), these nonuse values reflect 

values to nonrecreation stakeholders and should be used 

by managers in decisionmaking. 

Discussion and Future 

Research Needs

Based on past studies, it appears that economic values for natural and cultural resources in Grand Canyon 

and Glen Canyon amount to nearly $40 million of use 

values and several billion dollars worth of nonuse values 

each year. With 12-yr waiting lists to raft Grand Canyon 

National Park and tens of thousands of anglers and 

boaters using Glen Canyon National Recreation Area 

each year, recreation is obviously an important public 

use of these two areas. The now nearly 10-yr-old study 

Table 2. Estimates of nonuse value for three flow scenarios (2004 dollars) (updated from Welsh and others, 1995).

 National sample Market area sample

Flow scenario Per Annual value Per Annual value

 household (millions) household (millions)

Moderate fluctuations $17.06 $2,858 $27.94 $79

Low fluctuations $25.19 $4,219 $27.17 $77

Steady flow $25.69 $4,303 $36.57 $103
Recreation Use Values and Nonuse Values of Glen and Grand Canyons 163

suggested that preservation of natural environments in 

Grand Canyon provides nonuse values (e.g., existence 

and bequest values) to citizens nationwide. Given that 

nonuse values are nonrival goods available to all, the 

rapid population growth in the Southwestern United 

States suggests that nonuse values have probably risen 

substantially as well. 

Unfortunately, studies of the recreation and nonuse 

values are between 10 and 20 yr old and reflect the flow 

regimes in place before the Grand Canyon Protection 

Act of 1992. Recreation use values and nonuse values 

in the current flow regime and how they change with 

experimental flow regimes have not been studied. With 

proposed increases in whitewater rafting use levels in 

Grand Canyon National Park, it is time to begin regularly monitoring recreation satisfaction and use value. 

The Grand Canyon Protection Act specifically lists recreation monitoring, and the omission of systematic recreation use value monitoring has been previously pointed 

out by the National Research Council (1999). Behan 

(2000) provided detailed suggestions on the type of recreation monitoring that is needed. If the spirit of adaptive 

management and the function of the Grand Canyon 

Monitoring and Research Center are to be more fully 

realized, the call in the Grand Canyon Protection Act of 

1992 to monitor recreation needs to be given the same 

consideration that has been given to other resources discussed in this report. If integrated adaptive management 

of all resources is to be performed, then it is important 

to monitor recreation satisfaction and value since recreation is directly affected by flow regimes, changes in sand 

deposits for beaches, and trout size and abundance. For 

example, a better understanding of changes in angler use 

in Lees Ferry could result from an integrated analysis of 

use in relation to catch rate and fish condition class. 

The importance of recreation use and nonuse values 

is becoming more apparent as trade-offs are explored 

between desired flows to recover endangered humpback 

chub and those desired for recreation and hydropower. 

Although the Endangered Species Act of 1973 requires 

that all reasonable and prudent recovery alternatives be 

considered, having economic valuation information on 

recreation, endangered fish, and hydropower may aid in 

finding a suitable balance. Federal oversight agencies such 

as the Office of Management and Budget may require 

benefit-cost information on temperature control devices 

being considered at Glen Canyon Dam to aid recovery of 

humpback chub. Adaptive management requires putting 

monitoring in place prior to new management actions. 

Now is the time to do the same for recreation use economic values so that the intended and unintended effects 

of future management actions can be assessed. 

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Colorado River Storage Project, Arizona—nonuse value study final report: Madison, Wis., Hagler 

Bailly Consulting, Inc., September 8, 1995, 400 p.: 

Springfield, Va.: National Technical Information Service, NTIS no. PB98-105406.

Contact Information:

John Loomis

Professor

Colorado State University

Department of Agriculture and Resource Economics

Fort Collins, CO

John.Loomis@colostate.edu

Aaron J. Douglas

Natural Resource Economist

U.S. Department of the Interior

U.S. Geological Survey

Fort Collins, CO

aaron_douglas@usgs.gov

David A. Harpman

Natural Resource Economist

U.S. Department of the Interior

Bureau of Reclamation

Denver, CO

First page photograph credit: © 2005 Geoff Gourley; used with permission dharpman@do.usbr.gov
Chapter 10

Status and 

Trends of 

Hydropower 

Production at 

Glen Canyon 

Dam

David A. Harpman

Aaron J. Douglas

Introduction

Approximately 7% of the electricity in the United 

States is generated by hydroelectric powerplants (Energy 

Information Administration, 2004, p. 2). These plants 

are an invaluable component of the Nation’s interconnected electric power system in which generation 

resources consist of thermal, nuclear, solar, wind, and 

other sources. Hydroelectric powerplants are characterized by exceptionally low costs of operation, are highly 

reliable, and produce electricity without burning fossil fuels and producing air pollution. In addition, they 

provide voltage control, system regulation, and other 

ancillary services which help ensure the reliability and 

electrical integrity of the system. 

Although they play an important role in the electric 

power system, hydroelectric powerplants, such as the 

one at Glen Canyon Dam, have some widely recognized 

environmental effects. Large hydro facilities have blocked 

the spawning of anadromous and migratory aquatic species, eliminated the downstream transport of sediment, 

fundamentally altered the seasonal hydrograph, affected 

water chemistry, and changed the downstream temperature regime (Collier and others, 1996; Poff and others, 

1997; Van Steeter and Pitlick, 1998 a, b). Furthermore, 

the operation of these plants, particularly those used to 

produce peaking, or variable, power, causes hourly variations in stream flow and elevation, thereby adversely 

affecting downstream aquatic and riparian communities (Nilsson and others, 1997; Parasiewicz and others, 

1998) and recreation (Bishop and others, 1987; Kearsley 

and others, 1994; Welsh and Poe, 1998). The unveiling of plans to construct Glen Canyon Dam spurred 

a nationwide protest (Bradley, 1964; Martin, 1989). 

Construction of the dam started a public environmental 

discourse which continues to this day (McPhee, 1971; 

Elfring, 1990; Brower, 1997; Long and Essick, 1997; 

Jacobs and Wescoat, 2002). 

Glen Canyon Dam

Glen Canyon Dam, shown in figure 1, is a 710-fthigh (216 m) concrete thick arch dam. It is the second 

highest dam in the United States (Hoover Dam is 16 ft 

(5 m) higher). Construction of Glen Canyon Dam began 

on October 1, 1956; Lake Powell started filling on March 
166 The State of the Colorado River Ecosystem in Grand Canyon

13, 1963; and the first electric power was generated on 

September 4, 1964. 

The outlet works at Glen Canyon Dam are composed of four hollow “jet tubes” and two spillways. 

These outlet works are used only under special conditions, primarily to accommodate releases from the dam 

that exceed the amount of water which can be released 

through the powerplant. Such releases may occur when 

the reservoir is full and tributary inflows exceed the 

capacity of the powerplant or they may be ordered for 

environmental purposes such as the 1996 beach/habitatbuilding flow. 

The hollow jet tubes consist of four 96-inch-diameter (244 cm) pipes. The combined release capacity of the 

four hollow jet tubes is 15,000 cubic feet per second (cfs). 

The intake elevation of the jet tubes is approximately 

3,374 ft (1,028 m), or 326 ft (99 m) below the surface 

when the reservoir is full. These elevations are illustrated 

in figure 2.

Each of the two spillways consists of an intake structure with two 40- by 52.5-ft (12- by 16-m) radial gates 

and a concrete-lined spillway tunnel. These spillways 

are located on both sides of the dam, each of which is 

capable of releasing 104,000 cfs when the reservoir is full 

(3,700 ft (1,128 m)). The elevation of the spillway crest 

is 3,648 ft (1,112 m). The spillways cannot be used to 

release water from the reservoir when the lake elevation

falls below 3,648 ft (1,112 m).

Powerplant 

The powerplant at Glen Canyon Dam is made up 

of eight hydroelectric generation units. Since 1964, these 

units have been uprated and rewound several times. As 

of August 2003, the combined generation capability of 

the powerplant (at unity power factor) is 1,320.0 megawatts (MW) (Seitz, 2004). A separate penstock feeds each 

of the eight Francis type turbines, which each produce 

approximately 155,000 horsepower. Current operating rules require at least 40 ft (12 m) of submergence to 

prevent the entrainment of air into the penstocks, which 

would cause damage to the turbines. As a result, the 

powerplant cannot be operated at lake elevations below 

3,490 ft (1,064 m). Each turbine has a release capacity of 

approximately 4,150 cfs when the reservoir is full. The 

nominal powerplant release capacity is approximately 

33,200 cfs. 

Electricity Background 

Electricity cannot be efficiently stored on a large 

scale by using currently available technology. It must be 

produced as needed. Consequently, when a change in 

demand occurs—such as when an irrigation pump or a 

central air conditioner is turned on—somewhere in the 

interconnected power system, the production of electricity must be increased to satisfy this demand. In the 

language of the utility industry, the demand for electricity is known as “load.” Load varies on a monthly, weekly, 

daily, hourly, and even second-by-second basis. During 

the year, the aggregate demand for electricity is highest 

when heating and cooling needs are greatest. During a 

given week, the demand for electricity is typically higher 

on weekdays and lower on weekends, particularly holiday 

weekends. 

The maximum amount of electricity which can be 

produced by a powerplant is called its capacity. Capacity is typically measured in megawatts. The capacity 

of thermal powerplants is determined by their design, 

their location, and the ambient temperature. In the case 

of hydroelectric powerplants, capacity varies over time 

because it is a function of reservoir elevation, the amount 

of water available for release, and the design of the facility. Because the capacity at hydroelectric powerplants is 

highly variable, the amount of dependable or marketable capacity is of particular significance. The amount 

of dependable or marketable capacity is determined by 

using various probabilistic methods (e.g., Ouarda and 

others, 1997).

The large variation in loads has important implications for the electrical generation system. In particular, 

it greatly influences the amount of generation capacity 

required and, therefore, the capital cost of the system. 

The implications of large variation in loads can be 

Figure 1. Aerial view of Glen Canyon Dam (photograph courtesy 

of the Bureau of Reclamation).
Status and Trends of Hydropower Production at Glen Canyon Dam 167

readily illustrated by two extreme cases. First, assume 

the demand for electricity is constant and is 1.0 MW at 

all times. This assumed demand would imply (ignoring 

security and reliability concerns) that a utility supplier 

could supply this demand by building a 1.0-MW powerplant and operating it continuously. For a month (30 

d), this situation would imply generation of 1.0 MW for 

720 h, which would generate 720 megawatthours (MWh) 

of electricity. Now assume that the demand for electricity is more variable: assume that it is 1 MW for 1 h of 

the month and 0.5 MW for the rest of the hours in the 

month. In this case, the costs of constructing a 1-MW 

powerplant must also be incurred, but the plant generates only 360.5 MWh of energy (1 MW * 1 h + 0.5 MW 

* 719 h), or approximately 50% of its potential output. 

The highly variable nature of the demand for electricity 

results in the following observable characteristics of the 

electrical power system: (1) some powerplants are idle 

for part, or all, of the day or season, and (2) the capital 

costs of electricity production are quite high relative to 

operational costs.

Electric energy is most valuable when it is most 

in demand—during the day when people are awake 

and when industry and businesses are operating. This 

period, when the demand is highest, is called the 

“onpeak period.” In the West, the onpeak period is 

typically defined as the hours from 7 a.m. to 11 p.m., 

Monday through Saturday. All other hours are considered to be offpeak.

Hydroelectric Power and the 

Interconnected Power System

The two most commonly encountered types of 

hydroelectric powerplants are run-of-river plants and 

peaking plants. Run-of-river plants typically have little 

water storage capability. Consequently, generation at 

run-of-river plants is proportional to water inflow, resulting in little variation in electrical output during the day. 

Peaking hydroelectric powerplants, such as Glen Canyon 

Dam, often have significant water storage capability and 

are designed to rapidly change output levels to satisfy 

changes in the demand for electricity. Peaking hydroelectric powerplants are particularly valuable because they 

can be used to generate power during onpeak periods, 

thereby avoiding the cost of operating more expensive 

thermal plants such as gas turbine units. 

In addition to furnishing capacity and energy, 

hydroelectric powerplants play an important role in 

the interconnected electric power system by supplying 

ancillary services. They contribute to system reliability 

by furnishing reactive power, voltage support, and system 

regulation services. These facilities also fulfill part of the 

regional reserve requirements and provide backup generation in the event of unexpected outages. In addition, 

they provide extra energy during extremely hot or cold 

weather periods and help maintain transmission stability 

during system disturbances. 

Figure 2. Important operating elevations of Glen Canyon Dam and Lake Powell in feet above mean sea level (msl).
168 The State of the Colorado River Ecosystem in Grand Canyon

The Economic Value

of Hydropower

The economic value of operating an existing hydroelectric powerplant is measured by the avoided cost of 

doing so. In this context, avoided cost is the difference 

between the total power system cost of satisfying the 

demand for electricity “with” and “without” operating 

the hydroelectric powerplant. Conceptually, avoided 

cost is the savings realized by supplying electricity from 

a low-cost hydroelectric power source rather than from a 

higher cost thermal source. These savings arise, in part, 

because the cost of operating a hydroelectric powerplant is relatively low in comparison to thermal units. 

For example, the average operating expense for a typical 

hydroelectric powerplant in 2003 was $7.51/MWh. In 

contrast, the average cost of operating a typical fossil-fuel 

steam plant was $22.59/MWh, and the average cost of 

operating a typical gas turbine unit was approximately 

$48.93/MWh (Energy Information Administration, 

2004, p. 49, table 8.2).

The economic value of operating an existing 

hydroelectric powerplant varies considerably with time 

of day. The variable cost of meeting demand varies on a 

second-by-second basis depending on the load, the mix 

of plants being operated to meet load, and their output 

levels. During offpeak periods, demand is typically satisfied with lower cost coal, run-of-river hydropower, and 

nuclear units. During onpeak periods, the additional load 

is met with more expensive sources such as gas turbine 

units. Consequently, the economic value of hydropower 

is greatest during hours when the demand for electricity, 

and the variable cost of meeting demand, is the highest.

If the cost of purchasing an additional 

megawatthour of electricity from a least cost source were 

observable in the market, then the economic value of 

producing hydroelectricity could be readily determined. 

For example, assume that in a particular hour the cost 

of purchasing a megawatthour of electricity from the 

least cost source was $30 and that the cost of producing a megawatt of hydroelectricity was $6. The avoided 

cost, or economic value, of producing an additional 

megawatthour of hydroelectric power at that time would 

be $24 ($30 - $6). 

In addition to operating costs, the fixed investment 

(or capital) costs of alternative sources of electricity supply may contribute to the economic value or avoided cost 

of an existing hydroelectric powerplant. If a hydroelectric 

powerplant were decommissioned or its operations were 

restricted, the generation capacity in the system would be 

reduced. A new powerplant, probably a thermal plant, 

would need to be constructed to replace this lost capacity. 

If there were initially excess capacity in the system, the 

construction of a new powerplant could be deferred until 

a future date but would be constructed sooner than would 

otherwise be the case. If there were little or no excess 

capacity in the system, the need would be more immediate. All other factors being the same, the time of the 

required replacement capacity has a significant effect on 

the present worth of these additional costs. The sooner 

the replacement capacity is required, the higher the present worth of the costs incurred. The further out in the 

future these construction expenditures occur, the smaller 

the present worth of the costs incurred. 

Federal Power

There is a long history of Federal involvement in 

the provision, operation, and regulation of the electric 

power system. The foundations for this involvement are 

based on three factors: first, the electric power industry is 

a natural monopoly (produces a product most efficiently 

supplied by one supplier in a given area) and hence is 

subject to regulation under the Sherman Antitrust Act 

of 1890, the Interstate Commerce Act of 1887, and 

other applicable statutes; second, the Federal Government owns most of the Nation’s large-scale hydroelectric 

resources; and third, Federal economic development 

programs facilitated the provision of electricity to large 

areas of the rural United States (Energy Information 

Administration, 1996). 

During the Great Depression and the Dust Bowl 

years (ca. 1930s), the Federal Government became much 

more active in the provision and regulation of electricity. This involvement was consistent with the widespread 

belief that electricity should be inexpensive and readily 

available to the public. Federal power- and water-development projects were also promulgated for purposes of 

creating employment, resettling the landless, improving agricultural production, and fostering business and 

industry. A number of large public works projects were 

begun during this period; among these were the construction of Hoover Dam and the construction of Grand 

Coulee Dam, which remains the Nation’s largest hydroelectric facility. A hallmark of the era was creation of the 

Rural Electrification Administration (REA). The Rural 

Electrification Act of 1936 (Public Law 74-605) established the REA to provide loans and assistance to organizations providing electricity to rural areas and towns with 

populations under 2,500. REA-backed cooperatives were 

instrumental in tripling the proportion of rural homes 
Status and Trends of Hydropower Production at Glen Canyon Dam 169

and farms served by electricity between 1932 and the 

entry of the United States into the Second World War in 

1941 (Energy Information Administration, 1996). 

The Bureau of Reclamation constructed and 

operates Glen Canyon Dam. The Western Area Power 

Administration (hereafter Western), an entity established 

by the Department of Energy Organization Act of 1977, 

now markets and transmits the electricity produced by 

the dam. 

In compliance with the Colorado River Storage 

Project (CRSP) Act of 1956, power generated at Glen 

Canyon Dam and other CRSP facilities is first provided 

to CRSP-participating projects. These are typically 

Bureau of Reclamation irrigation projects, and generation is used to meet their pumping needs. Generation 

that is surplus to these “project uses” is then marketed 

by Western to about 200 wholesale power customers 

entitled to preference allocations. These preference 

customers are generally municipal and county utilities, 

rural electric cooperatives, Federal reservations, Indian 

Tribes, and certain other authorized entities (see General 

Accounting Office, 2001, for further details on preference). A list of current preference customers and their 

allocations can be found in Western Area Power Administration (2004a). These preference customers, in turn, 

sell electricity to approximately 1.7 million residential, 

commercial, industrial, and agricultural users located 

predominantly in a six-State region comprising Nevada, 

Utah, Arizona, Wyoming, Colorado, and New Mexico. 

 Each of the preference customers is allocated an 

amount of energy and capacity based on Western’s 

determination of the marketable power resource. The 

most recent such determination, the “Post-2004 Determination of Marketable Resource,” is described in 

Western Area Power Administration (2004b). If these 

preference customers require additional energy and there 

is additional energy available, Western may sell additional power to them on a short-term basis. If energy is 

available beyond the needs of the preference customers, 

Western may exchange energy with other suppliers or 

make sales on the spot market. If generation is insufficient to meet the allocations of preference customers, 

Western must exchange energy with another supplier or 

purchase additional energy on the market.

Western’s rate-setting procedure for power differs 

from that of a non-Federal utility (General Accounting 

Office, 2000). By statute, power must be marketed at 

the lowest possible rates that are consistent with sound 

business practice. Administratively, Glen Canyon Dam 

is located in Western’s Salt Lake City Area/Integrated 

Projects (SLCA/IP) region. The preferred customer rate 

is known as the SLCA/IP rate. The SLCA/IP rate is set 

to ensure that revenues are sufficient to repay all assigned 

costs within a prescribed period. These costs include 

annual power operation and maintenance costs, power 

facility construction costs including interest, certain environmental costs, and other nonpower-related costs that 

power users were assigned by Congress to repay (including irrigation costs that water users are unable to repay). 

The current SLCA/IP (F7) rate is $20.72/MWh. 

The SLCA/IP rates charged by Western are 

designed to recover the taxpayer investment. They are 

lower than current wholesale market rates for electric 

power. Comparisons of the electric power rates are 

relatively difficult because of differences in contractual 

terms, commitment lengths, products provided, and fluctuations in electricity market conditions. Because of these 

complexities, the average revenue per megawatthour of 

wholesale electricity sold is widely used in the industry as 

a metric for comparison purposes. Two studies compared 

Western’s average revenue received per megawatthour 

of wholesale electricity sold against those of other utilities. In a 1994 study, the General Accounting Office 

(1996b, p. 105, table V.1.) estimated Western’s average 

revenue per megawatthour of wholesale power sold to be 

47% of the revenue received by publicly owned utilities and 52% of the revenue received by investor-owned 

utilities. A 1999 assessment by the Energy Information 

Administration estimated Western’s average revenue per 

megawatthour of wholesale power sold to be 42% of the 

average revenue received by utilities in the West (Energy 

Information Administration, 2000, p. 44, table 13).

Monitoring of the

Hydropower Resource

The hydropower resource at Glen Canyon Dam 

is monitored rather intensively. Since the plant’s supervisory control and data acquisition (SCADA) system 

was installed in the early 1980s, it has monitored and 

recorded a voluminous amount of information about 

power production and operations. At each of the eight 

units, these data include generation, release, reservoir 

afterbay water-surface elevations, voltage, frequency, circuit breaker status, gate positions, bearing temperatures, 

transformer settings, and the temperatures and operational status of a variety of appurtenant equipment. 
170 The State of the Colorado River Ecosystem in Grand Canyon

These data are sampled at time increments ranging 

from 1 to 4 s. Selected data are recorded and archived at 

larger time intervals. This level of monitoring is standard 

business practice in the electric power industry.

The Effects of

Environmental Constraints

The single most important determinant of hydropower production and economic value at Glen Canyon Dam is the amount of water released during the 

month. Based on projected hydrologic conditions, 

monthly and annual release volumes for Glen Canyon 

Dam and all major CRSP facilities are established by 

the annual operating plan (AOP) at the beginning of 

the water year (see www.usbr.gov/uc/water/rsvrs/ops/

aop/aop05draft.pdf for an example). Monthly release 

volumes under the modified low fluctuating flow (MLFF) 

operating regime are identical to those under historical 

operations (1963–91) except for water years in which 

an experimental flow occurs. In years when experimental releases occur, monthly release volumes may vary 

substantially from the historical pattern, and there may 

be differences in monthly releases across the entire water 

year. 

The environmental constraints that are part of the 

MLFF operating regime affect hydropower production 

at Glen Canyon Dam to varying degrees under different circumstances. Typically, these constraints are most 

limiting for moderate monthly release volumes and 

less constraining at either very high or very low release 

volumes. The maximum release constraint of 25,000 cfs 

is binding or constraining only when the reservoir elevations and monthly release volumes are sufficiently high 

to permit releases of 25,000 cfs or greater. The upramp 

rate of 4,000 cfs/h and the downramp rate of 1,500 

cfs/h limit the hour-to-hour ability of the powerplant to 

respond to changes in load the majority of the time. At 

extreme high and low monthly release volumes, these 

ramp-rate restrictions have less effect. The maximum 

daily change in flow constraint limits the 24-h change 

in flow to 5,000; 6,000; or 8,000 cfs depending on the 

monthly release volume. This constraint greatly reduces 

the ability of the powerplant to respond to load changes 

within any given 24-h period. At very high and very low 

release volumes, the maximum daily change constraint 

has less of an effect on hydropower operations. Readers 

wishing to explore the effects of the MLFF constraints 

at Glen Canyon Dam in conjunction with different 

monthly release volumes and reservoir elevations can do 

so by using an interactive computer model developed by 

Harpman (2002). A rigorous mathematical exposition of 

hourly hydropower models is presented in Edwards and 

others (1999). A Microsoft Excel spreadsheet example of 

such a model is described in Edwards (2003). 

The relative effects of the hourly constraints at 

Glen Canyon Dam and changes in monthly release 

volumes are illustrated in figure 3, which is constructed 

from Harpman (1997, 1999b) and from unpublished 

results. The interim low fluctuating flow (iLFF) operation regime (interim operating criteria elsewhere in 

this report) was a precursor to the MLFF. The iLFF 

maximum release constraint was 20,000 cfs, and the 

iLFF upramp rate was 2,500 cfs. Otherwise, these two 

operational regimes are identical. Figure 3 compares the 

monthly economic value of hydroelectricity produced 

at Glen Canyon Dam in water year 1996 under three 

conditions: historical operations of Glen Canyon Dam, 

operation under the iLFF, and operation under the iLFF 

with the beach/habitat-building flow (BHBF) of 1996. 

Although the 1996 BHBF experiment started on March 

22 and ended on April 8, to facilitate this 7-d highrelease experiment, changes in water-release volumes 

were required over much of the water year. As shown in 

figure 3, the changes in monthly release volumes necessitated by the 1996 BHBF created a significant effect. 

Relative to historical operations, the economic effects of 

the iLFF hourly constraints are less pronounced.

The Costs of

Environmental Constraints

Relative to historical operations, the MLFF hourly 

environmental constraints on hydropower operations (see 

Overview, this report) have both short-run and long-run 

effects. In the short run, the MLFF reduces the maximum generation ability of the powerplant to respond to 

changes in load. As a result, more of the load must be 

met by other generators in the system, typically thermal 

generators. Since operation of thermal powerplants is 

more expensive than hydropower, additional costs are 

incurred. In the long run, new or replacement powerplants are needed earlier than would otherwise be the 

case. Increased capital costs associated with the construction of new and replacement powerplants may be 

substantial.

A number of economic analyses of changes in the 

operation of Glen Canyon Dam have been undertaken. 

Only three of these contain analyses of the MLFF alternative. Of these, only one study contains an estimate of 
Status and Trends of Hydropower Production at Glen Canyon Dam 171

both the short-run and long-run power system impacts 

of the MLFF. The Bureau of Reclamation Power 

Resources Committee (PRC) estimated the long-run cost 

of several alternative operating regimes at Glen Canyon 

Dam by using a production expansion model. Using 

regression analysis to interpolate between modeled alternatives, the PRC estimated that the annualized economic 

cost of changing from historical operations to the MLFF 

was $36.1 million (annualized value, 1996 dollars) per 

year. Because of excess capacity in the system, most of 

these costs were projected to be incurred late in the 50-yr 

analysis period (Power Resources Committee, 1995). 

Two very detailed short-run studies of MLFF effects 

at Glen Canyon have been undertaken to date. These 

studies differ in conceptual approach and intended application. To correctly interpret their results, it is important 

to distinguish between the approaches they employ. 

A study by Harpman (1999a) estimated the shortrun economic effect of the MLFF relative to historical 

operations. This effect is conceptually equivalent to the 

difference between the historical line and the iLFF line 

in figure 3. The purpose of this study was to estimate the 

economic cost to all power users in the interconnected 

system. Using an hourly constrained nonlinear optimization model and spot market prices, Harpman estimated 

that the economic cost of the MLFF was $6.173 million 

(1996 dollars) for a representative 11.3 million acre-feet 

(maf) (13,933 million m3

) water year. 

A short-run, ex post study of the 2000 low steady 

summer flow (LSSF) experiment was undertaken by 

Palmer and others (2004). Their approach was to estimate the difference in hydropower value between the 

MLFF and the 2000 LSSF relative to MLFF operations 

in the absence of the LSSF. The approach employed 

is conceptually equivalent to estimating the difference 

between the BHBF and the iLFF in figure 3. The goal of 

the Palmer and others (2004) study was to estimate the 

ex post financial cost of the LSSF to Federal power users, 

a subset of the power users in the interconnected system. 

Early in water year 2000, additional water was released 

from Lake Powell to create storage space in advance of 

expected spring inflows and to facilitate low steady flows 

during the summer experiment period. Because of the 

onset of the drought, approximately 605,000 acre-feet 

(af) (745,965,000 m3

) of this water was not replaced by 

inflows during water year 2000. Although some of the 

inflow deficit was subsequently recovered as of December 2004, a 228,000-af (281,124,000 m3

) storage deficit 

remains, and the reservoir elevation is 2.9 ft (0.9 m) lower 

than it would have been without the LSSF (Thomas 

Ryan, Bureau of Reclamation, written commun., 2005). 

Technically, the effects of the LSSF experiment are 

still ongoing. In order to complete their study, Palmer 

and others (2004) were forced to make several assumptions about when this inflow deficit would be recovered 

and the LSSF experiment would be concluded. Using 

these assumptions, observed prices, and an hourly linear 

programming model, Palmer and others (2004) estimated 

that the ex post cost of the LSSF experiment to Federal 

power users was approximately $32 million (2000 dollars).

Status and Trends in 

Hydropower Production

The average release at Glen Canyon Dam from 

water year 1978 through 1999 was approximately 10.93 

maf (13,477 million m3

). Since the onset of the drought 

in 2000, releases have been much lower than average, 

and hydropower production has fallen annually. In water 

year 2000, the annual release was 9.38 maf (11,566 

million m3

). The annual release from 2001 to 2004 has 

reflected the minimum objective release of approximately 8.23 maf (10,148 million m3

). As illustrated in 

figure 4, diminished inflows to Lake Powell combined 

with this nearly constant annual release have resulted in 

markedly lower reservoir elevation levels. 

The average annual hydropower production from 

1978 to 1999 was approximately 5,196,113 MWh. As 

shown in table 1, in recent years the production of 

hydropower at Glen Canyon Dam has been considerably below average. Generation has diminished since 

the onset of the drought in 2000. Although the annual 

Figure 3. Comparison of the monthly economic value in water 

year 1996 of hydroelectricity produced at Glen Canyon Dam 

under different operating regimes (interim low fluctuating 

flows, iLFF; beach/habitat-building flow, BHBF; and historical 

operations, 1963–91).
172 The State of the Colorado River Ecosystem in Grand Canyon

release over the last 4 yr has been approximately the 

same (8.23 maf (10,148 million m3

)), decreasing head, 

or the distance water falls, has adversely affected annual 

generation. In 2001, approximately 3,940,247 MWh 

were generated. By water year 2004, only 3,320,196 

MWh were generated. 

Water year 2004 unregulated inflows to Lake Powell 

were approximately 51% of the 30-yr average (1971–

2000). By the end of December 2004, the elevation of 

Lake Powell had fallen to 3,564.6 ft (1,086.5 m), and 

reservoir contents were approximately 8,678.0 thousand acre-feet (kaf) (10,699,974 thousand m3

), or 36% 

of capacity. This decrease in storage capacity is 135.4 

ft (41.3 m) below full pool and 74.6 ft (22.7 m) above 

the minimum elevation necessary for power generation. 

These conditions last occurred in 1969 when the reservoir was being filled. Hydropower generation at Glen 

Canyon Dam is a function of both the amount of water 

released through the turbines and the head (for example, 

see Harpman, 2002, appendix 5). 

As the reservoir elevation has fallen, the head available for hydropower production has declined, and this 

decrease has adversely affected generation capacity. 

When the reservoir is full (elevation 3,700 ft (1,128 m)), 

at a release level of 25,000 cfs the generation capability 

at Glen Canyon Dam is approximately 1,017 MW. At 

an elevation of 3,564.6 ft (1,086.5 m) and a release level 

of 25,000 cfs, the generation capacity at Glen Canyon 

Dam is approximately 749 MW, a reduction of 268 

MW or approximately 26.4% (John Brooks, Bureau of 

Reclamation, written commun., 2005). 

The Basin Fund

The Colorado River Storage Project, authorized by 

the CRSP Act of 1956 (Public Law 84-485), is a program to develop, and make available for use, the water 

resources of the upper Colorado River Basin. The CRSP 

is composed of 4 multipurpose storage units—Flaming 

Gorge Dam, Wayne N. Aspinall Unit, Navajo Unit, and 

Glen Canyon Dam, often referred to as the “mainstem 

units”—and 21 authorized participating projects in the 

States of Colorado, New Mexico, Utah, and Wyoming.

Financial exchanges necessary to the operation 

of the CRSP are facilitated by the Upper Colorado 

River Basin Fund (Basin Fund). The Basin Fund was 

established by section 5 of the CRSP Act. The Basin 

Fund is a financial instrument that obviates the need for 

congressional actions to pay for project operation and 

maintenance. It is a dedicated Treasury account for the 

deposit of project revenues, which would otherwise be 

deposited in the general fund, and a source of funds for 

the payment of project expenses, which would otherwise 

need to be appropriated. Existence of the Basin Fund 

greatly streamlines the financial operations of CRSP and 

participating projects. 

As described in the CRSP Act, all revenues collected in connection with the operation of the CRSP 

and participating projects are credited to the Basin Fund. 

Revenues are available, without further appropriation 

for defraying the cost of operation, maintenance, and 

replacements of and emergency expenditures for all 

CRSP facilities, with the exception that for participating 

projects, such costs are paid for with revenues generated from that project. Funds in excess of those needed 

for project operation and maintenance needs, certain 

environmental programs, and assigned costs of the 

salinity control program are paid annually to the general 

Figure 4. Lake Powell end of month (EOM) elevation in feet 

above mean sea level (msl).

Table 1. Average and recent net hydropower 

production at Glen Canyon Dam.

Water year Net generation (MWh)

1978–99 average 5,196,113 

2000 4,600,453

2001 3,940,247

2002 3,772,544

2003 3,518,297

2004 3,320,196
Status and Trends of Hydropower Production at Glen Canyon Dam 173

fund of the Treasury to repay CRSP investment costs 

with interest. Funds in excess of those requirements are 

then apportioned to each of the upper Colorado River 

Basin States to help defray investment costs associated 

with developing the irrigation components of the 21 

authorized participating projects. There are a number 

of complex provisions, clauses, and details associated 

with the CRSP Act, the Basin Fund, and project repayment, but they are all outside the scope of this chapter. A 

description of these aspects of the CRSP Act and their 

implications for the recovery of the taxpayer investment 

can be found in General Accounting Office (1996a).

As dictated by prudent business practice, the 

SLCA/IP power rate is set at a level sufficient to meet 

operational and repayment needs while accommodating 

foreseeable variations in generation and resultant revenue. Sometimes unforeseen, adverse hydrologic conditions occur, necessitating greater than expected replacement power purchases. Purchases of replacement power, 

in excess of revenues, are made with funds from the 

Basin Fund. If adverse hydrologic conditions continue, 

the Basin Fund may be depleted and an increase in the 

SLCA/IP rate would be required. Conversely, favorable 

hydrologic conditions can result in greater than expected 

revenues from hydropower sales; these additional revenues are deposited into the Basin Fund. Funds in excess 

of those needed for project operations are treated as 

described in the CRSP Act. If favorable hydrologic 

conditions persist for a number of years, repayment on 

investment is accelerated, and a downward adjustment to 

the SLCA/IP rate may be initiated. 

The status of the Basin Fund fluctuates monthly, 

thereby reflecting the timing of project revenues and 

expenditures. Exclusive of replacement power purchases, 

about $95 million is needed to fund CRSP project 

operational needs on an average annual basis. The vast 

majority of these revenues are derived from hydropower 

sales in the CRSP system. Since 2000, extensive and 

unplanned purchases of replacement power have been 

required because of the drought. These purchases have 

drawn down the cash reserve balance in the Basin Fund. 

In mid-January 2005, the Basin Fund held approximately $50 million (David Taylor, Western Area Power 

Administration, oral commun., 2005).

In addition to replacement power purchases, the 

Basin Fund is used to meet CRSP payroll and other 

operational and maintenance expenses. Certain environmentally related expenditures, including the costs of 

the Glen Canyon Dam Adaptive Management Program, 

are also made from the Basin Fund. To the extent that 

the Basin Fund is depleted during a period of adverse 

hydrologic conditions, funding for all of these purposes 

could be jeopardized. Western has taken two steps to 

bolster CRSP revenues and improve the status of the 

Basin Fund. First, Western has lowered their determination of marketable power resource (Western Area 

Power Administration, 2004b). The lower determination 

reduces the amount of replacement energy that they are 

required to purchase and shifts more of the risk of being 

energy-short in times of adverse hydrologic conditions to 

power users. Second, Western has initiated a substantial 

rate-increase action (Western Area Power Administration, 2005). As proposed, the new rate is $25.77/MWh, 

a 24.4% increase over the current rate. Western’s rate 

proposal also includes a provision for cost recovery 

charge, which can be implemented when revenue shortfalls are projected. These steps will increase the amount 

of revenue derived from hydropower sales and bolster 

the position of the Basin Fund.

The Grand Canyon Protection Act of 1992 contains special provisions establishing and funding the 

Glen Canyon Dam Adaptive Management Program. 

Section 1805 of the act establishes the long-term monitoring program now carried out by the Glen Canyon 

Dam Adaptive Management Program, and section 1807 

of the act declares expenditures for this program to be 

nonreimbursable. These provisions shift the burden of 

paying these costs from project beneficiaries, such as 

water and power users, to taxpayers in general. Expenditures by the Glen Canyon Dam Adaptive Management 

Program are drawn from the Basin Fund. These payments are then credited towards project repayment, 

thereby reducing the repayment obligation of all project 

beneficiaries. As a result, these environmentally related 

expenditures are indirectly paid for by all taxpayers in 

the United States.

Outlook for the Future

In the years to come, the amount of hydropower 

generated at Glen Canyon Dam is largely dependent on 

future hydrologic conditions. While future hydrologic 

conditions can never be known with certainty, probabilistic forecasts can provide some important insights. The 

Bureau of Reclamation uses the RiverWareTM modeling 

system (Zagona and others, 2001) for CRSP multiyear 

planning studies. These multiyear planning studies 

employ the indexed sequential method (described in 

Ouarda and others, 1997) for simulating future hydrologic conditions. RiverWareTM modeling runs completed 

in January 2005 use December 2004 CRSP end-ofmonth reservoir elevations as starting conditions and 
174 The State of the Colorado River Ecosystem in Grand Canyon

simulate monthly operations for 90 different monthly 

inflow sequences, each of which is 90 yr long. Statistical 

analysis of these simulations suggests that the median 

time required to refill Lake Powell (achieve an elevation of 3,695 ft (1,126 m) in July) is approximately 19 yr 

(Thomas Ryan, Bureau of Reclamation, written commun., 2005). This evidence suggests that several decades 

may pass before hydropower production at Glen Canyon 

returns to the level of the late 1990s.

The scheduled replacement of the turbine runners at Glen Canyon Dam will increase hydropower 

generation from 1% to 7% (John Brooks, Bureau of 

Reclamation, written commun., 2005). Turbine runners are the large fan-like blades turned by the force of 

water falling through the penstocks. The existing turbine 

runners at Glen Canyon Dam have reached the end of 

their design life and are now scheduled for replacement. 

Improvements in runner technology, design, and fabrication methods now allow for improvements in efficiency 

(more electricity produced for a given amount of water 

released), greater generation capacity (higher generation level for a given head and water release level), or 

both. Analysis of alternate turbine runner designs, their 

costs, and benefits is now underway. Installation of new 

turbine runners on two of the generation units at Glen 

Canyon Dam is expected to begin within 2 yr, and all of 

the turbine runners are expected to be replaced within 

the next 10 yr. 

The potential installation of temperature control 

devices (TCDs) at Glen Canyon Dam is expected to 

reduce generation by less than 1% (Bureau of Reclamation, 1999). Thermal and chemical stratification develops 

in Lake Powell during the summer months. The penstock 

intakes are located at an elevation of 3,476 ft (1,059 m) 

and are typically within the cold, hypolimnetic strata (see 

chapter 4, this report). As a result, releases from Glen 

Canyon Dam remain at about 50oF (10oC) all year long. 

Native fish populations persist at these relatively low 

temperatures, but it is believed that their spawning and 

rearing success is drastically reduced (see chapter 2, this 

report). The Bureau of Reclamation is studying the feasibility of installing TCDs at Glen Canyon Dam (Bureau 

of Reclamation, 1999). The purpose of these TCDs is to 

allow for the management of downstream temperatures 

to benefit existing and remnant populations of native 

fish. Current plans call for installation of TCDs on two 

of the eight generation units at Glen Canyon Dam. 

Given current drought conditions, the outlook for 

hydropower production at Glen Canyon Dam is somewhat bleak in the near term; however, conditions are 

expected to improve in the future. Easing of the current drought and subsequent gradual improvements in 

hydrologic conditions in the Colorado River Basin are 

expected to refill reservoirs and increase the amount of 

hydropower that can be generated at Glen Canyon Dam 

and other CRSP units. At any given reservoir elevation

and release, the planned installation of new turbine 

runners will result in an increase in hydropower generation. Although the amount of CRSP generation has 

been reduced by the drought in recent years and CRSP 

rates are slated to increase, the hydroelectric energy 

produced at Glen Canyon Dam has been, and continues 

to be, one of the lowest cost sources of electric energy 

available in the West.

Acknowledgments

This effort was supported by the U.S. 

Geological Survey, Grand Canyon Monitoring and 

Research Center, interagency agreement DCN5IA93953031. The assistance of Michael W. Loring, 

regional economist, Bureau of Reclamation, Salt Lake 

City, Utah, is gratefully acknowledged.

References

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Circular 1126.

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update. DOE/EIA-D562 (96): Washington, D.C., 

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Report no. GAO-01-373: Washington, D.C., U.S. 

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Harpman, D.A., 1997, Glen Canyon Dam beach/habitat-building test flow—an ex post analysis of hydropower cost. U.S. Department of the Interior, Bureau 

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National Technical Information Service, NTIS no. 

PB97-159321, 53 p.

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Harpman, D.A., 1999b, The economic cost of the 

1996 controlled flood, in Webb, R.H., Schmidt, J.C., 

Marzolf, G.R., and Valdez, R.A., eds., The controlled 

flood in Grand Canyon: Washington, D.C., American 

Geophysical Union, Geophysical Monograph Series, v. 

110, p. 351–357.

Harpman, D.A., 2002, Understanding hourly operations at Glen Canyon Dam, Colorado River Storage 

Project, AZ-GCPSE95 version 2.0. U.S. Department 

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resources: lessons from Glen Canyon Dam: Environment, v. 44, no. 2, p. 8–19.

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Effects of Glen Canyon Dam on sand deposits used as 

river campsites in the Grand Canyon of the Colorado 

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117–135.

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Canyon and the struggle for the soul of the West (1st 

ed.): New York, Henry Holt & Company.

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York, Farrar, Straus & Giroux.
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responses of river-margin vegetation to water-level 

regulation: Science, v. 276, no. 5313, p. 798–800.

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1997, Indexed sequential hydrologic modeling for 

hydropower capacity estimation: Journal of the 

American Water Resources Association, v. 33, no. 6, p. 

1337–1349.

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financial impacts of the low summer steady flow 

experiment at Glen Canyon Dam: Salt Lake City, 

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Power Administration, 32 p.

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effect of managed hydropower peaking on the physical 

habitat, benthos, and fish fauna in the River Bregenzerach in Austria: Fisheries Management and Ecology, 

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paradigm for river conservation and restoration: BioScience, v. 47, no. 11, p. 769–784.

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impacts of potential changes in Glen Canyon power 

plant operations. Phase III final report: Denver, Colo., 

Stone and Webster Management Consultants, Inc. 

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Service, NTIS no. PB96-114004, 305 p.

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Denver, Colo., Bureau of Reclamation, Technical 

Service Center.

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Colorado River 1. Historic changes in streamflow, 

sediment load, and channel morphology: Water 

Resources Research, v. 34, no. 2, p. 287–302.

Van Steeter, M.M., and Pitlick, J., 1998b, Geomorphology and endangered fish habitats of the upper 

Colorado River 2. Linking sediment transport to 

habitat maintenance: Water Resources Research, v. 34, 

no. 2, p. 303–316.

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in contingent valuation: comparisons to a multiple 

bounded discrete choice approach: Journal of Environmental Economics and Management, v. 36, no. 1, 

p. 170–185.

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report 2003: U.S. Department of Energy, Western 

Area Power Administration.

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69, no. 98, Thursday, May 20, 2004, p. 29135–29136.

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Contact Information:

David A. Harpman

Natural Resource Economist

U.S. Department of the Interior

Bureau of Reclamation

Denver, CO

dharpman@do.usbr.gov

Aaron J. Douglas

Natural Resource Economist

U.S. Department of the Interior

U.S. Geological Survey

Fort Collins, CO

aaron_douglas@usgs.gov

First page photograph credit: Bureau of Reclamation
Chapter 11

Cultural 

Resources in 

the Colorado 

River Corridor 

Helen C. Fairley

Introduction

Cultural resources along the corridor of the 

Colorado River include archaeological sites and other 

types of historic properties, as well as resources that are 

of traditional concern to Native American peoples such 

as springs, landforms, sediment and mineral deposits, 

native plant concentrations, and various animal species. 

All of these resources have the potential to be affected 

indirectly, and in some cases directly, by the operations 

of Glen Canyon Dam. 

The principal cultural resource goal of the Glen 

Canyon Dam Adaptive Management Program is to 

“preserve, protect, manage and treat cultural resources 

for the inspiration and benefit of past, present and 

future generations” (Glen Canyon Dam Adaptive 

Management Program, 2001). The National Park 

Service (NPS) goal for managing archaeological and 

historic resources in the Colorado River corridor is inplace preservation with minimal impact to the integrity 

of the resources. When in-place preservation is not possible, the NPS and other Federal agencies consider data 

recovery through excavation of archaeological remains 

to be an appropriate alternative in certain cases. The six 

Native American tribes who actively participate in the 

Glen Canyon Dam Adaptive Management Program and 

have long-standing traditional ties to the Grand Canyon 

region—Hopi Tribe, Hualapai Tribe, Kaibab Band of 

Paiute Indians, Paiute Indian Tribe of Utah, Navajo 

Nation, and Pueblo of Zuni—are generally supportive 

of in-place preservation goals for cultural resources, 

but they have widely varying opinions regarding the 

appropriateness of undertaking intervention measures to 

mitigate dam and visitor impacts, such as installing check 

dams to control erosion or conducting excavations to 

recover information from archaeological sites. 

This chapter describes research, monitoring, and 

mitigation activities during the past 15 yr that have 

evaluated and addressed ongoing impacts to cultural 

resources in the Colorado River corridor because of 

dam operations and other agents of deterioration, such 

as visitation and rainfall-induced erosion. The chapter begins with a summary of research and inventory 

activities prior to the early 1990s, which is followed by 

a summary of the monitoring and research activities 

initiated in response to the Operation of Glen Canyon 

Dam Final Environmental Impact Statement (EIS) and 

the Secretary of the Interior’s Record of Decision (ROD) 
178 The State of the Colorado River Ecosystem in Grand Canyon

(U.S. Department of the Interior, 1995, 1996). The chapter ends with some recommendations for the future. 

Background

Current information concerning cultural resources is 

based on a number of previous investigations within the 

Colorado River corridor in Glen and Grand Canyons. 

Comprehensive overviews of previous investigations are 

included in Ahlstrom and others (1993), Fairley and others (1994), and Fairley (2003). These studies documented 

evidence of human occupation in the Grand Canyon 

region extending back as far as the Paleo-Indian period, 

at least 11,000 yr before present. Starting around 4,500 

yr ago, petroglyphs, spear points, and uniquely crafted 

artifacts known as “split twig figurines” were placed at 

various locations along and near the Colorado River 

in Glen and Grand Canyons, indicating intermittent 

use of the area by Late Archaic hunters and gatherers. 

Sparse and somewhat controversial evidence for use of 

the canyon by early farming cultures before 1000 B.C. 

is present in eastern Grand Canyon (Davis and others, 

2000); however, the best documented and most intensive use of Grand Canyon by farmers occurred during 

the 11th and early 12th centuries A.D., during a time 

period known as Pueblo II. During this time, people with 

ancestral ties to modern Puebloan cultures built numerous small masonry dwellings, irrigation ditches, erosion 

control features, and granaries for storing corn, squash, 

and cotton throughout Grand Canyon (fig. 1). For reasons not yet fully understood (but that are likely due at 

least in part to climate-induced stressors), these ancestral 

Puebloan farmers moved away from Grand Canyon 

around the end of the 12th century, although small 

groups continued to visit the canyon for seasonal hunting, plant gathering, trading, and ceremonial pilgrimages 

into the first decades of the 20th century. Following the 

departure of the ancestral Puebloan occupants, ancestors 

of the Hualapai, Havasupai, and Southern Paiute moved 

into the region. Numerous campsites and food processing areas dating to the late prehistoric (A.D. 1300–1540), 

proto-historic (A.D. 1540–1776), and early historic (A.D. 

1776–1850) periods testify to their extensive, and at 

times intensive, seasonal use of inner Grand Canyon. 

Descendants of these Yuman and Numic speakers were 

still residing in and around Grand Canyon when the 

first European settlers arrived in the region in 1848, 

and Havasupai, Hualapai, and Southern Paiute people 

continue to live in and near Grand Canyon to this day. 

After 1880, however, as Euro-Americans became increasingly enamored with the spectacular scenery and economic potential of the region, the archaeological record 

becomes increasingly dominated by the material remains 

of Euro-American miners, trappers, homesteaders, government explorers, and tourists (fig. 2).

Previous Cultural Resource 

Research and Inventories 

Euro-Americans first noted archaeological remains 

in the river corridor during the Powell expeditions 

of 1869 and 1871–72 (Powell, 1875). Powell and his 

crew found traces of previous human occupation in 

the canyon in the vicinity of the Little Colorado River, 

Unkar Creek, Bright Angel Creek, and Shinumo Creek. 

In the 1950s and 1960s, investigations of archaeological remains in the river corridor became more focused 

under the direction of the NPS, in part because of 

anticipated dam developments in Grand Canyon (Taylor, 

1958; Euler, 1967a). In the late 1960s and early 1970s, 

researchers affiliated with the School of American 

Research, New Mexico, and Prescott College, Arizona, 

conducted surveys and excavations in the river corridor and adjacent areas to investigate prehistoric settlement patterns (Schwartz, 1965; Euler and Taylor, 1966; 

Euler, 1967b; Schwartz and others, 1979, 1980, 1981). 

Figure 1. Pottery dating to the Pueblo II period is commonly 

found at archaeological sites along the Colorado River. These 

formerly buried pot sherds have become exposed by wind 

(photograph by Amy Draut, U.S. Geological Survey).
Cultural Resources in the Colorado River Corridor 179

Together, these studies provided the foundational information concerning the numerous and diverse cultural 

resources existing within the river corridor.

In 1984, NPS archaeologists conducted test excavations at five archaeological sites along the Colorado 

River in advance of their stabilization (Jones, 1986). 

Three of these sites were deteriorating primarily because 

of visitor use, one was deteriorating because of both 

human and natural impacts, and one was being damaged by erosion from a side channel, all resulting in the 

need for stabilization measures. At all of these sites, excavations revealed the presence of subsurface stratigraphic 

cultural deposits extending back many centuries earlier 

than surface evidence indicated. For example, at one site 

(AZ B:10:4) near Deer Creek, a buried roasting feature 

produced calibrated radiocarbon dates ranging between 

A.D. 610 and 380 B.C., whereas the overlying surface 

structure contained ceramics dating to the Pueblo I-II 

period (about A.D. 800–1150) (Jones, 1986, p. 105). At 

another site (AZ B:16:1) near Whitmore Wash, surface 

materials of Southern Paiute affiliation were found in 

association with a fire pit that was radiocarbon dated to 

A.D. 1230–1340, overlying a roasting feature with a calibrated radiocarbon age of 1365–905 B.C. (Jones, 1986, 

p. 51). This project was important for highlighting the 

presence of deeply buried cultural deposits at numerous 

archaeological sites in the river corridor (fig. 3). These 

older, underlying deposits are often invisible on the surface because floods, slope wash, and aeolian (wind) processes have deposited sediment on top of earlier cultural 

remains, obscuring them from view. 

In 1990–91, an intensive archaeological inventory was conducted by NPS archaeologists (Fairley 

and others, 1994) in preparation for the writing of 

the EIS (U.S. Department of the Interior, 1995). 

This inventory located 475 sites within the assessed 

area, which extended from Glen Canyon Dam to 

Separation Canyon, about 255 RM, and up to the 

estimated 300,000 cubic feet per second (cfs) flood 

level. The sites ranged in type from isolated hearths and 

dispersed lithic scatters to complex multiple-component 

habitations, many with associated roasting features or 

masonry structures. The sites ranged in age from Late 

Archaic, about 2500–1500 B.C., to the mid-20th century. 

Many sites date to the ancestral Puebloan occupation 

between A.D. 950 and 1200, while many other sites are 

affiliated with the ancestral Pai and Paiute use of Grand 

Canyon from about A.D. 1250 to 1870. 

Of the sites within the surveyed area, approximately 

336 were considered to be situated within the area of 

potential effect from dam operations, and many of these 

sites had identifiable impacts that were believed to be 

related to dam operations (Fairley and others, 1994, p. 

148). Dam-related impacts were categorized as direct, 

indirect, or potential. Direct impacts included sites 

where inundation or bank cutting from dam-controlled 

Figure 2. The remains of Bert Loper’s boat came to rest on 

the banks of the Colorado River in 1948, shortly after Loper 

drowned in 24 and 1/2-mile Rapid. This boat is an example of the 

varied historic artifacts found along the Colorado River in Grand 

Canyon (photograph by Jeff Sorensen, Arizona Game and Fish 

Department).

Figure 3. The curved masonry wall of a deeply buried prehistoric 

structure was uncovered during excavations at site AZ C:13:10 in 

April 1984. No evidence of this structure was visible on the site 

surface prior to excavation (photograph by Helen Fairley, U.S. 

Geological Survey).
180 The State of the Colorado River Ecosystem in Grand Canyon

river flows had occurred within the site in recent years. 

Indirect impacts included (1) bank slumpage or slope 

steepening from river flows immediately adjacent to the 

site, (2) arroyo cutting or other erosion phenomena tied 

to the effects of dam-controlled flows, and (3) effects of 

visitor impacts at sites because of changes in recreational 

use patterns related to recent dam operations. Potentially impacted sites included all those located within the 

estimated area of inundation from a 300,000-cfs flood. 

This flood level reflected the former estimated volume of 

the highest historical flood on record (in 1884) (Hereford 

and others, 1993; cf. Topping and others, 2003, p. 31) 

and also the maximum release level possible from Glen 

Canyon Dam, estimated at 256,000 cfs, combined with 

a hypothetical 40,000-cfs flood event from the Little 

Colorado River and other tributary streams. 

Of the 336 sites considered to be within the area of 

potential effect, 33 showed evidence of direct impacts, 

138 revealed evidence of indirect impacts, and 238 were 

categorized as potentially impacted based on their location below the estimated 300,000-cfs level or based on 

their location in or on unconsolidated Holocene sedimentary deposits (Fairley and others, 1994, p. 148). (The 

impact numbers exceed total number of sites because 

many sites exhibited more than one category of impact.)

Participating Native American tribes also conducted 

cultural resource inventories in the early to mid-1990s 

to identify resources with important traditional cultural values, including but not limited to prehistoric 

Native American archaeological sites. These studies 

were conducted by the Hopi Tribe, the Hualapai Tribe, 

the Navajo Nation, the Southern Paiute Consortium 

(comprising the Kaibab Band of Paiute Indians and the 

Paiute Indian Tribe of Utah), and the Pueblo of Zuni 

(Stoffle and others, 1994, 1997; Hart, 1995; Roberts 

and others, 1995; Ferguson, 1998; Stevens and Mercer, 

1998). Numerous locations of cultural importance were 

identified and evaluated by the individual tribes, including areas with culturally important biological resources, 

significant landscape features, mineral locations, and 

specific archaeological resources. Assessments were 

conducted by the tribes to identify potential impacts 

resulting from dam operations and to formulate possible 

treatment options. These studies have subsequently been 

used by the Bureau of Reclamation for the identification 

and preliminary evaluation of traditional cultural properties within the area of potential effect as defined by the 

Programmatic Agreement for Cultural Resources (see discussion below) (U.S. Department of the Interior, 1995, p. 

Att-23); however, studies to formally define and evaluate 

traditional cultural properties have yet to be completed.

Monitoring and Research

Monitoring of Cultural Resources 

Before the 1990s, the NPS annually monitored a 

sample of archaeological sites in the Colorado River 

corridor. These initial monitoring efforts focused primarily on sites prone to impacts from visitors. After the comprehensive inventory of the river corridor was completed 

in 1991, the NPS expanded its monitoring program to 

encompass the full suite of resources located within the 

hypothetical area of potential effect from dam operations. In the mid-1990s, the Southern Paiute Consortium 

and the Hualapai and Hopi Tribes also initiated their 

own monitoring programs to track changes in resource 

conditions at culturally important locations in the river 

corridor (Stoffle and others, 1995; Ferguson and others, 

1997).

Since 1994, monitoring of historic properties that 

are eligible to be listed in the National Register of 

Historic Places—nationally, regionally, and locally significant prehistoric and historic sites, structures, objects, 

and places of traditional cultural importance—has 

been conducted under the auspices of a Programmatic 

Agreement for Cultural Resources. The agreement exists 

between the Bureau of Reclamation, NPS, Advisory 

Council on Historic Preservation, Arizona State Historic 

Preservation Office, and six affiliated Native American 

tribes. Current monitoring protocols, established under 

interim guidelines of the Programmatic Agreement for 

Cultural Resources, document the presence of all types 

of impacts occurring to archaeological resources in the 

river corridor, regardless of ultimate cause.

In addition to the NPS monitoring efforts, the Hopi 

Tribe, Hualapai Tribe, and Southern Paiute Consortium 

conduct annual monitoring trips to assess changes to 

their traditional cultural resources and to assess the 

general health of the ecosystem through their own traditional value system. Tribal monitoring has been conducted both through and outside of the programmatic 

agreement, as not all resources of tribal concern meet 

the established definitions of National Register-eligible 

historic properties. 

The Programmatic Agreement for Cultural 

Resources is concerned with tracking and mitigating 

dam effects at approximately 318 National Registereligible archaeological sites in the river corridor: 54 in 

Glen Canyon National Recreation Area and 264 in 

Grand Canyon National Park (Leap and others, 2000, 

p. I-8). Approximately 160 of these sites are actively 
Cultural Resources in the Colorado River Corridor 181

monitored at the present time. All sites currently monitored fall within the affected environment as defined by 

the EIS (U.S. Department of the Interior, 1995). The 

monitoring is carried out by staff from the NPS, working with cooperators from Northern Arizona University. 

National Park Service archaeologists conduct monitoring trips several times each year and produce annual 

monitoring reports, which are submitted to the Bureau 

of Reclamation in partial fulfillment of an ongoing cooperative agreement (Leap and others, 2000). Currently, 

archaeological sites above Lees Ferry are not being monitored (Chris Kincaid, Glen Canyon National Recreation 

Area, oral commun., 2004); only sites downstream of 

Lees Ferry are routinely monitored. 

Archaeological sites are currently selected for 

monitoring and remedial treatments based on interim 

protocols established under the programmatic agreement. These protocols include judgmentally selecting 

sites for monitoring based on perceived susceptibility or 

likely vulnerability to erosion or visitor impacts. Sites are 

monitored on a cycle that varies from semiannually to 

annually, biennially, or once every 3, 4, or 5 yr (Leap and 

others, 2000). Monitoring cycles are assigned on the basis 

of perceived levels of stability or visitor use, but the monitoring cycles are not rigidly adhered to and frequently 

change. For example, of the 91 sites monitored in fiscal 

year 2001 (FY01), 55% (n = 50) were monitored more or 

less frequently than their assigned monitoring cycle, and 

18% were reassigned to a new monitoring cycle based on 

perceived changes in their stability. Sites that are stable 

or show no signs of visitor use are not included in the 

current monitoring program. This bias in site selection 

was intentionally designed to focus attention on those 

sites that were theoretically at greatest risk from damage from visitor use and erosion and were most likely to 

require preservation treatments in the foreseeable future; 

however, the deliberate emphasis placed on monitoring 

sites that are assumed to be most threatened limits the 

usefulness of the resulting data for drawing systemwide 

conclusions about status and trends of site condition, 

rates of impacts, or overall effects of dam operations on 

historic properties. 

The main goals of the current monitoring program 

are to document site impacts and evaluate the need for 

site protection measures such as erosional control check 

dams. Changes in the numbers, types, and locations of 

site impacts are documented in yearly reports prepared 

by the NPS and Northern Arizona University cooperators (see Leap and others, 2000, for a listing of annual 

reports through 1999; see also Leap and Kunde, 2000; 

Dierker and others, 2001, 2002; Leap and others, 2003). 

These reports discuss the results of site-specific evaluations, identify specific changes occurring at individual 

sites, and make recommendations about future protection measures, including data recovery. The reports do 

not track systemwide trends in site condition or evaluate 

changes in site condition relative to dam-controlled flows.

Currently, archaeological site-monitoring activities 

conducted under the programmatic agreement involve 

repeat site visits, visual assessments of site impacts, and 

qualitative assessments of overall condition, which are 

documented through the use of repeat photography 

and completion of a two-page checklist of impacts. As 

described in the FY02 annual monitoring report (Dierker 

and others, 2002, p. 2), 

Archaeologists qualitatively assess impacts to sites 

via repeat observations. The degree of impact is 

categorized as “present” or “absent,” with physical erosion further categorized as “active” or 

“inactive.” Active erosion is defined as obvious 

recent movement, disturbance, or rearrangement of sediment or artifacts onsite. Inactive 

erosion is defined as a (less obvious) perception 

that past geophysical processes are discernable 

at the site, but are not presently at work. 

Visitor impacts are recorded as present or absent 

in five categories: social trails, artifact collection piles, 

evidence of onsite camping, criminal vandalism, and 

other impacts (fig. 4). Physical impacts are recorded as 

present or absent and either active or inactive within 

the following eight categories: surface erosion, gullying, 

Figure 4. Visitors frequently remove artifacts from their original 

locations and concentrate them in “collection piles,” resulting 

in loss of information about the original context of the artifacts 

(photograph by Helen Fairley, U.S. Geological Survey).
182 The State of the Colorado River Ecosystem in Grand Canyon

arroyo cutting, bank slumpage, aeolian/alluvial erosion or deposition, side canyon erosion, animal-caused 

erosion, and other erosion (fig. 5). Impacts that the NPS 

views as being directly related to dam operations include 

bank slumpage and gullying/arroyo cutting in locations 

where drainage systems are actively entrenching to 

achieve grade with the present-day “highest discharge” 

terrace levels formed under dam-controlled flows. The 

precise role of dam operations relative to other erosional 

forces—precipitation events, human trampling, wind, 

and other “natural” and “cultural” agents of erosion—in 

causing or exacerbating erosion of archaeological sites in 

the river corridor remains a topic of continuing controversy in the scientific community. 

Because the current archaeological site monitoring program does not measure or otherwise attempt 

to quantify impacts or rates of change in either a relative or absolute sense, it is difficult to draw any specific 

conclusions about overall trends in resource condition in 

relation to either the interim operating flows of 1991–95 

or the modified low fluctuating flow (MLFF) alternative 

implemented in 1996 (U.S. Department of the Interior, 

1996). Two conclusions can be drawn, however, by 

using the currently available monitoring information: (1) 

archaeological sites continue to receive impacts from visitor use and erosion, and (2) archaeological site conditions 

are likely to continue to deteriorate (at an unknown rate) 

because impacts from visitor use and erosion are ongoing 

and not likely to diminish in the foreseeable future. 

Erosion Control with Check Dams

Beginning in 1995, the NPS began installing rock 

and brush check dams at selected archaeological sites in 

the river corridor in an attempt to control erosion. Check 

dams were first piloted as an erosion control measure 

in an area below the Little Colorado River known as 

“Palisades,” where gullies bisect two archaeological sites 

(AZ C:13:99 and AZ C:13:100). National Park Service 

archaeologists had monitored and documented a progressive deepening and widening of the drainages in the 

Palisades area since 1978. Continuing channel erosion 

caused the collapse and disappearance of numerous 

slab-lined cists and portions of masonry structures in the 

late 1980s and early 1990s, hence the decision to initiate 

erosion control measures. With assistance and supervision from the Zuni Conservation Project (a team of soil 

conservation experts from the Pueblo of Zuni), NPS 

archaeologists installed 70 check dams at the two sites by 

using a variety of local materials and construction styles 

(Leap and Coder, 1995; Leap and others, 2000). 

Since the initial pilot project in 1995, NPS archaeologists have installed approximately 280 check dams at 

29 different archaeological sites (Leap and others, 2000). 

Currently, 260 erosion-control features are actively monitored and maintained at 27 sites (Leap and others, 2003, 

p. 58). An evaluation of check dam effectiveness conducted in 2002 (Pederson and others, 2003) found that 

the brush checks built with a “basket weave” technique 

seemed to work best and caused less damage to surrounding terrain upon failure than did check dams built 

Figure 5. An erosional gully cutting headward into a 

prehistoric roasting feature (photograph by Amy Draut, U.S. 

Geological Survey).

Figure 6. Members of the Zuni Soil Conservation Project 

constructing a check dam by using the basket-weave technique 

(photograph courtesy of Grand Canyon National Park).
Cultural Resources in the Colorado River Corridor 183

with rocks or logs (fig. 6). The researchers observed that 

brush checks tended to fail in their central sections or get 

ripped out as a woody mass, whereas the more rigid rock 

and log checks were often flanked by the gullies via lateral slope erosion, further exacerbating erosional impacts 

to the sites. Pederson and others (2003) concluded that 

brush checks were less damaging than stone checks and 

that check dams, without routine maintenance, could 

cause more harm than good; however, check dams could 

temporarily slow rates of erosion provided they were 

routinely maintained. These findings are considered 

somewhat tentative and in need of further verification 

because they were based on observations conducted over 

the course of a single monsoon season in 2002, which 

was one of the driest monsoon seasons on record. 

Test Flow Impacts on 

Cultural Resources

Many of the archaeological resources along the corridor of the Colorado River are situated on or contained 

within the Holocene sedimentary deposits, which form 

dunes and terraces (fig. 7). The sediment resource has 

declined, and the alluvial terraces have eroded since the 

completion of Glen Canyon Dam. A systemwide method 

for regenerating the river terraces and redistributing sediment is considered an essential component to maintaining integrity of cultural resources in place.

The 1996 beach/habitat-building flow, or controlled 

flood, presented an opportunity to study the effects of 

high-flow discharge from Glen Canyon Dam on alluvial 

terraces and margin deposits along the river corridor. 

Although the effects of the 1996 beach/habitat-building 

flow of 45,000 cfs on archaeological sites could not be 

predicted, the hope was that it could provide systemwide 

mitigation to most cultural sites in the Colorado River 

corridor through the accumulation of additional sediment at higher elevations than normally would occur 

under the MLFF alternative (Balsom and Larralde, 1996, 

p. 3). Mitigation and monitoring of archaeological sites, 

ethnobotanical resources, and sediment accumulation at 

the mouths of arroyos were undertaken to evaluate the 

effects of this experimental high flow. In addition, rates 

of terrace retreat were studied in the Glen Canyon reach 

to determine whether terraces containing archaeological 

sites were negatively affected by the high flows (Balsom 

and Larralde, 1996).

The overall findings of the cultural resource studies 

done in conjunction with the 1996 beach/habitat-building flow were that the 45,000-cfs flow had either no 

effect, no adverse effect, or in some instances a beneficial 

effect on cultural resources (Balsom and Larralde, 1996, 

p. 25). In a few locations, however, especially in the Glen 

Canyon reach, loss of sediments occurred in a manner 

that, in the long run, could be detrimental to cultural 

resources. Follow-up studies conducted by the Hopi 

Tribe (Yeatts, 1998) and by Northern Arizona University 

researchers (Hazel and others, 2000) found that sediment 

deposited in arroyo mouths by the 1996 beach/habitat-building flow persisted in some locations for several 

years, especially where brush check dams had been 

installed in the lower reaches of the drainages. These 

studies, however, did not specifically evaluate whether 

the sediment plugs diminished the rate of down-cutting 

in upper reaches of the affected gullies. Nevertheless, the 

studies demonstrated that backfilling of some erosional 

channels could be accomplished by periodic high-flow 

events and that high, sediment-enriched flows offer one 

potential means of conducting systemwide mitigation for 

effects of dam operations on cultural resources. 

Since 1996, several additional test flows have taken 

place, including the 2000 low summer steady flows 

experiment, the 2003–05 fluctuating nonnative fish 

suppression flows, and the November 2004 experimental high flow. No specific cultural resource monitoring 

programs were conducted in conjunction with the 2000 

low summer steady flows or the 2003–05 fluctuating 

nonnative fish suppression flows; however, analysis of 

the sediment mass balance under the 2003 and 2004 

winter nonnative fish suppression flows showed that the 

Figure 7. A buried Pueblo II structure is becoming exposed 

by erosion near Unkar Delta (photograph by Amy Draut, U.S. 

Geological Survey).
184 The State of the Colorado River Ecosystem in Grand Canyon

fluctuating flows eroded material above and beyond 

the amount supplied by tributaries within the previous year. Also, analysis showed that higher levels of 

very fine grained material were entrained during the 

initial few days of the 2003 and 2004 high fluctuating 

flows, suggesting the possibility that fine sediments that 

were derived from predam deposits contributed to the 

sediment being transported out of the system (David 

Topping, U.S. Geological Survey, oral commun., 2005). 

Although not conclusive, this information suggests that 

there may be some additional loss of predam terrace 

deposits, where most archaeological sites are situated, 

occurring under the experimental fluctuating nonnative 

fish suppression flows. 

During the 2004 experimental high flow, one of the 

areas monitored during the 1996 beach/habitat-building flow and two additional sites upstream in Marble 

Canyon were monitored to determine whether sandbars 

would be created in the vicinity of archaeological sites, in 

locations where the new sandbars could serve as sources 

of sediment for windborne redeposition on downwind 

archaeological sites. An additional aim of this research 

was to document whether sediment would be deposited in the mouths of arroyos that currently bisect some 

archaeological sites and whether these sediment “plugs” 

would be retained long enough to help reduce the rate of 

downcutting in affected arroyos because of the temporary elevation increase at the arroyo mouths. Preliminary observations indicate that large sandbars did form 

upwind of the site areas and that sediment did backfill 

arroyos; however, at the time this report was being written, it was still too early to determine whether the 2004 

experimental high flow would benefit the archaeological 

sites over the long term. U.S. Geological Survey scientists will be monitoring the fate of the new sandbars 

and associated archaeological sites over the next year to 

determine whether and to what degree the newly formed 

sandbars contribute sediment to the windborne deposits 

that blanket sites located at higher elevations. 

The Role of Windborne Sediment 

in Preserving Archaeological Sites

In addition to the monitoring activities previously 

described, several research projects have been initiated 

and supported through the Glen Canyon Dam Adaptive 

Management Program over the past decade to improve 

overall understanding of how Glen Canyon Dam 

operations may be affecting archaeological sites in the 

Colorado River corridor below the dam (Thompson and 

others, 2000; Draut, 2003; Pederson and others, 2003; 

Draut and others, 2005; Wiele and Torizzo, 2005).

Of particular interest is whether and how postdam 

changes in overall sediment supply and flow regimes 

downstream of the dam may be contributing to the erosion of high-elevation dunes and terraces bordering the 

Colorado River, where many archeological sites occur. 

These terraces are bisected by numerous arroyos and 

gullies draining to the river, and many of the erosion 

channels pass through or by archaeological features. 

Pieces of anecdotal and empirical (aerial photography) 

evidence indicate that these gullies have been increasing 

in size, depth, and abundance over the past four decades 

(Hereford and others, 1993). The question that both dam 

managers and NPS managers are seeking to answer is, 

“To what extent is the ongoing erosion of these higher 

elevation terraces related to dam operations?”

Scientists generally agree that gullies are formed in 

response to specific, and often unusually intense, precipitation events (e.g., Webb, 1985). If precipitation is 

responsible for the establishment of gullies and arroyos, 

what might dam operations have to do with the ongoing erosion of the predam terraces? The answer lies in 

understanding the dynamic nature of the predam fluvial 

system in contrast with the present-day, dam-controlled 

hydrologic system, which is also very dynamic but in 

fundamentally different ways (Topping and others, 

2003). The predam system was generally characterized 

by high seasonal variability and low daily variability. 

Flows during the winter months were typically quite low, 

often running at less than 3,000 cfs. The flows usually 

stayed low until late April or early May, when runoff 

from the Rocky Mountains started making its way to the 

Gulf of California. Spring flows typically peaked in June 

or early July, with additional spikes in late summer in 

response to localized monsoon storm events. The annual 

spring snowmelt floods ranged between about 35,000 and 

120,000 cfs and averaged around 55,000 cfs, with peak 

flows of 120,000 cfs reoccurring about once every 6 yr 

(Topping and others, 2003). The highest known flood in 

historic times occurred in 1884 with an estimated flow of 

210,000 ± 30,000 cfs. In 1921, a flood of 170,000 cfs was 

measured at the Grand Canyon gage (Topping and others, 

2003, p. 31). Floods even larger than this are known from 

the geomorphic record (O’Connor and others, 1994).

The high spring flows typically carried huge sediment loads. An analysis of the historical predam sediment transport records from the Lees Ferry gage and 

Grand Canyon gage shows that the monthly sediment 

loads during May averaged around 13.9 to 17.6 million 

tons (12.6 and 16 million Mg) per month, respectively, 
Cultural Resources in the Colorado River Corridor 185

which is close to 20% of the annual amount transported 

each year (Topping and others, 2000b). As flood waters 

receded, sand was deposited at and below the flood stage 

along the river banks. After these flood deposits dried 

out, wind transported the fluvial sediment farther inland, 

where some of it covered archaeological sites and formed 

coppice dune fields around mesquite thickets. 

Today, virtually 100% of the sediment load that 

used to be transported by the river through Grand 

Canyon is trapped upstream in Lake Powell. Two major 

tributaries below Glen Canyon Dam, the Paria River 

and the Little Colorado River, contribute the bulk of the 

current sediment coming into the river system. Taken 

together, the contributions of sand from various sources 

provide Grand Canyon with approximately 16% of its 

predam levels (see chapter 1, this report). Compounding the effects of this drastic reduction in sediment, the 

dam is operated to meet peak power demands, so in the 

postdam era prior to the Record of Decision (1963–96) 

the Colorado River fluctuated by as much as 25,000 cfs 

on a daily basis, and the daily discharge range exceeded 

10,000 cfs on 43% of all days (before the dam, daily 

ranges in excess of 10,000 cfs occurred on only about 

1% of all days) (Topping and others, 2003). Furthermore, flows higher than 9,000 cfs essentially guarantee 

that any fine-grained sediment coming into the system 

will be transported downstream to Lake Mead in a 

period of a few weeks to a few months (Topping and 

others, 2000 a, b; Rubin and others, 2002), and analysis of the continuous discharge record from Lees Ferry 

gage and the Grand Canyon gage demonstrates that 

flows greater than this level have dominated the postdam 

record (Topping and others, 2003, p. 48). 

The reduction in sand supply translates into a 

reduction in the size, height, and volume of sandbars 

throughout the river corridor (Hazel and others, 1999; 

Schmidt and others, 2004). Furthermore, because damcontrolled flows are generally constrained below 25,000 

cfs, sand and silt are no longer being deposited at higher 

elevations where fine sediment would be less susceptible 

to riverine transport, more readily available for inland 

transport by wind, and able to backfill the lower reaches 

of arroyos and gullies that dissect the terraces.

Aeolian (windborne) sediment has previously been 

shown to play an important role in the formation and 

subsequent reworking of terraces where many archaeological sites are located (Hereford and others, 1993, 

1996), and aeolian deposition has been hypothesized 

to play an important role in mitigating the effects of 

runoff erosion (Lucchitta, 1991). In order to improve 

our understanding of how changes in sediment supply 

and river flow dynamics are affecting the archaeological 

sites, a study was initiated in 2003 to examine the role of 

aeolian sediment in preserving archaeological sites from 

several different perspectives: (1) the relative importance 

of aeolian sedimentation in the past compared to today, 

both as a terrace-forming process and also in backfilling 

incipient rills and gullies; (2) the extent to which aeolian 

sand cover may be diminishing throughout the ecosystem 

under current sediment-limited conditions; (3) the extent 

to which current rates of aeolian transport vary at different locations under varying ecological and geomorphic 

parameters throughout the river corridor; and (4) the 

extent to which aeolian transport rates and downwind 

aeolian sand cover could potentially increase when new 

bars are formed in optimal locations relative to the areas 

where archaeological sites occur.

To study these issues, Draut (2003; Draut and 

Rubin, in press) established wind-transport monitoring 

instruments at six locations throughout the corridor of 

the Colorado River (fig. 8). These monitoring stations 

measure the amount of sand being transported by varying wind speeds at different times of the year and under 

different sediment supply conditions. Preliminary results 

from the first year of data (Draut and Rubin, in press) 

indicated that wind speeds and predominant directions 

vary widely throughout the river corridor and that transFigure 8. U.S. Geological Survey scientists installing a weather 

station near RM 65. These devices measure wind velocity and 

direction at six locations along the Colorado River (photograph by 

Amy Draut, U.S. Geological Survey).
186 The State of the Colorado River Ecosystem in Grand Canyon

port conditions are also highly variable, limiting efforts to 

model sediment-transport rates for the system as a whole.

To determine the extent to which aeolian sediment formed the material in which archaeological sites 

are embedded, Draut worked collaboratively with NPS 

archaeologists, U.S. Geological Survey scientists, and 

other researchers to evaluate subsurface deposits at a 

nonrandom sample of archaeologically rich locations 

throughout the river corridor (Draut and others, 2005). 

These investigations focused on describing the various geomorphic processes that have contributed to the 

formation and preservation of archaeological sites by 

closely examining the sedimentary structures preserved 

in subsurface contexts (fig. 9). Preliminary findings indicated that aeolian deposits were common throughout 

the prehistoric landscape of the river corridor and were 

important factors in the formation of many sites but that 

wind deposition was clearly not the only matrix-forming process at work in the past. Aeolian sediment blankets the surface of many archaeological sites, but often 

these windborne deposits cover substrates that are both 

fluvially (from the river) and colluvially (from the slope) 

derived (Draut and others, 2005). 

In terms of understanding how dam operations 

could be altered to enhance the sediment supply available 

for redeposition by the wind, Draut is tracking the fate of 

several sandbars that formed during the November 2004 

experimental high flow near previously established sediment-transport monitoring stations (Draut and Rubin, in 

press). During 2005–06, Draut will evaluate the extent 

to which aeolian processes may contribute to the erosion 

of the newly formed sandbars and track the amount of 

sediment transported by the wind from the sandbars to 

nearby archaeological features. This information will 

allow researchers to assess the relative importance of 

fluvial and aeolian processes in maintaining the sedimentary matrices of archaeological sites located above the 

level of dam-controlled flows in the river corridor.

Ethnobotanical Resources and 

Other Tribally Valued Resources

The Hopi and Hualapai Tribes and Southern 

Paiute Consortium have monitored a variety of culturally 

important resources in the Colorado River corridor since 

the mid-1990s. The monitored resources of concern 

include culturally valued plants and plant gathering locations, traditionally valued mineral resources, landscape 

features, traditional use areas, and archaeological sites. 

Since 1995, the Southern Paiute Consortium has 

monitored culturally important resources in the corridor 

of the Colorado River to assess their condition relative to 

Glen Canyon Dam operations and visitor use, to educate and train tribal members in resource monitoring, 

and to educate tribal members and the general public 

about the traditional importance of Grand Canyon to 

the Southern Paiute people (Stoffle and others, 1995; 

Drye and others, 2001; Bullets and others, 2003, 2004). 

Approximately 20 individual locations are monitored by 

the Southern Paiute Consortium on a 6-yr cycle. Some 

locations are visited every year, while others are visited 

only once or twice over the 6-yr period. Locations of 

importance include traditional plant areas, rock art, 

specific perennial tributaries, and archaeological sites 

with evidence of use by Southern Paiute ancestors. The 

general assessment from Southern Paiute Consortium 

monitoring during the past 5 yr is that most of the monitored resources appear to be in relatively good condition, 

although concerns about visitor trails at archaeological 

sites, visitor behavior around certain traditionally significant locations in Grand Canyon, and drought stress on 

plants have been noted. To date, no specific recommendations regarding Glen Canyon Dam operations have 

been forthcoming from these monitoring efforts.

The Hualapai Tribe monitored traditional cultural 

resources in conjunction with the 1996 beach/habitatbuilding flow (Jackson and others, 1997) and again in 

2001, 2002, 2003, and 2004 (Jackson and others, 2004 

a, b). Beginning in 2001, the baseline conditions of 15 

previously documented traditional cultural locations 

in Grand Canyon and of 5 previously undocumented 

Figure 9. A U.S. Geological Survey scientist examines 

stratigraphy exposed in the wall of an arroyo near RM 209 

(photograph by Amy Draut, U.S. Geological Survey).
Cultural Resources in the Colorado River Corridor 187

locations were evaluated by Hualapai tribal members 

and consultants using a numerical condition index rating system. Several additional sites were added to the 

monitoring program in 2002–04, resulting in a total 

of 28 traditional cultural locations receiving 1 or more 

years of monitoring. Natural and cultural impacts were 

ranked on a 5-point scale, from 0 (absent) to 4 (severe). 

Impact ratings averaged over all 20 sites indicated that 

human impacts, both visitor- and dam-related, were 

more problematic than were natural impacts. Negative 

dam-related impacts included water stress on vegetation 

because of the lack of periodic inundation of higher 

elevation plant communities, continuing nonnative plant 

encroachment, and the loss of beach area from damcontrolled flows. Effects from the diminishing surface 

elevation of Lake Mead, accompanied by vegetation 

encroachment of nonnative plants, primarily tamarisk 

(Tamarix ramosissima), were also noted. Throughout the 

corridor of the river, human impacts from trailing, 

artifact movement, and onsite camping were observed 

to be a problem, with the latter impacts rated as heavy 

to severe in several cases. On Lake Mead, the wakes of 

motor boats are also thought to contribute significantly 

to beach erosion (Jackson and others, 2004b). Based only 

on the 2001 monitoring results, human impacts were the 

most significant impacts observed at most sites, with 13 

of the 20 locations rated as having heavy (3) to severe (4) 

human impacts. In contrast, only 5 of these 20 locations monitored in 2001 were assigned similar natural 

impact ratings, and most of the impacts were from 

rodent burrowing and side canyon flash floods. Overall 

in 2001, the average rating for all natural impacts was 

1.8, whereas the average human impact rating was 2.6. 

In future years, repeated analyses of the same sites will 

allow Hualapai tribal members to determine whether or 

not conditions are improving or deteriorating relative to 

2001 baseline conditions, but at this time (2005), comparative data are insufficient for conducting this analysis.

The Hopi Tribe initiated an ethnobotanical project in 1998 to evaluate traditional plant resources in 

the corridor of the Colorado River (fig. 10). This study, 

completed in 2001, identified over 128 plant species in 

Grand Canyon that were traditionally used for ceremonies, medicines, food, and other necessities of daily life 

(Lomaomvaya and others, 2001). Beginning in 2002, 

the Hopi Tribe initiated a multiyear project to evaluate whether terrestrial ecosystem data currently being 

collected by cooperating university scientists (Kearsley 

and others, 2002) could be useful to the Hopi Tribe for 

assessing resource conditions from a Hopi perspective. 

The results of this initial study indicated that the terrestrial ecosystem data could be usefully interpreted from a 

Hopi perspective, provided that the data were translated 

from scientific categories and terminology into Native 

American categories and terminology (Huisinga and 

Yeatts, 2003). In 2003, the Hopi Tribe initiated a pilot 

study to begin assessing the terrestrial ecosystem data 

from a native Hopi perspective. Results of this pilot 

study are anticipated to be available in spring 2005. 

Summary

Resource monitoring of archaeological and traditional cultural resources suggests that archaeological 

resources continue to be impacted both by physical 

processes such as surface erosion and gullying and by 

recreational visitors. Although surface erosion and visitor 

impacts would undoubtedly be occurring without the 

presence or continuing operation of Glen Canyon Dam, 

the manner in which the dam is currently operated 

prohibits the retention of sediment within the river corridor. The diminishing supply of sediment appears to be 

contributing to and exacerbating the rate and amount 

of erosion occurring at all levels within the ecosystem. 

In addition to impacts from Glen Canyon Dam operations, visitor impacts such as trailing, trampling, and collection of artifacts are contributing to the degradation 

of many archaeological sites in the river corridor and 

of several locations of traditional importance to Native 

American people.

Figure 10. Hopi elders discuss and document the uses of 

culturally valued plants in the Colorado River corridor. The 

photograph was taken at RM 43 below the 100,000-cfs watersurface elevation (photograph courtesy of Michael Yeatts and the 

Hopi Tribe). 
188 The State of the Colorado River Ecosystem in Grand Canyon

Tribal assessments of dam-related impacts to ethnobotanical resources in the river corridor offer somewhat 

mixed results. For the most part, the Southern Paiute 

Consortium has identified satisfactory conditions for traditional plant resources, although there is some evidence 

of plant deterioration, probably from ongoing drought 

conditions. Meanwhile, the Hualapai Tribe has expressed 

concern over the condition of certain key botanical 

resources (e.g., the willow tree (Salix sp.) at Granite Park), 

and they have noted an apparent increase in nonnative plants such as Bermudagrass (Cynodon dactylon) and 

camel thorn (Alhagi maurorum) at specific locations in the 

river corridor. 

Under the current NPS monitoring program, 

frequency of monitoring is tied to perceived levels of 

erosion or visitor use, with those sites showing more 

evidence of active erosion or more frequent visitor use 

being monitored more frequently than those showing less 

impact. As one would expect from a monitoring program 

that is weighted towards tracking impacts at the more 

threatened and heavily visited sites, annual monitoring 

results show relatively high levels of physical and visitorrelated impacts.

Because the current data set is lacking measurements related to rates or degrees of erosion, it is not 

possible to determine whether or to what degree rates 

of erosion at archaeological sites may have changed in 

recent years under the MLFF regime. The NPS monitoring program is undergoing a reevaluation and redesign in 

2005 to better meet the needs of the Glen Canyon Dam 

Adaptive Management Program for information related 

specifically to effects of Glen Canyon Dam operations 

on National Register-eligible historic properties. Once 

revised monitoring protocols are implemented in 2006, it 

will be possible to track rates and trends in gully formation and downcutting relative to different flow regimes 

and to make systemwide assessments of resource condition over time. 

The limited monitoring, in conjunction with the 

1996 beach/habitat-building flow and 2004 experimental high flow, indicates that the creation of sandbars 

above the level of normal dam operations may have 

beneficial effects on archaeological sites in two respects: 

(1) by creating sources of sediment for subsequent windborne redeposition at archaeological sites located upwind 

of the newly formed sandbars and (2) by temporarily 

raising the effective base level to which terrace channels 

are downcutting in the short term, thereby temporarily 

slowing the rates of downcutting and headward migration of erosional gullies. For these measures to be effective over the long term, however, periodic high flows 

under sediment-enriched conditions would need to be 

repeated at relatively frequent intervals. 

References

Ahlstrom, R.V.N., Purcell, D.E., Zyniecki, M., Gilpin, 

D.A., and Newton, V.L., 1993, An archaeological overview of Grand Canyon National Park. Archaeological 

Report No. 93-92: Flagstaff, Ariz., SWCA.

Balsom, J.R., and Larralde, S., eds., 1996, Mitigation 

and monitoring of cultural resources in response to the 

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Contact Information:

Helen C. Fairley

Social Scientist

U.S. Department of the Interior

U.S. Geological Survey

Southwest Biological Science Center

Flagstaff, AZ

hfairley@usgs.gov

First page photograph credit: Amy Draut, U.S. Geological Survey Courtesy of Michael Yeatts and the

Hopi Tribe
Chapter 12

Recreational 

Values and 

Campsites in 

the Colorado 

River 

Ecosystem

Matt Kaplinski

Jeff Behan

Joseph E. Hazel, Jr.

Roderic A. Parnell

Helen C. Fairley

Introduction

Grand Canyon National Park is one of the bestknown wildland preserves in the world. Its designation 

as a national park in 1919 sought to protect it for the 

benefit of human visitors as well as to safeguard the 

physical, biological, and cultural resources contained 

within its borders. Interest in recreation on the Colorado 

River has risen dramatically since the mid-1960s, and a 

226-mi (364 km) journey through Grand Canyon by boat 

is now regarded as one of the world’s premier wild-river 

experiences. Recreational use of the Colorado River 

corridor through Grand Canyon is closely regulated by 

the National Park Service (NPS), and demand for the 

corridor, particularly for river trips, greatly exceeds availability. 

Beginning with the initial explorations of John 

Wesley Powell in 1869, river runners and hikers have 

used sandbars along the Colorado River below presentday Glen Canyon Dam as campsites. These camps, and 

their associated activities, make up an important element 

of the modern-day recreational experience within Glen 

and Grand Canyons. Because of their crucial role, the 

relative size, distribution, and quality of campsites along 

the Colorado River are of particular concern to river 

managers (Bureau of Reclamation, 1995; Stewart and 

others, 2000; Glen Canyon Dam Adaptive Management 

Program, 2001; National Park Service, 2004). 

This chapter presents an assessment of the current state of knowledge concerning the impacts of Glen 

Canyon Dam operations on the changing condition of 

campsite areas and sandbars and the implications of 

physical changes of the Grand Canyon ecosystem for 

visitor capacity and quality of experience. After defining the study area and some key concepts, the chapter 

briefly reviews the relationships between the condition 

and extent of Colorado River sandbars and the quality of the visitor recreation experience. An overview of 

historical status and trends of the number and size of 

campsites along the Colorado River is followed by a 

summary of recent findings. Discussion focuses on the 

effects of the modified low fluctuating flow (MLFF) alternative and high-volume experimental flows on campsite 

area. The chapter concludes with an evaluation of these 

results relative to the stated recreation goals and management objectives of the Glen Canyon Dam Adaptive 

Management Program (GCDAMP).
194 The State of the Colorado River Ecosystem in Grand Canyon

Background

The Colorado River flows approximately 293 RM 

from Glen Canyon Dam to the Grand Wash Cliffs, the 

physical feature that marks the western boundary of 

Grand Canyon National Park. The focus in this chapter is on the portion of the river from Lees Ferry to 

Diamond Creek (RM 0–226), even though recreational 

use of the river corridor extends another 50 mi (80 km) 

downstream to Lake Mead. Lees Ferry is the launching point for river trips through Grand Canyon, and 

Diamond Creek, on the Hualapai Indian Reservation, is 

the typical takeout point.

Geomorphic Characteristics 

of Campsites

Debris fans are sloping deposits of boulders, gravel, 

and sand that form at the mouth of a tributary as the 

result of flash flood events that constrict the main channel and increase the local bed elevation of the river 

(Schmidt and Graf, 1990). At most constrictions, recirculation zones or eddies (currents of water moving against 

the main current in a circular pattern) are formed in the 

river, and because of lower flow velocities, sand is deposited within eddies. Flow patterns within an eddy define 

the configuration of sand that is deposited (Schmidt and 

Graf, 1990). Typically, eddies contain a primary recirculating zone and often have secondary zones of separated 

flow where the current rotates in the opposite direction 

of the primary zone or is virtually stagnant. Sand deposits are classified based upon where they are deposited 

in relation to the primary and secondary recirculating 

patterns. Two types of deposits are the highest in elevation and are most typically associated with campsites: 

separation deposits and reattachment deposits. Separation deposits mantle the downstream part of the debris 

fan near the point where the main current separates to 

form the eddy. Reattachment deposits are located at the 

downstream end of the primary recirculating zone where 

the main-channel current reattaches to the bank. 

Channel-margin deposits are not associated with 

tributary debris fans and occur along the channel banks. 

These deposits form within small eddies associated with 

bank irregularities caused by talus and rock outcropping. 

A small number of these channel margin deposits are 

used as campsites. Within some reaches of the corridor, 

flat-lying units of rock crop out along the river, and the 

ledges are also used as campsites. 

Visitor Capacity and 

Wilderness Experience 

In the context of recreation, carrying capacity is 

now referred to as “visitor capacity,” defined recently 

as “. . . a prescribed number and type of people that 

an area will accommodate, given the desired natural/

cultural resource conditions, visitor experiences, and 

management program” (Haas, 2001). Ecological aspects 

of visitor capacity usually relate to cumulative ecological 

impacts, but in places such as Colorado River campsites, 

actual physical space available, impacted or not, is also 

a critical factor. These categories have been referred 

to as “ecological” capacity (e.g., plant, animal, and soil 

impacts) and “physical” capacity (e.g., people per unit 

area of flat sleeping area; camping parties per beach) 

(Shelby and Heberlein, 1986). This distinction is useful for understanding visitor capacity for the Colorado 

River, where both physical space available and resource 

impacts are important management considerations. 

In addition to resource protection, primary objectives for recreation management include minimizing 

impacts on, enhancing, and preserving the quality of recreation experiences. Experience quality is complex and 

affected by an array of factors, some of which are social 

rather than ecological or physical, so visitor capacity also 

has a “social” component. Social variables that affect 

experience quality include the number of people visible 

at one time in a given area and the number of encounters of one group with other parties or with groups of 

a particular type or size (Shelby and Heberlein, 1986). 

Resource protection will always be a primary concern 

in highly sensitive areas, such as desert riparian zones, 

where even moderate human activity may cause significant ecological impacts, and in settings where heavy 

use produces damage. Thus, in ecologically sensitive 

areas, visitor capacity is limited by the need to protect 

resources. For a broad range of less sensitive areas, 

however, social variables that affect recreation experience 

quality may limit visitor capacity at levels below those at 

which unacceptable resource impacts occur. 

On the Colorado River in Grand Canyon, legal 

guidelines call for not only resource protection and a 

quality recreational experience but also an undeveloped, 

uncrowded, wilderness-type experience. It is clear that 

on the Colorado River, outstanding opportunities for 

wilderness experiences are indeed a key factor in the 

river’s popularity and something that the general public 

has come to expect when visiting Grand Canyon. This 

popularity was confirmed during the initial phase of 

river recreation research in the 1970s (Shelby, 1976) and 
Recreational Values and Campsites in the Colorado River Ecosystem 195

in subsequent studies (Bishop and others, 1987; Hall and 

Shelby, 2000). 

The National Park Service has explicitly expressed 

an intention to manage for wilderness-type experiences for Colorado River visitors within Grand Canyon 

National Park (National Park Service, 1995, p. 11). In 

addition to the opportunity to experience natural ecological conditions, one of the most important attributes 

of a wilderness experience is solitude (Hendee and others, 1990). Because the operation of Glen Canyon Dam 

has eroded sandbars used for camping and has reduced 

the sand available for maintaining them (see chapter 1, 

this report), the area available for camping in the river 

corridor has declined significantly since construction of 

Glen Canyon Dam in 1963 (Kearsley and others, 1994; 

Kaplinski and others, 2005). The decrease in campsite 

area can affect solitude by increasing the level of crowding along the corridor. Crowding reduces the ability of 

separate river trips to camp out of sight and hearing of 

one another and also reduces the ability of individuals or 

small groups within a particular trip to camp out of sight 

and hearing of one another. 

Because tracking changes in recreation resources 

and experience quality means identifying quantifiable 

parameters, campsite area has emerged as the preferred 

parameter for measuring these attributes for recreational 

monitoring programs in Grand Canyon. Campsite area 

is not the only factor in Colorado River recreation that 

affects experience quality, but it is a readily measurable 

factor that has arguably changed more than any other 

facet of the river experience in the past 40 yr.

Status and Trends

Studies of campsite area have been conducted by 

Weeden and others (1975), Brian and Thomas (1984), 

Kearsley and Warren (1993), Kearsley and others (1994), 

Kearsley (1995), and Kearsley and Quartaroli (1997). 

These studies evolved from qualitative estimates of 

campsite carrying capacity to quantitative aerial photographic measurements. Weeden and others (1975) and 

Brian and Thomas (1984) focused on developing an 

inventory of the size and number of campsites throughout the river corridor. Both of these studies estimated 

the capacity of each site with dam releases above the 

24,000–28,000 cubic feet per second (cfs) stage elevation,

with capacity defined as the number of campers that 

could occupy a campsite for an overnight stay. Researchers have focused on high-elevation campsites because 

summer demand for energy produces medium to high 

releases. As a result, lower sandbar elevations are inundated during the height of the commercial rafting season 

(mid-May through mid-September) and therefore are 

not available for camping during the time of year when 

campsites are in highest demand. Kearsley and Warren 

(1993) repeated the inventory and improved the campsite 

area measurements by developing techniques to quantitatively measure camp area from aerial photography and 

videography. Subsequent studies by Kearsley and others 

(1994), Kearsley (1995), and Kearsley and Quartaroli 

(1997) improved upon the aerial photographic mapping 

by using geographic information system software.

Kearsley and Warren (1993) studied camps between 

Lees Ferry and Diamond Creek and divided them into 

critical and noncritical reaches. A critical reach was 

defined as any contiguous stretch of the river in which 

the number of available campsites is limited because of 

geologic characteristics, high demand, or other logistical 

factors. Noncritical reaches were defined as any stretch 

of the river in which campsites are plentiful and little 

competition for the majority of sites occurs. These reach 

definitions closely parallel the geomorphic reach definitions of Schmidt and Graf (1990). 

Kearsley and Warren (1993) found that campsites 

had decreased dramatically in both number and size 

since Weeden’s team completed its initial survey in 1973. 

Reaches designated as critical because of limited availability of suitable campsites by Kearsley and Warren 

(1993) (Marble Canyon, RM 11–40.8; upper Granite 

Gorge, RM 76.5–116; and Muav Gorge, RM 139–164) 

are nearly the same as the critical sections identified by 

Weeden and others (1975). Campable area decreased 

primarily because of erosion in critical reaches; in noncritical reaches, decrease in campsite area was primarily 

the result of vegetation encroachment (Kearsley and 

Warren, 1993). An overall trend of increased campsite 

size and number between 1973 and 1983 was attributed 

to the high releases in 1983 needed to keep Lake Powell 

from spilling over Glen Canyon Dam. The 1983 high 

releases forced sand from the river channel onto sandbars, but the change was temporary; sandbars significantly decreased in size and number less than 1 yr later. 

Moreover, campsites in the upper Marble Gorge and 

upper Granite Gorge decreased between 1973 and 1983 

and between 1983 and 1991. The inventory documented 

226 campsites above 25,000 cfs, which represented a 

32% decrease in the number of campsites between 1973 

and 1991. The inventory also found a 51% decrease in 

large camps, resulting in a 44% decrease in campsite 

area between 1973 and 1991. Campsite area decreased 

an average of 9% between 1991 and 1994, with disproportionately larger decreases at camps in critical reaches 
196 The State of the Colorado River Ecosystem in Grand Canyon

(Kearsley, 1995). River-induced changes accounted for 

80% of lost campsite area above the Little Colorado 

River and 32% of loss below the Little Colorado River 

confluence.

Kearsley and others (1994) concluded that loss of 

Colorado River campsites was an ongoing process that 

was initiated with the installation of Glen Canyon Dam 

more than 30 yr ago and that the rate of decline had 

slowed over time. The overall pattern of change was one 

of initial systemwide decrease in sites (1965–73), variable 

change during years of regulated high flows because of 

high levels of precipitation (1983–86), and a systemwide 

decrease in campsites between 1984 and the mid-1990s. 

They noted that not all sandbars in Grand Canyon 

respond in the same manner to high flows, fluctuating 

flows, or vegetation encroachment and that campsite 

availability in critical reaches had decreased the most. 

To monitor changes in campsite availability resulting 

from the 1996 beach/habitat-building flow, 53 camping 

sandbars were randomly selected from the 218 remaining from the 1991 inventory by Kearsley and Warren 

(1993). The sites were physically measured 2 weeks 

before, 2 weeks after, and 6 mo after the 7-d, 45,000-

cfs experimental flow (Kearsley and Quartaroli, 1997). 

Float-by assessments were made of 200 sandbars, including the 53 that were also measured. Results showed a 

systemwide increase in campsite area. Half (100/200) of 

the sites assessed were at least 10% larger, 39% (77/200) 

were the same, and 12% (23/200) were smaller than 

before the experimental flow. For 53 sites directly measured, 62% (33/53) increased in size, 17% (9/53) were 

the same, and 21% (11/53) decreased in area. Float-by 

assessments were less sensitive to measuring change but 

not biased toward increase or decrease. At many sites, 

sand was deposited directly on top of existing campable 

areas and did not increase campsite area. At some sites, 

new sand was deposited as a mound over previously 

usable space, and the increase in slope angle resulted in 

decreased camping area.

Eighty-two new sites were created, in the sense that 

these sites were not usable just before the 1996 beach/

habitat-building flow. Although 33 of these “new” sites 

were included in previous campsite inventories, all had 

degraded to being unusable by the time of the experiment. Many new sites consisted of deposition on low-elevation sandbars with little sun or wind protection. These 

sandbars were theoretically usable but not highly valued 

as camps and were subject to rapid erosion. Forty of the 

new sites were between RM 40 and RM 65. More than 

twice as many sites were created in noncritical reaches 

than in critical reaches. Six months after the beach/

habitat-building flow, only 55% (45/82) of the new sites 

were still considered usable. The high flow obliterated 

three previously inventoried campsites. Overall, the 1996 

beach/habitat-building flow increased the number, size, 

capacity, and aesthetic qualities of campsites. These benefits were substantial, but degradation occurred quickly: 

within 6 mo, nearly half of the new campsites were 

unusable, remaining new sites were half their initial size, 

and most of the increased area on measured established 

sites had eroded. Relatively high-flow releases immediately following the 1996 beach/habitat-building flow in 

late 1996 and 1997 probably exacerbated erosion.

Recent Monitoring

Following the Record of Decision in 1996 (U.S. 

Department of the Interior, 1996) and the establishment of the Glen Canyon Dam Adaptive Management 

Program, a new campsite monitoring program was 

initiated by the U.S. Geological Survey’s Grand Canyon 

Monitoring and Research Center. The following discussion focuses on the results of 6 yr of campsite area monitoring beginning in 1998 and ending in 2003 (Kaplinski 

and others, 2005). 

Monitoring Objectives 

and Methods

The 1998–2003 campsite area monitoring program 

focused on describing changes in the size of camping areas in the Colorado River corridor. Monitoring 

included annually measuring campsite area at a series of 

long-term monitoring sites and evaluating the changes in 

campsite area among years and as the result of different 

dam releases.

Annual surveys were conducted every October from 

1998 through 2003 by crews from Northern Arizona 

University’s Department of Geology to quantify campsite area change. Surveys at the selected study sites 

were conducted by using standard total station survey 

techniques (U.S. Army Corps of Engineers, 1994). The 

surveyors adopted the criteria of Kearsley (1995) and 

Kearsley and Quartaroli (1997) to identify campable 

area. Campable area was defined as a smooth substrate, 

preferably sand, with no more than 8o of slope and 

with little or no vegetation. Not all campable areas were 

mapped at every site. Instead, representative camp spots 

were selected across a range of stage elevations. Camping 

areas not represented in the mapping were typically far 

(>328 ft (>100 m)) from the main mooring/cooking areas. 
Recreational Values and Campsites in the Colorado River Ecosystem 197

Study Sites

The study sites are located throughout the Colorado 

River corridor between Lees Ferry and Diamond Creek 

(fig. 1) and were selected to coincide with a subset of 

the long-term study sites used by the Northern Arizona 

University sandbar monitoring project, which monitors 

change in sandbar area and volume (Beus and others, 

1992; Kaplinski and others, 1995, 1998; Hazel and 

others, 1999, 2001, 2002). These sites were originally 

selected on the basis of (1) distribution throughout the 

geomorphic reaches, (2) size sufficient to guarantee 

persistence through the period of study, (3) geomorphic diversity within and between sites, (4) availability 

of historical data, and (5) variation in recreational use 

intensity and vegetation cover (Beus and others, 1992). 

Given these criteria, only a subset of the sandbar monitoring sites could be used to monitor campsite areas. 

These sites, although not chosen randomly, have proven 

to be representative of systemwide changes in terms of 

changes in sand volume and area at campsites located 

above high normal flows (above 20,000 cfs) (Schmidt 

and others, 2004). Therefore, it is reasonable to assume 

that changes to campsite areas at these sites are also representative of changes to campsite area systemwide. 

The study began with 31 study sites. In 2002, 6 sites 

were added, for a total of 37 sites. Only the original 31 

sites, which have been measured since 1998, were used 

to summarize the campsite areas, while all sites were 

used to calculate average percent change between years. 

Sixteen of these sites are located in Marble Canyon 

between the Paria River and the Little Colorado River 

confluence, and 21 are located in Grand Canyon below 

the Little Colorado confluence. There are 18 sites within 

critical reaches as defined by Kearsley and Warren 

(1993), and 19 are in noncritical reaches. 

Figure 1. The Colorado River corridor below Glen Canyon Dam and locations of 31 study sites from the 1998–2003 campsite area 

monitoring program. The shaded area represents Grand Canyon National Park. Study site locations are noted by river mileage.
198 The State of the Colorado River Ecosystem in Grand Canyon

Dam Releases Before and 

During the Study Period

Dam releases during the 1998–2003 study period 

included normal modified low fluctuating flow (MLFF) 

operations, plus a low summer steady flow (LSSF) experiment during 2000, the habitat maintenance flows (HMF) 

in May 2000 and September 2000, and fluctuating nonnative fish suppression flows from January to March in 

2002 and 2003 (fig. 2). Normal MLFF dam releases fluctuate diurnally and seasonally, based on power demand 

and water-delivery schedules. Typically, flow releases are 

higher in winter and summer months and lower during 

spring and fall months. In 1998 and 1999, daily mean 

flow releases ranged from an average of approximately 

19,400 cfs in high-volume months to approximately 

12,400 cfs in low-volume months. The LSSF experiment 

in 2000 consisted of two high-flow releases in spring 

and fall and a period of low steady (no diurnal fluctuation) flow during summer. The low steady flow during 

summer was lowered to a constant 8,000 cfs. The high 

flows were short-duration (4 d) dam releases of 31,000 

cfs. These were the only two flows large enough to reach 

above the 25,000-cfs stage elevation, or the upper limit 

for nonexperimental MLFF operation releases, during 

the study period. 

River flow levels during the 1998 and 1999 survey 

trips fluctuated from 10,000 to 18,000 cfs. Therefore, 

surveyors were only able to measure camp areas consistently at every site above the 15,000-cfs stage elevation.

Subsequent analysis of campsite area below 25,000-cfs 

stage elevation excluded the measurements made during 1998 and 1999. During the 2000 to 2003 surveys, 

low-volume releases allowed measurement of camp area 

above the 10,000-cfs stage elevations at some sites and 

above 15,000-cfs stage elevation at all sites. Fluctuating 

nonnative fish suppression flows were conducted from 

January through March in 2002 and 2003. During these 

experiments the flows fluctuated from 5,000 to 20,000 

cfs. Comparison of camp area change between surveys 

was conducted by using area measured only above the 

25,000-cfs stage elevation.

Findings

Recent analysis of the 1998–2003 monitoring results 

by Kaplinski and others (2005) demonstrated that the 

total camp area above the 25,000-cfs stage elevation 

significantly decreased during the study period (fig. 3). 

Total campsite area changes were derived by summing 

all of the campsite area measurements in a particular 

reach. Between 1998 and 2003, the total campsite area 

decreased by 55%. The average decrease was 15% 

between each survey (fig. 3).

Longitudinal changes were examined by comparing the total campsite area above and below the Little 

Colorado River (LCR) confluence (fig. 4). In the following discussion, the term Marble Canyon refers to sites 

above the LCR, while the term Grand Canyon refers to 

sites below the LCR. Campsite areas in Marble Canyon 

and Grand Canyon decreased at a similar rate and 

showed an overall loss of 57% and 53%, respectively. 

There was a longitudinal difference in the response to 

the powerplant capacity flows conducted as part of the 

2000 LSSF experiment. Camp area in Grand Canyon 

increased slightly (4%) following the high flows of the 

LSSF experiment, while campsites in Marble Canyon 

decreased by 24%. Area increases in Grand Canyon 

camps are possibly related to greater deposition downstream of the LCR where the sediment supply is presumably greater. 

The pattern of campsite area change was different 

in critical and noncritical reaches (fig. 5). Total campsite 

area within critical reaches decreased by 37% during 

the study period for an average decrease of 8% per year. 

In noncritical reaches the change was greater, with a 

total decrease of 63% and an average decrease of 18% 

per year. Campsite area increased slightly in the critical 

reaches (7%) following the LSSF experiment, whereas 

sites in noncritical reaches decreased by 18%. Critical 

reaches are generally narrower than noncritical reaches, 

and the campsites tend to be smaller and less vegetated 

Figure 2. Daily mean discharge hydrograph from the USGS 

gaging station on the Colorado River near Lees Ferry during the 

period of study. Note the daily and seasonal fluctuations in flow 

volume during 1998 and 1999 and during the low summer steady 

flow experiment in 2000 that included two high-flow events.
Recreational Values and Campsites in the Colorado River Ecosystem 199

25,000-cfs stage elevation, as well as above the 25,000-cfs 

stage elevation. In the most recent monitoring study, surveyors measured all campsite areas exposed at the time 

of the visit, allowing campsite area changes to be divided 

between discrete ranges of stage elevation (fig. 6). 

High-elevation campsite area (above 25,000 cfs) has 

progressively decreased during the study period, with the 

exception of a short-lived increase within the 25,000-

cfs to 30,000-cfs range following the LSSF experiment. 

Repeat surveys after 2000 showed that this slight increase 

in campsite area decreased to levels equivalent to those 

measured in 1998. 

Camp area at lower elevations has increased because 

of the deposition from high-flow events associated with 

the LSSF experiment in 2000, the fluctuating nonnative 

fish suppression flows from January to March 2003, and 

medium- to high-volume (10,000 to 25,000 cfs) summer 

dam operations. In fact, the amount of campsite area 

available at lower elevations is now greater than that 

available at higher elevations (fig. 7). Since the lower 

elevation areas are within the zone of flow fluctuation, 

these increases may not persist because lower elevation 

sandbars are more susceptible to bank erosion than sand 

at higher elevations (Hazel and others, 1999). 

Campsite area and sandbar volume both decreased 

during the study period; however, campsite area 

decreased at a greater rate than did sandbar volume (fig. 

8), which indicates that other factors contributed to the 

Figure 4. Total camp area above the 25,000-cfs stage elevation in 

Marble and Grand Canyons.

Figure 3. Total camp area above the 25,000-cfs stage elevation. 

Figure 5. Total camp area above the 25,000-cfs stage elevation in 

critical and noncritical reaches.

because the steep bedrock channels provide little space 

for sediment deposition.

Campsite area exists across the entire range of 

normal Glen Canyon Dam releases (5,000 to 25,000 

cfs), and the amount of camp area available is greatly 

dependent on flow levels. Some GCDAMP management objectives are specifically concerned with measuring sandbar area and volume between the 5,000-cfs and 

Figure 6. Distribution of total campsite area above the 25,000-cfs 

stage elevation in three different stage ranges.
200 The State of the Colorado River Ecosystem in Grand Canyon

loss of high-elevation campsite area. These factors presumably include vegetation growth, surface water runoff, 

aeolian processes (wind-caused sediment movement), and 

human impact. 

Although not quantitatively addressed in this study, 

visual observations and photographic documentation 

compiled by the Grand Canyon River Guides, Inc., 

Adopt-a-Beach Program during the same period (1998–

2003) indicate that, excluding sandbar erosion, vegetation growth contributes most significantly to the loss of 

high-elevation campsite area (Thompson and others, 

1997; O’Brien and others, 1999, 2000; Thompson, 2001, 

2002). Unfortunately, a direct comparison of campsite 

area change and vegetation colonization during the 

1998–2003 study period was not possible because of the 

incompatibility of vegetation monitoring protocols (M. 

Kearsley, Northern Arizona University, oral commun., 

2004). Kaplinski and others (2005) recommended that, 

in the future, vegetation coverage changes at campsites should be tracked by using remotely sensed aerial 

imagery to provide a quantified assessment of the role of 

vegetation in reducing total campsite area. 

Surface runoff events that significantly decreased 

campsite area were observed at only three sites during 

the 6 yr of monitoring. Human impacts were generally 

minimal, except at locations where vegetation pruning and removal had increased or maintained campsite 

areas. Aeolian reworking of sandbars did not appear to 

be a significant factor in affecting campsite area because 

of the amount of vegetation established along higher 

elevation sandbar areas. 

Discussion and Conclusions

Campsites within the Colorado River ecosystem 

exist primarily on sandbars. The size and capacity of 

camping area are directly related to the areal extent of 

sandbars and the amount of vegetation colonizing the 

sandbars (Kearsley and others, 1994). Previous studies 

by Kearsley and Warren (1993) and Kearsley and others 

(1994) established that substantial losses in open areas 

used for camping had occurred because of sandbar erosion and colonization by vegetation. Although both erosion and vegetation reduce campsite area, the processes 

and their effects are not identical.

Erosion of sandbars is caused primarily by operations of Glen Canyon Dam. The magnitude of daily 

fluctuations, the ramping rates, and the increased ability 

of clearwater releases to transport sediment have all been 

identified as contributing factors (Beus and others, 1992; 

Rubin and others, 2002). 

Vegetation encroachment is leading to higher rates 

of campsite area decrease than can be attributed to 

erosion alone. Encroachment by nonnative species such 

as tamarisk (Tamarix ramosissima) and camel thorn (Alhagi 

maurorum), as well as by native species such as arrowweed 

(Pluchea sericea) and coyote willow (Salix exigua), has led to 

colonization on previously open sections of sandbars, 

thus further decreasing campsite area. In some larger 

and less frequently visited sites, dense patches of vegetation now make the sites essentially unusable for camping 

activities. Although this process has substantially reduced 

available space at many campsites, the effects of these 

changes on visitor capacity are somewhat less clear than 

when area is lost to erosion. For example, in some frequently used camps, individual sleeping sites are cleared 

Figure 7. Total campsite area above and below the 25,000-cfs 

stage elevation.

Figure 8. Percent change between surveys of sandbar volume 

and campsite area in Marble and Grand Canyons. 
Recreational Values and Campsites in the Colorado River Ecosystem 201

of and kept free of vegetation by constant use. These 

individual sites are also often separated by vegetation 

“screens” that may actually serve to reduce the distance 

that recreationists feel they need to be separated from 

one another in order to achieve privacy. Additionally, 

clumps of mature tamarisk trees along steep riverbanks 

may serve to reduce sandbar erosion by anchoring sediment in place.

Rainfall-induced flash flooding also reduces campsite area. Flash flood impacts were transient before 

completion of the dam because the effects of tributary 

erosion were erased every year or two as flood flows from 

high spring runoff deposited sediment and reworked 

sandbars. Today, the effects of tributary flash floods are 

cumulative and tend to be long lasting because periodic 

high-flow events that are capable of transferring tributary-derived sediment to higher elevations are infrequent.

The monitoring results of Kaplinski and others 

(2005) showed that between 1998 and 2003 more than 

half of the available campsite area at the study sites was 

lost. Camping area above the 25,000-cfs stage elevation decreased by 55% during this 6-yr period, and the 

average rate of change was 15% per year. The decrease 

in high elevation campsite area occurred both in Marble 

Canyon and in Grand Canyon (above and below the 

LCR) as well as within critical and noncritical reaches. 

Notably, lower elevation campsite areas increased after 

2000, and the total campsite area below the 25,000-cfs 

stage elevation now exceeds the area available at higher 

elevations. The rate of decrease in high-elevation campsite area greatly exceeds the decrease in sandbar volume. 

This difference indicates that other factors—probably 

vegetation encroachment—have contributed to the 

recent loss of high-elevation campsite area. Unfortunately, Kaplinski and others (2005) could not undertake 

a quantitative comparison of campsite area change and 

vegetation colonization from 1998 to 2003 because the 

vegetation study is designed to detect systemwide, rather 

than site-specific, changes in vegetation cover (Mike 

Kearsley, Northern Arizona University, oral commun., 

2004).

In order to construct a longer term view of changes 

to campsites in Grand Canyon, Kaplinski and others 

(2005) compiled the percent change between surveys 

from the campsite inventories conducted by Brian 

and Thomas (1984), Kearsley and others (1994), and 

Kearsley and Quartaroli (1997) and combined them 

with the results from the 1998–2003 campsite monitoring program. Between 1973 and 2003, the only observable periods of increases in either the number of camps 

or the size of camps occurred after the high flows of 

1983–84, which were needed to keep Lake Powell from 

overtopping Glen Canyon Dam, and the 1996 beach/

habitat-building flow, when flows were greater than powerplant capacity. During years between flood events, both 

before and after the implementation of the 1996 Record 

of Decision (U.S. Department of the Interior, 1996), 

sandbars declined in area, volume, and total number. 

The campsite monitoring results showed that current operations of Glen Canyon Dam are not meeting 

the goals of the GCDAMP with respect to the recreational resources of the Colorado River corridor. Specifically, the GCDAMP seeks to “maintain or improve 

the quality of recreational experiences for users of the 

Colorado River Ecosystem, within the framework of 

the GCDAMP ecosystem goals” (Glen Canyon Dam 

Adaptive Management Program, 2001). With this goal in 

mind, the Adaptive Management Work Group developed the following management objectives to maintain 

or improve recreational resources:

Maintain or improve the quality and range of recreational opportunities in Glen and Grand Canyons 

within the capacity of the Colorado River ecosystem 

to absorb visitor impacts in ways consistent with NPS 

and tribal river corridor management plans (objective 9.1) (Glen Canyon Dam Adaptive Management 

Program, 2001).

Increase the size, quality, and distribution of camping beaches in critical and noncritical reaches in the 

mainstem within the capacity of the Colorado River 

ecosystem to absorb visitor impacts in ways consistent with NPS and tribal river corridor management 

plans (objective 9.3) (Glen Canyon Dam Adaptive 

Management Program, 2001).

Maintain or enhance the wilderness experience in 

the Colorado River ecosystem in consideration of 

existing management plans (objective 9.4) (Glen 

Canyon Dam Adaptive Management Program, 

2001).

The significant decrease in campsite area during 

the study period indicates that the second management 

objective above (objective 9.3) is not being met. While 

this study does not explicitly link changes in campsite area to the recreational/wilderness experience in 

Grand Canyon, the significant decrease in campsite area 

indicates that other management objectives are possibly 

not being met. For example, a significant decrease in 

campsite area may indicate a decrease in the range and 

quantity of recreational opportunities. Also, because 

existing campsites are smaller and thus more crowded, 

the quality of campsites is not being maintained or 

improved. The decrease in campsite area leads to more 

1.

2.

3.
202 The State of the Colorado River Ecosystem in Grand Canyon

crowding and less choice for camps, which can negatively 

affect the wilderness experience (Hendee and others, 

1990). On the other hand, conclusions about the inherently sociological aspects of the river experience based 

solely on changes to campsite area at a limited number 

of sites are tenuous. Future research and monitoring 

should be expanded to include sociological aspects of 

the recreational experience. In addition, future research 

should investigate the linkages between the sociological aspects of the recreational experience and physical 

parameters, such as campsite area, in order to evaluate 

whether GCDAMP goals are being achieved. 

The continued existence of sandbars suitable for 

camping in the Colorado River ecosystem will depend 

on periodic high flows to redeposit sediment lost through 

incremental erosion, scour, and vegetation encroachment; therefore, the continuing availability of campsite 

area is necessarily linked with the frequency and magnitude of flood events from Glen Canyon Dam. Unless 

vegetation is physically removed, and provided that 

enough sediment is available for deposition, high-flow 

events are the only mechanism by which sandbars used 

as campsites above the 25,000-cfs stage elevation can be 

built and maintained.

In order to properly address the management 

objectives of the GCDAMP, the recreational monitoring 

program should be expanded to include monitoring and 

research of both physical and psychological parameters 

of the river experience. Currently, the program consists 

only of campsite area measurements at a limited number 

of sites. This limited amount of information makes a 

complete assessment of the stated goals and objectives 

of the program currently impossible. One of the largest 

gaps in the current knowledge base is a complete, systemwide inventory of campsites in the Colorado River 

ecosystem in Grand Canyon. Since the last inventory 

in 1991, significant changes have occurred, and a new 

measurement of the number and size of camps is essential to evaluate the current state of the resource. O’Brien 

and Roberts (1999) and Roberts and Bieri (2001) used 

a modified version of the 1991 campsite inventory to 

develop a numerical river trip simulator model to predict 

visitor-use dynamics on the Colorado River. This model 

has been subsequently used by the NPS to assist in developing a management plan of the river corridor (National 

Park Service, 2004). Unfortunately, an up-to-date measurement of campsite carrying capacity was not available 

for the planning efforts. O’Brien and Roberts (1999) and 

Roberts and Bieri (2001) recommended that a method 

be developed to convert campsite area measurements 

to carrying capacity of a site; they suggested addressing 

this key information need by developing a standardized 

method of estimating carrying capacity for a beach so 

that onsite estimates are performed consistently. They 

also recommended that an interdisciplinary team comprising a statistician, a sociologist, a geologist, and surveyors should be included in order to develop an empirically verifiable and repeatable method of measuring and 

interpreting the campsite area, location, and abundance 

in relation to other variables such as trip length, attraction sites, number of people, and social aspects of visitor 

use. Campsite area measurements clearly indicate that 

campsite area has declined. How does this measured 

decline relate to carrying capacity of the river corridor? 

Does the decline in campsite area relate directly to a 

decline in the quality of the recreational and wilderness 

experience? Are these parameters linked and, if so, how? 

Future research aimed at addressing both the physical 

parameters and the psychological aspects of recreation 

along the Colorado River would help answer these types 

of questions and more clearly assess whether or not the 

goals and management objectives of the GCDAMP are 

being achieved. 

© 2005 Geoff Gourley; used with permission
Recreational Values and Campsites in the Colorado River Ecosystem 203

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the Grand Canyon Monitoring and Research Center, 

cooperative agreement no. 98-FG-40-0190. 

Thompson, K., 2001, Effects of low steady summer flows on Grand Canyon camping sand bars, 

2000: Report to the Grand Canyon Monitoring and 

Research Center, Flagstaff, Ariz.

Thompson, K., 2002, Long term monitoring of camping 

beaches in Grand Canyon: a summary of results from 

1996–2001: Report to Grand Canyon Monitoring and 

Research Center, Flagstaff, Ariz., 15 p.

Thompson, K., Burke, K., and Potochnik, A., 1997, 

Effects of the sand bar-habitat building flow and 

subsequent interim flows from Glen Canyon Dam on 

Grand Canyon camping sand bars, 1996: a repeat 

photography study by Grand Canyon River Guides 

adopt-a-sand bar program: Report to the Grand 

Canyon Monitoring and Research Center.

U.S. Army Corps of Engineers, 1994, Engineering and 

design-topographic accuracy standards: EM1110-1-

1005, p. 2-1 to 2-12.

U.S. Department of the Interior, 1996, Record of 

Decision, Operation of Glen Canyon Dam Final 

Environmental Impact Statement.

Weeden, H., Borden, F., Turner, B., Thompson, D., 

Strauss, C., and Johnson, R., 1975, Grand Canyon 

National Park campsite inventory, contract no. CX 

001-3-0061 with the National Park Service: University 

Park, Pennsylvania State University.
Recreational Values and Campsites in the Colorado River Ecosystem 205

Contact Information:

Matt Kaplinski

Research Associate

Northern Arizona University

Department of Geology

Flagstaff, AZ

Matt.Kaplinski@nau.edu

Jeff Behan

Faculty Research Assistant

Oregon State University 

Institute for Natural Resources

Corvallis, OR 

jeff.behan@oregonstate.edu

Joseph E. Hazel, Jr.

Research Associate

Northern Arizona University

Department of Geology

Flagstaff, AZ

Joseph.Hazel@nau.edu

Roderic A. Parnell

Professor, Director of the Center for Environmental 

Sciences and Education

Northern Arizona University

Department of Geology

Flagstaff, AZ

Roderic.Parnell@nau.edu

Helen C. Fairley

Social Scientist

U.S. Department of the Interior

U.S. Geological Survey

Southwest Biological Science Center

Flagstaff, AZ

hfairley@usgs.gov

Jeff Sorensen, Arizona Game and Fish

Department

First page photograph credit: © 2005 Geoff Gourley; used with permission

Chapter 13

Lessons from 

10 Years of 

Adaptive 

Management 

in Grand 

Canyon 

Jeffrey E. Lovich

Theodore S. Melis

Introduction

Almost a decade ago, the Secretary of the Interior 

issued a Record of Decision (ROD) (U.S. Department 

of the Interior, 1996) regarding the operation of Glen 

Canyon Dam and its impacts on natural, cultural, and 

recreational resources of the Colorado River below Glen 

Canyon Dam. This decision was largely in response to 

mandates of the Grand Canyon Protection Act of 1992. 

Adaptive management (see Overview, this report, for 

discussion of adaptive management), sometimes known 

as “learning by doing,” was deemed to be the method 

of choice after a multiyear environmental impact statement process that included extensive public involvement. 

Practitioners of adaptive management intentionally see 

management policies as experimental because of the 

scientific uncertainties inherent in such large, complex 

ecosystems (Walters and Holling, 1990; Clark, 2002). 

The intent in selecting this style of management is 

reflected in the Operation of Glen Canyon Dam Final 

Environmental Impact Statement (hereafter EIS) with 

the following language: 

It is intended that the ROD will initiate a process 

of adaptive management, whereby the effects of 

dam operations on downstream resources would 

be assessed and the results of those resource 

assessments would form the basis for future modifications of dam operations. Many uncertainties 

still exist regarding the downstream impacts of 

water releases from Glen Canyon Dam. The 

concept of adaptive management is based on 

the need for operational flexibility to respond 

to future monitoring and research findings and 

varying resource conditions (U.S. Department of 

the Interior, 1995, p. 34).

The very language in the EIS acknowledges that 

adaptive management is a process of experimentation, 

monitoring, and evaluation.

The end of a decade of research and monitoring 

provides an important opportunity to evaluate the effects 

of Glen Canyon Dam operations on resources of concern and to determine if the desired outcomes are being 

achieved and whether they are compatible with one 

another or not. In this concluding chapter we present 

a summary of adaptive management of the Colorado 

River ecosystem below Glen Canyon Dam by reviewing predictions contained in the EIS. In table II-7 of the 

EIS (summarized here in table 1), resource categories 
208 The State of the Colorado River Ecosystem in Grand Canyon

are listed and associated with predictions for how those 

resources would respond under the preferred alternative of modified low fluctuating flows (MLFF) (see table 

2 for generalized operating rules). During preparation 

of the EIS, the best scientific data available were used 

to generate those prognostications; however, a decade 

later we have significant new information for evaluating 

the operation of Glen Canyon Dam in relation to the 

objectives of the 1995 EIS and the 1992 Grand Canyon 

Protection Act. 

This report is the first systematic attempt to conduct an assessment of the changing state of resources in 

the Colorado River ecosystem in Grand Canyon over a 

decadal timeframe. Our objectives are to (1) encapsulate 

what we have learned about the Colorado River ecosystem over a decade of scientific inquiry, summarizing the 

most salient conclusions of this report, and (2) discuss, in 

very general terms, research and monitoring challenges 

and questions facing the program. 

While this report is not meant to be prescriptive 

with respect to future action for the Glen Canyon Dam 

Adaptive Management Program, the authors of the 

preceding chapters presented evidence that dam operations during the last 10 yr under the preferred alternative 

of the MLFF have not restored fine-sediment resources 

or native fish populations in Grand Canyon, both of 

which are resources of significant importance to the 

program. Some resources of concern, however, have 

improved under the MLFF, as shown below and also in 

the preceding chapters.

What Have We Learned 

from 10 Years of Adaptive 

Management?

Adaptive management is an integrated, multidisciplinary approach for confronting uncertainty in natural 

resources issues. It is adaptive because it acknowledges 

that managed resources will always change as a result of 

human intervention, that surprises are inevitable, and 

that new uncertainties will emerge. Active, experimental 

learning is the way in which the uncertainty is minimized 

(Walters and Holling, 1990). Adaptive management 

acknowledges that policies must satisfy social objectives 

but also must be continually modified and flexible for 

adaptation to surprises. Adaptive management, therefore, views policy as hypothesis. That is, most policies are 

really questions, and management actions become treatments in an experimental sense. Our summary of what is 

known about the influence of the MLFF on downstream 

resources examines many facets of the Colorado River 

ecosystem, especially those resources deemed as most 

important during the EIS process. Substantial importance is also ascribed to those resources affected by the 

Endangered Species Act of 1973.

Fish Response 

The Grand Canyon population of the federally 

endangered humpback chub (Gila cypha) has declined 

during the past decade under MLFF operations. Only 

eight native fish species were historically found in Grand 

Canyon. Six of these were desert species endemic (not 

found elsewhere) to the Colorado River ecosystem, making this one of the most unusual fish communities in the 

world (Mueller and Marsh, 2002). Of the original eight, 

only four remain in Grand Canyon, namely the humpback chub, the bluehead sucker (Catostomus discobolus),

the flannelmouth sucker (Catostomus latipinnis), and the 

speckled dace (Rhinichthys osculus). Of these four, only the 

humpback chub is endangered, and its numbers have 

dropped dramatically in the last decade. At the same 

time, nonnative fish have increased in both diversity and 

abundance. The reasons for the decline of native fish 

are commonly cited to include dramatic changes in the 

thermal, sediment, and hydrologic regimes of the river 

because of the construction and operation of numerous 

dams in the basin, the introduction of nonnative predatory and competitive fishes, and the introduction of 

diseases and parasites (Mueller and Marsh, 2002). The 

actual mechanisms of decline and extirpation are poorly 

known, in part because of a lack of early data on population numbers. 

Our knowledge about the cause and effect between 

dam operations and chub decline is incomplete; we 

do know, however, that the current MLFF operation 

has not resulted in increased survival and recruitment 

of humpback chub, despite the prediction of the EIS 

(table 1). Although there is no basis for claiming that 

the current operation at Glen Canyon Dam resulted in 

recent and repeated low recruitment and the continued 

decline of the humpback chub, it is clear that the restrictions on dam operations since 1991 have not produced 

the hoped-for restoration and maintenance of this 

endangered species (see chapter 2, this report). During 

the MLFF, basin hydrology has varied from drought to 

wet conditions and then back to drought conditions. 

Through these conditions, the decline of the humpback 

chub has continued. This trend leads to questions about 

whether daily, monthly, or even annual patterns of dam 
Lessons from 10 Years of Adaptive Management in Grand Canyon 209

Table 1. Natural and cultural resources of the Grand Canyon ecosystem and predictions from the Operation of Glen 

Canyon Dam Final Environmental Impact Statement (EIS) table II-7 (U.S. Department of the Interior, 1995) on how 

these resources would respond under the modified low fluctuating flow (MLFF) alternative, which is the preferred 

alternative in the EIS. 

[Glen Canyon Dam Adaptive Management Program (GCDAMP) is a federally authorized initiative to ensure that the 

primary mandate of the Grand Canyon Protection Act of 1992 is met through advances in information and resource 

management. Resources are ordered in this table as they appear in the EIS. A plus sign (+) indicates that the 

prediction was correct or exceeded expectations, a minus sign (-) indicates that the prediction was not entirely 

correct or did not achieve the desired outcome, and plus and minus signs together (+/-) indicate a mixed outcome. 

Data unavailable are indicated by a question mark (?) and may imply a total absence of data or that the data are not 

available to the U.S. Geological Survey’s Grand Canyon Monitoring and Research Center through the research and 

monitoring program under the GCDAMP]

Resource Prediction Outcome Comments

Sediment and aquatics

Fine sediment (sandbars and 

related physical habitats linked 

to native fishes (backwaters), 

terrestrial vegetation, marshes, 

campsites for recreation, and in 

situ preservation of archeological resources)

Modest improvement through 

implementation of constrained 

daily powerplant operations 

and periodic implementation 

of experimental high flows 

following accumulation of new 

tributary sand supplies in the 

main channel of the ecosystem. 

Sand accumulation was predicted to occur under averageto-below-average hydrology and 

associated hydroelectric power 

operations.

-

Sandbars continued to erode, 

and new sand inputs were not 

accumulated within the main 

channel. Experimental high 

flows were conducted, but the 

lack of flexibility in the timing and frequency of these 

controlled floods limited their 

effectiveness.

Coarse sediment (debris flow 

impacts from tributaries and 

their influence on the navigability of rapids and terrestrial 

sandbars)

This resource was not included 

in table II-7 of the EIS (U.S. 

Department of the Interior, 

1995), but predictions regarding the fate of this material 

were given on p. 104–105 of 

the document. It is included in 

this tabulation for the sake of 

completeness.

Inputs of coarse-grained sediment 

from tributary debris flows will 

continue to accumulate in the 

main channel under constrained hydropower operations, 

causing rapids to worsen and 

burying sandbars under coarse 

deposits. High-flow releases 

may partially rework the new 

deposits and improve navigation 

within rapids.

+

The influence of ongoing, naturally occurring debris flows, in 

terms of aggradation of rapids 

and burial of sandbars, has 

been partially mitigated by 

occasional experimental high 

flows. The ability of high dam 

releases to rework new debris 

flow deposits is related more 

to peak discharge and timing 

after debris-flow events than it 

is to the duration of the high 

releases.
210 The State of the Colorado River Ecosystem in Grand Canyon

Aquatic food web “Potential major increase”

+/-

Increases were apparent in Lees 

Ferry reach but not canyonwide. Fine-sediment inputs 

from tributaries below the Lees 

Ferry reach are most likely 

the limiting factor in primary 

productivity.

Native fish “Potential minor increase”

+/-

Recruitment and population of 

adult humpback chub decreased; native suckers may be 

stable or slightly increasing.

Nonnative fish “Potential minor increase”

+

Rainbow trout population increased substantially following 

the operational change in the 

Lees Ferry reach and within 

Marble Canyon.

Interactions between native 

and nonnative fish

“Potential minor increase in 

warm, stable microhabitats”

An increase in warm, stable microhabitats would favor native 

fish and nonnative warmwater 

fish.

-

Warmer dam releases because of 

drought-lowered Lake Powell 

levels may have increased warm 

microhabitats, but this situation 

is not directly related to dam 

operations.

Trout “Increased growth potential, 

stocking-dependent”

-

Rainbow trout numbers have 

increased in the Lees Ferry 

reach, but condition factor 

has declined. Stocking is not 

required.

Vegetation

Woody plants Modest increase

Exotic species included (tamarisk, 

camel thorn (Alhagi maurorum)).

+

Woody vegetation has increased, 

especially arrowweed (Pluchea 

sericea) and nonnative tamarisk, 

in the riparian zone that was 

formerly inundated frequently 

under the no action period 

(1963–91) of hydropower 

operations.

Emergent marsh plants “Same as or less than no action”

+/-

Wet marsh species decreased, and 

dry marsh species increased, 

likely because of the reductions 

of daily inundation and without 

periodic rejuvenation through 

floods.

Table 1. Natural and cultural resources of the Grand Canyon ecosystem and predictions from the Operation of Glen 

Canyon Dam Final Environmental Impact Statement (EIS) table II-7 (U.S. Department of the Interior, 1995) on how 

these resources would respond under the modified low fluctuating flow (MLFF) alternative, which is the preferred 

alternative in the EIS. —Continued
Lessons from 10 Years of Adaptive Management in Grand Canyon 211

Wildlife

Wintering waterfowl Potential increase

+/-

Trends vary by species and are 

difficult to distinguish from 

background variation.

Endangered and other 

special status species

Native fish (humpback chub, 

razorback sucker, flannelmouth 

sucker)

“Potential minor increase”

+/-

Recruitment and population of 

adult humpback chub decreased; native suckers may be 

stable or slightly increasing.

Bald eagle “Potential increase”

?

Numbers in Arizona have increased overall.

Peregrine falcon No effect

+

Numbers have been stable in 

Grand Canyon since 1988.

Kanab ambersnail “Some incidental take”

+/-

Snail habitat increased since 1998, 

but not snail numbers, which 

are relatively stable.

Southwestern willow 

flycatcher

“Undetermined increase” - No increase, but the flycatcher is 

uncommon in Grand Canyon. 

Cultural resources

Archaeological sites affected “Moderate (less than 157)”

?

Subsequent analyses have not 

been conducted to fully assess.

Traditional cultural 

properties affected

“Moderate”

?

Subsequent analyses have not 

been conducted to fully assess.

Traditional cultural 

resources affected

“Increased protection”

?

Subsequent analyses have not 

been conducted to fully assess.

Air quality

Effect of emissions 

on regional air quality

“Slight reduction”

?

Not Addressed by Glen Canyon 

Dam Adaptive Management 

Program (GCDAMP).

Recreation

Angler safety “Moderate improvement” ? No long-term monitoring data.

Day rafting “Major improvement”

?

Pre-EIS study suggests that net 

willingness-to-pay values were 

insensitive to flows. More studies are needed.

Table 1. Natural and cultural resources of the Grand Canyon ecosystem and predictions from the Operation of Glen 

Canyon Dam Final Environmental Impact Statement (EIS) table II-7 (U.S. Department of the Interior, 1995) on how 

these resources would respond under the modified low fluctuating flow (MLFF) alternative, which is the preferred 

alternative in the EIS. —Continued
212 The State of the Colorado River Ecosystem in Grand Canyon

Whitewater boating safety “Minor improvement”

?

NPS responsibility—not monitored as part of GCDAMP.

Whitewater boating camping beaches (average area at 

normal peak stage)

“Minor increase”

-

Camping areas have been diminished because of vegetation 

expansion and sandbar erosion, 

despite the fact that the new operating policy has limited daily 

peaking release to 25,000 cfs.

Whitewater boating 

wilderness values

“Moderate to potential to become 

major increase” ?

Potential decrease and decline in 

campable areas (see chapter 12).

Economic benefits (not related 

to hydropower revenue)

Positive

+

Increase to both locally and 

regionally.

Power

Annual economic cost

(foregone hydroelectric 

power revenue)

Acceptable costs relative to other 

alternatives ?

Subsequent studies are not available to fully assess.

Wholesale rate of power Acceptable costs relative to other 

alternatives

?

 Not monitored as part of 

GCDAMP. See Western Area 

Power Administration (WAPA) 

for data.

Retail rate of power (70% of 

end users)

“No change to slight decrease”

?

 Not monitored as part of 

GCDAMP. See WAPA for data.

Retail rate of power (23% of 

end users)

“Slight decrease to moderate 

increase” ?

 Not monitored as part of 

GCDAMP. See WAPA for data.

Retail rate of power (7% of 

end users)

Acceptable costs relative to other 

alternatives ?

 Not monitored as part of 

GCDAMP. See WAPA for data.

Nonuse value “No data”

+

Substantial nonuse value, $3–$4 

billion, has been demonstrated 

as willingness to pay for flows to 

protect fish.

Table 1. Natural and cultural resources of the Grand Canyon ecosystem and predictions from the Operation of Glen 

Canyon Dam Final Environmental Impact Statement (EIS) table II-7 (U.S. Department of the Interior, 1995) on how 

these resources would respond under the modified low fluctuating flow (MLFF) alternative, which is the preferred 

alternative in the EIS. —Continued
Lessons from 10 Years of Adaptive Management in Grand Canyon 213

Table 2. Operating limits and general likelihood of occurrence under the preferred alternative (U.S. Department of 

the Interior, 1995) of modified low fluctuating flows (maf = million acre-feet; cfs = cubic feet per second).

General range of hydrologic conditions for Glen Canyon Dam

Dam releases under 

operating rules, as well 

as constrained by annual 

hydrology

Dry 

(minimum of 8.23–10 maf of 

annual release)

Normal

(10–15 maf of 

annual release)

Wet

(15–20 maf of annual release)

Minimum releases

7 a.m.–7 p.m. (cfs)

8,000

(likely only during weekends)

8,000

(unlikely to occur)

8,000

(very unlikely to occur)

Minimum releases

7 p.m.–7 a.m. (cfs)

5,000

(very likely to occur 

on weekends)

5,000

(unlikely to occur)

5,000

(very unlikely to occur)

Maximum peak under 

diurnal releases (cfs)

25,000

(daily peaks reach about 

18,000–19,000 cfs, mainly 

during summer)

25,000 1

(daily peaks reach about 

20,000–24,000 cfs, mainly 

during summer)

25,000

(steady flows at this level 

occur for 1.5 maf 

monthly releases)

Daily fluctuations (cfs/24 h) 5,000 (possible)

6,000 (possible)

8,000 2

 (possible)

5,000 (unlikely)

6,000 (possible)

8,000 2 (most likely)

5,000 (unlikely)

6,000 (unlikely)

8,000 2 (most likely)

Ramp rate (cfs/h) 4,000 up (always)

1,500 down (always)

4,000 up (always)

1,500 down (always)

4,000 up (always)

1,500 down (always)

Monthly volume (maf) 480,000–900,000 700,000–1,200,000 800,000–2,000,000

1

 Exceeded during habitat maintenance flows.

2

 Maximums represent normal or routine limits and may necessarily be exceeded during high water years.
214 The State of the Colorado River Ecosystem in Grand Canyon

operation alone are relevant to native fish recruitment or 

whether changes in the sediment and thermal regimes 

of the river imposed by regulation have had the greatest 

influence on native fishes. Further, the issue of nonnative 

fishes and their potential to limit recruitment of native 

fish through predation and competition (although highly 

suspected by scientists as a significant factor) remains 

unresolved in Grand Canyon.

Populations of both bluehead and flannelmouth 

suckers appear to have remained relatively stable under 

the MLFF operating policy. The reasons for this persistence are also unknown. Conversely, the relatively stable 

habitat conditions created under the MLFF during 

protracted drought conditions, coupled with a coarsening of substrate in the river channel (see section below on 

fine sediment), appear to have greatly favored rainbow 

trout (Oncorhynchus mykiss), particularly in the Lees Ferry 

reach, as reflected in their increasing numbers during the last decade.

Sediment Response

Research and monitoring have conclusively demonstrated a net loss of fine sediment from the Colorado 

River ecosystem under the MLFF. Closure of Glen 

Canyon Dam eliminated about 84% of the sand that 

historically entered Grand Canyon (see chapter 1, p. 18, 

this report). Managing the remaining supply below the 

dam will apparently require carefully managed experimental high flows that are strategically released immediately following tributary sand inputs. It is not yet clear 

whether even this strategy will succeed in sustainable 

restoration of sand resources throughout Grand Canyon 

(Rubin and others, 2002). 

Because physical processes related to hydrology and 

sediment transport were relatively well studied, a logical question is, “Why did the writers of the EIS predict 

this outcome incorrectly?” Again, the EIS writers did 

a commendable job of using the best science available, 

but three critical monitoring programs for measuring 

suspended-sediment flux throughout Grand Canyon 

were discontinued in the early 1970s. This situation, and 

a lack of analytical or conceptual models (Marzolf and 

others, 1999), forced the EIS team to evaluate only a 

limited set of sand-transport data after the dam was built 

and then work with flawed assumptions rather than with 

continuous data records of flow and sediment concentration. Clearly, long-term monitoring efforts and good 

models (see Conceptual Modeling text box, Overview, 

this report) are essential to the success of adaptive management and accurate predictions.

The EIS assumption that sand would accumulate 

on the bed of the river over multiple years has been 

transformed through learning and adaptive management experimentation. Recent research suggests that 

future management of sediment should involve high-flow 

releases immediately following inputs of sand and finer 

sediment from tributaries below the dam. While such 

releases may be controversial because they bypass the 

hydroelectric powerplant, recent studies also suggest that 

the duration of such flows may need to be only a small 

fraction of what was originally suggested. Such fine tuning in the prescription of experimental high flows that 

are used for achieving habitat restoration could reduce 

the financial impacts and controversy associated with 

such management actions.

A physical habitat component of ecosystem restoration tied to the EIS strategy for restoration of native 

fish depended on the outcome of modest improvement 

in fine-sediment resources. As originally proposed in 

the EIS, restoration of sand-based, nearshore habitats, 

termed “backwaters,” has also not been realized (see 

chapter 1, this report) under the strategy of MLFF 

and hydrologically triggered experimental high flows. 

Detailed synthesis studies of sediment inputs and outputs 

to the system (fine-sediment mass balance), intensive 

field monitoring, and change detection analyses from 

remote-sensing data all point to a decrease in fine-sediment resources in Glen, Marble, and Grand Canyons in 

the time since the EIS was implemented. These changes 

have resulted in smaller and coarser grained sediment 

deposits that are associated with a net loss of systemwide 

sand supply and no evidence of accumulating sand from 

tributary inputs, even under protracted drought hydrology and constrained hydropower operations. 

We also know from research on coarse sediment 

dynamics that there has been an overall trend for the 

Grand Canyon reach to experience coarsening of the 

substrate in the river channel since completion of Glen 

Canyon Dam. As fine sediment is eroded because of 

dam operations, gravel and larger material remain. The 

impact of this “coarsening” of the river substrate has 

two potential biological implications: first is the creation 

of preferred habitat for benthic invertebrates, which are 

an important component of the aquatic ecology of the 

system, and second is the creation of spawning substrate 

for the nonnative rainbow trout. Both of these changes 

move the system farther from predam conditions and 

potentially benefit nonnative species like trout at the 

expense of natives.

Loss of sand habitats was documented under the 

no action period (1963–91), and loss has continued since 

dam operations were altered to reduce sandbar erosion. 
Lessons from 10 Years of Adaptive Management in Grand Canyon 215

Sand-transport data collected from 1999 through 2004 

indicate that whenever the monthly flow regime from 

the dam forces daily peak discharges significantly above 

10,000 cfs for extended periods, new and existing sand 

and finer sediments are being exported relatively quickly 

(weeks to months), rather than accumulating in the main 

channel over multiple years.

One alternative test of the MLFF concept for multiyear accumulation of sand supply might be to equalize 

monthly volumes during droughts in order to further 

limit daily peaks over such periods. If such a test failed to 

increase sand supply through accumulation of tributary 

inputs, then objectives for sand-habitat restoration might 

have to be reconsidered, or more proactive strategies, 

such as sediment augmentation, might need to be implemented. Meanwhile, release of short-duration, habitatbuilding flows following significant tributary sand inputs 

appears to be the most certain option for restoration of 

sand habitats below the dam.

Water Quality and Climate

The presence and design of Glen Canyon Dam 

caused major environmental changes to the Colorado 

River ecosystem, including (1) alterations in the timing 

and variability of the annual, seasonal, and daily flow 

patterns of the river; (2) drastic reduction of fine-sediment supply to the reaches of Glen, Marble, and Grand 

Canyons; and (3) reduced variability in water temperature. On the basis of current science information, the 

MLFF operating alternative has not effectively mitigated 

the influence of regulation with respect to either the 

thermal and hydrologic changes or the fine-sediment 

supply limitation of the downstream ecosystem. 

Given the importance of Lake Powell as the major 

source of water for the Grand Canyon ecosystem below, 

the lake monitoring program serves as an early warning 

system for changes in water quality. Although data from 

Lake Powell indicate that dam operations affect some 

resources downstream from Glen Canyon Dam, water 

quality in this large reservoir appears to be largely unaffected by the new dam operations since 1991. Overall, 

the water quality of the reservoir appears to be strongly 

linked to climatic annual to decadal variability governing spring inflow events and to the aging of the reservoir. 

The reservoir does have the potential to exert substantial 

impacts on downstream resources, however. For example, 

the current drought that started in the late 1990s reduced 

the level and volume of Lake Powell to elevations not 

seen since the reservoir began filling in the 1960s. The 

lower storage level of the reservoir has brought warmer 

surface (epilimnetic) waters to the penstocks, causing 

higher temperature water to be discharged downstream.

The effects of warmer water on downstream biological resources are currently difficult to predict with 

certainty and potentially include both positive and negative ecological consequences. Potential consequences 

include the creation of conditions that support the mainstem spawning of native fish, the invasion and dominance of warmwater fishes from Lake Mead, undesirable 

alteration of the food base, and unknown effects on the 

coldwater fishery in the Lees Ferry reach. This “natural” 

warming of Glen Canyon Dam releases that result from 

falling reservoir levels provides an important opportunity 

to test system responses to the possible installation of a 

temperature control device on the dam. 

Current understanding of global climate drivers 

provides little ability to predict the timing or extent of 

droughts over much of the Colorado River Basin. The 

current drought may or may not be a so-called “mega 

drought” because of the limited duration to date. At the 

time of this writing we are cautiously optimistic about 

precipitation and runoff predictions for the basin, but the 

final outcome will have little to do with dam operations. 

Human Use of the River

During the latter part of the 20th century, societal 

values associated with river regulation began shifting 

away from a policy focused solely on water supply and 

energy development to one in which preservation of 

natural resources was also valued. This shift in values 

occurred only after river regulation by mainstem dams 

was well underway on the Colorado River.

River regulation in itself has facilitated the development of an economically significant business associated with whitewater rafting (see chapter 9, this report) 

by reducing the predam variability in flow extremes of 

the river and thereby allowing such activities to continue with relative economic safety throughout the year. 

Recreational use of the river is one of the resources of 

concern that appears to have benefited most from the 

stabilizing influence of the MLFF, relative to more variable dam operations. By eliminating very high and very 

low discharges, the MLFF favors year-round recreational 

boating and fishing. Although the most comprehensive regional economic study of river-based recreation 

is now 10 yr old, the figures are impressive: over $46 

million (2004 dollars) in nonresident total expenditures 

and maintenance of 586 jobs, with 438 jobs in commercial rafting alone. Presumably those figures are 

even higher today.
216 The State of the Colorado River Ecosystem in Grand Canyon

Camping

Between 1998 and 2003, camping area above the 

25,000-cfs stage elevation decreased by 55%, and the 

average rate of change was 15% per year. The decrease 

in high-elevation campsite area occurred in Marble 

Canyon and in Grand Canyon as well as within critical (campsite-limited) and noncritical reaches. Losses 

are thought to be attributable both to net sediment 

exports under current dam operations (see chapters 

1 and 12, this report) and to encroachment of woody 

vegetation (see chapter 6, this report). Notably, lower 

elevation campsite areas increased after 2000, and the 

total campsite area below the 25,000-cfs stage elevation

now exceeds the area available at higher elevations. The 

rate of decrease in high-elevation campsite area greatly 

exceeds the decrease in sandbar volume. Vegetation 

encroachment most likely contributed to the recent loss 

of high-elevation campsite area.

The exact relationship and interaction among camping areas, vegetation expansion, and dam operations are 

unknown and provide a challenge for future researchers. 

For example, increased vegetation in sandbar areas may 

also provide greater substrate stability and shade, both 

limiting campsite erosion rates and enhancing camping 

areas from an aesthetic perspective. Tradeoffs between 

vegetation expansion and sandbar stability must also be 

considered from the perspective that increased vegetation might also limit the potential for wind processes to 

beneficially blow sand deposits upslope onto cultural sites 

that are subject to rainfall and runoff erosion.

Overall, in terms of recreation, future research 

should focus on detailed analyses of how fishing use, 

catch rates, and fish condition in Glen Canyon National 

Recreation Area are related to flows. Economic data on 

fishing and recreational rafting need to be updated to 

establish current baseline data before new flow regimes 

are initiated. Finally, studies to quantify the wilderness 

experiences of recreational users must be initiated so that 

the benefit of eventually achieving ecosystem restoration 

can be fully evaluated.

Nonuse Values

Survey efforts tied to the EIS process found that 

households across the Nation, including those that might 

never visit Grand Canyon, were willing to pay additional taxes for flows that benefited native fish and trout. 

Although the amount that people were willing to pay for 

these benefits was quite reasonable, when aggregated up 

to the number of households in the population, it produced estimates in the $3 billion to $4 billion (2004 dollars) range (see chapter 9, this report). The public at large 

is willing to pay to have flows and other management 

actions that benefit Glen Canyon National Recreation 

Area, Grand Canyon National Park, and the resources 

found in both. While there are those who question the utility of nonuse valuation of Grand Canyon resources, these 

dollar amounts reflect the iconic values that make Grand 

Canyon National Park famous throughout the world.

Hydroelectric Power Generation

Hydroelectric power and revenue associated with 

its production and marketing are also highly valued 

resources of concern to society, and environmental 

constraints on Glen Canyon Dam operations under the 

MLFF policy have large, annual economic costs (see 

chapter 10, this report). The acceptability of those costs 

has to be interpreted within the context of societal values 

associated with both electrical energy and environmental conservation objectives. Environmental constraints 

on dam operations have regional economic impacts on 

power revenue that is generated to pay back the cost of 

Glen Canyon Dam and to fund related water-resource 

and energy development. EIS studies on recreational use 

and nonuse values, however, suggest that dam operations under the MLFF benefit both local and regional 

economies through stabilization of flows, despite information that suggests that the ecological objectives of the 

program (e.g., retention of fine sediment, recovery of the 

humpback chub) remain unfulfilled. 

Because of the constraints imposed on dam operations under the MLFF, the economic value of hydropower that was foregone is unknown. Existing scientific 

data suggest, however, that the policy, no matter how 

costly, has not resulted in the level of environmental benefits predicted or desired in the EIS for natural resources 

below the dam.

An ex post facto cost-benefit analysis of Glen 

Canyon Dam operations is needed to fully assess the economic value of the MLFF operation versus documented 

environmental benefits below the dam. Additional 

experimental designs and the eventual implementation 

of alternative, longer-term changes in the current operating strategy would benefit from such an assessment.

Water Resources

Water allocation in the Colorado River Basin is 

governed by the Colorado River Compact of 1922 and 

subsequent laws and treaties. None of the laws pertain-
Lessons from 10 Years of Adaptive Management in Grand Canyon 217

ing to water management in the basin were superseded 

by the Grand Canyon Protection Act of 1992 or the 

1996 Record of Decision; therefore, delivery of the volume of water required under these laws, including under 

the MLFF, has remained unaffected by the Glen Canyon 

Dam Adaptive Management Program.

Cultural Resources

Under the MLFF operation, sand that is being 

exported from Grand Canyon is coming not only from 

new tributary inputs but also from existing beaches and 

river terraces that contain archaeological sites. Many 

archaeological sites in Grand Canyon have been covered 

with windborne (aeolian) sand for centuries. This sand 

was transported from lower elevation beaches that were 

frequently resupplied with new sand sources derived 

from annual floods during the predam era. Before dam 

operations were constrained, operations in the no action 

period (1963–91) were optimized for maximum water 

storage and power revenue (within the constraints of 

existing law and policies) rather than for strategically 

conserving limited sand supplies remaining downstream 

for restoration of sandbars (and, presumably, long-term 

preservation of cultural sites). The MLFF operation has 

not mitigated sand export, and therefore the sand supply 

remains critically limited. With more sand leaving the 

ecosystem than being supplied, more and more of these 

archaeological sites are being exposed to the ravages 

of erosion. As sites are eroded, artifacts and structures 

are exposed, making them more susceptible to visitor 

impacts and destabilization because of the loss of the 

surrounding sedimentary matrix in which they are buried. Such changes make it difficult, if not impossible, for 

archaeologists to reconstruct and interpret the historical 

and cultural information contained within these important settings within Grand Canyon. 

In addition to being valued by scientists for the 

information that the archeological sites provide, the 

sites are also valued by many Native American people 

who have traditional affiliations with these sites and the 

Grand Canyon area in general. Other resources along 

the Colorado River that are also valued by the tribes of 

the region include traditionally used plants, minerals, 

water sources, and significant landscape features. The 

effects of the MLFF on these tribally valued resources 

remain uncertain; the Native Americans engaged in the 

Glen Canyon Dam Adaptive Management Program 

would like to see more emphasis placed on monitoring 

the effects on these resources in the future.

Vegetation in the River Corridor

Glen Canyon Dam operations under the MLFF 

have stabilized flow conditions that were in effect before 

the EIS in Grand Canyon and that had significant 

impacts on riparian vegetation. The EIS predicted a 

modest increase in woody vegetation (table 1), and that 

prediction has proven largely correct, if not understated. 

The EIS also predicted that marsh communities would 

be the same as or less than expected under the no action 

alternative, which is also largely correct. Since implementation of the MLFF, there has been a decrease in wet 

marsh vegetation and an increase in dry marsh vegetation.

The stability of flows has encouraged an increase in 

vegetation density in and near the wetted zone. While an 

increase in vegetation may appear to be desirable, one of 

the impacts is a decrease in available camping area for 

recreational users (see chapter 12, this report). Furthermore, the increase is partially attributable to expansion 

of nonnative tamarisk (Tamarix ramosissima) and arrowweed into the riparian zone. In the terrestrial realm, 

future research should focus on identifying the responses 

of wildlife to this fundamental change in habitat structure, striving to understand the relationship between 

riparian vegetation and insects as related to the food web 

of the river, and examining the effects of human-mediated removal of nonnative vegetation versus natural disturbance. Understanding the complex interface between 

dam operations and overlapping elements of both the 

terrestrial and aquatic parts of the river ecosystem provides even greater challenges.

Integrated Ecological Factors

Although linkages between native fish recruitment 

and backwaters are not well documented and strategies 

for achieving sandbar habitat restoration are still being 

investigated, it is clear that physical habitat availability 

does not reduce or diminish the need by native fish for a 

sustainable food supply. Dam operations under the MLFF 

have resulted in steadier flows and greater minimum discharges of clearer water than operations in the no action 

period, and this situation has probably led to increases in 

the standing mass of algae and invertebrates (table 1). 

A critical future research need is to develop a better 

understanding of the linkages between the organic matter and invertebrates and the actual prey base of fish, 

both native and nonnative. A large amount of data has 

been collected on the food items consumed by nonnative 
218 The State of the Colorado River Ecosystem in Grand Canyon

rainbow trout, but only limited data are available for the 

humpback chub because of the endangered status of 

this fish. Use of stable-isotope analysis will be critical to 

assessing the energy sources and trophic pathways that 

are important to fish. Also, critical tests of the hypothesis 

that competition between nonnative trout and humpback 

chub is negatively affecting humpback chub populations 

are not possible because of inadequate data. Eating the 

same food items is only the first criterion to establish that 

competition is negatively impacting a particular species. 

Further research will be required to determine if this 

hypothesis is supported by data.

Recent Management Experiments

With respect to native fishes, we have learned that, 

under the MLFF, focused efforts are still required to 

understand the importance of the sediment and thermal aspects of physical habitat in the early life history 

of humpback chub and other species. Focused efforts 

are also needed to understand the influence of introduced nonnative species on the successful recruitment 

of humpback chub to the adult life stage. Additional 

experiments in these areas will require even more commitment to the adaptive management approach by using 

repeated implementation of both flow and nonflow 

treatments over an extended period of perhaps 10 or 

more years coupled with long-term monitoring. 

Mechanical removal of nonnative fish, especially 

rainbow trout, is currently in the third year of a 4-yr 

implementation strategy to test the hypothesis that reduction of predatory and competitive fish species will result 

in an increase in survival and recruitment of humpback 

chub (see U.S. Department of the Interior, 2002, for 

details of the current experimental design). While the 

adaptive management program has demonstrated that 

mechanical removal is an effective way to significantly 

reduce the number of nonnative trout in the removal 

reach, it has yet to detect the desired increase in the 

number of spawning-age humpback chub.

In addition to understanding how aquatic ecology, 

dam operations, and fish populations of the Colorado 

River ecosystem are interrelated, scientists need to focus 

future research on the effects of warming discharges of 

water from Glen Canyon Dam. An experimental temperature control device has been proposed for the dam 

later in this decade, and substantial questions remain 

unanswered as to the efficacy of this experimental treatment with respect to both its risk and its cost benefit.

As linkages between the aquatic ecology of the river 

and its native and nonnative fishes are defined, new 

efforts for tracking critical elements of water quality will 

need to evolve to track bioenergetic pathways and fish 

responses to flow treatments such as temperature control 

device operations. Also, as more information becomes 

available about the role of fine sediment in the preservation of cultural sites, aquatic ecology, and fish recruitment, there will need to be a commitment to modeling and monitoring of long-term fluxes of suspended 

organic and inorganic materials through the river system.

Conclusions and 

Future Challenges

Research and monitoring conducted by U.S. 

Geological Survey scientists and their cooperators have 

conclusively demonstrated a net loss of sediment from 

the system and have documented the decline of the 

federally endangered humpback chub during the last 

decade. At this first milestone, both findings are critical 

pieces of information to assess conditions and adjust 

management actions in the spirit of adaptive management. It is important to note that water-delivery requirements continued to be met throughout the decade after 

the EIS, despite increased costs associated with environmental and experimental regulation of flows.

Although incomplete, a substantial body of knowledge now exists for the Colorado River ecosystem in 

Grand Canyon. The overarching question is, “What will 

society do with the knowledge now available to move 

into the next active phase of the Glen Canyon Dam 

Adaptive Management Program?” The complexity of 

the natural system presents enormous challenges for 

determining how resources and population numbers 

vary in time and space and underscores the importance 

of long-term studies to describe patterns and processes. The next critical phase of adaptive management 

requires strategic action on the part of both managers 

and scientists.

Along with future action come the continuing challenge and need for greater integration of monitoring and 

research studies. As the complexity of issues in the Glen 

Canyon Dam Adaptive Management Program becomes 

more obvious, so does the need for interdisciplinary, not 

just multidisciplinary, science. Good examples of recent 

interdisciplinary science include (1) the interface between 

fine-sediment studies and cultural resources as the result 

of research efforts to understand the deposition of windcarried sediment, (2) water-quality studies related to 

temperature and fish biology, and (3) the developing link 

between aquatic ecology studies and fish diets. Contin-
Lessons from 10 Years of Adaptive Management in Grand Canyon 219

ued efforts will be required to integrate knowledge across 

disciplines and scales and to develop a more robust conceptual model for the Grand Canyon ecosystem. 

A continued adaptive management approach below 

Glen Canyon Dam, one focused on systematic experimentation, is recommended as a more efficient strategy 

for learning than the approach initially undertaken of 

monitoring the MLFF operation without comparing it 

to other flow and nonflow (e.g., mechanical removal of 

nonnative fish) alternatives. Future experimental treatments (flow or otherwise) must be evaluated within a 

strategic framework of periodic milestones and with 

rigorous scientific review so as to effectively identify 

viable management options for achieving the desired mix 

of resource responses. Success in this approach relies first 

and foremost on managers and stakeholders identifying 

what is desired, as well as determining whether identified objectives are measurable by science and attainable 

through dam operations. 

In the context of an adaptive ecosystem management process, the information identified at this milestone should lead to a dialog between managers and 

scientists about what other flow or nonflow alternatives 

might be considered to achieve the desired environmental outcomes. 

Because of the uncertainty about the cause and 

effect of MLFF operations on Grand Canyon resources 

and the even greater uncertainty about other conservation options, such as mechanical removal or thermal 

modification, the next steps in the process seem best 

approached as ongoing management policy experiments 

in the spirit of adaptive management, punctuated with 

frequent milestones at which the state of knowledge 

gained is assessed by all interested parties.

Critical Issues for 

Further Research

The synthesis of knowledge from over a decade of 

research and monitoring in Grand Canyon provides an 

opportunity to identify critical research needs in the Glen 

Canyon Dam Adaptive Management Program. The 

purpose of this report is to establish another milestone 

in our understanding of the state of resources in Grand 

Canyon; however, we do not attempt herein to provide 

an exhaustive or prescriptive list of management options. 

That effort will require another set of processes and 

products. Instead, we focus on the drivers, or major science questions, that will need to be addressed in the next 

phase of adaptive management: 

Why is the humpback chub population in Grand 

Canyon declining? Specifically, what factors or 

combination of factors are most influential in this 

downward trend? 

What is the linkage between native and nonnative fish population dynamics and the aquatic and 

terrestrial food base, and how are these factors 

related to dam operations? 

Under a potentially continuing drought scenario 

for the upper basin of the Colorado River, what 

are the impacts of warmwater discharges on the 

ecosystem?

If additional research demonstrates that sediment 

inputs from tributaries below the dam cannot be 

manipulated to achieve the desired conservation 

of sandbars and backwaters with dam operations, 

is sediment augmentation a viable option? 

What are the specific linkages between dam 

operations and archaeological site erosion, and 

what are the options for preserving the significant 

variety of culturally important resources and the 

information values associated with nonrenewable 

heritage resources?

Experimentation and research are needed to determine what role Glen Canyon Dam operations have had 

in these issues and whether further changes in those 

operations can benefit key resources.

References

Clark, M.J., 2002, Dealing with uncertainty: adaptive 

approaches to sustainable river management: Aquatic 

Conservation: Marine and Freshwater Ecosystems, v. 

12, p. 347–363.

Marzolf, G.R., Jackson, W.L., and Randle, T.J., 1999, 

Flood releases from dams as management tools: interactions between science and management, in Webb, 

R.H., Schmidt, J.C., Marzolf, G.R, and Valdez, R.A., 

eds., The controlled flood in Grand Canyon: Washington, D.C., American Geophysical Union, Geophysical 

Monograph Series, v. 110, p. 359–367.

Mueller, G.A., and Marsh, P.C., 2002, Lost, a desert 

river and its native fishes: a historical perspective of 

the lower Colorado River: Denver, Colo., U.S. Government Printing Office, Information and Technology 

Report USGS/BRD/ITR-2002-0010, 69 p.

•

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220 The State of the Colorado River Ecosystem in Grand Canyon

Contact Information:

Jeffrey E. Lovich

Deputy Center Director

U.S. Department of the Interior

U.S. Geological Survey

Southwest Biological Science Center

Flagstaff, AZ

jeffrey_lovich@usgs.gov

Theodore S. Melis

Physical Scientist

U.S. Department of the Interior

U.S. Geological Survey

Southwest Biological Science Center

Flagstaff, AZ

tmelis@usgs.gov

Jeff Sorensen, Arizona Game

and

Fish

Department

Rubin, D.M., Topping, D.J., Schmidt, J.C., Hazel, J., 

Kaplinski, M., and Melis, T.S., 2002, Recent sediment 

studies refute Glen Canyon Dam hypothesis: Eos, 

Transactions of the American Geophysical Union, v. 

83, p. 273, 277–278.

U.S. Department of the Interior, 1995, Operation of 

Glen Canyon Dam Final Environmental Impact Statement: Salt Lake City, Utah, Bureau of Reclamation, 

Upper Colorado Region, 337 p., appendices. 

U.S. Department of the Interior, 1996, Record of Decision, Operation of Glen Canyon Dam: Washington, 

D.C., Office of the Secretary of Interior, 13 p.

U.S. Department of the Interior, 2002, Proposed experimental releases from Glen Canyon Dam and removal 

of nonnative fish; environmental assessment: Salt Lake 

City, Utah, Bureau of Reclamation, Upper Colorado 

Region, 112 p., appendices.

Walters, C.J., and Holling, C.S., 1990, Large-scale management experiments and learning by doing: Ecology, 

v. 7, p. 2060–2068.

First page photograph credit: © 2005 Christopher Taesali; used with permission