U.S. Department of the Interior

U.S. Geological Survey

Circular 1366

Effects of Three High-Flow Experiments 

on the Colorado River Ecosystem 

Downstream from Glen Canyon Dam, Arizona
Cover photographs:

Background—Glen Canyon Dam during the March 2008 high-flow experiment (Anne Phillips, U.S. Geological Survey). 

Inset top—Sandbar and boats (Matt Kaplinski, Northern Arizona University). 

Inset middle—Humpback chub (Gila cypha; George Andrejko, Arizona Game and Fish Department). 

Inset bottom—Rainbow trout (Oncorhynchus mykiss; copyright Eric Engbretson, used with permission). 

Bottom background—Sand grains (David M. Rubin, U.S. Geological Survey).
Effects of Three High-Flow Experiments 

on the Colorado River Ecosystem 

Downstream from Glen Canyon Dam, Arizona

Edited by Theodore S. Melis

Circular 1366

U.S. Department of the Interior

U.S. Geological Survey

National Park Service
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2011

This report and any updates to it are available online at:

http://pubs.usgs.gov/circ/1366/

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natural hazards, and the environment, visit http://www.usgs.gov or call 1-888-ASK-USGS

For an overview of USGS information products, including maps, imagery, and publications, 

visit http://www.usgs.gov/pubprod

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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:

Melis, T. S., ed., 2011, Effects of three high-flow experiments on the Colorado River ecosystem downstream from Glen 

Canyon Dam, Arizona: U.S. Geological Survey Circular 1366, 147 p. 
 iii

Contents

CHAPTER 1

Introduction and Overview ..........................................................................................................................1

Lara M. Schmit and John C. Schmidt

CHAPTER 2

Understanding Physical Processes of the Colorado River ..................................................................17

John C. Schmidt and Paul E. Grams

CHAPTER 3 

The High Flows—Physical Science Results ..........................................................................................53

John C. Schmidt and Paul E. Grams

CHAPTER 4

Biological Responses to High-Flow Experiments at Glen Canyon Dam ............................................93

Theodore A. Kennedy and Barbara E. Ralston

Text boxes

Richard A. Valdez and illustrations by Joseph R. Tomelleri

Richard A. Valdez, Christopher T. Robinson, and Theodore S. Melis

CHAPTER 5

Science-Based Strategies for Future High-Flow Experiments at Glen Canyon Dam ....................127

Scott A. Wright and Theodore A. Kennedy

©John Annerino, used with permission
iv 

Conversion Factors and Datum

Inch/Pound to SI

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

acre 4,047 square meter (m2

)

square foot (ft2

) 929 square centimeter (cm2)

square foot (ft2

) 0.09290 square meter (m2)

square mile (mi2

) 2.590 square kilometer (km2) 

Volume

cubic foot (ft3

) 0.02832 cubic meter (m3) 

cubic yard (yd3

) 0.7646 cubic meter (m3) 

acre-foot (acre-ft) 1,233 cubic meter (m3

)

Flow rate

foot per second (ft/s) 0.3048 meter per second (m/s)

cubic foot per second (ft3

/s) 0.02832 cubic meter per second (m3/s)

cubic foot per day (ft3

/d) 0.02832 cubic meter per day (m3/d)

Mass

ton, short (2,000 lb) 0.9072 megagram (Mg) 

SI to Inch/Pound

Multiply By To obtain

Length

centimeter (cm) 0.3937 inch (in)

millimeter (mm) 0.03937 inch (in)

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

Area

square meter (m2

) 0.0002471 acre 

square centimeter (cm2

) 0.001076 square foot (ft2)

square meter (m2

) 10.76 square foot (ft2) 

square centimeter (cm2

) 0.1550 square inch (ft2) 

square kilometer (km2

) 0.3861 square mile (mi2)

Volume

cubic meter (m3

) 35.31 cubic foot (ft3)

cubic meter (m3

) 1.308 cubic yard (yd3) 

cubic meter (m3

) 0.0008107 acre-foot (acre-ft) 

Flow rate

cubic meter per second (m3

/s) 70.07 acre-foot per day (acre-ft/d) 

cubic meter per second (m3

/s) 35.31 cubic foot per second (ft3/s)

cubic meter per day (m3

/d) 35.31 cubic foot per day (ft3/d) 

Mass

megagram (Mg) 1.102 ton, short (2,000 lb)

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows: °F =(1.8×°C) +32

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows: °C=(°F −32)/ 1.8

In this report, horizontal and vertical coordinate information is referenced in feet above the GRS80 

 ellipse defined by the North American Datum of 1983 (NAD83).

Elevation, as used in this report, refers to NAD83/GRS80 ellipsoid heights and not traditionally defined 

 North American Vertical Datum of 1988 (NAVD88) orthometric heights.
 v

Project Staff

Editor

Theodore S. Melis, Ph.D.

U.S. Geological Survey

Southwest Biological Science Center

Grand Canyon Monitoring and Research Center

Flagstaff, Arizona

Project Coordinator

Lara M. Schmit

Northern Arizona University

Colorado Plateau Research Station

Flagstaff, Arizona

Production Staff

Kay E. Hedrick

U.S. Geological Survey 

Science Publishing Network

Raleigh, North Carolina

David R. Jones

U.S. Geological Survey

Science Publishing Network

Menlo Park, California

Additional Production Support

James E. Banton

U.S. Geological Survey

Science Publishing Network

Nashville, Tennessee

Darlene Ryan

U.S. Geological Survey

Science Publishing Network

Flagstaff, Arizona

Research Assistants

Kyrie L. Fry

Northern Arizona University

Colorado Plateau Research Station

Flagstaff, Arizona

Meredith A. Hartwell

Northern Arizona University

Colorado Plateau Research Station

Flagstaff, Arizona

Special Acknowledgment

The assistance of Matthew E. Andersen, U.S. Geological 

Survey; Colden Baxter, Idaho State University; Wyatt Cross, 

Montana State University; Amy E. Draut, U.S. Geological 

Survey; Helen C. Fairley, U.S. Geological Survey; Graham 

D. Foster, U.S. Geological Survey; Thomas M. Gushue, 

U.S. Geological Survey; Robert O. Hall, Jr., University of 

Wyoming; John F. Hamill, U.S. Geological Survey; Joseph 

E. Hazel, Jr., Northern Arizona University; Matt Kaplinski, 

Northern Arizona University; Kate Kitchell, U.S. Geological 

Survey; Ronald E. Kirby, U.S. Geological Survey; Josh Korman, Ecometric Research, Inc.; Milada Majerova, Utah State 

University; Andrew S. Makinster, Arizona Game and Fish 

Department; Jonathan Nelson, U.S. Geological Survey; William R. Persons, U.S. Geological Survey; Emma Rosi-Marshall, Cary Institute of Ecosystem Studies; Robert Ross, U.S. 

Geological Survey; David M. Rubin, U.S. Geological Survey; 

David J. Topping, U.S. Geological Survey; Marijke van 

Heeswijk, U.S. Geological Survey; Carl J. Walters, University 

of British Colombia; and Michael D. Yard, U.S. Geological 

Survey, is gratefully acknowledged.

Peer and Technical Reviewers

The content of the chapters contained in this report was 

greatly enhanced by the wisdom and research of many colleagues and cooperators. In particular, the project team and 

the authors gratefully recognize the indepth and thoughtful 

review of the independent and anonymous scientists who 

served as peer reviewers for the individual chapters.

Financial Support

This report was funded by hydropower revenues provided by 

the Bureau of Reclamation as part of the Glen Canyon Dam 

Adaptive Management Program (GCDAMP). The GCDAMP 

was established in 1997 to provide for long-term research and 

monitoring of downstream resources affected by Glen Canyon 

Dam. Scientific information, such as that contained in this 

report, is used as the basis for developing recommendations 

for dam operations and management actions. More information on the GCDAMP is available at http://www.gcdamp.gov/.

©David Edwards, used with permission
Mike Quinn, National Park Service
CHAPTER 1

Introduction and Overview 

By Lara M. Schmit1 and John C. Schmidt2

Three high-flow experiments (HFEs) were conducted by the U.S. Department of the Interior at 

Glen Canyon Dam, Arizona, in March 1996, November 2004, and March 2008 (figs. 1 and 2). 

These experiments, also known as artificial or controlled floods, were large-volume, scheduled releases of water from Glen Canyon Dam that were designed to mimic some aspects of pre-dam 

Colorado River seasonal flooding. The goal of these experiments was to determine whether high 

flows could be used to benefit important physical and biological resources in Glen Canyon National 

Recreation Area and Grand Canyon National Park (fig. 2) that had been affected by the operation of 

Glen Canyon Dam. Efforts such as HFEs that seek to maintain and restore downstream resources are 

undertaken by the U.S. Department of the Interior under the auspices of the Grand Canyon Protection 

Act of 1992 (GCPA; title XVIII, secs. 1801–1809, of Public Law 102-575). Scientists conducted a 

wide range of monitoring and research activities before, during, and after the experiments. Initially, 

research efforts focused on whether HFEs could be used to rebuild and maintain Grand Canyon 

sandbars, which provide camping beaches for hikers and whitewater rafters, create habitats potentially used by native fish and other wildlife, and are the source of windborne sand that may help to 

protect some archaeological resources from weathering and erosion. As scientists gained a better 

understanding of how HFEs affect the physical environment, research efforts expanded to include 

additional investigations about the effects of HFEs on biological resources, such as native fishes, 

nonnative sports fishes, riverside vegetation, and the aquatic food web. The chapters that follow 

summarize and synthesize for decisionmakers and the public what has been learned about HFEs to 

provide a framework for implementing similar future experiments. 

This report is a product of the Glen Canyon Dam Adaptive Management Program 

(GCDAMP), a Federal initiative authorized to ensure that the primary mandate of the GCPA 

(GCPA sec. 1802 (a)) is met through advances in information and resource management. The 

program and its research efforts focus on a study area that encompasses the Colorado River 

corridor from the forebay of Glen Canyon Dam to the western boundary of Grand Canyon 

National Park, which is identified as the Colorado River ecosystem elsewhere in this report. The 

study area includes the approximately 16-mile river corridor between the dam and Lees Ferry 

within Glen Canyon National Recreation Area and the entire 277-river mile corridor downstream 

from Lees Ferry and within Grand Canyon National Park (fig. 2). The U.S. Geological Survey’s 

Grand Canyon Monitoring and Research Center (GCMRC) is responsible for the scientific monitoring and research efforts of the GCDAMP, including the preparation of this report. The GCMRC 

gratefully acknowledges the contributions of those scientists with Federal and State resourcemanagement agencies, academic institutions, and private consulting firms who undertook much of 

the research presented in the chapters that follow. 

1

 Northern Arizona University, Colorado Plateau Research Station, P.O. Box 5614, Flagstaff, AZ 86011.

2

 Intermountain Center for River Rehabilitation and Restoration, Department of Watershed Sciences, Utah State University, 

Logan, UT 84322–5210. 
2 Effects of Three High-Flow Experiments on the Colorado River Ecosystem Downstream from Glen Canyon Dam, Arizona

The Colorado River and Grand Canyon

The Colorado River is one of the most iconic rivers in the United States, and its watershed 

includes parts of seven States—Arizona, California, Colorado, Nevada, New Mexico, Utah, and 

Wyoming. The river is noted for its striking canyons, especially the part of the river that crosses 

the southern Colorado Plateau and forms Grand Canyon. Grand Canyon is one of the world’s most 

spectacular canyon systems, reaching depths of more than 6,000 feet (ft). Recognizing the need to 

protect its world-class geologic wonders and awe-inspiring natural beauty for future generations, 

Theodore Roosevelt issued Presidential Proclamation 794 in 1908 establishing Grand Canyon 

National Monument. The monument became the United States’ 17th national park on February 26, 

1919, when President Woodrow Wilson signed the authorizing bill. 

The Colorado River and Grand Canyon also are central to the traditional values and histories 

of many of the region’s Native Americans. In fact, some places in what is now Grand Canyon 

National Park are identified in multiple Tribal creation stories as the place of origin for the Tribe’s 

ancestors (Fairley, 2003; Dongoske and others, 2010). More than 4,300 archaeological resources 

have been documented in Grand Canyon National Park, including about 336 sites in the river 

corridor potentially affected by dam operations (Fairley, 2005). The oldest human artifacts found 

in the park date to the Paleo-Indian period and are almost 12,000 years old (National Park Service, 

2009). In addition to its geologic and cultural legacy, the Colorado River ecosystem downstream 

from Glen Canyon Dam boasts a diverse array of plants and animals, including federally endangered species, such as the humpback chub (Gila cypha) and the southwestern willow flycatcher 

(Empidonax traillii extimus).

Grand Canyon is part of an extensive canyon network through which the Colorado River 

and its major headwater tributaries flow. The Colorado River watershed covers 15 percent of the 

conterminous United States, and its channel network is the conduit by which snowmelt originating in the middle and southern Rocky Mountains reaches the Gulf of California (fig. 3). Three 

large headwater tributary systems—the upper Colorado River, the Green River, and the San Juan 

River—drain the western slope of the Rocky Mountains and contribute flow to the Colorado 

River. Southwest of the Rocky Mountains, these rivers cross the Colorado Plateau, a vast uplifted 

assemblage of sedimentary rocks. The upper Colorado and Green Rivers join just upstream from 

Cataract Canyon in Canyonlands National Park, Utah. The San Juan River joins the Colorado 

Figure 1. Glen Canyon Dam during the 2008 high-flow 

experiment on the night of March 6, 2008. Three highflow experiments—water releases above 

powerplant capacity—were conducted in 1996, 2004, 

and 2008 to determine if water releases designed to 

mimic some aspects of natural pre-dam seasonal 

flooding can be used to improve a wide range of natural resources in Glen Canyon National Recreation Area 

and Grand Canyon National Park. Photograph courtesy 

of Thomas Ross Reeve, Bureau of Reclamation.
Chapter 1—Introduction and Overview 3

River in Glen Canyon National Recreation Area in a part of Glen Canyon inundated by Lake 

Powell, the reservoir formed by Glen Canyon Dam.

Glen Canyon Dam

Glen Canyon Dam, located just south of the Arizona-Utah border, forms a reservoir that is 

one of four main-stem water-storage units authorized in 1956 under the Colorado River Storage 

Project (CRSP) Act (Ch. 203, Public Law 485). The CRSP reservoirs allow the upper basin 

States—Colorado, New Mexico, Utah, and Wyoming—to store water in wet years and release 

water during drier periods, thereby allowing the upper basin States to meet their obligations to 

the lower basin States under the 1922 Colorado River Compact while also maximizing opportunity for future water development. Historically, a minimum of 8.23 million acre-feet (MAF) has 

been released annually from Lake Powell, the second largest reservoir in the United States, to 

satisfy Colorado River Compact obligations to the lower basin and also provide the upper basin’s 

share of water to Mexico under a 1944 treaty. Annual releases have exceeded 8.23 MAF during 

periods of average to above average precipitation levels in the upper Colorado River Basin and to 

balance storage between Lakes Powell and Mead. In response to recent drought conditions in the 

Colorado River Basin, interim guidelines have been established to address the possibility of water 

shortages (U.S. Department of the Interior, 2007).

The CRSP Act included hydropower facilities at three of the four water-storage units it 

authorized, including Glen Canyon Dam, to produce salable power. The powerplant at Glen 

Canyon Dam is made up of eight hydroelectric generation units. In 2007, the powerplant 

produced 3.5 million megawatt hours. Power generated at CRSP facilities is first provided to 

CRSP participating projects, which typically are irrigation projects (Harpman and Douglas, 

2005). The power that is surplus to project uses is sold to approximately 240 wholesale 

customers—municipal and county utilities, rural electric cooperatives, Federal and State 

facilities, Native American Tribes, and nonprofit organizations—primarily located in seven 

States. The revenues generated by the CRSP units are used to pay for annual operation and 

maintenance costs, including environmental programs such as the GCDAMP, power facility 

construction costs, and other nonpower-related costs assigned by Congress (Harpman and 

Douglas, 2005).

Figure 2. The Glen Canyon 

Dam Adaptive Management 

Program study area includes 

the Colorado River corridor 

that stretches approximately 

293 river miles from the forebay of Glen Canyon Dam to 

the westernmost boundary of 

Grand Canyon National Park, 

Arizona.

Lake Mead

Lake Powell

Paria

Riv e r

Kanab

Creek

Diamond Cr Havasu Cr

Bright Angel Cr

Glen Canyon Dam

GRAND CANYON

NATIONAL PARK

GLEN CANYON

NATIONAL RECREATION

AREA

Hoover Dam

Lees Ferry

Little Colorado River

0 20 40 miles

0 20 40 60 kilometers

UTAH

ARIZONA

NEVADA

Colorado

R i v e r

M

A R B L E C A N Y O N

Hualapai

Indian 

Reservation

Hopi

Indian 

Reservation

Havasupai

Indian 

Reservation

Navajo

Indian 

Reservation

Kaibab

Indian 

Reservation

Paiute

Indian 

Reservation

N

ARIZONA

Map

Area
1956 1957

Last completely unregulated spring 

snowmelt flood, which had a peak flow 

of about 126,000 ft3

/s, to pass through 

Grand Canyon is measured at the Lees 

Ferry gaging station.

Glen Canyon Dam

construction begins.

Events Relevant to High-Flow Experiments

1963

Glen Canyon Dam is closed and 

complete regulation of the river begins; 

U.S. Supreme Court held in Arizona v. 

California that, as the result of the 

Boulder Canyon Project Act, California 

held an allocation of 4.4 MAF, Arizona 

2.8 MAF, and Nevada 300,000 acrefeet of Colorado River water.

4 Effects of Three High-Flow Experiments on the Colorado River Ecosystem Downstream from Glen Canyon Dam, Arizona

Figure 3. The Colorado River 

watershed covers 15 percent of the 

conterminous United States. The 

channel network of the watershed is 

the conduit by which snowmelt originating in the middle and southern 

Rocky Mountains reaches the Gulf of 

California.

Time line photograph credits: Glen Canyon Dam construction, Bureau of Reclamation; Glen Canyon Dam, Anne 

Phillips, USGS; Colorado pikeminnow, George Andrejko, 

Arizona Game and Fish Department; Glen Canyon Dam 

with open jet tubes, Anne Phillips, USGS; Razorback 

sucker, George Andrejko, Arizona Game and Fish 

Department; jet tubes, Anne Phillips, USGS; scientists, 

Paul Alley, USGS; Humpback chub, George Andrejko, 

Arizona Game and Fish Department.

NEVADA

Glen Canyon Dam

Hoover

Dam

CALIFORNIA ARIZONA

MEXICO

COLORADO

NEW MEXICO

UTAH

Colorado River drainage basin

WYOMING

R o c k y

M o u n t a i n s

Colorado

Plateau

San

Ju a n Ri ver

Littl e Color ado River

G r e e n

R i v e r

U p p e r C o l o r a d o

R i v e r

Lake Powell

Lake Mead

L

o

w e r C o l o r a d o R i v e r

Gulf

of 

California

Las Vegas

Phoenix

0 100 200 miles

N

0 100 200 kilometers
1965 1966 1967 1968 1969 1972 1973

Bureau of Reclamation released a series of 14 short-duration high flows,

6 of which were in excess of 50,000 ft3

/s, from Glen Canyon Dam. These 

high flows scour fine and coarser sediment from the Lees Ferry reach, 

which combined with the clear and cold water released from the dam 

created habitat conditions ideal for a nonnative trout fishery below the 

dam in Glen Canyon National Recreation Area.

National Historic 

Preservation Act passed.

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.

Last verified record of Colorado pikeminnow 

caught in Grand Canyon at Havasu Creek.

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 Service (marine species). Humpback chub and Colorado 

pikeminnow federally listed as 

endangered.

Colorado River 

Basin Project 

Act passed.

Chapter 1—Introduction and Overview 5

Changing Societal Values and 

Efforts to Protect Grand Canyon

Until the last half of the 20th century, the goal of Federal policy was to control and use the 

waters of the Colorado River to create wealth and new opportunities through dam building and 

hydropower generation. The early 20th century view of the value of the Colorado River was 

expressed in a 1946 report prepared by the U.S. Department of the Interior entitled “The Colorado 

River—A Natural Menace Becomes A National Treasure,” which stated: 

In their present state this land, this water, and these minerals are not wealth because 

they are not being utilized economically. * * * Water can be brought to this land to 

produce crops; these minerals can be mined and processed with an abundance of lowcost hydroelectric energy made available; trade can be established; and in general, the 

wealth produced can be converted into more and better opportunities for the American 

people (U.S. Department of the Interior, 1946, p. 211).

At the time Glen Canyon Dam was constructed (1956–63), little consideration was given to how 

dam operations might affect downstream resources in Grand Canyon National Park (Babbitt, 1990). 

In fact, the dam was completed before enactment of the National Environmental Policy Act of 1969 

(42 U.S.C. § 4321 and § 4331–4335, Public Law 91-190) and the Endangered Species Act of 1973 

(16 U.S.C. § 1531–1544, 87 Stat. 884, Public Law 91-135). By the late 1950s, public values began 

to shift, and throughout the 1960s and 1970s recognition of the environmental consequences of Glen 

Canyon Dam and its operation grew. National Park Service and U.S. Geological Survey scientists 

and river recreationists observed the physical transformation of the river in Grand Canyon, including 

the loss of large beaches used for camping, narrowing of rapids so as to reduce navigability, and 

changes in the distribution and composition of riparian vegetation (Dolan and others, 1974; Cooley 

and others, 1977; Turner and Karpiscak, 1980; Howard and Dolan, 1981). The humpback chub and 

Colorado pikeminnow (Ptychocheilus lucius), species found only in the Colorado River Basin, were 

listed as endangered in 1967 under the Endangered Species Preservation Act (Public Law 89-669, 80 

Stat. 926) by the U.S. Fish and Wildlife Service, which concluded in 1978 that the dam and its operation jeopardized the continued existence of humpback chub in Grand Canyon. 

The status of Grand Canyon as a crown jewel of the National Park System and concerns 

about the effects of Glen Canyon Dam have inspired lawsuits, agency actions, and legislation. The 

GCPA, legislation authorizing Federal efforts to protect resources downstream from Glen Canyon 

Dam, directs the Secretary of the Interior to operate the 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)). The act 

also clearly states that it was to be implemented in accordance with existing laws, treaties, and 
1974 1975 1978 1979 1980 1982

First lawsuit filed over Glen Canyon Dam operations by commercial river raft operators 

contending that the disruption of normal 

flows was interfering with their ability to conduct river trips.

U.S. Fish and Wildlife Service files 

jeopardy opinion on the effects of 

Glen Canyon Dam on endangered 

fishes.

Grand Canyon National Park designated a United Nations Educational, 

Scientific and Cultural Organization 

World Heritage Site.

Lake Powell reaches full pool (3,700 ft); bonytail 

chub (Gila elegans) federally listed as endangered.

Glen Canyon Environmental

Studies created to study effects 

of Glen Canyon Dam operations. 

Grand Canyon National Park 

Enlargement Act passed. 

6 Effects of Three High-Flow Experiments on the Colorado River Ecosystem Downstream from Glen Canyon Dam, Arizona

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

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

The “Operation of Glen Canyon Dam: Final Environmental Impact Statement” (hereafter 

referred to as EIS), which outlined alternative dam-operation strategies for meeting GCPA requirements, was filed in March 1995, and the Record of Decision was signed by the Secretary of the 

Interior in October 1996. The Record of Decision noted that the goal of selecting the preferred alternative “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 capability and flexibility only to the extent necessary to achieve recovery 

and long-term sustainability” (U.S. Department of the Interior, 1996b, p. G-11). Having established 

this goal, the Secretary’s decision was to implement the modified low fluctuating flow (MLFF) 

alternative, the preferred alternative described in the EIS, with minor changes (U.S. Department of 

the Interior, 1996b). The Record of Decision also formally established the GCDAMP.

The Colorado River Before and After the Construction 

of Glen Canyon Dam

Annual spring snowmelt floods were the defining attribute of the pre-dam flow regime (White 

and others, 2005). Before the Colorado River was regulated by dams, streamflow gradually 

increased from mid-December to March, precipitously increased in April and May, and reached 

its peak in early June. The timing and magnitude of the annual pre-dam snowmelt flood depended 

on how much snow had accumulated in the Rocky Mountains during the preceding winter and 

the rate at which the snowpack melted in the spring. The largest recorded flood in Grand Canyon 

occurred in June 1884 and was approximately 210,000 cubic feet per second (ft3

/s; Topping and 

others, 2003). Approximately every 2 years, however, the largest annual snowmelt flood measured 

about 85,000 ft3/s (Topping and others, 2003), which is almost three times greater than flows that 

occur when Glen Canyon Dam is operated at powerplant capacity. Smaller floods of shorter duration 

occurred in late summer and fall in many years during the North American monsoon season when 

moisture from the Pacific Ocean moves northward through Mexico and up to the Colorado Plateau. 

Although most of the water in the Colorado River originates from the distant Rocky 

Mountains, most of the sediment carried by the river originates in nearby desert watersheds 

of the Colorado Plateau. Tributaries in Colorado Plateau watersheds contribute large amounts 

of sand, silt, and clay to the Colorado River, and this sediment load is then trapped in Lake 

Powell. Before extensive European settlement, the Colorado River delivered about 100 million 

tons of sand per year to its delta at the head of the Gulf of California (Meade and others, 1990) 

and transported approximately 60 million tons of sand per year past Lees Ferry (Topping and 

others, 2000). 
1983 1984 1989 1990

1990–91

1987 1991

Glen Canyon Dam releases a 

peak flow of 97,300 ft3

/s to 

stop Lake Powell from overtopping Glen Canyon Dam.

One of the last razorback suckers 

(Xyrauchen texanus) seen in Grand 

Canyon is caught and released at 

Bass Rapids.

National Research Council completes review of Glen 

Canyon Dam Environmental Studies, publishing “River 

and Dam Management: A Review of the Bureau of 

Reclamation’s Glen Canyon Environmental Studies.”

Secretary of the Interior Manuel Lujan, Jr., orders an Environmental Impact Statement on Glen Canyon Dam operations. 

Research flows used to evaluate a variety of fluctuating and 

steady flow patterns.

Interim operation criteria for Glen 

Canyon Dam implemented; razorback sucker and Kanab ambersnail 

federally listed as endangered.

The National Research Council sponsors symposium that reviews existing knowledge of the 

Colorado River ecology and dam management.

Chapter 1—Introduction and Overview 7

Figure 4. The endangered 

humpback chub (Gila cypha) 

is a freshwater fish found 

only in the Colorado River 

Basin. Reaching 20 inches in 

length, the humpback chub 

has features that distinguish 

it as a native Colorado River 

fish—large adult body size, 

small depressed skull, and 

small eyes. 

Pre-dam floods disturbed the aquatic ecosystem, and native fish species developed strategies 

to survive periods when the velocity in the main part of the channel was high and large amounts 

of suspended sediment were being transported. For example, several of the native fish species 

share unusual body shapes, including a large adult body size, small depressed skulls, large humps 

on their backs, and small eyes, which presumably developed as adaptations to life in a turbid and 

seasonably variable riverine environment (fig. 4; chapter 4, this volume; Minckley, 1991). During 

typical floods, the mean velocity of the Colorado River exceeded 10 feet per second (ft/s) at Lees 

Ferry, five times greater than the velocity during typical pre-dam base flows (Burkham, 1986). 

Base flow is that part of the stream discharge that is not attributable to direct runoff from precipitation or melting snow and usually is sustained by groundwater. Sandbars, riverbanks, and their 

accompanying aquatic habitats were reshaped during floods. Additionally, the increased elevation 

of the river surface during floods provided water to native riparian vegetation otherwise principally dependent on precipitation. 

The Regulated River

The regulation of rivers by dams, such as Glen Canyon Dam, results in multidimensional 

physical and ecological changes to the system. For example, Ward and Stanford (1995) emphasize that river regulation not only alters river characteristics, such as temperature and flow, but 

also results in changes in interactions between the river and other water bodies and between 

aquatic and riparian systems, including changes in the movement of nutrients, sediment, fish, 

and other organisms. Flow regulation of the Colorado River by Glen Canyon Dam effectively 

Photograph courtesy of George Andrejko, Arizona Game and Fish Department
1992 1994 1995 1996 1997

Grand Canyon Protection 

Act of 1992 passed.

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 federally listed as 

endangered; U.S. Department of the Interior establishes the Grand Canyon Monitoring and Research 

Center in Flagstaff, Arizona.

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.

First high-flow experiment of 45,000 ft3

/s conducted at 

Glen Canyon Dam; Record of Decision for the operation 

of Glen Canyon Dam signed by Secretary of the Interior 

Bruce Babbitt.

8 Effects of Three High-Flow Experiments on the Colorado River Ecosystem Downstream from Glen Canyon Dam, Arizona

replaced relatively high and low flows with a greater frequency of moderate flows. The presence 

and operation of the dam altered the natural timing and magnitude of floods, the most important and 

dynamic attribute of the pre-dam flow regime. For example, if post-dam floods are defined as “[dam] 

releases exceeding powerplant capacity [33,200 ft3

/s] for a month or more,” a definition used by the 

Glen Canyon Environmental Studies (U.S. Department of the Interior, 1988, p. A-21), the science 

organization that preceded the GCMRC, then post-dam floods only occurred in 1983, 1984, 1985, 

and 1986. The largest of these post-dam floods occurred on June 29, 1983, when a peak flow 

of 97,300 ft3

/s was measured at Lees Ferry. Additionally, soon after the completion of the dam, 

the Bureau of Reclamation released a series of six short-duration flows in excess of 50,000 ft3

/s 

in April and June 1965, which scoured the river channel in the tailwater (the 16-mile section of 

river downstream from Glen Canyon Dam) and created conditions suitable for a nonnative trout 

fishery (chapter 2, this volume).

High-Flow Experiments

Beginning in the late 1970s, scientists started exploring and documenting ecological 

responses to alterations in components of natural flow regimes, including the loss of sensitive 

species and disruption of spawning and migration signals for fish (Poff and others, 1997). This 

new information generated a number of projects to restore elements of natural flow regimes 

throughout the country (Poff and others, 1997). In Grand Canyon, the 1996 Record of Decision 

included the use of beach/habitat-building flows, which according to the EIS were “scheduled high releases of a short duration designed to rebuild high elevation sandbars, deposit 

nutrients, restore backwater channels, and provide some of the dynamics of a natural system” 

(U.S. Department of the Interior, 1995, p. 40). Essentially, resource managers sought to benefit 

key terrestrial and aquatic resources by simulating one aspect of the pre-dam river—floods. 

Specifically, beach/habitat-building flows were defined as infrequent high releases that are at least 

10,000 ft3

/s greater than the allowable peak discharge in a minimum release year (25,000 ft3/s for 

MLFF) but not greater than 45,000 ft3/s for 1 to 2 weeks. More recently, “high-flow experiment” 

(HFE), the term used in this report, has been used to describe experimental flows from Glen 

Canyon Dam ranging from powerplant capacity (33,200 ft3/s) to 45,000 ft3/s. 

In accordance with the principles of adaptive management, also known as “learning by doing,” 

the EIS identified uncertainties about sandbar building in response to high releases of short duration and required a test of a beach/habitat-building flow before long-term implementation of this 

element of the EIS (U.S. Department of the Interior, 1995). In reaching a finding of no significant 

impact, which paved the way for the first test of experimental high flows, the 1996 final environmental assessment reached the following conclusion: “Because all impacts of the proposed action 

on downstream resources are consistent with natural processes, they are considered to be beneficial 

to the overall ecosystem” (U.S. Department of the Interior, 1996a, p. iv). Although no significant 
2000 2001 2002 2004 2005

2003–5

2003–6

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 (high, 

steady flows within powerplant capacity) 

conducted in spring and summer.

Draft strategic plan for the Glen Canyon Dam Adaptive Management Program developed by program members.

The U.S. Department of the Interior approves an environmental assessment 

that outlines a new strategy for releasing high-flow experiments that created 

a triggering threshold linked to sand 

inputs and retention.

Experimental nonnative fish suppression flows 

were implemented at Glen Canyon Dam to 

reduce the abundance of rainbow trout because 

of concerns about negative effects of high trout 

abundance on endangered native fish.

Nonnative fish were experimentally removed from critical humpback chub habitat in the Colorado River to evaluate the feasibility of controlling nonnative 

fish and effects of control on the 

fish community.

Fall sediment inputs to Marble Canyon 

from the Paria River result in the initiation 

of the second high-flow experiment on 

Sunday, November 21.

Grand Canyon Monitoring and Research Center published “The State of the Colorado River 

Ecosystem in Grand Canyon,” which evaluated 

how downstream resources responded to 

modified low fluctuating flows and showed 

that some of the benefits expected under Record of Decision operations have not occurred. 

Chapter 1—Introduction and Overview 9

impacts were identified, the final environmental assessment identified predicted impacts of the test to 

resources on the basis of the scientific knowledge that existed at the time (table 1; U.S. Department 

of the Interior, 1996a). The first HFE was conducted between March 26 and April 7, 1996, just 1 

year after completion of the EIS, and involved a 7-day steady release of 45,000 ft3/s that was preceded 

and followed by low steady flows of 8,000 ft3/s for 4 days each to photograph river shorelines (fig. 5; 

Schmidt and others, 1999). A coordinated team of scientists documented the effects of the 1996 HFE 

on physical, biological, cultural, and recreational resources; the findings were well publicized and 

documented, serving to substantially improve the scientific understanding of the effects of the experiment (Webb and others, 1999; Patten and Stevens, 2001). 

On the basis of improved scientific knowledge resulting from the 1996 HFE, the timing and 

duration of the 2004 and 2008 experiments were modified from the 1996 experimental design; 

projections made for the 2004 and 2008 HFEs (table 1) also were revised to reflect improved 

understanding. The second HFE occurred between November 22 and 24, 2004, and involved a 

60-hour release of about 41,700 ft3/s. The third HFE occurred between March 6 and 8, 2008, and 

included a 60-hour release of about 42,800 ft3

/s. The 2004 and 2008 HFE water releases increased 

from base to peak flows over 30-hour periods, a 50-percent slower rate of rise than for the 1996 

event. For comparison, peak flows for each of three HFEs conducted to date have been lower and 

of shorter duration than the peak flows typical of the pre-dam period or for the unregulated inflow 

to Lake Powell; the three HFEs also occurred earlier or later in the year than pre-dam seasonal 

flooding periods (fig. 5). 

Figure 5. Discharge of the 

Colorado River at Lees Ferry 

(U.S. Geological Survey gaging station 9380000) for the 

calendar years during which 

the three high-flow experiments occurred—1996, 2004, 

and 2008—and 1937. The 1937 

discharge record is shown 

because it represents a typical 

pre-dam year. The 1937 instantaneous peak discharge of 

84,800 cubic feet per second 

was equal in magnitude to an 

average pre-dam flood likely 

to occur approximately every 

2 years.

Discharge, in thousand cubic feet per second 20

0

40

60

80

Jan Feb Mar Apr May June July Aug Sept Oct Nov Dec Jan

2008 2004

1996

1937
2006 2007 2008 2009 2010

The Bureau of Reclamation began the process to develop an 

environmental assessment for nonnative fish control in March.

USGS estimates that the number of Grand Canyon adult humpback 

chub increased to between 6,000 and 10,000 fish. The Bureau of 

Reclamation begins the process of developing an environmental 

assessment for a protocol for future high-flow experiments.

In November, sediment supplies in the Colorado 

River reach once-in-a-decade levels, initiating 

National Environmental Policy Act compliance 

activities in preparation for a third high-flow experiment. In response to basinwide drought conditions, interim shortage guidelines and coordination operations for Lakes Powell and Mead 

are approved in December.

Paria River sand inputs exceed the annual 

average in October and reach levels in excess of those that triggered the 2004 experiment but do not result in a high-flow 

release. The Long-Term Experimental Plan 

Environmental Impact Statement was begun to address flow and nonflow actions in 

the Colorado River below Glen Canyon Dam. Third high-flow experiment occurred between March 6 and 8.

10 Effects of Three High-Flow Experiments on the Colorado River Ecosystem Downstream from Glen Canyon Dam, Arizona

High-Flow Experiments and Adaptive Management

More than 15 years ago, the Secretary of the Interior issued the Record of Decision regarding 

the operation of Glen Canyon Dam that selected adaptive management as the means of managing 

the dam and other efforts to meet the GCPA mandate. In selecting adaptive management, the intent 

of the EIS was to create a process “whereby the effects of dam operations on downstream resources 

would be assessed and the results of those resource assessments would form the basis of future 

modifications of dam operations,” because it was recognized that “many uncertainties still exist 

regarding the downstream impacts of water releases from Glen Canyon Dam” (U.S. Department 

of the Interior, 1995, p. 34). Monitoring and evaluation of the response of downstream resources to 

the HFEs led to the modification of elements of the experiments over time to incorporate improved 

scientific knowledge (Lovich and Melis, 2007).

Some of the findings documented in the following chapters were unanticipated. For example, 

it was concluded in the 1995 EIS that tributary inputs of sand would accumulate over multiple 

years on the channel bed of the Colorado River in Marble Canyon (fig. 2) and eastern Grand 

Canyon under the MLFF operating regime during minimum release years (8.23 MAF; U.S. 

Department of the Interior, 1995), making sand available for redistribution to sandbars by using 

HFEs. Monitoring following the 1996 HFE indicated, however, that tributary-supplied sand does 

not accumulate on the channel bed over multiyear periods under typical dam operations; rather, 

it is carried downstream (Topping and others, 2000; Rubin and others, 2002). Typically, there is 

insufficient sand on the channel bed in Marble and Grand Canyons to rebuild sandbars by using 

HFEs. Scientists and resource managers subsequently focused on the need to strategically time 

HFEs to take advantage of episodic tributary floods that supply new sand to the river downstream 

from the dam (Rubin and others, 2002). In November 2004, an HFE was timed to follow tributary 

floods for the first time (see chapter 3, this volume). In the fall of 2004, flooding in the Paria River 

(fig. 2) enriched the Colorado River with fine sediment in Marble Canyon in advance of the HFE. 

As a result, the 2004 HFE resulted in an increase in the total area and volume of sandbars in the 

upper half of Marble Canyon but produced results downstream similar to those seen in 1996. The 

March 2008 HFE occurred following above-average sand inputs from the Paria River in fall 2006 

and fall 2007, but the new sand supplies were partially depleted and coarsened because they were 

exposed to normal dam operations in the months between inputs and the HFE. Because of these 

relatively rare multiyear sand inputs, the 2008 HFE was different from both the 1996 and 2004 

HFEs (U.S. Department of the Interior, 2008) and resulted in widespread increases in the area and 

volume of sandbars similar to or greater than the two previous HFEs.

Biological research also produced unanticipated results and the opportunity for increased 

learning (see chapter 4, this volume). Research conducted in conjunction with the 2008 HFE provided 

valuable new information about how nonnative rainbow trout (Oncorhynchus mykiss) respond to 

the disruption to flow, habitat, and food sources caused by spring HFEs. For example, the March 
Chapter 1—Introduction and Overview 11

Table 1. Effects on resources downstream from Glen Canyon Dam predicted to result from three high-flow experiments (HFEs) 

conducted in 1996, 2004, and 2008 on the basis of the science available at the time of each experiment (U.S. Department of the Interior, 

1996a, 2002, and 2008). Changes over time in predicted resource responses reflect improved understanding of the effects of high flows 

on affected resources.

Resource 1996 2004 2008

Water and 

hydropower

No effect on end-of-year water storage 

in Lakes Powell and Mead. Two-percent 

less energy generated during test flow. 

Little or no effect on wholesale or retail 

power rates. Total financial cost: $3.1 

to $4.3 million; economic cost: $0.5 to 

$2.2 million.

Annual dam releases will be the same as 

no action. Bypass of about 93,000 acrefeet of water (about 1 percent of annual output), additional power purchase 

requirements during steady 8,000 cubic 

feet per second. Aerial photography flows 

included in cost of all sediment input 

scenarios. 

2008 water year release unchanged. 

Predicted changes in levels of Lakes Powell and Mead are minor temporary effects. 

Projected cost of the high flow test is 

$4.1 million, or a 9.4-percent increase in 

purchase power requirement for 2008. 

Sediment 

and sandbars 

One to 3 feet sand deposition on most 

sandbars followed by erosion over time. 

Net erosion on some sandbars during test flow. Sand transport upstream 

above Little Colorado River estimated at 

850,000 tons in 1996.

More likely to rebuild sandbars and 

beaches than in 1996, with more diverse 

grain size; downstream sediment export 

would be less than in 1996.

Significant positive sandbar building will 

occur, with likely increase in number and 

size of sandbars immediately after the 

event; uncertainty about where and how 

long the effects will persist. Potential 

beneficial change in backwaters.

Fish and 

aquatic food 

base

Temporary reduction in Cladophora

biomass with increased drift downstream. 

Backwaters re-formed. Nonnative populations temporarily disrupted by high 

flows; interactions between native and 

nonnative fish rapidly return to no-action 

conditions. Some trout (Oncorhynchus 

mykiss) eggs, fry, and young lost downstream; mitigation through stocking. 

Adult trout may be affected for a period 

following test flow.

Reduction in benthos species; increased 

drift; primary producers expected to 

rapidly recover; improved production 

following removal of detritus.

May disrupt ongoing spawning but 

improve spawning habitat; may displace 

small-bodied fish. Anticipated positive 

effect on nearshore rearing habitats used 

by humpback chub (Gila cypha) through 

rejuvenation; short-term negative effect 

on food base; limited displacement of 

juvenile fish, very little displacement of 

subadults and adults. 

No significant adverse effects on movement, habitat use, or diet of humpback 

chub. No significant impact to standing 

biomass of benthic invertebrates over 

long term. For Lees Ferry, temporary 

reduction in abundance of Lees Ferry 

rainbow trout smaller size classes, with 

some downstream displacement of 

age-1 fish, but no lasting impacts to 

abundance or condition. Creation and 

improvement of backwater rearing 

habitats for native fish.

Vegetation, 

habitat, and 

special status 

species

Some woody and emergent marsh 

vegetation lost through scouring or 

burial; vegetation recovery to no-action 

levels in months/years following test 

flow. Some wildlife habitat lost; recovery 

to no-action levels following test flow. 

No long-term effects on aquatic food 

base; few wintering waterfowl present 

during test flow. Habitat improvement 

for southwestern willow flycatcher 

(Empidonax traillii) and humpback 

chub. Some Kanab ambersnail (Oxyloma 

haydeni ssp. kanabensis) and northern 

leopard frog (Rana pipiens) habitat 

inundated by test flow; leopard frog 

population may be lost.

Twenty-percent reduction in cover in 

riparian zone; drowning of xeric-adapted 

species and burying low-lying grasses 

and herbs; scouring weakly rooted plants; 

increased distribution of seeds; seed 

scour may initiate germination of native 

riparian plants; potential ice damage 

of marshes and wetlands in Glen and 

Marble Canyons; minimum effects on 

lake riparian vegetation. Loss of up to 

17 percent of primary Kanab ambersnail 

habitat if not already removed by 

previous high release.

Minor effects; short term burial of seeds 

and plants on existing sandbars, some 

scouring of riparian vegetation, and a 

short-term increase in groundwater and 

soil nutrient concentrations. Newly 

exposed sediment may be subject to 

exotic species colonization. Moving 

Kanab ambersnails and their habitat 

for mitigation could result in adverse 

effect. Effects on northern leopard frogs 

uncertain.

Cultural 

resources

High terrace erosion rates may be reduced 

in short term. Temporary restoration of 

natural processes generally beneficial.

No historic properties affected; adverse 

effect on Tribal cultural resources 

(marshes, herpetofauna). 

No adverse effects expected; one historic 

property in Glen Canyon National Recreation Area could be adversely affected.

Recreation

River-based recreation activities affected 

to some degree during test flow. Number 

and size of camping beaches increased.

The high-flow period will have negligible effect on recreation because of 

short duration. National Park Service to 

forewarn boaters and campers; no effect 

on float trips. 

Short-term disruption of Lees Ferry angling 

(3 days to a week). Loss of about 190 boating user days; improved boater experience 

for groups on water during high flows. 

Potential to improve camping beaches.
12 Effects of Three High-Flow Experiments on the Colorado River Ecosystem Downstream from Glen Canyon Dam, Arizona

2008 HFE and the dam operations that followed resulted in more than a 400-percent increase in the 

survival and growth rates of rainbow trout hatched in the month following the experiment and more 

than a 200-percent increase in expected trout abundance by 2009 (Korman and others, 2010; Korman 

and Melis, 2011; Korman and others, in press). Additionally, annual fish monitoring detected an 

800-percent increase in rainbow trout catch rates between 2007 and 2009 downstream from Lees Ferry 

at the confluence of the Colorado and Little Colorado Rivers (Makinster and others, 2010) where 

most of the Grand Canyon population of endangered humpback chub are found. The influx of nonnative trout, which prey on (Yard and others, in press) and are thought to compete with native fishes 

for habitat and limited food resources, could reverse recent improvements in adult humpback chub 

numbers and recruitment rates of juvenile fish into the adult population (Coggins and Walters, 2009).

Although it has been 15 years since the first HFE, uncertainties still remain about downstream 

impacts of water releases from Glen Canyon Dam. Continued scientific research and long-term 

monitoring will help address some of the most pressing questions. For example, ongoing monitoring 

will allow scientists to determine if increased numbers of trout near the confluence of the Colorado 

and Little Colorado Rivers result in a corresponding decline of adult humpback chub. But some of the 

answers also raise critical questions: Can dam operations achieve resource goals over the long term? 

What are the resource tradeoffs of different operations? In the context of the adaptive management 

program, scientific research can be used to reduce, but not eliminate, the uncertainties inherent in 

management decisions. The design of future HFEs (see chapter 5, this volume) will require flexibility 

to respond to some of the unanticipated outcomes that have already been identified since the EIS was 

completed and others that are likely to emerge as the result of improved scientific knowledge.

Report Organization

The chapters that follow provide a summary and synthesis of the extensive number of studies 

conducted since 1996 on the effects of HFEs on the physical, biological, recreational, and cultural 

resources found in the Colorado River corridor downstream from Glen Canyon Dam. Chapter 2 

provides an overview of what was known about how floods affect physical processes and resources 

in the Colorado River ecosystem downstream from the dam before the first HFE in 1996. Chapter 3 

examines the experimental use of three high flows greater than powerplant capacity from Glen 

Canyon Dam in 1996, 2004, and 2008 to rehabilitate the physical setting of the Colorado River in 

Grand Canyon, tracing the evolution in understanding that has occurred over time about the duration, timing, and frequency of HFEs. This chapter also examines how HFEs affect sandbar size, 

wind transport of river sand from new sandbars toward archaeological sites, camping beaches, and 

backwater channel habitats. Chapter 4 considers how HFEs influence the biological components of 

the system, including the aquatic food web, the native and nonnative fish communities, riverside 

vegetation, and the endangered Kanab ambersnail (Oxyloma haydeni ssp. kanabensis). On the basis 

of the findings presented in the preceding chapters, the concluding chapter outlines what scientists have identified as some of the key resource-management implications associated with HFEs, 

suggests a strategy for initiating future HFEs, and considers experimentation needed to address 

outstanding research questions previously identified by river managers.

Units and Place Names

U.S. customary units are used for all measurements provided in this report to facilitate understanding by the general reader. Metric conversions are provided in a table at the front of the report. 

River mile (RM) is used to describe distances along the Colorado River in the study area. The use 

of the river mile has historical precedent and provides a reproducible method for describing location. Lees Ferry is considered the reference point, RM 0, with mileage measured for both upstream 

and downstream locations. Locations upstream from Lees Ferry in the Glen Canyon National 

Recreation Area (referred to as the Lees Ferry reach) are assigned negative river mile designations; 
Chapter 1—Introduction and Overview 13

thus, Glen Canyon Dam is located at RM −16. For purposes of this report, the entire canyon system 

between Lees Ferry and the Grand Wash Cliffs (RM 277) is referred to as Grand Canyon National 

Park or simply Grand Canyon. Shorter segments of this canyon system are referred to as Marble 

Canyon (RM 0 to RM 61), which ends at the confluence of the Colorado and Little Colorado 

Rivers; eastern Grand Canyon (RM 61 to RM 88); central Grand Canyon (RM 88 to RM 166); and 

western Grand Canyon (RM 166 to RM 277). For some purposes, Marble Canyon is further divided 

into upper Marble Canyon (RM 0 to RM 30) and lower Marble Canyon (RM 30 to RM 61). 

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Impact Statement, Colorado River Storage Project, Coconino County, Arizona: Salt Lake City, 

Utah, Bureau of Reclamation, Upper Colorado Regional Office, 337 p., accessed November 18, 

2010, at http://www.usbr.gov/uc/library/envdocs/eis/gc/pdfs/Cov-con/cov-con.pdf.

U.S. Department of the Interior, 1996a, Glen Canyon Dam, Beach/habitat-building test flow, Final 

environmental assessment and finding of no significant impact: Salt Lake City, Utah, Bureau of 

Reclamation, Upper Colorado Region, 68 p. + attachments. 

U.S. Department of the Interior, 1996b, Record of Decision, Operation of Glen Canyon Dam—

Final Environmental Impact Statement: Washington, D.C., Office of the Secretary of the Interior, 

Bureau of Reclamation, 15 p., accessed May 26, 2010, at http://www.usbr.gov/uc/rm/amp/pdfs/

sp_appndxG_ROD.pdf.

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

removal of non-native fish—Environmental assessment: Salt Lake City, Utah, Bureau of Reclamation, Upper Colorado Regional Office, 112 p.

U.S. Department of the Interior, 2007, Record of Decision, Colorado River interim guidelines for 

lower basin shortages and the coordinated operations for Lake Powell and Lake Mead, Final 

Environmental Impact Statement: Washington, D.C., Office of the Secretary of the Interior, 59 p. 

accessed November 23, 2010, at http://www.usbr.gov/lc/region/programs/strategies/

RecordofDecision.pdf.

U.S. Department of the Interior, 2008, Final environmental assessment—Experimental releases 

from Glen Canyon Dam, Arizona, 2008 through 2012: Salt Lake City, Utah, Bureau of 

Reclamation, Upper Colorado Region, 59 p. + apps., accessed January 13, 2010, at 

http://www.usbr.gov/uc/envdocs/ea/gc/2008hfe/index.html.

Ward, J.V., and Stanford, J.A., 1995, The serial discontinuity concept—Extending the model to 

floodplain rivers: Regulated Rivers, Research & Management, v. 10, p. 159–168.

Webb, R.H., Schmidt, J.C., Marzolf, G.R., and Valdez, R.A., eds., 1999, The controlled flood 

in Grand Canyon: Washington, D.C., American Geophysical Union, Geophysical Monograph 

Series, v. 110, 367 p.

White, M.A., Schmidt, J.C., and Topping, D.J., 2005, Application of wavelet analysis for monitoring the hydrologic effects of dam operation—Glen Canyon Dam and the Colorado River at Lees 

Ferry, Arizona: River Research and Applications, v. 21, p. 551–565.

Yard, M.D., Coggins, L.G., Baxter, C.V., Bennett, G.E., and Korman, Josh, in press, Trout piscivory 

in the Colorado River, Grand Canyon—Effects of turbidity, temperature, and fish prey availability: Transactions of the American Fisheries Society.
Mark Lellouch, National Park Service
CHAPTER 2

Understanding Physical Processes 

of the Colorado River 

By John C. Schmidt1 and Paul E. Grams2

The spring 1996 high-flow experiment (HFE) initiated a new era of scientifically based 

adaptive management of the Colorado River in the 16 miles of Glen Canyon between the 

dam and Lees Ferry and the 277 miles of the Colorado River ecosystem within Marble 

and Grand Canyons (fig. 1). Since the 1996 HFE, releases from Glen Canyon Dam have been 

used to advance natural-resource goals, such as maintaining populations of endangered fish and 

rebuilding sandbars, while meeting requirements to transfer water from the upper Colorado River 

Basin to the lower basin, and to generate hydroelectricity. 

The management objectives and the associated scientific hypotheses of the 1996 HFE, as 

well as the subsequent HFEs in 2004 and 2008, were founded in decades of scientific study of 

the changing physical and ecological attributes of the Colorado River ecosystem (Carothers and 

Brown, 1991; Webb, 1996; Webb, Wegner, and others, 1999). The purpose of this chapter is to 

describe that foundation by summarizing some of the long-standing scientific lines of inquiry 

related to physical processes. These scientific themes have guided Colorado River scientists for 

more than 50 years. In the next chapter, the insights gained from the three HFEs are presented 

within the context that is set in this chapter. Together, the two chapters describe the ongoing 

Colorado River research program as a continuum that began with observations and measurements 

made a century ago and now includes new analytical approaches and measurement techniques. 

In many cases, the questions asked today are the same questions considered by the pioneers of 

Colorado River science.

Historical Overview

Modern development of the Colorado River began in the late 19th century when diversions 

were constructed to support agriculture in the Grand Valley in Colorado, the Colorado River 

valley near Blythe, California, and Yuma, Arizona, and in tributary valleys elsewhere in the basin 

(Mueller and Marsh, 2002). Construction of the Alamo Canal, which connects the Colorado River 

to the Alamo River, began in 1900 and allowed part of the lower Colorado River to flow into 

California’s Salton Trough to create an “Imperial Valley” (deBuys and Myers, 1999). Laguna 

Dam near Yuma was the first dam built on the main stem of the Colorado River and was the first 

1

 Intermountain Center for River Rehabilitation and Restoration, Department of Watershed Sciences, Utah State University, 

Logan, UT 84322–5210.

2

 U.S. Geological Survey, Southwest Biological Science Center, Grand Canyon Monitoring and Research Center, 

2255 N. Gemini Drive, Flagstaff, AZ 86001.
Lake Powell

Lake Mead

Glen Canyon Dam

Upper Marble Canyon

(UMC)

Lower Marble Canyon

(LMC)

Eastern 

Grand Canyon

(EGC)

Central

Grand Canyon

(CGC)

Western

Grand Canyon

(WGC)

GRAND CANYON

NATIONAL PARK

Lava Falls Rapid

GLEN CANYON

NATIONAL RECREATION

AREA

Lees Ferry gage

30-mi gage

61-mi gage

Diamond Creek gage

166-mi gage Grand

Canyon

gage

Paria

River

Kanab Creek

Diamond Creek Havasu Creek

Bright Angel Cr.

Little Colorado River

0 20 40 miles

0 20 40 60 kilometers

UTAH

ARIZONA

NEVADA

N

Colorado

River

Sandbar study sites

Streamflow and sediment 

transport measurement locations

ARIZONA

Map

Area

permanent dam built by the Bureau of Reclamation (then called the Reclamation Service). The dam 

was completed in 1909 (Mueller and Marsh, 2002). 

Early scientific studies of the Colorado River were used to assess the potential for agricultural 

development and hydropower production (for example, Davis, 1897; Lippincott, 1900). The first scientific observations of the Colorado River in Grand Canyon were made by John Wesley Powell during his 

1869 exploration (Powell, 1875, 1895). Although it may not have been realized at the time, the photographs taken during the second Powell expedition in 1871, as well as photographs taken by early expeditions of the U.S. Geological Survey (Birdseye, 1924), became the basis for subsequent monitoring of 

geomorphic and ecological change in Grand Canyon when Turner and Karpiscak (1980) and Stephens 

and Shoemaker (1987) replicated many of the original images from those expeditions (fig. 2). A similar 

approach to describing long-term change in the Colorado River ecosystem was taken by Webb (1996), 

who replicated the 1889–90 photographs of Robert Brewster Stanton (Smith and Crampton, 1987). 

Streamflow and sediment transport have been measured throughout the Colorado River Basin 

for more than a century. These data were needed to plan irrigation districts, to strategically locate 

reservoirs, and to anticipate reservoir sedimentation rates. The first gaging station in the watershed 

was established on the Gila River at Buttes, Arizona, in 1889. More than 200 gaging stations had 

already been established, and some had already been abandoned, in the Colorado River Basin when 

Eugene C. LaRue (1916) summarized the potential for developing the river system. Water-surface 

elevation measurements were first made at Lees Ferry in May 1921 (Topping and others, 2003). 

Figure 1. The Grand Canyon region, showing locations of gaging stations and data-collection 

sites for sandbar topography and channel bathymetry. The Lees Ferry gage (U.S. Geological 

Survey station 093820000) continuously measures stage and streamflow; supplemental sediment 

transport measurements have been made during some post-dam floods and all high-flow experiments (HFEs). The 30-mile, 61-mile, Grand Canyon (U.S. Geological Survey station 09402500), 166-

mile, and Diamond Creek (U.S. Geological Survey station 09404200) gages continuously measure 

stage, streamflow, and suspended-sand concentration. During the 1996 HFE, data were collected 

at the Lees Ferry, 61-mile, Grand Canyon, and 166-mile gages. During the 2004 HFE, data were 

collected at the Lees Ferry, 30-mile, 61-mile, Grand Canyon, and Diamond Creek gages. Data 

were collected at all six gages during the 2008 HFE. See Topping and others (2010) for detailed 

information on the data collected at each gage. The reaches for sand mass-balance calculations 

between the gages are also identified. Modified from Hazel and others (2010).

18 Effects of Three High-Flow Experiments on the Colorado River Ecosystem Downstream from Glen Canyon Dam, Arizona
The first measurements of suspended-sediment concentrations were made in 1892 at Yuma by C.B. 

Collingwood of the University of Arizona (LaRue, 1916). The U.S. Reclamation Service made the 

first suspended-sediment transport measurements near Yuma in 1903.

Data concerning the physical structure of the Colorado River channel network were also 

required to evaluate potential dam sites. The downstream profile of the water surface of the Colorado 

River in Grand Canyon was the last part of the channel network surveyed (Birdseye, 1924). These 

data were remarkably accurate and subsequently were used to describe the geomorphology3

 of the 

channel (Leopold, 1969), as a reference datum in hydraulic models (Randle and Pemberton, 1987), 

and in an evaluation of long-term changes of rapids (Magirl and others, 2005). McKee (1938) 

described sedimentary structures typical of Colorado River sandbars in Grand Canyon.

Attributes of the fish community also were described in the late 1800s and first half of the 

20th century (Evermann and Rutter, 1895; Miller and others, 1991). These early studies provided 

the first evidence that many aspects of the life history of native fish species were adapted to the 

large floods and winter low flows characteristic of the Colorado River, as well as the river’s 

large suspended-sediment load, wide-ranging temperatures, and segmented downstream profile 

of steep canyons and wide, low-gradient valleys (chapter 4, this volume). The first systematic 

observations of riparian vegetation in Grand Canyon were made during a river expedition in 

1938 (Clover and Jotter, 1944).

As dams and diversions were completed in the Colorado River watershed, scientists and 

engineers observed changes in channel geomorphology as well as associated changes in the distribution and abundance of native fish species and native vegetation. Using channel survey data, 

Stanley (1951) and Borland and Miller (1960) demonstrated that erosion of fine sediment occurred 

immediately downstream from Hoover and Parker Dams on the lower Colorado River, similar 

to what Cory (1913) observed downstream from Laguna Dam. The sediment eroded from the 

channel near each dam and accumulated farther downstream (Stevens, 1938). At Hoover Dam, the 

zone of fine-sediment evacuation extended about 90 miles (Schmidt and Wilcock, 2008, fig. 5).

Completion of Glen Canyon Dam in 1963 initiated similar physical and ecological changes 

to the Colorado River in Grand Canyon. Scientific studies conducted in the 1970s indicated that 

alteration of the flow and temperature regimes, complete trapping of the upstream sediment 

supply, and introduction of nonnative aquatic and riparian species had transformed the pre-dam 

Colorado River ecosystem (Dolan and others, 1974). The riparian vegetation zone expanded, 

3

 Geomorphology is the study of the earth’s natural landscapes and how they form. The geomorphology of a river involves 

descriptions of physical attributes, including channel shape (width and depth), bed material size and distribution, downstream 

gradient, and meander pattern, as well as the study of physical processes.

Figure 2. Downstream view from the left side of the Colorado River showing changes in channel conditions, including the dramatic increase

in riparian vegetation along the Colorado River in Grand Canyon near river mile 217.7. A, Photograph taken September 30, 1923, by E.C. LaRue, 

U.S. Geological Survey. B, First matched photograph taken July 4, 1972, by R.M. Turner. C, Second matched photograph taken October 24, 1991, 

by David Edwards. Photographs courtesy of the U.S. Geological Survey, Desert Laboratory Repeat Photography Collection.

A. September 30, 1923 B. July 4, 1972 C. October 24, 1991

Chapter 2—Understanding Physical Processes of the Colorado River 19
nonnative species assumed a dominant role in both the riparian and aquatic communities, populations of several native fish species were lost in Grand Canyon, and humpback chub (Gila cypha) 

were reduced to a small population in an area near the confluence with the Little Colorado River 

and in the tributary itself (Turner and Karpiscak, 1980; Carothers and Brown, 1991; Webb, 1996). 

These studies also indicated that the size of bare sandbars was smaller in the post-dam period.

Effects of Dams on Downstream River Channels

Dams cause both erosion and deposition in downstream river channels (Williams and Wolman, 

1984). The tendency for a channel downstream from a dam to evacuate or accumulate sediment 

depends on the river’s capacity to transport sediment in relation to the amount of sediment available to be transported (Lane, 1955; Williams and Wolman, 1984; Andrews, 1986; Brandt, 2000a, b; 

Grant and others, 2003; Schmidt and Wilcock, 2008). Because the greatest amount of sediment 

transport occurs during floods and because sediment supplied to post-dam channels typically comes 

only from tributaries that enter the river downstream from the dam, changes in flood characteristics 

caused by dams and the sediment-supply characteristics of downstream tributaries play an important role in determining channel adjustment. 

The relative amount of sediment supply to a given reach and transport out of that reach is 

commonly referred to as the sediment mass balance. The mass balance of a channel is in deficit 

downstream from those dams that trap a large proportion of the pre-dam sediment supply; in these 

cases, sediment is evacuated from the channel (Schmidt and Wilcock, 2008). In contrast, the sediment 

mass balance is in surplus downstream from a dam where the magnitude of floods is greatly reduced, 

but sediment continues to enter the channel from tributaries; in these cases, channels tend to accumulate sediment.

At the time Glen Canyon Dam was being constructed, river engineers anticipated that the 

disruption of the natural flow regime and sediment supply would perturb the mass balance of the 

channel into deficit immediately downstream from the dam. In 1956, the Bureau of Reclamation 

began a program of resurveying channel cross sections and measuring bed-material size between 

the dam and Lees Ferry in order to measure the anticipated evacuation of sediment from this river 

segment (Pemberton, 1976; Grams and others, 2007). Scientists and engineers did not know, 

however, if the zone of sediment evacuation would extend farther downstream into Marble or 

Grand Canyons. As explained below, it took decades of measurement and analysis, including 

critical observations made during the HFEs, for scientists and engineers to understand that a large 

part of Grand Canyon was in deficit with respect to fine sediment and that a large amount of fine 

sediment had been evacuated from much of the river corridor. It also took decades of study to 

understand the relative roles of changes in flow regime, including reduction in floods and changes 

in sediment supply, in creating the sediment-deficit conditions. 

The Streamflow Regime and How It Has Changed

The magnitude, frequency, duration, timing, and rate of change of hourly, daily, decadal, and 

century-scale discharge constitute a river’s flow regime, and floods are typically an essential attribute of that regime. Native aquatic and riparian ecosystems have developed in response to these 

attributes (Poff and others, 1997). In some river systems, the flow regime is relatively constant 

throughout the year, and the amount of sediment transported by the river is small. The native 

aquatic and riparian ecosystem of Grand Canyon, however, developed in and along a river whose 

discharge changed greatly throughout the year. The Colorado River typically had a predicable 

spring snowmelt flood, unpredictable smaller late summer and fall floods, low winter flows, and 

periods when the transported load of sediment was very large. 

20 Effects of Three High-Flow Experiments on the Colorado River Ecosystem Downstream from Glen Canyon Dam, Arizona
50,000

0

100,000

150,000

1920 1930 1940 1950 1960 1970 1980 1990 2000 2010

Discharge, in cubic feet per second

1965

flood

1983

flood

1984–86

floods

1996

HFE

2004

HFE

2008

HFE

Closure of

Glen

Canyon

Dam

The Pre-Dam Colorado River

Before completion of Glen Canyon Dam in March 1963, peak flows of 50,000 cubic feet 

per second (ft3/s), which is slightly larger than the magnitude of the post-dam HFEs, occurred 

approximately every year (fig. 3). In the drought decades of the 1930s and 1950s, however, 

five annual peak floods were less than the magnitude of the HFEs.4 Topping and others (2003) 

estimated that peak flows of 31,500 ft3/s had a recurrence of approximately 8 months, that the 

2-year recurrence peak flow (meaning a flow of that magnitude was equaled or exceeded every 

2 years, on average) had a magnitude of about 85,000 ft3/s, and that a flood of about 120,000 ft3/s 

occurred, on average, every 6 years. The largest observed and reported flood in Grand Canyon 

occurred in June 1884 and was approximately 210,000 ft3/s (Topping and others, 2003). The 

second-largest observed flood occurred in June 1921 and was approximately 170,000 ft3/s. The 

large magnitude of the pre-dam floods stands in contrast to the low flows that occurred much of 

the rest of the year. The median flow of the Colorado River for the entire pre-dam period was 

7,980 ft3

/s, which means that half of the time flows were smaller than this magnitude, and half 

of the time flows were larger than this magnitude.

The Post-Dam Colorado River

Flow regulation provided by Glen Canyon Dam “effectively replaced relatively high and 

low flows with a greater frequency of moderate flows” (Topping and others, 1999, p. 72), because 

moderate flows are routed through the Glen Canyon Dam powerplant to produce hydroelectricity 

as water is transferred to the lower basin States. The role of flow regulation provided by Glen 

Canyon Dam is illustrated in figure 4, which shows the contrast between inflows to Lake Powell 

and dam releases in the 3 years when HFEs occurred. In 1996 and 2008, HFEs were conducted 

before the natural spring snowmelt flood. In these years, the magnitude of the HFE releases was 

less than the magnitude of the natural flood, and the duration of the HFE was much less. In these 

years, the part of the natural flood not immediately released downstream was used to increase 

water storage in Lake Powell. In 2004, the HFE release was much larger in magnitude than the 

natural snowmelt flood for that year. During a typical year, the daily range of flows is large, 

because usual dam operations consist of daytime releases of water to generate hydroelectricity 

and smaller nighttime releases. As a result, releases from Glen Canyon Dam typically exceed the 

magnitude of natural low flows.

4

 These floods occurred in 1931 (34,600 ft3/s), 1934 (25,300 ft3/s), 1954 (34,300 ft3/s), 1955 (35,600 ft3/s), and 1959 (38,900 ft3/s).

Figure 3. Continuousdischarge record for the 

Colorado River at Lees Ferry 

(river mile 0) for 1921 through 

2009. With the exception of the 

first 2 years, the observations 

used to construct this record 

are at intervals of 30 minutes 

or less. Closure of Glen Canyon 

Dam and post-dam floods and 

high-flow experiments (HFEs) 

are indicated. Modified from 

Topping and others (2003).

Chapter 2—Understanding Physical Processes of the Colorado River 21
20,000

0

40,000

60,000

20,000

0

40,000

60,000

80,000

Jan Mar May July Sept Nov Jan

20,000

0

40,000

Discharge, in cubic feet per second

High-flow

experiment

A. 1996

B. 2004

C. 2008

Combined flow of upper Colorado, 

Green, and San Juan Rivers

Lees Ferry

Lees Ferry

Lees Ferry

High-flow

experiment

High-flow

experiment

The frequency of large floods has greatly decreased since completion of the dam, and the 

magnitude of typical low flows has increased. The 2-year recurrence peak flow of the post-dam 

period was 31,500 ft3/s, a 62 percent reduction from the magnitude of the 2-year recurrence peak 

flow of the pre-dam measurement period (Topping and others, 2003). The median flow of the 

post-dam period was 12,000 ft3

/s, 58 percent larger than that of the pre-dam period.

Post-dam peak flows are not only small relative to the pre-dam peak flows, but they are also 

small relative to the magnitude of flows that occur during the remainder of the year. For example, the 

pre-dam 2-year recurrence peak flow was approximately 77,000 ft3/s larger than the pre-dam median 

flow, but the post-dam 2-year recurrence peak flow was only 19,500 ft3/s greater than the post-dam 

median flow. Thus, pre-dam peak flows were about 970 percent larger than typical base flows, but 

post-dam peak flows were only about 150 percent larger than typical base flows.

Glen Canyon Dam has not been operated uniformly since 1963. Changes in operation reflect 

changing objectives of reservoir and river management, as well as wet and dry cycles of watershed 

precipitation. High flows that have exceeded the capacity of the powerplant have occurred for several 

reasons. Some of these high flows are considered “floods,” as defined by the Glen Canyon [Dam] 

Environmental Studies (GCES) program (U.S. Department of the Interior, 1988, p. A-21). This 

program defined post-dam floods as dam “releases exceeding powerplant capacity for a month or 

more,” and these occurred in 1965, 1983, 1984, 1985, and 1986 (table 1). 

The 1965 flood (fig. 3) occurred between April and June, when a large volume of water needed 

to be released downstream in order to equalize the storage contents of Lakes Powell and Mead. For 

reasons that were not documented at the time (see text box, Channel Change in the Glen Canyon 

Reach), the high flows were released as a series of short-duration pulses; six of these pulses exceeded 

50,000 ft3

/s, and between each pulse, flows were greatly reduced. Releasing this water as a series 

of pulses maximized fine-sediment evacuation from the channel bed in Glen Canyon, resulting in a 

dramatic transformation of this segment of the river (Grams and others, 2007).

Floods also occurred each year from 1983 to 1986 (fig. 5); watershed runoff was unusually 

large in these years. Lake Powell and other reservoirs in the upper Colorado River Basin filled to 

Figure 4. Discharge of the 

Colorado River at Lees Ferry 

and the estimated inflow to 

Lake Powell for the years 

when the three high-flow 

experiments occurred. A, 1996. 

B, 2004. C, 2008. Inflow to Lake 

Powell was estimated as the 

sum of measured discharge 

each day at the Green River at 

Green River, Utah (U.S. Geological Survey station 09315000), 

the Colorado River near Cisco, 

Utah (U.S. Geological Survey 

station 9180500), and the San 

Juan River near Bluff, Utah 

(U.S. Geological Survey station 

9379500). 

22 Effects of Three High-Flow Experiments on the Colorado River Ecosystem Downstream from Glen Canyon Dam, Arizona
Table 1. Summary of post-dam floods and short-duration high flows.

[ft3/s, cubic feet per second]

Year

Number of 

consecutive days mean 

daily discharge 

exceeded 31,500 ft3

/sa

Dates Instantaneous 

peak, in ft3

/s Purpose

Post-dam floods

1965 34 May 21–June 25 60,200 Reservoir equalization and channel cleaning

1983 68 June 3–August 10 97,300 Excess runoff

1984 76 May 5–July 20 45,300 Excess runoff

1985 39 May 17–June 28 47,900 Excess runoff

1986 46 May 8–June 24 53,200 Excess runoff

Post-dam short-duration high flows

1965 1 April 11 34,200 Reservoir equalization and channel cleaning

1965 8 April 23–30 52,700 Reservoir equalization and channel cleaning

1965 7 May 4–11 52,900 Reservoir equalization and channel cleaning

1965 1 May 16 51,800 Reservoir equalization and channel cleaning

1980 6 June 24–July 1 45,400 Facilities test

1984 3 August 12–15 58,200 Spillway test

1996 8 March 26–April 2 45,900 High-flow experiment

2004 3 November 22–24 42,500 High-flow experiment

2008 3 March 6–8 42,800 High-flow experiment

a

 Consecutive is defined as occurring within 5 days when flows of similar magnitude occurred.

0

20,000

40,000

60,000

80,000

100,000

1980 1985 1990 1995 2000 2005 2010

Discharge, in cubic feet per second

1986

flood 2004

HFE

2008

HFE

1996

HFE

1984

flood

1985

flood

1990–91

experimental flows

2000

experimental flows

1986

flood 2004

HFE

2008

HFE

1996

HFE

1996

HFE

1997

experimental flows

1984

flood

1983

flood

1985

flood

1990–91

experimental flows

2000

experimental flows

Figure 5. Continuous 

discharge record for the 

Colorado River at Lees 

Ferry for 1980 through 2009. 

The observations used to 

construct this record are at 

15-minute intervals. Post-dam 

floods, experimental flows, 

and high-flow experiments 

(HFEs) are indicated. Modified from Topping and others 

(2003).

capacity, and spring snowmelt could not be entirely stored in the reservoirs. The largest dam release 

occurredon June 29, 1983, when a flow of 97,300 ft3/s was measured in the Colorado River at Lees 

Ferry. In addition to being unusually large, dam releases in each of these years were of long duration 

(table1). Discharge exceeded 31,500 ft3/s for 68 consecutive days in 1983 and for 76 consecutive 

days in1984.

Other large post-dam releases were of shorter duration. Two of these releases—in June 1980 

and August 1984—occurred so that the Bureau of Reclamation could conduct critical engineering 

tests of the dam spillways. The other short-duration high-flow releases from the dam were associated with the HFEs that occurred in spring 1996, fall 2004, and spring 2008. 

Chapter 2—Understanding Physical Processes of the Colorado River 23
Experimental Flows and High-Flow Experiments

Since 1990, occasional experimental releases from Glen Canyon Dam allowed scientists to 

measure key physical and biological processes, measure the response of key resources to controlled 

flow conditions, and test hypotheses. During a 13-month period in 1990 and 1991, different flow 

patterns were released in 2-week blocks. Measurements, especially of changing sandbar topography, 

were made during intervening low-flow periods between the blocks. In 2000, a 6-month period of 

experimentation was executed that included two short-duration pulses at powerplant capacity with 

an intervening 3-month period when flows were held steady at 8,000 ft3

/s. Very short periods of low 

releases of approximately 5,000 ft3

/s occurred in October 1984 and June 1990 during acquisition of 

aerial photography. Subsequent aerial photographs have been taken, and remote data acquisition has 

occurred during flows of approximately 8,000 ft3

/s.

The first HFE was conducted between March 26 and April 7, 1996, and involved a 7-day 

steady release of 45,000 ft3

/s that was preceded and followed by low steady flows of 8,000 ft

3

/s 

for 4 days each (fig. 6; Schmidt and others, 1999; see text box, Modeling the Colorado River 

Shoreline). The coordinated effort of scientists to evaluate the effects of the 1996 HFE on 

physical, biological, cultural, and recreational resources was documented by Webb, Schmidt, 

and others (1999). The second HFE was conducted between November 22 and 24, 2004, and 

involved a 60-hour release of about 41,700 ft3

/s. The third HFE was conducted between March 

6 and 8, 2008, and included a 60-hour release of about 42,800 ft3

/s. The rise in discharge 

from normal dam operations for the 2004 and 2008 HFEs took place over 30-hour periods, a 

50-percent slower rate of rise than for the 1996 HFE (fig. 6). 

24 Effects of Three High-Flow Experiments on the Colorado River Ecosystem Downstream from Glen Canyon Dam, Arizona

Channel Change in the Glen Canyon Reach

The reach between Glen Canyon Dam and Lees Ferry, 

locally known as the “Glen Canyon” or “Lees Ferry” reach, 

is distinct from downstream Marble and Grand Canyons in 

several important ways, including the jurisdictional boundary 

that places the upstream reaches in 

Glen Canyon National Recreation Area 

and the downstream reaches in Grand 

Canyon National Park. The differences 

in channel and canyon character result 

from the change in geology at Lees 

Ferry from less resistant Mesozoic rock 

formations to more resistant Paleozoic 

rock formations in Marble Canyon. This 

geologic contact is the reason that 

Lees Ferry is the point of departure 

for both downstream travel into Grand 

Canyon and upstream travel into Glen 

Canyon. The change in bedrock geology 

results in a break in the landscape 

that provides a rare access point to 

the river, which is engulfed in canyons 

both upstream and downstream 

from Lees Ferry. More importantly for the geomorphic 

character of the river, the change in geology marks the 

break between the upstream river segment that lacks large 

debris fans and the downstream segments where debris 

fans are common. Tributaries to the Colorado River in 

Glen Canyon supply sand and mud almost exclusively. 

Beginning with the first small tributary downstream 

from Lees Ferry, tributaries in Marble 

Canyon and downstream Grand Canyon 

deliver boulders composed of resistant 

rocks, in addition to sand and mud. 

Although geology is an important factor 

in the site selection and design of all 

dams, in this case geologic context also 

played a major role in determining how 

the completed dam affected the river 

downstream. The fact that the channel in 

the Glen Canyon reach would be affected 

by the completion of Glen Canyon Dam 

was realized by engineers in the 1950s at 

least as early as when construction began 

in 1956. Engineers recognized that the bed 

in Glen Canyon would be subject to scour 

and that the lack of replenishment from 

upstream would likely cause progressive bed lowering. It 

was essential that the bed lowering in the tailwater immediately below the dam be accounted for in the design and 

Lake Powell

Glen Canyon Dam

GRAND CANYON

NATIONAL PARK

GLEN CANYON

NATIONAL RECREATION

AREA

Lees Ferry

Little Colorado R.

0 20 mi

0 20 km

M

A R B L E C A N Y O N

Colorado R.
Pre-Dam and Post-Dam Sediment Supply

Before completion of Glen Canyon Dam, the major sources of sand and mud transported by 

the Colorado River through Grand Canyon were the sedimentary rocks of the Colorado Plateau 

upstream from Lees Ferry (Thomas, 1960; Iorns and others, 1965; Howard and Dolan, 1981; 

Andrews, 1990, 1991b). Sand is defined as particles that are finer than 2 millimeters (mm) in diameter and coarser than 0.0625 mm; mud is finer than 0.0625 mm and can be either silt or clay. The 

term fine sediment is used to refer to sediment that consists of sand and finer particles. Topping, 

Rubin, and Vierra (2000) estimated that between 35 and 50 percent of the pre-dam suspendedsediment load of the Colorado River through Grand Canyon was sand and the remainder was mud 

(table 2). Although sandbars in Grand Canyon may contain discrete mud lenses or layers, primarily 

they are composed of sand (Schmidt and Graf, 1990; Topping and others, 2005). Thus, sand is the 

grain size of greatest interest to Grand Canyon researchers and managers. 

Many investigators recognized that trapping the entire fine-sediment load in Lake Powell 

that once was transported past Lees Ferry would create a sediment deficit that was likely to scour 

the 16 miles of the river between the dam and Lees Ferry and result in changes in the size and 

abundance of sandbars and other sand deposits farther downstream in Grand Canyon. Scientists 

disagreed, however, in their predictions of the downstream extent of the zone of sediment deficit 

and on whether conditions of sediment surplus would exist in central and western Grand Canyon 

(RM 88 to RM 277). The different predictions about downstream sand mass balance conditions 

arose, in part, because scientists made different assumptions about the amount of fine sediment 

supplied by tributaries to the Colorado River downstream from the dam. 

Chapter 2—Understanding Physical Processes of the Colorado River 25

construction of the dam, otherwise powerplant performance and efficiency could be severely compromised.

To be able to predict the scour in this reach with the 

highest level of accuracy possible, Bureau of Reclamation 

engineers collected the necessary field data to construct 

a flow and sediment-transport model—state of the art 

for the time—that predicted the magnitude of bed scour 

(Pemberton, 1976). This field campaign included measurement of channel cross sections, water-surface elevations, 

and samples of surface and subsurface bed material. The 

effort went so far as to take core samples of deeper bed 

material. The study was limited to the reach between the 

dam site and Lees Ferry, because it was already understood that the transition in geology at Lees Ferry meant 

that scour of the bouldery rapids and riffles downstream 

was unlikely, even after dam construction. Engineers and 

scientists at the time were not concerned about the fate 

of fine sediment, because only the coarse fraction of the 

bed material controls the gradient of the water surface 

and can thereby affect the performance of the dam. This 

effort resulted in predictions that as much as 10.76 x 106

cubic yards of bed material would be evacuated, and 

this predicted scour was largely realized soon after the 

dam was completed when a series of pulsed flows were 

released in the spring and summer of 1965 (Pemberton, 

1976; Grams and others, 2007). Bed scour of about 27 feet 

during the 1965 floods was also documented at the cableway 

at Lees Ferry by Burkham (1986). Repeated measurements 

indicated that the bed subsequently filled by 12 feet, such 

that the bed permanently was degraded by about 15 feet at 

the Lees Ferry cableway (Burkham, 1986; Grams and others, 

2007). Although the primary reason for these high flows was 

to transfer water from Lake Powell to Lake Mead, it remains 

unclear why such an extreme and repeated high-flow 

release pattern was used. The release pattern may have had 

some secondary purpose, such as testing the water outlet 

facilities of the newly completed dam and, possibly, testing 

the prediction of bed scour. Regardless of the purpose at the 

time, the 1965 pulsed flows (14 in total) were significant in 

that they were an intentional release of high flows (or floods) 

from a dam that resulted in a change in the downstream 

channel that was both predicted and carefully monitored.

Resurveys of the cross sections established for the modeling 

exercise were completed soon after the 1965 pulsed flows 

and again in the 1970s, 1980s, 1990s, and finally in 2000. 

These surveys, together with photographs and measurements of bed-material size, document the transition of the 

Glen Canyon reach from a highly dynamic sand-bedded 

channel to a stable gravel- and cobble-bedded channel, 

which in many respects is more similar to a cold Alpine 

headwaters trout stream than a lowland desert river 

(see chapter 4, this volume; Grams and others, 2007).
26 Effects of Three High-Flow Experiments on the Colorado River Ecosystem Downstream from Glen Canyon Dam, Arizona

Modeling the Colorado River Shoreline

“How high will the water get?” is probably one of the 

most basic and frequently asked questions about high 

flows in Grand Canyon. Scientists and managers have 

long relied on the experience of veteran river runners 

as well as observations made at discrete locations, 

such as streamgaging stations and sandbar study sites, 

for information about expected levels of inundation for 

particular flows. Observations of driftwood lines and other 

evidence have also provided guidance. Lacking for many 

years, however, was a means for predicting the level of 

inundation systematically for any point of interest along the 

river. Such a depiction of the shoreline requires detailed 

knowledge of both the water-surface elevation for the flow 

of interest and the topography of the surface that will be 

inundated. While models for predicting discharge (Wiele 

and Smith, 1996; Wiele and Griffin, 1997) and water-surface 

elevation (Randle and Pemberton, 1987) have been in use 

for many years, these models lack the level of resolution 

and coupling with topographic information that would 

allow their application to the prediction of shorelines. 

Completion of high-resolution (about 3 foot (ft)) digital 

elevation models (DEMs) of the river corridor that were 

constructed from remote sensing missions in 2000 and 

2002 provided data that allowed the construction of a 

more finely resolved flow model (Magirl and others, 2005, 

2008). The model is a traditional one in that it employs 

basic principles of mass conservation, energy transfer, 

and flow resistance to predict water-surface elevations. 

For application in Grand Canyon, Magirl and others (2008) 

used 2,680 channel cross sections extracted from the 

remotely sensed topography. Despite the high quality of the 

topographic data available, below-water channel-depth 

data were (and still are) lacking. Thus, it was necessary 

to construct the model by using an approach that relies 

on the creation of estimated or “synthetic” cross sections 

for the below-water parts of the channel where remotely 

sensed data were not available. Randle and Pemberton 

(1987) used the same approach but with far fewer cross 

sections. The synthetic cross sections were created by 

adjusting them such that the model-predicted watersurface elevation matched a water-surface elevation that 

was measured by airborne lidar1

 during a steady flow of 

8,000 cubic feet per second (ft3

/s) in 2000. Observations 

of water-surface elevations for other discharges were 

used to further calibrate and verify the model.

The completed model provides predictions of water-surface 

elevation at spacings of about 300 to 500 ft from Lees Ferry 

to Diamond Creek (RM 0–225), with uncertainties of about 

1.3 ft for flows as high as 45,000 ft3

/s and 3.3 ft for flows as 

high as about 90,000 ft3

/s (Magirl and others, 2008). The 

final step in providing maps depicting areas of expected 

inundation was the overlay of the predicted water-surface 

elevations onto the DEM to create “virtual shorelines” 

for the entire river corridor (see figure below). These 

maps were used by scientists, the National Park Service, 

and the general public to anticipate areas of potential 

inundation for the March 2008 high-flow experiment.

Virtual shorelines at Eminence Break camp (river mile 44). The 

blue shaded area (the river) shows the observed shoreline at a 

flow of 8,000 cubic feet per second (ft3

/s) in the Colorado River 

at Lees Ferry, the blue line shows the predicted shoreline at 

45,000 ft3/s, and the red line shows the predicted shoreline at 

97,000 ft3

/s. Modified from Magirl and others (2008).

1

Lidar (light detection and ranging) instruments employ a laser scanning 

mechanism to measure distances from the instrument, which may be airborne 

or on the ground, to the ground surface thereby creating a highly detailed 

topographic map of the area of interest.

0 100 200 feet

0 100 200 meters

Predicted shorelines

EXPLANATION

45,000 ft3/s

97,000 ft3

/s

N
0

20,000

40,000

–15 –10 –5 0 5 10 15 20

Discharge, in cubic feet per second

Time, in days relative to the time that each HFE began

1996

2004

2008

There was reasonable agreement about the fine-sediment supply rate from the Paria and Little 

Colorado Rivers. The average annual sand supply from the Paria River was estimated to be between 

1.4 and 1.9 million tons, and the supply from the Little Colorado River was estimated to be between 

3.3 and 3.4 million tons (table 2). Laursen and others (1976), Howard and Dolan (1981), and 

Andrews (1991b) documented that the Paria River augmented the amount of pre-dam fine sediment supplied from the upper basin by approximately 4 to 6 percent (table 2). These studies 

showed that between 77 and 79 percent of the sand and mud transported past the Grand Canyon 

gage, located 88 miles downstream from Lees Ferry (fig. 1), was supplied by the Colorado River 

watershed upstream from Lees Ferry. Between 14 and 16 percent of the sand and mud transported 

past the Grand Canyon gage was supplied by the Paria and Little Colorado Rivers. 

Scientists disagreed in their estimates of the amount of fine sediment delivered by the 

numerous smaller tributaries. Howard and Dolan (1981) estimated that the average annual total 

sand and mud supply from these tributaries was about 4.4 million tons, but Randle and Pemberton 

(1987) estimated that this rate was only 700,000 tons, with an unknown percentage of sand (table 2). 

On the basis of the larger estimate, Howard and Dolan (1981) predicted that sand would accumulate 

in Marble and Grand Canyons. 

Following the 1996 HFE, measurements of the amount of sand delivered by the Paria and 

Little Colorado Rivers and by the smaller tributaries were resumed. These estimates demonstrate 

that the supply rate from small tributaries is much less than was assumed by Howard and Dolan 

(1981) and that the balance between tributary sand supply and main-stem sand-transport capacity, 

which determines whether sediment evacuation or accumulation is more likely, is very delicate 

(Webb and others, 2000; Topping and others, 2010).

Fine-Sediment Evacuation or Accumulation?

The management of dam releases for the purpose of maintaining sandbars and related 

downstream natural resources not only depends on accurate measurements of the amount of sand 

delivered by the Paria and Little Colorado Rivers and smaller tributaries, but also on the prediction 

of the rate at which sand is transported by the main current and the prediction of the rate of the 

transfer of sand and mud between the main current and eddies. The rate and magnitude of eddysandbar deposition depends on the magnitude of suspended-sand concentration. Higher suspendedsand concentrations result in higher sand deposition rates and generally larger sandbars.

Predicting the Amount of Suspended-Sand Transport

Scientists have struggled to predict the amount of sand transported by the Colorado River, 

especially during floods and other high flows. Because the measurement of suspended-sand 

Figure 6. Continuous 

discharge record for the 

Colorado River at Lees Ferry 

for three high-flow experiments (HFEs). The 1996 HFE 

began at 12:00 a.m. on 

March 26, 1996; the 2004 HFE 

began at 12:00 a.m. on 

November 21, 2004; and the 

2008 HFE began at 6:00 p.m. 

on March 4, 2008.

Chapter 2—Understanding Physical Processes of the Colorado River 27
Table 2. Estimates of the mean annual suspended-sediment load transported in the Colorado 

River and its tributaries in the Grand Canyon region.

[ns, not specified; na, not available]

Suspended-sediment

load, in tons per year

Period 

of estimate Source Percentage of sand

Colorado River at Lees Ferry gage (pre-dam)

101,300,000 1929–1957 Iorns and others, 1965 na

65,690,000 1948–1962 Laursen and others, 1976 na

73,019,000 ns Howard and Dolan, 1981 na

66,100,000 1941–1957 Andrews, 1991b na

62,800,000 1949–1962 Topping, Rubin, and Vierra, 2000 40

Colorado River at Lees Ferry gage (post-dam)

8,600,000 1963–1965 Laursen and others, 1976 na

260,000 1966–1970 Topping, Rubin, and Vierra, 2000 na

Paria River near Lees Ferry

2,655,000 1948–1957 Iorns and others, 1965 na

2,506,000 1949–1957 Laursen and others, 1976 na

3,732,000 1949–1969 Laursen and others, 1976 na

3,214,000 ns Howard and Dolan, 1981 na

3,020,000 1947–1976 Andrews, 1991b na

3,300,000 1949–1970 Topping, Rubin, and Vierra, 2000 50

Little Colorado River

9,836,000 1948–1969 Laursen and others, 1976 na

9,377,000 ns Howard and Dolan, 1981 na

9,270,000 1941–1957 Andrews, 1991b na

9,480,000 1949–1970 Topping, Rubin, and Vierra, 2000 30–40

Ungaged tributaries

4,363,000 ns Howard and Dolan, 1981 na

700,000 ns Randle and Pemberton, 1987 na

790,000 ns Topping, Rubin, and Vierra, 2000; 

from Griffiths and others, 1996 na

Colorado River at Grand Canyon gage (pre-dam)

168,000,000 1926–1950 Smith and others, 1960 na

142,000,000 1935–1948 Smith and others, 1960 na

83,610,000 1948–1962 Laursen and others, 1976 na

92,453,000 ns Howard and Dolan, 1981 na

195,000,000 1925–1940 Andrews, 1991b na

85,900,000 1941–1957 Andrews, 1991b na

91,500,000 1948-1962 Topping, Rubin, and Vierra, 2000 35

Colorado River at Grand Canyon gage (post-dam)

20,490,000 1963–1969 Laursen and others, 1976 na

15,790,000 1966–1969 Laursen and others, 1976 na

11,000,000 1970–1990 Andrews, 1991b, stable rating curve 

estimation na

15,430,000 1966–1972 Topping, Rubin, and Vierra, 2000 na

28 Effects of Three High-Flow Experiments on the Colorado River Ecosystem Downstream from Glen Canyon Dam, Arizona
10 –1

100

10 –2

10 –3

10 –4

10 –5

1,000 10,000 100,000

July 21 to May 31

June 1 to July 20

Concentration, in percent by volume

Water discharge, in cubic feet per second

transport is labor intensive and, therefore, costly, scientists usually make occasional measurements 

and calculate an average relation between river flow and the concentration of sand transported 

(fig. 7). Typically, scientists assume that scatter in measured data represents statistically random 

measurement uncertainty and that an average relation may be fit through the center of the cloud 

of field data. However, early investigations of sediment transport showed that a simple relation 

was not applicable to the pre-dam Colorado River. For example, Leopold and Maddock (1953) 

showed that much more suspended sediment was transported during the rising stage of the 1941 

spring snowmelt flood than during the receding stage. Scientists refer to such a pattern in which 

there is a lag or “memory” effect as hysteresis. Because the mechanical forces caused by turbulent streamflow in large rivers are approximately the same at the same discharge, whether during 

rising or falling stage, scientists explain hysteresis in sediment-transport relations as being the 

result of changes in the amount of sediment available for transport. The condition is referred to as 

sediment-supply limitation. 

Hysteresis occurred in the pre-dam river, because the months of highest streamflow—spring 

and early summer—and the months when most sand was supplied to the channel—late summer 

and fall—were asynchronous. Although sand was brought into Grand Canyon from upstream 

during the spring snowmelt flood, a large proportion of the sand transported through Grand 

Canyon during the spring snowmelt flood was locally scoured from the bed and banks during the 

rising part of the annual flood. This sand had accumulated during the previous late summer and 

fall when flow in the river was relatively low. 

Colby (1964) also described this seasonal pattern of sand accumulation in the summer and 

fall followed by evacuation during the subsequent spring. He observed that “the quantity and 

particle size of the available sand generally change with time,” and he suggested that the wide 

range of concentrations measured at the same discharge was caused by changes in the amount of 

sand on the channel bed and the size of the sand (Colby, 1964, p. D30). Howard and Dolan (1981) 

analyzed all pre-dam sand-transport data collected between 1948 and 1957 and identified the 

same seasonal pattern in periods of sand accumulation and evacuation. Burkham (1986) showed 

similar sand-transport relations in the post-dam river, and Laursen and others (1976) used theoretical transport relations to estimate that the Colorado River could transport more sand than was 

measured in the years immediately after the dam was closed. Despite these studies, the importance 

of sand-supply limitation was not explicitly considered by Howard and Dolan (1981), Andrews 

(1990, 1991b), Smillie and others (1993), or Randle and others (1993), nor was it considered in the 

analysis of sediment transport in the “Operation of Glen Canyon Dam: Final Environmental Impact 

Statement” (hereafter referred to as EIS; U.S. Department of the Interior, 1995). 

Figure 7. Suspended-sand concentration 

and discharge of the Colorado River near the 

Grand Canyon gage at river mile 88 during 

the pre-dam period (1944–62). Concentrations 

shown are the average for the measurement 

section based on depth-integrated samples. 

The red symbols represent data collected from 

June 1 to July 20, and the green symbols represent data collected from July 21 to May 31. 

As described in the text, the systematic nature 

of the time when measured concentrations 

were relatively high or low indicates that the 

scatter is not random and is related to the 

amount and size of sand available for transport and its distribution on the bed and along 

the channel margins. Modified from Topping, 

Rubin, and Vierra (2000, fig. 4b).

Chapter 2—Understanding Physical Processes of the Colorado River 29
The relation between river flow and suspended-sand concentration is further complicated by the 

existence of downstream variations in sand concentration. Suspended-sand concentration increases 

downstream when net erosion from the channel bed and from eddy sandbars occurs or when the 

rate of sand entrainment from the bed is large. Conversely, a downstream decrease in suspendedsand concentration occurs when there is net deposition of sand onto the channel bed or into eddies. 

Because of these complex interactions, the ability to predict sand transport and concentration at 

different places in Grand Canyon is essential for anticipating the effects of high flows on sandbars. 

Calculations of Post-Dam Sand Mass Balance

The mass balance of sand for any river segment, also referred to as the change in sand storage 

in a river segment, is the difference between sand input, or sand supply, at the upstream end of the 

segment and sand output, or sand export, at the downstream end of the segment. In the Colorado 

River in Grand Canyon, the change in sand storage includes changes in the amount of sand on the 

channel bed and in the sandbars that are recreational and ecological resources.

Taking into account the characteristic size of the sand delivered by the Paria River, the flow 

regime of the pre-dam Colorado River, and the anticipated long-term operating rules to generate 

hydropower at the dam, Laursen and others (1976) predicted that the capacity of the Colorado 

River to transport sand was relatively high. They also predicted that there would be a decrease in 

sand storage in the post-dam Colorado River in Marble and Grand Canyons, meaning those river 

segments would be in sediment deficit; thus, they predicted that sand would be eroded from the 

river’s sandbars and channel bed. Laursen and others (1976, p. 4–86) concluded:

the beaches of the Colorado River * * * could be in danger of being washed away 

since the transport capacity of the regulated river is in excess of the amount of beachbuilding material being supplied from the tributaries * * * How long they will last 

cannot as yet be estimated; certainly more than 10 years, probably less than 

1,000 years; but how much more or less than 100 years is a matter for continued study.

In contrast, subsequent investigators assumed that the average amount of sand transported by 

the Colorado River was much less than the amount assumed by Laursen and others (1976), and 

these investigators ignored the role of supply limitation. Depending on estimates of the amount of 

sand supplied by the tributaries entering the Colorado River downstream from Glen Canyon Dam, 

researchers concluded that (1) the post-dam river was in sediment surplus and had accumulated 

sand (Howard and Dolan, 1981), (2) the post-dam river was in equilibrium and the rate of sand 

export was approximately equal to the rate of sand supply (Andrews, 1990, 1991b), or (3) sand 

was accumulated when dam releases were less than about 31,500 ft3/s, and sand was evacuated when floods occurred (U.S. Department of the Interior, 1995). It was estimated in the EIS 

(U.S. Department of the Interior, 1995) that sand had accumulated between 1966 and 1982 (fig. 8) 

because there were no floods and only one short-duration flow in 1980 (table 1) during that period. 

This hypothesis led to the assumption that sand could be managed and conserved on the channel 

bed for decades and that occasional short-duration high flows could mobilize the sediment and 

redistribute some sand to sandbars while transporting the remainder downstream to Lake Mead. 

The latter perspective was the working hypothesis of the 1996 HFE and of the beach/habitat 

building flows proposed in the Record of Decision (U.S. Department of the Interior, 1996).

As described in the next chapter, some of the most important results from the HFEs concern 

reevaluating the role sand-supply limitation plays in determining how much sediment is transported during each high flow. The central finding of Rubin and others (1998), Rubin and Topping 

(2001), Topping and others (1999, 2005), Topping, Rubin, and others (2000), and Topping, Rubin, 

and Vierra (2000) was that the amount of sand stored on the channel bed and in sandbars—now 

termed sand enrichment—determines the amount of sand transported by subsequent HFEs. These 

findings indicate that Laursen and others (1976) generally were correct in their assessment of the 

deficit of sand in the post-dam river. Today, river managers strive to understand the relation among 

30 Effects of Three High-Flow Experiments on the Colorado River Ecosystem Downstream from Glen Canyon Dam, Arizona
30

Lees Ferry to

Phantom Ranch

Net

gain

Net

loss

Lees Ferry to

Little Colorado River

Little Colorado River to

Phantom Ranch

Cumulative sand storage, in millions of tons 20

10

0

1964 1968 1972 1976 1980 1984 1988

sand supplied from tributaries, short-term sand enrichment in the Colorado River, sand transport 

during HFEs, sand transport between HFEs during normal operations, and the resultant sand mass 

balance. Crucial measurements have been made during HFEs and incorporated into numerical 

models. Collectively, these insights have led to ever-improving predictive tools to anticipate shortterm and long-term change in sandbars and associated aquatic and riparian habitats.

Physiography of the Colorado River and Its Valley

Rivers are not uniformly shaped conduits. Instead, natural river channels vary in width, 

depth, bed material, pattern, and slope—depending on the characteristics of the bedrock and 

hillslope deposits through which the river has excavated its channel and the geology and relief 

of the upstream watershed. Variation in channel form is important in determining how a natural 

river channel responds to conditions of sediment evacuation or accumulation and how the channel 

responds to floods. Differences in the bedrock exposed on the banks and near the bed of the 

Colorado River have a substantial effect on channel form and on the downstream distribution of 

many natural resources, such as the number of campsites or the abundance of archaeological sites. 

Longitudinal, or downstream, differences in channel form also affect where erosion and deposition occur during HFEs (see text box, Longitudinal Organization of Grand Canyon).

Powell “noted many changes in the dominant ‘mood’ of Grand Canyon * * * such as the ‘oppressive constriction’ of the granite gorges” (cited by Howard and Dolan, 1981, p. 273). During the 1980s 

and 1990s, researchers followed the lead of Howard and Dolan (1981) and characterized the large-scale 

longitudinal characteristics of Grand Canyon. Schmidt and Graf (1990) divided Grand Canyon into 

11 segments (long parts of the channel whose shape is similar) on the basis of rock type at river level 

and whether the channel was “narrow” or “wide.” Schmidt and Graf (1990) showed that the frequency 

of sandbar campsites and the number and size of sandbars varied by segment (U.S. Department of the 

Interior, 1988). Griffin (1997) defined 10 “geomorphically similar” segments on the basis of attributes 

of channel cross-section shape. Brian and Thomas (1984) and Kearsley and others (1994) took a 

different approach and defined segments on the basis of the frequency of sandbar campsites; “critical” 

segments were defined as parts of the river with very few campsites.

The longitudinal profile of the Colorado River is a series of long, flat reaches interrupted by 

short, steep rapids and riffles (see figure in text box, Longitudinal Organization of Grand Canyon). 

Leopold (1969) reported that 50 percent of the total elevation drop of the river occurred in only 

9 percent of the downstream distance, and Magirl and others (2005) found that 66 percent of the 

total drop, as measured in 2000, occurred in rapids and riffles. The spacing between rapids is 

determined by the spacing of tributary canyons (Dolan and others, 1978), because debris from 

each tributary partially blocks the river. 

Figure 8. Cumulative sand 

storage between the Lees 

Ferry and Grand Canyon gaging stations (river miles 0–88), 

based on the assumption that 

the same discharge always 

transports the same amount of 

sand. From U.S. Department of 

the Interior, 1995, fig. III-15. 

Chapter 2—Understanding Physical Processes of the Colorado River 31
32 Effects of Three High-Flow Experiments on the Colorado River Ecosystem Downstream from Glen Canyon Dam, Arizona

Longitudinal Organization of Grand Canyon

Grand Canyon researchers and river managers have a 

wide array of spatially robust and detailed geographic 

data from which to characterize the longitudinal, or 

downstream, variation of the Colorado River corridor. 

These data have been used to estimate the width of the 

river at different discharges and thereby define river 

segments as to being narrow or wide and segments that 

have relatively steep or flat gradients (Magirl and 

others, 2005, 2008). Other researchers have measured 

the distribution of eddy sandbars by measuring the 

debris fans that create fan-eddy complexes (Melis, 

1997) or the number of eddy deposition zones (Schmidt 

and others, 2004). These data provide a picture of the 

large-scale, longitudinal variation in the physical 

structure of the Colorado River in Grand Canyon (see 

figure below) and have resulted in the identification of 

geomorphic reaches that are based on these characteristics and serve as a basis for stratifying research and 

monitoring sampling designs (see table3). Channel width, in feet

Elevation, in feet above datum

0

200

400

600

800

1,000

1,200

0

500

1,000

1,500

2,000

2,500

3,000

Water-surface elevation

at 8,000 ft 3

/s

Channel width at 45,000 ft 3

/s

Channel width at 8,000 ft 3

/s

3,500

0 50 100 150 200

Distance downstream from Lees Ferry, in miles

1.81.1 2.6 1.2 2.2 1.9

Number of debris fans per river mile

UPPER

MARBLE

CANYON

LOWER

MARBLE

CANYON

EASTERN

GRAND

CANYON

CENTRAL

GRAND

CANYON

WESTERN

GRAND

CANYON

More accommodation space is available for sand 

deposition in wider reaches than in relatively narrow 

reaches. The relatively wide reaches include lower 

Marble Canyon, the upstream part of eastern Grand 

Canyon, and much of western Grand Canyon. Not only 

do flood flows inundate much more area relative to 

low flows in these segments of Grand Canyon, but the 

frequency of debris fans and of eddy deposition zones 

is greater than elsewhere. Thus, there is greater area 

for sand deposition during high-flow experiments in 

these segments than elsewhere in Grand Canyon. The 

flattest part of the river’s profile is also in lower 

Marble Canyon where the river-level bedrock is Muav 

Limestone and Bright Angel Shale (Beus and Morales, 

2003) between about river mile (RM) 35 and RM 55. 

The steepest and narrowest part of the river is upper 

Granite Gorge in eastern and central Grand Canyon 

between about RM 75 and RM115. There is relatively 

little available space for sand deposition during floods 

between RM 87 and RM 100, and the fewest number 

of debris fans in Grand Canyon occur in the same 

segment. There also is relatively little available space 

for sand deposition between RM 10 and RM 35.

Downstream variations in channel width and water-surface elevation, and the occurrence 

of debris fans and eddy deposition zones between Lees Ferry and Diamond Creek. 

From data described by Magirl and others (2008).
2,860

2,880

2,900

21 21.5 22 22.5 23 23.5 24

Elevation,

in feet above datum

Distance from Lees Ferry, in river miles

Water surface at 8,000 cubic feet per second

Because debris flows and debris fans exert a strong influence on the Colorado River’s 

longitudinal profile and its cross-sectional form, Melis (1997) measured debris fans and used 

these data to develop a longitudinal classification that incorporated river width measurements 

and characteristics of debris fans, such as their number, size, shape, and ability to constrict the 

channel, into the previous classifications on the basis of changes in river-level bedrock (table 3). 

Using this approach, Melis (1997) divided Grand Canyon into six segments. Melis’s (1997) 

approach was novel, because he recognized the effects of downstream differences in debris-fan 

characteristics on the distribution of some river resources that occur near debris fans. 

Today, the longitudinal variation in canyon characteristics is the framework within which 

the impacts of HFEs are described, because it is recognized that different parts of Grand 

Canyon respond differently to the same HFE. The classification proposed by Schmidt and Graf 

(1990) primarily is used when describing the longitudinal distribution of natural resources or 

in aggregating site-scale data (table 3). A different classification is used when describing the 

fine-sediment mass balance of segments of the river corridor. The latter classification is based on 

the location of sediment-transport measurement stations (fig. 1), because these stations are used 

to calculate the mass of fine sediment that has accumulated or been evacuated from different 

parts of the river corridor. These “sediment-mass-balance segments” are upper Marble Canyon 

(river mile [RM] 0 to RM 30), lower Marble Canyon (RM 30 to RM 61), eastern Grand Canyon 

(RM 61 to RM 88), central Grand Canyon (RM 88 to RM 166), and western Grand Canyon 

(RM 166 to RM 225).

Landforms of the River Corridor

While boating the river or hiking along the channel banks, one observes that sand is 

intermittently distributed and other parts of the river shoreline are talus (rock debris), bouldery debris-flow deposits, or gravel bars. Specific landforms that have been the focus of most 

management and scientific interest include eddy sandbars, pre-dam river terraces, eolian (windblown) deposits, and debris-flow deposits. 

Eddy Sandbars and Channel-Margin Deposits

 Dolan and others (1978) and Howard and Dolan (1981) showed that most sandbars form 

in eddies that occur immediately downstream from debris fans. Schmidt and Rubin (1995) more 

precisely described the shorter-scale, or reach-scale, organization of the channel and defined the 

fan-eddy complex as the fundamental hydraulic and geomorphic feature of Grand Canyon and other 

debris-fan-affected canyons of the Colorado River watershed. The core of each fan-eddy complex is 

the debris fan that constricts the channel. A rapid often, but not always, exists here, because velocity 

is greater in the steeper, narrower, and shallower channel (fig. 10). Upstream from this rapid, streamflow is ponded and is relatively slow. The ponded flow may extend several miles upstream from 

Figure 9. Longitudinal profile of 

the Colorado River water surface 

from river mile 21 to 24 in Marble 

Canyon showing the relatively 

flat pool segments separated by 

short and relatively steep rapids. 

The water-surface profile is 

based on data collected by light 

detection and ranging (lidar) in 

March 2000, when the release 

from Glen Canyon Dam was 

steady at 8,000 cubic feet per 

second (Magirl and others, 2005). 

Chapter 2—Understanding Physical Processes of the Colorado River 33
Schmidt and Graf (1990)

Reach 

number River mile Reach type

1 RM 0–11 W

2 RM 11–23 N

3 RM 23–40 N

4 RM 40–62 W

5 RM 62–77 W

6 RM 77–118 N

7 RM 118–126 N

8 RM 126–140 N

9 RM 140–160 N

10 RM 160–214 W

11 RM 214–225 N

Melis (1997)

Reach 

number River mile Reach type

1 RM 0–8 W

2 RM 8–38 N

3 RM 38–77 W

4 RM 77–170 N

Subreach 

4A RM 87–100 Widened narrow

N

Subreach 

4B RM 116–128 Widened narrow

N

5 RM 170–213 W

6 RM 213–225 N

Table 3. Classifications of large-scale organization of the Colorado River on the basis of geomorphic characteristics. 

Classifications are those of Schmidt and Graf (1990), Kearsley and others (1994), and Melis (1997). Length of columns is scaled to 

reflect the river length for each reach. Reach type designation is narrow (N) and wide (W) for Schmidt and Graf (1990) and Melis (1997) 

and critical (C) and noncritical (NC) for Kearsley and others (1994). River mile designations are rounded to the nearest mile. From 

Kearsley and others (1999).

Kearsley and others (1994)

Reach 

number River mile Reach type

1 RM 0–11 NC

2 RM 11–41 C

3 RM 41–76 NC

4 RM 76–116 C

5 RM 116–131 NC

6 RM 131–139 C

7 RM 139–164 C

8 RM 164–225 NC

a rapid. Kieffer (1985) showed that flow upstream from Crystal Rapids (RM 98), one of the largest 

rapids in Grand Canyon, is ponded for approximately 1 mile. The banks of the ponded flow sometimes 

are channel-margin deposits, which are narrow sand deposits that resemble flood plains and sometimes 

have natural levees on top. Immediately downstream from each rapid, channel width increases in what 

is termed the expansion (Schmidt, 1990). In the expansion, downstream-directed flow separates from 

the bank and forms a jet of high-velocity flow in the center of the channel. Recirculating eddies occur 

between the jet and the channel banks, and sandbars occur in these eddies. The length of the recirculating eddy changes with discharge, and the recirculating zone typically gets longer and narrower 

as discharge increases (Schmidt, 1990). The channel bed is typically very deep immediately downstream from the rapid, forming a deep scour hole. Downstream from the expansion, the channel width 

narrows, the bed becomes shallow, and a gravel bar sometimes is formed. If the gravel bar is sufficiently large, a small rapid may occur where the river flows around the bar (Webb and others, 1988). 

34 Effects of Three High-Flow Experiments on the Colorado River Ecosystem Downstream from Glen Canyon Dam, Arizona
Separation zone Reattachment zone

Reattachment bar

Return-current

channel and 

backwater habitat

Debris fan

Separation bar

River flow

Recirculating eddies are effective traps of sand and mud that are transported in suspension 

by the main flow. Observations during the mid-1980s demonstrated that the eddy sandbars have 

a consistent topography that can be subdivided into separation bars and reattachment bars on the 

basis of their specific location within each eddy (Schmidt, 1990; Schmidt and Graf, 1990; see text 

box, The Fan-Eddy Complex). Separation bars mantle the debris fans that form rapids and are 

deposited in zones of low velocity near where the downstream-directed flow of the rapid separates 

from the channel bank (Schmidt and Graf, 1990). The reattachment bar forms within the primary 

cell of recirculating eddy flow, and the highest part of the reattachment bar occurs where downstream-directed flow reattaches to the channel bank. Between the reattachment and separation bar 

is the primary eddy return-current channel, a deep channel that is maintained by the high-velocity 

flow of water that exits the primary cell of recirculating eddy flow. 

Eddies constitute about 20 percent of the water-surface area (Schmidt and others, 2004), which 

is a large proportion of the river. There are approximately five eddies per river mile in Marble 

Canyon, but the frequency of eddies varies between 3.5 and 9.3 per river mile depending on canyon 

width (Schmidt and others, 2004; Hazel and others, 2006). Measurements of changes in eddy-bar 

topography suggest that eddies have the potential to accumulate or evacuate very large volumes of 

sand and mud. Schmidt and others (2004) estimated that about 50 percent of the pre-dam annual 

accumulation and evacuation of fine sediment occurred by the increase and decrease of eddy-bar 

size. The volume of fine sediment potentially stored in eddies in Marble and eastern Grand Canyon 

is 15.8 million tons and 7.4 million tons, respectively (Schmidt and others, 2004). This estimated 

volume is much larger than the annual pre-dam accumulation and evacuation of fine sediment in 

Marble and eastern Grand Canyon estimated by Topping, Rubin, and Vierra (2000)—7 million tons. 

Because the volume of potential deposition in eddies is much larger than the typical amount of fine 

sediment annually accumulated and evacuated, parts of eddy sandbars must have persisted for many 

years and have been reworked only by the largest pre-dam floods. 

Although the general pattern of flow at the water surface was well understood by the late 

1980s, researchers had a poor understanding of the three-dimensional attributes of flow within 

eddies or the mechanics by which sediment is exchanged between the main current and eddies. 

Streamflow in rapids is highly turbulent, and the mechanical forces associated with this turbulence suspend sand and mud throughout the water column. Qualitative observations made in the 

mid-1980s showed that some of the water and associated suspended sediment moves from the 

main current into the adjacent eddy, and Schmidt and others (1993) simulated this process in a 

flume. Researchers in the mid-1980s made crude measurements of hydraulics and bathymetry 

(water depth) in and near eddies. Most observations, however, were qualitative and were based on 

Figure 10. Diagrammatic illustration 

of a typical fan-eddy complex, including 

reattachment sandbar, returncurrent channel, and backwater habitat. Main channel flow is from left to 

right. The separation zone is the area 

where downstream flow separates 

from the bank, and the reattachment 

zone is the area where downstream 

flow reattaches to the bank. The small 

arrows indicate the primary cell of 

recirculating flow within the eddy at 

flows during which the sandbar would 

be submerged. Modified from Schmidt 

and Graf (1990).

Chapter 2—Understanding Physical Processes of the Colorado River 35
36 Effects of Three High-Flow Experiments on the Colorado River Ecosystem Downstream from Glen Canyon Dam, Arizona

The Fan-Eddy Complex

Topography of the bed and of eddy sandbars near Eminence Break camp (RM 44) in lower Marble 

Canyon. These maps show the typical topography of the channel near tributary debris fans that 

determine the local hydraulics and shape the channel bed. A, This map shows the topography of the 

lowest elevation of each point in the fan-eddy complex between 1990 and 2008. The three main parts 

of an eddy sandbar are shown: separation bar, primary eddy return-current channel, and reattachment 

bar. B, This map shows the topography of the highest elevation of each point in the same fan-eddy 

complex. The difference in topography between the maps in A and B represents the sediment that was 

gained, or lost, during this survey period. Note that the distinctive topography of the three main parts 

of the eddy sandbar can be observed in A and B. Flow is from top to bottom.

minimum elevations surveyed (figure A below) are greatest 

for the reattachment-bar part of the eddy sandbar, indicating that a large amount of sand can be stored or 

evacuated from these landforms. The primary eddy 

return-current channel is much smaller when the reattachment bar is large (figure B below). The scour hole persists 

as a topographic feature of the bed even when the bed has 

a large amount of sediment (figure B below).

Flow conditions at this site were measured during the 2008 

high-flow experiment (HFE) by Wright and Kaplinski (in 

press). The recirculating cell of eddy flow is clearly 

depicted in these measurements, as is the strong upstream 

flow near the left bank (figure top right). Comparison 

among figures shows that separation and reattachment 

bars form in low-velocity conditions in the zones where 

downstream-directed currents separate and reattach to 

the left bank. 

Repeated measurements of the channel bed and eddy 

sandbars made by using modern topographic and 

bathymetric (water-depth) mapping techniques (Hazel 

and others, 2008; Kaplinski and others, 2009) precisely 

describe the fan-eddy complex. These survey data were 

combined to develop two maps near Eminence Break 

camp at river mile 44 in lower Marble Canyon. One such 

map represents the topography of the lowest elevations 

surveyed between 1990 and 2008 at each point in the 

survey area (figure A below). A companion map represents the topography of the highest elevations surveyed 

between 1990 and 2008 at each point in the survey area 

(figure B below).

Debris fans that partly block the channel and eddy 

sandbars downstream from these debris fans occur in 

repeating patterns. The difference in topography between 

the maximum elevations surveyed (figure B below) and the 

0 300 600 feet

0 300 600 meters

Separation bar

Scour hole

Primary eddy

return-current

channel

Reattachment bar

Water-surface

elevation at 

8,000 ft3

/s

DEM surface

elevation, 

in feet

2,780

2,700

A B

River

flow
Chapter 2—Understanding Physical Processes of the Colorado River 37

Depth-average horizontal velocities measured at peak flow during the 2008 HFE 

at the fan-eddy complex shown on the 

previous page. Scale bars indicate the 

speed of the flow. Modified from Wright 

and Kaplinski (in press).

One aspect of the flow in eddies that had not been observed 

before the measurements made during the March 2008 HFE is 

that the downstream-directed flow remains confined to a narrow 

part of the channel near the right bank downstream from the 

debris fan (figure below), and this narrowing and forcing of the 

flow to the right bank is caused by the convergence of recirculating eddy flow at the upstream end of the eddy. At the downstream end of the fan-eddy complex, a portion of the flow is 

oriented into the eddy. Thus, sand in suspension in the main flow 

first moves onto the reattachment bar.

Collectively, these topographic and flow-velocity measurements demonstrate the close link between the topography 

of debris fans that cause the Colorado River’s rapids, 

the behavior of streamflow in the lee of debris fans that 

create large recirculating eddies, and the development 

of distinct separation and reattachment bars near the 

upstream and downstream ends of each eddy. The details 

and channel morphology of each debris fan differ, as 

do the characteristics of the flow entering each rapid. 

Because of these challenges, it is impossible to predict 

the detailed behavior during high flows at every sandbar 

in Grand Canyon. Thus, scientists strive to generalize 

results by using the average channel characteristics, 

average debris fan shape, and average sediment-transport 

characteristics in different segments of Grand Canyon.

Cross-section view illustrating the three-dimensional characteristics of streamflow during 

the 2008 HFE. The view is looking downstream and shows the core of high-velocity flow far 

on the right side of the river. Filled contours are the downstream velocity component; downstream is red and upstream is blue. Arrows show vectors of cross-section and verticalvelocity components. Transect is located at the downstream end of the eddy depicted on 

the previous page (Wright and Kaplinski, in press).

0 350 700 1,000 feet

0 350 700 1,000 meters

10 ft/s

Distance across channel from right bank, in feet

Depth below water 

surface, in feet

300 100 0

0

15

30

45

−3

0

3

7

10

400 200

Velocity,

in feet per second
surface-flow conditions, measurements of bed topography, or interpretation of sedimentary structures of exposed bedforms and bars during periods of low flow (Rubin and others, 1990; Schmidt 

and Graf, 1990; Andrews, 1991a; Nelson and others, 1994; Rubin and McDonald, 1995).

Research conducted during and since the 1996 HFE provided greater insight about river hydraulics 

and resultant changes in channel bed and eddy-bar topography, because the technology for making river 

measurements has greatly improved. Technologies and expanded measurement programs now provide 

scientists with a better understanding about where sand is distributed on the channel bed and how much 

of that sand is mobilized and subsequently deposited in eddies during high flows. The identification of 

fan-eddy complexes as the fundamental building blocks of the river ecosystem led scientists to devise 

mapping strategies with which to characterize the spatial distribution of these complexes and the distribution of eddy sandbars, campsites, and fish habitat. An ever-improving technology has allowed scientists to understand how fine sediment moves downstream and into eddies. Collectively, these insights 

provide an unprecedented perspective on how the Colorado River functions mechanistically, and new 

data contribute to the effort to numerically predict the river’s behavior. As described in later chapters, 

these improvements in understanding have led to better predictions about processes, such as sandbar 

deposition rates and patterns, which in turn can provide guidance on specific high-flow parameters, such 

as peak magnitude and duration, for high-flow releases.

Pre-Dam River Terraces and Wind-Blown Deposits

Pre-dam river terraces and wind-blown, or eolian, deposits that occur throughout the river 

corridor extend to higher elevations than the water surface of the HFEs. Thus, the potential effects 

of HFEs on these deposits are indirect. Eolian deposits typically occur on or near terraces, and the 

two units often are interbedded (Hereford and others, 1993). These deposits are host to many of 

the archeological sites in Grand Canyon (Fairley, 2005). The pre-dam river terraces were originally 

deposited as eddy sandbars and channel-margin deposits during large pre-dam floods. The terraces 

are composed primarily of sand and mud, although they may contain layers of coarser tributary and 

hillslope deposits. The eolian deposits typically are composed of fine sand and silt. These deposits 

initially were described and mapped in an attempt to understand whether archeological resources 

were affected by dam operations (Hereford and others, 1993). 

In addition to mapping the deposits and categorizing them by age and archeological context, 

Hereford and others (1993) identified potential mechanisms by which these deposits might be affected by 

post-dam floods and shorter-duration high flows. One of the most severe outcomes to archeological sites 

results from the formation and expansion of gullies or rills that form by overland flow across alluvium 

(sediment deposited by the river) during intense rains (Melis and others, 1995, p. 86; Draut and others, 

2004; Pederson and others, 2006; Hazel and others, 2008). Hereford and others (1993) suggested that 

gully erosion might be arrested or slowed by deposition of sand at gully mouths during HFEs. 

Continued research on the pre-dam terraces led to the observation that eolian deposits may play an 

important role in archeological site preservation. In some cases, the eolian deposits are derived from the 

pre-dam terraces and are largely inactive. In other cases, the eolian deposits are derived from sandbars 

close to the river and are active (Draut and Rubin, 2008). Because many of the active dunes are located 

near archeological sites, this observation led to the hypothesis that increased activity of those dunes might 

be accomplished by building eddy sandbars, the sand of which might be entrained by canyon winds. In 

turn, the wind-transported sand might enhance archaeological site preservation by directly burying sites 

or by filling in small gullies that were eroding into sites (Draut and Rubin, 2008). 

Debris-Flow Deposits

Debris fans are maintained by occasional debris flows that emanate from steep, tributary canyons 

(Graf, 1979). The role of debris flows in creating rapids was first demonstrated by Cooley and others 

(1977), who described debris flows in December 1966 in Crystal Creek that dramatically changed Crystal 

Rapids (RM 98) into one of the most difficult rapids of the post-dam river to navigate. Webb and others 

(1988) described the characteristics of debris flows in Monument Creek (RM 91) in 1984 that changed 

38 Effects of Three High-Flow Experiments on the Colorado River Ecosystem Downstream from Glen Canyon Dam, Arizona
the configuration of Granite Rapids, and they reexamined the 1966 debris-flow deposits in Crystal and 

Lava Chuar (RM 65) Creeks (Webb and others, 1989). Researchers in the late 1980s and early 1990s 

determined that debris flows occurred in all of the nearly 800 steep, ephemeral tributaries in Grand 

Canyon but that debris flows are more common in some places—upper Marble Canyon and western 

Grand Canyon—than elsewhere (Griffiths, 1995; Griffiths and others, 1996, 1997; Melis and Webb, 

1993; Melis and others, 1995, 1996; Melis, 1997; Webb, Melis, and others, 1999).

Howard and Dolan (1979, 1981) and Graf (1980) predicted that the reduced magnitude of 

floods following completion of Glen Canyon Dam and other large dams of the Colorado River 

system might cause accumulation of coarse, bouldery debris in rapids, thereby making rapids more 

difficult to navigate. Howard and Dolan (1979) further stated that the rapids of Grand Canyon 

were likely to become increasingly difficult to navigate because of boulder accumulation. Kieffer 

(1985) estimated that only floods of about 100,000 ft3

/s could transport bouldery debris delivered 

from tributary flash floods, and a post-dam flood of this magnitude has only occurred once. From 

measurements made during the 1996 HFE, described in the next chapter, it has been determined that 

smaller-magnitude floods are capable of transporting recently deposited boulders from debris flows, 

thus suggesting that boulder accumulation in rapids is not inevitable.

Measurements of Bed-Material Size and Distribution

The bed material of the Colorado River ranges in size from boulders to mud, which reflects the 

diverse sources of sediment delivered to the river. Bedrock occurs as islands in some places and has 

been identified on the channel bed by underwater imaging (Anima and others, 1998). Coarse sediments in the river include boulders delivered to the channel bed by debris flows and rockfalls. Wilson 

(1986) pioneered the use of oceanographic hydroacoustic instruments in Grand Canyon and showed 

that gravel is widely distributed over much of the channel. He showed that, in fall 1984, between 

30 and 81 percent of the channel bed was composed of bedrock or boulders (U.S. Department of the 

Interior, 1988, table A-2). 

Estimates of the proportion of the bed covered by sand vary widely. Because the channel bed and 

eddies are the only source of sand that creates eddy sandbars during HFEs, scientists have spent much 

time and effort increasing their understanding of the distribution of sand in the river and how it changes 

with time. Howard and Dolan (1981) assumed that 75 percent of the bed in Grand Canyon was sand, a 

value much larger than subsequently estimated by Wilson (1986). Anima and others (1998) used various 

methods of underwater imaging to show that less than 30 percent of the bed was covered with sand in 

the early 1990s, and most of this sand was located upstream from rapids and adjacent to eddies near 

scour holes. Rubin, Anima, and Sanders (1994, p. 6) estimated that the mean thickness of sand in Marble 

Canyon was “at least a few tens of centimeters and not more than a few meters.” As described in the next 

chapter, measurements made before and during each HFE now enable scientists to better constrain the 

amount of sand stored on the bed and the amount available for transport during an HFE. These insights 

are essential to predicting the outcome of high-flow releases, because river managers seek to transfer 

new tributary sand inputs from the channel bed to eddies rather than merely moving sand from upstream 

eddies to those farther downstream. 

Adjustment of the Channel Bed at Annual and Decadal Time Scales

If controlled high flows are to be used in the post-dam period to maintain or increase the size of 

eddy sandbars, it is fundamental to understand the source of sand that is deposited at high elevations 

during an HFE. Rubin, Anima, and Sanders (1994) provided a conceptual framework that explicitly 

linked changes in the volume of sandbars with changes in the overall sand mass balance of the river. 

They argued that two factors control the downstream variations in sandbar erosion: (1) the difference between the amount of sand entering and exiting a given reach and (2) the net rate of sand 

exchange between the channel bed and eddies. In other words, sandbar erosion (and deposition) 

Chapter 2—Understanding Physical Processes of the Colorado River 39
depends on how much sand is in the channel and how much of that sand is exchanged with the 

sand that forms eddy sandbars. The model of Rubin, Anima, and Sanders (1994) is a framework 

that remains applicable and demonstrates that it is essential to (1) measure large-scale longitudinal 

changes in sand mass balance, (2) understand the proportion of the total sand supply that is stored 

in eddies and on the channel bed, and (3) understand the processes by which sand is exchanged 

between the channel and eddies. 

Thus, it is fundamental to understand how the Colorado River adjusts its channel bed at the short 

time scale of individual floods and also how the river adjusts its form at the multidecadal time scale 

in response to the large post-dam decrease in sand supply and changes in flow regime. For example, it 

is not known whether eddy sandbars have been more resistant to change than the sand deposits on the 

channel bed or whether more sand has been evacuated from the channel bed or eddy sandbars.

Early researchers focused on changes in channel-bed topography, because data were available at 

the Lees Ferry and Grand Canyon gages (fig. 1). These data (Leopold and Maddock, 1953; Leopold 

and others, 1964) show that the channel bed had been scoured and filled substantially during the 

passage of floods and that most of the sand and mud delivered to the Colorado River corridor during 

the pre-dam period was transported downstream. However, a small, and imprecisely measured, 

proportion of fine sediment was stored temporarily on the channel bed and banks. These observations 

were the basis for speculation about whether there were large seasonal or decadal changes in sand 

accumulation and evacuation. Between the 1950s and the early 1990s, scientists debated whether 

measured bed changes at the Lees Ferry and Grand Canyon gages were representative of large-scale 

bed changes throughout Grand Canyon or if those changes were merely local phenomena. Following 

many years of repeated measurements of bed topography and bed-material size, the current understanding of how the river accumulates and evacuates sand is consistent with the interpretation by 

Colby (1964). The understanding is that (1) sand is temporarily stored in deep scour holes, other deep 

parts of the riverbed, and eddies; (2) there are parts of the bed that change little during floods and 

parts that change greatly; and (3) bed locations that change greatly are an important source of the sand 

that is entrained by the river and deposited in eddies during HFEs.

In contrast to speculation about changes to the riverbed in Grand Canyon, bed changes in Glen 

Canyon between the dam and Lees Ferry have been well measured and were well understood at the time 

of the 1996 HFE. Pemberton (1976) showed that the bed had been substantially scoured in Glen Canyon 

and that by 1963 the zone of scour had extended about 7 miles downstream from the dam. Grams and 

others (2007) described in detail the timing and downstream extent of this erosion and bed coarsening 

that ultimately resulted in the transformation of that reach from a predominantly sand-bedded river to a 

gravel and cobble-bedded river (see text box, Channel Change in the Glen Canyon Reach).

Adjustment of Sandbars at Annual and Decadal Time Scales

Sandbar erosion in Grand Canyon was observed by river runners and scientists in the early 

1970s (Dolan and others, 1974; Laursen and Silverston, 1976). The cause of the erosion was uncertain. Some researchers attributed erosion to the river’s tendency to evacuate sand in response to the 

diminished sand supply. Other researchers focused on the effects of the daily pulses of streamflow 

caused by the peak production of hydroelectric power because it was assumed that the channel 

bed was accumulating sand even while the eddy sandbars were eroding. Concerns about erosion 

prompted the National Park Service (NPS) to establish a sandbar-monitoring program in the mid1970s. This monitoring program was maintained in the late-1970s and early 1980s by NPS cooperators (see review by Webb, Wegner, and others, 1999). Although Howard (1975) and Howard and 

Dolan (1976, 1979) initially concluded that sandbar erosion was substantial, these findings were 

revised by Howard and Dolan (1981, p. 284) who concluded “during the first ten years since the 

dam, sandy channel banks have suffered only a very slight erosion, with individual cases of both 

pronounced erosion and marked deposition.” Occasional topographic measurements of sandbars 

were made throughout the early 1980s (Beus and others, 1982, 1984).

40 Effects of Three High-Flow Experiments on the Colorado River Ecosystem Downstream from Glen Canyon Dam, Arizona
Increased funding and scientific attention associated with the initial GCES program coincided 

with the floods of the mid-1980s (fig. 5). The effects of the summer 1983 flood were not well 

measured, however, because the flood was underway when the GCES field program began. Channel 

changes caused by this flood primarily were inferred from observed changes in campsites. Brian 

and Thomas (1984) reported that some campsites were eroded and others were created by this flood. 

Most of the campsites that were affected by sand deposition were in western Grand Canyon. The 

effects of the 1984 flood on sandbars were measured by Beus and others (1985).

The effects of the 1985 flood on sandbars were measured by Schmidt and Graf (1990), who 

measured sandbar topography during peak flood and flood recession in May and August. They also 

measured widespread and substantial erosion of flood-formed bars during an experimental period 

of powerplant fluctuations between October 1985 and mid-January 1986. Occasional topographic 

measurements of sandbars were made in the late 1980s and as part of the 1990–91 experimental 

flow periods (Beus and others, 1992). 

Schmidt and Graf (1990) and Kearsley and others (1994) evaluated sandbar conditions in the 

late 1980s within a historical context of topographic changes that had occurred since dam closure by 

making measurements on aerial photographs taken in 1965, 1973, 1980, 1984, and 1990. Kearsley 

and others (1994) concluded that approximately one-half of the sandbars used as campsites were 

smaller in 1990 than in 1965 and only 2 percent were larger (fig. 11). They also concluded that the 

“benefit” of sandbar rebuilding caused by the 1983 flood had been short lived because of subsequent 

erosion. 

By the mid-1990s, it was generally agreed that large post-dam floods or floods with very large 

suspended-sand concentrations could rework and rebuild eddy sandbars. Researchers concluded that 

eddy sandbars had been substantially reworked by the 1983 flood and that large amounts of riparian 

vegetation had also been removed. Rubin, Schmidt, and others (1994) showed that the smaller 

floods of 1984 through 1986 had rearranged the 1983 flood deposits and left a relatively thin veneer 

of reworked sand on platforms of 1983-deposited sand. In January 1993, approximately 4.6 million 

tons of sand were delivered by flooding in the Little Colorado River, and Wiele and others (1996) 

showed that the associated high concentrations of sand in the main stem of the river could create 

large eddy sandbars. The sedimentology of separation bars and reattachment bars formed by the 

floods of the 1980s was relatively consistent throughout Grand Canyon (Rubin, Schmidt, and others, 

1994), providing indication that there may have been some consistency in how eddy sandbars 

responded to a subsequent HFE.

Progress in monitoring sandbar change has continued during the past 15 years. Today, comprehensive measurements of changes in topography are measured by ground surveys and remote-sensing 

methods, such as lidar (light detection and ranging; see text box, Modeling the Colorado River 

Shoreline) and photogrammetry. The spatial and temporal resolution of these measurements is unique 

among the world’s large rivers, yet scientists still struggle to make generalizations about sandbar 

changes because of the wide range of controlling factors that affect sandbar size and frequency.

Erosion of Eddy Sandbars

By the mid-1990s, the research community realized that sandbar erosion was caused by many 

processes: (1) nearshore currents, (2) waves generated by rapids (Bauer and Schmidt, 1993), 

(3) seepage erosion caused when river flows decreased quickly and groundwater in sandbars 

emerged as springs along the streamward side of the bars (Budhu and Gobin, 1994), (4) wind (Dolan 

and others, 1974), (5) tributary floods and hillslope runoff (Melis and others 1995; Melis, 1997), 

(6) mass failures termed rapid failure events (Cluer, 1995), and (7) trampling by hikers and boaters 

(Valentine and Dolan, 1979). The rapid erosion events observed by Cluer (1995) occurred during 

fluctuating or steady flows, when a large portion of an eddy sandbar slumped into the main channel 

and thereby caused a large decrease in sandbar area. Cluer (1995) referred to these events as “rapid 

failure events” and photographed these events using time-lapse cameras. Rapid-failure events were 

later measured in detail during HFEs.

Chapter 2—Understanding Physical Processes of the Colorado River 41
100

Change in campsite area, in percent 80

60

40

20

0

1965 to 1973 1973 to 1983 1983 to 1984 1984 to 1991

Although researchers in the mid-1970s had thought that erosion primarily was caused by 

large-ranging fluctuations that occurred during days of production of peaking hydroelectric power, 

researchers in the late 1980s and early 1990s came to realize that eddy sandbar erosion occurred 

whenever main-stem sand concentrations were low and the flow regime changed. Researchers also 

realized that the expansion of riparian vegetation led to reduced campsite areas. Although some 

research has continued concerning the processes and rates of sandbar erosion, the realization that 

erosion was widespread and inevitable caused researchers to focus more of their attention on how 

to rebuild sandbars during high releases rather than on how to control the rates of erosion.

Spatial Variability in Patterns of Sandbar Change

Variability in patterns of sandbar change has been observed at both large and small scales. 

Schmidt and Graf (1990) observed different patterns of sandbar change during the mid-1980s 

where the channel was narrow and where the channel was relatively wide. Measurements made 

in the late-1980s and early 1990s showed that different shapes and sizes of debris fans caused 

eddies and the associated sandbars to respond differently to the same high flow. Recognition of 

the inherent variability in how sandbars responded to floods subsequently forced researchers 

to develop robust monitoring programs that would allow recognition of large-scale patterns of 

sandbar change that would not be overwhelmed by small-scale variability.

Although the study design of the June 1990 through July 1991 test flows sought to explicitly 

link sandbar erosion with specific flow regimes, Beus and others (1992, p. 59) concluded that “no 

single test flow alternative affected all sandbars in the same manner.” Repeat measurements of 

29 sandbars made during the 13-month test period showed that some sandbars were eroded, others 

had new sand deposition, and others changed little during the entire study period. Study sites did 

not respond consistently to test flows, and some topographic changes were not expected. Beus and 

others (1992) found that erosion was widespread and substantial during some steady flows and 

that deposition of new sand was widespread during some highly fluctuating flows. Such findings 

were not anticipated by the research and management community, and scientists were inspired 

to examine the sand-supply conditions during this period. Although the current high-resolution, 

suspended-sediment monitoring program was not yet in place, Beus and others (1992) speculated 

that erosion or deposition more likely was determined by the recent occurrence, or lack of occurrence, of tributary sand-supplying floods, than by the flow regimes that were being evaluated.

The Need for High-Flow Experiments

The need to test the potential use of high flows was a significant part of the EIS 

(U.S. Department of the Interior, 1995) and Record of Decision (U.S. Department of the Interior, 

1996), the basis of which was a management objective to maintain or improve, if possible, 

Figure 11. History of changes in campsites in Grand Canyon between 1965 

and 1991 based on a composite of results from campsite inventories and 

aerial photograph interpretation. For each of four time intervals, the percentages of sites are shown that remained approximately the same size (tan), 

increased in size (brown), or decreased in size (red). Decreases in campsite 

size occurred in every time interval and the greatest decrease occurred 

between 1965 and 1973. Some campsites increased in size between 1983 and 

1984 and between 1984 and 1991 in response to post-dam floods that occurred 

between the observations. Modified from Kearsley and others (1994).

42 Effects of Three High-Flow Experiments on the Colorado River Ecosystem Downstream from Glen Canyon Dam, Arizona
the condition of sandbars and related resources in Grand Canyon National Park. The studies 

summarized in this report provided the scientific basis for that objective and identified the gaps in 

knowledge that high-flow experimentation might fill. Although sandbar erosion was considered 

the “problem,” these studies recognized that some degree of sandbar erosion was an inevitable 

natural process, even under restricted flows designed to minimize sand transport and reduce 

erosion rates. Thus, the idea of using high flows to “replenish” sandbars included the concept that 

the high flows would need to be repeated with some frequency to compensate for erosion between 

the events. It was clearly recognized that experimentation was needed. Although the floods of the 

1980s and the winter 1993 floods from the Little Colorado River demonstrated that high flows 

can build sandbars, prescribed high flows require specificity in terms of flow magnitude, duration, hydrograph shape, and the frequency required to meet management objectives. An additional 

overarching question was whether there was enough sand enrichment to provide the elevated 

suspended-sediment concentrations needed to build and also maintain sandbars. The following 

chapter describes and summarizes studies conducted during these high flows that were designed 

to address these questions and additional studies that were undertaken to monitor the results of the 

HFEs and evaluate the degree to which they were meeting management objectives. 

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and Colorado Rivers: Transactions of the American Geophysical Union, v. 19, p. 653–659.

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Topping, D.J., Rubin, D.M., Grams, P.E., Griffiths, R.E., Sabol, T.A., Voichick, Nicholas, Tusso, 

R.B., Vanaman, K.M., and McDonald, R.R., 2010, Sediment transport during three controlledflood experiments on the Colorado River downstream from Glen Canyon Dam, with implications for eddy-sandbar deposition in Grand Canyon National Park: U.S. Geological Survey 

Open-File Report 2010–1128, 111 p. (Also available at http://pubs.usgs.gov/of/2010/1128/.)

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between grain-size evolution and sediment depletion during Colorado River floods, 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. 71–98.

Topping, D.J., Rubin, D.M., Nelson, J.M., Kinzel, P.J., III, and Corson, I.C., 2000, Colorado 

River sediment transport, 2. Systematic bed-elevation and grain-size effects of sand supply 

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http://dx.doi.org/10.1029/1999WR900286.

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River by changes in the surface grain size of eddy sandbars over multi-year timescales: Sedimentology, v. 52, no. 5, p. 1133–1153, doi:10.1111/j.1365-3091.2005.00738.x.

Topping, D.J., Rubin, D.M., and Vierra, L.E., Jr., 2000, Colorado River sediment transport, 

1. Natural sediment supply limitation and the influence of Glen Canyon Dam: Water Resources 

Research, v. 36, no. 2, p. 515–542, doi: 10.1029/1999WR900285.

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September 30, 2000: U.S. Geological Survey Professional Paper 1677, 118 p. (Also available at 

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Turner, R.M., and Karpiscak, M.M., 1980, Recent vegetation changes along the Colorado River 

between Glen Canyon Dam and Lake Mead, Arizona: U.S. Geological Survey Professional 

Paper 1132, 125 p.

U.S. Department of the Interior, 1988, Glen Canyon Environmental Studies Executive Review 

Committee, Final report: Bureau of Reclamation, Glen Canyon Environmental Studies, 

84 p. + apps. (Available from National Technical Information Service, PB88-183348/AS.)

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Impact Statement, Colorado River Storage Project, Coconino County, Arizona: Salt Lake City, 

Utah, Bureau of Reclamation, Upper Colorado Regional Office, 337 p., accessed November 18, 

2010, at http://www.usbr.gov/uc/library/envdocs/eis/gc/pdfs/Cov-con/cov-con.pdf.

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Environmental Impact Statement: Washington, D.C., Office of the Secretary of the Interior, 

Bureau of Reclamation, 15 p., accessed May 26, 2010, at http://www.usbr.gov/uc/rm/amp/pdfs/

sp_appndxG_ROD.pdf.

Valentine, Susan, and Dolan, Robert, 1979, Footstep-induced sediment displacement in the Grand 

Canyon: Environmental Management, v. 3, no. 6, p. 531–533, doi:10.1007/BF01866322.

50 Effects of Three High-Flow Experiments on the Colorado River Ecosystem Downstream from Glen Canyon Dam, Arizona
Webb, R.H., 1996, Grand Canyon—A century of change—Rephotography of the 1889–1890 

Stanton expedition: Tucson, The 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: U.S. Geological Survey Water-Resources 

Investigations Report 00–4055, 67 p.

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Pizzuto, J.E., 1999, 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.

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River, Grand Canyon National Park, Arizona: U.S. Geological Survey Professional Paper 1492, 

39 p.

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Grand Canyon: Washington, D.C., American Geophysical Union, Geophysical Monograph 

Series, v. 110, 367 p.

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effects of Glen Canyon Dam on the Colorado River 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: Washington, D.C., American Geophysical Union, Geophysical Monograph Series, v. 110, 

p. 1–21.

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Canyon from flooding of the Little Colorado River: Water Resources Research, v. 32, no. 12, 

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flow in the Colorado River through Grand Canyon, Arizona: U.S. Geological Survey WaterResources Investigations Report 97–4046, 17 p.

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no. 5, p. 1375–1386, doi:10.1029/96WR00199.

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U.S. Geological Survey Subcommittee on Sedimentation—Interagency Advisory Committee on 

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during a controlled flood, Colorado River, Arizona: Journal of Geophysical Research.

Chapter 2—Understanding Physical Processes of the Colorado River 51
Paul Grams, U.S. Geological Survey
CHAPTER 3

The High Flows—Physical Science Results 

By John C. Schmidt1 and Paul E. Grams2

As described in chapters 1 and 2, both the existence of Glen Canyon Dam as a physical 

barrier and the regulation of streamflow and water temperature by operation of the dam 

caused changes to the downstream environment of the Colorado River (fig. 1). The undesirable effects from the existence of the dam and Lake Powell, the reservoir formed by the dam, 

could be mitigated only by costly alterations or construction of new facilities. Those effects that 

result completely from the operation of the dam, however, could possibly be mitigated by changes 

in the schedule of dam releases that generate hydroelectricity and by the release of additional flow 

using tubes that bypass the powerplant’s turbines. Some changes to the Colorado River result from 

both the existence and operation of the dam, such as the temperature of waters released downstream, and these effects could be partially mitigated by changes in the elevations at which water 

is extracted from Lake Powell.

Many changes have occurred in the physical structure and geomorphic processes of the 

Colorado River downstream from the dam since it was completed in 1963. The reduced magnitude 

and frequency of floods have caused accumulation of gravel, cobbles, and boulders in bars and 

in rapids. The greatly reduced supply of sand and mud once delivered from the upstream watershed led to bed incision and removal of most of the sand that once covered the Colorado River in 

the 16 miles between the dam and Lees Ferry (fig. 1). Downstream from Lees Ferry, the greatly 

reduced sand and mud supply has led to a decrease in the number and size of sandbars.

High-flow experiments (HFEs) have been implemented in order to make critical measurements 

of physical processes of the post-dam river, especially regarding how the Colorado River transports 

and reworks the remaining small supply of sand and mud, primarily contributed by the Paria and 

Little Colorado Rivers. Additionally, HFEs have been implemented as management actions in order 

to evaluate if short-duration high flows could significantly restructure some of the valued natural 

resources of the Colorado River ecosystem. In this chapter, the scientific findings and insights gained 

from the three HFEs conducted in 1996, 2004, and 2008 are summarized as they pertain to important 

physical processes and related natural resources. This chapter primarily focuses on processes and 

attributes related to sand, because that is the physical attribute of the system most closely associated 

with target natural and cultural resources. The movement of gravel and boulders is briefly summarized as are the changes in associated natural resources, such as characteristics of rapids.

The decision to implement the first HFE in 1996 followed extensive consideration of the 

effects of post-dam floods and short-duration high flows. As described in chapter 2, post-dam 

floods, as defined by the Bureau of Reclamation, are those dam releases that exceed the capacity 

of the hydroelectric powerplant and last longer than a month; these floods occurred in 1965, 

1

 Intermountain Center for River Rehabilitation and Restoration, Department of Watershed Sciences, Utah State University, 

Logan, UT 84322–5210.

2

 U.S. Geological Survey, Southwest Biological Science Center, Grand Canyon Monitoring and Research Center, 

2255 N. Gemini Drive, Flagstaff, AZ 86001.
Lake Powell

Lake Mead

Glen Canyon Dam

Upper Marble Canyon

(UMC)

Lower Marble Canyon

(LMC)

Eastern 

Grand Canyon

(EGC)

Central

Grand Canyon

(CGC)

Western

Grand Canyon

(WGC)

GRAND CANYON

NATIONAL PARK

Lava Falls Rapid

GLEN CANYON

NATIONAL RECREATION

AREA

Lees Ferry gage

30-mi gage

61-mi gage

Diamond Creek gage

166-mi gage Grand

Canyon

gage

Paria

River

Kanab Creek

Diamond Creek Havasu Creek

Bright Angel Cr.

Little Colorado River

0 20 40 miles

0 20 40 60 kilometers

UTAH

ARIZONA

NEVADA

N

Colorado

River

Sandbar study sites

Streamflow and sediment 

transport measurement locations

ARIZONA

Map

Area

Figure 1. The Grand Canyon region, showing locations of gaging stations and data-collection 

sites for sandbar topography and channel bathymetry. The Lees Ferry gage (U.S. Geological 

Survey station 093820000) continuously measures stage and streamflow; supplemental sediment 

transport measurements have been made during some post-dam floods and all high-flow experiments (HFEs). The 30-mile, 61-mile, Grand Canyon (U.S. Geological Survey station 09402500), 166-

mile, and Diamond Creek (U.S. Geological Survey station 09404200) gages continuously measure 

stage, streamflow, and suspended-sand concentration. During the 1996 HFE, data were collected 

at the Lees Ferry, 61-mile, Grand Canyon, and 166-mile gages. During the 2004 HFE, data were 

collected at the Lees Ferry, 30-mile, 61-mile, Grand Canyon, and Diamond Creek gages. Data 

were collected at all six gages during the 2008 HFE. See Topping and others (2010) for detailed 

information on the data collected at each gage. The reaches for sand mass-balance calculations 

between the gages are also identified. Modified from Hazel and others (2010).

1983, 1984, 1985, and 1986. Short-duration high flows also exceed powerplant capacity but last 

only a few days. Before 1996, short-duration high flows occurred in June and July 1980 and 

August 1984. These high flows were engineering tests conducted to evaluate the dam’s spillway. 

Measurements and observations made during and after the post-dam floods of the mid-1980s 

convinced the research and resource management communities that long-duration floods eroded 

sandbars and caused damage to other resources (U.S. Department of the Interior, 1988). Other 

studies, however, indicated that the physical and ecological responses to short-duration high flows 

were potentially beneficial. Thus, the National Research Council (1996) encouraged the use of 

short-duration high flows as a tool in rehabilitating desired attributes of the riverine ecosystem 

(Schmidt, Andrews, and others, 1999). 

Streamflow Characteristics of Three High-Flow Experiments

The first HFE occurred between March 26 and April 7, 1996, and was a 7-day steady 

release of 45,000 cubic feet per second (ft3/s) that was preceded and followed by low, steady 

flows of 8,000 ft3

/s for 4 days (chapter 2, fig. 6, this volume). The second HFE occurred between 

54 Effects of Three High-Flow Experiments on the Colorado River Ecosystem Downstream from Glen Canyon Dam, Arizona
November 22 and 24, 2004, and was a 60-hour steady release of about 41,700 ft3

/s. The third HFE 

occurred between March 6 and 8, 2008, and was a 60-hour release of about 42,800 ft3

/s. The 2004 

and 2008 high flows rose over 30-hour periods, a 50-percent slower rate of rise than for the 1996 

high flow. 

The magnitude of floods on wide rivers decreases downstream if a substantial amount of the 

floodwaters temporarily occupies the flood plain. However, the magnitude of a flood can increase 

downstream if there are large inflows from tributaries. In narrow-bottomed canyons, such as Grand 

Canyon, flood attenuation caused by overbank flooding is small, and the primary downstream 

changes in flood magnitude occur when a high flow coincides with tributary inflows.

During the 1996 HFE, tributary inflow was minimal, and flow of the Colorado River at 

Diamond Creek (fig. 1), 241 miles downstream from the dam, exceeded the release at the dam 

by about 500 ft3/s, which was only a 1-percent increase in flood magnitude. In contrast, substantially more tributary inflow was measured in 2004 and in 2008, and discharge at Diamond Creek 

exceeded the magnitude of the dam release by about 5 percent.

Purposes and Design of the High-Flow Experiments

The HFEs had two purposes, one related to management of riverine resources and another 

related to improving scientific understanding of physical and biological processes in the river 

(Marzolf and others, 1999; Schmidt, Andrews, and others, 1999; Patten and others, 2001). Thus, 

each HFE was a management action with natural resource responses that were predicted and 

evaluated through environmental assessments (U.S. Department of the Interior, 2002, 2004). Each 

HFE was also a scientific experiment during which critical measurements were made and scientific 

hypotheses were tested. The dual purposes of the HFEs were closely linked, because each prediction about a resource response was itself a hypothesis that could be tested.

 As management actions, the goals of the HFEs were to improve those river resources that are 

well adapted to floods, control undesirable physical and biological attributes of the river ecosystem, 

and not damage desirable natural resources less well adapted to floods. Four of the primary objectives of the 1996 HFE focused on anticipated positive responses of resources well adapted to floods 

(Schmidt, Andrews, and others, 1999; Patten and others, 2001): 

•	 rejuvenate low-velocity habitats for native fishes 

•	 enlarge sand deposits 

•	 preserve and restore sandbars used as campsites 

•	 provide water to vegetation in the upper riparian zone 

Two other objectives of the 1996 HFE—reduce nearshore vegetation and remove nonnative fish 

downstream from Lees Ferry—sought to control undesirable species believed to be less well 

adapted to floods. In addition, it was predicted that desirable resources less well adapted to floods, 

such as nonnative rainbow trout (Oncorhynchus mykiss) that live in the Colorado River immediately downstream from Glen Canyon Dam, would not be adversely affected. 

Although any HFE might be perceived as “failing” if specific desired resource responses are 

not achieved, the success of an HFE may also be judged by the scientific understanding gained 

from the experiment and comparisons with other HFEs. Marzolf and others (1999, p. 367) observed

[The 1996 HFE] was conducted to demonstrate management utility. At the same time, 

the flood was [an] * * * experiment to test specific ideas about what had been learned 

about the physics of flow, sediment transport, and sediment deposition. As a management demonstration, the flood might have resulted in failure: that is, the expected 

beneficial effects might not have been realized. As [an] * * * experiment, the flood could 

Chapter 3—The High Flows: Physical Science Results 55
not fail, because no matter what happened new knowledge would have been gained as 

long as appropriate observations were made. Ideas would have been either reinforced 

and understood more certainly because the result was as expected and the causes and 

effects more clearly documented, or concepts would be rejected, and knowledge would 

have changed because the results were not as expected. In fact, science proceeds most 

certainly when incorrect ideas are rejected.

Between 1996 and 2008, scientific hypotheses for each HFE evolved as learning 

progressed. In 1996, predictions about sediment transport were very explicit. It was predicted 

that 840,000 tons of sand would be transported past the 61-mile gage (fig. 1) during the 7-day 

HFE. New deposition on sandbars was predicted to be 1 to 3 feet thick (Schmidt, Andrews, 

and others, 1999). The 2004 HFE was predicted “to create sandbars more efficiently and with a 

more diverse grain size distribution * * * and * * * to transport a smaller percentage of sediment downstream” (U.S. Department of the Interior, 2002, p. 43). It was also hypothesized that 

the 2004 HFE would result in sandbars with larger proportions of mud and very fine sand than 

sandbars formed in 1996. It was predicted that these sandbars “would likely be more resistant 

to erosion and retain more nutrients than coarser grained sandbars” because clay and silt add 

strength to sand deposits (U.S. Department of the Interior, 2002, p. 43). 

The duration and timing of the 2004 and 2008 HFEs were partly designed to clarify 

findings gained from the previous HFEs. The magnitude of each HFE was similar because of 

constraints inherent in the design of the dam (when the reservoir is full, no more than about 

31,500 ft3/s may be released through the powerplant and an additional 15,000 ft3/s may be 

released through tubes that bypass the powerplant). The similarity of flood magnitude among 

the HFEs allowed scientists to replicate some of their measurements (U.S. Department of the 

Interior, 2002). The duration of the HFEs was adjusted, however, because results from the 

1996 experiment demonstrated that high concentrations of suspended sand in the river could be 

sustained for only a few days. The timing of the 2004 HFE was adjusted to evaluate the benefit 

of scheduling a high flow in the fall to more closely coincide with the tributary floods that had 

delivered large amounts of fine sediment to the Colorado River. The 2008 HFE occurred after 

2 consecutive years of unusually large tributary inputs of fine sediment and before most of the 

new sand was exported downstream to Lake Mead by typical dam releases.

Research and evaluation efforts conducted during HFEs can be categorized into two 

groups (Marzolf and others, 1999). Process/response studies monitor resource response and are 

designed to encourage learning about the mechanisms and processes by which high flows affect 

resources. In contrast, negative-impact studies simply monitor the target resource and have 

limited ability to explain the processes and links that caused the response. Negative-impact 

studies typically are undertaken when it is believed that there will not be a substantial resource 

response, when there is insufficient understanding to design a process/response study, or if there 

is insufficient funding to implement such a study. In 1996, process/response studies focused 

on measuring the downstream passage of the flood wave, the amount of sediment transported 

by the river, changes in the amount of sediment on the channel bed, scour of tributary debrisflow deposits near debris fans that form rapids, sandbar erosion and deposition, organic-matter 

transport, changes in riparian vegetation, and water quality. 

Negative-impact studies in 1996 included studies of the aquatic food web, native and 

non-native fish populations, Kanab ambersnails (Oxyloma haydeni kanabensis), and changes in 

marshes (see chapter 4, this volume). Studies of these biological resources were “primarily to 

document resource losses or changes” (Marzolf and others, 1999, p. 365) and did not attempt to 

measure physical or biological ecosystem processes. Over time, biologically oriented process/

response studies evolved. Food-web monitoring, for example, shifted from negative-impact 

studies to process/response studies. Fish studies were increasingly tied to questions associated 

with the effects of specific dam operations and specific life-history stages.

56 Effects of Three High-Flow Experiments on the Colorado River Ecosystem Downstream from Glen Canyon Dam, Arizona
Evaluation of Physical Effects of the High-Flow Experiments: 

Key Measurements and Analyses

Most sandbars in Grand Canyon are present in eddies downstream from obstructions that 

partially block the channel. These obstructions are primarily caused by accumulations of boulders, 

cobbles, and gravel at the mouths of steep, ephemeral tributaries that partly block the Colorado 

River and form its rapids. Large, recirculating eddies are present downstream from virtually 

every rapid, and these eddies are effective traps for the fine sediment carried in suspension by 

the Colorado River. Eddy sandbars form when the flooding river carries suspended sediment into 

eddies at a higher rate than the sand and mud can be circulated within the eddy or be returned to 

the main flow. Large eddy sandbars form when suspended-sediment loads are transported in high 

concentrations by the main flow. High sandbars are constructed by large magnitude floods that rise 

to relatively high elevations. 

Observations made during the HFEs determined that the concentration and amount of sand 

transported by high flows is a function of the volume and areal distribution of sand on the channel 

bed immediately before each high flow. If the areal extent of sand is the same, higher concentrations of suspended sand occur if the sand is finer. Thus, the highest suspended-sand concentrations 

occur when there is a large amount of fine sand covering a large area of the channel bed. The lowest 

suspended-sand concentrations occur when there is a small volume of medium or coarse sand spread 

over a small area of the channel bed. In order to compare the conditions during HFEs, the amount of 

suspended-sand concentrations and loads and how these amounts changed throughout Grand Canyon 

must be determined. In turn, such a comparison necessitates evaluation of the amount of sand on the 

channel bed and in eddies before each HFE and the characteristic size of that sand. 

As scientific insight and understanding evolved after the 1996 HFE, scientists changed the 

measurement strategies and techniques used to characterize river conditions prior to each experiment as well as the techniques used to measure river hydraulics and sediment transport during 

high flows. New analytical approaches also allowed retrospective analyses; thus, inferences about 

conditions that existed immediately before the 1996 HFE have been made even though there were 

few direct measurements made immediately before that high flow (see text box, Monitoring Fine 

Sediment in Grand Canyon).

Description of River Segments

Nearly all of the sandbar and sediment-transport data collected during the three high flows 

were collected at monitoring sites located between Lees Ferry and Diamond Creek (fig. 1). This 

225-mile river corridor is subdivided into segments. As explained in chapter 2, these subdivisions 

vary depending on the purpose of the classification. For sediment budgeting, the primary distinction 

is between Marble Canyon and Grand Canyon. Marble Canyon begins at Lees Ferry and extends 

downstream to the Colorado River’s confluence with the Little Colorado River. Grand Canyon 

extends from the mouth of the Little Colorado River downstream to the confluence with Diamond 

Creek, although physiographically, Grand Canyon extends to the Grand Wash Cliffs at river mile3

 

(RM) 277. Diamond Creek, however, is used for sediment budgeting purposes, because Lake Mead 

reservoir inundates much of the Colorado River farther downstream. Marble Canyon is subdivided 

into upper and lower Marble Canyon at RM 30, which is the location of the 30-mile gage (fig. 1). 

Eastern Grand Canyon extends from the confluence with the Little Colorado River (RM 61) downstream to the Grand Canyon gage (RM 88; fig. 1). The two largest tributaries downstream from the 

Little Colorado River—Kanab Creek and Havasu Creek—join the Colorado River in central Grand 

Canyon. The downstream end of central Grand Canyon is designated at RM 166, the location of 

the 166-mile gage. Western Grand Canyon extends from this point downstream to the Grand Wash 

3

 Locations along the Colorado River in Grand Canyon are referenced by the convention of river mile, which is the distance 

downstream from Lees Ferry, Arizona, along the channel centerline.

Chapter 3—The High Flows: Physical Science Results 57
58 Effects of Three High-Flow Experiments on the Colorado River Ecosystem Downstream from Glen Canyon Dam, Arizona

Monitoring Fine Sediment in Grand Canyon

The ability to use high-flow experiments (HFEs) to build 

sandbars in Grand Canyon depends on the delicate 

balance between the amount of sand that is supplied to 

the river by tributaries downstream from Glen Canyon Dam 

and the amount that is transported 

downstream to Lake Mead. The 

difference between these quantities 

is called a sediment budget and, 

if positive, is the amount of sand 

“surplus” that may be redistributed 

from the river’s channel bed to sandbars by high flows. If negative, that 

difference is the amount of sand that 

is being eroded from the channel 

bed and banks. HFEs conducted 

while maintaining a positive sand 

budget should continue to cause increases in sandbar 

size. HFEs conducted in the context of a negative sand 

budget may result in erosion of sand from the channel bed 

with temporary increases in sandbar size at the expense of 

sand stored on the channel bed. Overall, HFEs conducted 

during such sand “deficit” conditions would eventually 

eliminate the antecedent conditions that presently cause 

the river to build sandbars during high flows in the future. 

Scientists monitor the sand budget, also known as a 

sand “mass balance,” by measuring the amount of sand 

and mud that is transported past monitoring stations on 

the Colorado River and some of the tributaries. Although 

some sand is transported by rolling or bouncing on the 

channel bed, more than 90 percent of the sand and all 

of the mud is carried in suspension above the bed. The 

standard method for measuring suspended-sediment 

transport in rivers has been essentially the same since 

the time the first samples were 

collected in the Colorado River in 

the 1920s. This method involves 

collecting a sample of water 

and analyzing the sample to 

determine the concentration 

and grain size of sand and mud. 

Samples may be collected by 

a wide array of sampler types, 

which have improved over time 

resulting in more accurate and 

more consistent measurements. 

One drawback with these 

“conventional” samples is that 

even the most robust sampling 

schemes that collect samples at 

multiple locations and depths across the channel actually 

sample a relatively small fraction of the water passing 

by any given point. Conventional samples also require 

the presence of a field crew and use of either a fixed 

cableway (bottom photograph) or a 

motor boat (top photograph), which 

greatly limits how often samples 

can be collected. While use of an 

automated pump sampler may allow 

a somewhat greater frequency 

of sampling, these samplers also 

have strict limitations in the number 

and size of samples that may be 

collected. Because suspendedsediment concentrations can change 

rapidly and dramatically and these 

changes do not necessarily correspond to changes in 

streamflow, accurate estimates of sand loads require 

a more robust sampling program than is possible 

with any of these conventional sampling methods.

Beginning in 2002, sediment researchers initiated a 

program to overcome some of these obstacles by the use 

of “surrogate” technologies for measuring suspendedsediment concentrations in water. This effort evolved into 

the current monitoring program, which relies heavily on 

the use of a combination of acoustic and optical instruments that measure suspended-sediment concentrations 

and grain size at 15-minute intervals at five locations along 

the Colorado River in Marble and Grand Canyons (fig. 1). 

While these instruments provide the frequency of sampling 

that is needed, they still require periodic maintenance and 

calibration with conventional samples. All of these surrogate 

samplers are connected to field computers, many of which are 

integrated into a two-way, broadband, satellite-communication 

system. This allows access to 

the data from the office and 

remote instrument observation 

and maintenance. In its current 

form, the monitoring network 

provides the ability, in virtually 

real-time, to monitor suspendedsediment concentrations and 

determine the status of the sand 

budget. This information on 

sand-enrichment levels can then 

be made available to scientists 

and managers for planning 

potential future high flows.

David J. Topping, U.S. Geological Survey

Nancy Hornewer, U.S. Geological Survey
Cliffs (RM 277), but Diamond Creek is the last point where streamflow and sediment measurements are made.

The river is subdivided differently for the purposes of evaluating the response of sandbars 

and campsites to HFEs. In some reaches, sandbars and associated camping areas are common 

relative to demand for recreational campsites while in other reaches demand for campsites 

frequently exceeds the available supply. Kearsley and others (1994), therefore, divided the 

river into critical reaches, where campsites are relatively scarce, and noncritical reaches, where 

campsites are relatively abundant (see chapter 2, table 3, this volume). 

Sand Enrichment 

Colorado River scientists and managers define the supply of sand delivered by tributaries 

prior to an HFE as the antecedent sand enrichment (Topping and others, 2010). Antecedent sand 

enrichment is calculated as the difference between the amount of sand entering and leaving a 

river segment for a specified time period (see chapter 2, this volume). The objective of each 

HFE was to redistribute this antecedent sand supply, because that sand primarily is present as 

submerged eddy sandbars or covers other submerged parts of the channel bed. The goal of the 

HFEs was to mobilize this sand and build larger eddy sandbars that extend to higher elevations, 

while minimizing the associated transport of sand downstream toward Lake Mead reservoir. 

Antecedent sand enrichment is a powerful tool that was used in planning the 2004 and 2008 

HFEs and can be used in planning future high flows. Antecedent mud enrichment is not typically calculated and is of less interest in evaluating the effects of high flows on rebuilding eddy 

sandbars.

One possible long-term experiment Colorado River managers could conduct is to schedule 

as many high flows in Grand Canyon as can be sustained by the supply of sand delivered from 

the tributaries. As described below, scientists have learned that each HFE mobilized all, or 

nearly all, of the antecedent sand enrichment. Thus, to avoid long-term sand depletion, the 

duration of future high flows can only be for the time it takes to deplete this antecedent supply. 

Also as explained below, scientists have learned that flood-constructed sandbars erode during 

the periods between high flows. Thus, a desired outcome of the HFE experimental program is to 

quantify the balance among the rate of supply of sand from tributaries that increases the antecedent sand enrichment, the quantity of sand mobilized by high flows of different magnitude 

and duration, and the rate at which sandbars erode during the periods between high flows.

However, not all of the fine sediment on the channel bed and in eddies immediately before 

an HFE is antecedent sand enrichment. Additionally, there is sand that was in the river channel 

before Glen Canyon Dam was completed in 1963. The amount of this nonrenewable supply 

of sand that was in the river at the time of dam completion is unknown. Because monitoring 

was sparse, the change in the nonrenewable supply that occurred between 1963 and the beginning of the HFEs in 1996 is also unknown, but Topping and others (2000) concluded that an 

increase in sand storage during this period was unlikely (see chapter 2, this volume). Evolving 

technology since the late 1990s, however, has provided increasingly precise ways to measure 

the recent deliveries of sand by tributaries, primarily from the Paria and Little Colorado Rivers. 

Scientists also are able to more accurately and precisely measure the rate at which that sediment is transported downstream after entering the Colorado River. Because the antecedent sand 

enrichment is determined by the recent delivery of tributary sand minus the export of that sand 

farther downstream toward Lake Mead prior to the HFE, improved measurement of main-stem 

sand transport is a critical component of the calculation of the sand enrichment of each part of 

Marble and Grand Canyons.

In upper Marble Canyon (fig. 1), the antecedent sand enrichment is calculated as the sum 

of all sand inputs that enter the Colorado River between Lees Ferry and the 30-mile gage, 

minus the measured export of sand past the 30-mile gage. Sand inputs from the Paria River are 

estimated on the basis of measurements of discharge and sand concentrations, and the same 

Chapter 3—The High Flows: Physical Science Results 59
procedure is used to estimate sand inflow from the Little Colorado River (Topping and others, 

2010). For smaller tributaries, including Kanab and Havasu Creeks, inputs are estimated on the 

basis of regional relations (Webb and others, 2000; Topping and others, 2010).

Antecedent sand enrichment can be calculated for any time period for which measurements 

are available. Topping and others (2010) used different time periods to calculate enrichment 

before the 2004 and 2008 HFEs. They justified using different time periods because each HFE had 

a different experimental design and planning period. 

Field Measurements of Suspended Sediment

Suspended-sediment data were collected at four gages during the 1996 HFE, and findings 

from that HFE prompted expansion of sediment-monitoring efforts in tributaries and the Colorado 

River (see text box, Monitoring Fine Sediment in Grand Canyon). Five gages measured suspended 

sediment in the main stem during the 2004 HFE, and six gages were in operation during the 2008 

HFE. These data, combined with data collected in the tributaries, allowed direct calculation of 

antecedent sand enrichment in 2004 and 2008. The monitoring program is described in detail by 

Topping and others (2010). 

Longitudinal, or downstream, patterns in sand transport during the HFEs were measured in 

two ways. First, scientists compared the concentrations of suspended sand at the different gages 

in 1996, 2004, and 2008. Second, scientists made measurements of sand concentration at approximately 0.5-mile intervals between the gages on downstream sampling trips in 2004 and 2008. 

The objective of the latter sampling scheme was to examine how suspended-sand concentration changed within one parcel of the HFE’s water as it passed downstream from Lees Ferry 

through Marble and Grand Canyons. This type of sampling is informative, because it can be used 

to identify which parts of the river contributed relatively more or less sand to the river’s transported load during the HFE. Because no sand is released with the water at the dam and tributary 

sand input during the HFEs has been negligible, all of the sand in transport downstream comes 

from the channel bed and from eddy sandbars. The suspended-sediment concentration increases 

rapidly as sand and mud that had been deposited on the bed and in eddies is entrained, or picked 

up, by the high flow. At some distance downstream, however, the quantity of fine sediment 

entrained by the flow from some parts of the channel is approximately balanced by the quantity 

deposited in other locations, and beyond this point, there are reaches where there are only localized imbalances between erosion and deposition. Downstream increases in suspended-sand 

concentration reflect net erosion of sand from the channel bed and eddy sandbars; conversely, 

downstream decreases in suspended-sand concentration reflect net deposition. 

During the 2004 HFE, scientists collected samples between the Lees Ferry and Grand Canyon 

gages (fig. 1) while boating downstream. During the 2008 HFE, two groups of scientists collected 

samples between Lees Ferry and the Grand Canyon gage and between the Grand Canyon and 

Diamond Creek gages (Topping and others, 2010).

The advantage of the downstream sampling trips was that direct measurements of suspendedsand conditions were made along the entire river course. The disadvantage, however, was that the 

sampled conditions only represented the characteristics of the flow at the beginning of each HFE 

when the sampling was conducted. In contrast, the continuous sampling at the gages described the 

ever-changing suspended-sand concentration but only at a few locations.

Sandbar Monitoring

Since the mid-1990s, the sandbar-monitoring effort has been led by the Northern Arizona 

University Sandbar Studies Group (see text box, Measuring Colorado River Sandbars). This 

program was initiated during the experimental flow program of 1990 and 1991 (Beus and others, 

1992) when sandbar-surveying protocols were first established and measurement sites were identified. Since that time, the number of measurement sites has expanded, and methods have evolved 

(Hazel, Kaplinski, Parnell, Kohl, and Schmidt, 2008; Kaplinski and others, 2009). 

60 Effects of Three High-Flow Experiments on the Colorado River Ecosystem Downstream from Glen Canyon Dam, Arizona
Because the number of sandbar monitoring sites has increased over time and the 2004 HFE 

included monitoring only in Marble Canyon and eastern Grand Canyon, there are generally fewer 

data from which to make comparisons among the HFEs than there are data to analyze individual 

HFEs. Detailed measurements of sandbars and the adjacent channel were made before and after 

the HFE at 32 sites in 1996, 12 sites in 2004, and 40 sites in 2008 (Hazel and others, 1999; Hazel 

and others, 2010). Of these sites, seven in Marble Canyon were monitored before and after each 

of the three HFEs, and 15 in Grand Canyon were monitored before and after the 1996 and 2008 

HFEs (Grams, Hazel, and others, 2010). 

These monitoring sites represent a small subset of the total number of all eddy sandbars 

in Grand Canyon. Schmidt and others (2004) estimated that there are approximately 300 large 

eddies, each greater than about 11,000 square feet (ft2

) in area, with eddy sandbars in upper and 

lower Marble Canyon. Extrapolation of measurements from the relatively few monitoring sites 

to the rest of Marble and Grand Canyons has been a longstanding challenge. For example, no 

more than about 8 percent of all sandbars in upper or lower Marble Canyon have been measured 

annually. The number of measured sandbars is an even smaller percentage of the total number of 

sandbars in eastern, central, and western Grand Canyon. 

Collecting detailed topographic information about a significantly greater number of the 

sandbars on an annual basis has never been feasible, because such sampling would require a great 

many resources. Although monitoring by aerial photographs has been used to measure additional 

sandbars (Schmidt, Grams, and Leschin, 1999), this method yields less precise measurements 

and has not been repeated during every HFE. Nevertheless, one of the strengths of the existing 

sandbar-monitoring program in Grand Canyon is its longevity; there are few rivers in the world 

where such precise annual measurements of channel morphology have been made over a 20-year 

period.

The amount of sand that can accumulate in an eddy during any high flow is at least partly 

controlled by the antecedent sandbar volume. Beus and others (1992) found that if eddy sandbars 

are relatively large just before a high flow, less new sand is deposited, presumably because there is 

less accommodation space where new sand deposition can occur (Hazel and others, 1999). 

The topography of sandbars was measured immediately before each HFE. Soon after each 

HFE (fig. 2), the topography was measured again to determine the amount of sand deposited or 

eroded during each experiment (see text box, Measuring Colorado River Sandbars). Sandbar 

volume is computed above a “minimum surface.” This minimum surface is computed from all 

available survey data for each monitoring site and reflects the topography of the lowest elevation 

Figure 2. Data collection at 

the sandbar study site near 

river mile 175 following the 2008 

high-flow experiment (HFE). 

View is downstream from the 

shoreline on the right bank of 

the river. Photograph courtesy 

of Grand Canyon Youth.

Chapter 3—The High Flows: Physical Science Results 61
62 Effects of Three High-Flow Experiments on the Colorado River Ecosystem Downstream from Glen Canyon Dam, Arizona

Measuring Colorado River Sandbars

Field-based monitoring of sandbars in Grand Canyon 

dates back to the early 1970s, about a decade after Glen 

Canyon Dam was completed, when it was first recognized 

that sandbars were at risk of erosion. The monitoring 

methods have evolved from simple topographic profiles, 

to complete topographic mapping of exposed sandbars, 

and finally to comprehensive topographic-bathymetric 

(water-depth) mapping of both the exposed and 

submerged portions of sandbars (fig. 3). The primary challenge of sandbar monitoring has always been in satisfying 

the need for precise, or closely spaced, measurements at 

each monitoring site, while at the same time monitoring 

a large and representative number of sandbars. Precise 

measurements are required to adequately characterize 

the topography of each site, and a large sample size is 

required because sandbars can independently change 

in shape and size. It was found that imprecise, widely 

spaced measurements could erroneously indicate a 

change in sandbar size when in fact sand was just redistributed within the same site. The topographic surveys 

collected as part of the current monitoring protocol can 

be used to detect both changes in sandbar shape and 

size. These measurements are made by scientists using 

conventional ground-based survey methods (Hazel and 

others, 2010). The number of study sites has increased 

gradually since 1990, with detailed measurements made 

before and after the HFEs at 32 sites in 1996 and 40 sites 

in 2008. In 2008, topographic surveys were collected at an 

additional 71 sites to better characterize the response of 

aquatic backwater habitats to high flows (Grams, Schmidt, 

and Andersen, 2010; Hazel and others, 2010). In 2004, only 

12 of the sites in Marble Canyon and eastern Grand Canyon 

that were surveyed in 1996 and 2008 were surveyed before 

and after the HFE. The more comprehensive surveys that 

include bathymetry are typically conducted near the time 

of high flows and not as part of the annual sandbar monitoring. While these surveys provide much more information 

than the topographic surveys alone, they require a much 

more extensive data-collection effort and substantially 

more post processing (Kaplinski and others, 2009).

The elevation data collected as part of the sandbar monitoring may be used in several different ways, depending 

on the specific science or resource-management 

question. Typically, the sandbar data are analyzed to 

determine sandbar area and volume within three elevation zones. It is important to track the sandbars by 

elevation zone because, for example, a large volume 

of sand that is low in the channel may not contribute to 

a large campsite but it may create a large backwater 

habitat. The elevation zones are separated at the river 

stage represented by a discharge of 8,000 cubic feet 

per second, also called the reference stage (fig. 3).

Analyses may be conducted to determine how changes 

in sandbars affect specific resources, such as backwater habitat abundance and campsite area. Changes 

in backwater habitat are determined by calculating 

the area and volume of backwater habitat for each 

sandbar survey that would exist at a particular river 

discharge (fig. 3). Campsite area is calculated on 

the basis of a field assessment of the area within a 

particular sandbar that is suitable for camping.

River mile 22, December 2004

David J. Topping, U.S. Geological Survey
Backwater

Sand volume below reference stage

River flow

Sand volume above

reference stage

8,000 ft3

/s = reference stage 

8,000 ft3

/s = reference stage 

Sand volume above reference stage

Sand volume below reference stage

River

Minimum surface

B

A

Backwater

A

A A’

A’

surveyed for each part of the sandbar (fig. 3). The volumes of sand contained in the sandbars 

typically are reported in two categories that distinguish between sand that is usually submerged 

below the water surface and sand that is usually exposed above the water surface. The elevation 

at which this distinction is made is called “the reference stage,” defined as the elevation of the 

water surface at a river discharge of 8,000 ft3

/s. This discharge was chosen because sand below 

the corresponding river stage is almost always submerged; flows of 8,000 ft3/s or greater have 

occurred 75 percent of the time in the Colorado River since 1963 (Topping and others, 2003). 

Sand above the reference stage is either always exposed above the water surface or is intermittently exposed during periods of diurnally fluctuating dam releases and during HFEs.

River Conditions Prior to Each High-Flow Experiment

River conditions prior to each of the three HFEs can be compared on the basis of the antecedent sand enrichment and by comparing estimates of how that enrichment was distributed over 

the channel bed and estimates of the grain size of that sand (Topping and others, 2010). The latter 

two factors were estimated from the concentration and grain size of suspended sand transported on 

the first day of each HFE using techniques developed by Rubin and Topping (2001) and Topping 

and others (2010), but can only be applied to the channel near each gage.

Figure 3. A, Perspective 

view based on 3-foot resolution digital elevation model of 

a typical eddy sandbar and the 

adjacent channel showing pattern of recirculating flow, the 

reference stage of 8,000 ft3

/s in 

white, and a typical backwater 

as defined by aquatic ecologists. The portions of the sandbar above the reference stage 

and below the reference stage 

are distinguished. Main-stem 

flow is from bottom right to top 

left. B, Cross-section view of 

an eddy sandbar and channel 

illustrating the region for sand 

above the reference stage and 

total sand (sand above the reference stage plus sand below 

the reference stage). Modified 

from Grams, Hazel, and others 

(2010).

Chapter 3—The High Flows: Physical Science Results 63
Table 1. Conditions affecting sand enrichment for high-flow experiments (HFEs) 

conducted in 1996, 2004, and 2008. Modified from Topping and others (2010).

[ft3/s, cubic feet per second; nd, no data]

1996 2004 2008

Conditions of streamflow and tributary sand supply

Median dam release for year preceding 

HFE (ft3

/s) 15,400a 10,500b 11,300

Paria River sand supply for year preceding HFE (million tons) 0.42 0.69 1.01

Little Colorado River sand supply for 

year preceding HFE (million tons) 0.04 0.21 1.23

Sand enrichment for accounting periodc

 (million tons)

Upper Marble Canyon nd 0.42 ± 0.12 1.32 ± 0.69

Lower Marble Canyon nd 0.13 ± 0.05 0.59 ± 0.30

Eastern Grand Canyon nd 0.02 ± 0.05 0.92 ± 0.73

Central and western Grand Canyon nd 0.17 ± 0.11 1.01 ± 0.44

Bed sand area (ranking 1 = greatest relative bed sand area)

Upper Marble Canyon nd 2 1

Lower Marble Canyon 1d 3 1

Eastern Grand Canyon 1 3 2

Central Grand Canyon 1 nd 2

Western Grand Canyon nd 2 1

Bed sand grain size (ranking 1 = finest relative grain size)

Upper Marble Canyon nd 1 2

Lower Marble Canyon 3 2 1

Eastern Grand Canyon 3 2 1

Central Grand Canyon 2 nd 1

Western Grand Canyon nd 2 1

a

 This discharge would result in either no accumulation of the tributary-supplied sand or net 

scour of sand already stored in the Colorado River during the year prior to the CFE (from Topping and others, 2000).

b

 This discharge is low enough to be within the range in Topping and others (2000) under 

which net sand accumulation is most likely to occur.

c

Accounting period for 2004 HFE extends from July 1, 2004, to rising limb of 2004 HFE. 

Accounting period for the 2008 HFE extends from recession of the 2004 HFE to the rising limb 

of the 2008 HFE.

d

 The bed sand area for lower Marble Canyon was approximately the same in 1996 and 2008.

Antecedent Sand Enrichment: Volume, Distribution, and Grain Size

The sediment-transport measurement program in place in the early 1990s precludes estimation of 

the antecedent sand enrichment immediately before the 1996 HFE, but enrichment in Marble Canyon 

was probably less than before the 2004 and 2008 HFEs. Tributary inputs of sand had been relatively 

small and average dam releases relatively large in the year preceding the 1996 HFE (table 1). In 

contrast, tributary inputs to Marble Canyon were larger, had occurred more recently, and were of a 

finer grain size before the 2004 and 2008 HFEs.

Interpretation of the conditions in eastern Grand Canyon before the 1996 HFE are also complicated by the lingering effect of Little Colorado River floods that had occurred in the winter of 1993. 

64 Effects of Three High-Flow Experiments on the Colorado River Ecosystem Downstream from Glen Canyon Dam, Arizona
Topping and others (2010) estimated that a similar or larger amount of sand probably was on the 

channel bed in eastern Grand Canyon in 1996 than in 2004 or 2008, but this sand was of coarser 

size than sand on the bed prior to the 2004 and 2008 HFEs (table 1).

Antecedent sand enrichment can be calculated for the 2004 and 2008 HFEs, and these calculations indicate that there was more sand available for transport in 2008 except in upper Marble 

Canyon. Additionally, before the 2008 HFE, the sand was of a finer size except in upper Marble 

Canyon (table 1). Upper Marble Canyon had between 630,000 and 2,010,000 tons of sand immediately before the 2008 HFE, primarily resulting from floods in the Paria River that had delivered 

between 3,300,000 and 4,100,000 tons during the preceding 31⁄2 years (Topping and others, 2010). 

In October 2006, two floods in the Paria River with a recurrence interval of 7 years occurred in 

1 week and supplied about 1,400,000 tons of sand. 

Sand enrichment in upper Marble Canyon in 2004 was about 70 percent less than in 2008, 

because the Paria River only delivered between 610,000 and 750,000 tons of sand during the 

41⁄2 months immediately before that experiment. There is no evidence that the Colorado River had 

been accumulating significant amounts of sand prior to the summer of 2004. The enriched sand in 

upper Marble Canyon was much finer in 2004, because the 2004 HFE was conducted soon after 

floods occurred in the Paria River.

Antecedent Sandbar Volume

The amount of new sandbar deposition depends not only on the antecedent sand enrichment 

but also on the antecedent sandbar volume, because less deposition can occur if the eddy sandbars are relatively large when a high flow occurs (Hazel and others, 1999). There was less room 

for new sandbar deposition in upper Marble Canyon in 2008, because eddy sandbars were larger 

than before the previous HFEs (Grams, Hazel, and others, 2010). Eddy sandbars in lower Marble 

Canyon, in eastern Grand Canyon, and farther downstream were all approximately the same size 

before each experiment. 

Insights About River Processes 

Sediment Concentration, Load, and Grain Size

Measurements made during the 1996 HFE confirmed that only a limited amount of sand is 

available to be transported by the post-dam Colorado River. Documentation of this phenomenon, 

called sediment supply limitation, is one of the most important findings of the HFE physicalscience research program (Schmidt, 1999). Measurements in 1996 showed that the 7-day high 

flow quickly depleted the antecedent sand enrichment (fig. 4A). The concentration of suspended 

sand and mud was greatest on the first day of the HFE and quickly decreased thereafter (Rubin and 

others, 1998; Topping and others, 1999, 2000; Rubin and Topping, 2001). These findings, coupled 

with modeling that showed that the highest rates of sandbar deposition occurred during the first 

days of the high flow (Wiele and Torizzo, 2005), led to the decision to reduce the duration of the 

2004 and 2008 HFEs to less than one-half the duration of the 1996 HFE. The last days of the 1996 

HFE transported much less fine sediment and did not substantially increase deposition of eddy 

sandbars (Schmidt, 1999).

The observations made during the 1996 HFE inspired Topping and others (2000) to comprehensively reanalyze the pre-dam sediment-transport data, which led to confirmation of the early 

observations of Leopold and Maddock (1953) and Colby (1964). This reanalysis demonstrated 

that sand-supply limitation had also occurred in the pre-dam river. Sand had accumulated during 

the pre-dam period between Lees Ferry and the Grand Canyon gage (fig. 1) only when flows were 

less than about 9,000 ft3

/s (Topping and others, 2000). The management implication of this finding 

was that sand contributed by tributaries can only be stored in the river channel when flows are less 

Chapter 3—The High Flows: Physical Science Results 65
Suspended-sand concentration,

in milligrams per liter

0

1,000

2,000

3,000

1996

0.08

0.12

0.16

0.20

0.24

0 2 4 6 8

Days after beginning of high, steady discharge

Suspended-sand median grain size,

in millimeters

Lees Ferry gage

61-mi gage

166-mi gage

Grand Canyon gage

Lees Ferry gage

61-mi gage

166-mi gage

Grand Canyon gage

Figure 4A. Suspended-sand data collected during the 1996 high-flow experiment (HFE) at all study sites. The 

upper graph shows suspended-sand concentration; the lower graph shows suspended-sand median grain size. 

Data were shifted in time such that zero time (indicated by the leftmost vertical gray line) is the beginning of high, 

steady discharge during the HFE. The right vertical gray line indicates the end of the high, steady discharge part 

of the HFE. The symbols represent discrete samples. Error bars indicate the 95-percent confidence interval associated with these measurements. Modified from Topping and others (2010). Gage locations are shown in figure 1.

than about 9,000 ft3/s. This finding was tested as a management approach and demonstrated to be 

effective in 2000 during the low summer steady flow (LSSF) experiment when dam releases were 

about 8,000 ft3/s for 3 summer months (Schmidt and others, 2007). Sand does not accumulate in 

the channel over multiyear periods in the post-dam period, because post-dam flows usually are 

greater than 9,000 ft3

/s (Topping and others, 2003).

Scientists also learned that there is good correlation between the magnitude of antecedent sand enrichment and of suspended-sediment concentration (Topping and others, 2010). 

Antecedent sand enrichment and suspended-sand concentration were greatest in 2008 (Topping 

and others, 2010). Average suspended-sand concentration was lowest in Marble Canyon and in 

western Grand Canyon in 1996 when enrichment was less than before the other HFEs.

Scientists also learned that the antecedent conditions of bed-sand area and bed-sand grain size 

may compensate for each other. In 2008 in upper Marble Canyon, high suspended-sand concentration was promoted by a large area of the channel bed covered by sand (table 1). In 2004, high 

suspended-sand concentration was promoted by a smaller proportion of the channel bed covered 

by finer sand. In lower Marble Canyon, the channel bed was less extensively covered, and the 

grain sizes were slightly coarser in 2004 than in 2008, thereby causing lower concentrations of 

suspended sand. Sand concentrations in eastern Grand Canon were lower in 1996 than in 2008, 

because the sand on the channel bed was very coarse.

66 Effects of Three High-Flow Experiments on the Colorado River Ecosystem Downstream from Glen Canyon Dam, Arizona
Suspended-sand concentration,

in milligrams per liter

0

1,000

2,000

3,000 2004

Days after beginning of high, steady discharge

Suspended-sand median grain size,

in millimeters

–2 –1 0 1 2 3 4 5

0.08

0.10

0.12

0.14

0.16

Lees Ferry gage

30-mi gage

61-mi gage

Grand Canyon gage

Diamond Creek gage

Lees Ferry gage

30-mi gage

61-mi gage

Grand Canyon gage

Diamond Creek gage

Figure 4B. Suspended-sand data collected during the 2004 high-flow experiment (HFE) at all study sites. The upper graph 

shows suspended-sand concentration; the lower graph shows suspended-sand median grain size. Data were shifted in 

time such that zero time (indicated by the leftmost vertical gray line) is the beginning of high, steady discharge during the 

HFE. The right vertical gray line indicates the end of the high, steady discharge part of the HFE. The symbols represent discrete samples. Error bars indicate the 95-percent confidence interval associated with these measurements. Modified from 

Topping and others (2010). Gage locations are shown in figure 1.

Longitudinal (Downstream) Patterns in Sediment Transport

Downstream trends in the concentration and size of suspended sand measured during the 2004 

and 2008 HFEs indicate that neither the antecedent sand enrichment nor the grain size of that sand 

was uniformly distributed on the channel bed immediately before the HFEs. Because most of the 

sand delivered to the Colorado River enters at two discrete locations—the Paria and Little Colorado 

Rivers—the distribution and characteristics of the antecedent sand enrichment is primarily determined 

by three factors. The first factor is the amount and sizes of the sand that enters from these tributaries. 

The second factor is the Colorado River’s flow regime during the period preceding each HFE, because 

dam releases control how much and how far sand delivered from each tributary is transported downstream. Dam releases also affect the amount of sand that resides in eddies as compared to the amount 

in the main channel. The third factor is the geomorphic organization of the canyon into narrow and 

wide segments, because sand has a tendency to accumulate in the wide parts of Marble and Grand 

Canyons where there are more eddies and the channel gradient is relatively flat.

Topping and others (2010) used the longitudinal data to show that much more of the channel 

bed between RMs 15 and 35 and between RMs 51 and 85 was covered by sand before the 2008 

HFE than before the 2004 HFE. Upstream from RM 15, the antecedent sand enrichment was finer 

in 2004 than in 2008.

Chapter 3—The High Flows: Physical Science Results 67
Suspended-sand concentration, in milligrams per liter

0

1,000

2,000

3,000

4,000

5,000 2008

Days after beginning of high, steady discharge

Suspended-sand median grain size, in millimeters

–2 –1 0 1 2 3 4 5

0.08

0.10

0.12

0.14

0.16

0.18

0.20

Lees Ferry gage

30-mi gage

61-mi gage

Grand Canyon gage

166-mile gage

Diamond Creek gage

Figure 4C. Suspended-sand data collected during the 2008 high-flow experiment (HFE) at all study sites. The upper 

graph shows suspended-sand concentration; the lower graph shows suspended-sand median grain size. Data 

were shifted in time such that zero time (indicated by the leftmost vertical gray line) is the beginning of high, steady 

discharge during the HFE. The right vertical gray line indicates the end of the high, steady discharge part of the HFE. 

The symbols represent discrete samples. Error bars indicate the 95-percent confidence interval associated with 

these measurements. Modified from Topping and others (2010). Gage locations are shown in figure 1.

68 Effects of Three High-Flow Experiments on the Colorado River Ecosystem Downstream from Glen Canyon Dam, Arizona
2004 parcel 1

2008 parcel 1

2008 parcel 2

2004 parcel 2

Sand concentration, in milligrams per liter

0

1,000

2,000

3,000

Upper

Marble

Canyon

Lower

Marble

Canyon

Eastern

Grand

Canyon

Glen

Canyon

River mile

River gages

Net sand erosion Net sand deposition Net sand erosion

0

Lees Ferry

20 40 60 80

Figure 5. Downstream changes in suspended-sand concentration measured during high-flow 

experiments in 2004 and 2008. Tan shading indicates reaches of net sand erosion, and blue shading indicates reaches of net sand deposition. “Parcel 1” indicates sampling on the first day of the 

steady high flow, and “parcel 2” indicates sampling on the second day of the steady high flow. 

Vertical lines indicate locations of the Lees Ferry, 30-mile, 61-mile, and Grand Canyon gages. 

Modified from Topping and others (2010). Gage locations are shown in figure 1.

These measurements also show that the geomorphic organization of Marble and Grand 

Canyons plays a large role in determining the distribution of where erosion or deposition occurs 

during each HFE. Despite different proportions of the channel bed covered by sand of different 

grain sizes in 2004 and 2008, the spatial patterns of erosion and deposition were similar early 

in each event when the downstream sampling was conducted. In 2004 and 2008, net erosion of 

sand was measured at the beginning of the HFE in upper Marble Canyon between RMs 0 and 24 

(fig. 5). Farther downstream to approximately RM 50 in lower Marble Canyon, net deposition was 

measured in 2004 and 2008. Net erosion was measured in 2004 between RMs 50 and 85, where 

measurements ceased. In 2008, net erosion was measured between RMs 50 and 105, and net 

deposition was measured between RMs 105 and 179. Farther downstream, erosion and deposition 

were in balance.

These measurements only describe conditions of the initial stages of each high flow and 

cannot be used to determine net sediment budgets for the HFEs. Nevertheless, the findings demonstrate that the downstream effects of high flows vary and evolve over space as well as time. Thus, 

the physical processes that drive eddy-sandbar deposition or erosion evolve during high flows and 

differ in various parts of the canyons. Different parts of Marble and Grand Canyons respond to the 

same HFE differently. 

The Relation Between Main-Stem Sediment Transport

and Eddy-Sandbar Characteristics 

Designing high flows to build sandbars requires understanding how the characteristics of the 

sandbars that are being deposited evolve with the characteristics of the sediment transported by 

the river. Rubin and others (1998) and Topping and others (1999) described how the grain-size 

distribution of the suspended sediment, the grain-size distribution of the fine sediment on the 

channel bed, and the grain-size distribution of the flood deposits that make up eddy sandbars 

co-evolved during the 1996 HFE. Rubin and others (1998) also showed that the concentration 

of suspended sand in one eddy evolved over time with the progressively declining main-channel 

Chapter 3—The High Flows: Physical Science Results 69
FINE

COARSE

suspended-sand concentration. They compared measurements at an eddy at RM 122 and at 

the Grand Canyon gage (RM 88; fig. 1) and found that the concentrations of suspended sand 

in the eddy and in the main channel during the HFE both decreased by a factor of two. The 

concentration of mud in suspension decreased by a factor of five, and the size of the silt and 

clay particles that constitute the mud also increased. 

Mud travels downstream faster than sand; thus, mud is the first sediment deposited 

into eddies. As a result, the newly formed eddy deposit is muddiest at its base, and this mud 

subsequently is buried by coarser sand (Rubin and others, 1998). In 1996, flood deposits 

coarsened upward by a factor of two (fig. 6). Similar sedimentological patterns of upward 

coarsening of sandbar deposits were observed after the 2004 HFE (Topping and others, 2006) 

and the 2008 HFE (Draut, Topping, and others, 2010). One implication of these observations 

is that manipulating high flows for the purpose of increasing the content of mud in newly 

formed sandbars, as hypothesized before the 2004 HFE, is difficult because mud is quickly 

transported downstream, and muddy sand is subsequently buried by coarser sand deposited in 

the last days of each high flow. 

The studies demonstrating the links between main-stem suspended-sediment concentration and eddy-sandbar characteristics showed that, “For identical flows, an eddy deposit can 

either aggrade or erode, depending on the concentration of sediment in the main-channel 

flow” (Rubin and others, 1998, p. 99). Field measurements made in 1996 and computer 

models that simulated the hydraulics and sediment transport in eddies (Wiele and others, 

1999) showed that the magnitude of suspended-sand concentration determines the sand 

deposition rate. Larger sandbars are formed by high flows that have high suspended-sand 

concentrations.

Sandbar Response to High-Flow Experiments

 Scientists have struggled to generalize about the patterns of sandbar change caused by 

high flows. As described in chapter 2, this volume, there is large variability in how specific eddy 

sandbars respond to a discrete high flow. Observations made in 1996 demonstrated that not all 

eddy sandbars in the same river segment responded in the same way, despite the fact that the 

discharge and main-stem suspended-sand concentration were similar (Rubin and others, 1998; 

Schmidt, Grams, and Leschin, 1999; Topping and others, 1999).

Daily measurements of sandbar building in 1996 and 2008 indicate that most eddysandbar deposition occurred during the first few days of each high flow. Measurements of daily 

Figure 6. Cross-sectional 

view of a sand deposit formed 

by the 1996 high-flow experiment. The image was taken following the high flow and shows 

the upper 4.5 feet of a 15-footthick deposit. The deposit consists of fine-grained sand in the 

lower left and coarser crossbedded layers in the upper 1.5 

feet of the deposit. Photograph 

taken by David M. Rubin, U.S. 

Geological Survey.

70 Effects of Three High-Flow Experiments on the Colorado River Ecosystem Downstream from Glen Canyon Dam, Arizona
Table 3. Styles of sandbar response (from 

Hazel and others, 2010).

Response 

style

Sandbar

response above 

reference stage

Sandbar

response below 

reference stage

1 Deposition Deposition

2 Deposition Erosion

3 Erosion Erosion

4 Erosion Deposition

Table 2. Summary of changes in sandbar and channel elevation for three high-flow experiments 

(HFEs). Values are average elevation change for indicated storage component, in feet.

Storage component 1996 HFE 2004 HFE 2008 HFE

Eddy sandbars above reference stage 0.59 ± 0.16 0.52 ± 0.13 0.49 ± 0.13

Eddy sandbars below reference stage –1.84 ± 0.59 –1.57 ± 0.46 –1.35 ± 0.56

Channel-margin sandbars above 

reference stagea 0.98 ± 0.33 0.98 ± 0.33 0.98 ± 0.33

Main channel below reference stage –1.61 ± 0.43 –1.74 ± 0.95 0.72 ± 0.72

a

 Although thickness changes in channel margin deposits are large, these deposits occupy much less area than 

eddy sandbars and contribute much less to the change in sand storage (Hazel and others, 2006). These changes 

were measured separately from eddy sandbars only in 1996 (Hazel and others, 2006), and the same change in 

elevation was assumed for 2004 and 2008.

topographic change were made at five sites in 1996 and two sites in 2008. In 1996, most deposition occurred during the first 3 days of the experiment, and erosion tended to dominate thereafter 

(Andrews and others, 1999; Schmidt, 1999). Wright and Kaplinski (in press) showed that deposition rates above reference stage were high for the entire 60 hours of the 2008 HFE. Scientists have 

concluded that efficient sandbar building during a high-flow event can occur in 1 to 3 days, but 

longer high flows have the potential to cause sustained sandbar deposition if the antecedent sand 

enrichment is unusually large.

Measurements made during the 1996 HFE were also used to examine the elevation of sandbars. Andrews and others (1999) reported that sandbars accumulated sand to within about 1 foot of 

the water surface at the peak flow, but did not aggrade much more than this. During the subsequent 

HFEs, sandbar elevations increased similarly because the magnitude of those HFEs was similar 

to that in 1996 (table 2). Hazel and others (2006) reported that less sand was deposited by smaller 

floods; a 31,000 ft3/s dam release in 2000 caused increases in sandbar volume above the 8,000 ft3/s 

reference stage that were about 80 percent less than the sandbar changes caused by the 1996 HFE.

As described in chapter 2, previous analyses of sandbar change had detected few common 

patterns of topographic change caused by post-dam floods or short-duration high flows. Hazel and 

others (2010) distinguished four styles of sandbar change (table 3) on the basis of whether or not 

the net topographic change of the eddy sandbar and adjacent channel was erosion or deposition 

above the reference stage and below the reference stage. The four styles represent the four possible 

combinations: net deposition above and below the reference stage (style 1; figs. 7, 8), net erosion 

below the reference stage and net deposition above (style 2; figs. 9, 10), net erosion above and 

below the reference stage (style 3; figs. 11, 12), and net deposition below the reference stage and 

net erosion above (style 4). 

Styles 1 and 2—where net deposition occurred above the reference stage—are the predominant style of eddy sandbar change caused by HFEs. The relative proportion of eddy sandbars in 

style 1 or style 2 varied among the HFEs. The 2008 

HFE had the highest measured suspended-sediment 

concentrations and the greatest proportion of measured 

sandbars with a style 1 response. Forty-five percent of 

surveyed sandbars in Marble and Grand Canyons had a 

style 1 response, and 37 percent had a style 2 response 

(fig. 13C). Somewhat lower suspended-sand concentrations during the 2004 HFE caused only 26 percent of 

measured sandbars to follow style 1 (fig. 13B). Even 

lower suspended-sand concentrations during the 1996 

HFE caused only 9 percent of the measured sandbars to 

Chapter 3—The High Flows: Physical Science Results 71
Elevation change, in feet

−1.6 to 0.0

−3.3 to −1.6

−4.9 to −3.3

−6.6 to −4.9

−9.8 to −6.6

< −9.8

Erosion Deposition

0 150 300 feet

0.0 to 1.6

1.6 to 3.3

3.3 to 4.9

4.9 to 6.6

> 6.6

N

Figure 7. Matched photographs of river mile 172 where style 1 net 

depositional response occurred. View is from the right bank of the 

river and shows the eddy sandbar (A) before, (B) during, (C) immediately after, and (D) 2 months after the 2008 high-flow experiment. 

Streamflow is from left to right. From Hazel and others (2010).

A. Before the 2008 HFE 

B. During the 2008 HFE 

C. Immediately after the 2008 HFE 

D. Two months after the 2008 HFE Figure 8. Style 1 net depositional 

response to the 2008 high-flow experiment 

at river mile 172. The dashed lines show 

the study site boundary and the approximate location of the boundary between 

the main channel and the eddy. Streamflow is from right to left. Modified from 

Hazel and others (2010).

Flow

Flow

72 Effects of Three High-Flow Experiments on the Colorado River Ecosystem Downstream from Glen Canyon Dam, Arizona
Elevation change,

in feet

−1.6 to 0.0

−3.3 to −1.6

−4.9 to −3.3

−6.6 to −4.9

−9.8 to −6.6

−13.1 to −9.8

−16.4 to −13.1

<−16.4

Deposition

Erosion

0.0 to 1.6

1.6 to 3.3

3.3 to 4.9

4.9 to 6.6

6.6 to 9.8

>9.8

0 150 300 feet

N

Elevation change,

in feet

−1.6 to 0.0

−3.3 to −1.6

−4.9 to −3.3

−6.6 to −4.9

−9.8 to −6.6

−13.1 to −9.8

−16.4 to −13.1

<−16.4

Deposition

Erosion

0.0 to 1.6

1.6 to 3.3

3.3 to 4.9

4.9 to 6.6

>6.6

0 150 300 feet

N

Flow

Flow

Figure 9. Style 2 deposition and erosion 

during 2008 high-flow experiment at river 

mile 44. The dashed lines show the study site 

boundary and the approximate location of the 

boundary between the main channel and the 

eddy. Streamflow is from top to bottom. Modified from Hazel and others (2010).

Figure 10. The large reattachment bar that existed after the high flow 

on March 10, 2008, at river mile 44. View of the sandbar is from across 

the river, and streamflow is from left to right. Modified from Hazel and 

others (2010).

Figure 11. Erosion and deposition illustrating style 3 sandbar response at river 

mile 3. The dashed lines show the study site 

boundary and the approximate location of the 

boundary between the main channel and the 

eddy. Streamflow is from top to bottom. Modified from Hazel and others (2010).

Figure 12. Matched photographs of river mile 3 showing style 3 

response of the eddy sandbar. A, Before the 2008 high-flow experiment 

(HFE). B, Immediately after the 2008 HFE. Streamflow is from left to right. 

Modified from Hazel and others (2010).

A. Before the 2008 HFE

B. Immediately after the 2008 HFE

After the 2008 HFE

Flow

Flow

Chapter 3—The High Flows: Physical Science Results 73
−6 

0 

6 

−6 

0 

6 

−6 

0 

6 

−6 

0 

6 

Distance downstream from Lees Ferry, in river miles 

C. 2008 HFE 

Style 1 

Style 2 

Style 4 

Style 3 

−6 

0 

6 

−6 

0 

6 

−6 

0 

6 

−6 

0 

6 

Net change in sand thickness, in feet 

B. 2004 HFE 

Style 1 

Style 2 

Style 4 

Style 3 

−6 

0 

6 UMC LMC EGC CGC WGC 

−6 

0 

6 

−6 

0 

6 

−6 

0 

6 

A. 1996 HFE 

Style 1 

Style 2 

Style 4 

Style 3 

Figure 13. Downstream variations in response style at study sites for the 1996, 

2004, and 2008 high-flow experiments (HFEs). Upper Marble Canyon (UMC), lower 

Marble Canyon (LMC), eastern Grand Canyon (EGC), central Grand Canyon (CGC), 

and western Grand Canyon (WGC) segments are shown. A, The distribution of totaleddy net change in thickness of sand for each response style with distance downstream for the 1996 HFE. B, The distribution of total-eddy net change in thickness of 

sand for each response style with distance downstream for the 2004 HFE. Data were 

not collected in central Grand Canyon or western Grand Canyon. C, The distribution 

of total-eddy net change in thickness of sand for each response style with distance 

downstream for the 2008 HFE. Modified from Hazel and others (2010).

74 Effects of Three High-Flow Experiments on the Colorado River Ecosystem Downstream from Glen Canyon Dam, Arizona
be classified as style 1 (fig. 13A). In 2004 and 1996, the predominant style of sandbar response in 

Grand Canyon was style 2. In each of the three HFEs, between 14 and 18 percent of the monitoring sites exhibited net erosion—a style 3 response. Style 4 response represents sand accumulation below the reference stage and is uncommon during HFEs.

Despite the insight gained by describing styles of topographic change in sandbars during the 

HFEs, the large variability inherent in how river processes create sandbars continues to make it 

difficult to predict how individual sandbars respond to high flows. For example, while style 1 and 

style 2 responses were most common in each reach for each HFE, there were nearly always at 

least one or two sites that exhibited a style 3 response (fig. 13). Thus, at the same time that most 

sandbars build during an HFE, it is always possible that a nearby sandbar may erode.

Redistribution of River-Deposited Sand to Higher Elevations by Wind 

Although wind and blowing sand are common phenomena in Grand Canyon, the connection 

between sandbar deposition caused by HFEs, eolian (windblown) sand transport, and deposition 

of windblown sand at archeological sites is complex and difficult to predict and monitor. It is even 

more difficult to predict if the deposition of windblown sand near archaeological sites effectively 

protects those sites from erosion. Links among these processes were not investigated until the 

2004 HFE (Draut and Rubin, 2008). This study and continued investigation during the 2008 HFE 

(Draut, Hazel, and others, 2010) documented that, in at least some cases, newly deposited sandbars exposed to the wind caused elevated wind-blown sand transport rates and that wind-deposited 

sand dunes were rejuvenated (fig. 14). Some of these newly formed dunes subsequently covered 

archaeological sites or filled small gullies that threatened to erode archaeological resources (Draut 

and Rubin, 2008; Draut, Hazel, and others, 2010). No data, however, are available with which to 

demonstrate if the occurrence of these processes was widespread or isolated to a few locations.

Sand Deposition in Tributary Gullies

Archaeological sites adjacent to gullies and small ephemeral streams are potentially threatened by bank erosion. In some locations, incision of deep gullies that traverse through archaeological sites cause soil erosion and loss of cultural artifacts. Hereford and others (1996) speculated 

that deposition of sand in the mouths of these gullies during high flows and floods might arrest 

continued gully erosion and thereby protect archaeological sites.

These gullies have been present for many years, and pre-dam rates of gully erosion are 

unknown. Additionally, it is not known if the changes in flood regime and sediment supply caused 

by Glen Canyon Dam have significantly changed the magnitude of deposition of flood sands in 

Figure 14. Eolian dune 

crest that formed on a highflow-experiment-deposited 

sandbar taken on July 29, 

2008. From Draut, Hazel, 

and others (2010).

Chapter 3—The High Flows: Physical Science Results 75
the mouths of gullies. Monitoring of a few of these gullies before and after the HFEs showed 

that sand deposition occurs in the mouths of some gullies (Hazel, Kaplinski, Parnell, and Fairley, 

2008). Those sand deposits, however, do not usually persist, because they are eroded whenever 

flow occurs in the gullies and ephemeral streams. Thus, it is unlikely that deposition of flood sands 

in the mouths of gullies can stem gully erosion and contribute to archaeological site protection.

Mobilization of Debris-Flow Deposits at Debris Fans 

Debris fans are accumulations of deposits formed by debris flows and flash floods emanating 

from steep tributary canyons (Webb and others, 1989). These deposits include very large boulders 

that constrict the flow of the river and create the rapids of Marble and Grand Canyons (Howard 

and Dolan, 1981). Measurements made during the 1996 HFE showed that recent debris-flow 

deposits can be mobilized by short-duration, high-flow releases. Pizzuto and others (1999) 

measured movement of boulders from a new debris-flow deposit at the Prospect Creek debris fan 

that forms Lava Falls rapid. Webb and others (1999) measured debris-fan reworking caused by the 

same HFE at other sites. These studies confirmed Kieffer’s (1985) speculation that large flows are 

needed to prevent debris-flow aggradation at rapids. However, the HFE studies show that bouldermobilizing flows can be much less than those estimated by Kieffer (1985) if the high flows occur 

within a few months of a substantial debris flow. 

The Cumulative Effect of the High-Flow Experiments 

on Sandbars and Related Resources 

Clearly, HFEs have allowed vast improvements in understanding the physical processes of 

the Colorado River in Grand Canyon. At the same time, each of these high flows has affected the 

natural resources of the river corridor, especially those resources formed by sand. In this section, 

the long-term, cumulative changes in three resources are described: (1) eddy sandbars, because 

these sandbars are a distinctive attribute of the pre-dam river landscape, (2) campsites on eddy 

sandbars, because campsites are a recreational resource, and (3) backwaters or embayments of 

stagnant or slow-moving water that occur at the edges of sandbars, because these backwaters 

provide aquatic habitat for some valued fish species.

The removal of fine sediment caused by sediment-deficit conditions in the post-dam river 

caused at least a 25-percent decrease in the average size of eddy sandbars in Grand Canyon 

between completion of Glen Canyon Dam in 1963 and the early 1990s (Schmidt and others, 

2004). This decrease altered the river landscape, decreased the number and size of campsites, and 

changed the distribution and characteristics of aquatic habitat. Restoring, or enlarging, sandbars 

has been suggested as a goal of the Glen Canyon Dam Adaptive Management Program (National 

Research Council, 1999), although there are other potential restoration targets for the Colorado 

River in Marble and Grand Canyons (Schmidt and others, 1998; Schmidt, 2010).

 The cumulative effect of the three HFEs cannot be uniquely isolated to the deposition caused 

by each high flow. Although eddy sandbars are constructed during high flows, they are eroded 

by the intervening flows. Over the long-term, average sandbar size either increases or decreases 

depending on (1) the magnitude of deposition during each high flow, (2) the frequency of those 

high flows, and (3) the rate of sandbar erosion that occurs between the high flows (fig. 15).

A sustainable river-management program that rebuilds eddy sandbars should seek to reverse 

the long-term erosional trend that began in 1963 through the redistribution of the tributary-derived 

sand supply—the only renewable supply of sand to the post-dam Colorado River. An alternative sustainable river-management program would be to augment the sand supply at Lees Ferry 

using sand accumulated in Lake Powell (Rubin and others, 2002; Randle and others, 2007). An 

example of an unsustainable management strategy would be to use high-flow releases to progressively mobilize and export the nonrenewable supply of sand that had been delivered to Marble and 

76 Effects of Three High-Flow Experiments on the Colorado River Ecosystem Downstream from Glen Canyon Dam, Arizona
Relative sandbar size

 Time

1. HFE-caused deposition and post-HFE erosion with no long-term net increase in sandbar size.

2. Increased deposition during HFEs leading to net increase in sandbar size.

3. Increased frequency of high flows leading to net increase in sandbar size.

4. Increased rate of erosion between high flows leading to net decrease in sandbar size.

Grand Canyons before the dam was constructed. The success of such a program would be unsustainable, because eventually the nonrenewable sand supply would be exhausted. 

Thus, the three HFEs are also evaluated in terms of the degree to which each experiment 

represents “sustainable” river management. The amount and distribution of eddy-sandbar deposition in different river segments during different HFEs are also compared in relation to the amount 

of sand transported to Lake Mead and the sources of the sand entrained by each high flow.

Sandbar Size

Assessment of the cumulative effects of the three HFEs on eddy sandbars involves segmentscale evaluation of the long-term changes of eddy sandbars. Thus, this assessment involves 

averaging the topographic changes measured at specific sites in order to make segment-scale 

generalizations. 

Average Sandbar Changes Above Reference Stage During and After 

Each High-Flow Experiment

More than 80 percent of the post-dam fine sediment is stored in eddies below the reference 

stage (8,000 ft3/s; Hazel and others, 2006). Nevertheless, deposition above the reference stage 

determines the amount of sand that can be seen by visitors to Grand Canyon National Park and 

how much sand is potentially available as campsites. Every HFE caused widespread new sandbar 

deposition above the reference stage. Deposition occurred at most eddy sandbars, and most sandbars changed consistent with styles 1 or 2 of sand deposition.

Eddy-sandbar deposition above the reference stage was greater in parts of Marble and Grand 

Canyons where the suspended-sand concentration was greatest. In upper Marble Canyon, the 2008 

HFE had the highest suspended-sand concentration and created the largest eddy sandbars (fig. 16). 

The other HFEs had lower suspended-sand concentrations and deposited smaller sandbars. These 

patterns are illustrated by the ranking of the post-HFE to pre-HFE ratios of sandbar volume 

(table 4); four of the five greatest proportional increases in sandbar volume occurred in or immediately downstream from Marble Canyon during either the 2004 or 2008 HFEs.

Sandbar erosion rates were high immediately following each of the three high flows. Higher 

erosion rates occurred when the average dam releases were high or when there was little or no fine 

Figure 15. Conceptual diagram illustrating the dependency of net sandbar size on 

potential variations in the amount of deposition that occurs during a high-flow experiment (HFE), the frequency of HFEs, and the 

rate of post-HFE erosion. The case shown 

at the top shows a hypothetical amount of 

HFE deposition followed by an equal amount 

of erosion. Cases two and three result in 

net increases in sandbar size by increasing 

the amount of deposition during HFEs and 

increasing the frequency of HFEs, respectively. Such responses would require sufficiently great antecedent sand enrichment 

to support either larger or more frequent 

HFEs. In the final example, the rate of erosion 

following HFEs is increased, resulting in net 

decreases in sandbar size.

Chapter 3—The High Flows: Physical Science Results 77
Table 5. Summary of changes in sandbar size between February 1996 and October 2008.

Reach Number

of sites

Number of sites that are larger than 

February 1996 measurements

April 2008 October 2008

Upper Marble Canyon 6 4 2

Lower Marble Canyon 6 6 5

Eastern Grand Canyon 6 5 2

Central and 

western Grand Canyon 16 16 16

Entire study area 34 31 25

Table 4. The ten largest changes in sandbar 

volume between pre-high-flow experiment 

(HFE) and post-HFE conditions among 34 sites 

monitored before and after the 1996 and 2008 

HFEs and 9 sites monitored before and after the 

2004 HFE.

Ranking

Ratio of 

post-HFE to

 pre-HFE 

sandbar volume

Site HFE year

1 4.42 RM 65R 2004

2 4.21 RM 65R 2008

3 2.89 RM 63R 2008

4 2.51 RM 30R 2004

5 2.33 RM 63R 1996

6 2.20 RM 139R 1996 

7 2.18 RM 119R 1996

8 1.89 RM 30R 2008

9 1.84 RM 30R 1996

10 1.80 RM 47R 2008

sediment input from tributaries. Thus, erosion rates were highest after the 

1996 HFE (Grams, Hazel, and others, 2010) when tributary sediment inputs 

were low and average release volumes were high (fig. 17). Erosion rates 

were lowest following the 2004 HFE when tributary sediment inputs were 

high and average release volumes were relatively low. The 2008 HFE had 

intermediate conditions with intermediate erosion rates. Following the 1996 

and 2008 HFEs, high erosion rates occurred during the summer season when 

the average volume of dam releases was higher than in 2004. Erosion rates 

were lower during the 6-month period after the 2004 HFE (Grams, Hazel, and 

others, 2010). 

Long-Term, Cumulative Changes in Sandbar Topography

Long-term rehabilitation of eddy sandbars can occur only if the 

increases in sand volume caused by high flows exceed the erosion that 

occurs during the intervening periods (fig. 15). Alternatively, if there are 

only small amounts of deposition during high flows and large volumes of 

erosion during intervening periods, a long-term decrease in sandbar size 

will result. 

On average, the net effect, over the long term, of the HFE program 

has been to rebuild eddy-sandbar study sites above the reference stage in 

the downstream part of Marble Canyon and in most of Grand Canyon. 

Eighty-three percent of the long-term monitoring sites in lower Marble 

Canyon were larger in October 2008 than in March 1996. All of the 16 long-term monitoring sites 

in central and western Grand Canyon were larger in October 2008. However, in upper Marble 

Canyon and in eastern Grand Canyon, only one-third of the long-term monitoring sites were 

larger in October 2008 than they were in February 1996 prior to implementation of the first HFE 

in March 1996 (table 5). Two-thirds of the sites were smaller.

The temporal pattern leading to cumulative increase in sandbar size is illustrated by the 

changes that have occurred near RM 22 (fig. 18). Here, erosion that occurred during the periods 

between the HFEs removed only a small proportion of the high-flow deposits, and new deposition 

during each HFE exceeded the erosion that occurred during the previous period. Thus, the volume 

of sand at the eddy sandbar near RM 22 was much greater in 2010 than in 1996 (fig. 19). Longterm decrease in sandbar volume occurred where erosion during the intervening periods was large 

and deposition during high flows was small and limited to a small area. For example, the sandbar 

at RM 68 was progressively eroded, and the long-term changes at RM 16 and RM 47 have been 

relatively small. 

78 Effects of Three High-Flow Experiments on the Colorado River Ecosystem Downstream from Glen Canyon Dam, Arizona
0.0

1.5

1.0

0.5

2.0

2.5

0.0

1.5

1.0

0.5

2.0

2.5

A. Marble Canyon

Normalized sandbar volume above reference stage

1996

2/13

4/15

9/12

2/15

4/21

9/1

4/15

5/6

3/18

8/19

9/9

5/1

6/1

11/15

12/1

5/7

10/15

2/1

4/1

1997 1998 1999 2000 2002 2004 2005 2006 2008

Normalized total sandbar volume

B. Grand Canyon

C. Marble Canyon D. Grand Canyon

1996

10/15

2/13

4/15

9/12

2/15

4/21

9/1

4/15

5/6

3/18

8/19

9/9

5/1

6/1

11/15

12/1

5/7

10/15

2/1

4/1

10/15

10/15

1997 1998 1999 2000 2002 2004 2005 2006 2008

1996

2/13

4/15

9/12

2/15

4/21

9/1

4/15

5/6

3/18

8/19

9/9

5/1

6/1

11/15

12/1

5/7

10/15

2/1

4/1

1997 1998 1999 2000 2002 2004 2005 2006 2008 1996

2/13

4/15

9/12

2/15

4/21

9/1

4/15

5/6

3/18

8/19

9/9

5/1

6/1

11/15

12/1

5/7

10/15

2/1

4/1

10/15

1997 1998 1999 2000 2002 2004 2005 2006 2008

Figure 16. Sandbar volume that is normalized by dividing the volume for each indicated date by the volume measured in February 1996, before the 1996 high-flow experiment. Thus, the initial data point for each plot is equal to 

one, because it is the initial survey divided by itself (thick blue line). Where values are greater than one, sandbar 

size is larger than February 1996; where values are less than one, sandbar size is smaller than February 1996. 

A, Normalized sandbar volume above reference stage (8,000 cubic feet per second) in Marble Canyon. B, Normalized sandbar volume above reference stage in Grand Canyon. C, Normalized total sandbar volume in Marble 

Canyon. D, Normalized total sandbar volume in Grand Canyon. In each plot, the shaded region shows the upper 

and lower quartiles, the line shows the median value, the whiskers show the range of all data within 1.5 times 

the distance between the bounds of the upper and lower quartiles. Outliers outside this range are shown with 

unique symbols. Pre-flood measurements are shown in blue, post-flood measurements are shown in red, 6-month 

post-flood measurements are shown in green, and measurements made between controlled floods are shown in 

brown. Modified from Grams, Hazel, and others (2010).

Chapter 3—The High Flows: Physical Science Results 79
0

1,000

2,000

3,000

0 10,000 20,000

Erosion rate, in cubic feet per day

Mean of daily discharge for period

between surveys, in cubic feet per second 

A

0 400,000 800,000

2004 2004

2008 2008

1996 1996

Paria River sand input, in tons

B

Marble Canyon

Grand Canyon

Sediment Mass Balance

One of the key management issues regarding the use of high flows as a tool in redistributing sand from the channel bed to eddy sandbars is minimizing the amount of sand transported 

to western Grand Canyon and Lake Mead and maximizing the amount of sand deposited in 

sandbars. A comparison of the sediment mass balance of each HFE allows for the evaluation of 

this ratio. 

The quantity of sand transported into and out of each segment of Grand Canyon has been 

well measured since about 2000. However, measurements of changes in the distribution and 

size of eddy sandbars and of sand on the channel bed are not nearly as numerous, because few 

places have been repeatedly measured. The few measurements of sandbar deposition or erosion 

were averaged for each segment, and uncertainty was assigned on the basis of the variability 

and number of sites measured in each segment. Average changes in topography were multiplied 

by the estimated total area of eddies and of the channel in order to estimate the volume of sand 

deposited or eroded from each part of Grand Canyon (table 2) using methods developed by 

Schmidt (1999) and Hazel and others (2006). The sand budget calculations are reported with the 

appropriate degree of uncertainty.

Upper and Lower Marble Canyon

New deposition of eddy sandbars and channel-margin deposits above the reference stage 

in Marble Canyon was between 490,000 and 870,000 tons in 1996, between 450,000 and 

770,000 tons in 2004, and between 420,000 and 740,000 tons in 2008 (table 6). Where did 

this sand come from? How much was transported downstream in relation to the amount that 

created these new deposits?

Topping and others (2010) showed that, in 2008, the total amount of new deposits in 

Marble Canyon and sand exported farther downstream was probably less than the antecedent 

sand enrichment. Thus, the 2008 HFE probably mobilized most, or all, of the antecedent 

sand enrichment in Marble Canyon, and it is unlikely that the 2008 HFE mobilized any of 

the sand supply that had entered the river corridor before 2004. New deposits in upper and 

lower Marble Canyon in 2008 (table 6) were between 14 and 76 percent of the antecedent 

sand enrichment (table 1). Export of sand to eastern Grand Canyon was between 920,000 and 

1,020,000 tons, which was between 32 and 111 percent of the antecedent sand enrichment. 

The wide range of estimates of each part of the mass balance—the antecedent sand enrichment, the volume of new deposits, and the amount of sand exported farther downstream—

yields large uncertainty in these estimates. 

Figure 17. Rate of sandbar erosion following 

the 1996, 2004, and 2008 high-flow experiments 

(HFEs). A, The relation between sandbar erosion and the average of mean daily discharge 

for the period between surveys conducted 

immediately after HFEs and 6 months after 

HFEs for sites in Marble Canyon, eastern 

Grand Canyon, and combined central and 

western Grand Canyon. B, The relation 

between sandbar erosion and the total magnitude of Paria River sand inputs for the same 

time period and sites shown in A. From Grams, 

Hazel, and others (2010).

80 Effects of Three High-Flow Experiments on the Colorado River Ecosystem Downstream from Glen Canyon Dam, Arizona
Flow

A. Before the 1996 HFE

Figure 18. Matched views looking across the Colorado River at the study site near river mile 22 showing conditions before and after each 

high-flow experiment (HFE). A, Before the 1996 HFE. B, After the 1996 HFE. C, Before the 2004 HFE. D, After the 2004 HFE. E, Before the 2008 

HFE. F, After the 2008 HFE. The images show deposition by each HFE, and a sandbar that is progressively larger following each HFE. Photographs by Joseph Hazel and Matt Kaplinski, Northern Arizona University.

B. After the 1996 HFE

C. Before the 2004 HFE D. After the 2004 HFE

E. Before the 2008 HFE F. After the 2008 HFE

Chapter 3—The High Flows: Physical Science Results 81
Normalized volume of sand 

above reference stage

0.6

1.0

1.4

1.8

1996 1998 2000

High flow High flow High flow

2002 2004 2006

River mile

3

16

22

47

55

68

2008

In contrast, the 2004 HFE probably mobilized all of the antecedent sand enrichment in 

Marble Canyon and probably also mobilized older, remnant sand deposits. New deposits in 

Marble Canyon were between 450,000 and 770,000 tons—between 63 and 200 percent of the 

antecedent sand enrichment. The total amount of sand exported from Marble Canyon—between 

690,000 and 760,000 tons—was large in relation to the antecedent sand enrichment—between 96 

and 205 percent of the antecedent supply—because the enrichment was only between 380,000 and 

720,000 tons. Thus, the 2004 HFE probably mobilized more sand than just the amount that had 

entered upper and lower Marble Canyon in the 41⁄2 months immediately before that HFE. 

In 1996, the total amount of sand transported from lower Marble Canyon to eastern Grand 

Canyon was large, but there are no data about the antecedent sand enrichment to compare with 

transport data. The sum of the total amount of sand transported downstream plus the amount of 

sand that formed new sand deposits in upper and lower Marble Canyons probably exceeded the 

antecedent sand supply, because little sand was contributed by the Paria River in the year before 

that first HFE.

Eastern, Central, and Western Grand Canyon

Comparison of antecedent sand enrichment and the volume of sand entering and exiting 

eastern, central, and western Grand Canyon in 2004 and 2008 indicates that the 2004 HFE mobilized a higher proportion of the antecedent supply than did the 2008 HFE. Neither HFE, however, 

is likely to have mobilized older sand deposits.

General Conclusions

The sediment mass balance calculations show that each HFE mobilized most, or all, of the 

antecedent sand enrichment in Marble Canyon. Some of this mobilized sand was deposited in 

eddies to form larger sandbars; the remainder of the sand was exported farther downstream. In 

other words, the “checking account” of accumulated sand enrichment in Marble Canyon was 

“re-zeroed” by each HFE. This conceptual model of accumulation, mobilization, redistribution, 

and re-zeroing was previously envisioned by the U.S. Department of the Interior (1995). The 

difference between the conceptual model proposed in the mid-1990s and the model based on 

insights gained from the three HFEs is that scientists now know that the interval over which 

new tributary sand accumulates is relatively short; new sand is generally exported downstream 

within a year. Accumulation only occurs when tributary delivery of sand is relatively large and 

dam releases are relatively low volume (Rubin and others, 2002; Topping and others, 2010).

Figure 19. Normalized sandbar volume above the reference stage at selected sites for 

February 1996 to October 2009. 

Normalized means that the values shown are relative values; 

they are shown as the ratio of 

sandbar volume measured on 

the indicated date to sandbar 

volume measured in February 

1996, before the 1996 high-flow 

experiment. Each high flow is 

shown by a vertical dotted 

gray line.

82 Effects of Three High-Flow Experiments on the Colorado River Ecosystem Downstream from Glen Canyon Dam, Arizona
R2

 = 0.27

R2

 = 0.72

−25,000

0

25,000

−25,000 0 25,000 50,000

Change in campsite area above reference stage, in square feet

−25,000

0

25,000

Change in sandbar area above reference stage,

in square feet 

−100,000 0 100,000 200,000

Change in sandbar volume above reference stage,

in cubic feet 

February to April

April to October

Campsites

While campsites in Grand Canyon are not officially designated, a large number of eddy 

sandbars are frequently used for camping, such that river runners expect the sandbars to be 

available for use by boating parties. Many of these sites are marked on river running guide maps 

(Stevens, 1990; Belknap and Belknap Evans, 2001). Thus, these sandbars constitute a population of informally designated campsites, and a large number of these campsites are partially or 

completely affected by high flows. For monitoring purposes, campsites are defined as relatively 

flat areas that are bare or sparsely vegetated (Kearsley and others, 1999). 

Since most eddy sandbars increased in area and volume after each HFE, it is not surprising 

that the areas of most campsites increased similarly. Kearsley and others (1999) reported that 

the 1996 HFE caused a 37-percent increase in the total number of campsites in Grand Canyon. 

They also reported that the campsite area of 50 randomly selected sites increased by nearly 

50 percent (Kearsley and others, 1999). Hazel and others (2010) reported that the 2008 HFE 

caused a 100-percent increase in the campsite area of 37 sites.

The proportional changes in campsite area caused by the HFEs differ from the proportional 

changes in sandbars, however. The relation between geomorphic changes in eddy sandbars and 

campsites is confounded by two factors. One factor solely concerns geomorphology; changes in 

sandbar volume do not necessarily cause commensurate changes in sandbar area (fig. 20). For 

example, increases in sandbar thickness may increase the elevation of the sandbar, but the area 

that is useful as a campsite may not change 

substantially (Kearsley and others, 1999). 

A second confounding factor is that sand 

deposition amongst vegetation often does 

not increase campsite area, because river 

runners often avoid densely vegetated areas 

(fig. 21).

Although each HFE caused shortterm increases in campsite area, erosion 

and vegetation invasion led to decreased 

campsite area during the intervening 

periods (fig.22; Hazel and others, 2010; 

Kaplinski and others, 2010). Kearsley and 

others (1999) reported that 44 percent of the 

campsites created by the 1996 HFE were 

not usable 6 months later. Hazel and others 

(2010) reported substantial erosion and 

campsite area decline in the 6-month period 

following the 2008 HFE. Between HFEs, 

campsite area declined more in noncritical 

areas than in critical areas (fig. 22), owing to 

more extensive expansion of riparian vegetation (see chapter 4, this volume).

Table 6. Estimated total sediment deposition in Marble Canyon (all values in millions 

of tons).

Storage location 1996 HFE 2004 HFE 2008 HFE

Eddy above reference stage 0.58 ± 0.16 0.51 ± 0.13 0.48 ± 0.13

Channel-margin above reference stage 0.10 ± 0.03 0.10 ± 0.03 0.10 ± 0.03

Sum of deposits above reference stage 0.68 ± 0.19 0.61 ± 0.16 0.58 ± 0.16

Figure 20. Relation 

between change in campsite 

area and change in sandbar 

area and volume for two 

time intervals in 2008, pre- to 

post-flood (February–April) 

and the following 6 months 

(April–October). A linearregression (straight-line) fit 

for both time intervals combined is shown. Modified 

from Hazel and others (2010).

Chapter 3—The High Flows: Physical Science Results 83
Figure 21. Repeat views looking across 

the Colorado River from the left bank at the 

sandbar and campsite located at river mile 

202.3. Flow in main channel is from right to 

left. Between 1998 and 2006, woody vegetation, primarily arrowweed (Pluchea sericea), 

had expanded to cover large areas of the 

formerly sandy, unvegetated sandbar. A, 

Photograph taken April 30, 1998, at a river 

discharge of about 14,000 ft3/s. B, Photograph 

taken October 25, 2006, at a river discharge 

of about 11,700 ft3

/s.

A. April 30, 1998

B. October 25, 2006

Implications to Aquatic Habitat: Response of Sandbar-Created Backwaters to 

High-Flow Experiments

The redistribution of sand during high flows not only leads to eddy-sandbar deposition but also 

causes changes in sandbar topography that substantially increases the number and size of backwaters. These areas of low or stagnant flow are nursery rearing habitat for humpback chub (Gila 

cypha). These desirable topographic changes primarily occur where there is substantial deposition 

of the reattachment bar within an eddy. Reattachment bars are the part of an eddy sandbar that 

underlies most of the recirculating flow (Rubin and others, 1990). Grams, Schmidt, and Andersen 

(2010) documented changes in sandbar topography at 78 reattachment bars and reported that 

deposition on the bar and erosion in the adjacent eddy return-current channel occurred at one-half 

of these sites during the 2008 HFE. These changes accentuated bar relief (see chapter 2 text box, 

The Fan-Eddy Complex) and led to increases in the area and volume of backwater habitat. These 

changes in sandbar morphology were summarized by Grams, Schmidt, and Andersen (2010) in 

plots that show backwater area as a function of discharge (fig. 23). The March 2008 HFE resulted 

in increases in the area of backwater habitat that would be present at all discharges between 8,000 

and 20,000 ft3

/s. The increase persisted for at least 2 months, but returned to conditions similar to 

those before the HFE by about 6 months after the HFE.

Conclusions

Over the long term, eddy-sandbar size can only be increased if (1) high flows cause substantial 

deposition, (2) high flows occur frequently, and (3) erosion that occurs between high flows is less 

than the deposition. Thus, the net effect of high flows in building eddy sandbars results from the 

Flow

84 Effects of Three High-Flow Experiments on the Colorado River Ecosystem Downstream from Glen Canyon Dam, Arizona
4

Noncritical reaches

Critical reaches

Total campsite area within

study sites, in acres 3

1

2

0 1998 2000 2002 2004 2006 2008

160,000

February

April

May

October

Total backwater area at

indicated discharge, in square feet 120,000

40,000

80,000

0

5,000 10,000

Discharge, in cubic feet per second

15,000 20,000

magnitude and the frequency of high flows and the deposition they cause. Erosion ensues rapidly 

after each high flow, and the rate of erosion declines thereafter but persists. The longer the time 

period between HFEs, the more erosion occurs. 

The HFE research program demonstrated that eddy sandbars are quickly constructed by 

high flows if those flows have high suspended-sand concentrations. In turn, high suspended-sand 

concentrations are caused by large antecedent sand-enrichment conditions or by smaller antecedent 

sand enrichment with very fine grain sizes. Thus, every time that enrichment is large in quantity 

or fine in grain size, an opportunity exists to build sandbars and to reverse erosional trends (see 

chapter 5, this volume).

Although there has been no experimentation with substantially larger HFE magnitudes, it is 

likely that high flows larger in magnitude than the three HFEs conducted to date would produce 

larger sandbars than those measured between 1996 and 2008, provided that the antecedent sand 

enrichment is sufficiently large to sustain high concentrations of suspended sand for 1 to 3 days. 

Larger sandbars would be expected to form, because a larger area would be inundated, and the 

water-surface elevation would be higher. Available data are consistent with the perception that the 

high-flow deposits would be thicker when high flows are larger in magnitude. 

The duration of high flows should typically be only a few days if the sole objective is to 

increase eddy sandbar size. The duration of the 1996 HFE was too long, because the supply of sand 

was exhausted before the end of the 7-day event (Topping and others, 1999). In contrast, elevated 

sand concentrations were maintained throughout the 2008 HFE (Topping and others, 2010). 

Moreover, daily measurements made during the 2008 HFE indicated that sandbars above the reference stage were still getting larger up to the end of the high flow (Wright and Kaplinski, in press). 

This indicates that there may be sand-enriched conditions in which peak-flow durations longer 

than 60 hours could result in a greater sandbar-building response than has yet been observed. This 

Figure 22. High-elevation campsite area in critical and 

noncritical reaches between 1998 and 2008. The error 

bars show plus and minus 10 percent uncertainty. Critical reaches are reaches where campsites are in scarce 

supply, and noncritical reaches have more abundant 

campsites. Modified from Kaplinski and others (2010).

Figure 23. The relation 

between total backwater 

habitat area and discharge 

for the sandbar and backwater monitoring sites in 

2008. Modified from Grams, 

Schmidt, and Andersen 

(2010). 

Chapter 3—The High Flows: Physical Science Results 85
potential benefit of longer-duration flows must be balanced against the greater net sand export from 

Grand Canyon that would also occur.

The HFE research program also showed that high flows that are released when antecedent 

sand enrichment is low have the potential to remove more sand from the channel bed than just 

the antecedent enrichment. Thus, high flows have the potential to remove some of the nonrenewable sand resource if the high flows are not properly scheduled. This scenario probably happened 

in 1996 and may have happened in 2004. Thus, a sustainable program of HFEs must be based on 

accurate accounting of the antecedent enrichment and of the grain sizes of that enrichment. Simply 

scheduling high flows on a regular basis and ignoring the magnitude of the antecedent supply risks 

accelerating long-term erosion of eddy sandbars. See chapter 5, this volume, for possible future 

HFE options.

The evidence that supports this conclusion is the comparison of antecedent sand enrichment 

and sand budgets for the three HFEs. The sand enrichment prior to the 2008 HFE was more than 

sufficient to cause widespread increases in sandbar sizes. The sand enrichment prior to the 1996 

HFE was too small to support the high-elevation deposition that was measured; some of the new 

sand deposited at high elevation by this flood was eroded from low-elevation parts of eddy sandbars and from the main channel and was remnant sand delivered to the Colorado River a few years 

to a decade before March 1996. Sand enrichment before the 2004 HFE was sufficiently large and 

was probably the source for all of the new high-elevation sandbars in upper Marble Canyon, but 

remnant sand that was not part of the antecedent sand enrichment was deposited in high-elevation 

sandbars farther downstream. These differences in antecedent sand conditions in different parts of 

Marble and Grand Canyons present another challenge for sustainable river management—the need 

to make assessments of antecedent conditions in different segments of the river corridor before 

scheduling HFEs. Additionally, resource-management goals for future high flows could potentially differ in various parts of Marble and Grand Canyons if sand enrichment is not uniformly 

widespread.

Rubin and others (2002) described the management alternatives faced at those times when 

tributaries supply large amounts of sand to the Colorado River. Dam releases of less than 9,000 ft3

/s 

are needed to retain sand so that it is not exported to western Grand Canyon and Lake Mead. This 

low-flow regime would have to be maintained as the primary flow regime until the next high flow 

was scheduled in order to retain that sand in upstream parts of Marble and Grand Canyons, thereby 

ensuring the largest possible antecedent sand enrichment.

Measurements also demonstrate that there is substantial site-to-site variability in the magnitude and distribution of sandbar deposition caused by any high flow. The topographic response 

of specific sandbars to specific high flows cannot be predicted with precision. Thus, the predicted 

response of sandbars to specific future high flows must to be characterized by average, large-scale 

river segment behavior.

Some physical processes measured during the HFEs are well understood but not easily 

generalized to all of Marble and Grand Canyons. Scientists understand that discharges of about 

45,000 ft3/s can mobilize recently deposited debris flows and that canyon winds may redistribute 

sand from sandbars to higher elevations and sometimes bury archaeological sites. Unfortunately, 

neither process is easily generalized nor are there quantitative estimates of the river-segment-scale 

importance of these processes. 

Thus, there is no question that high flows similar in magnitude to those that occurred during 

the HFEs of 1996, 2004, and 2008 effectively mobilize accumulated fine sand delivered by 

tributaries downstream from Glen Canyon Dam and rebuild eddy sandbars in Marble and Grand 

Canyons. Short-duration dam releases mobilize this sand and either redistribute it to eddy sandbars 

or to western Grand Canyon and Lake Mead. Sand-enrichment conditions similar to those prior to 

the 2008 HFE afford great potential to build sandbars with relatively small volumes of water.

Limited research has been conducted on the magnitude of sandbar erosion that occurs 

during periods between HFEs. Grams, Schmidt, and Andersen (2010) reported that more erosion 

occurs when total flow is large. Therefore, large dam releases that are entirely within powerplant 

86 Effects of Three High-Flow Experiments on the Colorado River Ecosystem Downstream from Glen Canyon Dam, Arizona
capacity and that occur between HFEs are likely to undo the positive effects of sandbar 

building, backwater creation, and campsite expansion caused by HFEs. Even though high flows 

are an important part of an adaptive management program to rebuild sandbars, they remain only 

one part of a complex strategy.

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Origins of 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. 23–36.

Schmidt, J.C., Grams, P.E., and Leschin, M.F., 1999, Variation in the magnitude and style of deposition and erosion in three long (8–12 km) reaches as determined by photographic analyses, 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. 185–203.

Schmidt, J.C., Topping, D.J., Grams, P.E., and Hazel, J.E., 2004, System-wide changes in the distribution of fine sediment in the Colorado River corridor between Glen Canyon Dam and Bright 

Angel Creek, Arizona—Final report: Logan, Utah, Utah State University, submitted to U.S. Geological Survey, Grand Canyon Monitoring and Research Center, cooperative agreement 

no. 1425-98-FC-40-22640, 107 p., accessed January 31, 2011, at http://www.gcmrc.gov/library/

reports/Physical/Fine_Sed/Schmidt2004.pdf.

Schmidt, J.C., Topping, D.J., Rubin, D.M., Hazel, J.E., Jr., Kaplinski, Matt, Wiele, S.M., and Goeking, S.A., 2007, Streamflow and sediment data collected to determine the effects of low summer 

steady flows and habitat maintenance flows in 2000 on the Colorado River between Lees Ferry 

and Bright Angel Creek, Arizona: U.S. Geological Survey Open-File Report 2007–1268, 79 p. 

(Also available at http://pubs.usgs.gov/of/2007/1268/.)

Schmidt, J.C., Webb, R.H., Valdez, R.A., Marzolf, G.R., and Stevens, L.E., 1998, Science and 

values in river restoration in the Grand Canyon: BioScience, v. 48, no. 9, p. 735–747, accessed 

August 17, 2010, at http://www.jstor.org/stable/1313336.

Stevens, Larry, 1990, The Colorado River in Grand Canyon—A comprehensive guide to its natural 

and human history (3d ed.): Flagstaff, Arizona, Red Lake Books, 115 p.

Topping, D.J., Rubin, D.M., Grams, P.E., Griffiths, R.E., Sabol, T.A., Voichick, Nicholas, Tusso, 

R.B., Vanaman, K.M., and McDonald, R.R., 2010, Sediment transport during three controlledflood experiments on the Colorado River downstream from Glen Canyon Dam, with implications 

for eddy-sandbar deposition in Grand Canyon National Park: U.S. Geological Survey Open-File 

Report 2010–1128, 111 p. (Also available at http://pubs.usgs.gov/of/2010/1128/.)

Topping, D.J., Rubin, D.M., Nelson, J.M., Kinzel, P.J., III, and Bennett, J.P., 1999, Linkage between 

grain-size evolution and sediment depletion during Colorado River floods, 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. 71–98.

Topping, D.J., Rubin, D.M., Schmidt, J.C., Hazel, J.E., Melis, T.S., Wright, S.A., Kaplinski, Matt, 

Draut, A.E., and Breedlove, M.J., 2006, Comparison of sediment-transport and bar-response results 

from the 1996 and 2004 controlled-flood experiments on the Colorado River in Grand Canyon, in 

Proceedings of the Eighth Federal Interagency Sedimentation Conference, April 2–6, 2006, Reno, 

Nevada: Advisory Committee on Water Information, p. 171–179 (available on CD-ROM).

Topping, D.J., Rubin, D.M., and Vierra, L.E., Jr., 2000, Colorado River sediment transport, 

1. Natural sediment supply limitation and the influence of the Glen Canyon Dam: Water 

Resources Research, v. 36, no. 2, p. 515–542, doi: 10.1029/1999WR900285.

90 Effects of Three High-Flow Experiments on the Colorado River Ecosystem Downstream from Glen Canyon Dam, Arizona
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. (Also available at 

http://pubs.usgs.gov/pp/pp1677/.)

U.S. Department of the Interior, 1988, Glen Canyon Environmental Studies Executive Review 

Committee, Final report: Bureau of Reclamation, Glen Canyon Environmental Studies, 

84 p. + apps. (Available from National Technical Information Serivce, PB88-183348/AS.)

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., accessed November 18, 

2010, at http://www.usbr.gov/uc/library/envdocs/eis/gc/pdfs/Cov-con/cov-con.pdf.

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

and removal of non-native fish—Environmental assessment: Salt Lake City, Utah, Bureau of 

Reclamation, Upper Colorado Regional Office, 112 p., accessed January 31, 2011, at http://

www.usbr.gov/uc/envdocs/ea/gc/PropExpReleasesEA-09-2002.pdf.

U.S. Department of the Interior, 2004, Proposed experimental actions for water years 2005–2006 

Colorado River, Arizona, in Glen Canyon National Recreation Area and Grand Canyon 

National Park—Draft supplemental environmental assessment: Salt Lake City, Utah, Bureau 

of Reclamation, Upper Colorado Regional Office, 26 p., accessed March 25, 2010, at 

http://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=1419&context=govdocs.

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: U.S. Geological Survey Water-Resources 

Investigations Report 00–4055, 67 p.

Webb, R.H., Melis, T.S., Griffiths, P.G., and Elliott, J.G., 1999, Reworking of aggraded debris 

fans, 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. 37–51.

Webb, R.H., Pringle, P.T., and Rink, G.R., 1989, Debris flows from tributaries of the Colorado 

River, Grand Canyon National Park, Arizona: U.S. Geological Survey Professional Paper 1492, 

39 p.

Wiele, S., and Torizzo, M., 2005, Modelling of sand deposition in archaeologically significant 

reaches of the Colorado River in Grand Canyon, USA, in Bates, P.D., Lane, S.N., and Ferguson, R.I., eds., Computational fluid dynamics—Applications in environmental hydraulics: 

Hoboken, New Jersey, John Wiley & Sons, chap. 14, p. 357–394, accessed November 29, 2010, 

at http://onlinelibrary.wiley.com/doi/10.1002/0470015195.ch14/summary.

Wiele, S.M., Andrews, E.D., and Griffin, E.R., 1999, The effect of sand concentration on depositional rate, magnitude, and location in the Colorado River below the Little Colorado River, 

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. 131–145.

Wright, S.A., and Kaplinski, M., in press, Flow structures and sandbar dynamics in a canyon river 

during a controlled flood, Colorado River, Arizona: Journal of Geophysical Research.

Chapter 3—The High Flows: Physical Science Results 91
National Park Service
CHAPTER 4

Biological Responses to High-Flow 

Experiments at Glen Canyon Dam 

By Theodore A. Kennedy1,2 and Barbara E. Ralston1,2

Sandbars are a prominent geomorphic feature of the Colorado River (see chapters 2 

and 3, this volume, for description). Closure of Glen Canyon Dam in 1963 (fig. 1) and 

subsequent operations have eroded these features. The Glen Canyon Dam Adaptive 

Management Program (GCDAMP) has released artificial floods (termed high-flow experiments 

(HFE)) to reestablish sandbar building processes in the hopes of benefitting a variety of biological, physical, and cultural resources that are dependent on floods or the sandbars and associated 

geomorphic features that are created by the floods (see chapters 2 and 3, this volume). High-flow 

experiments were conducted in March 1996, November 2004, and March 2008. The purpose 

of this chapter is to synthesize the biological resource responses to these three HFEs to assess 

whether this policy option is having a measurable effect on biological resources.

This chapter focuses on five biological resources that are of importance to the GCDAMP: 

(1) riparian vegetation, (2) Kanab ambersnail (Oxyloma haydeni kanabensis), (3) rainbow trout 

(Oncorhynchus mykiss), (4) endangered humpback chub (Gila cypha), and (5) other native fish 

populations (flannelmouth sucker (Catostomus latipinnus), bluehead sucker (Catostomus discobolus), and speckled dace (Rhinichthys osculus); see text box, Descriptions of Selected Fish 

Species in Grand Canyon. Riparian vegetation is valued because it provides high-quality habitat 

for terrestrial wildlife. Kanab ambersnails are federally listed as endangered, and portions of 

their habitats may be destroyed during HFEs. Rainbow trout were introduced to the tailwater 

below Glen Canyon Dam shortly after the dam was closed, and maintaining a sport fishery in 

the tailwater is an important GCDAMP goal (Gloss and Coggins, 2005). Humpback chub are 

native to the Colorado River Basin, and the Little Colorado River population in Grand Canyon is 

the largest anywhere (Coggins and Walters, 2009). Because many GCDAMP policies, including 

HFEs, are intended specifically to benefit humpback chub, much of this chapter focuses on 

describing links between HFEs and this imperiled species. The response of the three other native 

fish species still present in Grand Canyon is evaluated separately from humpback chub (four 

other species of fish native to the Colorado River have become extirpated since closure of Glen 

Canyon Dam; Gloss and Coggins, 2005). In addition to rainbow trout, other nonnative species 

are present in the Colorado River (for example, common carp Cyprinus carpio; see text box, 

Descriptions of Selected Fish Species in Grand Canyon), but these species are not discussed 

in this chapter because a GCDAMP goal for these species has not been established, the link 

between these nonnative species and HFEs is highly uncertain, and the effects of these nonnative 

species on native species are relatively unknown (see Yard and others, in press, for analysis of 

1

 U.S. Geological Survey, Southwest Biological Science Center, Grand Canyon Monitoring and Research Center, 

2255 N. Gemini Drive, Flagstaff, AZ 86001.

2

 Both authors contributed equally to the development of this chapter.
Lake Mead

Lake Powell

Paria

Riv e r

Kanab

Creek

Diamond Cr Havasu Cr

Bright Angel Cr

Glen Canyon Dam

GRAND CANYON

NATIONAL PARK

GLEN CANYON

NATIONAL RECREATION

AREA

Hoover Dam

Lees Ferry

Little Colorado River

0 20 40 miles

0 20 40 60 kilometers

UTAH

ARIZONA

NEVADA

Colorado

R i v e r

M

A R B L E C A N Y O N

Hualapai

Indian 

Reservation

Hopi

Indian 

Reservation

Havasupai

Indian 

Reservation

Navajo

Indian 

Reservation

Kaibab

Indian 

Reservation

Paiute

Indian 

Reservation

N

ARIZONA

Map

Area

the effects of brown trout (Salmo trutta) on humpback chub). Although a GCDAMP goal has been 

established for the aquatic food base (algae and invertebrates), food-base data are only discussed 

in this chapter in the context of the fish population responses. This approach is consistent with 

the GCDAMP goal for food base, which is simply to maintain an adequate food supply for fish. 

For simplicity, the terrestrial ecosystem is considered separately from the aquatic ecosystem 

throughout this chapter.

Terrestrial Ecosystem

Background

Riparian vegetation along shoreline slopes below Glen Canyon Dam reflects the progressive 

reduction in annual flood peaks that occurred with regulation of the Colorado River (Carothers 

and Brown, 1991; Topping and others, 2003; Kennedy and Ralston, 2010). Vegetation cover 

increased substantially following the reduction in annual flood peaks associated with the closure 

of Glen Canyon Dam (Turner and Karpiscak, 1980; Kennedy and Ralston, 2010). Numbers of 

nonnative tamarisk (Tamarix spp.) expanded dramatically in Grand Canyon following river regulation, although the species was present in the river as early as 1938 (Clover and Jotter, 1944). Four 

distinct vegetation zones currently exist and reflect the frequency of inundation and disturbance, 

which is dependent on elevation above river level (fig. 2). Plants closest to the shore and up to 

about the 20,000 cubic feet per second (ft3/s) stage elevation experience daily inundation for at 

least some periods of the year. Wetland species common in this zone include sedges (Carex spp.), 

cattail (Typha latifolia), horsetail (Equisetum spp.), and common reed (Phragmites australis). 

The lower riparian zone (between 20,000 and 31,000 ft3

/s stage elevations) has not been regularly inundated since 2000 because of a long-term drought in the upper Colorado River Basin. 

Vegetation in this zone includes woody riparian species, such as tamarisk, seep willow (Baccharis 

spp.), coyote willow (Salix exigua), and arrowweed (Pluchea sericea). The middle riparian zone 

is inundated during HFEs when discharge is between 31,000 and 45,000 ft3

/s. Plants found in 

this zone are similar to those in the lower riparian zone, but also include bunch grasses (sand 

dropseed, Sporobolus cryptandrus) and perennial shrubs (spiny aster, Chloracantha spinosa). The 

Figure 1. The Colorado 

River below Glen Canyon 

Dam, Arizona.

94 Effects of Three High-Flow Experiments on the Colorado River Ecosystem Downstream from Glen Canyon Dam, Arizona
6-year return flood pre-dam

Upper riparian zone

>45,000–200,000 ft3/s

Middle riparian zone

>31,000 –45,000 ft3

/s

Lower riparian zone

>20,000–31,000 ft3

/s

Hydro riparian zone

8,000–20,000 ft3

/s

Bedrock

Pre-dam 

alluvial/fluvial terraces

River

sand

Figure 2. Vegetation zones according to 

inundation frequency and flow magnitude. 

In the post-dam period since 1991, when 

restrictions constrained dam operations, 

flows actively influence riparian vegetation 

up to the 45,000 cubic feet per second (ft3/s) 

water-surface elevation. The higher elevation 

benches above this elevation are subject to 

floods up to the 6-year return flood frequency 

(200,000 ft3/s). Vegetation follows a moisture 

gradient from water-tolerant plants located 

near the shoreline to species that tolerate 

drier upslope conditions (Ralston, 2010).

upper riparian zone includes the pre-dam riparian faculative vegetation, those species found in 

both riparian and upland habitats, and desert species associated with the Colorado Plateau, Great 

Basin, and Sonoran Desert floristic areas (McLaughlin, 1989). Honey mesquite (Prosopis glandulosa), catclaw acacia (Acacia greggii), mountain pepperweed (Lepidium montanum), Mormon tea 

(Ephedra nevadensis), prickly pear cactus (Opuntia spp.), creosote (Larrea tridentata), ocotillo 

(Fouquieria splendens), and brittlebush (Encelia farinosa) are common in this zone (Carothers and 

Brown, 1991). The cover of riparian vegetation is highest in wide, alluvial reaches and lowest in 

narrow, bedrock-confined reaches (Ralston and others, 2008). 

Freshwater springs are present along the river channel, and their locations are tied to fractures 

in the underlying geology that allow groundwater discharge (Huntoon, 1981). Springs support a 

high diversity of riparian and wetland plant species. Vaseys Paradise is a spring located 32 miles 

downstream from Lees Ferry (fig. 1), the source of which is groundwater that is discharged about 

125 feet above the Colorado River from a cave in the Redwall Limestone (fig. 3; Huntoon, 1981). 

Plant species that are common in this spring include nonnative watercress (Rorippa nasturtiumaquaticum), native cardinal monkey flower (Mimulus cardinalis), and water sedge (Carex sp.). The 

amount of discharge in the spring affects the lateral extent of vegetation growth along the canyon 

wall, and peak discharges in the Colorado River determine the lower extent that vegetation occupies 

(Ralston, 2005). 

Vaseys Paradise is designated as a critical habitat for the endangered Kanab ambersnail 

(England, 1992). This species was listed as endangered in 1992 (England, 1992); however, uncertainties regarding the taxonomy (it may not be a unique species) may eventually affect its status as 

an endangered species (Meretsky and others, 2002). At Vaseys Paradise, the snail occupies habitats 

from well above historic flood elevations down to the river shoreline (Ralston, 2005). 

High-Flow Experiments

The geomorphic features that are reworked and rebuilt during HFEs—sandbars, rocky slopes, 

debris fans, and return-current channels—are the substrate for growth of riparian vegetation. Thus, 

HFEs can affect vegetation through changes in habitat and also through burial and removal during 

the HFE itself. The vegetation response to HFEs affects terrestrial wildlife, through changes in 

habitat, and sociocultural resources, through changes in campsite area. The structural complexity 

of riparian vegetation is particularly important for nesting birds (Sogge and others, 1998). 

Plant-dwelling arthropods are food resources for some riparian bird species (Yard and others, 

2004). The quantity and type of vegetation that colonizes sandbars affect campsite area in Grand 

Canyon (Kearsley and others, 1994). Increasing campsite area through sandbar building is one 

Chapter 4—Biological Responses to High-Flow Experiments at Glen Canyon Dam 95
of the primary motivations for conducting HFEs. Vegetation also affects the rate of sediment 

erosion and deposition during floods in general (Malanson, 1993; Simon and others, 2004). In 

this section, two aspects of riparian vegetation response are evaluated: (1) whether HFEs affect 

vegetation cover, which has implications for both wildlife and campsite area, and (2) whether 

HFEs differentially affect the cover of native as opposed to nonnative vegetation. No published 

studies are available regarding the 2004 HFE, so the 1996 and 2008 spring-timed HFEs are the 

focus of this review. 

Riparian Vegetation 

The magnitude and duration of both the 1996 and 2008 HFEs were insufficient to remove 

woody riparian plants, but some wetland plants close to the shoreline were removed (Kearsley 

and Ayers, 1999; Stevens and others, 2001; Ralston, 2010). Sediment deposition completely or 

partially buried plants, including coyote willow, seepwillow (Baccharis spp.), tamarisk, and some 

low-lying grasses and forbs. The 1996 HFE caused a 20-percent reduction in the total areal cover 

of woody and herbaceous plants on sandbars but this reduction was short lived, lasting less than 

12 months (Kearsley and Ayers, 1999; Stevens and others, 2001). Herbaceous plant cover on 

sandbars doubled within 6 months of the 2008 HFE (Ralston, 2010). 

Burial of vegetation during HFEs may affect riparian community structure by favoring 

plants adapted to burial and growth through vegetative reproduction. Plants that recovered 

quickly following the 2008 HFE were ones that are well adapted to burial, such as coyote 

willow (Ralston, 2010). Clonal wetland plants, such as common reed, also quickly occupied 

Figure 3. A, Kanab ambersnail 

(Oxyloma haydeni kanabensis), 

with a dime for perspective. B, 

The spring at Vaseys Paradise, 

one of three locations where the 

snails are found, showing the 

high discharge released from the 

caves high above the Colorado 

River. A scour line from the 1996 

HFE is present in the photograph, 

with vegetation re-growing below 

the line in places. C, Watercress 

(Rorippa nasturtium-aquaticum). 

D, Cardinal monkeyflower (Mimulus cardinalis), the primary plant 

species associated with Kanab 

ambersnail. (Ambersnail photograph by Roy Averill-Murray, 

Arizona Game and Fish Department; other photographs by Jeff 

Sorensen, Arizona Game and Fish 

Department.) 

A

C

B

D

96 Effects of Three High-Flow Experiments on the Colorado River Ecosystem Downstream from Glen Canyon Dam, Arizona
bare sandbars and shorelines following both the 1996 and 2008 HFEs (Kearsley and Ayers, 1999; 

Ralston 2010). Seed banks were immediately depleted during the 1996 HFE, which resulted in 

a localized reduction in sources of seeds for germination (Kearsley and Ayers, 1999). Nutrients 

derived from decomposing vegetation buried in sandbars during HFEs may also facilitate colonization of sandbars by vegetation (Parnell and others, 1999). 

The timing of an HFE can affect the success of plants that are propagated with seeds. One of 

the primary concerns regarding HFE timing is the risk of distributing seeds of invasive species, 

especially tamarisk. The 1996 and 2008 HFEs occurred during the time of year before tamarisk 

begins producing seeds—seed production generally occurs between April and September. As 

such, the establishment of tamarisk seedlings was low (less than 2 percent) in 1996 and 2008 

(Kearsley and Ayers, 1999; Stevens and others, 2001; Ralston, 2010). Tamarisk is a poor competitor 

in dense vegetation (Sher and others, 2000). Following the 2008 HFE, tamarisk seedlings were 

most commonly found in the lower riparian zone where vegetative cover was less than 15 percent 

(Ralston, 2010). The combination of sparse vegetation and consistent water availability throughout 

the growing season should have created ideal conditions for tamarisk seedling establishment, 

yet establishment was still low (Ralston, 2010). High-flow experiments that are coincident with 

tamarisk seed production could favor seedling establishment by tamarisk, but native species such 

as willows are also producing seeds at this time of year, so tamarisk would not be the only species 

poised to take advantage of favorable germination conditions following HFEs. 

Kanab Ambersnail

The 1996 HFE resulted in the loss of 16 percent of the habitat used by Kanab ambersnail at 

Vaseys Paradise (Stevens and others, 2001). Conservation measures associated with HFEs in 2004 

and 2008 included temporary removal of snails and their habitat prior to the HFE. Snails were 

released above the inundation zone, while mats of vegetation were temporarily held above the 

zone of inundation until the HFE was over and then returned to their original location. Vegetation 

recovery occurred within 6 months (U.S. Department of the Interior, 2008a). A snail census 

following the 2004 HFE documented no substantial decline in abundance, perhaps due to these 

mitigation measures (Sorensen, 2005, 2009). 

Synthesis

High-flow experiments conducted in early spring appear to be a useful tool for meeting GCDAMP 

objectives for riparian vegetation, including maintaining native marsh and riparian communities and 

reducing nonnative species. However, reductions in campsite area due to vegetation recovery and 

expansion following HFEs might offset the temporary increases in campsite area that have previously 

occurred because of sandbar building during HFEs (Kaplinski and others, 2005). Vegetation may also 

influence sandbar building because the presence of vegetation along shorelines reduces water velocities 

and decreases the capacity of the river to rework and redistribute sediment (Simon and others, 2004). 

The effect that increased post-dam vegetation has on sediment deposition and erosion dynamics in the 

Colorado River is largely unknown. Future sediment studies might consider incorporating mechanical 

vegetation removal from shorelines to better understand the effects of vegetation on sediment deposition and sandbar building. Lastly, the effects of HFE timing on riparian vegetation is highly uncertain 

because no data were collected during the fall-timed 2004 HFE. 

High-flow experiments lead to temporary reductions of Kanab ambersnail habitat, which in 

the absence of mitigation measures lead to direct reductions in the number of snails. Conservation 

measures associated with recent HFEs apparently limited the loss of snail habitat and snail mortality. 

However, Kanab ambersnails are distributed well above the stage elevation of HFEs, so these shortduration disturbances only affect a relatively small proportion of the overall habitat and population 

(about 15 percent). Furthermore, this species clearly survived and persisted despite natural pre-dam 

floods that were much larger in magnitude and duration than HFEs. Thus HFEs may not represent a 

substantial threat to the persistence of Kanab ambersnail populations at Vaseys Paradise. 

Chapter 4—Biological Responses to High-Flow Experiments at Glen Canyon Dam 97
Table 1. Average water temperature and turbidity data for selected sites in the Colorado River, Arizona. Locations are denoted 

by river mile (RM) with the place name for each site noted parenthetically. River miles increase in the downstream direction. Lees 

Ferry is RM 0 and is 16 miles downstream from Glen Canyon Dam. The section of river between Glen Canyon Dam and Lees Ferry, 

within Glen Canyon National Recreation Area, is managed as a rainbow trout sport fishery. Sections of river below Lees Ferry that 

are within Grand Canyon National Park are managed for native fish populations. Values are the average ± 1 standard deviation (an 

indicator of the amount of variation inherent in the measurement). The period of record for both water temperature and turbidity is 

2006 through 2009, with the exception of turbidity data for RM 0 (2006 through July 2009) and RM 88 (2008 through 2009). Turbidity is 

a measure of water clarity with low turbidity corresponding to high water clarity. Data courtesy of Nick Voichick, U.S. Geological 

Survey, unpub. data, 2010.

[°F, degrees Fahrenheit]

River mile 0 

(Lees Ferry)

River mile 30 

(Fence fault)

River mile 61 

(Upstream from confluence 

with Little Colorado River)

River mile 88 

(Phantom Ranch)

River mile 225 

(near Diamond Creek)

Water temperature (°F) 50.7  ±  2.7 51.3  ±  2.7 52.3  ±  3.0 53.5  ±  3.6 56.9  ±  5.5

Turbidity (nephelometric 

turbidity units) 2 ± 10.5 50  ±  347 71 ±  478 225  ±  672 347  ±  1,070

Aquatic Ecosystem

Background

The serial discontinuity concept (Ward and Stanford, 1983) is a useful framework for 

describing the aquatic ecosystem downstream from Glen Canyon Dam (fig. 1). Dams such as Glen 

Canyon affect fish populations because of changes in the physical template, creating a “discontinuity” in the whole ecosystem. For the purposes of this report, the physical template of the 

Colorado River is defined as the physical habitat (backwaters and other channel features; Schmidt 

and Graf, 1990) and parameters such as water temperature, suspended sediment, and flow regime. 

The physical template of the Colorado River changes with distance downstream as climate and the 

cumulative influence of tributaries gradually cause the physical aspects of the ecosystem to shift 

to those more typical of unimpounded segments (Carothers and Brown, 1991). Water temperature and water clarity are two aspects of this changing physical template that appear to have a 

pronounced effect on fish assemblages below Glen Canyon Dam (Carothers and Brown, 1991; 

Gloss and Coggins, 2005). 

Glen Canyon Dam has an overriding influence on native fish populations because of its 

effects on water temperatures. Water released from Glen Canyon Dam is cold (table 1) because 

it is drawn from deep within Lake Powell, the reservoir formed by the dam. These low water 

temperatures are often too low for successful native fish reproduction in the main stem (Minckley 

and Deacon, 1991; Valdez and Ryel, 1995; Voichick and Wright, 2007) and effectively restrict 

native fish spawning to warm-water tributaries. For example, water temperatures must be at least 

61 degrees Fahrenheit (°F) for humpback chub to initiate spawning (Hamman, 1982). Cold water 

temperatures also limit growth rates for native fish that are rearing in the main stem (Clarkson 

and Childs, 2000); growth is a strong predictor of survival for animal populations (Krebs, 2008). 

Water temperatures gradually warm with distance downstream (table 1). 

Glen Canyon Dam also has an overriding influence on fish populations because of its 

influence on water clarity (table 1; fig. 4). Suspended sediment settles in Lake Powell, so 

water released from the dam is much clearer than water that flowed through Glen Canyon 

during pre-impoundment conditions (Topping and others, 2000). Tributaries periodically flood, 

contributing vast quantities of sediment (from boulders to fine clay) to the river (Topping, 1997; 

98 Effects of Three High-Flow Experiments on the Colorado River Ecosystem Downstream from Glen Canyon Dam, Arizona
Figure 4. Downstream 

patterns of sediment concentration and biomass of 

Cladophora (a type of algae) 

and macroinvertebrates along 

the Colorado River ecosystem. 

Modified from Carothers and 

Brown (1991).

Relative

density

Cladophora

Macroinvertebrate

(Gammarus, midges, snails, annelid worms)

Increasing turbidity

with downstream flow

Decreasing density

Colorado River

Kanab

Creek

Paria 

River

Glen

Canyon

Dam

Little

 Colorado 

 River

Sediment input

SEDIMENT CONTENT

Clear

Low

Medium

High

Topping and others, 2007). The first major tributary that enters below Glen Canyon Dam is the 

Paria River (RM 0; fig. 4). The next major tributary is the Little Colorado River (RM 61; fig. 4). 

Algae biomass (Carothers and Brown, 1991; Stevens and others, 1997) and production (Hall 

and others, 2010) decrease as water clarity decreases downstream (table 1; fig. 4). The progressive decrease in algae production drives a downstream decrease in aquatic invertebrate biomass 

(Gammarus, midges, snails, annelid worms; fig. 4; Carothers and Brown, 1991; Stevens and 

others, 1997; Kennedy and Gloss, 2005; Rosi-Marshall and others, 2010). Aquatic invertebrates 

are a high-quality food resource consumed by every species of fish present in the Colorado River 

(Valdez and Ryel, 1995; McKinney and Speas, 2001; Kennedy and Gloss, 2005). Increasing 

turbidity caused by increasing suspended-sediment concentrations also affects some fish populations, particularly rainbow and brown trout by influencing their ability to forage and detect food 

resources that are becoming scarce (Yard and others, in press). 

 The fish assemblage in the Colorado River below Glen Canyon Dam changes predictably with 

water temperature and water clarity along the downstream gradients (Makinster and others, 2010). 

Immediately below the dam, in the Lees Ferry reach (fig. 1), the Colorado River supports a rainbow 

trout sport fishery (Makinster and others, in press). Low water temperatures in the tailwater reach 

are close to ideal for rainbow trout spawning and growth (McKinney and others, 2001). The quality 

of overall rainbow trout habitat decreases precipitously below the Little Colorado River (Makinster 

and others, 2010). In the reach between Glen Canyon Dam and the Little Colorado River, the 

aquatic food web is fuelled almost exclusively by microscopic algae (Angradi, 1994; Shannon 

and others, 1994), invertebrate biomass is high (Stevens and others, 1997), and rainbow trout 

completely dominate the fish catch (Makinster and others, 2010).

Native flannelmouth sucker, bluehead sucker, and nonnative common carp dominate reaches 

far downstream from Glen Canyon Dam that are warm and turbid (Makinster and others, 2010). 

Main-stem water temperatures that are close to optimal for spawning and growth of these species 

likely contribute to their dominance there (Gloss and Coggins, 2005). These bottom-feeding 

species are also well adapted to foraging on the river bed for scarce food items in turbid conditions 

(Gloss and Coggins, 2005). 

Tributaries also directly influence the fish assemblage in the main stem by supporting large, 

self-sustaining source populations of native and nonnative fishes. Humpback chub densities in the 

main-stem Colorado River are highest in the vicinity of its confluence with the Little Colorado 

Chapter 4—Biological Responses to High-Flow Experiments at Glen Canyon Dam 99
River, in spite of water temperatures that are about 20 °F below optimal for their growth (Hamman, 

1982), because the Little Colorado River is their primary spawning habitat (Gorman and Stone, 

1999). A great deal of humpback chub juvenile rearing occurs in the Little Colorado River itself, 

but the quality of rearing habitat in the main-stem river at the confluence with the Little Colorado 

River probably also contributes to humpback chub adult recruitment (Gloss and Coggins, 2005). 

Large numbers of rainbow trout in the vicinity of the Little Colorado River confluence are of great 

concern to resource managers because rainbow trout are known to prey on humpback chub (Yard 

and others, in press). Dietary overlap between humpback chub and rainbow trout is also high—both 

eat predominantly aquatic invertebrates and algae (Valdez and Ryel, 1995; McKinney and Speas, 

2001)—so all of the conditions necessary for strong negative effects of competition on juvenile 

humpback chub that are rearing in the main stem also exist. The effects of competition and predation are inseparable—if competition for limited food resources leads a fish to spend more time 

foraging, that fish is more vulnerable to predation (Walters and Korman, 1999). The GCDAMP 

has long recognized the threats that competition and predation by rainbow trout pose to juvenile 

humpback chub rearing in the main stem near the confluence with the Little Colorado River 

(Minckley, 1991; Gloss and Coggins, 2005). In fact, more than 20,000 rainbow trout were experimentally removed from this reach of the Colorado River during 2003–06 to improve the quality 

of the rearing environment for juvenile humpback chub (Coggins, 2008; Coggins and Yard, 2010; 

Coggins and others, in press). 

Other tributaries and various aspects of geography also influence fish populations throughout 

Grand Canyon. For example, Bright Angel Creek (at RM 88, fig. 1) is the primary spawning and 

rearing habitat for nonnative brown trout (Gloss and Coggins, 2005). Densities of brown trout in 

the main-stem Colorado River are highest near Bright Angel Creek and decrease precipitously 

upstream and downstream from this tributary (Makinster and others, 2010). Large numbers of both 

native suckers (flannelmouth and bluehead) are known to spawn in Kanab Creek and Havasu Creek 

(fig. 1; Gloss and Coggins, 2005), and densities of these species in the Colorado River are high 

near these spawning grounds (Makinster and others, 2010). Proximity to Lake Mead and the large 

source populations of warm-water species that it supports (for example, striped bass Morone saxatilis and common carp) may also influence fish assemblages in the Colorado River far downstream 

from Glen Canyon Dam (Gloss and Coggins, 2005; Makinster and others, 2010).

High-Flow Experiments

Hydrology and the seasonal and daily patterns of discharges are significant aspects of the 

Colorado River’s physical template. Indeed, HFEs represent an attempt by resource managers 

to restore one aspect of the pre-dam flow regime—a dynamic flow pattern that featured annual 

floods—because providing “some of the dynamics of a natural system” might benefit native fish 

populations (U.S. Department of the Interior, 1995, p. 40). Additionally, HFEs are the only known 

means of creating backwaters through sandbar building. Backwaters are areas of low velocity in 

the lee of an eddy return-current channel (see chapter 3, this volume, for a description of backwaters and how they are formed by HFEs). Backwaters have at least some of the ingredients of 

high-quality native fish rearing habitat (for example, warm water temperatures and moderate levels 

of food resources; Brouder and others, 1999; Behn and others, 2010). An extensive fish capture 

dataset is available, documenting native fish use of these habitats (Brouder and others, 1999; Grams 

and others, 2010). 

Synthesizing the response of humpback chub and other fish populations to previous HFEs 

is complicated by at least two factors: (1) long and variable generation times (time required for a 

fish to become capable of reproduction), and (2) the numerous direct and indirect pathways that 

connect HFEs to fish populations. Because it takes humpback chub at least 4 years to reach maturity, it takes years for a change in dam operations to lead to a response in the adult humpback chub 

population trends that are monitored (monitoring of juveniles is relatively imprecise; Coggins and 

Walters, 2009). If the effects of an experimental management action do not persist for more than 

100 Effects of Three High-Flow Experiments on the Colorado River Ecosystem Downstream from Glen Canyon Dam, Arizona
1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010

Mean rainbow trout relative abundance

(catch per minute)

0

2

4

6 Figure 5. Rainbow trout 

mean relative abundance 

(catch per minute) in the Lees 

Ferry tailwater fishery, 1991–

2009. Figure represents data 

from all size classes in both 

fixed and random transects. 

Points represent the average 

among sites and seasons, and 

bars represent ± 2 standard 

errors of the average, an 

approximation for a 95-

percent confidence interval. 

See Makinster and others (in 

press) for details.

a year, or they are weak relative to other factors, then the effects are unlikely to be seen in the 

humpback chub adult recruitment trends (Coggins and Walters, 2009). Long generation times of 

humpback chub also makes it critical to consider their status prior to HFE implementation because 

prior conditions can affect the eventual response. Therefore, documenting the effects of a single 

HFE on humpback chub populations requires, at a minimum, long-term data documenting adult 

population trends. 

High-flow experiments can affect humpback chub both directly and indirectly, which further 

complicates the ability to understand whether the infrequent HFEs conducted to date have affected 

humpback chub. Downstream displacement and mortality of humpback chub during HFEs are 

examples of a direct effect that is negative. High-flow experiments create backwaters, so HFEs 

may also have indirect and positive effects on humpback chub populations through the creation of 

high-quality habitat. The creation of backwaters might also benefit nonnative fish species as well 

and, thus, would have an indirect and negative effect on humpback chub. Numerous other direct 

and indirect effects connecting HFEs to humpback chub have been hypothesized and studied 

(Shannon and others, 2001; Valdez and others, 2001). Understanding why an HFE affects humpback chub, therefore, requires primary research studies to evaluate the full array of direct and 

indirect effects that HFEs have on humpback chub. 

In this section, evidence for a population-level response of fishes to HFEs is discussed first. 

Data from the March 2008 HFE are the focus of these population-level assessments for three 

reasons: (1) a major change in annual release volumes during 1995–99 confounded and complicated interpretation of fish population response to the 1996 HFE (Shannon and others, 2001; 

Valdez and others, 2001), (2) methods currently used for monitoring fish below RM 0 were 

only established in 2000, so adult population data for rainbow trout and native fish in Grand 

Canyon are not available in association with the 1996 HFE (Makinster and others, 2010), and 

(3) essentially no primary biology research studies were conducted during the 2004 HFE, so it 

is not possible to assess the strength of direct and indirect pathways connecting that HFE to any 

population-level responses. After reviewing population-level responses to the 2008 HFE, the 

evidence supporting potential direct and indirect pathways linking HFEs to these fish species is 

evaluated. 

Rainbow Trout

Rainbow trout populations throughout Glen, Marble, and Grand Canyons increased dramatically in 2008 and 2009, dates for which the most recent data are available (figs. 5 and 6; Makinster 

and others, 2010). The 2008 rainbow trout cohort spawned in the Lees Ferry reach is the largest 

Chapter 4—Biological Responses to High-Flow Experiments at Glen Canyon Dam 101
0

100

200

300

Reach 1

River mile 0–56

0

15

30

45

60

75 All reaches

Mean catch per unit effort (fish per hour)

Reach 3

River mile 69.1–109

0

25

75

50

100

0

25

75

50

100 Reach 2

River mile 56–69

2000 2002 2004 2006 2008 2010

Reach 5

River mile >179

2000 2002 2004 2006 2008 2010

0

10

30

20

40 20

10

0

15

5

Reach 4

River mile 109.1–179

A B

C D

E F

on record (Korman and others, 2010; Korman and Melis, 2011; Korman and others, in press), and 

the 2009 cohort was also relatively strong compared to other years in the 20-year record (Korman 

and others, 2010; Makinster and others, in press; Korman and others, in press). Rainbow trout 

populations at the confluence of the Little Colorado River (RMs 56 to 69) were 800 percent larger 

in 2009 than in 2007 because of the large number of rainbow trout spawned in 2008 and 2009 

(fig. 6). It should be noted that rainbow trout populations also increased in 1997, the year after the 

March 1996 HFE (Makinster and others, in press). Collectively, these data indicate that springtimed HFEs benefit rainbow trout populations. 

Rainbow trout response to the one fall-timed HFE that was conducted in November 2004 is 

poorly understood. Rainbow trout populations began declining in 2001–2002 and continued to 

decline until 2007 (Makinster and others, in press). Thus, the November 2004 HFE occurred in 

the midst of a population decline that had started 2 years before. This preexisting downward trend 

limits our ability to make inferences regarding the influence of fall-timed HFEs on rainbow trout 

populations. 

The March 2008 HFE appears to have improved the quality of the spawning habitat used 

by rainbow trout in the Lees Ferry reach. Abundance of age-0 rainbow trout in July 2008 was 

more than four times greater than expected given the number of viable eggs that produced these 

fish (fig. 7). Expectations for the relation between viable eggs and age-0 trout were developed 

by quantifying the relation between egg abundance and juvenile rainbow trout among four 

pre-HFE years (Korman and others, 2010; Korman and Melis, 2011; Korman and others, in 

press). Comparison of the 2008 and 2009 data with these pre-HFE years (2003–04, 2006–07) 

Figure 6. Mean catch per unit 

effort (fish per hour) of rainbow 

trout captured during electrofishing surveys on the Colorado 

River between Lees Ferry and 

Lake Mead, 2000–2009. A, All 

reaches. B, Reach 1. C, Reach 2. 

D, Reach 3. E, Reach 4. F, Reach 

5. Points represent the average among sites and seasons, 

and bars represent ± 2 standard errors of the average, an 

approximation for a 95-percent 

confidence interval. From Makinster and others (2010).

102 Effects of Three High-Flow Experiments on the Colorado River Ecosystem Downstream from Glen Canyon Dam, Arizona
Adult rainbow trout 

(Oncorhynchus mykiss).

Copyright Eric Engbretson, 

used with permission.

0 0.0

0.4

0.8 2008

2009

2004

2010

2007

2006 2003

2 4

Eggs, in millions

Age-0 abundance in July,

in millions

6

demonstrates that spawning conditions improved in 2008. The 

amount of fine sediment (sand, silt, and clay) exported from the

Lees Ferry reach during the 2008 HFE was higher than in 2004 

and comparable to 1996, even though the 1996 HFE was 2.8 

times longer in duration and had slightly higher peak discharge 

than the 2008 HFE (Melis and others, in press). In fact, enough 

fine sediment was exported from this reach to roughly cover the 

entire 16-mile-long Lees Ferry reach to a depth of 1.6 inches 

(Scott Wright, U.S. Geological Survey, written commun., 2010), 

which undoubtedly increased the porosity and overall quality of 

the rainbow trout spawning habitat (Korman and others, 2010; 

Korman and Melis, 2011; Korman and others, in press). 

The growth and survival rates for juvenile fish that hatched 

after the 2008 HFE were also higher than expected (Korman and others, 2010; Korman and 

Melis, 2011; Korman and others, in press), indicating that the HFE improved the rearing environment for juvenile rainbow trout. Average growth rates of age-0 trout in the summer of 2008 

were virtually the same as in 2006, even though abundance was eight times greater in 2008. 

The growth of juvenile salmonids generally declines at higher densities (Jenkins and others, 

1999; Imre and others, 2005; Ward and others, 2007), so the unusually strong growth in 2008 

that occurred under high densities indicates that the quality of the rearing environment for age-0 

trout in the Lees Ferry reach was improved by the spring-timed HFE (Korman and others, 2010; 

Korman and Melis, 2011; Korman and others, in press). Interestingly, both age-0 growth and 

abundance in 2009 were also higher than expected given age-0 abundance and the number of 

viable eggs deposited in that year, which suggests that the effect of the 2008 HFE on early life 

stages persisted into 2009. Recent monitoring data suggest that the effect of the 2008 HFE on 

rainbow trout has subsided and did not persist into 2010 (fig. 7).

Increases in the amount of invertebrate prey available to rainbow trout in the Lees Ferry 

reach (Rosi-Marshall and others, 2010; Melis and others, in press) may be the ultimate cause 

of improvements in the rearing environment that was observed (Korman and others, 2010; 

Korman and Melis, 2011; Korman and others, in press). Rainbow trout consume food items that 

drift in the water column (Radar, 1997). Concentrations of invertebrate prey in drift increased 

following the 2008 HFE, and this effect persisted for at least 15 months (fig. 8; Cross and 

others, in press; Melis and others, in press). The increase in invertebrate drift that occurred after 

the 2008 HFE was due to large increases in the concentrations of chironomid midges and black 

flies (400 to 800 percent increase, depending on species and which post-HFE dataset is used for 

comparison; Melis and others, in press). Biomass and production of both species increased after 

the HFE (Rosi-Marshall and others, 2010; Cross and others, in press), suggesting the increases 

in drift that were observed were due to changes in abundance, but not drift vulnerability. This 

Figure 7. The relations between the number of viable rainbow trout eggs deposited in 

the Lees Ferry reach and the resulting population size of age-0 trout on July 15, 2003–09 

(no data collected in 2005). The thick black line shows the best-fit curve (relation) 

between the number of viable eggs and age-0 trout abundance using data from 2003 to 

2007 only. In general, this relation and similar ones for fish populations throughout the 

world suggest that at some point fish populations are not limited by the number of eggs 

produced, hence the constant juvenile abundance (flat line) once egg numbers exceed 

about 250,000. Conditions in 2008 and 2009 were much different and more favorable 

than pre-HFE years because the egg-juvenile data fall well above the flat line. Note 

that in 2010 the egg-juvenile relation again falls on this line, indicating that the positive 

effects of the 2008 HFE persisted for just 2 years. The vertical lines show the 95-percent 

confidence limits for the age-0 abundance estimates. From Korman and Melis (2011).

Chapter 4—Biological Responses to High-Flow Experiments at Glen Canyon Dam 103
Invertebrate drift concentration,

in milligrams per 1,000 liters

0.0

0.10

0.20

0.30

Pre-HFE Post-HFE1 Post-HFE2

is consistent with findings in other river systems—small-bodied and fast-growing species such 

as these often benefit from flood-type disturbances (Fisher and others, 1982; Robinson and 

Uehlinger, 2008; see text box, The Swiss Experience with High-Flow Experiments—Implications 

for the Management of Glen Canyon Dam). In addition, large quantities of these invertebrates are 

eaten by rainbow trout in other rivers (Radar, 1997). 

Direct negative effects of HFEs on rainbow trout are probably limited to displacement of 

eggs and young fry because larger fish (greater than 4 inches) do not appear vulnerable to downstream displacement (Hilwig and Makinster, 2010). Korman and others (2010) found indirect 

evidence that mortality of eggs and young fry was relatively high, probably due to scouring and 

downstream displacement. Spawning and emergence are strongly seasonal (Korman, 2009), so 

slight differences in HFE timing could have a dramatic effect on direct mortality of these vulnerable young life stages. 

 Humpback Chub

Humpback chub population dynamics cannot definitively be attributed to any of the three 

HFEs (fig. 9; Coggins and Walters, 2009). It is premature to make any definitive conclusions 

regarding the effect of the 2008 HFE on humpback chub adult recruitment because it takes chub 

at least 4 years to recruit into the adult population (Coggins and Walters, 2009). Nevertheless, it 

seems reasonable to anticipate that the March 2008 HFE may ultimately have a measureable negative effect on populations of adult humpback chub because of the substantial increase in rainbow 

trout populations that resulted from the HFE. 

Although the exact mechanisms underlying the trends in adult humpback chub abundance 

(fig. 9) remain unclear, evidence indicates that rainbow trout are a contributing factor; when 

rainbow trout populations are large, humpback chub populations generally decline, probably 

because of a combination of increased competition and predation. Numbers of adult humpback 

chub in 2009 were 50 percent greater than in 2002 (about 7,650 adult fish in 2009 as opposed to 

about 5,000 in 2002). However, data on rainbow trout populations in Marble Canyon and near the 

confluence with the Little Colorado River are only available beginning in 2000 (Makinster and 

others, 2010), so it is impossible to evaluate whether rainbow trout populations were affecting the 

long-term humpback chub population decline of the 1990s. 

The creation of sandbar-bounded backwaters is the primary hypothesized benefit of HFEs for 

humpback chub (U.S. Department of the Interior, 2008a, b). Data and analyses regarding juvenile 

humpback chub growth and survival among habitat types, including backwaters, have not been 

published (William Pine, University of Florida, unpub. data, 2010). Without these data, assessing 

the importance of backwaters in humpback chub rearing and adult recruitment can only be 

accomplished with a weight-of-evidence approach. Note that because cold water temperatures in 

the main stem restrict humpback chub spawning to the Little Colorado River and other tributaries, 

Figure 8. Average invertebrate drift concentrations (milligrams per 1,000 liters) from daytime collections at the downstream end of the Lees Ferry reach before and after the March 2008 HFE. PreHFE drift data were collected monthly between October 2007 and March 2008. Two different postHFE drift averages are presented: (1) Post-HFE 1 is the average of samples collected from October 

2008 to March 2009, which matches the exact seasonal timing of the pre-HFE dataset, and (2) PostHFE 2 is the average of samples collected from April 2008 to October 2009, which represents the 

entire post-HFE dataset. The vertical lines that extend above and below the top of the colored bars 

represent 95-percent confidence intervals (from Melis and others, in press). The concentration of 

invertebrate drift was significantly higher after the 2008 HFE relative to before, regardless of which 

post-HFE dataset is used for comparison; confidence intervals for the pre-HFE data do not overlap 

with either post-HFE set of data. From Melis and others (in press).

104 Effects of Three High-Flow Experiments on the Colorado River Ecosystem Downstream from Glen Canyon Dam, Arizona
Figure 9. Estimated adult 

humpback chub (Gila cypha) 

abundance (age 4+). Point 

estimates are average values, 

and error bars represent maximum and minimum 95-percent confidence intervals. 

From Coggins and Walters 

(2009). 

Adult humpback chub (Gila 

cypha). Photograph courtesy 

of George Andrejko, Arizona 

Game and Fish Department.

1990 1992 1994 1996 1998 2000 2002 2004 2006 2008

Adult abundance, in thousands

4

2

0

8

6

10

12

HFEs in the context of cold water temperatures will not lead to 

improvements in main-stem spawning habitat. 

The weight of evidence indicates that backwaters are not 

exceptionally high-quality rearing habitat for juvenile humpback 

chub compared to other potential rearing habitats. The availability 

of prey items (algae and invertebrates) in backwaters is comparable to their availability in other geomorphic features, such as 

rocky shorelines, that are more common and persistent than backwaters (Brouder and others, 1999; Behn and others, 2010; RosiMarshall and others, 2010), perhaps because daily fluctuations in 

discharge associated with changing demand for hydropower lead 

to frequent flushing and turnover of water in these habitats (Grand and others, 2006; Behn 

and others, 2010). On average, water completely turns over six times per day in backwaters 

when discharge fluctuates daily, compared to twice per day when discharge is stable (Behn and 

others, 2010). Daily fluctuations in discharge also lead to a rapid decrease in backwater area 

(Grams and others, 2010). In addition, backwater occurrence is low and areas are small near 

the confluence with the Little Colorado River where the majority of juvenile humpback chub 

main-stem rearing occurs (Grams and others, 2010). 

The effects of HFEs downstream from the Little Colorado River do not appear to be as 

dramatic as changes that were observed in invertebrate biomass or production in the Lees Ferry 

reach (Shannon and others, 2001; Rosi-Marshall and others, 2010). Invertebrate biomass and 

production are approximately 10 times greater in the Lees Ferry reach relative to invertebrate 

biomass and production downstream from the Little Colorado River (Rosi-Marshall and others, 

2010), so trends in invertebrate populations are easier to measure and quantify in the Lees Ferry 

reach. Potential benefits of the 2008 HFE on invertebrate biomass and production will likely be 

short lived because turbidity resulting from suspended sediment delivered by tributaries such 

as the Paria River represents a major constraint on the production of algae on which invertebrates rely (Stevens and others, 1997; Hall and others, 2010). Tributary flooding and suspended 

sediment inputs may mask or prevent a long-term and sustained invertebrate response to HFEs, 

similar to what was observed in the Lees Ferry reach, from ever occurring. This is not to say 

that HFEs do not have beneficial effects on the invertebrate prey base at downstream locations. 

Indeed, research from segments of the Colorado River upstream from Lake Powell suggests that 

natural floods increase production of food items (algae and invertebrates) by moving fine sediment off the bed and onto the shoreline (Osmundson and others, 2002), because sandy unstable 

substrates generally support lower densities of algae and invertebrates than hard and stable 

Chapter 4—Biological Responses to High-Flow Experiments at Glen Canyon Dam 105
substrates (see text box, Channel Change in the Glen Canyon Reach). Any benefits of HFEs along 

downstream segments in Grand Canyon likely will be smaller in magnitude than were observed in 

the Lees Ferry reach, which means potential changes will be more difficult to detect statistically, 

and benefits to fish populations will also be smaller.

The 2008 Biological Opinion (U.S. Department of the Interior, 2008a) considers direct 

mortality of humpback chub one of two primary mechanisms linking HFEs to humpback chub 

(the other is the creation of backwaters). During HFEs, the shoreline habitat that young humpback chub occupy changes (Korman and others, 2004; Grams and others, 2010), and water 

velocities can exceed the swimming ability of humpback chub (Korman and others, 2004; 

Protiva and others, 2010). On the basis of laboratory investigations, it is known that juvenile 

humpback chub (about 4 inches in length) become fatigued much sooner when water temperatures are cool (57 °F) as opposed to when water temperatures are warm (68 °F; U.S. Department 

of the Interior, 2008a). If juvenile humpback chub are assumed to be completely vulnerable to 

habitat changes and higher velocities associated with an HFE (that is, the fish do not move to 

occupy lower velocity habitat with the onset of flooding, although this is an untested assumption), a single HFE could result in the mortality of as many as 900 juveniles (U.S. Department 

of the Interior, 2008a). 

To mitigate the potential effects of downstream displacement and mortality, it has been 

suggested that future HFEs could be conducted in the spring when juvenile humpback chub 

rearing in the main stem would be larger relative to the fall (U.S. Department of the Interior, 

2008a). Increases in humpback chub size due to the growth that would occur between November 

(preferred HFE timing based on tributary sand inputs) and March, it was reasoned, could help 

compensate for the poor-swimming performance that would be expected with overall cold mainstem water temperatures (U.S. Department of the Interior, 2008a). However, the temperature of 

water released from Glen Canyon Dam peaks in the fall (Vernieu and others, 2005; Voichick and 

Wright, 2007), so delaying HFEs until the spring to mitigate potential displacement mortality 

could actually have the undesired effect of reducing swimming performance (and potentially 

increasing mortality) due to colder water temperatures; water temperatures at Lees Ferry in 

November 2004, when an HFE was conducted, were 56 °F, but temperatures were just 47 °F in 

March 2005 (Voichick and Wright, 2007). 

Other Native Fish

The catch-rate data indicate no evidence of a population-level response by flannelmouth 

sucker, bluehead sucker, or speckled dace to the two HFEs (2004 and 2008) conducted since 

continuous fish monitoring activities began in 2000 (figs. 10 and 11; Makinster and others, 2010); 

however, a lack of focused research studies on these species makes this conclusion highly uncertain. Because these species of fish dominate downstream reaches where rainbow trout are less 

abundant than near the Little Colorado River, the rainbow trout population increase following 

the 2008 HFE appears to pose less of a risk to other native fish relative to humpback chub. 

Populations of both species of sucker underwent periods of substantial expansion beginning about 

2005 (figs. 10 and 11; Makinster and others, 2010). Data needed to evaluate potential mechanisms 

underlying this increase and also potential links between these fish and HFEs are lacking, because 

there have been no focused research studies in the tributaries that serve as important spawning 

and rearing habitat for these species (for example, the Little Colorado River, Kanab Creek, and 

Havasu Creek; Minckley and Deacon, 1991). 

High-flow experiments do not appear to represent a major source of direct mortality to 

native fish, but the lack of data makes evaluating this relation difficult. Suckers and speckled dace 

dominate downstream reaches (figs. 10 and 11) where water temperatures are seasonally warmer 

relative to near the Little Colorado River confluence (Voichick and Wright, 2007). Swimming 

performance at the water temperatures present along downstream reaches is relatively high (Ward 

and others, 2003). 

106 Effects of Three High-Flow Experiments on the Colorado River Ecosystem Downstream from Glen Canyon Dam, Arizona
0

2

4

Reach 1

River mile 0–56

0

10

20

All reaches

A B

Mean catch per unit effort (fish per hour)

Reach 3

River mile 69.1–109

0

4

12

8

0

8

4

Reach 2

River mile 56–69

C D

2000 2002 2004 2006 2008 2010

Reach 5

River mile >179

2000 2002 2004 2006 2008 2010

0

10

20

30

0

40

20

Reach 4

River mile 109.1–179

E F

Native fish (excluding humpback chub) could be spawning in the main-stem river because 

they are commonly found in downriver sections where water temperatures are suitable for 

spawning (Makinster and others, 2010); however, very little is known about sucker or speckled 

dace spawning in the Colorado River and its tributaries. It is impossible, therefore, to assess 

whether HFEs affect the dynamics of these species through improvements in spawning habitats. 

Backwaters created during HFEs may represent an important improvement in the quality of 

rearing habitat available for suckers and dace because these fish dominate downriver sections of 

the Colorado River that experienced large and comparatively persistent increases in backwater 

area after the 2008 HFE, at least relative to the Little Colorado River confluence (Grams and 

others, 2010). Suckers, in particular, have been captured in backwaters in extremely high numbers 

(Hoffnagle and others, 1999; Grams and others, 2010). Focused research studies similar to the 

GCDAMP’s nearshore ecology project (a conservation measure associated with the 2008 HFE 

that seeks to understand the relative importance of backwaters to overall humpback chub rearing 

and is scheduled from 2009 to 2012) will be necessary to determine whether backwaters are an 

important rearing habitat for other native fish. 

High-flow experiments do not appear to lead to large or persistent improvements in available 

food at downriver locations (Shannon and others, 2001; Rosi-Marshall and others, 2010). For 

additional information, refer to the discussion in the humpback chub section. 

Figure 10. Mean catch per 

unit effort (fish per hour) of 

flannelmouth sucker (Catostomus latipinnus) captured during electrofishing surveys on 

the Colorado River between 

Lees Ferry and Lake Mead, 

2000–2009. The bars represent ± 2 standard errors of 

the mean, a close approximation of 95-percent confidence 

intervals. From Makinster and 

others (2010).

Chapter 4—Biological Responses to High-Flow Experiments at Glen Canyon Dam 107
0

1

2 Reach 1

River mile 0–56

0

2

4

All reaches

A B

Mean catch per unit effort (fish per hour)

Reach 3

River mile 69.1–109

0

4

8

0

8

4

Reach 2

River mile 56–69

C D

2000 2002 2004 2006 2008 2010

Reach 5

River mile >179

2000 2002 2004 2006 2008 2010

0

4

8

0

4

6

2

Reach 4

River mile 109.1–179

E F

Figure 11. Mean catch per unit 

effort (fish per hour) of bluehead 

sucker (Catostomus discobolus) 

captured during electrofishing 

surveys on the Colorado River 

between Lees Ferry and Lake 

Mead, 2000–2009. The bars 

represent ± 2 standard errors of 

the mean, a close approximation of 95-percent confidence 

intervals. From Makinster and 

others (2010).

Synthesis 

The strong rainbow trout response to the March 2008 HFE was not predicted or anticipated 

by resource managers or scientists (U.S. Department of the Interior, 2008a, b). A similarly counterintuitive response occurred with flow stabilization (a decrease in the daily range in discharge 

from the dam) that was implemented beginning August 1991 and was intended to benefit native 

fish populations, but it actually had the opposite effect (Pine and others, 2009). Before 1991, 

discharge from Glen Canyon Dam varied considerably over the course of a day due to changing 

demand for hydroelectricity. Restrictions were placed on hydroelectricity generation in 1990 and 

yet humpback chub populations declined for about 12 years after these constraints were in place, 

with populations only leveling off in about 2002 (Coggins and Walters, 2009; Pine and others, 

2009). This same change in dam operations proved to be beneficial to rainbow trout recruitment. 

Although stocking continued until 1998, the Lees Ferry sport fishery went from being completely 

dependent on stocking to self recruiting and self sustaining in a matter of years (McKinney and 

others, 2001). 

These kind of counterintuitive prediction failures are an indication that resource managers 

and scientists may have failed to identify key processes or variables when devising and developing management options to be evaluated as part of the adaptive management process (Pine and 

others, 2009). In Grand Canyon, these prediction failures may have occurred because the importance of geomorphology and flow regimes to humpback chub populations has been overestimated, 

108 Effects of Three High-Flow Experiments on the Colorado River Ecosystem Downstream from Glen Canyon Dam, Arizona
Figure 12. Path diagram depicting some of the influences that Glen Canyon Dam and its operations have 

on rainbow trout (Oncorhynchus mykiss) and humpback chub (Gila cypha) populations. Line width denotes 

the strength of scientific evidence for a given interaction; thick lines indicate effects that are well supported 

by data whereas thin lines are not well supported by 

data. Positive effects are shown as solid lines, and 

negative effects are shown as dashed lines. Glen 

Canyon Dam has a strong and direct negative effect 

on humpback chub due to its effect on water-release 

temperature. In contrast, this same alteration of water 

temperature has a strong and positive effect on rainbow trout populations. Spring high-flow experiments 

appear to have strong positive effects on rainbow 

trout due to improvements in spawning gravels and 

available food. Spring high-flow experiments may have 

positive effects on humpback chub due to increases in 

backwater habitat or increases in available food, but 

these effects are not well supported by data. Likewise, 

spring high-flow experiments may have a negative 

effect on humpback chub due to downstream displacement and mortality of juveniles, but this is not well supported by data either. The weight of evidence indicates 

humpback chub populations are affected by rainbow 

trout populations; when rainbow trout populations are 

large, humpback chub populations generally decline, 

probably due to a combination of increased competition and predation. Note that many other potential 

interactions that are weak or not supported by data are 

left out of this schematic. Studying strong pathways 

connecting high-flow experiments to populations of 

rainbow trout and humpback chub would be a logical 

focus for future research and monitoring efforts. 

Glen Canyon Dam

High-flow experiments (spring)

Pathway positive, well supported by data

Pathway positive, not well supported by data

Pathway negative, well supported by data

Pathway negative, not well supported by data

Prey base Habitat

Rainbow trout Humpback chub

Shifts in

prey species 

Cleansing

spawning

gravels

Backwater

creation

Predation

competition

Displacement

mortality

Displacement

mortality

Water

temperatures

Water

temperatures

Adaptive management Adaptive management

while the overriding influence that water temperature has on humpback chub has been underestimated (Olden and Naiman, 2009; fig. 12). Stabilizing discharge and conducting HFEs were intended 

to improve two aspects of the physical template (geomorphology, specifically backwaters, and flow 

regime), but a different aspect of the physical template (water temperature) may severely constrain 

and limit humpback chub populations, such that improvements in geomorphology and flow regime 

are unlikely to elicit a positive response. In the context of cold water, the potential beneficial effects 

of HFEs on humpback chub appear to be weak and short lived. Furthermore, if these benefits exist, 

they appear to be completely overwhelmed by the negative effects of a large and persistent increase 

in nonnative rainbow trout. Thus, HFEs in the context of cold water do not appear to be a useful tool 

for sustaining humpback chub populations in Grand Canyon. 

Chapter 4—Biological Responses to High-Flow Experiments at Glen Canyon Dam 109
110 Effects of Three High-Flow Experiments on the Colorado River Ecosystem Downstream from Glen Canyon Dam, Arizona

Descriptions of Selected Fish Species in Grand Canyon

Text by Richard A. Valdez and illustrations by Joseph R. Tomelleri

Humpback chub (Gila cypha)

(Native)

Endemic to Colorado River Basin. Federally listed as endangered. Maximum size 16–18 inches, 21⁄2 pounds, with prominent fleshy hump behind head and large fins. Occurs as six 

populations, including largest of about 7,650 adults in 

Grand Canyon (Coggins and Walters, 2009). Warmwater species that reproduces at 61–72 °F 

(Hamman, 1982). Matures at 3–4 years of age 

and lives to 40 years. Eats primarily drift and 

bottom aquatic and terrestrial invertebrates, 

algae, plant material, and small fish (Valdez and others, 

2001). Nine aggregations in Grand Canyon, including the 

largest at the mouth of the Little Colorado River. Most spawning 

occurs in the Little Colorado River, but main-stem spawning is 

suspected, as evidenced by young found in Middle Granite 

Gorge and near warm springs approximately 30 miles upstream 

from the Little Colorado River (Andersen and others, 2010).

Flannelmouth sucker (Catostomus latipinnis)

(Native)

Indigenous to Colorado River Basin. Maximum size 30 inches, 

4 pounds, with prominent fleshy mouth. Occurs in most middle 

and lower elevation rivers and large tributaries. Warm-water 

species that reproduces at 48–64 °F (Weiss and others, 1998). 

Matures at 3–4 years of age and lives to 20 years. 

Eats primarily bottom vegetarian, benthic 

invertebrates, algae, organic detritus, and 

seeds. Found throughout Grand Canyon with 

concentrations at tributary mouths, including 

Little Colorado River and Bright Angel, Shinumo, Havasu, 

and Kanab Creeks. Evidence of successful reproduction in 

tributaries; main-stem spawning is suspected in western 

Grand Canyon (Weiss and others, 1998; Makinster and others, 

2010).
Chapter 4—Biological Responses to High-Flow Experiments at Glen Canyon Dam 111

Bluehead sucker (Catostomus discobolus)

(Native)

Indigenous to Colorado River Basin. Conservation species in range-wide 

plan. Maximum size 16 inches, 1 pound, with hardened cartilaginous 

mouth ridges. Warm-water species that reproduces and grows best at 

59–68 °F. Matures at 1–3 years of age and lives to 18 years. 

Uses mouth ridges to scrape periphyton, debris, 

benthic invertebrates, and plant materials from 

submerged rocks. In Grand Canyon, locally 

common to abundant from the Little Colorado 

River to Lake Mead inflow with concentrations in and around 

tributary mouths, including the Little Colorado River and 

Shinumo, Havasu, and Kanab Creeks. There is evidence of 

successful reproduction in tributaries in Grand Canyon (Maddux and 

Kepner, 1988).

Razorback sucker (Xyrauchen texanus) 

(Native)

Endemic to Colorado River Basin. Federally listed as endangered. 

Maximum size 36 inches, 13 pounds, with hardened cartilaginous dorsal 

ridge behind head and large fleshy mouth. Historically, found in 

middle and lower elevation rivers, tributaries, and flood-plain 

habitats. Presently found in small numbers in 

rivers and reservoirs. Warm-water species that 

reproduces and grows best at 54–64 °F. Matures 

at 1–3 years of age and lives to 44 years. Young 

feed on zooplankton (cladocerans, copepods, and rotifers), 

juveniles consume algae and bottom ooze, and adults eat 

immature mayflies (Baetidae), stoneflies (Plecoptera, 

Protonemoura), and midges (Chironomidae), and algae and detritus (Tyus, 

1998). Historically, small numbers reported from Grand Canyon but none 

reported since early 1990s (Makinster and others, 2010). Recently found 

spawning at the Colorado River inflow area to Lake Mead (Bureau of 

Reclamation, 2010). 

Speckled dace (Rhinichthys osculus)

(Native)

Native to Colorado River Basin. Maximum size 5 inches, with distinct 

body speckles and black face mask. Found 

throughout western North America in cool 

mountain streams, medium and large rivers, 

small impoundments, and small isolated 

desert springs. Warm-water species that reproduces and grows best at 52–66 °F. Matures at 2 years of age 

and lives to 5 years. Young feed on mid-water zooplankton and algae, 

and juveniles and adults are bottom dwellers and feed on benthic insects 

or plant material (Carlander, 1969; Scott and Crossman, 1973). 
112 Effects of Three High-Flow Experiments on the Colorado River Ecosystem Downstream from Glen Canyon Dam, Arizona

Rainbow trout (Oncorhynchus mykiss)

(Nonnative)

Introduced as sport fish into cold rivers, streams, and reservoirs of the 

Colorado River Basin, including Grand Canyon. Maximum size 30 inches, 

20 pounds, with distinct black body spots and small fleshy fin behind 

dorsal fin. Native to Pacific coast streams, but stocked in cold waters 

throughout North America. Typically spawn in late winter and early 

spring at temperatures of 43–55 °F with optimum of 50 °F. 

Matures at 3–5 years of age and lives to 12 years. 

Adults and juveniles feed mainly on aquatic and 

terrestrial invertebrates, and large adults may eat 

mostly fish. First introduced into Grand Canyon by 

National Park Service at Tapeats Creek in 1923 

(National Park Service, unpub. report, 1932). Tailwater of Glen 

Canyon Dam was stocked regularly by Arizona Game and Fish 

Department from 1964 to mid-1990s. Natural reproduction in late winter 

and early spring supports self-sustaining population and sport fishery 

(Gloss and Coggins, 2005). Currently the primary species for the blue 

ribbon trout fishery below Glen Canyon Dam and locally abundant 

between Lees Ferry and the Little Colorado River and in tributary inflows 

at Nankoweap, Bright Angel, Tapeats, and Deer Creeks (Makinster and 

others 2010).

Brown trout (Salmo trutta)

(Nonnative)

Introduced as sport fish into cold rivers, streams, and reservoirs of the 

Colorado River Basin, including Grand Canyon. Maximum size 36 inches, 

25 pounds, with distinct large black spots and small red spots 

surrounded by blue halos; small fleshy fin behind dorsal fin. 

Native to Europe, but stocked in cold waters 

throughout North America. Typically spawn in fall 

and early winter at 45–54 °F, with optimum of 52 °F. 

Matures at 3–5 years of age and lives to 15 years. 

Aggressive and highly predaceous; consumes other 

fish early in life; also eats insects, amphibians, and fish eggs. First 

introduced into Grand Canyon by National Park Service at Shinumo 

Creek in 1926 and shortly after at Bright Angel Creek. Last reported 

stocking in 1934 (National Park Service, unpub. report, 1932). Presently in 

Bright Angel, Shinumo, and Tapeats Creeks and the main stem primarily 

near Bright Angel Creek inflow and occasionally in Glen Canyon Dam 

tailwater (Makinster and others, 2010).
Chapter 4—Biological Responses to High-Flow Experiments at Glen Canyon Dam 113

Red shiner (Cyprinella lutrensis)

(Nonnative)

Introduced as bait fish into reservoirs of the Colorado River Basin. 

Maximum size 3 inches, with dark red fins in spawning males. Native to 

mid-west and southern United States. Spawns March through June in 

backwaters, small riffles, and crevices over a variety of substrates, 

including fine gravel, boulders, logs, brush, roots, and aquatic 

vegetation at temperatures of 59–86 °F (Carlander, 1969). 

Present from Glen Canyon Dam to Diamond Creek 

into mid-1960s, but by 1968, only in small numbers 

above Lake Mead (Gloss and Coggins, 2005). 

Currently found in small numbers near warm 

tributary inflows and in backwaters (Grams and others, 

2010). Occurs in large numbers in Lake Mead inflow (Makinster 

and others, 2010). Feeds on planktonic algae, crustaceans, and benthic 

invertebrates, but is highly competitive, aggressive, and is predaceous on 

young fish in isolated habitats (Bestgen and others, 2006).

Fathead minnow (Pimephales promelas)

(Nonnative)

Introduced as bait fish into various waters of the Colorado River Basin. 

Maximum size 3 inches, with enlarged head and dark bands on 

adult males. Native to central North America. First 

introduced to lower Colorado River in about 1940 

from Lake Mead area bait shops, presumably 

moved into Grand Canyon shortly after. Presently 

absent or uncommon in main-stem Colorado River 

above Little Colorado River but locally common or 

abundant in warm tributary inflows, nearshore habitats, backwaters in lower Grand Canyon and upper Lake Mead (Makinster and others, 

2010; Grams and others, 2010). Mature in 4–5 months and live 2–3 years. 

Fractional spawner May to August at 61–86 °F with optimum of 77 °F. A 

rapid colonizer that can survive in isolated pools with low oxygen and poor 

water quality. Feeds on algae, detritus, and small aquatic invertebrates 

(Carlander, 1969).

Common carp (Cyprinus carpio)

(Nonnative)

Introduced into warm rivers, streams, ponds, and reservoirs of the 

Colorado River Basin. Maximum size 48 inches, 100 pounds, 

with large scales, mouth barbells, and serrated 

dorsal spine. Matures at 2–4 years of age and 

lives to 20 years. Native to Asia. Imported to 

United States in mid to late 1800s and stocked into 

lower Colorado River in the late 19th century 

(Mueller, 2005). Presently found throughout Grand Canyon 

downstream from Lees Ferry. Spawns May to June at 64–86 °F with 

optimum of 73 °F. Eats variety of foods, including algae, seeds, and other 

plant matter and invertebrates. Efficient at finding and vacuuming small 

fish and eggs from substrate. 
114 Effects of Three High-Flow Experiments on the Colorado River Ecosystem Downstream from Glen Canyon Dam, Arizona

The Swiss Experience with High-Flow Experiments—Implications for the 

Management of Glen Canyon Dam?

Richard A. Valdez, Christopher T. Robinson, and Theodore S. Melis

Planning is currently underway by the U.S. Department 

of the Interior’s Bureau of Reclamation for a long-term 

experimental plan that will implement a structured, long-term 

program of experimentation at Glen Canyon Dam over the 

coming decade. The program will include criteria for triggering repeated high-flow experiments (HFEs) on the basis 

of relatively frequent sand inputs to the Colorado River from 

tributaries below the dam (see chapter 5, this volume).

If the experimental triggering protocol suggested by sediment scientists is adopted, then the frequency of HFEs 

during a future Glen Canyon Dam experiment is anticipated 

to be annual or possibly more frequent in some years. In 

all likely cases, the HFEs intended to conserve limited sand 

supply below the dam will include flows from the dam’s 

hydropower plant in combination with varying magnitude 

bypass releases. As planning continues, results from other 

experimental river programs where repeated high-flow dam 

releases have also been studied may be informative.

Nearly two-thirds of the electricity produced in Switzerland 

comes from hydropower. It is estimated that over 80 percent 

of Switzerland’s hydroelectric potential has already been 

exploited. With few suitable dam sites remaining, developing 

the remaining 10 to 20 percent of hydroelectric potential will 

have increasingly greater environmental consequences and 

costs, although climate change is expected to increase precipitation in Europe and provide greater flexibility for hydropower 

production. The Swiss Federal Institute of Environmental 

Science and Technology (EAWAG) has developed criteria for 

“green labeling” hydropower production (Bratrich and others, 

2004). The so-called “Green Hydro” concept establishes ecological criteria for certification that generally are consistent with 

the concept of “environmental flows,” which are designed to 

sustain select or key ecological and societal values (Acreman 

and Ferguson, 2010). By 2004, 13 Swiss facilities had successfully passed the certification procedure, producing a total of 186 

gigawatts (GW) of green electricity per year, which is sufficient 

to power almost 40,000 households. Although this green energy 

is produced by hydropower, it is important to know that hydropower causes other ecological effects on regulated rivers.

To test whether an annual artificial flood (hereafter, high-flow 

experiment or HFE) regime can improve the ecology of regulated 

rivers below dams, a program of experimental high flows was 

implemented on the Spöl River in southeastern Switzerland in 

2000 (see map) in cooperation with the Swiss National 

Park and the Engadiner Kraftwerke Power Company 

(Scheurer and Molinari, 2003). The Spöl River flows through 

a confined channel surrounded by mountainous terrain 

(see photographs on opposite page). About 20 separate 

high flows have been released between 2000 and 2010 

from Punt dal Gall Dam that forms Livigno Reservoir on 

the Swiss-Italian border (see figure at end of text block).

A comprehensive study of the ecological effects of these 

repeated HFEs on the Spöl River was initiated in 1999 

(Robinson and Uehlinger, 2003). The study included aquatic 

food production (periphyton, stream metabolism, and 

benthic macroinvertebrates; Robinson and others, 2003; 

Uehlinger and others, 2003), brown trout (Salmo trutta; 

Ortlepp and Mürle, 2003), longitudinal patterns (Jakob and 

others, 2003), and river morphology and riparian vegetation (Mürle and others, 2003). A major finding from these 

studies that may be relevant to a long-term Glen Canyon 

The study area on the Spöl River. The main study reach is 

located about 1.4 miles downstream from Punt dal Gall Dam. 

The dashed lines show the tunnel infrastructure for water 

transport for hydropower production. This infrastructure allows 

the frequently repeated high-flow experiments to be a no-cost 

management strategy for the hydropower company. Modified 

from Scheurer and Molinari (2003).

Punt dal Gall Dam

Punt Periv

Water tunnel

Pradella

Power

Station

Spöl River

Livigno Reservoir

Inn River

ITALY SWITZERLAND

0 5 Miles

0 5 Kilometers
Chapter 4—Biological Responses to High-Flow Experiments at Glen Canyon Dam 115

Dam HFE protocol was that one or two high-flow events 

per year can enhance and sustain the long-term ecological 

integrity of the river (Scheurer and Molinari, 2003) and that 

flow releases must be repeated on a regular basis (annually) 

to maintain their benefits (Robinson and Uehlinger, 2008).

The HFEs in the Spöl River scour and immediately reduce 

primary producer biomass, which then recovers quickly 

after each event (Uehlinger and others, 2003), similar 

to the effects of natural floods observed in unregulated 

rivers. Importantly, the high flows shifted primary aquatic 

food producers from a moss-dominated streambed to 

one of diatoms and filamentous algae. There was also a 

longitudinal downstream effect of scouring on periphyton 

and benthic organic matter as a consequence of the 

degree of disturbance with generally greater effects in 

the most upstream reaches (Jakob and others, 2003). 

After the first 3 years of HFEs, periphyton assemblages 

now resemble those of unregulated Alpine mountain 

rivers where a snow and ice-melt-driven flow pulse 

results in a typical seasonal pattern of low periphyton 

biomass in summer and high biomass in autumn.

The HFEs significantly reduced macroinvertebrate densities, 

although densities typically recovered to pre-experiment 

levels within a matter of weeks (Robinson and Uehlinger, 

2008). Some taxa have decreased in abundance since 

beginning the experimental program, including amphipods 

(Gammaridae) and flatworms (Turbellaria), whereas others 

have increased in abundance, including mayflies (Baetidae), 

midges (Chironomidae), and stoneflies (Plecoptera, 

Protonemoura). Other taxa, such as blackflies (Simuliidae), 

caddisflies (Trichoptera), and mayflies (Heptageniidae) 

that were negatively affected by high flows in 2000, have 

subsequently increased in abundance. The sequence of 

annual high flows imposed on this regulated river revealed 

that the response of macroinvertebrates to repeated 

HFEs occurs over a period of years rather than months, as 

species composition shifts to the new and more variable 

habitat template. The Swiss results showed that the highflow experimental regime must be maintained if resource 

managers wish to sustain the development of a more natural 

macroinvertebrate assemblage, especially if it is important 

in maintaining a food web to support native brown trout.

The flow experiments on the Spöl River were intended to 

improve the fisheries potential (brown trout) of the river within 

the Swiss National Park. Despite increased food resources 

after flow regulation, the reproduction and recruitment of 

brown trout had declined, primarily because spawning areas 

were greatly reduced by the clogging of coarse sediments 

(Ortlepp and Mürle, 2003). Trout abundance was not reduced 

by the high flows, and relatively few fish (less than 2 percent) 

were killed or stranded. The quality of fish habitat, spawning 

grounds in particular, has noticeably improved, even though 

food resources (macroinvertebrate composition) have 

changed since the experiments began. The results showed 

that the condition of trout in the Spöl River has remained 

relatively constant, but the number of redds have increased 

sixfold since initiation of the experimental program, which 

presumably translates to increases in fish recruitment.

One important finding of the high flows was the lack 

of extensive effects on riparian vegetation. Mürle and 

others (2003) found that a lack of flow disturbance on the 

Spöl River had allowed woody vegetation to develop on 

Left, the Spöl River at one of the study reaches (Punt Periv) under low-flow conditions (56 ft3/s). Right, during one of the high-flow 

experimental releases (1,483 ft3

/s) on July 5, 2000. Photographs courtesy of Urs Uehlinger.
116 Effects of Three High-Flow Experiments on the Colorado River Ecosystem Downstream from Glen Canyon Dam, Arizona

previously exposed gravel banks. Young trees developing on the river bank were only slightly affected by 

the high flows, suggesting that colonization of trees was 

not constrained by the experimental releases. Thus far, 

the number and duration of high flows have not been 

sufficient to restore former flood plains free of trees; 

however, the coverage (burial) of grass areas by sand 

and gravel partially created locations for pioneer plants. 

These effects on riparian vegetation may be related 

to the canyon-confined nature of the river valley in 

contrast to other rivers with more dynamic flood plains.

An evaluation of modified flow regimes worldwide 

(Murchie and others, 2008) and the experience of the Spöl 

River (Robinson and Uehlinger, 2008) reveal important 

issues in the evaluation of relatively frequent HFEs on river 

ecology. Study designs could include methods that target 

physical as well as all biological levels, including aquatic 

food producing organisms, fishes, and riparian vegetation. The most effective experimental designs also would 

provide sufficient data to allow for detailed statistical 

analyses to be performed. Rigorous study designs that 

include the use of appropriate controls and replicates are 

essential whenever feasible. Data on physical variables 

that respond to changes in flow can also be collected 

and examined to add explanatory power to results.

The HFEs on the Spöl River highlight some major effects 

of artificial floods on regulated river ecosystems. While 

the experiments affected the existing aquatic communities, recovery of primary and secondary producers 

The flow regime of the Spöl River (on the Swiss-Italian border) for 2 representative years 

before regulation, the year before the high-flow experimental program (1999) showing the 

typical seasonal regulated flow, and the flow after the experimental program, including each 

of the flow events. Data for 1999 and the years before regulation are daily values. Data are 

instantaneous values after 2000. Modified from Robinson and Uehlinger (2008). 

(periphyton and macroinvertebrates) was rapid after 

each flood. Biotic assemblages have shifted to the 

new habitat template of the river, but these changes 

in slower variables have occurred over years as novel 

biological thresholds were passed and new ecosystem 

states were established. Whether biotic assemblages 

return to pre-dam levels is unknown, but the frequently 

repeated high flows have clearly increased the physical 

dynamics of the river, which has been translated to 

changes in biotic assemblages. It also is unknown, 

however, if repeated and continued high flows over 

time will eventually restore much of the ecosystem 

structure and function to those of a typical mountain 

river. Without consistent monitoring, the gradual nature 

and timing of this shift might not have been apparent. 

The findings on the Spöl River may have general 

ecological application. Some of the learning that has 

occurred there is likely transferrable to the current Glen 

Canyon Dam HFE protocol planning process. However, 

it is clear that each river system is different because of 

its unique ecological setting and the specific manner in 

which each system is regulated. The Swiss experience 

clearly illustrates the benefits of cooperative efforts 

among stakeholders, especially power producers, water 

users, environmental groups, and State and Federal 

officials. The Spöl River experiment and other programs 

(Murchie and others, 2008) show how a good understanding of the relation of dam operations and resulting 

river flows to ecosystem components can help manage 

hydropower facilities to achieve large ecological benefits 

through relatively small modifications and costs.

Discharge, in cubic feet per second

Pre-regulation Post-regulation

1961 1962 2000 2002 2004 2006 2008 2010

0

706

1,412

2,119

1974–1999
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Ward, J.V., and Stanford, J.A., 1983, The serial discontinuity concept of lotic ecosystems, 

in Fontaine, T.D., III, and Bartell, S.M., eds., Dynamics of lotic ecosystems: Ann Arbor, 

Michigan, Ann Arbor Science Publishers, chap. 2, p. 29–42.

Weiss, S.J., Otis, E.O., and Maughan, O.E., 1998, Spawning ecology of flannelmouth suckers, 

Catostomus latipinnis (Catostomidae), in two small tributaries of the lower Colorado River: 

Environmental Biology of Fishes, v. 52, no. 4, p. 419–433.

Yard, H.K., Van Riper, Charles, III, Brown, B.T., and Kearsley, M.J., 2004, Diets of insectivorous birds along the Colorado River in Grand Canyon, Arizona: The Condor, v. 106, no. 1, 

p. 106–115, accessed December 28, 2009, at http://www.jstor.org/stable/pdfplus/1370520.pdf.

Yard, M.D., Coggins, L.G., Baxter, C.V., Bennett, G.E., and Korman, Josh, in press, Trout 

piscivory in the Colorado River, Grand Canyon—Effects of turbidity, temperature, and fish 

prey availability: Transactions of the American Fisheries Society.
Anne Phillips, U.S. Geological Survey
Lake Mead

Lake Powell

Paria Riv e r

Kanab Creek

Diamond Cr Havasu Cr

Bright Angel Cr

Glen Canyon Dam

GRAND CANYON

NATIONAL PARK

GLEN CANYON

NATIONAL RECREATION

AREA

Hoover Dam

Lees Ferry

Little Colorado River

0 20 40 miles

0 20 40 60 kilometers

UTAH

ARIZONA

NEVADA

Colorado

R i v e r

M

A R B L E C A N Y O N

Hualapai

Indian 

Reservation

Hopi

Indian 

Reservation

Havasupai

Indian 

Reservation

Navajo

Indian 

Reservation

Kaibab

Indian 

Reservation

Paiute

Indian 

Reservation

N

ARIZONA

Map

Area

CHAPTER 5

Science-Based Strategies for Future High-Flow 

Experiments at Glen Canyon Dam 

By Scott A. Wright1,2 and Theodore A. Kennedy2,3

As detailed in the preceding chapters of this report, the presence and operation of Glen 

Canyon Dam has drastically altered physical and biological processes along the 

Colorado River corridor downstream from the dam (fig. 1). These changes have led to 

a variety of effects on Colorado River fluvial geomorphology and biological resources in Grand 

Canyon. The Glen Canyon Dam Adaptive Management Program (GCDAMP) has conducted 

three high-flow experiments (or HFEs)—in 1996, 2004, and 2008—to benefit sandbar resources 

and increase learning about the role that HFEs play in the geomorphology and ecology of the 

Colorado River. The results of these HFEs are not reviewed in detail in this chapter; instead, the 

reader is referred to chapters 2–4 of this report as well as other recent review articles (Lovich 

and Melis, 2007; Melis and others, 2010). The purpose of this chapter is to provide a brief 

overview and synthesis of physical and biological findings from previous HFEs and, on the 

basis of key physical and biological findings, develop a strategy for future HFEs that seeks to 

1

 U.S. Geological Survey, California Water Science Center, 6000 J Street, Sacramento, CA 95819.

2

 Both authors contributed equally to the development of this chapter. 

3

 U.S. Geological Survey, Southwest Biological Science Center, Grand Canyon Monitoring and Research Center, 

2255 N. Gemini Drive, Flagstaff, AZ 86001.

Figure 1. The Glen Canyon 

Dam Adaptive Management 

Program study area includes 

the Colorado River corridor 

that stretches approximately 

293 river miles from the forebay of Glen Canyon Dam to 

the westernmost boundary of 

Grand Canyon National Park, 

Arizona.
Pre-HFE

During HFE

Post-HFE

River mile 30 River mile 45

Flow Flow

128 Effects of Three High-Flow Experiments on the Colorado River Ecosystem Downstream from Glen Canyon Dam, Arizona

achieve two goals: (1) benefit sandbar resources while maintaining neutral trends in key biological resources and (2) increase learning about important resource responses to HFEs. 

Physical Processes

The links between dam operations and geomorphology are direct and strong and, therefore, 

can be well understood. Two major effects on geomorphology are a reduction in sand supply due 

to trapping of sediments in Lake Powell, the reservoir formed by Glen Canyon Dam (fig. 1), and a 

reduction in natural flood peaks caused by multiyear water storage in Lake Powell. In the post-dam 

period, shoreline sandbar deposits are dependent on sand supplied from the tributaries downstream 

Figure 2. Repeat views looking across the Colorado River from the right shore of two sandbars in Marble Canyon 

before and after the March 2008 high-flow experiment (HFE). These examples are for illustrative purposes only (that 

is, these results are not necessarily representative of the system response); for detailed sediment-response studies, 

see chapter 2, this volume; Hazel and others (2010); Grams, Hazel, and others (2010); and Topping and others (2010). 

Flow direction is from left to right at both locations.

USGS
Chapter 5—Science-Based Strategies for Future High-Flow Experiments at Glen Canyon Dam 129

from the dam and occasional HFEs to redistribute sand from the river bottom to the shoreline and 

riparian corridor. With reduced sand supply and HFEs, which have been less frequent and smaller 

than natural floods, sandbars have persistently eroded during the post-dam period (chapters 2–3, 

this volume). Sandbars were a prominent geomorphic feature of the pre-dam river, and maintaining these features has been deemed desirable because they provide ideal campsites for river 

rafters (Hazel and others, 2010; Kaplinski and others, 2010), low-velocity shoreline habitat that is 

used by native fishes (hereafter referred to as backwaters; see chapters 2–4 for further description), 

and a source of sand that is transported upslope through eolian processes and may help preserve 

archaeological sites (Draut and Rubin, 2008; Draut and others, 2010). 

High flows from Glen Canyon Dam are the only known tool for rebuilding sandbars using the 

existing post-dam sand supply (fig. 2). Three HFEs have been released from Glen Canyon Dam, in 

1996, 2004, and 2008, as detailed in chapters 2–3 of this report. Although these HFEs were much 

smaller in magnitude and duration than typical natural pre-dam snowmelt floods in the spring 

(chapters 2–3, this volume), the HFEs were comparable in size to many natural pre-dam floods 

that resulted from rainfall events in the late summer and early fall. Numerous focused research 

projects were conducted during these HFEs, each resulting in a wealth of knowledge about key 

processes. For example, on the basis of research conducted during the 1996 HFE, the two subsequent HFEs were conducted under the general concept of sand-input “triggering” and in response 

to substantial inputs of sand from natural Paria River floods. The hypothesis that was evaluated 

in these two recent events was that HFEs conducted soon after new sand had been delivered by 

tributaries would enhance sand deposition on sandbars and minimize erosion of sand stored on the 

channel bed prior to the tributary inputs. Continuous suspended-sediment monitoring established 

in 2000 yielded the data needed to identify links between sand storage and sandbar building during 

HFEs, as well as sandbar erosion afterward (Grams, Hazel, and others, 2010; Hazel and others, 

2010; Topping and others, 2010; chapter 3, this volume). 

Three definitive conclusions that have important implications for designing future sedimentmanagement strategies can be drawn from these studies: 

• HFEs are effective at building sandbars by transferring sand from the channel bed to 

sandbars along the channel margins

• HFEs conducted soon after tributary-derived sand has accumulated on the channel 

bed are more effective at building sandbars and less likely to result in erosion of sand 

stored on the channel bed and in sandbars prior to the tributary inputs compared to 

HFEs conducted when sand is depleted

• Sandbars tend to erode quickly in the weeks and months following HFEs, depending 

on flow releases from the dam as well as ongoing tributary sand supply

Biological Processes

Glen Canyon Dam has directly contributed to the decades-long declines and local extinctions of native fish populations in Grand Canyon. For example, water storage in Lake Powell 

has substantially reduced summer water temperatures (Vernieu and others, 2005), which restricts 

native fish spawning and early rearing to tributaries or segments of the river that are far downstream from the dam (Gloss and Coggins, 2005). Humpback chub (Gila cypha), one of four native 

fish present in Grand Canyon and federally listed as endangered, now spawn almost exclusively 

in the Little Colorado River (fig. 1; Gorman and Stone, 1999). In addition, four other species of 

native Colorado River fish are no longer present in Grand Canyon (Gloss and Coggins, 2005). 

In contrast, cold water temperatures create ideal spawning and rearing conditions for nonnative 

rainbow trout (Oncorhynchus mykiss) in the tailwater below Glen Canyon Dam (Korman, 2009), 

and this reach of the river is managed as a sport fishery. Monitoring, research, and ecosystem 

modeling are beginning to identify some of the complex, indirect pathways linking dam 
130 Effects of Three High-Flow Experiments on the Colorado River Ecosystem Downstream from Glen Canyon Dam, Arizona

operations to native fish populations that are mediated by (linked to) changes in the distribution and 

abundance of rainbow trout.

Rainbow Trout

High-flow experiments conducted in the spring (1996 and 2008) have been associated with 

strong increases in rainbow trout populations in the Lees Ferry tailwater reach and downriver 

sections of the Colorado River in Grand Canyon (Makinster and others, 2010). The effect of the 

March 2008 HFE was particularly strong, resulting in the largest cohort, or fish born in the same 

year, of new rainbow trout on record (monitoring began in 1991; Makinster and others, 2010, 

Korman and Melis, 2011; Korman and others, in press). High-flow experiments apparently increase 

hatch rates for rainbow trout eggs because of improvements in spawning habitats (Korman and 

others, 2010; Korman and Melis, 2011; Korman and others, in press). Increases in growth and 

survival of young trout were also observed in 2008 (Korman and others, 2010; Korman and others, 

in press), likely due to improvements in available food (Rosi-Marshall and others, 2010; Cross and 

others, in press). 

The response of rainbow trout to the one fall-timed HFE that was conducted in 2004 is poorly 

understood. The November 2004 HFE did not benefit rainbow trout populations, but several factors 

limit the ability to understand why this occurred. Rainbow trout populations started declining in 

2001/2002 and continued to decline until 2007 for reasons that are unknown (Makinster and others, 

2010). Thus, the November 2004 HFE occurred in the midst of a population decline that had started 

2 years before. In addition, two studies that identified key indirect pathways linking major changes 

in adult rainbow trout to the 2008 HFE (detailed studies of rainbow trout eggs and juveniles, and 

food-web studies) were not conducted during 2004/2005. 

Humpback Chub

High-flow experiments have had no measurable positive effects on humpback chub populations (Coggins and Walters, 2009; chapter 4, this volume). The majority of humpback chub early 

juvenile rearing occurs in the Little Colorado River (Gorman and Stone, 1999), but the quality of 

main-stem rearing habitats is probably also important because large numbers of juvenile humpback 

chub are regularly captured in the vicinity of the Little Colorado River confluence (Valdez and 

Ryel, 1995; Gloss and Coggins, 2005). High-flow experiments were thought to benefit humpback 

chub through the creation of backwaters, a potentially high-quality rearing habitat formed in the lee 

of an eddy reattachment sandbar (see chapters 2–3, this volume). Daily fluctuations in discharge 

due to changing demand for hydroelectricity, however, lead to frequent flushing and turnover 

of water in backwaters (Behn and others, 2010), which likely limits the quality of backwaters 

as rearing habitats (Grand and others, 2006; Behn and others, 2010). These same fluctuations in 

discharge also lead to rapid reductions in the area and volume of this habitat type (Grams, Hazel, 

and others, 2010; Grams, Schmidt, and Andersen, 2010). High-flow experiments do not lead to 

substantial improvements in available food at downriver locations (Shannon and others, 2001; 

Rosi-Marshall and others, 2010). Because humpback chub spawning occurs predominantly in the 

Little Colorado River, HFEs also do not improve the quality of their spawning habitat. Collectively, 

these studies indicate that potential benefits of HFEs on the quality of rearing habitats used by 

humpback chub are weak and short lived. 

Although the exact mechanisms underlying trends in adult humpback chub abundance remain 

unclear, the weight of evidence indicates that rainbow trout are playing an important role; when 

rainbow trout populations are large, humpback chub populations generally decline, probably due to 

a combination of increased competition and predation (fig. 3; Coggins, 2008; Coggins and Walters, 

2009; Coggins and Yard, 2010; Coggins and others, in press; Yard and others, in press). Downriver 

migration of the large 2008 rainbow trout cohort spawned in the Lees Ferry tailwater reach, 

together with local recruitment along downriver sections, has contributed to an approximately 
2000

2001

2002

2003

2004

2005

2006

2007

2008

2009

1 fish = 250 individuals for the 12.5 miles of Colorado River near the Little Colorado River confluence

Rainbow trout Humpback chub

Mechanical removal

Mechanical removal

Mechanical removal

Mechanical removal

Chapter 5—Science-Based Strategies for Future High-Flow Experiments at Glen Canyon Dam 131

Figure 3. Changes in the abundance of rainbow trout and adult 

humpback chub in the Colorado 

River near the confluence with 

the Little Colorado River and the 

number of rainbow trout mechanically removed from this reach 

during 2003–06. The March 2008 

HFE created favorable spawning and rearing conditions for 

rainbow trout in the Glen Canyon 

Dam tailwater, which resulted in 

the largest cohort of rainbow trout 

on record (monitoring began in 

1991), which contributed to the 

800-percent increase in rainbow 

trout densities observed near the 

Little Colorado River since 2007. 

Rainbow trout densities in 2006 

were the lowest on record, in part 

because of mechanical removal of 

rainbow trout that occurred from 

2003 to 2006. Density estimates for 

rainbow trout are from Makinster 

and others (2010), number of rainbow trout removed is from Coggins (2008), and adult humpback 

chub abundance estimates are 

from Coggins and Walters (2009). 

Note: It is possible for the number 

of rainbow trout removed in any 

given year (for example, 2003) to 

exceed the estimated abundance 

of rainbow trout in that same year 

because of immigration of new 

individuals into the reach between 

the first removal trip of the year 

(January) and the last (September). 

800-percent increase in rainbow trout densities in the vicinity of the Little Colorado River since 

2007 (fig. 3; Makinster and others, 2010). This increase is of concern because the Little Colorado 

River population of humpback chub is the largest anywhere in the Colorado River Basin (Gloss 

and Coggins, 2005). In fact, over 20,000 rainbow trout were removed from this section of the 

river from 2003 to 2006 to reduce the threats that competition and predation pose to the persistence of humpback chub populations (Coggins, 2008; Coggins and others, in press). It is too early 

to draw any definitive conclusions regarding the effect of the 2008 HFE on humpback chub adult 

recruitment because it takes chub at least 4 years to recruit into the adult population (Kaeding and 

Zimmerman, 1983; Coggins and Walters, 2009). Nevertheless, it seems reasonable to anticipate 

the 2008 HFE may ultimately have a measureable negative effect on populations of adult humpback chub because of the substantial increase in rainbow trout populations that resulted from the 

HFE. The rainbow trout population increase is strong and persistent and represents a substan-
Figure 4. Path diagram depicting some of the influences 

that Glen Canyon Dam and its operations have on rainbow 

trout and humpback chub populations. Line width denotes 

the scientific evidence for a given interaction; thick lines 

indicate effects that are well supported by data whereas 

thin lines are not well supported by data. Positive effects 

are shown as solid lines and negative effects are shown as 

dashed lines. Glen Canyon Dam operations have a strong 

and direct negative effect on humpback chub due to its 

effect on water release temperature. In contrast, this same 

alteration of water temperature has a strong and positive 

effect on rainbow trout populations. Spring HFEs appear 

to have strong positive effects on rainbow trout due to 

improvements in spawning gravels and the prey base. Spring 

HFEs may have positive effects on humpback chub due to 

increases in backwater habitat or improvements in the prey 

base, but these effects are not well supported by data. Likewise, spring HFEs may have a negative impact on humpback 

chub due to downstream displacement and mortality of 

juveniles, but this is not well supported by data either. The 

weight of evidence indicates humpback chub populations 

are linked to rainbow trout populations; when rainbow trout 

populations are large, humpback chub populations generally 

decline, probably due to a combination of increased competition and predation. Note that many other potential interactions that are weak or not supported by data are left out of 

this schematic. Studying strong pathways connecting HFEs 

to populations of rainbow trout and humpback chub is a logical focus for future research and monitoring efforts. 

Glen Canyon Dam

High-flow experiments (spring)

Prey base Habitat

Rainbow trout Humpback chub

Shifts in

prey species 

Cleansing

spawning

gravels

Backwater

creation

Predation

competition

Displacement

mortality

Displacement

mortality

Water

temperatures

Water

temperatures

Adaptive management Adaptive management

Pathway positive, well supported by data

Pathway positive, not well supported by data

Pathway negative, well supported by data

Pathway negative, not well supported by data

132 Effects of Three High-Flow Experiments on the Colorado River Ecosystem Downstream from Glen Canyon Dam, Arizona

tial decrease in the quality of the rearing environment for humpback chub, while the potential 

beneficial effects of HFEs on the rearing environment appear weak and short lived (fig. 4). 

Three definitive conclusions that have important implications for the design of future sedimentmanagement strategies can be drawn from these studies:

• HFEs conducted in the spring benefit rainbow trout populations as a result of improvements in 

spawning and rearing habitat

• HFEs have had no measurable positive effects on humpback chub populations

• Large increases in rainbow trout populations near the Little Colorado River that occurred after 

the 2008 HFE are inconsistent with both the GCDAMP goals for humpback chub and rainbow 

trout, as well as native fish management objectives of Grand Canyon National Park

Other Resources 

The GCDAMP has established goals for several other Colorado River resources, including 

recreation (for example, rafting and fishing), riparian vegetation and spring ecosystems, cultural 

properties (for example, archaeological sites), the Kanab ambersnail (Oxyloma haydeni kanabensis), and water quality (for example, salinity). These other resources were not considered in the 

development of this HFE strategy because there are few definitive studies to guide HFE planning 

on the basis of these other resources. There are, however, direct or indirect pathways connecting 

dam operations to virtually every resource of the river. Thus, resources and pathways that appear 
Chapter 5—Science-Based Strategies for Future High-Flow Experiments at Glen Canyon Dam 133

linked to HFEs could continue to be studied so that information on these resources may help 

inform adaptive management. Additional monitoring and research data are necessary, however, 

before a meaningful analysis of the complex tradeoffs between sandbars, fish, recreation, archaeological sites, and other resources can be conducted.

Synthesis

High-flow experiments are an important tool for rebuilding sandbars. The three previous 

HFEs have demonstrated the effectiveness of individual HFEs for rebuilding sandbars, particularly when they occur after sand has been stored on the channel bed downstream from the dam. A 

logical next step in the adaptive-management process of the GCDAMP is to evaluate the cumulative effects of multiple HFEs over longer periods of time. Although many sandbars at high elevation were larger in October 2008 than in February 1996 (before the first HFE), an equal number of 

sites in Marble and eastern Grand Canyon were smaller. This finding reflects the uncertainty as to 

whether sandbar building during HFEs can offset or exceed the sandbar erosion that occurs during 

periods of typical dam operations between HFEs. Thus, it is important to consider the frequency 

of HFEs and the erosion of sandbars between HFEs for future HFE planning. The fundamental 

sandbar-related science question therefore is: 

Can sandbar building during HFEs exceed sandbar erosion during

periods between HFEs, such that sandbar size can be increased and

maintained over several years?

On the basis of studies that have been conducted to date, HFEs do not appear to be a tool that can 

be used to benefit humpback chub. Rainbow trout pose a threat to juvenile humpback chub rearing 

in the main stem near the confluence with the Little Colorado River because of increased competition and predation. Beneficial effects of the March 2008 HFE on rainbow trout populations appear 

to be largely responsible for the 800-percent increase in rainbow trout observed near the confluence 

between 2007 and 2009. A large increase in rainbow trout near the confluence with the Little Colorado 

River also occurred in the year following the 1996 HFE. The November 2004 HFE did not benefit 

rainbow trout populations, but a preexisting downward trend in rainbow trout populations and the 

absence of data make this finding highly uncertain. Thus, natural-resource managers might consider 

proceeding with caution when implementing any HFE strategies, particularly those involving frequent 

spring-time events, because currently the biological response to HFEs appears to be inconsistent with 

management goals for humpback chub. A logical next step in the HFE process is evaluating whether 

the seasonal timing of HFEs affects the rainbow trout recruitment response. If fall-timed HFEs do not 

lead to increases in rainbow trout populations near the confluence with the Little Colorado River (or it 

is later demonstrated that rainbow trout do not exert strong influence on humpback chub rearing), then 

managers might be able to balance goals for sandbars and native fish without the need for substantial 

rainbow trout mitigation or removal. The fundamental fish-related science question therefore is: 

Does the seasonal timing of HFEs influence the rainbow trout response?

An adaptive-management process for HFE decisionmaking would be flexible and incorporate relevant scientific information, such as near real-time information about sediment conditions 

downstream from the dam and information on adult population trends for rainbow trout and 

humpback chub, as well as other resources. Indeed, as more HFEs are conducted, strong links 

connecting other resources to dam operations may be identified and incorporated into subsequent 

HFE strategies. An integrated science-based strategy would allow for effective management of the 

available post-dam sand supply while considering the effects of the strategy on other resources 

within an adaptive-management framework. The objective of this chapter is to outline such a 

science-based strategy for future HFEs at Glen Canyon Dam. 
134 Effects of Three High-Flow Experiments on the Colorado River Ecosystem Downstream from Glen Canyon Dam, Arizona

Sand Accounting and High-Flow Experiments 

A useful analogy for sand storage in the Colorado River downstream from Glen Canyon Dam 

is a checking account. Deposits to the account come from natural tributary flooding that brings 

sand into the main stem, increasing sand storage and the account balance. Withdrawals occur 

continuously during typical dam operations (that is, export of sand downstream to Lake Mead), 

and the rate of the withdrawals depends on the volumes of water released from the dam and the 

pattern of the releases. Even though HFEs help build sandbars, they also cause a withdrawal 

because the higher flows export sand downstream. That is, there is a cost in terms of overall sand 

storage that must be paid in order for the process of sandbar building to occur. An important 

aspect of any sustainable HFE strategy is to attempt to remain budget neutral, such that the total 

withdrawals do not exceed deposits over the long term. A long-term sand-budget deficit would 

lead to persistent erosion of the remaining sand storage, thus making it progressively more difficult to rebuild sandbars. However, it may be desirable to have an initial negative sand budget for 

several years while the river adjusts to the effects of frequent HFEs; that is, the river may quickly 

evolve to have less sand in storage overall but with more sand at high elevation in sandbars. 

Although this may be acceptable, if the negative sand budget continues indefinitely, the strategy 

would not be sustainable.

The preceding analogy illustrates why HFEs cannot be considered in a vacuum. Sand-storage 

conditions are closely linked to sand supply from the tributaries downstream from the dam, basin 

hydrology, and dam operations (for example, Wright and others, 2008; Wright and Grams, 2010). 

Thus, all of these factors must be taken into consideration during HFE planning. The supply of 

sand from tributaries is highly variable with large quantities delivered during episodic natural 

flooding events. Sand storage in the main stem typically is greatest immediately following these 

natural tributary floods. The rate at which this “new” sand is exported from the river reaches of 

interest depends on basin hydrology as well as dam operations. Basin hydrology (for example, 

upper Colorado River Basin snowpack) and reservoir-storage conditions in Lake Powell and Lake 

Mead determine the required amount of water to be released from Glen Canyon Dam in a given 

year. Because sand-transport rates depend on river flow rates, more sand is exported during years 

with larger releases, and less sand is exported during years with smaller releases. Sand-export 

rates also depend on the seasonal and hourly patterns of water releases that typically are scheduled 

to meet hydroelectricity demand. As shown by Wright and Grams (2010), flows that fluctuate 

monthly and daily tend to export more sand than steady flows.

Because these various controls on sand storage are not predictable with a high degree of 

accuracy, it is not possible to determine far in advance whether or not a given month or year is 

the optimal time for an HFE. If sand-storage conditions are monitored continually, however, this 

information can be used in the decisionmaking process throughout the year. Numerical modeling 

tools could also be used within the planning process to help estimate sand-storage conditions and 

to design the optimal magnitude and duration of the HFE for a given event. Several tools have 

been developed for the reach of the Colorado River downstream from Glen Canyon Dam specifically to support future HFE planning. The most important of these is the sand-transport monitoring program that was implemented by the Grand Canyon Monitoring and Research Center in 

the early 2000s (Topping and others, 2010). This program provides high-resolution, near real-time 

measurements of sand transport in the major tributaries and at several main-stem sites. These data 

are currently used to estimate sand-storage conditions in the main stem and thus already provide 

much of the information necessary for HFE decisionmaking. Information from other monitoring 

and research programs, such as sandbar monitoring, can also be used to make adjustments as more 

is learned about how the river responds to repeated HFEs.

In addition to monitoring data, several modeling tools exist that can provide useful information for the decisionmaking process. For example, a numerical model of sand transport and 

storage (such as Wright and others, 2010) is currently being used to predict sand-storage conditions into the near future, both with and without an HFE. Also, because HFEs would be conducted 
Average monthly sand load, 

in thousand metric tons 300

400

100

200

0

July Sept

Summer/fall

accounting/HFE period

Paria River

Little Colorado River

Winter/spring

accounting/HFE period

Nov Jan Mar May

Chapter 5—Science-Based Strategies for Future High-Flow Experiments at Glen Canyon Dam 135

under a variety of sand-storage conditions, numerical models could be used to design the optimal 

HFE magnitude and duration to achieve the desired outcome. In a year with relatively high sand 

storage, a “larger” HFE may be warranted compared to a year with less sand storage. Wright and 

Grams (2010) demonstrated how the sand-storage model can be used together with a numerical 

model of flow releases (Wiele and Smith, 1996) to estimate sand-storage conditions for a range of 

dam operations. A similar approach would be useful for supporting HFE decisionmaking in near 

real time, as described in the following section.

Options for High-Flow Experiment Strategies

Numerous factors are considered in the design of an HFE strategy, such as timing, magnitude, duration, and frequency. The seasonal timing of HFEs is especially relevant given our 

understanding of biological resource responses. A range of possible options for HFEs that vary 

in frequency and by season is presented in table 1. The options in table 1 generally trend top 

to bottom from options based solely on “sand triggers” (that is, HFEs are tied to the amount of 

sand stored in the river) to those that are less dependent on sand storage but more dependent 

on seasonal timing (for example, HFEs occur every spring, which would approximate pre-dam 

patterns of natural floods). Each of these options has strengths and weaknesses and large uncertainties in outcomes, particularly in relation to biological resources. For example, option 1 would 

allow for HFEs any time of the year when there is sufficient new sand stored in the main stem of 

the Colorado River, whereas option 7 specifies a fall HFE every year regardless of sand storage in 

the river. 

Previous research indicates that HFEs that are tied to antecedent (preexisting) sand-storage 

conditions in the river (that is, sand-triggered HFEs) probably result in greater sandbar building 

and are less likely to contribute to long-term erosion of sand stored on the channel bed (chapter 3, 

this volume). Thus, options such as options 4, 6, and 7 (table 1) that include annual HFEs that are 

not linked to sand storage are unlikely to be as effective as sand-triggered HFEs at increasing and 

sustaining sandbar sizes. Also, because the majority of tributary-derived sand inputs occur in the 

summer and fall (fig. 5), sand-triggered fall HFEs are most likely to result in sustainable increases 

in sandbar sizes. Fall timing is considered in options 1, 2, and 3, but not in option 5, which only 

includes sand-triggered spring HFEs. Sand-triggered HFEs that occur only in the spring would 

be less likely than sand-triggered fall HFEs to result in sustainable increases in sandbar sizes. 

However, spring HFEs have been shown to increase (1) campsite area preceding the peak river 

recreation season (Hazel and others, 2010), (2) eolian sand transport prior to the windy season 

(Draut and others, 2010), and (3) rainbow trout population in the Lees Ferry reach. Also, because 

occasional large tributary sand inputs do occur in the winter and spring, spring HFEs are a potentially useful tool for sandbar building. Thus, because both fall and spring HFEs have potential 

Figure 5. A year with two 

sand-budget accounting 

periods and two HFE windows 

(fall and spring). Vertical axis 

shows the average monthly 

sand loads from the major 

tributaries. The presence of 

two main periods of tributary 

activity supports the concept 

of two accounting/HFE periods 

per year.
Table 1. Various options for high-flow experiments (HFEs). The options in this table generally 

trend from top to bottom, from options based solely on “tributary sand-input triggers” (HFEs 

are tied to the amount of sand stored in the river) to those that are less dependent on sand 

storage but more dependent on seasonal timing.

Option 

number HFE timing and frequency HFE magnitude and duration

1 Immediately following sand inputs, if sand 

storage in the river reaches the trigger level

Based on sand storage conditions in 

the river before the HFE

2 Fall and spring, if sand storage in the river 

reaches the trigger level

Based on sand storage conditions in 

the river before the HFE

3 Fall, if sand storage in the river reaches the 

trigger level

Based on sand storage conditions in 

the river before the HFE

4 Every spring; fall, if sand storage 

in the river reaches the trigger level

Spring HFEs unknown because not 

based on sand storage conditions in 

the river before the HFE; fall HFEs 

based on sand storage conditions

5 Spring, if sand storage in the river reaches 

the trigger level

Based on sand storage conditions in 

the river before the HFE

6 Every spring

Unknown because not based on sand 

storage conditions in the river before 

the HFE

7 Every fall

Unknown because not based on sand 

storage conditions in the river before 

the HFE

136 Effects of Three High-Flow Experiments on the Colorado River Ecosystem Downstream from Glen Canyon Dam, Arizona

positive and negative resource responses, an adaptive strategy that includes options for sandtriggered fall and spring HFEs is most likely to result in sustainable increases in sandbar sizes and 

contribute information about other resource responses. In addition, this strategy is consistent with 

pre-dam flow data, which indicate that natural floods comparable in magnitude to previous HFEs 

occurred in both the fall and spring (fig. 6). 

From a purely sediment perspective, the scientific evidence suggests that a strategy based on 

sand-triggered HFEs that combines options 1 and 2 (table 1) would be a logical starting point for 

future experimentation, as described in more detail below. Option 1 would allow for sand-triggered 

HFEs any time of the year, depending on tributary sand inputs, with the possibility of multiple HFEs 

occurring in a given year. Option 2 is similar in that HFEs would also be sand triggered, the difference being that HFEs would only occur at certain times of the year (fall and spring, with the potential 

for two HFEs in a given year). Without additional research, it is difficult to determine which of these 

approaches would be most effective for long-term, sustainable sandbar building. Option 1 is the most 

proactive for managing new tributary-derived sand inputs but would likely lead to more frequent, 

smaller HFEs because sand inputs would not be allowed to accumulate. Option 2 is less proactive 

but allows for potential accumulation of multiple tributary-derived sand inputs and, thus, HFEs with 

larger peaks and longer durations. The most effective strategy for the purpose of sandbar building 

likely would be a combination of these approaches. Option 1 may be more effective during years 

when release volumes are high because the residence time of new sand in the river would be shorter, 

justifying the need to act quickly to manage the resource. During years of lower release volumes, 

it may be beneficial to allow for sand accumulation during the tributary input season with an HFE 

occurring following multiple inputs. This hybrid approach is outlined in more detail below through 

specific questions about HFE timing, triggering, peak and duration, and frequency. 
160

120

80

40

0

Flow at Lees Ferry,

in thousand cubic feet per second

Oct

Flows that exceed 

1.5 times the box length

Median flow for a given month

Flow exceeded 25 percent of the time

Flow exceeded 75 percent of the time

Nov Dec Jan Feb Mar Apr May June July Aug Sept

Chapter 5—Science-Based Strategies for Future High-Flow Experiments at Glen Canyon Dam 137

An HFE strategy that combines options 1 and 2 would also assist scientists in better understanding the effects of HFEs on biological resources, such as humpback chub and rainbow trout. 

For example, HFEs conducted thus far (through 2008) do not appear to have benefited humpback 

chub populations likely because the HFEs benefited nonnative rainbow trout that compete with 

humpback chub. Because mechanical removal of rainbow trout is costly (costs have averaged 

$150,000 per removal trip (Runge and others, 2011)), it would be useful to determine whether 

seasonal timing of HFEs affects rainbow trout response. If rainbow trout response can be minimized, for example by conducting HFEs at a time of year other than in the spring, this would 

greatly reduce the cost of rainbow trout mitigation efforts. A strategy that combines options 1 and 

2 would allow scientists to study rainbow trout population response to HFEs conducted in the fall, 

which currently is not well known. 

Strategy Based on Sand-Triggered High-Flow Experiments 

A strategy for HFEs that is based on sand triggers would consider the amount of sand storage 

in the river prior to HFEs, and HFEs would only be scheduled once sand storage reached a 

threshold (the “trigger level”). The design (magnitude and duration) of the HFE would be based 

on the sand-storage conditions in the river prior to the HFE.

What time of year would high-flow experiments occur?

An HFE strategy that combines options 1 and 2 (table 1) would result in HFEs that could 

occur essentially any time of year or only at the end of the fall or spring sand-accumulation 

periods. Such a strategy would be consistent with previous HFEs that occurred during both fall 

(November 2004) and spring (March 1996 and 2008). The fall and spring are logical “windows” 

for HFEs on the basis of the seasonal pattern of sand supply from the tributaries (fig. 5). Natural 

tributary flooding typically is greatest in the late summer and early fall, and thus the summer/fall 

accounting/HFE period (July–December) is most advantageous for managing the sand that enters 

the main stem. The specific timing of HFEs within this period would need to be evaluated on a 

year-to-year basis with incorporation of new information as it becomes available. For example, if 

Figure 6. The pre-dam flow regime on the Colorado River at Lees Ferry (data from Topping and 

others, 2003). The plot shows box-and-whisker diagrams for each month of all instantaneous 

flow measurements from the beginning of the record (1921) to the beginning of flow regulation 

by Glen Canyon Dam (1963). The plot illustrates the strong snowmelt signal from April to July as 

well as the higher flows in the late summer and early fall.
138 Effects of Three High-Flow Experiments on the Colorado River Ecosystem Downstream from Glen Canyon Dam, Arizona

flow releases from the dam under standard operating procedures are expected to be low through 

the fall, it likely would be most advantageous to wait until the end of the accounting/HFE period 

(fig. 5) to allow for the accumulation of tributary-derived sand inputs. However, if flow releases 

are expected to be high, it may be more advantageous to conduct HFEs immediately following, or 

even during, tributary input events (Lucchitta and Leopold, 1999). Use of a numerical model to 

predict possible outcomes prior to an HFE would be valuable for evaluating these scenarios.

Natural tributary flooding can also occur during the winter and spring (fig. 5), and thus a 

winter/spring accounting period (January–June) is also justified. The strategy would be identical 

for this period as for the summer/fall period. The same general guidelines for dry as opposed to 

wet periods would apply, with HFEs likely being more effective at the end of the period during 

low-flow conditions. A more proactive approach, however, may be justified during wet conditions because of the short residence time of new tributary sand under high-flow conditions (for 

example, figure 4B in Rubin and others, 2002).

How would high-flow experiments be “triggered”?

All HFEs would be triggered on the basis of tributary sand inputs and resultant sand storage 

along the main-stem Colorado River. During the accounting periods (fig. 5), HFEs would be 

triggered and designed on the basis of recent tributary-derived sand inputs and sand storage. 

For example, if summer and fall tributary sand inputs were small, then a fall HFE would not be 

conducted. High-flow experiments would only be conducted when enough new sand supply could 

be documented such that a short-duration powerplant-capacity release (about 31,000 cubic feet per 

second (ft3/s)) could be conducted without exporting more sand from Marble Canyon (fig. 1) than 

had accumulated during the preceding accounting period (that is, a net positive sand budget would 

be maintained for Marble Canyon). Thus, the two accounting periods would be treated separately 

for HFE-triggering purposes. By requiring maintenance of a net positive sand budget for each 

accounting period, HFEs would be conducted with the goal of avoiding erosion of sand stored on 

the channel bed prior to the accounting period. The magnitude and duration of the HFEs would be 

designed on the basis of the sand-storage conditions in each accounting period, as described in the 

next section. Existing near real-time monitoring data and numerical modeling tools could be used to 

evaluate the effect of HFEs on the sand budget for a range of potential magnitudes and durations.

What high-flow experiment magnitude and duration would be used?

The magnitude and duration of HFEs would be designed on the basis of sand storage in the 

river. In years with large tributary sand inputs and low-volume water releases, sand storage is 

likely to be greatest and would allow for HFEs with higher peaks and longer durations. Longerduration HFEs with larger sand supply should result in increased sandbar building. In other years, 

when sand is stored in the river but the amounts are not as large, an HFE with a lower peak and 

shorter duration would be appropriate. The overarching goal would be to maximize sand transport 

during higher magnitude HFEs for the purpose of sandbar building, without exporting more sand 

than is available in order to avoid erosion of the sand stored on the channel bed at the end of the 

previous accounting period. As with the trigger evaluations, near real-time data and numerical 

models could be used to support the decisionmaking process in designing any long-term experiment of repeated HFEs intended to optimize sandbar rebuilding and maintenance.

Because the magnitude and duration of the HFE would be tied to sand-storage conditions 

in the main-stem river, it is not possible to specify HFE details far in advance. Also, additional 

experimentation is necessary to determine the most effective magnitudes and durations of HFEs 

for building sandbars. Previous HFEs had peak flows of 42,000–45,000 ft3

/s and durations of 

7 days (1996) and 2.5 days (2004 and 2008). To facilitate the proposed adaptive strategy for future 

HFEs, it would be desirable to experiment with a broader range of peak flows and durations. In 

the past, several powerplant-capacity (about 31,000 ft3/s) flows have also been released from the 
Chapter 5—Science-Based Strategies for Future High-Flow Experiments at Glen Canyon Dam 139

dam. Although it is known that these flows are not nearly as effective at building sandbars as 

the larger HFEs (chapter 3, this volume), further experimentation would be required to evaluate 

their effectiveness in a long-term, multiple HFE strategy. The strategy outlined in this chapter 

would allow for short-duration powerplant-capacity releases, at least initially, when sand-storage 

conditions in the river do not support larger peaks and longer durations. The maximum peak and 

duration would depend on sand-storage and water-supply conditions. Testing of peak flows greater 

than 45,000 ft3

/s is scientifically justified, but is constrained by current low reservoir levels such 

that the spillways at Glen Canyon Dam are inaccessible. Higher peak flows could be considered in 

the future if reservoir levels permit. As outlined in the section How would high-flow experiments 

be “triggered”?, the maximum duration would be constrained so as not to export more sand from 

Marble Canyon than is available. On the basis of the sand-transport data from the 2004 and 2008 

HFEs (Topping and others, 2010), most HFEs would warrant durations of only a few days or less. 

How frequently would high-flow experiments occur?

The frequency at which future HFEs would occur is difficult to estimate because it would be 

based on future tributary sand supply, hydrologic conditions in the basin, and dam operations; there 

is uncertainty and unpredictability regarding all of these conditions. A rough estimate of frequency, 

however, can be derived from historical data. Such an estimate requires information about tributaryderived sand inputs and sand-export rates downstream to Lake Mead (fig. 1) during and between HFEs. 

Long-term records of Paria River sand inputs (1925–2008, see Topping and others (2010) and Topping 

(1997) for methods) suggest an average sand supply during the summer/fall accounting period (July–

December) of about 900,000 metric tons with about 300,000 metric tons on average during the winter/

spring (January–June) accounting period. Both periods experience substantial variability in tributary 

sand inputs from year to year. The rate of sand export downstream to Lake Mead is also highly variable 

depending on hydrologic conditions in the basin and dam operations. During the period 2002–09, for 

which sand-transport monitoring data are available, the average sand-export rate from Marble Canyon 

(fig. 1) was about 250,000–300,000 metric tons for each accounting period. Thus, if 2002–09 can be 

considered representative of future conditions (an uncertain and likely incorrect assumption because 

flow releases were relatively low during this period, but nonetheless useful for illustrative purposes 

here), the summer/fall accounting period would be expected to have substantial sand accumulation (inputs of about 900,000 and export of about 300,000 metric tons), whereas the winter/spring 

accounting/HFE period would not (inputs and export both about 300,000 metric tons). The smallest 

HFE that would be conducted, a powerplant-capacity release, would likely export 200,000 metric 

tons of sand or less (depending on HFE duration) on the basis of the releases in May and September 

2000, which exported about 340,000 and 220,000 metric tons, respectively, from Marble Canyon 

(Schmidt and others, 2007). These releases had peak durations of about 3 days (Schmidt and others, 

2007); thus, the total export could be reduced substantially by reducing the duration of the release. 

These approximate numbers suggest that fall HFEs would be triggered frequently, nearly every year, 

on the basis of historical averages of sand accumulation, but that spring HFEs would be triggered much 

less frequently, for example, only in years when winter/spring tributary-derived sand inputs are well 

above average. However, these historical average conditions do not tell the whole story because of the 

variability in tributary inputs. This variability would almost certainly result in multiyear periods when 

HFEs would not be justified on the basis of sand-trigger considerations, as well as multiyear periods 

when one or two HFEs per year would be justified. The actual frequency of HFEs would vary with 

future tributary sand supply, future hydrologic conditions in the basin, and future dam operations. 

Little Colorado River Contingencies

The HFE-trigger strategy described above is based on sand storage in Marble Canyon, which 

primarily depends on sand supplied by the Paria River (fig. 1). This approach would be scientifically 

justified because Marble Canyon is the reach most at risk of progressive erosion because it is closest 
140 Effects of Three High-Flow Experiments on the Colorado River Ecosystem Downstream from Glen Canyon Dam, Arizona

to the dam and thus has the smallest post-dam sand supply (not including Glen Canyon, see discussion 

in chapter 3, this volume). However, it would be desirable to have contingencies built into the strategy 

by also considering the Little Colorado River (fig. 1), which joins the main stem at the lower end of 

Marble Canyon, because it is the other major sand-supplying tributary downstream from the dam. 

In many instances, natural flooding on the Little Colorado River occurs simultaneously with natural 

flooding on the Paria River. In cases where the Marble Canyon trigger is close to being met, it may be 

useful to consider recent Little Colorado River sand inputs in the decisionmaking process. Also, if an 

extremely large natural flood were to occur on the Little Colorado River, for which there is historical 

precedent (the area drained by the Little Colorado River is about 18 times larger than that of the Paria 

River), the effects of such a natural flood should be taken into consideration. Under this scenario, it 

may be possible to time an HFE release to coincide with natural flooding on the Little Colorado River 

as a means of producing a higher HFE peak downstream from the confluence (Lucchitta and Leopold, 

1999). In order to take advantage of such a situation, it may be deemed acceptable to conduct an HFE 

that results in a slightly negative sand budget for Marble Canyon. 

Decision Flow Chart

An example flow chart of the decisionmaking and implementation process for the strategy 

described above is shown in figure 7. The chart illustrates the flow of information through time for 

1 year. Phase numbers are shown in the squares in figure 7.

Phase 1—Summer/fall accounting period

Beginning on July 1 each year, currently available sand-transport monitoring data would be used to track 

tributary-derived sand inputs to, and sand-storage conditions in, Marble Canyon.

Phase 2—Summer/fall HFE deliberations

Once tributary-derived sand inputs occur, deliberations could begin to determine if and when to conduct a fall 

HFE. These deliberations would include evaluating water forecasts (that is, what are the flow releases from 

the dam expected to be in the near future) and conducting numerical modeling simulations. On the basis of 

these analyses, a decision would be made regarding the most appropriate HFE timing for the current year. If 

tributary-derived sand inputs are small throughout the accounting period, no HFE would be conducted. 

Phase 3—Summer/fall HFE implementation

If the decision is made to conduct an HFE, this phase would consist of further refining the magnitude and 

duration of the HFE and conducting scientific studies in association with the HFE.

Phase 4—Winter/spring accounting period

As during the summer/fall accounting period, sand-transport monitoring data would be used to track the 

sand-storage conditions in Marble Canyon. If a fall HFE occurred, the sand budget would be reset to 

zero. If no fall HFE occurred, then the sand-budget information from the summer/fall accounting period 

would be carried forward to the winter/spring accounting/HFE period.

Phase 5—Winter/spring HFE deliberations

The decisionmaking process in this phase would be identical to that of phase 2; that is, water forecasts 

and numerical model simulations would be used to evaluate the most appropriate HFE timing, if tributary-derived sand inputs are large enough to support a spring HFE.

Phase 6—Winter/spring HFE implementation

This phase would be identical to phase 3.
Revise strategy

1

2

3

4

5

6

Change strategy to mitigate

unacceptable resource trend(s)

Summer/fall

Monitor sand budgets,

update models

Summer/fall

Evaluate HFE trigger

and potential timing

Summer/fall

Refine hydrograph,

conduct HFE

Winter/spring

Monitor sand budgets,

update models

Trigger met

Trigger met

Winter/spring

Evaluate HFE trigger and

potential timing

Winter/spring

Refine hydrograph,

conduct HFE

Annual status check

Evaluate status of sandbars, 

native fish, and other resources

Resource

trend(s)

unacceptable

Resource trends 

desirable or acceptable

Chapter 5—Science-Based Strategies for Future High-Flow Experiments at Glen Canyon Dam 141

Annual Status Check and Strategy Revision

As described above, an integrated and science-based adaptive strategy would allow for 

effective evaluation of flow options for managing the available post-dam sand supply while 

considering the effects of the strategy on biological, physical, and cultural resources within an 

adaptive-management framework. This adaptive approach is incorporated in the flow chart (fig. 7) 

through an annual status check of river resources and a subsequent step that allows for revisions 

of the strategy. The annual status check would consist of reviewing available monitoring data and 

scientific interpretations. If any resource trends are deemed to be unacceptable by natural-resource 

managers (for example, rainbow trout response), other HFE implementation options could be 

considered (some examples are presented in table 1), and (or) mitigation strategies could be 

devised (for example, removal of nonnative trout). Such an approach also allows for new information to be incorporated into the strategy, even if resource trends are considered acceptable. 

Adaptive Management, Monitoring, and Research

Because there is substantial uncertainty about the outcome that may result from implementation of the outlined HFE strategy, other reasonable options are available (table 1). In particular, the 

biological responses to HFEs conducted in the fall, and to a lesser extent those conducted in the 

spring, are difficult to predict. Thus, modification of the strategy may be required on the basis of 

knowledge gained from biological responses to future HFEs. High-flow experiments appear to be a 

critical tool for managing the post-dam sand supply to rebuild and maintain sandbars, so if a strong 

rainbow trout response occurs following HFEs in the fall, it may be necessary to explore whether 

dam operations between HFEs could be changed as a means of mitigating rainbow trout response to 

Figure 7. The decisionmaking

process for a science-based 

experimental strategy for tributary sand-input-triggered HFEs 

with two sand-budget accounting periods and two HFE windows per year. Each box and 

decision point is described in 

detail in the text.
142 Effects of Three High-Flow Experiments on the Colorado River Ecosystem Downstream from Glen Canyon Dam, Arizona

HFEs. Modification of the strategy in response to sandbar-monitoring results may also be required, 

and a different HFE strategy may be justified during wet and dry climatic periods. Also, if sandbars 

erode too quickly after HFEs are released, it may be necessary to explore whether altering dam 

operations in between HFEs (for example, Wright and others, 2008; Hazel and others, 2010; Wright 

and Grams, 2010) or augmenting the post-dam sand supply (Randle and others, 2007), or both, would 

allow natural-resource managers to meet goals for sand-based river resources. Because of these 

uncertainties, the annual “status check” would be a critical component of an adaptive HFE strategy. 

This status check could include reviewing recent monitoring data for sand budgets, sandbar size, 

and native and nonnative fish population trends. On the basis of the findings of these reviews, the 

HFE strategy could be adapted, if necessary, to incorporate new knowledge and learning. Adaptive 

management could only be achieved, however, by integrating a robust monitoring and research 

program into a science-based strategy. 

Robust monitoring and research projects supported by the GCDAMP have allowed scientists 

to develop a clear understanding of the pathways linking previous HFEs to the key river resources 

of sandbars and rainbow trout populations. Implementation of an HFE strategy on the basis of 

sand-input triggers requires continued monitoring of tributary-derived sand inputs to, and sand 

transport in, the main stem, so that sand budgets can be computed for the evaluation of triggers. 

Sandbar and sand-storage monitoring are also required to evaluate the cumulative long-term influence of HFEs and dam operations between HFEs in support of adaptive management. Other more 

process-based studies may also be warranted, for example daily surveys of sandbars during HFEs 

to improve numerical modeling tools, such as eddy-sandbar simulations.

Continued monitoring of humpback chub, rainbow trout, and other fish populations is critical to 

understanding how these important resources are affected by repeated HFEs. Process-based studies 

focused on evaluating whether rainbow trout response is affected by HFE timing would be a logical 

focus for future biological research (see figure 3). Food-base response might be strongly affected by 

HFE timing because of seasonal differences in recovery times of organisms, whereas cleansing of 

gravels by flushing of fine sediments may benefit survival of rainbow trout eggs and fry, even if HFE 

cleansing of spawning gravels occurs in fall, months prior to initiation of spawning (Topping and others, 

2010; Melis and others, in press). Research on the ecology of nearshore habitats near the Little Colorado 

River is critical for better understanding the complex interactions between flow regimes and habitat use 

by humpback chub and rainbow trout and other nonnative fish in Marble and Grand Canyons. 

The links between HFEs and cultural resources warrant further study. More information is 

needed to evaluate the tradeoffs between, for example, potential lost revenue from guided fishing 

as opposed to improved experience due to increased trout populations. Future HFE strategies could 

include studies of recreation economics. Eolian-transport studies were started in 2001 and documented only weak benefits of the November 2004 HFE on transport rates relative to the March 2008 

HFE. The weak response following the November 2004 HFE, however, was at least in part driven 

by the larger experimental fluctuating flows evaluated for rainbow trout suppression during January 

through March 2005 (Korman and Melis, 2011), which quickly eroded the sandbars that provide a 

source of windblown sand during the spring. Thus, continued eolian-transport monitoring would 

be useful, particularly following fall HFEs without the large experimental fluctuations that occurred 

during the winter of 2005 (with more typical dam operations following the fall HFE).

Importance of Dam Operations Between High-Flow Experiments 

In this chapter a possible strategy for future HFEs at Glen Canyon Dam that incorporates the 

findings of previous scientific research has been outlined. Where appropriate, it has been noted that 

the ecosystem response to HFEs is strongly dependent on the “intervening dam operations,” that 

is, the flow releases that occur between the HFEs. For sandbars, the intervening dam operations are 

important because they determine the rate of post-HFE sandbar erosion, the rate of export of sand 

from the system following tributary-derived sand inputs, and thus the amount of sand available for 
Chapter 5—Science-Based Strategies for Future High-Flow Experiments at Glen Canyon Dam 143

building sandbars during a given HFE. This link led Wright and others (2008) to conclude that the 

optimal intervening dam operation for rebuilding and maintaining sandbars is year-round steady 

flows, which would export the least amount of sand compared to other potential dam operations 

(see also Wright and Grams, 2010). Thus, if implementation of the strategy outlined in this chapter 

led the GCDAMP to conclude that the goal for sandbars was not being achieved, the next step in 

the adaptive-management process logically would be to reduce daily and seasonal flow fluctuations—that is, to move to a steadier flow regime. 

The changes, however, in intervening dam operations that occurred in the early 1990s, which 

substantially reduced the daily-flow fluctuations that are caused by varying demand for hydroelectricity, allowed rainbow trout populations in the Lees Ferry tailwater reach to become self sustaining 

and self recruiting (Pine and others, 2009). Therefore, it is reasonable to assume that further stabilizing 

flows to conserve sand would lead to further increases in rainbow trout juvenile growth and survival. 

The national importance of the Grand Canyon humpback chub population (it is the largest anywhere), 

combined with the current information about rainbow trout and humpback chub dynamics, suggests 

a move toward increasing the number of rainbow trout would be unacceptable to natural-resource 

managers and inconsistent with GCDAMP and National Park Service goals for native fish. 

Thus, it is apparent that changes in intervening dam operations must be considered carefully 

and in the context of an ecosystem approach. For example, as outlined above, intervening dam 

operations that may conserve sandbars may not benefit native fish. Also, other resources, such as 

recreation, riparian vegetation, and hydropower, would be affected by a change in intervening dam 

operations and would need to be considered. Because of these complexities, evaluation of alternative intervening dam operations was beyond the scope of this chapter. The issue was raised in this 

chapter to illustrate the tradeoffs that would likely need to be evaluated in order to develop any 

new experimental strategy for intervening dam operations, if a new strategy is deemed necessary. 

For example, one tradeoff that may be necessary to balance sand and native fish goals is the use 

of large daily fluctuations in flow, for short periods at certain times of the year following HFEs, to 

disadvantage rainbow trout spawning and juvenile survival (Korman and Melis, 2011). This could 

potentially occur from late spring through mid-summer and would likely reduce egg and juvenile 

survival without adversely affecting the invertebrate prey base that supports the adult fish that are a 

valued recreational resource. Trout-suppression flows could be stopped at the onset of the summer 

monsoon season in the Southwestern United States, when substantial sand inputs typically begin 

to occur and juvenile humpback chub begin entering the main stem because of natural flooding in 

the Little Colorado River. While it is known that these types of flows erode sandbars and export 

sand at high rates, these fluctuations may be required to mitigate the potential strong positive effect 

of HFEs (which are necessary for sandbar building) on the rainbow trout population. Short-term 

reductions in sandbar area could potentially be offset by stable and low flows at other times of the 

year, if such dam operations were also part of a future long-term experimental design.

Conclusions 

The most effective strategy for future releases from Glen Canyon Dam is one that provides 

flexibility and adaptability—flexibility would allow the best scientific information to be used in 

decisionmaking, and adaptability would allow ongoing learning to be readily incorporated in the 

process. Previous HFEs have provided a wealth of information and dramatically increased our 

knowledge of key physical and biological processes. However, it is still not possible to write a 

detailed HFE prescription with a known outcome, particularly in relation to biological resource 

responses. Experimentation, monitoring, research, and adaptive management are the necessary 

tools for implementing a long-term science-based strategy for improving sandbar resources while 

simultaneously ensuring that trends for native fish are at least neutral. 

The HFE-triggering strategy outlined in this chapter strives to achieve these goals of flexibility and adaptability. The various components of the strategy are derived from previous 
144 Effects of Three High-Flow Experiments on the Colorado River Ecosystem Downstream from Glen Canyon Dam, Arizona

monitoring and research during HFEs, and the process is specifically designed to use, where appropriate, the monitoring and numerical modeling tools that have already been developed for the Colorado 

River downstream from Glen Canyon Dam. The strategy is designed to rebuild sandbars while 

eliminating or minimizing long-term progressive erosion of sand from Marble Canyon. Uncertainty in 

future climate and dam operations, however, dictates that this outcome is not guaranteed, and thus it is 

critical that long-term monitoring be conducted and evaluated within the context of HFEs. Maximizing 

benefits to sandbars was the primary goal of the strategy outlined here, with potential effects to 

biological resources being considered within the context of the sand-based strategy. The design strategy 

outlined allows for new data on physical, biological, and cultural resources to be incorporated into 

the decisionmaking process on a regular basis. Finally, implementation of HFEs in the future does 

not guarantee progressive sandbar building through time. However, sandbar trends without HFEs are 

one of the few outcomes that can be predicted with absolute certainty: without HFEs, sandbar size 

(above typical shorelines) will decrease through time because HFEs are the only documented tool for 

rebuilding sandbars (Rubin and others, 2002; Wright and others, 2005). 

Acknowledgments 

Many people have contributed to the information in this chapter through the countless discussions that the authors have had over the past 8 years in formal workshops and meetings, informal 

meetings, hallway discussions, phone calls, and evening sessions. The following list is likely not 

comprehensive and is in no particular order: Ted Melis, David Topping, David Rubin, Jack Schmidt, 

Paul Grams, Matt Kaplinski, Joe Hazel, Amy Draut, Helen Fairley, Jonathan Nelson, Steve Wiele, 

Barbara Ralston, Josh Korman, Colden Baxter, Wyatt Cross, Bob Hall, Emma Rosi-Marshall, 

Matthew Andersen, Mike Yard, Bill Persons, Rich Valdez, and Carl Walters. 

References

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