U.S. Department of the Interior

U.S. Geological Survey

USGS Fact Sheet 2009–3033

2009

Status and Trends of Resources Below Glen Canyon Dam 

Update—2009

The protection of resources 

found in Glen Canyon National 

Recreation Area and Grand Canyon 

National Park, Arizona, emerged as 

a significant public concern in the 

decades following the completion of 

Glen Canyon Dam in 1963. The dam, 

which lies about 15 miles upstream 

from the park, altered the Colorado 

River's flow, temperature, and 

sediment-carrying capacity, resulting 

over time in beach erosion, expansion 

of nonnative species, and losses 

of native fish. During the 1990s, in 

response to public concern, Congress 

and the Department of the Interior 

embarked on an ongoing effort to 

reduce and address the effects of dam 

operations on downstream resources.

In 2005, the U.S. Geological Survey 

produced a comprehensive report 

entitled “The State of the Colorado 

River Ecosystem in Grand Canyon,” 

which documented the condition 

and trends of resources downstream 

of Glen Canyon Dam from 1991 to 

2004. This fact sheet updates the 

2005 report to extend its findings to 

include data published through April 

2009 for key resources.

Background

Congress passed the Grand Canyon Protection Act of 1992, which directs the Secretary 

of the Interior to operate the dam “to protect, 

mitigate adverse impacts to, and improve 

values for which Grand Canyon National Park 

and Glen Canyon National Recreation Area 

were established ....” 

In response to the 1992 legislation and 

an extensive environmental review, the 

Secretary of the Interior signed a formal 

decision in 1996 that altered historical flows 

from the dam and established the Glen Canyon Dam Adaptive Management Program 

(GCDAMP). Adaptive management, also 

known as “learning by doing,” is a process 

for evaluating and revising management actions as new information becomes available. 

The U.S. Geological Survey (USGS) Grand 

Canyon Monitoring and Research Center is 

responsible for the scientific monitoring and 

research efforts of the program.

During the past 13 years, the modified 

low fluctuating flow (MLFF) alternative 

has governed dam operations. Under MLFF 

operations, the historical range of daily flow 

fluctuations and peak releases associated 

with hydropower production from the dam 

have been reduced. Additionally, in keeping 

with the precepts of adaptive management, 

the Department of the Interior implemented 

a series of flow and nonflow experiments 

intended to improve downstream resource 

conditions. After more than a decade, two 

questions remain: What are the effects of 

Glen Canyon Dam operations on those resources of concern? Are desired outcomes 

being achieved? The current condition of 

each key resource is summarized below and 

in the table on the following pages.

A view of Marble Canyon (top) in Grand Canyon National Park, Arizona. Grand Canyon is home to a diverse 

range of wildlife, including the endangered humpback chub (Gila cypha; lower left), and is a premier whitewater rafting (lower right) destination. The flow of Colorado River through the canyon is controlled by Glen Canyon 

Dam. Federal efforts to protect natural, cultural, and recreational resources affected by the dam are ongoing. 

© 2009 Geoff Gourley

Arizona Game and Fish Department Mark Lellouch, National Park Service
Water Quality

Because the water-intake structures of Glen 

Canyon Dam are well below the surface of 

Lake Powell, where the warmth of the sun 

cannot penetrate, water released from the 

dam is cold, with an average temperature of 

about 48°F (9°C) for the period from 1988 

to 2005. Warming occurs as the water moves 

downstream, reaching an average annual 

high of about 64°F (18°C) at Diamond Creek 

between 1990 and 2002. Low temperatures 

have threatened the survival of endangered 

humpback chub (Gila cypha). Beginning in 

2003, an ongoing drought in the upper Colorado River Basin has reduced the water level 

in Lake Powell, meaning releases are drawn 

from closer to the surface of the reservoir 

where the water is warmer. As a result, water 

temperatures have increased substantially, 

reaching an annual high of 70°F (21°C) in 

2005 at Diamond Creek. The warmer releases 

may be playing a role in recent increases 

in native fish, including humpback chub in 

Grand Canyon (see below).

Specific-conductance data are collected as 

a cost-effective method for estimating salinity, or salt inputs, which affect ecosystem 

functioning as well as municipal, industrial, 

and agricultural water users. Drought conditions, prevalent since 1999, generally result 

in an increase in specific conductance in 

Lake Powell and water released downstream, 

owing in part to the decrease in the volume 

of water in the reservoir. 

Sediment

Throughout Grand Canyon National Park, 

sandbars create habitat for native plants and 

animals, provide camping beaches for river 

runners and hikers, and act as the source of 

sediment needed to protect archeological 

resources. Sandbars in the park have been 

significantly eroded because Glen Canyon 

Dam traps all of the upstream sediment supply and eliminates natural flooding. 

Today, Colorado River tributaries below 

the dam are the only sources of sediment 

available to maintain the park's sandbars, 

providing just 16% of the sand supply available before the dam's construction. In 1996, 

2004, and 2008, short-duration high-flow 

experimental water releases, which simulate 

mild natural floods, were tested for their 

ability to maintain sandbars with this postdam sand supply. High-flow releases are required to transfer sand from the riverbed and 

low-elevation parts of sandbars to higherelevation environments that are only inundated during higher than normal releases.

The 1996 high-flow experiment resulted in limited areas of sandbar building 

at the expense of larger areas of erosion, 

whereas the 2004 release produced net 

sandbar building in the upper reaches of the 

277-mile-long river system. As a result of 

the 2004 high-flow experiment, scientists 

confirmed that increases in total sandbar 

area and volume can occur when high-flow 

releases follow tributary floods that enrich 

sand supplies in the main channel below 

the dam. In 2008, when sand supplies in the 

main channel of the Colorado River were 

approximately three times larger than in 

2004, a high-flow experiment was done to 

determine whether the conditions of greater 

sand enrichment would result in more sandbar building than occurred in 2004. 

Building on what has been learned 

through experimentation, scientists have 

concluded sandbars can be potentially rebuilt 

using short-duration high flows following 

each average to above-average input of sand 

from tributaries; this approach would move 

sand from the riverbed to sandbars before it 

can be carried downstream. The effectiveness of this strategy rests on minimizing 

sand export and sandbar erosion during 

periods between high flows. Export and erosion rates are strongly dependent on water 

release volume and daily release patterns. 

For a given annual volume of water to be released from Glen Canyon Dam, the optimal 

dam operation for accumulating tributarysupplied sand is a constant, steady flow over 

the entire year. 

Aquatic Food Web

Aquatic food resources play an important 

role in the distribution, population density, 

and growth of native and nonnative fish. 

Found in Lakes Mead and Havasu in January 

2007, the nonnative quagga mussel (Dreissena bugensis) will in all likelihood make 

its way to Lake Powell. Introductions of 

invasive quagga and zebra mussels (Dreissena polymorpha) into lakes and rivers in 

the Eastern United States have caused broad 

ecosystem-scale changes, including shifts in 

the location of aquatic food resources. 

A risk assessment by the USGS found that 

it is likely that the mussel will establish in 

moderate densities at Lees Ferry. A moderate 

presence of quagga mussels in Lees Ferry 

might actually increase food availability for 

fish by stimulating algae and invertebrate 

production. High suspended-sediment 

concentrations and other conditions make it 

unlikely that quagga mussels will become 

well established in the mainstem Colorado 

River below Lees Ferry or its tributaries.

Native Fish

The humpback chub is an endangered 

freshwater fish found only in the Colorado 

River Basin. Humpback chub have been affected not only by changes in hydrology since 

the construction of the dam but also by predation by and competition with nonnative fish. 

The USGS has developed and used a 

computer model to assess the Grand Canyon 

humpback chub population status and trends 

from 1989 to 2008. Reproductively mature 

humpback chub, those 4 years old and older, 

appear to have decreased from 1989 to about 

2001, when the population stabilized at about 

5,000 adults. From 2001 to 2008, the adult 

population increased by approximately 50%. 

When possible model error is considered, the 

estimated number of adult chub in the Grand 

Canyon population is between 6,000 and 

10,000. The most likely number is estimated 

at 7,650 individuals.

The factors driving the estimated increase in 

adult humpback chub numbers in Grand Canyon are not easy to determine. Between 2000 

and 2008, both human-caused and natural 

events have occurred that could be independently or in combination contributing to the 

increase. Scientists hypothesize that humpback 

chub may have benefited from experimental 

water releases from Glen Canyon Dam, removal of nonnative fish, and drought-induced 

warming of dam releases since 2002, particularly during late summer and fall of 2005.

Grand Canyon populations of the flannelmouth sucker (Catostomus latipinnis) and 

bluehead sucker (Catostomus discobolus) are 

stable, and both species may have increased 

in the reach of the Colorado River upstream 

and downstream from the mouth of the Little 

Colorado River. In this reach, scientists have 

found juvenile, young, and adult fish of both 

species, which suggests that more successful 

reproduction is occurring.

Nonnative Fish

The number of nonnative rainbow trout 

(Oncorhynchus mykiss) found in Lees Ferry, 

which is immediately downstream from 

the Glen Canyon Dam and supports a recreational fishing industry, began declining 

about 2002. Research to better understand 
Update of key downstream natural, cultural, and recreational resources affected by Glen Canyon Dam, summarizing data published through April 2009. 

[The goals presented were established by the Glen Canyon Dam Adaptive Management Program (GCDAMP; http://www.usbr.gov/uc/rm/amp/strategic_plan.html). Predictions are 

drawn from the 1995 Operation of Glen Canyon Final Environmental Impact Statement (EIS) on how key resources would respond to the selected flow alternative]

Resource and GCDAMP Goal 1995 EIS Prediction 2009 Summary

Natural resources

Water quality (water temperature)

(Goal: Establish water temperature, 

quality, and flow dynamics to achieve 

GCDAMP ecosystem goals)

Specific conductance (salinity) 

No effect

No effect

Since 2003, downstream water temperatures have increased 

in response to drought conditions (see http://pubs.usgs.gov/

ds/2007/251/).

Drought conditions, prevalent since 1999, generally result in increases 

in specific conductance (see http://pubs.usgs.gov/ds/364/).

Sediment (sandbars and related physical 

habitats) (Goal: Maintain or attain 

levels of sediment storage within the 

main channel and along shorelines)

Modest improvement Sandbars erode during periods between high flows. Increases in 

total sandbar area and volume are only possible when high-flow 

releases follow large tributary floods that enrich sand supplies 

in the main channel (see http://pubs.usgs.gov/fs/2007/3020/).

Aquatic food web (Goal: Protect or 

improve the aquatic food base)

Potential major increase Increases were apparent in Glen Canyon Dam tailwater reach, but the 

trend is unclear along downstream reaches. Unlikely that quagga 

mussels (Dreissena bugensis) will become well established in the 

mainstem Colorado River below Lees Ferry or its tributaries (see 

http://pubs.usgs.gov/of/2007/1085/).

Native fish (humpback chub) (Goal: 

Maintain or attain viable populations 

of existing native fish)

Potential minor increase The population of adult humpback chub (Gila cypha) decreased 

between 1989 and 2001; however, adult abundance has 

increased more than 50% since 2001 (see http://pubs.usgs.gov/

of/2009/1075/). 

Trout (Goal: Maintain a naturally 

reproducing population of rainbow 

trout above the Paria River)

Increased growth potential, 

dependent on stocking

Rainbow trout (Oncorhynchus mykiss) numbers have decreased in 

the Lees Ferry reach. 

Riparian vegetation (Goal: Protect 

or improve the biotic riparian and 

spring communities)

Modest increase Native and nonnative woody vegetation continues to expand in 

the river corridor. Nonnative tamarisk (Tamarix ramosissima) is 

the dominant species, making up 24% of vegetation (see http://

pubs.usgs.gov/of/2008/1216/).

Kanab ambersnail (Goal: Maintain or 

attain viable populations of Kanab 

ambersnail)

Some incidental take Snail habitat increased since 1998 (see chapter 6 at http://www.

gcmrc.gov/products/score/2005/).

Cultural resources

Archeological sites affected (Goal: 

Preserve, protect, manage, and treat 

cultural resources)

Moderate degradation

(less than 157 sites affected)

Archeological site condition continues to decline because of 

a combination of factors including erosion, gravity, visitor 

impacts, and insufficient sediment (see chapter 11 at http://

www.gcmrc.gov/products/score/2005/).

Traditional cultural resources 

affected (Goal: Preserve, protect, 

manage, and treat cultural resources)

Increased protection Tribes have developed protocols for monitoring the condition of 

cultural resources in accordance with tribal values.

Recreation resources

Whitewater boating camping beaches

(average area at normal peak stage) 

(Goal: Maintain or improve the 

quality of recreational experiences)

Minor increase Areas suitable for camping have decreased on average 15% per 

year between 1998 and 2003 (see chapter 12 at http://www.gcmrc.

gov/products/score/2005/).
For more information contact:

U.S. Geological Survey

Southwest Biological Science Center

Grand Caynon Monitoring and Research Center

Flagstaff, AZ 86001

928-556-7094

This Fact Sheet and any updates to it are available 

online at 

http://pubs.usgs.gov/fs/2009/3033/

the early life-history dynamics of rainbow 

trout in Lees Ferry showed that hourly flow 

fluctuations do affect the neashore habitat 

use and growth of young trout (less than 

1 year old). For example, nearshore catch 

rates increased two to four times at the 

daily minimum flow compared to the daily 

maximum. The effects of winter experimental fluctuating flows of 2003–2005, flows 

designed to disrupt spawning activity to 

reduce trout numbers and increase their size, 

were also evaluated for Lees Ferry. Survival 

rates increased as egg deposition decreased, 

meaning that incubation mortality rates 

owing to experimental flows were insufficient to reduce the numbers of young fish. 

In 2006, for example, early survival rates 

posted a six-fold increase despite a ten-fold 

decrease in egg deposition. 

In early 2003, a major effort was begun 

by the GCDAMP to remove nonnative fish, 

particularly rainbow and brown trout (Salmo 

trutta), from the Colorado River near the 

confluence of the Little Colroado River, 

which is considered important habitat for the 

humpback chub. Not only do trout rely on the 

same food sources—aquatic and terrestrial 

invertebrates, algae, and small fish—trout are 

also thought to prey on juvenile humpback 

chub. Between 2003 and 2006 the rainbow 

trout population in the Colorado River near 

the Little Colorado River was reduced by 

more than 80%.

Riparian Vegetation

Historically, the flow of the Colorado 

River varied greatly by season, swelling 

with snowmelt in the spring and slowing to a 

relative trickle by winter. Today, the regulation of the river by Glen Canyon Dam has 

reduced disturbance to vegetation caused by 

predam floods, and MLFF operations provide a constant source of water for riparian 

plants (plants living near a waterway). These 

conditions have resulted in the expansion 

of woody and herbaceous vegetation, both 

native and nonnative species, immediately 

along the river channel. 

A limited study of vegetation change 

between 1992 and 2002 indicated that vegetation expansion appears to be the greatest 

along shorelines. Additionally, monitoring 

of existing vegetation showed an increase in 

diameter, suggesting that individual woody 

plants are increasing in size. Increases in 

both the number and size of riparian plants 

may contribute to campsite area loss. 

Recent vegetation mapping efforts indicate 

that tamarisk (Tamarix ramosissima) is the 

dominant nonnative species, representing 

24% of the vegetation community. The 2008 

high-flow experiment was timed for early 

spring to decrease the likelihood of spreading tamarisk seeds, which would result in 

increased tamarisk colonization. 

Riparin habitat for the endangered Kanab 

ambersnail has increased since 1998; habitat is used as a surrogate for snail numbers 

because they fluctuate widely by season. 

The snail, which is associated with wetland 

and spring vegetation, is currently found 

at three locations, including two in Grand 

Canyon National Park (Vaseys Paradise and 

Elves Chasm). Vaseys Paradise is a small 

patch of spring-fed vegetation that is surveyed by the USGS in cooperation with the 

Arizona Game and Fish Department.

Archeological Sites

Grand Canyon has been used by humans 

for at least 13,000 years. Today, more than 

nine contemporary Native American tribes 

have cultural ties to the area. Grand Canyon 

National Park contains more than 2,600 documented prehistoric sites; 336 sites are within 

the area potentially affected by Glen Canyon 

Dam operations.

Cultural resource monitoring suggests 

that archeological resources are affected 

both by erosion and recreational visitors. 

Natural erosion patterns would happen 

whether the dam existed or not; however, 

the dam and its operations have limited the 

sediment available within the river corridor. 

The diminished sediment supply appears to 

be exacerbating the rate and amount of erosion affecting cultural resources. 

Sandbars created by a 2004 high-flow experimental release from Glen Canyon Dam 

increased the windborne transport of new 

river deposited sand toward some archeological sites found near the Colorado River in 

Grand Canyon. Increasing the availability of 

sand that can be transported by the wind to archeological sites in the river corridor, following sandbar building high flows (as described 

above) may reduce erosion and improve the 

condition of some archeological sites.

Camping Beaches

Camping area above the maximum water 

level permitted under MLFF dam operations (the water level at a flow rate of 25,000 

cubic feet per second) decreased by 55% 

between 1998 and 2003, which is an average 

annual decline of 15% per year. Relative to 

total sandbar area, area suitable for camping 

continues to decline, indicating that factors 

other than sandbar erosion—particularly 

vegetation encroachment—contribute to loss 

of campable area. 

Conclusion

The regulated Colorado River below Glen 

Canyon Dam is a dynamic system affected 

by a range of factors such as dam operations 

and other conditions, including drought. The 

timely and high-quality scientific monitoring 

and research provided by USGS scientists 

and their cooperators about the effects of 

dam operations and other natural and humancaused actions on downstream resources 

provides information essential to effective 

adaptive management of Colorado River resources below Glen Canyon Dam.

Scientists with the USGS Grand Canyon Monitoring and Research Center collecting water samples 

during the 2008 high-flow experiment (USGS photo 

by Paul Alley).

John F. Hamill