U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2010–3009

2010

2008 High-Flow Experiment at Glen Canyon Dam Benefits 

Colorado River Resources in Grand Canyon National Park

chub (Gila cypha) and other native fish. 

Today, the Colorado River downstream 

from Glen Canyon Dam usually runs 

clear because its reservoir, Lake Powell, 

traps all of the upstream sediment. Tributaries downstream from the dam, primarily the Paria and Little Colorado Rivers, 

provide Grand Canyon with 6 to 16% of 

its pre-dam sand supply.

Because of concerns about the effects 

of Glen Canyon Dam on downstream 

resources, the Grand Canyon Protection Act was enacted in 1992 “to protect, 

mitigate adverse impacts to, and improve 

the values for which Grand Canyon 

National Park and Glen Canyon National 

Recreation Area were established . . .” 

In 1996, the Secretary of the Interior 

signed a formal decision that changed the 

operating rules of the dam, authorized 

experimental water releases from the dam, 

and established the Glen Canyon Dam 

Adaptive Management Program. Adaptive 

management is a way to evaluate and revise 

management actions as new information 

becomes available through research, monitoring, and experimentation.

2008 High-Flow Experiment

On March 5, 2008, the Secretary 

of the Interior pulled the levers at Glen 

Canyon Dam to release high flows into 

the Colorado River. Water was released 

through the powerplant and bypass tubes 

at the dam to a peak flow of approximately 41,500 cubic feet per second, 

about twice the normal peak flow, for 60 

hours. This high-flow experiment (HFE) 

and two previous experiments in 1996 

and 2004 were designed to mimic natural 

seasonal flooding. 

U.S. Geological Survey (USGS) 

scientists and their cooperators conducted 

On March 5, 2008, the Department of the Interior began a 60-hour highflow experiment at Glen Canyon Dam, 

Arizona, to determine if water releases 

designed to mimic natural seasonal 

flooding could be used to improve 

downstream resources in Glen Canyon 

National Recreation Area and Grand 

Canyon National Park. U.S.Geological 

Survey (USGS) scientists and their cooperators undertook a wide range of physical and biological resource monitoring 

and research activities before, during, 

and after the release. Scientists sought 

to determine whether or not high flows 

could be used to rebuild Grand Canyon 

sandbars, create nearshore habitat for 

the endangered humpback chub, and 

benefit other resources such as archaeological sites, rainbow trout, aquatic food 

availability, and riverside vegetation. 

This fact sheet summarizes research 

completed by January 2010.

Background

Glen Canyon Dam, which provides 

hydropower to customers in six states, 

regulates the Colorado River 15 miles 

upriver from Grand Canyon National 

Park, Arizona. Before the dam’s completion in 1963, the Colorado River swelled 

in most years with spring snowmelt from 

the Rocky Mountains, producing floods 

and transporting large quantities of sand 

that created and maintained sandbars in 

Grand Canyon. These sandbars provide 

camping beaches for hikers and whitewater rafters, are sources of windborne sediment needed to protect archaeological 

resources from weathering and erosion, 

and create habitats used by native fish 

and other wildlife. For example, sandbars 

create backwaters—areas of low-velocity 

flow—that are thought to be used as rearing areas by the endangered humpback 

Glen Canyon Dam releases high flows of Colorado River water on the night of March 6, 2008. A highflow experiment was undertaken to determine if water releases designed to mimic natural seasonal 

flooding could be used to improve a wide range of downstream resources in Glen Canyon National 

Recreation Area and Grand Canyon National Park. (Photograph courtesy of T. Ross Reeve, Bureau of 

Reclamation.)
Map of the Colorado River below Glen Canyon Dam showing the river corridor between Lake 

Powell and Lake Mead reservoirs.

Lake Mead

Lake Powell

Havasu

Creek

Paria

Riv er

Kanab

Cre ek

Diamond Creek

Bright Angel Cr

Glen Canyon Dam

GRAND CANYON

NATIONAL PARK

GLEN CANYON

NATIONAL RECREATION

AREA

Hoover Dam

Lees Ferry

Little Colorado Riv er

0 20 40 miles

0 20 40 60 kilometers

UTAH

ARIZONA

NEVADA

N

Colorado

R i v e r

M A R B L E C A N Y O N

A R I Z O N A

Map

Area

monitoring and research activities before, 

during, and after the 2008 HFE. This 

work focused on a wide range of questions, including: Are high flows capable 

of significantly rebuilding sandbars? 

Will windborne sand carried from rebuilt 

sandbars increase preservation potential 

at archaeological sites? Do such releases 

have the ability to create beneficial habitat for native fish? What effects do high 

flows have on riverside (riparian) vegetation, nonnative rainbow trout (Oncorhynchus mykiss), and the quality and abundance of aquatic foods that support native 

and nonnative fish?

Sandbars

The 2008 HFE resulted in widespread increases in the area and volume 

of sandbars, expansions of camping areas, 

and increases in the number and size of 

backwater habitats similar to or greater 

than those created by the 1996 and 2004 

HFEs. During the 1996 HFE, sandbars 

increased in height, but at the expense 

of eroding the lower parts of preexisting sandbars; such erosion occurred to a 

much lesser extent during the 2008 HFE 

than in previous experiments. By October 2008, 6 months after the high-flow 

experiment, the new sandbars had been 

largely eroded by typical fluctuating-flow 

dam operations. However, median sandbar elevation was still slightly higher and 

backwater habitat was still slightly more 

abundant than in February 2008.

Building on what scientists learned 

from the 1996 and 2004 experiments, 

the 2008 HFE was timed to take advantage of multiple large sand inputs to the 

Colorado River in 2006 and 2007 from 

the Paria River. In November 2007, sand 

in the Colorado River reached levels 

typically seen only once in a decade. The 

results of the 2008 experiment reconfirmed that high flows are most effective 

when they are strategically timed to take 

advantage of tributary floods that supply 

new sand to the Colorado River downstream of the dam. However, the results 

of all three HFEs show that building 

sandbars throughout the river corridor 

in Grand Canyon National Park requires 

making more sand available than is typically delivered by tributaries annually. 

One possibility is to augment the sand 

delivered by tributaries with sand trapped 

behind the dam. Alternatively, the sand 

supply might be indirectly increased 

through the use of short-duration high 

flows following each average or greater 

tributary sand input. This approach would 

move new sand from the riverbed to sandbars before it can be carried downstream. 

Scientists have also learned that sandbars, 

backwaters, and other habitats are most 

efficiently built when larger quantities 

of finer grained sand are available in the 

Colorado River.

The key to the long-term maintenance of sandbars is to ensure that the 

frequency and timing of high flows are 

coordinated such that sandbar rebuilding 

exceeds the erosion that occurs between 

high flows. The rapid erosion of sandbars in the 6 months following the 2008 

HFE highlights the importance of dam 

operations between high flows. For any 

annual release volume, the daily water 

release pattern and monthly volume of 

releases from Glen Canyon Dam directly 

affect the long-term sustainability of 

sandbars, because these factors strongly 

affect sandbar erosion and the retention 

of new sand from tributaries. Current and 

previous studies show that more stable 

and relatively lower monthly volume 

releases are most effective at limiting 

erosion of sandbars, retaining new sand, 

and improving access to backwaters by 

fish. However, the volume of water that 

must be released from Glen Canyon Dam 

annually is determined by basin hydrology and legal requirements to deliver 

water from the upper to the lower Colorado River Basin.

It may not be possible to rebuild 

and maintain sandbars over the long run 

solely through the manipulation of Glen 

Canyon Dam operations. Long-term 

experimentation that includes multiple, 

more frequent HFEs following tributary 

floods and that incorporates ongoing 

monitoring of intervening dam operations 

is required to resolve this question. 

Archaeological Sites

In addition to investigating the 

effectiveness of restoring sandbars using 

high flows, scientists have also sought 

to determine whether or not new sandbars contribute to the preservation of 

archaeological sites in the Colorado River 

corridor. Many archaeological sites found 

near the river are buried by windborne 

sand that protects them from weathering 

and erosion. The greatest potential for 

moving new sand deposited by high flows 

inland toward archaeological sites occurs 

in the spring, when winds are stronger 

and precipitation is less likely to occur. 

The sandbars created by the March 2008 

HFE were present during spring windy 

conditions, unlike sandbars created by the 

November 2004 HFE that were quickly 

eroded by experimental fluctuating flows 

in early 2005.

Scientists monitored windblown 

sand movement following the March 

2008 HFE at nine archaeological sites. 

At two of nine study sites, spring and 
summer winds reworked the new sandbars to form new dunes. The shape of the 

dunes in both cases indicated sand movement inland toward larger, well-established dune fields that contain archaeological sites. At the remaining seven sites, 

new sandbars did not result in sizeable 

dunes, and sand-transport rates after the 

2008 experiment were similar to or lower 

than in previous years. These results were 

anticipated because much higher flows 

would be required to deposit sand in areas 

suitable for wind transport at some of the 

study sites.

Riparian Vegetation

The 2008 HFE provided an opportunity to examine the effects of high flows 

on riparian vegetation (plants living near 

a waterway), particularly the susceptibility of native riparian plant communities to 

the invasion of nonnative species such as 

tamarisk (Tamarix sp.). Riparian vegetation was generally buried by sand rather 

than being removed from the ground by 

the HFE. Tamarisk seedling germination was relatively minor (less than 2% 

of measured cover) 6 months after the 

experiment. 

Conducting the 2008 HFE in March 

likely reduced successful tamarisk 

seedling germination because tamarisk 

produces flowers and seeds from April 

through September. Vegetative cover 

in areas above the highest water level 

reached during the HFE did not change 

significantly as a result of the March 

experiment. The continued coarsening of 

sandbars relative to pre-dam conditions 

and dam operations that wash finer sediment downstream appears to favor riparian species that grow through vegetative 

reproduction (clonal species), are adapted 

to partial burial, and that can persist in 

coarser sand, including many native species (for example, arrowweed (Pluchea 

sericea), seepwillow (Baccharis emoryi), 

and common reed (Phragmites australis)). 

Aquatic Food Web

Determining how HFEs affect algae 

and aquatic invertebrates, which are the 

principal food sources for fish, was an 

important component of the 2008 experiment. In the clear Lees Ferry reach (a 

recreational trout fishery), which extends 

15 miles downstream from Glen Canyon 

Dam to the Paria River confluence, the 

2008 HFE temporarily decreased the 

Matched photographs taken by a remote camera of a sandbar (Willie Taylor Camp) on the Colorado 

River about 45 miles downstream from Lees Ferry, Arizona. The series of photographs shows how 

the sandbar and its accompanying backwater (the area of water behind the sandbar thought to provide habitat for native fish) were affected by the 2008 high-flow experiment (HFE) and subsequent 

erosion in the 8 months following the HFE. All of the photographs were taken at about 4 P.M. at a 

water level associated with a flow rate of about 8,500 cubic feet per second from Glen Canyon Dam. 

April 11, 2008 

About a month after the HFE

March 4, 2008

Before the HFE

March 11, 2008 

Immediately after the HFE

November 2, 2008 

About 8 months after the HFE
quantity of aquatic invertebrates. However, the relative quality of food sources 

increased because the HFE reduced the 

New Zealand mud snail (Potamopyrgus 

antipodarum) population by approximately 

80% and increased midges and black flies. 

Midges and black flies are high-quality 

food items for trout. In contrast, the mud 

snail is a nonnative aquatic invertebrate 

that has invaded the Lees Ferry reach in 

large numbers and is considered a nuisance species because the snails cannot be 

digested when eaten by rainbow trout. 

The mud snail population was 

reduced for at least 15 months after the 

2008 HFE, whereas midges and black flies 

were more abundant after the HFE. The 

increased availability of high-quality food 

items may be partially responsible for the 

unusually high growth and survival rates 

of rainbow trout since the HFE. Research 

findings indicate that conducting high 

flows every 2 to 3 years may be an effective strategy for controlling mud snails. 

Rainbow Trout

Scientists have sought to understand 

how high flows influence spawning and 

survival of early life stages of rainbow 

trout in the Lees Ferry reach. The March 

2008 HFE resulted in a large increase in 

early survival of age-0 (fertilization to 

about 1 to 2 months from emergence) 

rainbow trout because of improved habitat 

conditions. Abundance of age-0 trout in 

July 2008 was more than four times higher 

than had been expected.

Early survival was much higher for 

trout hatched about a month after the 

2008 HFE than those hatched earlier in 

the year. Fish hatched a month after the 

HFE (about mid April) were not exposed 

to high flows and emerged into apparently 

better quality habitat. Scientists think that 

the 2008 experiment increased the tiny 

spaces between gravel on the river bottom 

and improved availability of high-quality 

food. As a result, recently emerged trout 

survived and grew better through the summer and fall. Trout abundance in 2009 was 

more than two times higher than expected 

given the estimated number of viable eggs 

deposited in that year, indicating that the 

effect of the high flows on young trout persisted for at least a year after the high flow.

Resource managers were concerned 

that high flows might cause downstream 

movement of rainbow trout from Lees 

Ferry to Grand Canyon, where nonnative 

trout prey on and compete with the endangered humpback chub and other native 

fish. Data collected before, during, and 

after the 2008 HFE indicate that trout did 

not move significantly as the result of the 

experiment. Additionally, surveys showed 

no change in the number of juvenile and 

adult trout before or after the experiment 

in the Lees Ferry reach. 

Conclusion

On the basis of results from the March 

2008 HFE, resources of the Colorado River 

in Grand Canyon appear to have generally 

benefitted from the high flows released 

from Glen Canyon Dam. Although sandbar 

building was widespread in response to the 

2008 HFE, which occurred under highly 

sand-enriched conditions, these gains were 

short lived owing to erosion that occurred 

once releases from the dam resumed normal fluctuating-flow operations.

Planning future HFEs will benefit 

from the insights gained from both the 

March 2008 HFE and a comprehensive 

synthesis of the results from the 2008, 

2004, and 1996 experiments that will be 

completed in 2010. Data from the 2008 

experiment are also being used to advance 

development of sediment models and 

various aquatic-ecosystem models for the 

Colorado River. The sediment models will 

help scientists and managers determine 

the best frequency, timing, duration, and 

magnitude of future HFEs, whereas the 

ecosystem models will be used to evaluate 

factors contributing to changes in native 

and nonnative fish communities.

Theodore S. Melis,

David J. Topping, 

Paul E. Grams,

David M. Rubin, 

Scott A. Wright,

Amy E. Draut, 

Joseph E. Hazel, Jr.,

Barbara E. Ralston,

Theodore A. Kennedy, 

Emma Rosi-Marshall,

Josh Korman,

Kara D. Hilwig,

and Lara M. Schmit

For more information contact:

U.S. Geological Survey

Southwest Biological Science Center

Grand Canyon 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/2010/3009/

This sandbar in Grand Canyon was created as the result of the 2008 high-flow experiment at 

Glen Canyon Dam. To the left of the sandbar is a newly created backwater. Backwaters are areas 

of low-velocity flow that are thought to be used as rearing areas by the endangered humpback 

chub (Gila cypha) and other native fish. Sandbars also provide camping beaches for hikers and 

whitewater rafters. (Photograph courtesy of Joseph E. Hazel, Jr., Northern Arizona University.)