U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2011–3012

February 2011

Three Experimental High-Flow Releases from Glen Canyon Dam, 

Arizona—Effects on the Downstream Colorado River Ecosystem 

hree 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. 

Also known as artificial or controlled 

floods, these scheduled releases of 

water above the dam’s powerplant 

capacity were designed to mimic 

pre-dam seasonal flooding on the 

Colorado River. The goal of the HFEs 

was to determine whether high flows 

could be used to benefit important 

downstream resources in Glen 

Canyon National Recreation Area 

and Grand Canyon National Park that 

have been affected by the existence 

and operation of Glen Canyon Dam. 

These downstream resources 

include native fish, particularly 

endangered humpback chub (Gila 

cypha), terrestrial and aquatic 

sandbar habitats, cultural sites, and 

recreational resources. This Fact 

Sheet summarizes HFE-related studies 

published since 1996 and outlines a 

possible strategy for implementing 

future HFEs.

Background

The construction and operation of dams 

results in numerous physical and ecological 

changes to river systems. Since its completion in 1963, Glen Canyon Dam, Arizona (lat 

36.9375o and long -111.4843o), traps in Lake 

Powell all of the upstream sediment—gravel, 

sand, silt, and clay—formerly carried by 

the Colorado River through Grand Canyon 

National Park. Additionally, the dam reduced 

the magnitude and duration of flooding 

downstream, and dam operations result in a 

dominance of moderate flows compared to 

pre-dam seasonal floods and periods when 

flows were reduced to a relative trickle. Before the dam was built, Colorado River flow 

gradually increased from mid-December to 

March, precipitously increased in April and 

May, and reached its peak in early June. This 

pre-dam seasonal flooding moved sand from 

the riverbed to the shoreline, creating and 

maintaining sandbars.

Dam-induced changes in the Colorado 

River’s temperature, flow, and sediment-carrying capacity have been implicated in losses 

of native fish, invasion of nonnative species, 

sandbar erosion, and the narrowing of rapids. 

Through the periodic use of high-flow experiments (HFEs), which are scheduled releases 

of water from the dam above powerplant 

capacity, managers have attempted to benefit 

key resources by simulating one aspect of 

the pre-dam river—floods. Three HFEs, also 

known as artificial or controlled floods, were 

conducted by the U.S. Department of the 

Interior at Glen Canyon Dam in March 1996, 

November 2004, and March 2008. Research 

and long-term monitoring have allowed 

scientists to unravel many, but not all, of the 

uncertainties that existed about how HFEs 

might affect downstream river resources.

Jet tubes at Glen Canyon Dam release Colorado River water on the morning of March 5, 2008, 

during a high-flow experiment (HFE). This and two similar HFEs sought to determine whether 

high flows could be used to move sand from the riverbed to Grand Canyon sandbars, used as 

camping beaches (lower left), and to benefit other resources. Following the 2008 HFE, rainbow 

trout (Oncorhynchus mykiss; lower right) numbers increased, which may adversely affect 

native humpback chub (Gila cypha), an endangered species. 

Anne Phillips, USGS

© David Edwards

© Eric Engbretson

T
Physical Processes 

Studies conducted in conjunction with 

each HFE have allowed scientists to better 

understand the physical processes of the 

post-dam Colorado River, especially how 

the river transports and reworks the now 

greatly reduced sand supply (about 10 percent of historical values). With the upstream 

sand supply trapped behind Glen Canyon 

Dam, the Paria and Little Colorado Rivers, 

tributaries that enter the river downstream 

of the dam, are the primary sources of sand 

to the system. Sand provided by these and 

other, smaller tributaries is deposited on 

the riverbed and eventually carried downstream to Lake Mead, particularly during 

high-volume dam releases. Because HFEs 

to some extent mimic natural flooding, they 

have been conducted to evaluate their ability to benefit sediment-dependent resources, 

including sandbars and camping beaches, 

marsh and riverside vegetation, and aquatic 

habitats such as backwaters, which are 

nearshore areas of low-velocity flow used 

as rearing habitat by native fish. Sandbars 

are of particular concern because they eroded from the time the dam was completed 

in 1963 to 1991. During this period, the 

dam was operated to meet required downstream water transfers and to maximize the 

generation of electricity around peak daily 

demand, causing daily flows to vary greatly 

(from ~1,000 to ~25,000 ft3

/s) and resulting 

in sandbar erosion. Constraints were placed 

on dam operations starting in 1991, in part 

to reduce sandbar erosion and increase retention of sediment from tributaries.

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

A R I Z O N A

Map

Area

Map of the Colorado River downstream of Glen Canyon Dam showing the river corridor 

between Lake Powell and Lake Mead reservoirs. 

The first HFE took place between March 

26 and April 7, 1996, including a 7-day 

steady peak release at 45,000 ft3

/s—a peak 

flow about 50 percent greater than powerplant capacity. Scientists learned after this 

HFE that the sand delivered from tributaries 

does not accumulate on the riverbed over 

multiple years during typical operations. 

It had been thought that tributary-supplied 

sand would be stored on the riverbed in 

response to the 1991 operational changes 

and that this stored sand would be the primary source of sand available to rebuild 

sandbars during HFEs. Although the 1996 

HFE demonstrated that high flows can 

build sandbars, sandbars at higher elevations were built from sand scoured from 

the lower elevation portions of existing 

sandbars rather than from sand stored on 

the riverbed. In other words, in these cases 

sandbars became higher but not wider. On 

the basis of these findings, the 2004 and 

2008 HFEs were shorter in duration and 

strategically timed to follow tributary floods 

that provided “new” sand to the system 

before it was carried downstream. Research 

indicates that from February 1996 to October 2008—the span of the three HFEs—75 

percent of the sandbars at long-term study 

sites in Grand Canyon experienced net increases in volume, despite ongoing sandbar 

erosion between HFEs. 

Three conclusions related to sediment 

have important implications for designing 

future HFEs. First, HFEs build sandbars by 

eroding existing low-elevation portions of 

sandbars or by using tributary-supplied sand. 

Second, HFEs conducted soon after new 

sand has been supplied to the river channel 

by tributary floods are effective at increasing 

sandbar area and volume and less likely to 

result in the erosion of low-elevation portions of sandbars. Sandbars are built relatively quickly (hours to a few days) under these 

sand-enriched conditions, but they also tend 

to erode quickly (days to several months) 

following an HFE. Third, monitoring data 

show that sandbars erode more quickly as 

release volumes and daily fluctuations increase, whereas the rate of erosion is reduced 

when tributary sand inputs continue to occur 

following sandbar building. 

Biological Processes

As the Colorado River flows downstream 

from Glen Canyon Dam, the management goals for aquatic resources shift from 

maintaining naturally reproducing populations of nonnative fish to maintaining or 

attaining viable populations of native fish, 

particularly the endangered humpback 

chub (Gila cypha). The Lees Ferry reach, 

a 16-mile-long stretch of the river immediately downstream from the dam in Glen 

Canyon National Recreation Area, supports 

a nonnative rainbow trout (Oncorhynchus 

mykiss) sport fishery. Despite management 

efforts to benefit native fish in the main

stem within Grand Canyon National Park, 

rainbow trout are the dominant fish in the 

main stem as far downstream as its confluence with the Little Colorado River. Most 

humpback chub are found in the Little Colorado River and near its confluence with the 

Colorado River. Native flannelmouth sucker 

(Catostomus latipinnis), bluehead sucker 

(Catostomus discobolus), and nonnative 

common carp (Cyprinus carpio) dominate 

downstream reaches of the Colorado River 

as it nears Lake Mead. 

 It had been thought that HFEs would 

benefit native fish by creating nearshore 

backwater habitats that might serve as important rearing environments. On the other 

hand, scientists also anticipated that HFEs 

would displace both native and nonnative 

fish downstream and that some rainbow 

trout eggs and juvenile fish would experience mortality. Research associated with 

the 2008 HFE, however, indicates that high 

flows actually benefit rainbow trout populations by improving spawning and rearing 

habitats in the Lees Ferry reach. Survival 

rates of juvenile rainbow trout in this reach 

in 2008 were more than four times higher 
than observed in years before the experiment for which data are available (2003 to 

2007). This response persisted into 2009, 

with juvenile survival rates that were twice 

those in pre-HFE years; in 2010, however, 

juvenile rainbow trout survival was much 

lower and similar to levels between 2003 

and 2007. This pattern indicates that the 

effect of an HFE on early life stages of 

trout may persist for as long as 2 years. 

Increased survival rates recorded in 2008 

and 2009 appear to be the result of HFEinduced increases in aquatic invertebrates, 

such as midges and black flies, which are 

high-quality food items preferred by trout. 

These high survival rates led to increases in 

adult populations of rainbow trout throughout the river.

Downstream migration of the large number of rainbow trout that were spawned in 

the Lees Ferry reach in 2008, as well as 

spawning that may have occurred downstream, contributed to a roughly 800 percent 

increase in rainbow trout densities between 

2007 and 2009 in the main stem near the 

confluence with the Little Colorado River, 

where most humpback chub are found. This 

large increase followed efforts to control 

nonnative fish in this reach that resulted in 

the removal of about 20,000 rainbow trout 

from 2003 through 2006. Because rainbow 

trout are known predators of young humpback chub and may also compete with them 

for limited food resources, the increase of 

rainbow trout in the vicinity of the Little 

Colorado River has been cause for concern. 

Although the HFEs have been shown to 

result in temporary increases in the number 

and size of backwater habitats, corresponding beneficial effects on humpback chub 

populations have not been documented.

 Research related to the 1996 and 2008 

experiments1

 indicates that HFEs conducted during early spring and late winter 

can be a tool for maintaining native marsh 

and riparian plant communities and reducing nonnative vegetation. One of the 

primary concerns regarding HFE timing is 

the risk of dispersing seeds of nonnative 

species, especially tamarisk (Tamarix spp.). 

The 1996 and 2008 HFEs occurred before 

tamarisk begins producing seeds—seed 

production generally occurs between April 

and September. Thus, the establishment 

of tamarisk seedlings was low (less than 

2 percent) in 1996 and 2008. Plants that 

1

 No published research is available about the 

effects of the November 2004 HFE on vegetation.

recovered quickly following the 2008 HFE 

were those well adapted to burial. Clonal 

wetland plants also quickly occupied bare 

sandbars and shorelines following both the 

1996 and 2008 HFEs. Therefore, reductions in campsite area because of vegetation recovery and expansion following 

HFEs might offset the temporary increases 

in campsite area that resulted from sandbar 

building during HFEs. 

Three biological conclusions have important implications for designing future HFEs. 

First, on the basis of 2008 HFE research, 

spring-timed HFEs have the potential to 

significantly increase the rainbow trout 

population in the Lees Ferry reach and in 

downstream reaches that support native fish. 

Second, the large increases of rainbow trout 

documented in the Colorado River near its 

confluence with the Little Colorado River 

may adversely affect adult populations of endangered humpback chub. Third, HFEs have 

had no measurable positive impacts on juvenile or adult humpback chub populations. 

A Science-Based Strategy for 

Future High-Flow Experiments

 The U.S. Department of the Interior directed the U.S. Geological Survey (USGS) 

to develop a science-based strategy for conducting future HFEs as part of the Department’s efforts to produce a new protocol 

for these experiments. The strategy outlined 

below is based on adaptive management, or 

“learning by doing,” meaning that the strategy is anticipated to change as new scientific findings improve the understanding of 

how HFEs affect the river ecosystem. The 

primary goal of the strategy is to sustainably rebuild and maintain Grand Canyon 

sandbars, but it would also assist scientists 

to better understand the effects of HFEs on 

biological resources, particularly rainbow 

trout and humpback chub. 

Although HFEs can rebuild sandbars by 

depositing a fraction of new tributary sand 

at higher elevations along shorelines, higher 

flows also efficiently export available 

sand supplies downstream. An important 

objective of any HFE strategy would be to 

achieve a neutral sand budget, so that the 

total sand exported downstream does not 

exceed ongoing tributary sand inputs over 

the long term. Sand storage in the main 

stem is greatest immediately following 

tributary floods, before downstream export 

results from daily dam releases. With only 

about 10 percent of the pre-dam sand supMarch 4, 2008 (before the HFE)

March 11, 2008 (immediately after the HFE)

September 30, 2008 (about 6 months after the HFE)

Repeat photographs of a long-term sandbar 

study site on the Colorado River about 45 

miles downstream from Lees Ferry, Arizona, 

showing how that sandbar was affected by 

the 2008 high-flow experiment (HFE) and by 

erosion in the subsequent 6 months. All of 

the photographs were taken by a remote 

camera at about 4 p.m. and at a water level 

associated with a flow rate from Glen Canyon 

Dam of about 8,500 cubic feet per second 

(ft3

/s). The river flows from left to right. Boat 

(18 feet long) in bottom photo indicates scale.

ply still entering Grand Canyon, primarily 

from the Paria and Little Colorado Rivers, 

the best possibility for rebuilding and maintaining sandbars is conducting frequent 

HFEs following tributary floods that deliver 

large quantities of sand to the river. This is 
transport occurs more slowly, allowing multiple new sand inputs to accumulate before 

an HFE would likely result in the greatest 

sandbar-building response. This accumulateand-release strategy is likely to be most effective if the magnitude and duration of each 

HFE are designed in response to the volume 

and location of new sand in the system. 

However, during years of average or wet upper Colorado River Basin hydrology, when 

releases from Glen Canyon Dam are higher 

and sand is exported downstream more rapidly, HFEs might be more effectively conducted immediately following or even during 

tributary flooding. This option would be particularly appropriate when new sand would 

otherwise be rapidly exported downstream 

(days to weeks) because of large-volume 

dam releases required to meet downstream 

water delivery requirements. Although the 

science-based strategy described above was 

developed on the basis of monitoring data 

and published results, uncertainties exist 

about its ability to maximize future sandbar 

building and how HFEs will affect other resources over the long term. Climate change 

and consequent changes to dam operations 

add to these uncertainties. 

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 

because (1) typical dam operations do not 

allow multiyear accumulation of tributary 

sand inputs on the riverbed and (2) new 

sandbars are eroded by typical dam releases following each HFE. If future HFEs 

are strategically timed to follow tributary 

floods, and the duration and magnitude of 

HFEs are designed to match the volume 

of new sand delivered to the river (shortduration, low-magnitude HFEs when sand 

inputs are small and long-duration, highmagnitude HFEs when sand inputs are 

large), then it may be possible to enlarge 

and maintain sandbars through time.

Paria River flooding is the primary 

source of new sand inputs, and these floods 

typically occur from mid-summer through 

early fall. Therefore, conducting HFEs in 

the fall, following the typical Paria River 

flood pattern, would likely maximize sandbar building. On rare occasions, the Paria 

River floods between December and April, 

so spring-timed HFEs would maximize 

sandbar building in that situation. Because 

of the typical timing of Paria River flooding, about two-thirds of future HFEs would 

occur during the fall, if resource managers 

were to implement this strategy. The Little 

Colorado River also delivers sand to Grand 

Canyon at various times during the year, so 

managers could also consider timing HFEs 

to coincide with flooding on that tributary. 

During years when dam release volumes 

are below average and downstream sand 

Historical data on sand delivery to the Colorado River from flooding on the Paria and 

Little Colorado Rivers support a high-flow experiment (HFE) strategy with spring and fall 

releases from Glen Canyon Dam for the purpose of building and maintaining sandbars in 

Marble and Grand Canyons.

least, neutral. For example, if monitoring 

indicates that sandbars continue to erode or 

cannot be rebuilt and sustained at a desired 

level using this strategy, then managers may 

choose other experimental options, such 

as further constraining daily and seasonal 

water-release patterns, augmenting the 

Colorado River’s sand supply from sources 

in Lake Powell, or both. Monitoring and research associated with other key resources, 

such as native and nonnative fish, cultural 

sites, and recreational resources, would allow managers to detect any adverse effects 

resulting from HFEs and make changes as 

appropriate. Managers, for example, might 

choose to alter the timing of future HFEs to 

try to reduce the rainbow trout response, if 

ongoing monitoring indicates that the large 

increase in rainbow trout associated with 

the 2008 HFE is negatively affecting the 

adult population of humpback chub or other 

native fish. Although the described strategy 

does not guarantee success, sandbar trends 

without HFEs are one of the few outcomes 

that can be predicted with certainty—sandbar size will decrease through time without 

HFEs that follow tributary sand inputs.

A fuller exposition of these HFE-related 

research results can be found in Melis, 

T.S., editor, (in press), Effects of Three 

High-Flow Experiments on the Colorado 

River Ecosystem Downstream from Glen 

Canyon Dam, Arizona (U.S. Geological 

Survey Circular 1366).

Theodore S. Melis, Paul E. Grams, 

Theodore A. Kennedy, Barbara E. 

Ralston, Christopher T. Robinson, John 

C. Schmidt, Lara M. Schmit, Richard A. 

Valdez, and Scott A. Wright

Edited by

Peter H. Stauffer

Graphic design by

Judy Weathers

For more information contact:

U.S. Geological Survey

Southwest Biological Science Center

Grand Canyon Monitoring and Research Center

Flagstaff, Arizona

928-556-7094

This Fact Sheet and any updates to it are 

available

online at 

http://pubs.usgs.gov/fs/2011/3012/

0

50

100

150

200

250

300

350

400

JUL AUG SEP OCT NOV DEC JAN FEB MAR APR MAY JUN

Average Monthly Sand Load

(thousands of metric tons)

Paria

Fall Accounting Period Spring Accounting Period

Paria River Little Colorado River