he Lees Ferry reach of the 

 Colorado River—a 16-mile 

segment from Glen Canyon Dam to 

the confluence with the Paria River—

supports an important recreational 

rainbow trout (Oncorhynchus mykiss)

fishery. In Grand Canyon, nonnative 

rainbow trout prey on and compete 

for habitat and food with native fish, 

such as the endangered humpback 

chub (Gila cypha). Experimental flow 

fluctuations from the dam during 

winter and spring 2003–5 dewatered 

and killed a high proportion of rainbow 

trout eggs in gravel spawning bars, 

but this mortality had no measurable 

effect on the abundance of juvenile 

fish. Flow fluctuations during summer 

months reduced growth of juvenile 

trout relative to steadier flows. A 

high-flow experiment in March 2008 

increased both trout survival rates for 

early life stages and fish abundance. 

These findings demonstrate that Glen 

Canyon Dam operations directly affect 

the trout population in the Lees Ferry 

reach and could be used to regulate 

nonnative fish abundance to limit 

potential negative effects of trout 

on native fish in Grand Canyon.

T

The Effects of Glen Canyon Dam Operations on Early Life 

Stages of Rainbow Trout in the Colorado River

U.S. Department of the Interior

U.S. Geological Survey

USGS Fact Sheet 2011–3002

2011

2008; Makinster and others, 2010). Adult 

rainbow trout are known predators of 

young chub (Yard and others, in press) and 

also compete with native fish for food and 

habitat. 

In the Colorado River downstream 

from Glen Canyon Dam, mature rainbow 

trout spawn during winter through spring.

Fertilized eggs are deposited in nests, 

known as “redds,” created by spawning 

fish in the gravel on the river bottom. 

Eggs hatch after 1 month, and young trout 

live as “alevins” in the gravel for another 

month, nourished by an egg yolk sac that 

remains attached to their bodies. As the 

yolk is depleted, the young trout emerge 

from the gravel as “fry” and begin to feed 

Increased numbers of nonnative rainbow trout (Oncorhynchus mykiss; top) may adversely affect 

native humback chub (Gila cypha; lower left), an endangered species whose largest population is found in 

Grand Canyon National Park. Operations of Glen Canyon Dam (lower right) could be used to regulate nonnative 

fish abundance. (Photos: rainbow trout, © Eric Engbretson, used with permission; humpback chub, George 

Andrejko, Arizona Game and Fish Department; Glen Canyon Dam, © Christopher E. Taesali, used with permission.) 

Background

The Lees Ferry reach of the 

Colorado River, a 16-mile segment from 

Glen Canyon Dam downstream to the 

confluence with the Paria River, supports 

an important recreational rainbow trout 

(Oncorhynchus mykiss) fishery. Federal 

and State agencies introduced rainbow 

trout to the Lees Ferry reach following 

construction of the dam. There is 

increasing evidence that some of the trout 

produced in this reach move downstream 

through Marble Canyon to the confluence 

with the Little Colorado River, an area 

used by the largest population of the native 

endangered humpback chub (Gila cypha) 

in the Colorado River Basin (Coggins, 

on drifting aquatic insects. Survival rates 

for these early life stages can influence 

the abundance of the adult rainbow trout 

population. However, reduced abundance 

at an early life stage can actually lead to 

increased survival at one or more latter 

stages because of lower density, which 

reduces competition for limited food 

and habitat. Such compensatory survival 

responses in early life stages are seen 

in many fish species and may limit the 

extent to which their abundance changes 

in response to a decrease in survival at one 

life stage. 

Because of potential negative effects 

of rainbow trout on humpback chub, 

fluctuations in flow from Glen Canyon 
Aerial photograph of a gravel bar in the Lees Ferry reach of the Colorado River, annotated to show by colored 

dots the locations of rainbow trout redds (nests for spawning) in winter and spring of 2004. The colors indicate 

the elevation of each redd (light blue are lowest elevations; green, orange, and white are middle elevations; red 

are highest elevations). Only the light blue redds located at the lowest elevations were permanently submerged 

during this period. Other redds were dewatered for part of the day because of experimental fluctuating flows, or 

fish-suppression flows, released from Glen Canyon Dam in January through March of 2003–5. These dewatered 

redds did not produce viable young because elevated temperatures during the day killed eggs and recently 

hatched fish.

Lake Powell

Havasu

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Diamond Creek

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Glen Canyon Dam

GRAND CANYON

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AREA

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UTAH

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Colorado

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A R B L E C A N Y O N

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Map

Area

Map of the Colorado River downstream of Glen Canyon Dam showing the study area.

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20 0 20 40 Meters

N

Dam were increased during January 

through March of 2003–5 with the intent 

of reducing the abundance of rainbow 

trout. In addition, rainbow trout were 

removed near the Little Colorado River 

during 2003–6 (Coggins and others, in 

press). High-flow experiments (HFEs) 

from Glen Canyon Dam have also been 

conducted to improve camping beaches 

and potentially increase the quality of 

shoreline habitats for native fish in Grand 

Canyon National Park (Melis and others, 

2010). These HFEs also have the potential 

to affect survival rates for early life stages 

of rainbow trout, which in turn may affect 

the abundance of adult trout, the Lees 

Ferry fishery, and perhaps the survival 

rates of humpback chub and other native 

fish. The U.S. Geological Survey (USGS) 

initiated cooperative research studies 

in 2003 to evaluate the effects of these 

experimental dam operations on early life 

stages of rainbow trout in the Lees Ferry 

reach. The purpose of this Fact Sheet is to 

summarize the main findings to date from 

this research.

Findings

Increased experimental flow 

fluctuations during January–March killed 

25 and 50 percent of the eggs deposited 

in the Lees Ferry reach in 2003 and 

2004, respectively, because the redds 

were dewatered and exposed to lethal air 

temperatures for part of the day (Korman 

and others, in press). However, the number 

of fry produced in those years was similar 

to years under normal operations (2006–7, 

2010), because fry from the eggs that 

were not dewatered had higher survival 

rates owing to a compensatory survival 

response. 

In contrast, survival rates of early 

life stages increased more than fourfold 

following the March 2008 HFE compared 

to survival rates before the experiment. 

Fry abundance in 2009 was more than 

twofold higher than expected, given the 

estimated number of viable eggs deposited 

that year, but fry abundance in 2010 was 

similar to levels between 2003 and 2007. 

This pattern could indicate that the effect 

of an HFE on early life stages of trout 

declines through time, with increased 

survival rates lasting for as long as 2 years. 

In 2008, survival rates were much higher 

for trout hatched about a month after the 

HFE than for those hatched earlier 
in the year. Fish that hatched a month 

after the HFE (about mid-April) were not 

exposed to high flows and emerged into 

higher quality habitat with greater food 

availability (Rosi-Marshall and others, 

2010). Increased abundance of fry in 2008 

eventually led to increased abundance 

of 1-year-old trout in 2009 in the Lees 

Ferry reach, and some of these fish likely 

moved downstream to the area near the 

confluence with the Little Colorado River 

(Makinster and others, 2010) used by 

humpback chub. 

Research findings indicate that typical 

flow fluctuations associated with the 

modified low fluctuating flow operations—

the operating regime in place since 1996—

also affect trout in the Lees Ferry reach. 

Hourly flow fluctuations in summer force 

rainbow trout fry to stay farther from shore, 

where it is harder to feed and where water 

temperatures are cooler, which in turn 

reduces their growth. Otoliths, minute bony 

structures found in the inner ear of fish that 

record patterns of growth, similar to tree 

rings, were used to evaluate the effects of 

flow fluctuations on rainbow trout growth. 

Otoliths examined in 2003 showed a weekly 

striping pattern, indicating increased trout 

growth on Sundays, when flows during the 

day were low and steady compared to other 

days of the week when flows fluctuated 

(Korman and Campana, 2009). In 2004, 

when flows on Sunday were similar to other 

days of the week, the weekly high-growth 

striping pattern was not observed. 

Management Implications

These research findings indicate 

that the operation of Glen Canyon Dam 

is directly linked to survival rates and 

production of rainbow trout downstream 

of the dam, which in turn may affect 

survival rates of native fish. Reducing 

rainbow trout abundance by dewatering 

redds during winter and spring through 

increased daily flow fluctuations is 

unlikely to be effective in reducing trout 

populations because of compensatory 

survival responses in later life stages. 

However, increased flow fluctuations 

during summer may be effective at 

reducing trout numbers, because these 

fluctuations negatively affect fry growth 

and habitat use. Experimental flow 

fluctuations during summer would likely 

have to be carried out over successive 

years to reduce the abundance of the adult 

trout population. The efficacy of such 

2003 

2007 

2006 

2004 

2008 

2009 

2010 

0.0 

0.2 

0.4 

0.6 

0.8 

1.0 

0 1 2 3 4 5 6 7 

Fry Abundance in July (millions) 

Eggs (millions) 

The relation between the number of viable rainbow trout eggs deposited in the Lees Ferry reach and the resulting 

population size of fry in mid-July for the years 2003–10 (no data for 2005). The thick black curve shows the best-fit 

relation between viable eggs and fry using data from all years except those affected by the March 2008 high-flow 

experiment (2008 and 2009), when survival was unusually high. The flat relation indicates that the survival rate 

from egg to fry stage increases with reduced numbers of eggs, a compensatory effect that minimizes the effect 

of egg losses on fry abundance. The green vertical lines show the 95-percent confidence limits of fry abundance 

estimates. (From Korman and others, in press).

Diagrams showing the effect of fluctuating flows from Glen Canyon Dam on the use of nearshore habitat 

by rainbow trout fry in the Colorado River (dots denote the locations of individual fish). The shaded area 

represents the riverbank in cross section. In 2003, when flows fluctuated (high during the day, low at night) 

from Mondays through Saturdays, most fry remained in the permanently submerged zone and did not 

migrate up to the immediate nearshore zone during the day. This occurred even though the habitat higher 

along the riverbank was more suitable for small fish because of warmer water temperatures and lower 

water velocities and depths. On Sundays, flows were low and steady and most fish were located in the 

immediate nearshore zone where conditions were better, resulting in improved growth.

River bank

River bank

Daytime Fluctuating Flows Monday through Saturday

Daytime Low and Steady Flows on Sunday

15°C 11°C

15°C 11°C

Daily maximum ow elevation

Sunday maximum and

minimum ow elevation

Permanently submerged zone

Daily minimum ow elevation

Permanently submerged zone
experimental operations would have to 

be carefully monitored in terms of their 

effects on trout abundance in both the 

Lees Ferry reach and downstream in the 

area of the confluence with the Little 

Colorado River.

Rainbow trout abundance increased 

following the March 2008 HFE, in part, 

because of improvements in habitat 

and increased food availability for fry. 

The increase in fry ultimately led to an 

increase in abundance of older trout 

in the Lees Ferry reach and likely also 

downstream in the area of the confluence 

with the Little Colorado River. If this 

effect is sustained under repeated 

HFEs, then expanded control efforts 

for nonnative fish may be warranted to 

preserve and restore native fish. On the 

basis of currently available research, 

it is uncertain whether future HFEs, 

with the same or different timing, will 

produce rainbow trout responses similar 

to those seen in 2008–9. However, this 

uncertainty could be resolved if fish 

responses to future HFEs are closely 

monitored.

References

Coggins, L.G., Jr., 2008, Active adaptive 

management for native fish conservation in the Grand Canyon—implementation and evaluation: Gainesville, 

University of Florida, Ph.D. dissertation, 173 p.

Coggins, L.G., Yard, M.D., and Pine, W.E., 

in press, Nonnative fish control in the 

Colorado River in Grand Canyon, 

Arizona—an effective program or 

serendipitous timing?: Transactions of 

the American Fisheries Society.

Korman, J., and Campana, S.E., 2009, 

Effects of hydropeaking on nearshore 

habitat use and growth of age-0 rainbow trout in a large regulated river: 

Transactions of the American Fisheries Society, v. 138, no. 1, doi: 10.1577/

T08-026.1, p. 76-87, accessed on 

January 11, 2010, at http://

afsjournals.org/doi/pdf/10.1577/T08-

026.1.

Korman, J., Kaplinski, M., and Melis, T.S., 

in press, Effects of fluctuating flows 

and a controlled flood on incubation 

success and early survival rates and 

growth of age-0 rainbow trout in a 

large regulated river: Transactions of 

the American Fisheries Society.

Makinster, A.S., Persons, W.R., Avery, 

L.A., and Bunch, A.J., 2010, Colorado River fish monitoring in Grand 

Canyon, Arizona—2000 to 2009 

summary: U.S. Geological Survey 

Open-File Report 2010-1246, 26 p., 

accessed on November 5, 2010, at 

http://pubs.usgs.gov/of/2010/1246/.

Melis, T.S., Topping, D.J., Grams, P.E., 

Rubin, D.M., Wright, S.A., Draut, 

A.E., Hazel, J.E., Jr., Ralston, B.E., 

Kennedy, T.A., Rosi-Marshall, E., 

Korman, J., Hilwig, K.D., and Schmit, 

L.M., 2010, 2008 High-flow experiment at Glen Canyon Dam benefits 

Colorado River resources in Grand 

Canyon National Park: U.S. Geological Survey Fact Sheet 2010-3009, 

4 p., accessed on July 27, 2010, at 

http://pubs.usgs.gov/fs/2010/3009/.

Rosi-Marshall, E.J., Kennedy, T.A., 

Kincaid, D.W., Cross, W.F., Kelly, 

H.A.W., Behn, K.A., White, T., Hall, 

R.O., Jr., and Baxter, C.V., 2010, 

Short-term effects of the 2008 highflow experiment on macroinvertebrates in the Colorado River below 

Glen Canyon Dam, Arizona: U.S. 

Geological Survey Open-File Report 

2010-1031, 28 p., accessed on August 

10, 2010, at http://pubs.usgs.gov/

of/2010/1031/.

Yard, M.D., Coggins, L.G., Baxter, C.V., 

Bennett, G.E., and Korman, J., in 

press, Trout piscivory in the Colorado 

River, Grand Canyon—effects of 

turbidity, temperature, and fish prey 

availability: Transactions of the American Fisheries Society.

Josh Korman and Theodore S. Melis

Edited by

Peter H. Stauffer

Graphic design by

Stephen L. Scott

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/2011/3002 

Cross section from an otolith of a 43-mm (1.7 inches) rainbow trout fry that was 81 days old from hatch when 

sampled on July 30, 2003. Otoliths are minute bony structures found in the inner ear that show daily growth 

patterns in many fishes. The image (magnified 400 times) shows the weekly striping pattern (identified by 

white arrows) caused by increased growth during low and steady flows on Sundays. (From Korman and 

Campana, 2009)