7423

• UCREFRP Catalog Number: 7423
• Author: U.S. Bureau of Reclamation.
• Title: Glen Canyon Environmental Studies Draft Report, September 1987.
• USFW Year: 1987.
• USFW - Doc Type: Washington, D.C.
• Copyright Material: NO

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and on the quality and safety of river recreation.
Concern over the short- and long-term impact of these
large releases provided another focus for the study.

Reduced River Temperature Resulting From
Construction Of Glen Canyon Dam Diminished Habitat
For Humpback Chub

On May 25, 1978, the FWS concluded that construction
and operation of Glen Canyon Dam had jeopardized the
continued existence of the humpback chub by reducing
water temperature and changing the aquatic system.
They also concluded that dam operations were limiting
the potential for recovering the humpback chub, the
Colorado squawfish, the bony tail chub, and the
razorback sucker. Because little information was
available on habitat needs of these fishes, the FWS was
unable to recommend any changes in dam operation which
would aid recovery of the fish. Additional study was
therefore requested.

Our Understanding Of The Relationships Between
Dam Operations And Downstream Resources Is Not
Complete

The GCES occurred at a critical juncture in the "life
history" of Glen Canyon Dam, when significant releases
above powerplant capacity were occurring with
regularity for the first time. When Lake Powell filled
in 1980, a 17-year period with virtually no releases
over 31,500 cfs came to an end. Because the reservoir
no longer had a vast amount of unfilled space to store
spring runoff, the likelihood of releases exceeding
powerplant releases in spring and early summer in-
creased substantially.

The filling of the reservoir corresponded with years of
unusually high basin runoff in 1983 through 1986. This
combination of events led to the flood releases seen in
five of the past seven years. Figure II-1 illustrates