8128

• UCREFRP Catalog Number: 8128
• Author: U.S. Bureau of Reclamation.
• Title: Operation of Glen Canyon Dam\
• USFW Year: 1995.
• Copyright Material: NO

Description of Alternatives
Since the only known humpback chub population in the Lower Colorado
River Basin depends on the LCR for survival, a catastrophic event or a
series of incidents that would reduce the viability of this spawning
habitat could cause the loss of this population. This possibility will persist
until or unless:
1. At least one more population is established in the mainstem or one or
more of the tributaries below Glen Canyon Dam, and/or
2. Mainstem water temperatures are sufficiently warmed to support
spawning and recruitment.
Therefore, in consultation with FWS, NPS, AGFD, and other land
management entities such as the Havasupai Tribe, Reclamation would
make every effort-through funding, facilitating, and technical support-
to establish a new population of humpback chub within Grand Canyon.
Further Study of Selective Withdrawal
Increasing mainstem water temperatures by means of selective withdrawal
structures installed at Glen Canyon Dam offers the greatest potential for
creating new spawning populations of humpback chub and other native
fish in Grand Canyon.
Water released from Glen Canyon Dam to produce hydroelectricity is
withdrawn from the cold depths of Lake Powell at an elevation of
3470 feet-230 feet below the water surface when the reservoir is full
(3700 feet). The river water temperature at Lees Ferry, 16 miles
downstream, is nearly constant year round and averages about 46 degrees
Fahrenheit (°F).
Only a few species of aquatic organisms thrive under these conditions, but
those few species are abundant. However, many native species cannot
reproduce in these constant, cold temperature conditions.
Except for draining the reservoir, no operational method could prevent the
continued release of cold water. Multilevel intake structures (a means of
selective withdrawal) could be built at Glen Canyon Dam to provide
seasonal variation in water temperature. A structure would be attached to
each of the eight existing 15-foot-diameter penstocks to withdraw warmer
water from upper levels of the reservoir.
Preliminary studies indicated that multilevel intake structures on each of
the eight existing penstocks could increase the downstream river
temperature 5 to 18 °F above present conditions (river temperatures
between 54 and 69 °F from May to October). This temperature increase is
still 7 to 16 °F cooler than predam conditions during the summer months
Glen Canyon Dam EIS Summary 15