WSP08403

• File Number: 8220.101.17
• Description: Glen Canyon Dam/Lake Powell
• State: AZ
• Basin: Colorado Mainstem
• Date: 1/1/1999
• Author: BOR
• Title: GCD Modifications to Control Downstream Temperatures: Plan and Draft Environmental Assessment
• Water Supply Pro - Doc Type: EIS

Chapter III Affected Environment and Environmental Consequences 25

competition and predation by non-native fish, and cold-water releases and habitat
fragmentation from Glen Canyon Dam.

The biological foundation of the aquatic system in the post-dam Colorado River below Glen
Canyon Dam is Cladophora glomera/a, a filamentous green alga. Clear river conditions
created by the dam make the abundant growth of Cladophora possible. Together,
Cladophora, diatoms, and associated invertebrates (Gammarus and insects) provide an
important food source for other organisms in the food chain.

River Temperatures - Average monthly river temperatures predating the dam ranged from
just above freezing to about 260C (780F). Records show that present release temperatures
from the existing power intakes vary
little from 80C (460F). As the river
flows downstream through the warm
summer desert environment, water
temperatures increase about I oC for
every 30 miles traveled. During the
summer, temperatures increase from
80C near the dam to about l60C
(61 OF) at mile 240. This is below the
last aggregation of chub near
Diamond Creek and far downstream
of most of the known aggregations of
humpback chub.

The proposed temperature control
alternative would allow a
summertime release of l50C (590F).
It is expected that this would greatly
improve growth rates for rainbow
trout in the river reach between the
dam and Lees Ferry. Then as the
water flows downstream,
temperatures would increase into the
range (160C-220C) where they are suitable for most all of the nine known aggregations of
humpback chub.

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Diamond
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Figure 6 - Water temperatures increase with
distance below Glen Canyon Dam (for 80C and
l50C release temperatures). Also shown are the
locations of the 9 aggregations of endangered,
humpback chub.

Lake Temperatures - A computer modeling study of Lake Powell was conducted by
Reclamation's Denver Technical Service Center to evaluate potential changes which might
impact critical resources in Lake Powell. Surface temperatures during the winter are critical
to the survival of the threadfin shad, an important forage fish for the Lake Powell sport
fishery. Threadfin shad are very near their temperature tolerance during the winter season.
Surface temperatures in Lake Powell vary from about 100C (500F) in winter to over 250C
(770F) in summer. Surface temperatures in the winter below 100C (50 OF) could cause
problems for threadfin shad.