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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Water temperature varied seasonally: near freezing in
December and January and reaching a peak of near 80
degrees F in July and August. The pattern of water
temperatures did not substantially change from year
to year even when volume of flow was greatly different.

Post-Dam River Flows Fluctuate Frequently, Carry
Little Sediment, And Are Colder

The pattern of post-dam flows is much different.
Seasonal changes in flow magnitude, temperature, and
sediment load are much less. However, daily
fluctuations in flow are much greater. Flow regulation
reduced the average annual peak flow from about 93,000
cfs in the pre-dam era to about 29,000 cfs for the
period 1963 to 1980, when Lake Powell was being filled.
During a representative post-dam, low-runoff year
(1982, 8.3 maf) , peak flow remained below the
powerplant capacity of 31,500 cfs (Figure IV-2). Daily
flows were released in response to power demand,
changing by as much as 20,000 cfs in a 24-hour period,
and resembling the pattern of daily releases shown in
Figure V-3. Flow was in the range of 16,000 to 27,500
cfs for 25 percent of the year, 10,000 to 16,000 cfs
for 34 percent of the year, and below 10,000 for 42
percent of the year. Annual flow volume past Lees
Ferry ranged from 2.4 to 20.5 maf.

In contrast, during the high-water year of 1986 (16.6
maf) , the river outlet works were used to bypass the
powerplant, releasing excess runoff, and daily flows
reached 51,600 cfs (Figure IV-2). Flows exceeded
31,500 cfs for 42 days in May and June. Flows fluc-
tuated during the rest of the year, but remained above
16,000 cfs about 70 percent of the time.

All sediment from upstream of the dam is now trapped in
Lake Powell, drastically reducing the sediment load of
post-dam flows. Annual suspended sediment load at Lees
Ferry, which is upstream of any major tributary, is
estimated to have been 0.4 million tons in 1982 and in
1986, a decrease of about 99.5 percent from pre-dam
conditions. Virtually all the sediment added to the
system must now be delivered by tributaries below Lees
Ferry. (Note the change in the axis scale for sediment
between Figures IV-1 and IV-2.)

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