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

An increase in amplitude or rate of change
fluctuations could increase the number of stranded
and loss of backwaters. Increase in the frequency
low flows could increase reproductive losses
rainbow trout.

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Use of the uprated capacity in the Glen Canyon
generators may lead to several changes in flow patterns
from the dam. These changes would be most apparent in
water years with moderate runoff, which occur
approximately 30 percent of the time. In these years,
the highest steady releases may be raised from 31,500
c{s to 33,100 cfs. This corresponds to a maximum rise
in river level of less than one foot. During periods of
fluctuation, the peak flows may also be increased to
33,100 cfs. This would require either lowering the
bottom end of fluctuations by approximately 2,000 cfs
or by increasing the rate of rise and fall in the
pattern of releases. In years of high runoff, which
also occur approximately 30 percent of the time, the
effect of the uprate would primarily be to raise the
peak steady releases from 31,500 cfs to 33,100 cfs
(Appendix D, section II.)

Changes in the level of steady releases, from 31,500 to
33,100 cfs, are not likely to affect recreation
significantly. However, increases in the range or the
rate of fluctuations would have a negative effect on
both fishing and white-water boating.

For some resources, the actual impact of the rewind and
uprate may be more than the change from 31,500 to
33,100 cfs. Before the uprate program, discharges
between 27,500 and 31,500 cfs were infrequent.
Sand-dependent resources and vegetation therefore may
have stabilized in many areas to a level corresponding
to a discharge closer to 27,500 than to 31,500 cfs.
The difference in water level between 27,500 and 33,100
cfs is between 1.0 and 1.5 feet, depending upon the
width of the reach.

A change in water level of this size would result in
significant loss of camping beach area, substrate,
backwaters, and areal extent of vegetation,
particularly in narrow reaches. No species of
terrestrial vegetation or wildlife would likely be lost
from the canyon, but the number of nesting birds and
other wildlife could decline.

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