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
Daily and Hourly
Operaflons
Habitat Main-
tenance Flows
alternative's parameters have changed slightly since the draft EIS was
published based on new information and public comments.
To reduce long-term flood frequency, a single method is advanced under
this alternative-raising the height of the four spillway gates by 4.5 feet
(see "Common Elements"). However, since other methods are available to
accomplish this same goal, a final decision about the method ultimately
used would not be made until additional NEPA compliance has been com-
pleted to evaluate impacts related to Lake Powell shoreline resources.
Lake Powell's current elevation is well below the level that would require
reserving additional storage space, thus accomplishing the objective of
reducing the frequency of flood releases. The lake level is not expected to
reach full elevation for another 4 or 5 years. Until the spillway gates were
installed, additional operational measures would be implemented through
the Annual Operating Plan to provide the recommended flood protection.
Minimum flows would be no less than 8,000 cfs between 7 a.m. and 7 p.m.
and 5,000 cfs at night. The maximum rate of release would be limited to
25,000 cfs during fluctuating hourly releases. Any releases greater than
25,000 cfs (other than for emergencies) would be steady on a daily basis
and would be made in response to high inflow and storage conditions.
Ramp rates would be limited to 4,000 cfs per hour for increasing flows and
1,500 cfs per hour for decreasing flows.
Fluctuations would be limited during any 24-hour period to either 5,000;
6,000; or 8,000 cfs, depending on monthly release volumes. The maximum
allowable release under this alternative was increased from 20,000 to
25,000 cfs since the draft EIS was published. However, flows are expected
to only rarely exceed 20,000 cfs during a minimum release year (less than
1 percent of the time). This is because the maximum allowable daily
change constraint overrides the maximum allowable release and because
monthly release volumes are lower during minimum release years.
The increased maximum allowable releases would not result in any
measurable difference in impacts on downstream resources.
Maximum releases under the Modified Low Fluctuating Flow Alternative
normally would not exceed 25,000 cfs during a minimum release year.
Without higher flows:
• Portions of sandbars above the normal peak stage could not be rebuilt.
• Sediment would accumulate at low elevations, including backwaters.
• Camping beaches and return-current channels would likely become
filled with sediment and eventually overgrown with vegetation.
Glen Canyon Dam EIS Summary 25