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
analysis, evaluation, and remedial protection actions necessary to preserve
the historic properties within Glen and Grand Canyons. Remedial actions
would be performed on an as-needed basis as the result of knowledge
gained through monitoring. NPS would be the lead agency in any
required NEPA compliance. The ongoing consultation process should
minimize the influence of Glen Canyon Dam operations on cultural
resources.
Flood Frequency Reduction Measures
Although infrequent floodflows may be considered beneficial to
downstream resources, frequent or unscheduled floods-particularly those
of long duration-are damaging to downstream resources. Under this
common element, the frequency of unscheduled floodflows greater than
45,000 cfs would be reduced to no more than 1 year in 100 years as a
long-term average. This would allow management of the habitat
maintenance flows and beach/habitat-building flows described later in
this section. Floodflow frequency of once in 100 years is considered rare
enough for resource needs, while not imposing unreasonable requirements
on Lake Powell water storage.
Two separate methods of reducing flood frequency have been identified.
These methods focus on reserving additional storage space for flood
control.
1. Increase the capacity of Lake Powell 0.75 maf by raising the height of
the four spillway gates 4.5 feet to elevation 3704.5 feet (currently, each gate
is 40 feet wide and 52.2 feet high). This additional capacity would be
nonviolable flood control space and would be used only in years when
existing flood protection measures were insufficient. Construction of this
project would cost about $3 million. No permits under the Clean Water
Act or Rivers and Harbors Act would be required.
2. Change releases to target a maximum reservoir content of 23.3 maf
(1 maf less than the current active capacity) in the spring until the runoff
peak has clearly passed. This additional space would allow improved
management of late-season forecast errors, the primary cause of flood
releases that exceed 45,000 cfs. The amount of required vacant space in the
spring months would eventually decrease as Upper Basin depletions
increase.
By implementing either flood protection measure, additional reserved
reservoir space would be available from January 1 through July 1 to store
any additional unforecasted inflow.
Glen Canyon Dam EIS Summary 13