9603

• UCREFRP Catalog Number: 9603
• Author: Clayton, R. and A. Gilmore.
• Title: Flaming Gorge Environmental Impact Statement Hydrologic Modeling Study Report - Results of Action and No Action Alternative Analysis.
• USFW Year: 2001.
• USFW - Doc Type: \
• Copyright Material: NO

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27

Summary

The results presented in this report describe three separate runs of the Green River model.
Two of these runs were controlled by rulesets that achieved the objectives of the Action
Alternative while the other run was controlled by a ruleset that achieved the objectives of the No
Action Alternative. The Action Alternative is an operational regime for Flaming Gorge Dam that
achieves the flow objectives of the Flow Recommendations while "maintaining" the resources
for which Flaming Gorge Dam was authorized. The No Action Alternative is an operational
regime that achieves the flow objectives of the 1992 Biological Opinion while also "maintaining"
the resources associated with the authorization of Flaming Gorge Dam. The rulesets "maintain"
the resources associated with the authorizing purposes of Flaming Gorge Dam by minimizing
bypass releases as much as possible while achieving the flow objectives for each of the proposed
alternatives.

The difference between the two rulesets of the Action Alternative is the degree to which
the flow objectives of the Action Alternative are achieved. The first version, referred to as the
Action(ALL) ruleset, achieved all of the flow objectives of the Action Alternative. Results from
this model run showed that the frequency and magnitude of bypass releases were much greater
than in the No Action model run. Bypasses in the Action(ALL) model run were 53% while the
No Action model run had a bypass frequency of 18%. The frequency and magnitude of the
bypasses in the Action(ALL) model run had a dramatic effect on the reservoir elevation when
compared to the No Action model results. The occurrences of the reservoir elevations below
6000 feet above sea level were significant for the Action(ALL) model run while there were no
occurrences of elevations below 6000 feet in the No Action model run. In general, the reservoir
elevations during the summer months, on average, were about seven feet lower in the
Action(ALL) model run than they were in the No Action model run.

It was discovered that one flow objective for the Action Alternative caused most of the
increase in the frequency and magnitude of the spring bypass releases. The Action Alternative
flow objective that requires flows in Reach Two in excess of 18,600 cfs for two weeks in 40% of
all years caused most of the increase in the frequency and magnitude of the bypasses that
occurred in the Action(ALL) results. To achieve this objective, 40% of all years were required to
have peak releases with magnitudes of 8600 cfs and durations of at least two weeks while 20% of
all years were required to have magnitudes of 10,600 cfs and durations of at least two weeks.
Achieving the other objectives of the Action Alternative did not require peak release magnitudes,
durations, and frequencies at these levels.

The second version of the Action Alternative model run, referred to as the Action(ALL-
1) model run, achieved all the flow objectives of the Action Alternative but did not specifically
make any attempt to achieve the 18,600 cfs objective. While achieving all other flow objectives,
the Action (ALL-I) model run was able to achieve 18,600 cfs for 2 weeks or greater in 18.2% of
all years. Reach Two flows did achieve 18,600 cfs in 40% of all years, but the duration was 6
days compared to the flow objective duration of 14 days. The results from the Action(ALL-l)
model run showed a significant improvement to the impacts that were observed in the reservoir