Windy Gap Firming Project Draft Environmental Impact Statement August 2008

• Description: Windy Gap Firming Project Draft Environmental Impact Statement August 2008 Reclamation Managing Water in the West
• State: CO
• Date: 8/1/2008
• Title: Windy Gap Firming Project Draft Environmental Impact Statement August 2008
• Water Supply Pro - Doc Type: Publication

EXECUTIVE SUMMARY
WINDY GAP FIRMING PROJECT DRAFT ENVIRONMENTAL IMPACT STATEMENT
Stream Morphology and Floodplains
Stream morphology refers to the form and structure of a
stream, including its channel, banks, floodplain and
drainage area, which could be altered as a result of
changes in flow. The upper Colorado River is a
morphologically stable stream. The changes in flow
expected from the WGFP are not expected to cause
measurable changes to stream morphology or to
sediment transport and deposition in the Colorado River
below Windy Gap Reservoir.
Table ES -3. Windy Gap Firming Project firm
yield.
Condition /Alternative
Firm Yield (AF)
Existing Conditions
0
Alt. 1 — No Action
1,229
Alt. 2 — Proposed Action
26,559
Alt. 3
25,849
Alt. 4
25,849
Alt. 5
26,629
• Under all alternatives, the 2 -year peak discharge on the Colorado River at the Hot Sulphur Springs
gage below the Windy Gap diversion would be exceeded about 3 percent of the time, or about 1
percent less frequently than under existing conditions. High volume channel maintenance flows
would also experience a slight decrease in frequency. The projected reduction in the frequency of
peak discharges and channel maintenance flows is unlikely to significantly affect stream morphology
or change sediment transport or deposition.
• Flushing flows in the Colorado River equal to or greater than 450 cfs, which occur about 45 days per
year on average under existing conditions, would decrease to 38 days per year under the No Action
alternative, 36 days under the Proposed Action, and 35 days under the other alternatives. The
reduction in the frequency of flushing flows would remain adequate to transport sediment and prevent
deposition.
• Increased flows in East Slope streams below the Participants WWTPs would have minimal effect on
stream morphology.
• The potential for flooding along the Colorado River and Willow Creek would decrease and the
potential for flooding along East Slope streams below the Participants WWTPs would increase
slightly.
Surface Water Quality
Water quality impacts from WGFP include changes in the Colorado River below Granby Reservoir, in
Willow Creek below Willow Creek Reservoir, and in several East Slope streams, including the Big
Thompson River, St. Vrain Creek, North St. Wain Creek, Coal Creek, Big Dry Creek, and the Cache la
Poudre River. Potential effects to water quality were also evaluated in the Three Lakes system (Granby
Reservoir, Shadow Mountain Reservoir, and Grand Lake), Carter Lake, and Horsetooth Reservoir, as well as
the predicted water quality for new reservoirs. Stream and reservoir water quality models were used to
estimate the following water quality effects.
• Under average flow conditions for a typical late July day below Windy Gap Reservoir, temperatures
in the Colorado River are predicted to increase 0.5 °C under the No Action alternative, 0.6 °C for the
Proposed Action, and 0.7 °C to 0.8 °C for the other alternatives. This would increase the potential for
exceedance of the maximum weekly average temperature standard (18.2 °C) for all alternatives.
• When Windy Gap diversions reduce Colorado River flow to the 90 cfs minimum flow in late July
(which occurs infrequently), temperatures are predicted to increase about 4 °C for all alternatives
ES -11