7750

• UCREFRP Catalog Number: 7750
• Author: U.S. Bureau of Reclamation.
• Title: Study of Alternative Water Supplies for Endangered Fishes in the "15-Mile Reach" of the Colorado River.
• USFW Year: 1992.
• USFW - Doc Type: \
• Copyright Material: NO

CHAPTER H BACKGROUND
t maximum water supply protection needed by West Slope users. That same 66,000 acre-feet
may not be adequate today without a guarantee that the "check" will be operated in the
historic rather than current pattern.
' Under the current strict river administration, the GVIC junior right for 120 cfs will call for
water from Green Mountain Reservoir while the OMID tailrace is allowed to drain freely
t into the Colorado River below the GVIC diversion.
' Impacts on the 15-Mile Reach of the New Operation
The relatively large and senior irrigation and power diversions in the Grand Valley, help
' bring water to the Reach as well as take water from the Reach. These senior rights are
capable of prohibiting upstream junior appropriators from diverting water and are also
capable of requesting releases from GMR. The significance of GMR to the Grand Valley
' water supply should not be understated. Without these releases, upstream water supply to
junior appropriators would be restricted and OMID would frequently be required to
implement the check in order to generate power. For example, in the fast 18 days of
' September 1991, GMR released over 25,000 AF of water at an average flow rate of over
700 cfs.

The CRWCD developed a computer spreadsheet that assessed the impact of the new check
operation on upstream stored water (most notably Green Mountain Reservoir). Current
estimates of the impact depend on the flow capacity of the bypass channel, but range from
about 20,000 to 30,000 AF. This number roughly corresponds to what the operation plan
for Green Mountain Reservoir says is available for sale. A total of 86,000 to 96,000 AF of
releases from GMR would have been required in 1977 if the check had been operated then as
it is under current river administration. The Reach benefits from the new operation of the
check because Grand Valley irrigators will place more demand on upstream storage to meet
diversion requirements while the tailrace of the Orchard Mesa pumping and powerplants
return all pumping and power generation flows to the Reach.
The aggregate of the Grand Valley irrigation and power diversions can place a demand of
about 2,260 cfs on the Colorado River during the irrigation season. When the river is at
2,260 cfs, theoretically 582 cfs will be released through the Orchard Mesa Power and
Pumping Plant to the Reach. Under current operations of the Orchard Mesa Irrigation
District check, this flow is maintained until the flow in the river falls below 2,141 cfs. For
every 1 cfs decrease below 2,141 cfs at the Cameo gage, the flow in the Reach decreases 1
cfs. The Reach can be essentially dry immediately downstream of GVIC's diversion dam
when the natural flow at Cameo falls below 1,559 cfs. However, the possibility of the flows
(Revised 3/92)
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