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. . GVWM Flow Protection Plan December 19, 1994 4:02pm DRAFT 25,000 acre-feet (at) of water at an average flow rate of over 700 cfs or about 1,400 afper day. The Colorado River Water Conservation District (CRWCD) developed a computer spreadsheet J.'",- 1471 that assessed the impact of the current 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. Consequently, a total of 86,000 to 96,000 af of releases from GMR woul,li have been required in 1977 if the check had been operated then as it currently. The ~ 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 the Grand Valley Power Plants return pumping and power generation flows to the RtllaMI\ ,~t.... 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 r~~ ~UR.h theOf(;I1~<t.M.eS1': Power and Grand Valley Pumping Plant to thel1ffeachpl Unde't.o~urrent opera'trans or-tlie vOrcna1t~esa lnigation District check, this flow is maintained until the natural flow in the river falls b~I"Q.:v 2,140 cfs. For every 1 cfs decrease below 2,140 cfs at the Cameo gage, the flow in the'Reach decreases by 1 cfs. The Reach can be essentially dry immediately downstream of GVIC's diversion dam when the natural flow at Cameo falls below 1,5~0 cfs. However, the possibility of the flows in the river decreasing to these levels is remote ~ long asAreleases are available..from GMR. j;J",,, Based upon the CRWCD spreadsheet model, the current operation of the check could deplete or nearly deplete all Western Slope storage in GMR in dryer years. If this happens, both water users and fish habitat would suffer. Consequently, in a very low water year, it may "a"ally be beneficial from a water supply standpoint to use the check to supply some of GVIC's junior 120 cfs water rig~.t._~, operation would save water in GMR for release later in the year. If the check is used~CiVfC's junior water right, the flow in the beginning of the Reach would fall below 582 cfs. Even though thi,iLfIRl':: rattsi~b~!ow the desired minimum, the saved water could prevent near zero flows irF thl1Reach"'rater tnrne irrigation season. '*" Green Mountain Reservoir - was constructed as'rompensatory storage facility for the diversion of water to the eastern slope of Colorado through the Colorado,Big Thompson Project. The 152,000 af reservoir has two main storage pools, a 52,000 af replacement pool to replace out-of- priority diversion by the Colorado Big Thompson Project and a 100,000 af power pool which (;;;, ~ also available for west slope irrigation and domestic purposes. Upstream junior diverters can be grouped into two main categories, beneficiaries and non-beneficiaries of Green Mountain Reservoir. In practice this means that those junior diverters who are not beneficiaries of GMR 10