Background Operation of Grand Valley Diversions

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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 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 first 18 days of September 1991, GMR released over 25,000 AF of water at an average flow rate of over 700 cfs. The Colorado River Water Conservation District ( CRWCD) developed a computer spreadsheet to assess 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 current operation plan for Green Mountain Reservoir says is available for sale. Adding the 20,000 to 30,000 AF to the 66,000 that was released in 1977, a total of 86,000 to 96,000 AF of releases from GMR would have been required if the check had been operated then as it is today. 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 in the river decreasing to these levels is remote as long as releases are available from GMR. The peak flow rate required to maintain the 600 cfs flow recommendation is 565 cfs based upon the monthly deliver requirements by the FWS. Based upon the CRWCD computer model, the current operation of the check could deplete or nearly deplete all Western Slope storage in GMR. If this happens, both water users and fish habitat would suffer. Consequently, in a very low water year, it may actually be 7