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APPENDIX B WATER SUPPLY AND HYDROLOGY ISSUES Fish Passage Structure Water Supply During the conceptual design and layout of the proposed fish passage, it was the goal of the design team members to minimize the effect the fish passage would have on the operation of the Grand Valley Irrigation Company's Diversion Dam and to have no effect on water supply to GVIC or any other water users. Since the total diversion requirement of GVIC is 640 cfs and the operational flow of the riffle and pool fish passage arrangement is 100 cfs, it is necessary to have a minimum of 740 cfs in the Colorado River immediately upstream of GVIC's Diversion Dam in order for the passage to operate during periods of peak demand. Early and late in the irrigation season, GVIC may not need a full water supply so a total flow of 740 cfs is not always required. The main point is regardless of GVIC's flow requirements, in order to have enough water in the riffle and pools there must be an additional 100 cfs available. The timing of fish migration depends on several factors, some of which are understood and others not as well understood. All fish migration and significant movement is thought to occur between April 1 and October 31. To evaluate the likelihood that there would be at least 100 cfs available in the Colorado River upstream of the GVIC Diversion Dam for fish passage operation, two different conditions were analyzed; historic and enhanced flows. Historic flows consider how much water would have been available based upon what was there historically, and enhanced flows consider opportunities, both existing and potential, to enhance river flows. Historic Flows The U.S. Fish and Wildlife Service's Report Biological Defensible Flow Recommendation for the. Maintenance and Enhancement of Colorado Squawfish Habitat in the 'IS Mile' Reach of the Upper Colorado River During July, August and September, May 1989 (May 1989 Report), presents calculated flows in the Colorado River below the GVIC Diversion Dam (Table 16 in the May 1989 Report). These include the 581 cfs return flows from the Orchard Mesa Power and Pumping Plants located downstream from the GVIC Diversion Dam, and therefore are not available for use by the passage structure. The 581 cfs return flow is the normal return flow during the irrigation season. To calculate the flow available for use for fish passage, the 581 cfs flow was subtracted from the May 1989 Report data. The resulting table is the calculated flow going over the GVIC Diversion Dam. A review of the resulting table indicated that it was only during the months of July through October that flows fell below 100 cfs. Table 1 presents a summary of the modified Table 16 from the May 1989 Report for the months when fish migration is thought to be necessary. B-1