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. - DOLORES RIVER FISHERY MONITORING STUDIES APRIL - OCTOBER 1991 , EXECUTIVE SUMMARY Study objectives in this modification of the cooperative agreement were: 1. Determine the effects oflow summer flows from McPhee Reservoir on the downstream fish population in the Dolores River. 2. Provide information essential for the proper management of downstream releases to protect and enbance aquatic habitat and its associated components. 3. Document any movement of trout due to water flows and temperature changes in the Dolores River from McPhee Dam to Bradfield Bridge. Since 1989, density (nIha) and biomass (kg/ha) of rainbow, brown, and cutthroat trout has decreased precipitously in the Dolores River between McPhee Dam and Bradfield Bridge. Density and biomass data for October 1990, April 1991, and October 1991 are statistically significantly lower (at the 95% level) than September 1989. Total trout (~ 15 cm or 6 in.) density and biomass were 50%-60% lower in October 1991 compared to September 1989. Density and biomass for 8-16 inch (20-40 cm) trout decreased 55%-71 % between September 1989 and October 1991. Average density and biomass data from four high quality tailrace fisheries (Blue, Fryingpan, Gunnison, and South Platte rivers) are 8 to 16 times higher than comparable data (1986-1991) from the Dolores River. Trout trapping and tagging operations in the summer of 1991 revealed very little trout movement. At flows ::;; 60 cfs, many riffle areas are 2-4 inches deep, which effectively blocks upstream migration of trout ~ 16 inches (40 cm). Summer air temperatures in the river corridor were unusually cool in 1991. Maximum air temperature exceeded 1000 F only twice and maximum summer water temperatures rarely exceeded 700 F at the Bradfield Bridge. As a result, resident trout probably avoided severe thermal stress in 1991. Air and water temperature data indicate 60-70 cfs flows are the minimum amount of water required to avoid short-term (3-6 months) losses of trout. Daytime high air temperatures exceeding 1000 F, require a 70 cfs flow to keep maximum daily water temperatures at Bradfield Bridge below 70-72 0 F. Downstream releases should be managed to keep the 24-hour average water temperatures ::;; 660 F. Research across the western U.S. and Colorado indicates that 20%-30% of the average annual discharge (AAD) into an impoundment is the minimum downstream release required to avoid significant damage to a trout fishery. Release of 20 cfs out of McPhee Reservoir for 100 days (March-June 1990) equates to 4% of the AAD. Twenty percent of the AAD into McPhee is 70,000 acre-feet (AP) of water. Judicious use of water in average and above average water years can maintain a trout fishery in the river below McPhee Dam. However, a 20 cfs minimum flow pattern will result in a severely depressed trout in fishery in the Dolores River between McPhee Dam and Bradfield Bridge.