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EXECUTIVE SUMMARY Introduction - The U.S. Bureau of Reclamation is considering whether on not to modify Glen Canyon Dam to allow downstream temperatures to be managed. It is believed by the U.S. Fish and Wildlife Service and Reclamation that the cold summer temperatures created by the dam are a constraint to native and endangered wannwater fish. but there are other ecological interactions complicating the issue that cannot be conclusively resolved without physical testing. Reclamation has developed a relatively inexpensive way to modify the dam to allow temperatures to be controlled and tested. The plan includes a monitoring program to further our understanding of the ecosystem's response to temperature and an adaptive management program to apply this knowledge. Background - Before Glen Canyon Dam was constructed. the temperature of the Colorado River in the Grand Canyon would increase from near freezing in the winter to about 300C (860F) in the late summer. Though several native fish were present in the river system. Leibfried and Zimmerman (1994) reported that the fish community was dominated by wannwater non-native species. Channel catfish comprised 90% of the fish community in Glen Canyon (Woodbury 1959). The dominant forms of non-native fish were channel catfish and carp, with red shiners. largemouth bass. green sunfish. and bullheads present but less abundant (Valdez and Carothers 1998). Once the dam was constructed, release temperatures became relatively cold during the summer season. Water is now drawn year round from the deep intakes used for power production. Release temperatures vary little and are typically 80C_lOoC (460F-500F). Cold mainstem temperatures and high water velocity are thought to have expatriated (or at least greatly limited) most non-native competitors in the mainstem. Of the non-native species, rainbow trout do well in the clear water near the dam. Carp and catfish are abundant further downstream. in and near wann tributaries like the Little Colorado River. Native, wannwater fish have also been greatly limited by cold-water releases. Some are no longer present below Glen Canyon Dam. The endangered humpback chub are essentially limited to the area in and near the Little Colorado River. Impacts to Fish - Cold-water releases from Glen Canyon Dam are below optimal for the existing trout fishery and far below those temperatures needed to allow native and endangered wannwater fish (such as the humpback chub) to thrive in the mainstem of the Colorado River. Thermal shock from cold mainstem temperatures has been recognized as a likely cause of mortality for young endangered fish leaving seasonally wanned tributaries (Lupher and Clarkson 1993, 1994; Valdez and Rye11995; Thieme 1998). In their integration report on studies in Glen and Grand Canyons. Valdez and Carothers (1998) concluded that, "We believe that most larval tlannelmouth suckers, bluehead suckers, and humpback chub descending from wann natal tributaries into the cold mainstem die of thermal shock or from predation elicited by erratic swimming behavior. For those fish old enough to survive the transition, swimming ability may be reduced by as much as 98 percent by cold mainstem temperatures."