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~.' -.,':/ . Glen Canyon Temperature Control Studies By: Dave 1hieman Temperature Control Study Manager Bureau of Reclamation Prior to construction of Glen Canyon Darn, the Colorado River warmed seasonally. Since constnJction of the dam, . . cold water is drawn from the depths of Lake Powell through fixed-level intakes and released downsb'eam. Cold water releases have inhibited native fish spawning and reduced the ability of young native fish to mature. Humpback chub and other native fishes now depend on the warmer tnbutaries for spawning. The tributaries, however, C3IUIOt generally support the native fish beyond the l;uval stages. Increasing the dam's discharge temperature is believed to be a key element in the recovel)' of native fish below the dam. HIS Biological Opinion In its biological opinion on the Operation of Glen Canyon Dam Final ElWirolU1lental Impact Statement, the U.S. Fish and Wildlife Service recommended that Reclamation investigate modification of river water temperature. As a result, Reclama- tion began to lay out plans to study potential ways to increase water temperatures to help recover the endangered fish popula- tions below the dam. Retrofitting the dam's deep, coldwater intakes with adjustable level intakes (selective level withdrawal structure) would allow the discharge temperatures from the darn to be seasonally managed,. Selective Level Withdrawal Structure Studies Four critical questions about the feasibility of temperature con- trols will be evaluated over the next 2 yean: Will there be enough warm water In Lake Powell to meet the needs of the downstreamfish? Would the switch from deep to surface withdrawal adversely Impact the nutrient/food base of the river below the dam? Is there a risk that more warm-water species (nonnative competitors) will Invade the river system? Will the limnology and productivity of Lake Powell be affected by changes in the withdrawal level? Temperature Modeling Study A computer model is being developed to evaluate water temper- atures in the reservoir and in the river downstream of the darn. The initial study will look at the range of seasonal temperatures potentially available from the reservoir, when this water should be available for release, and how much warming will accrue as the water moves downstream through the Grand Canyon. This study is being conducted by Reclamation's computer modeling group in Denver. Primary Productivity Study Releases from Glen Canyon Dam already have extremely low nutrients levels, and swface withdrawal of wann water may adversely impact algae and macro invertebrates growing below the darn by further reducing nutrient levels. Laboratol)' tests will be conducted through a cooperative agreement with Northern Arizona University to evaluate the tolerance of the aquatic food base to both wann water and changes in nutrient levels. Nonnative Competition Cold water in the main channel of the river has restricted the spawning of native, wann-water fishes to the Little Colorado River and other seasonally wann tributaries, and limited the invasion of nonnative wann water species. Wanning the river may allow additional nonnative fish to move up the river from Lake Mead or down from Lake Powell. If temperature is the only barrier preventing migration, Rec1amation will evaluate release patterns which encourage recmitrnent of native fishes and discourage the invasion of nonnatives. This issue will be evaluated by the Glen Canyon Environmental Studies office in Flagstaff. GolNo Go Decision Point These studies are expected to continue through 1997. If the issues are resolved successfully, Rec1amation will then proceed with preparation of a planning report and envirolU1lenta1 compli- ance document. 6