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l:' Selective Withdrawal Operations (Continued from Page 5) in the Grand Canyon. The U.S. Fish and Wildlife Service (FWS) believes the survival of the species is jeopardized in the region and has listed the humpback chub, as well as the razorback sucker, as endangered. FWS Biological Opinion The FWS has presented its evaluations on the effects that Glen Canyon Dam is having on native fIsh in the Grand Canyon in two biological opinions on dam operations: the initial one in 1978 and the latest draft in 1993. Measures designed to conserve and recover the hump- back chub included recommendations in 1978, 1988, and 1993 to investigate the feasibility of modifying the penstock intakes at Glen Canyon Dam with selective withdrawal capabilities to seasonally warm downstream releases to benefit endangered species. Selective with- drawal structure studies are scheduled to begin in 1995. Full reservoir elevation 3700 ft. Zone of withdrawal (200 ft.) Hypolimnion Lake Powell Thermal Stratification Sunlight's ability to warm Lake Powell water decreases with depth. Lake Powell is thermally stratified through- out much of the year. The warmest layer, the epilimnion, is near the reservoir's surface and ranges in thickness from 30 to 80 feet and between 800 F (270 C) in the SUmmer and 450 F (70 C) in winter. The metalimnion, or middle layer, ranges from 30 to 80 feet in thickness, receives less sunlight so it is not as warm as the epilimnion, and temperatures decrease with depth. The bottom layer, the hypolimnion, where the penstock intakes are located at about 230 feet below the maximum stage of the lake, receives no sunlight and remains near 460 F W C). To raise the temperature of releases, the Glen Canyon Dam power penstock intakes could be retrofItted for (Continued on Page 7) Multilevel intake structures Generator penstocks Generators ~ uUet works A selective withdrawal structure would provide the ability to withdraw warmer water from near Lake Powell's surface. 6