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Selective Withdrawal Structure (Continued from Page 6) selective withdrawal operations with a tower-like structure attached to the penstock intakes that would rise toward the lake surface. The structure, commouly referred to as a multi-level intake structure. would have intakes at various elevations above the current intakes to selectively provide access to warmer water from near the reservoir's surface. Reclamation has retrofitted other dams which produce hydroelectric power with multi-level intake structures. Flaming Gorge Dam on the Green River was modified in the early 1970's to increase summer downstream temperatures to improve trout growing conditions. Shasta Dam on the Sacramento River will be retrofitted to access colder water from deep in the reservoir to cool downstream temperatures for salmon. Selective with- drawal structures do not inhibit the ability to produce electricity. Selective Withdrawal Facilities A selective withdrawal structure would enhance down- stream water quality by withdrawing warmer water from Lake Powell to discharge downstream. Warmer mainstem temperatures would likely allow successful humpback chub spawning and larval growth. Suitable temperatures for humpback chub spawning range from 610 F to 720 F (160- 220 C). With selective withdrawal, the mainstem spawning period would be from May to early July. Mainstem temperatures need to remain warmer through mid-September to allow develop- ment of fish eggs and adequate time for growth of the young a1evins and larvae. The river temperatures at Lees Ferry have ranged between about 430 F (60 C) and 540 F (120 C) since the mid-1960's, compared to pre-dam temperatures of3ZO F (00 C) in the winter to 8ZO F (280 C) in the summer. Highest pre-dam temperatures occurred after spring runoff in June, July, and August. During spring and summer, the river warms about 20 F every 35 miles as it flows downstream, reaching maximum warming in July and August. For example, water released from the dam at 540 F (120 C) in July may warm to about 640 F (180 C) at Havasu Creek and 680 F (200 C) at Diamond Creek, 172 and 241 miles, respectively, downstream of the dam. Although the mainstem's lower sections may be within the suitable temperature range for humpback chub spawning, the timing and available spawning habitat limit total production potential. Glen Canyon Dam Selective Withdrawal Operations An operational scheme for temperature modification below the dam proposes to combine warmer releases from May through September with instream warming. producing suitably warm mainstem thermal conditions in reaches of the mainstem below the LCR. The timing would correspond with historical humpback chub mainstem spawning and rearing periods. The availability of warmer water in the reservoir would allow for maxi- mum instream warming. The potential river temperature improvements below the dam would also benefit young rainbow trout. Possible Impacts Withdrawing warmer water from the epilimuion in Lake Powell may assist recovery of endangered native fish species in Grand Canyon, but also may impact water quality and the aquatic ecology of the Lake Powell/Colorado River/Lake Mead system. Reservoir temperatures vary with depth, as do some constituent concentrations, such as dissolved oxygen (DO), nutrients (uitrogen and phosphorus), and salinity (TDS). Withdrawals made from nearer the surface of Lake Powell during the spring and summer are likely to be warmer and higher in DO concentrations, but have lower nutrient and salinity concentrations than hypolimnetic withdrawals at any time of the year. Possible impacts to the system include: (1) promotion and expansion of non-native fish populations from Lake Mead into the lower reaches of the Colorado River in the Grand Canyon, (2) changes in the algal and diatom communities and species composition in the system (important to the aquatic food base), (3) changes to macroinvertebrate species composition and abundance below Lake Powell, (4) increased incidents of parasites and other fish diseases, and (5) a possible detrimental effect on the heat budget of Lake Powell and Lake Mead. Although many potential impacts have been identified. they have not been quantified, so the extent and magnitude of the impacts are not completely known. Intermittent operation of the selective withdrawal (Continued on Page 8) 7