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<br /> <br /> <br /> <br />1 <br /> <br /> <br />were reduced because of reservoir storage (Fig. 4). Power plant operations increased base flow <br />levels and daily discharge fluctuations increased due to hydropower production. Naturally turbid <br />water was cleared because virtually all sediment was trapped behind the dam. Water drawn from <br />the hypolimnion of the reservoir reduced temperature of summer releases dramatically, <br />especially after reservoir filling and normal operations commenced in 1967. For example, mean <br />July water temperature of the Green River in two typical post-impoundment years reached only <br />6.1 °C, compared with > 20°C in the pre-impoundment period. Post-dam winter water <br />temperatures were increased compared to pre-impoundment conditions. <br />Cold tailwater releases begun after 1967 dramatically reduced growth of trout in the <br />fishery that established between 1963 and 1966, which prompted the second major change in <br />Flaming Gorge Dam operation. A multi-level penstock was installed and became operational in <br />June 1978 to allow water to be drawn from warmer upper layers of the reservoir in summer. <br />Temperature of releases during summer operations generally did not exceed 13°C, a level <br />thought to maximize growth of rainbow trout and the downstream extent of the tailwater fishery. <br />Effects of penstock modifications on native fishes were assessed from 1978 to 1980 by Holden <br />and Crist (1981). <br />The third major dam-related operation event was instituted in 1992 in response to <br />findings of a Biological Opinion on operation of Flaming Gorge Dam (Tyus and Karp 1991, <br />U. S. Fish and Wildlife Service 1992). Spring release levels from the dam were increased in an <br />attempt to emulate natural runoffpatterns and enhance floodplain inundation downstream of <br />Jensen, Utah. The spring through autumn flow manipulations implemented in 1992 were <br />designed mostly to enhance physical habitat in the Green River downstream of the Yampa River; <br />habitat enhancement in the regulated reach was not a priority because few endangered fishes <br />were thought to occur there. Releases up to maximum power plant levels (130 m3/sec, Fig. 2) <br />were made for up to six weeks in spring around the time when unregulated Yampa River <br />discharge peaked. During summer, release levels were designed to maximize nursery habitat for <br />Colorado pikeminnow near and downstream of Jensen, Utah (Tyus and Karp 1991). To achieve <br />13 <br />