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Naturally turbid water was cleared when virtually all sediment was trapped behind the dam. Water drawn from the hypolimnion of the reservoir reduced temperature of summer releases to about 6°C after reservoir filling was completed in 1967. Downstream warming occurred but mean monthly summer water temperatures reached a maximum of about 14°C in the post-dam period in Lodore Canyon compared with > 20°C in the pre-impoundment period (Fig. 3 in Bestgen and Crist 2000). Post-dam winter water temperatures in the tailwater were increased compared to pre-impoundment conditions from near 0°C to about 4°C. Cold tailwater releases begun after 1967 dramatically reduced trout growth in the fishery i i i that established after dam closure, which prompted the second major Green River change due to Flaming Gorge Dam (Holden and Crist 1981). A multi-level penstock was installed and became operational in June 1978 to allow water to be drawn from warmer upper layers of the reservoir in summer. Temperature of multi-level penstock releases during summer operations were targeted at about 13°C, a level thought to maximize growth of rainbow trout Oncorhynchus mykiss and maximize downstream extent of the tailwater fishery. Effects of penstock modifications on native fishes in reaches further downstream were assessed from 1978 to 1980 by Holden and Crist (1981). Penstock modifications raised water temperature in the Green River upstream of the Yampa River, but summer maxima rarely exceeded 17°C in the period 1978 to 1991 (Fig. 3 in Bestgen and Crist 2000). The third major dam-related operation event occurred in 1992 in response to a Biological Opinion on operation of Flaming Gorge Dam (Tyus and Karp 1991, U. S. Fish and Wildlife Service 1992). Spring release levels from the dam were increased in an attempt to simulate a more natural runoff pattern and enhance floodplain inundation downstream of Jensen, Utah (Fig. 2). The spring through autumn flows implemented in 1992 were designed to enhance physical habitat in the Green River downstream of the Yampa River confluence; habitat enhancement in the regulated Green River reach upstream of the Yampa River was not a priority because few endangered fishes were thought to occur there. Releases up to maximum power plant levels (130 m3/sec, Fig. 4) were made for up to six weeks in spring around the time when unregulated 15