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vi I I I I I I I I I I I I ,I I ,I I I I I EXECUTIVE SUMMARY Ice conditions were evaluated in backwaters of the Green River in winter of 1993-94 and 1994-95 to determine if overwinter survival of age-O Colorado squawfish is linked to ice processes and operations of Flaming Gorge Dam. The two winters studied were relatively mild and ice development was not as extensive as observed in previous, more severe winters. Despite different operational regimes in winter 1993-94 (high volume, high fluctuations) and winter 1994-95 (low volume, low fluctuations), ice conditions were similar for both years, suggesting that in mild winters, ice processes are largely independent of dam operations. Only the volume of water released seemed to be an operational variable that increased the duration of the ice, as a result ofa lower heat budget. Of 14 and 9 backwaters observed and monitored during the two winters of study, 11% and 14%, respectively, were eliminated by natural channel geomorphic changes, and 21 % and 22%were eliminated by geomorphic changes resulting from ice jams. None of the remaining backwaters were rendered unsuitable as a result of ice conditions. Although all backwaters were covered with ice and collapsing ice lenses and some frazi1 ice development were observed, all retained greater than 30 m2 surface area, greater than 9 em depth of free water, above _0.50 C, and greater than 5 mgll dissolved oxygen concentration. Overwinter decreases in density of the 1987 through 1995 year classes of Colorado squawfish were compared with flow, ice duration, and water and air temperature to identify the variables that most affect overwinter survival. No clear relationships were found suggesting that ice processes are not related to fish survival, or they are related through complex and poorly understood relationships. We offer the hypothesis that warm spike flows during ice breakup in February and March provide cues to downstream movement of significant numbers of age-O Colorado squawfish as part of a life history strategy to reach warmer, more productive downstream reaches. The hypothesis is supported by increased densities and numbers of age-O Colorado squawfish in the Lake Powell inflow in April and subsequent low numbers in late summer, which also indicates that survival of these fish is low. The relationships between dam operations, ice conditions, and survival of age-O Colorado squawfish are not well understood, and additional studies are needed to better identify and measure influencing variables. Interim winter releases from Flaming Gorge Dam are advised that reflect similar flow regimes currently in place for summer and fall in order to continue to protect nursery habitats of age-O Colorado squawfish.