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with high flows, have kept the river relatively wanner in winter than pre-dam. These thermal effects are ameliorated longitudinal by ambient air temperatures and by the Yampa River, which enters the Green River about 105 km downstream from the dam. Objective 1: Link Between Dam Operations and Ice Development Relatively warmer water from Flaming Gorge Dam, combined with a greater mass from higher than historic winter releases, have reduced the annual frequency of ice cover by 52% on the middle Green River. Historically, an ice cap usually formed on the Green River during winter. During the pre-dam period (1946-63), surface ice was reported near Jensen, Utah, in a1118 years ( 1 00%) for 1-5 months of each year (mean, 3.3 months/year), but during the post-dam period (1964- 92). surface ice was reported in only 14 of29 years (48%) for 0-4 months of each year (mean, 0.8 months/year) (Figure 7). Higher than historic flows, combined with fluctuating dam releases of relatively warm hypolimnetic water, have reduced the incidence of surface ice development and increased the incidence of jam and frazil ice in one of the primary nursery habitat areas of Colorado pikeminnow. The development and persistence of surface ice on the Green River differed substantially between the two winters of this study. Surface ice in winter 1993-94, under high fluctuating releases, persisted for only about 5 days, while surface ice in winter 1994-95, under low steady releases, persisted for 42 days, despite similar warm air temperatures. We attribute this difference in ice cover persistence to the low, steady releases from the dam in the second winter, indicating that a relationship exists between dam release volumes and flow fluctuations, and persistence of surface ice. The combination of a lower heat budget (low water volume) and lack of disturbance from stage changes (steady, non-fluctuating releases) allowed development of surface ice through much of the age~O Colorado pikeminnow nursery area downstream of Jensen. MontWy volumes released from Flaming Gorge Dam from November, 1994 through March, 1995 were only 40-75% of comparable monthly releases for 1993-94. A more persistent ice cover in the year of low steady releases is consistent with observations of ice formation in pre-dam years (Valdez 1995). We conclude that there is a linkage between dam operations and the formation and persistence of surface ice on the Green River in the nursery area. The relationship between dam operations and frazil ice development and formation of ice jams was not clear. During this study, 50 and 56% of backwaters in the upper of two primary n ursery areas were rendered unsuitable as nursery habitats for age-O Colorado pikeminnow in winters of 1993-94 and 1994-95, respectively, while no new backwaters were formed. The primary cause for loss of these backwaters was flow-through caused by ice jams increasing river stage and by unknown local channel changes. Collapse of multiple ice lenses also decreased available free water and reduced effective free water depth. All backwaters affected by flow-through remained unsuitable for the remainder of the winter, primarily because retaining sand bars were eroded. In all cases observed, low-lying shoreline backwaters became escape channels for water ponded by ice jams. Increased flow through the'backwater eroded the retaining sand bar and presumably flushed all fish from the backwater into the main river channel. 21