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which historic observations were available was frazil ice, transported at the water surface in the form of slush, floes, and pancake ice. A stationary ice cover formed initially near the Ouray Bridge and progressed upstream from that point. Ice cover in all years is probably formed primarily by juxtaposition of floes up to about RM 290. Upstream of RM 290, underturning of ice floes and a rougher ice surface were more typical during the 1996-1997 study and is probably similar under most winter conditions. The reported upstream extent of the ice cover was typically at least up to RM 302 and often extended upstream of this point. No complete ice cover was reported upstream of Chew Bridge (RM 316), except for short, isolated stretches during a particularly severe winter. Apparently the river gradient in the study reach is too steep to allow ice progression past this point during most winters. A numerical model of dynamic ice formation in the Green River was developed using empirical information and used to simulate ice cover formation on the Green River for the winters of 1989-90 through 1995-96. The ice model results were in general agreement with historical ice observations during these years. Analysis of hydraulic conditions that occurred during the winter of 1996-1997, together with the ice process model was used to evaluate the potential effects of daily fluctuations on ice formation. and breakup. The results indicated that daily fluctuations of releases similar to those observed during 1996-1997 (approximately 1,800 cfs) from Flaming Gorge Dam would be unlikely to affect ice cover in the main channel of the Green River downstream of RM 300 (Jensen Bridge) under most winter conditions. Upstream of the Jensen Bridge daily fluctuations have a more pronounced effect and are more likely to affect ice cover formation and breakup. During especially cold winters, when production of frazil ice would be high, large daily fluctuations in flow would probably transport frazil ice beneath the ice cover in the reach above the Jensen Bridge. This would result in an ice cover thicker than ice covers that would occur through this reach under steady flow. Frazil depositions several feet thick were observed in this portion of the study reach during the winter of 1987-1988 when water releases from Flaming Gorge Dam fluctuated daily. The ice cover that developed in the upper portion of the study reach under conditions of steady flow during the 1997 field survey was about 24 cm thick. Recommendations resulting from this study include: 1. To prevent ice breakup, or the transport and deposition of frazil ice in areas used by overwintering endangered fishes, large daily fluctuations at the Jensen gage should be avoided during extremely cold weather (mean daily air temperature about -7 °C or below) until surface ice has formed to approximately RM 310. During milder winters, when frazil ice production is reduced, less restrictive operations could occur without depositing frazil ice under the ice cover and without breaking up all but the thinnest ice covers past about RM 300 (Jensen Bridge). 2. Initiate a program to collect accurate hourly or sub-daily water temperatures during -xi-