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thicker than would develop in this reach under steady flow. Main channel frazil ice depositions <br />up to 90 cm thick occurred an 11-mile reach (RM 305 to RM 316) during the winter of <br />1987-1988 when fluctuating releases were being made from Flaming Gorge Dam. The ice cover <br />that formed in the same reach during steady releases from Flaming Gorge Dam had a mean <br />thickness of about 24 cm during the 1997 study, although this was also a milder winter than <br />during 1987-1988. The upstream five miles of ice cover broke up during the 1997 field study, <br />shortly after releases from Flaming Gorge Dam began fluctuating on a daily basis. Apparently, <br />the ice cover was not strong enough to resist the increase in hydraulic stress caused by the <br />fluctuations in this reach. <br />Operations of Flaming Gorge Dam that take frazil production and the upstream extent of <br />the ice cover into account could reduce the likelihood that daily fluctuations would affect ice <br />formation, ice breakup, or the transport and deposition of frazil ice beneath the ice cover in main <br />channel areas used by overwintering endangered fishes (i.e., areas downstream of the Jensen <br />Bridge). To avoid deposition of large quantities of frazil beneath a stationary ice cover, large <br />daily fluctuations at the Jensen gage should be avoided during extremely cold weather (e.g., <br />mean daily air temperatures of about -7 °C or less) until surface ice cover has progressed <br />approximately 10 river miles upstream of the Jensen Bridge (RM 310). Under such operations, it <br />is unlikely that frazil ice would be deposited farther than approximately the Jensen Bridge area <br />and this should protect areas farther downstream that are used by the majority of overwintering <br />adult Colorado pikeminnow and razorback suckers. Under these conditions, ice cover would be <br />extended upstream of the Jensen Bridge within a few days and fluctuations could be resumed. <br />During less severe weather, when frazil production is reduced, the likelihood of depositing large <br />quantities of frazil beneath surface ice is also reduced. In such cases, operations that <br />approximate those seen during the field study of 1996-1997 would not result in significant <br />deposition of frazil ice under the stationary ice cover and would be unlikely to affect breakup of <br />all but the thinnest ice covers past about RM 300 (Jensen Bridge). <br />A numerical model of dynamic ice formation in the Green River was developed and used <br />to simulate the ice cover formation on the Green River for the winters 1989-1990 through <br />1995-1996. The ice model results were in general agreement with the historical ice observations <br />and indicated the model could be used to evaluate the condition of the ice cover in the study area <br />of the Green River under a given set of meteorological and hydrological conditions. However, <br />collection of additional data pertaining to ice formation and breakup in the Green River would be <br />useful for improving calibration of the flow and ice formation sub-models and would also allow <br />the results of the model to be validated. We recommend that accurate water temperature data be <br />collected during winter within the study reach. Such information would be necessary for future <br />improvements to the ice process models and would provide information about the environmental <br />conditions that endangered fishes encounter within the study reach. <br />Our results indicated that daily hydropower operations at Flaming Gorge Dam have little <br />-25-