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• DRAFT February25, 1998 <br />point. The upstream extent of the ice cover was typically at least RK 483 and often extended <br />upstream of this point. No ice cover progression was observed upstream of Chew Bridge (RK <br />504), except for short, isolated stretches during a particularly severe winter. Apparently the river <br />gradient is too steep to allow progression past this point. <br />Analysis of the data collected during the 1997 field survey indicated that daily fluctuations in <br />releases from Flaming Gorge Dam have minimal effect on the hydraulic conditions in the Green <br />River downstream of RK 480 (Jensen Bridge). As a result, daily fluctuations are unlikely to affect <br />the formation of ice cover downstream of this point to any significant degree. Upstream of this <br />point the fluctuations have a more pronounced effect and can affect the ice cover formation. <br />The primary result of daily fluctuations would be to transport frazil ice beneath the ice <br />cover in the reach above the Jensen Bridge. As the cover progresses upstream above Jensen <br />Bridge, the deposition of frazil ice caused by the daily fluctuations would result in an ice cover <br />thicker than an ice cover that would develop in this reach under steady flow. Frazil depositions <br />several feet thick were observed throughout an 18-km reach (RK 488 to RK 506) during the <br />winter of 1987-88 when Flaming Gorge Dam was operating under a regime of fluctuating <br />releases. An ice cover that progressed through this reach under conditions of steady flow was <br />• observed to have a thickness of about 0.24 m during the 1997 field survey. The upstream eight <br />kilometers of this ice cover was observed to collapse during the 1997 field measurements shortly <br />after releases from Flaming Gorge Dam began fluctuating on a daily basis. The ice cover was <br />apparently not strong enough to resist the increase in stress caused by the fluctuations in this <br />reach. If the ice cover had propagated upstream during a fluctuating regime, however, the <br />increased thickness caused by frazil transport beneath the cover would have provided the cover <br />with increased strength, and produced a cover strong enough to resist the stresses induced by <br />the fluctuations. <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 1989-90 through <br />1995-•96. The ice model results are in general agreement with the historical ice observations. <br />is <br />15