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1 <br /> <br />INTRODUCTION <br />i This report presents the results of a detailed investigation of the ice processes on the <br />Green River downstream of Flaming Gorge Dam that was designed to examine the influence of <br />daily fluctuations in water releases from Flaming Gorge Dam on the formation, breakup, and <br />movement of river ice. In order to better understand the river ice processes, historical records of <br />water temperature measurements, flow measurements, and ice observations were reviewed and <br />analyzed. In addition, a field survey was conducted during the winter of 1996-97 to make <br />detailed measurements of the river ice conditions and to measure the ice cover response to <br />alternate release scenarios at Flaming Gorge Dam: 1) releases that were steady throughout the <br />day and 2) releases that fluctuated over a 24-hr period. To accomplish this, water releases were <br />first held constant fora 20-day period after ice formation began. The resulting period of steady <br />flows within the study area was followed by a period of flows that fluctuated over each day in a <br />24-hour pattern similar to the release pattern used for hydropower production during winter <br />months. The response of the ice cover to flow fluctuations was observed, and icP thickness <br />measurements were made before and after the period of fluctuating flows. Next, an unsteady <br />flow model of the study reach of the Green River was developed using available surveyed river <br />cross-section information. This model, calibrated using the 1996-97 survey data, allowed us to <br />evaluate the effects of daily fluctuations in flow on hydraulic conditions in the Green River and <br />to estimate the limits of the river influenced by the fluctuating releases. Finally, a model of river <br />ice processes in the Green River study reach was developed and used with the unsteady flow <br />model to simulate the formation of ice in the Green River during winters for which appropriate <br />' data were available: the winters of 1989-90 through 1995-96. <br /> <br />1 <br />1 <br />L <br />1 <br /> <br /> <br />ii <br /> <br />