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4 DISCUSSION <br />This study was a detailed investigation of the ice processes on the Green River <br />downstream of Flaming Gorge Dam. The overall goal of the study was to assess the influence of <br />the daily release schedule of Flaming Gorge Dam on the river ice processes in the study reach in <br />order to evaluate the potential for impacts to overwintering endangered fish. The first step was <br />to analyze historical measurements of water temperature, air temperature, and ice observations. <br />The water temperature entering the study reach near the Jensen gage and just upstream of the <br />Chew Bridge was often at 0 °C during the winter, and the daily average air temperatures were <br />consistently below 0 °C throughout December, January, and most of February. Ice was observed <br />in the Green River study reach during every winter for which reliable records were available. <br />The USGS observations of ice, made in conjunction with retrieval of discharge measurements <br />from the Jensen gage, were confounded by the retrieval schedule and were not reliable indicators <br />of ice presence. <br />Formation of ice cover in the Green River study reach followed a consistent pattern each <br />winter for which records were available and the daily release schedule of Flaming Gorge Dam, <br />whether steady or fluctuating as a result of hydropower demand, did not appear to affect the basic <br />outline of this pattern (Table 11). The initial ice observed each winter was frazil ice, transported <br />at the water surface in the form of slush, floes, and pancake ice. A stationary ice cover formed <br />initially near the Ouray Bridge and then progressed upstream. The ice cover was formed <br />primarily by the juxtaposition of floes up to RM 290. Underturning of ice floes and a rougher ice <br />surface seem to be more typical upstream of this point. The upstream extent of the ice cover was <br />typically reported to be at least RM 302 and often extended upstream of this point. No ice cover <br />progression has been reported between Chew Bridge (RM 316) and Split Mountain, except for <br />short, isolated stretches during a particularly severe winter, probably as a result of the steeper <br />gradient between Chew Bridge and the downstream end of Split Mountain. <br />Analysis of the 1997 field survey data and modeling of hydraulic conditions indicated <br />that daily fluctuations in releases from Flaming Gorge Dam have only a small effect on the <br />hydraulic conditions in the Green River downstream of RM 300 (Jensen Bridge). Consequently, <br />daily fluctuations are unlikely to significantly affect the formation or breakup of ice covers <br />further downstream. The results indicated that the fluctuations would be more pronounced and <br />could affect the formation and breakup of ice cover upstream of RM 300. <br />The general trend of ice cover formation in the Green River can be outlined based upon <br />historical observations, the 1996-97 field surveys, and the historical water and air temperature <br />data. First, construction of Flaming Gorge Dam undoubtedly had an influence on the ice regime <br />downstream of the dam. A primary influence of the dam was to increase the river water <br />temperature immediately downstream of the dam during winter. Winter stratification of the <br />reservoir causes colder, less dense water to overlay warmer, more dense water (pure water is <br />-22-