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• <br />DRAFr February 25, 1998 <br />during the summer. <br />Below the confluence with the Price, the Green River breaks out onto a second large alluvial <br />plain, where the city of Green River, Utah is situated. The river channel widens in this reach, flow <br />velocity decreases, and water temperature increases slightly. The Green River continues <br />southward from Green River, Utah, and is joined by the San Rafael River, enters Stillwater <br />Canyon and eventually joins the Colorado River. River temperaturesdownstream from the Price <br />is controlled by ambient air temperature. Generally in this reach, the increase in solar radiation is <br />significant; day and night temperature are higher and the river is warmer than upstream. This is <br />especially true of the canyon lands area where the massive rock structures warm during the day <br />and reflect heat back to the air and water at night. This process also moderates the diurnal <br />temperature variation in the reach as the river meanders, and water temperatures are moderated <br />by the canyon. <br />As a general rule, the wetter the water year, the colder the water remains into the summer period. <br />Years in which snowmelt and runoff occur early, such as 1962 when the peak occurred from mid <br />April to mid May, are exceptions to this rule. During 1995, which was a high-volume water year <br />with a long duration, water temperatures stayed low well into July. During lower water years, <br />runoff begins earlier and warming occurs earlier in June as temperatures rise quickly, water <br />is volumes drop, and ambient air temperatures increase. <br />Ice Formation and Breakup <br />Daly and Tuthill, et.al (1997) describe a detailed investigation of the ice processes on the Green <br />River downstream of Flaming Gorge Dam. The overall goal of the study was to assess the <br />influence of the daily release schedule of Flaming Gorge Dam on river ice processes. The first <br />step was to analyze historical measurements of water temperature and air temperature, and ice <br />observations. The water temperature entering the study reach (Ouray Bridge, RK 400, to the <br />Chew Bridge, RK 504) was often at 0 °C during the winter, and the daily average air <br />temperatures are consistently below 0 °C throughout December, January, and most of February. <br />These are appropriate conditions to expect ice to form and, in fact, ice was observed in the Green <br />River study reach during every winter for which direct observations were available. The U.S. <br />Geological Survey observations of ice, made in conjunction with their discharge measurements at <br />Jensen, are confounded by the discharge measurement schedule and were not reliable indicators of <br />historical ice presence. Formation of ice cover in the study reach followed a consistent pattern <br />each winter and the daily release schedule of Flaming Gorge Dam, whether steady or fluctuating <br />as a result of hydropower demand, did not affect the basic outline of this pattern. The initial ice <br />observed each winter was frazil ice, transported at the water surface in the form of slush, floes, <br />and pancake ice. A stationary ice cover formed initially near the Ouray Bridge and then <br />progressed upstream. The ice cover was formed primarily by the juxtaposition of floes up to RK <br />464. Underturning of ice floes and a rougher ice surface were more typical upstream of this <br />14