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EXECUTIVE SUMMARY <br />This report provides results and conclusions of a detailed investigation of ice processes in <br />the main channel of the reach of the Green River between the downstream end of Split Mountain <br />(River Mile [RM'] 320) and the Ouray, Utah Bridge (RM 248). The objective of the study was <br />to examine the influence of daily fluctuations in water releases from Flaming Gorge Dam on <br />river ice processes in this reach, which serves as an overwintering area for endangered Colorado <br />pikeminnow and razorback sucker. The objective of the study was met through examination of <br />historical records of winter water and air temperatures, flow measurements, and ice observations; <br />through measurements of differences in ice conditions under steady and fluctuating flow <br />regimes; and through calibration and use of an ice process model to compare hydraulic and ice <br />conditions expected under steady and fluctuating flow regimes. <br />Examination of historical measurements of water and air temperatures, and historical and <br />current (winter of 1996-1997) ice observations indicated that ice occurred within the Green River <br />study reach during every winter for which reliable observations were available. Historical <br />observations of ice recorded by the USGS during discharge measurements were determined to be <br />unreliable indicators of the duration of ice presence during past winters because of the <br />intermittent nature of the observations. <br />Measurements of ice thickness were made at 17 cross-section locations within the study <br />reach during the winter of 1996-1997 under steady flows and after several days of fluctuating <br />flows resulting from initiation of a peaking flow regime at Flaming Gorge Dam. Ice cover broke <br />up at the three upstream-most cross section locations in the study reach during the first few days <br />of fluctuating flows. These three sites were located upstream of the Jensen Bridge, at RM 307.0, <br />308.2,and 316.3. Mean ice thickness at the 14 remaining cross section locations (between Jensen <br />Bridge [RM 300] and Ouray Bridge [RM 248]) was not significantly different under steady flows <br />and fluctuating flows. <br />A change in flow of approximately 1,800 cfs at the Jensen gage resulted in measured <br />stage (surface elevation) changes at seven sample locations that ranged from 24 cm at the <br />upstream end of the study reach to 6 cm at the downstream end of the study reach. The upstream <br />5 miles of ice cover in the study reach broke up after several days of fluctuating flows. <br />Formation of ice cover in the study reach appeared to follow a consistent pattern during <br />winters for which historical observations were available and the daily release schedule of <br />Flaming Gorge Dam, whether steady or fluctuating as a result of hydropower demand, was found <br />to have no apparent effect on the basic pattern. The initial type of ice reported for each winter for <br />'Distances reported as RM represent the distance upstream from the confluence of the Green and Colorado <br />Rivers, unless otherwise noted. <br />-x-