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2 METHODS <br />2.1 THE STUDY REACH <br />The study reach of the Green River extended from the downstream end of Split Mountain <br />Canyon (about River Mile [RM'] 320) to the Ouray Bridge (RM 248) (Figure 2). Flaming Gorge <br />Dam is located about 90 river miles upstream of the study reach at RM 410. As presented in <br />Valdez and Masslich (1989), the study reach can be effectively subdivided into two segments, <br />based upon channel characteristics. The segment from Split Mountain Canyon to the Jensen <br />Bridge (RM 302) comprises the upstream segment of the study reach. This segment has a <br />moderate gradient with a substrate consisting primarily of gravel and cobble in the upstream <br />portion and sand in the downstream portion. The downstream segment of the study reach <br />extends from the Jensen Bridge to the Ouray Bridge and is generally a low-gradient, meandering <br />river with a substrate primarily composed of sand. Upstream of the study reach, where the Green <br />River passes through Split Mountain Canyon, the river has a high gradient, numerous rapids, and <br />a substrate of mostly cobble and boulders. <br />2.2 HISTORICAL CONDITIONS AND OBSERVATIONS <br />Information pertaining to conditions in the Green River during the winter was obtained <br />from historical records in order to assess whether these data provided evidence that daily <br />fluctuations had affected the historical formation or distribution of ice in the study area. These <br />records were also used to evaluate data collected during the 1996-1997 field study and for use in <br />modeling ice processes in the Green River. The data collected included historical information <br />about winter flows, water temperature, air temperature, and observations of ice formation and <br />distribution in the Green River. <br />2.2.1 Flow Rates <br />information about releases from Flaming Gorge Dam was primarily based upon records <br />for the gage located near Greendale, Utah (U.S. Geological Survey [USGS] Station No. <br />09234500) and power generation records for the dam. The period of record for the Greendale <br />gage, which is located 0.5 river miles downstream of the dam at RM 409.5, is October 1950 to <br />the present. Flow at this gage has been regulated by Flaming Gorge Dam since November 1, <br />1962. <br />'Distances reported as RM represent the distance upstream from the confluence of the Green and Colorado <br />Rivers, unless otherwise noted. <br />-6-