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Stage changes for the 1 and 2-hour scenarios were somewhat less. The attenuation phenomenon was <br />illustrated by diminished total stage change and stage change rate with distance from the dam, as well <br />as increased time for the change to occur. <br />Predicted stage change rates under the 8-hour scenario were 7.2 and 6.5 cm/hr at Mitten Park <br />and Jensen, respectively. This rate dissipated to 1.3 cm/hr by the time the flow reached Ouray. <br />Under the 4-hour scenario, rates of 5.7 and 5.2 cm/hr were predicted at Mitten Park and Jensen, <br />respectively. A change in stage caused by this sudden increase in discharge was expected approx- <br />imately 24, 38, and 58 hours later at Mitten Park, Jensen Bridge, and Ouray Bridge, respectively. The <br />rate at which a given change in flow travels downstream depends on water volume and flow duration <br />preceding as well as following the change. A greater volume of water following a flow change can <br />essentially push that change downstream faster. This investigation also showed that frazil ice <br />appeared to slow the downstream rate of change. <br />Temperature and Ice Condltlons <br />Ice conditions on the Green River were different during the two winters (Table 2), primarily <br />because of air temperatures. Winter 1 (1986-87) was warmer than normal with little ice formation, <br />while Winter 2 (1987-88) was colder than normal with substantial ice development. Water clarity <br />appeared higher in Winter 2, probably as a result of lower flow regimes. These contrasting conditions <br />provided an excellent opportunity to examine trsh response to different winters under similar dam <br />operation. <br />Table 2. Ice conditions on the Green River and Yampa River for December through March, <br />1986-87 (Winter 1) and 1987-88 (Winter 2). <br />Surface Ice Formed Spring Breakup <br />Filver Winter 1 Wlnter 2 Wlnter 1 Wiener 2 <br />Green River 1/15/87 12/20/88 1/30/87 2/15188 <br />Yampa River 12/15/86 12/15/86 3/9/87 4/1/88 <br />Wlnter 1 (1986-Sn. Higher than normal air temperatures in Winter 1 produced above normal <br />water temperatures near Jensen. Monthly maximum air temperatures for December through March <br />were from 0.7 to 4.1°C above normal, while minimum temperatures wcrc 1.2 to 6.1°C above normal. <br />Monthly water temperatures wcrc up to 2.3°C warmer in February of Winter 1 than in Winter 2 <br />(Figure 7). Higher releases from Flaming Gorge Dam in Winter 1 also moderated river temperatures <br />by delivering a larger thermal mass to downstream areas. <br />11 <br />