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3 RESULTS 3.1 HISTORICAL CONDITIONS AND OBSERVATIONS 3.1.1 Winter Flows Releases from the dam are determined by a number of factors, including basin hydrology, reservoir storage, maintenance of a downstream trout fishery, endangered fish needs, and generation of hydropower. Compared to pre-dam historical records, Flaming Gorge Dam has reduced the magnitude of peak spring flows and has increased the magnitude of flows in the fall and winter. This alteration in the seasonal distribution of flows is clearly depicted in Figure 14, which shows the mean daily discharge at the Greendale, Utah gaging station for each day of the year for the period 1950-1962 (pre-dam period of record) compared to the mean daily discharge for each day of the year for the period 1963-1997 (post-dam). The Green River is joined by its largest unregulated tributary, the Yampa River, 65 miles downstream of Flaming Gorge Dam (Figure 1). As a result, the difference between pre-dam and post-dam flows downstream of the confluence is less pronounced than for flows upstream of the confluence, although changes to the seasonal pattern and magnitude of flow levels are still evident (Figure 15). Mean daily winter flows (defined here as November through March) at the Jensen gage ranged from 314 cfs to 24,200 cfs (mean =1,507 cfs; SD =1,522 cfs) during the 16 years prior to construction of Flaming Gorge Darn (Figure 16). Following construction of Flaming Gorge Dam (January 1963 through March 1997), mean daily flows ranged from 415 cfs to 9,710 cfs (mean = 2,785 cfs; SD =1,214 cfs) (Figure 17). During both pre-dam and post-dam periods of record, flows tend to increase near the end of February reflecting the early beginnings of the spring runoff. The flow continues to increase through the month of March and typically reaches a peak in late May or early June. Use of Flaming Gorge Dam as a peaking hydropower facility has also led to increased daily fluctuations in flow at downstream locations during many seasons of the year. Within a 24-hr period, hydropower releases from Flaming Gorge Dam can range from a minimum flow of 800 cfs to a maximum flow of 4,600 cfs. Variation in channel morphology along the river and tributary input serve to dampen fluctuations that result from hydropower operations at the dam. The degree of attenuation of operations-induced fluctuations is dependent on specific release parameters including the ramp rate (time between minimum and maximum flow), the minimum and maximum flow levels, and the duration of peak releases. This attenuation or dampening becomes greater at increasing distances from the dam until fluctuations are in the range of natural flow variance at some distance. Under maximum daily peaking powerplant operations (800 cfs minimum, 4,600 cfs maximum, and 12 hours of peak releases during a 24-hr period), the -14-