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Final Report 3-20 September 2000 Table 3.7.- The effect of regulation on mean date of peak flow at USGS stream gages on the Green River. Gage Regulation Condition Mean Date of Peak Flow Greendale Unregulated Regulated 9 June Jensen Unregulated 3 June Green River Regulated Unregulated Regulated 27 May 6 June 31 May In this report, flow in August through February is referred to as base flow. The source of unregulated base flows is predominately groundwater, with occasional augmentation by rain events and snowmelt. As discussed in Section 3.4.1, regulation has produced base flows that are considerably higher than would have occurred without regulation. Flow duration curves that compare regulated and unregulated flows for all three reaches are displayed in Figure 3.9. Regulation has reduced the percentage of time that flows are either very high or very low. This result can be clearly seen by comparing flow-duration data at the Jensen gage. Without regulation, flows at Jensen are less than 28 m3/s about 17% of the time. With regulation, flows at Jensen are less than 28 m3/s only 3% of the time. Regulation and the establishment of the 23-m3/s minimum release from Flaming Gorge Reservoir (see Section 3.2) has resulted in higher base flows. The date when base flow is reached varies according to the hydrologic conditions. Base flows are reached at Jensen by the first of August in years with average conditions, by late June in dry years (90% exceedance), and by late August in wet years (10% exceedance). Although unregulated base flows in the Green River are generally considered stable, variability in flows occurs during the base-flow period even without hydropower-induced fluctuations. Variability can occur at a number of different time scales, including between years, within years, between days, and within days. Each type of variability can have different effects on geomorphic and ecological processes (e.g., between- and within-year variability can affect vegetation establishment on low-elevation sand bars; between- and within-day variability can affect conditions within backwaters used by the endangered fishes). The level of variability in the Green River basin at each of these time scales is discussed below. Mean daily flows at the Jensen gage during the base-flow period were used to determine the level of flow variability between and within years. Between-year differences are largely related