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<br />Final Report <br /> <br />3-14 <br /> <br />September 2000 <br /> <br />3.4.1 Seasonal Distribution of Green River Flows <br /> <br />Flow in the Green River is dominated by snowmelt; consequently, there was a great deal <br />of seasonal variability in the unregulated flow regime. To quantify how this variability has been <br />affected by regulation, a comparison was made between the monthly distribution of regulated and <br />unregulated flow in the three reaches; data used were from 1963 to 1996. These data are presented <br />in Figure 3.6, and the percentage change in mean monthly flow due to regulation is presented in <br />Table 3.4. <br /> <br />Regulation has resulted in a reduction of flows from April through July and an increase in <br />flows from August through March. Reach 1, whose flow is dominated by releases from Flaming <br />Gorge Dam, has been most affected. The effects of regulation have been reduced in Reaches 2 and <br />3, since intervening tributaries, especially the Yampa River, contribute flows whose seasonal <br />distributions have been less affected by regulation. Nevertheless, flow variability in the system has <br />been reduced in all three reaches. <br /> <br />3.4.2 Peak Flows in the Green River <br /> <br />Regulation has resulted in a substantial reduction in the magnitude of spring peak flows. <br />Before construction of Flaming Gorge Dam, median spring peak flows were about 330 m3/s <br />(Figure 3.7), but they were reduced to about 85 m3/s after the dam was built. Only five occurrences <br />of releases greater than 200 m3/s have occurred since construction of the dam was completed in 1962 <br />(1983, 1984, 1986, 1997, 1999). Daily time-series data for 1963 to 1996 were used4 to compare <br />measured regulated peak flows to estimated unregulated peak flows. For all but measured flows at <br />the Greendale gage, log Pearson ill distributions were created to enable comparisons of specific <br />exceedance periods. For flows at Greendale, effects of Flaming Gorge Dam preclude the use of a log <br />Pearson ill distribution. For these data, a distribution was created by ranking values. <br /> <br />The frequency of high peak flows has been reduced significantly by regulation (Figure 3.8). <br />The discrepancy between regulated and unregulated flows is greatest in Reach 1, with effects of <br />regulation diminishing downstream. Table 3.5 presents the same data, showing specific flow values <br />and recurrence intervals for the corresponding exceedance levels. At the Jensen gage, the median <br />peak flow is 669 m3/s without regulation and 448 m3/s with regulation. At the Green River gage, the <br />median peak flow has been reduced from 788 to 575 m3/s. The percent reduction in peak flow <br />between unregulated and regulated conditions is shown in Table 3.6. <br /> <br />4IdealIy, instantaneous peak flows would have been used in this analysis, but the simulation of unregulated <br />instantaneous peak flows is not feasible. However, the difference between daily average peak flows and instantaneous <br />peak flows is not great. From 1963 to 1996, the instantaneous peak flow at the Jensen gage was calculated to be only <br />3.8% higher than the daily average peak flow; consequently, daily average peaks adequately convey the character of <br />peak flows in the Green River basin. <br />