Laserfiche WebLink
<br />Draft Final Completion Report to UDWR for Contract #93-1070, Amendment 3 <br /> <br />Base Flows <br /> <br /> <br />The summer base flows for the period between 1970 and 1990 were much greater than had occurred prior to <br /> <br /> <br />construction of Flaming Gorge Dam. Since darn closure, September typically has had the lowest mean monthly <br /> <br /> <br />discharge of the year (Fig. 3), although prior to dam closure, December and January typically had the lowest mean <br /> <br /> <br />monthly discharge of the year. Prior to dam closure, September base flows were typically less than 40 m3fs (Fig. 6). <br /> <br /> <br />After closure of Flaming Gorge Dam, base flows were as high as 85 m3fs and were never less than 55 m3fs between <br /> <br /> <br />1970 and 1990 (Fig. 6). After 1990 and the adoption of the Fmal Biological Opinion for the Operation of Flaming <br /> <br /> <br />Gorge, September base flows have been about 42 m3fs. These September flows are more similar to pre-dam <br /> <br /> <br />conditions. <br /> <br /> <br />Winter base flows (i.e., December, January, and February) have also increased since closure of Flaming Gorge <br /> <br />Dam. The 5-yr moving average of the mean monthly discharge for these three months is shown in Figure 7, Since the <br /> <br /> <br />closure of Flaming Gorge Dam, the average winter discharge has more than doubled (Fig. 3), and is greater than the <br /> <br /> <br />current recommended summer discharges (31 - 51 m3fs). The current flow recommendations establish target summer <br /> <br /> <br />low flows, but allow a much greater range of winter flows (between 22.7 and 133.1 m3fs). This large range of <br /> <br />discharges ignores the fact that overwinter habitat stability and availability are components of successful recruitment. <br /> <br />4 <br /> <br />Sediment Transport <br /> <br /> <br />Within the Green River basin. the sources of water and sediment do not coincide. Most of the water originates <br /> <br /> <br />in the mountainous headwater regions, whereas most of the sediment is contributed by the lower elevation semi-arid <br /> <br /> <br />regions (Iorns and others. 1965). While the peak flows that have the capacity to transport large quantities of sediment <br /> <br /> <br />are reduced by Flaming Gorge Dam, the sediment contributions to the Green River are not strongly regulated by the dam <br /> <br />because sediment-laden tributaries enter further downstream. Consequently, the Green River downstream from the <br /> <br /> <br />umegulated Yampa River is not a sediment-starved system like the Colorado River immediately below Glen Canyon <br /> <br />Dam. <br /> <br />Sediment load has been estimated using sediment rating curves and discharge records. Andrews (1986) <br />demonstrated that the sediment rating curve near Jensen. Utah. had not changed since the closure of Flaming Gorge <br />Dam, but that peak discharge and duration of large discharges had been significantly reduced. As a consequence, the <br />mean annual sediment discharge near Jensen. Utah. had decreased by 54 percent to 2.91x109 kg. Andrews (1986) <br />