Laserfiche WebLink
<br />! <br /> <br />14 Chapter 2.0 Description of Alternatives <br /> <br />The five proposed release types are: <br /> <br />. 8,000 cfs steady flows, <br /> <br />. 6,500-9,000 cfs fluctuating flows, <br /> <br />. 5,000-20,0001 cfs fluctuating non-native fish suppression flows, <br /> <br />. 31,000-33,000 cfs habitat maintenance flow, and <br /> <br />. 42,000-45,000 cfs high flows. The magnitude of these short-term releases would <br />not exceed 45,000 cfs but they would vary below this level depending on Lake <br />Powell elevation and generator availability. <br /> <br />The order in which the releases would occur depends on the amount of sediment <br />inputs from the Paria River or ungaged tributaries in Glen Canyon and upper Marble <br />Canyon (GCMRC 2002b). However, under the Proposed Action the fluctuating non- <br />native fish suppression flows would occur independent of sediment availability. Given <br />the complexity of the proposal and the many decision points, the proposal is graphically <br />depicted in a flow diagram (figure 2.2). <br /> <br />The first release scenario is called the autumn sediment input scenario <br />(figure 2.3). It would occur if three conditions are met. First, if at least 500,000 metric <br />tons of fine sediment enters the Colorado River from the Paria River and ungaged upper <br />Marble Canyon tributaries between July 1 and October 31, then dam releases would <br />change from ROD operations to a series of alternating 2-week long steady 8,000 cfs <br />releases and 2-week long 6,500-9,000 cfs fluchtating releases. If the minimum sediment <br />input does not occur, dam releases would follow the prescription of the ROD (as <br />described in the No Action Alternative). <br /> <br />Second, if at least 1,000,000 metric tons of fine sediment are present in Marble <br />Canyon by October 31, the alternating steady and fluctuating releases would continue. If <br />the minimum sediment input does not occur by that date, dam releases would follow <br />the prescription of the ROD. By December 1, a comparison would be made of the <br />effectiveness of sediment conservation by the 8,000 cfs steady releases and the 6,500- <br />9,000 cfs low fluctuating releases. The action agencies within the Department of the <br />Interior would decide which flow is most effective at sediment conservation and <br />discontinue the less effective release. <br /> <br />1 However, maximum How, up ramp and downramp rates could be adjusted through the <br />adaptive management and environmentai compliance process during the second year of non- <br />native suppression Haws if the Proposed Action is not achieving the objectives of the experiment <br />or is creating unanticipated adverse effects. <br />