Laserfiche WebLink
<br />Chapter 2.0 Description of Alternatives 21 <br /> <br /> <br />01198 <br /> <br />The fourth hydrological scenario is the habitat maintenance flow scenario <br />(figure 2.6). This scenario would be implemented only under two conditions: 1) the <br />autumn sediment input scenario must have been completed, and 2) a minimum <br />tributary sediment input of 500,000 metric tons must occur between Julyl-December 31.' <br />This scenario is similar to the winter sediment scenario in that a high flow test <br />immediately follows the tributary input. The high release would be at powerplant <br />capacity, last two days, and have 4,000 cfs!hr upramp rates and 1,500 cfs!hr downramp <br />rates. <br /> <br />The Paria River flow necessary to provide the minimum sediment input would be <br />approximately 2,500 cfs, though rare events could be as high as 12,000 cfs. Thus, the <br />combined powerplant capacity and tributary flow would be in the approximate range of <br />33,500 cfs (31,000 cfs dam release + 2,500 cfs tributary inflow) to 43,000 cfs (31,000 dam <br />release + 12,000 cfs tributary flow). If the combined flows would exceed 45,000 cfs, then <br />dam releases would be reduced to constrain total flow to 45,000 cfs or less. The close <br />association in timing of the sediment input and the ensuing dam release would be <br />facilitated through installation of additional gages on the Paria River to serve as an early <br />warning system announcing the inflow. <br /> <br />The habitat maintenance flow would be followed by ROD operations with daily <br />fluctuations until January 1 unless another minimum 500,000 metric ton input occurred, <br />in which case the powerplant capacity releases would be repeated, followed again by <br />ROD operations. On January 1, if there was a minimum sediment retention of 800,000 <br />metric tons in the reach of the Colorado River between Glen Canyon Dam and the Little <br />Colorado River, a high flow of 42,000-45,000 cfs would be released from the dam having <br />the same features as that under the auturrm sediment input scenario or winter sediment <br />input scenario. If the minimum amount of sediment is not retained above the Little <br />Colorado River, fluctuating non-native fish suppression releases would be initiated <br />following the January 1 evaluation. These releases would continue until April 1 unless <br />additional sediment was received by the Colorado River sufficient to bring the sediment <br />retained up to the 800,000 metric ton minimum. This amount of additional sediment in <br />the system would trigger a two-day 42,000-45,000 cfs high flow having the same features <br />as in the winter sediment input scenario. Following this high flow, the non-native fish <br />suppression flows fluctuating between 5,000-20,000 cfs would continue through March <br />31. Dam releases would then revert to those prescribed under ROD operations. <br /> <br />2 If the minimum sediment input trigger does not occur during the first or ensuing years <br />of Proposed Action operations, the autumn sediment input scenario would continue to receive <br />the highest priority for completion in the following year. <br />