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<br />0;)2J94 <br /> <br />12 <br /> <br />The 1996 Test Flow deposited new sand in these arroyo mouths, deposits which <br />remained in place through 1996, temporarily protecting these sites. In combination with site <br />stabilization/mitigation efforts implemented by the NPS, the 1996 Test Flow appears to have <br />provided temporary protection against ongoing terrace drainage erosion at these archeological <br />sites. <br /> <br />Sediment Management: Both Glen and Marble Canyons may be in a persistent, long-term <br />state of sediment deficit, since former estimates of sand inputs from the Paria and outflow <br />from Marble Canyon did not take hysteresis into account. Based on existing model results, <br />sand outputs from Marble Canyon exceed sediment inputs from the Paria when hysteresis is <br />included in the model. This condition suggests that a "scour wave" may have slowly migrated <br />downstream from Glen Canyon Dam since 1963. . The scour wave may have slowed <br />temporarily after reaching the geomorphic boundary in Marble Canyon where the river widens <br />significantly (about river mile 38-40). <br /> <br />Future management flows may require consideration of sediment storage, in concert <br />with other resource needs in the various reaches, and also the particle-size distribution of <br />those sediments. Hence, future efforts should focus on monitoring sediment availability and <br />grain-size distribution. More frequent, higher magnitude (35,000 to 90,000 cfs) but shorter <br />duration flows may be recommended in the future to test the hypothesis of rapid deposition of <br />sand through secondary circulation along the Colorado River. In combination with coarsened <br />particle-sizes, such flows may allow for maximum sediment conservation under high flow <br />periods. Such floods used as sediment management tools would quickly deposit finer sand in <br />the channel at high elevations along the river's shorelines while coarsening sand stored on the <br />mainstream bed. Coarsened sand on the bed may act to reduce downstream transport to Lake <br />Mead under ROD operations by shifting the suspended sediment transport curve. <br /> <br />New research results noted above suggests that short duration floods of greater <br />magnitude than the 1996 Test Flow may conserve sand by storing it at higher stage elevations. <br />These high flows might be used when stored sediments are in a coarsened condition and may <br />not be effective at all sites. At other times, such as following a period of high flood frequency <br />from the Paria and LC Rivers occurring in dry years, smaller magnitude floods of somewhat <br />longer duration might be the proper prescription. Adaptive management releases would be <br />designed for the antecedent conditions and objectives preceding such a high flow. During <br />surplus years. one scenario might be to use annual or more frequent high flows to store <br />sediment. Once deposited at higher stages, even high sustained releases would have a limited <br />capacity to export sand out of the system because bed materials would be coarsened. Smaller <br />floods (perhaps in conjunction with tributary inflows) might be sufficient for sediment <br />management objectives during dry years. However, the ROD states that high flows are only to <br />be conducted in high inflow years, and use of the spillways for flows >45,000 cfs remains <br />controversial. <br /> <br />Final Draft - 12/12/97 - For AMWG Review <br />