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<br />8 <br /> <br />Avery (1993), and Beus, et. al (1995) provided additional data on ongoing <br />beach response. <br /> <br />Again the primary area of concern focused on the critical reach of river from <br />Lees Ferry to the confluence with the Little Colorado River. This reach is <br />the most sensitive to long-term erosion due to the limited sediment input when <br />compared to the potential for sediment transport. The reach above Lees Ferry <br />receives almost no sediment input, and is essienta11y armored with a layer of <br />cobble. There was little movement of this cobble substrate during the high <br />flows of 1983, and therefore the reach should remain stable under the flow <br />regime of the preferred alternative. <br /> <br />Final EIS Analysis.-Sandbars (including camping beaches) go through natural <br />cycles of.deposition and erosion. High riverflows will transport sand-if <br />available-from the river bed and deposit it as sandbars in eddies. <br />Riverflows, rain, wind, and foot traffic all tend to erode these sandbars with <br />time. The cycle of deposition and erosion continues as long as there is sand <br />available in the system. Thus, sandbars depend on the availability of sand <br />storage in the river and occasional high flows to redeposit the sand. <br /> <br />Long-term sand storage depends on the sand supply from tributaries and sand <br />transport capacity of the river. Resource managers have some control over the <br />sand transport capacity through dam operations, but no control over the sand <br />supply from tributaries. Sand in Grand Canyon is transported by nearly all <br />riverf1ows, the amount transported increases exponentially with riverf10w (see <br />figure 1). The total amount of sand transported over the long term depends on <br />the magnitude, duration, and frequency of high riverf10ws. Over the 10ng- <br />term, the smaller the sand transport the greater the sand storage. <br /> <br />Because there is no change in either the monthly release volumes or the <br />operating criteria for the maximum daily change in flow, the increase in the <br />maximum flow criteria would not result in a substantial increase in the number <br />of hours when dam releases are greater than 20,000 cfs. The number of hours <br />when dam release are greater than 25,000 cfs would not change. Therefore, <br />there would not be a substantial increase in long-term sand transport capacity <br />nor a substantial decrease in sand storage. This is supported by impact <br />analysis of the alternatives presented in the draft EIS (page 182 and Appendix <br />D, pages 4-5). <br /> <br />In the analysis for the draft EIS, future hourly-flow releases from Glen <br />Canyon dam were modeled by the EDF (Power Resource Committee, 1993) for each <br />fluctuating flow alternative using the EDF .Peak Shaving Model." For each <br />alternative in the draft EIS, sand transport capacity was then computed for <br />each hour of the 20-year simulation. Based on these results, long-term sand <br />storage was computed for each alternative using 85 hydrologic scenarios (each <br />being 50 years) from the CRSS model (Randle, Strand, and Streifel, 1993). <br /> <br />The operating criteria for range in daily flow fluctuations, up and down ramp <br />rates, and maximum flows of the Moderate Fluctuating Flow alternative were all <br />greater than those of the preferred. alternative in both the draft and final <br />EIS,Results from this analysis showed that the probability of a net gain in <br />riverbed sand after 20 and.50 years was 61 and 70 percent for the Moderate <br />