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<br />. <br /> <br />. <br /> <br />. <br /> <br />" <br /> <br />r . <br /> <br />Powell and Mead. Reservoir equalization and UpperlLower Basin consumptive uses all affect <br />the cyclic drawdown and refilling of Lake Powell. High runoff years when the reservoir is full <br />produces high powerplant releases and increased risk of powerplant bypasses. <br /> <br />Some sediment experts believe that there is sufficient regeneration of main channel sediment <br />supplies to allow BHBF's in all years that such events would be allowed by the 1996 agreement, <br />even every year if possible. Longer duration spills may have different effects than the short <br />duration BHBF's, so additional sediment transport modeling would help clarify the allowable <br />frequency of such spills. <br /> <br />The desired magnitude ofBHBF's <br /> <br />At a BHBF symposium held in April 1997, many researchers expressed the opinion that, while <br />the 45,000 BHBF of April 1996 achieved some of the intended results, periodic higher releases <br />would be helpful to scour non-native vegetation, rework backwater areas, and deposit sand high <br />enough on the beaches to be less vulnerable to succeeding flows. Estimates of 60,000 to 90,000 <br />cfs were discussed as appropriate flow levels for these purposes. <br /> <br />Additional modeling and analysis should be devoted to this question of the magnitude of high <br />releases. This should address both the short duration BHBF's and the potentially longer duration <br />uncontrolled powerplant bypasses. <br /> <br />GCDEIS expectations related to spillway gate extensions <br /> <br />One of the GCDEIS conclusions was to reduce the return period of bypasses above 45,000 cfs to <br />a long term average of not more than 1 in 100 years. Thus, releases below 45,000 cfs were <br />allowed as part of normal operations, but the extreme spillway releases such as occurred in 1983 <br />were essentially forbidden. This threshold level of 45,000 cfs seemed to indicate that flows <br />below that level were acceptable from an ecological perspective while higher flows were deemed <br />too damaging. It is interesting to note that the current opinion of at least the sediment researchers <br />is just the opposite. <br /> <br />The CRSS modeling which formed the hydrologic basis for many of the GCDEIS decisions <br />determined that bypasses were rare events, and if a small amount of buffer space were provided, <br />such releases greater than 45,000 cfs could be avoided. Since it uses a monthly time step, the <br />CRSS model could not really estimate the peak bypass release other than to average the release <br />over the month in which it occurred. Thus some judgment was used in estimating the frequency <br />of releases greater than 45,000 cfs. <br /> <br />The limited value of the spillway gate extensions <br /> <br />The GCDEIS cornmitment to install the 4.5-foot extensions would produce about 500,000 acre- <br />feet of surcharge storage space above the normal maximum water surface of 3700 feet. While <br />this is a large amount of reservoir space, it is small in comparison to either the 7.8 MAF April - <br />July inflow or the 2.1 MAF forecast error term for June I (5 percent exceedence level). A buffer <br />