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<br />002433 <br /> <br />recreational rafters and campers. The river's now reduced sediment loads are inadequate to <br />replenish beaches, even if flood releases occur once every twenty years. Flood releases destroy <br />riparian vegetation and birds." The Act did not specifY remedial measures, but seemed to imply <br />that even the aggressive spill avoidance strategy that had been implemented to reduce spill <br />frequency might be insufficient. <br /> <br />These conclusions produced the GCDEIS decision to reduce the return period of powerplant <br />bypasses above 45,000 cfs to no more than an average of I in 100 years. The option of installing <br />the spillway gate extensions was selected as part of the preferred alternative instead of the option <br />of targeting an additional 750,000 acre-feet of vacant storage space when the reservoir filled in <br />July. The extensions were determined to be 4.5 feet in height, in contrast to the 8-foot high <br />extensions installed during 1983. Additional questions about the need to reduce the frequency of <br />powerplant bypasses and the desired magnitude and impacts of sustained high releases during <br />extreme flood years now provide impetus to re-examine the original decision that an additional <br />750,000 acre-feet of vacant storage space is needed through the installation of the gate <br />extensions. <br /> <br />The Evolution of Understanding Regarding High Releases <br /> <br />Despite the enormous beaches created particularly by the 1983 spill event, the general thinking at <br />that time was that there was a very limited supply of sediment below Glen Canyon Dam and that <br />spills destructively moved much of this sediment out of the Grand Canyon. During the high flow <br />years of 1984 - 1986, the main channel sediment storage was likely much lower than prior to <br />1983, and the deposition rate during the 1984 - 1986 spills was lower as a result. Sediment <br />experts then believed that the river downstream of the dam was in a sediment-starved condition. <br />Sediment supply thus became one of the primary driving forces behind ecological <br />recommendations for changing powerplant operations. <br /> <br />Based upon continuing research, including evaluation of the BHBF test flow, sediment <br />researchers now believe that flood flows counteract the possible adverse impacts that fluctuations <br />have on beach erosion, thus rebuilding the deposits that would eventually slough back into the <br />eddies, regardless of the nature of the powerplant operations. Some suggested that more frequent <br />floods could allow higher levels of fluctuations. <br /> <br />The Agreement Contained in the 1996 AOP <br /> <br />With this evolving positive view towards spills, a desire for a test of the GCDEIS Beach Habitat <br />Building Flow was expressed by the Transition Work Group beginning in 1994. The Basin States <br />strongly opposed this request for a purposeful powerplant bypass because the 1968 Colorado <br />River Basin Project Act requires avoiding anticipated spills, interpreted as powerplant bypasses. <br />This opposition created an impasse that blocked such a test. <br /> <br />Additional discussions between members of the Transition Work Group and the Basin States <br />resulted in a proposal for a modification of the GCDEIS preferred alternative, that of moving <br />Beach Habitat Building Flows (BHBF) from years oflow reservoir conditions (when spills would <br />