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<br />002~37 <br /> <br />contained in the 1996 AOP. Additionally, there will an alternatives and impacts analysis on <br />various options that exist regarding the spillway gates extensions. <br /> <br />GCDEIS and GCPA Conclusions Regarding Powerplant Bypasses <br /> <br />The majority of the GeES Phase I research work took place in the mid-1980;s, when the releases <br />from Glen Canyon Dam were at an all time high since the construction of the dam. These flood <br />flows were so different than historic releases and caused such large effects downstream that they <br />had a great influence on GeES recommendations. <br /> <br />On page 83 of the final GeES Phase I report, the first and foremost conclusion was that" Adverse <br />downstream consequences are caused primarily by sustained flood releases significantly greater <br />than powerplant capacity and by fluctuating releases", noting the erosive effect of floods on sand <br />deposits and vegetation. Generally, these conclusions suggested the elimination or reduction of <br />flood flows. <br /> <br />In the committee report accompanying the GCP A legislation, the Congress continued this <br />thinking of adverse impacts by stating that "Flood releases from the dam erode beaches used by <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! 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 additional vacant storage space option was rejected by the Basin States on the basis of <br />reduced reservoir yield. The extensions were determined to be 4.5 feet in height, in contrast to <br />the 8-foot high extensions installed during 1983. Additional questions about the need to reduce <br />the frequency of powerplant bypasses and the desired magnitude and impacts of sustained high <br />releases during extreme flood years now provide impetus to re-examine the original decision to <br />install the 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 />