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0025.)4 Background - Evolution of Spill Expectations GCES conclusions The majority of the GCES phase I research work took place in the mid-1980's, when the releases from Glen Canyon Dam were at an all time high since the construction of the dam. Flows were generally high and constant, providing almost no opportUnity for the researchers to observe the impacts of fluctuating flows. The flood flows were so different than historic releases and caused such large effects downstream that they had a great influence on GCES recommendations. , On page 83 of the final GCES Phase I report, the first and foremost conclusion was that "Adverse downstream consequences are caused primarily by sustained flood releases signific:mtly greater than powerplant capacity and by fluctuating releases". noting the erosive effect of floods on sand deposits and vegetation. Generally, these conclusions suggested the elimination or reduction of flood flows. Significant effort was taken to understand the operational causes of spills, and in 1987 modifications to monthly release patterns were made by Reclamation to reduce the frequency of spills from about 1 year in 4 to about 1 year in 20. Due to the limited nwnber of years of actual dam operation and forecasts, there was uncertainty associated with these estimates, but a reduction in this frequency was certainly desired by the researchers involved in GCES. . Despite'the enormous beaches created particularly by the 1983 spill event, the general thinking at that time was that there was a vety limited supply of sediment below Glen Canyon Dam and that spills destrUctively moved much of this sediment out of the Grand Canyon. 1992 Grand Canyon Protection Act In the committee report accompanying this legislation, the Congress continued this thinking of adverse impacts by stating that "Flood releases from the dam erode beaches used by recreational rafters and campers. The river's now reduced sediment loads are inadequate to replenish beaches, even if flood releases occur once every twenty years. Flood releases destroy riparian vegetation and birds." The Act did not specify remedial measures, but seemed to imply that even the aggressive spill avoidance strategy that had been implemented to reduce spill frequency might be insufficient, GCDEIS Transition Work Group and Sediment Balance After the passage of the GCP A, the thinking of some sediment experts began to change, primarily as the result of the hypothesis that the sediment rating curves below the dam were not static with time, Additional thought was also being given to the location of stored sediment in the canyon and the mechanisms for moving sediment from the channel bottom to eddy areas. SignificantrnQqeling b:tthe sediment researchers changed to a great degree the way in which transport mechanisms ~ere viewed. The long term balance of sediment in the Grand Canyon continued to be an important issue in these discussions, 2