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I I I I I I I I I I I I I I I I I I I UNDER CURRENT OPERATIONS, FLOOD RELEASES WILL OCCUR IN ABOUT ONE OF EVERY FOUR YEARS. Flood releases occur about one in four years due to reservoir storage targets and errors in forecasted runoff (among other variables). Current data are sufficient to show that this frequency of flooding would be damaging to downstream resources, but are insufficient to determine precisely the frequency of flooding that resources can tolerate in the long-term. Based on observations of the natural system in Grand Canyon, flood releases should be avoided until a tolerable frequency can be better defined. Current knowledge indicates that even a frequency as low as one flood in twenty years will produce a net long-term loss of camping beaches and substrate, although at a rate reduced from that caused by current operations. Two methods of frequency analysis were used to arrive at the one-in-four-year flood frequency. Operating procedures and methods in place during the GCES study period were used in calculating the frequency of spills. FWCTUATING RELEASES PRIMARILY AFFECT RECREATION AND AQUATIC RESOURCES. Except during periods of very high runoff, the amount of water released from Glen Canyon Dam is varied on an hourly basis, often with two peaks and two troughs daily. This is done to provide electrical power when it is most needed during the day. These fluctuations can cause the river level to change by up to 13 feet. Fluctuating releases stay below 31,500 cfs and are therefore not as detrimental as floods for terrestrial resources. However, they have a deleterious effect on recreation and aquatic resources. The quality of fishing and white-water boating is reduced by approximately 15 percent under fluctuating releases as compared to steady releases. Fluctuating releases have a greater impact on aquatic than on terrestrial resources. Fluctuations at any time of the year strand fish. Fluctuations during the summer months reduce habitat for larval native fishes. Fluctuations in the winter months reduce the natural reproduction of trout by exposing spawning beds and denying access of reproducing adults to tributaries. However, short periods of fluctuations at other times may increase food availability and trout growth. Beaches deposited during high, steady flows are rapidly eroded when exposed to either fluctuating or steady lower flows, but the rate of erosion diminishes and equilibrium is reached after several years of similar 7