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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 at this rate 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, we would recommend that flood releases be avoided until the tolerable fre- quency 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. Fluctuating Releases primarily Affect Recreation~- . 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 fish- ing and white-water boating is reduced by approximately 15 percent under fluctuating, as compared to steady, releases. Fluctuating releases have a greater impact on aquatic than on terrestrial resources. 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. However, short periods of fluctuations at other times may increase food availability and trout growth. t T b ~ 1 t