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released from Brownlee quickly passes through the small Oxbow reservoir into Hells Canyon Reservoir. The company attempts to buffer fluctuations from Brownlee Dam by releasing water a bit more steadily from Hells Canyon Dam. When possible. a minimum of at least 6,530 ft'/s is released from Hells Canyon Dam to meet instream flow require- ments for fish and navigation (Dennis Womack. Idaho Power Company. oral commun.. 1993). The dams of the Hells Canyon Complex were not designed specifically for ftood control. and unexpectedly high flows certainly can occur on the Snake River. The consum- mate unexpected flow happened 15,000 years ago when Lake Bonneville (ancestor of today's Great Salt Lake) suddenly cut a new channel into the upper Snake basin. A peak ftow of20 to 33 million ft'ls exploded through Hells Canyon. almost three times greater than the largest flood ever measured on the Amazon River (Jarrett and Malde, 1987. O'Connor, 1993). Historically. the more typical pre-dam ftoods would roll through Hells Canyon in May and June. reaching peaks of75,000 to 95,000 ft'ls every few years. But even with all three dams in place. ftoods of74,000 ft'/s or more have passed through the Complex on at least five occasions since 1970. Peak flow past the dams in a 1982 flood was 87,780 ft'/s. The great difference between the pre.dam and post-dam floods lies not in their peak flow. but with their sediment content. The three dams of the Hells Canyon Complex act as very effective sediment traps. Most suspended sediment reaching Brownlee Reservoir drops to the bottom of the lake; what little passes through is trapped behind the two reservoirs immediately downstream. Water released by Hells Canyon Dam is usually crystal clear. And no significant sediment-bearing rivers join the Snake until the Salmon River comes in. 60 miles downstream. u c 8 '" w :;; a. 1>> .i!! .., :0 " ~ '" E" .. .c .., w is How much sediment are we talking about? The Snake River below its confluence with the Salmon carried as much as 5 million tons downstream each year before the dams were built (Jones and Seitz, 1980). The current status of the sediment balance in the system is unknown. One approach to quantifying sedi. ment on the Snake River would be to sound the bottoms ofthe three Hells Canyon Complex reservoIrs. espe. cially Brownlee. Annual ~ood series for the Snake River near Asoton, Washington. Regulation by Brownlee, Oxbow, and Hells Canyon Dams does not sigM niflCantly affect the size or frequency of ~oods on the Snake River. 100,000 80,000 80,000 40,000 20,000 o 1930 1940 1950 1980 1970 1980 24