1992 Reservoir Drawdown Test: Lower Granite and Little Goose Dams




U.S. Army Corps of Engineers, Walla Walla District


adult, anadromous fish, downstream migration, hydroelectric, juvenile, juvenile salmon, Lower Snake River, lowering reservoirs, monitoring, reservoirs, salmon, Snake River, water quality


None supplied. From introduction: A test of the reservoir drawdown concept was completed in March, 1992, using Lower Granite and Little Goose Neck Dams (also known as "projects") on the lower Snake River. The test was designed to gather information regarding the effects of lowering existing reservoirs substantially to potentially improve survival of downstream-migrating juvenile salmon, as proposed by various entities in the Pacific Northwest. Since the hydroelectric projects on the lower Snake River are not currently equipped to safely pass adult and juvenile fish at the dams at lowered pool elevations, this test was primarily designed to provide information regarding the physical effects of such an operation on dam and reservoir structures. Information on a variety of physical features was gathered, and environmental data, including some effects of drawdown on water quality and aquatic organisms, were also obtained. Some anadromous fish were present in the reservoirs during March, so efforts were also made to minimize test impacts on these and to gather information that will help evaluate the long-term effects of reservoir drawdown. This report presents: (1) background material on the salmon runs and the effects of dam operations, (2) what was accomplished during the drawdown test, including implementation procedures, (3) monitoring and evaluation objectives and procedures, and (4) results.

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