To help salmon pass eight federal dams on the Snake and Columbia rivers in the US, the US Army Corps of Engineers has been drawing down the reservoir at the 219m high Dworshak dam in Idaho by about 25m. The Corps’ action, which has been taking place over the past four years, follows a salmon recovery plan set by the National Marine Fisheries Service (NMFS).

Dworshak is just one of five storage reservoirs from which water is released each spring and summer to increase river flows for salmon and steelhead trout. The total drop at Hungry Horse and Libby reservoirs in Montana, for example, do not exceed 6m. The other two reservoirs are Brownlee, on the Snake, and Grand Coulee, on the Columbia. US Army Corps which operates the reservoirs says it must balance the many functions of its dams against the needs of salmon.

The effects of the flow augmentation programme, ordered by the NMFS in 1995, ripple widely. Dworshak, on the Clearwater river, is one of ten major hydro dams in the Columbia river basin, which also supports a large water based recreational market. Residents of nearby Orofino are furious that the boating season must end early each summer. The town, once reliant on timber, now relies increasingly on recreation.

The drawdown is also affecting power production. Before augmentation began, the Corps emptied the region’s storage reservoirs each winter, using the water to generate hydro power for distribution by the Bonneville Power Administration. Now some water is held back so it can be released during spring and summer, when young salmon head to sea.

Holding water back cuts annual hydroelectric generation in the Northwest from about 12,000MW to about 10,800 MW.