The federal government must keep spilling water over dams on the Columbia and Snake rivers this summer to help juvenile salmon migrate to the sea, a federal judge ruled Wednesday.

U.S. District Judge James Redden's decision came just three days before the Bonneville Power Administration planned to reduce those spills. Redden issued a preliminary injunction against the Army Corps of Engineers, blocking the proposed cutback.

The power agency had argued the effect on fish would be minimal, but reducing the spill at four key dams could save Western ratepayers $18 million to $28 million in electricity costs this year.

Redden rejected that argument, saying the long-term environmental health of the region outweighed the short-term economic benefits of increased hydroelectricity production this summer.

"I don't want anyone to walk out of here thinking I ignored the public interest in terms of ratepayer dollars," Redden said after announcing his ruling.

"It's a difficult case, but my job is to consider the Endangered Species Act and the fate of juvenile salmon," Redden said, calling the summer spill plan "arbitrary and capricious."

The power agency had received National Oceanic and Atmospheric Administration Fisheries approval to reduce spill in August at the Ice Harbor and John Day dams on the Snake, and at the Bonneville and The Dalles dams on the Columbia.

The Army Corps of Engineers, which operates the dams, also approved the agency's plan on July 6, prompting a lawsuit seeking the injunction.

Oregon Gov. Ted Kulongoski (D) joined the lawsuit on behalf of environmentalists, tribes and fishermen who argued that the power agency's summer spill plan offered little or no benefit to Northwest ratepayers while risking potential damage to the region's long-term environmental and economic health if salmon runs decline.

"I think the people in this region understand that wild salmon in their rivers are more valuable than a nickel or a dime on their electric bills," said Todd True, attorney for Earthjustice, one of the environmental groups that filed the lawsuit.

The governors of the other three Northwest states served directly by Bonneville -- Idaho, Montana and Washington -- supported the summer spill plan.