The court affirmed last month’s injunction, siding with environmental groups against federal agencies including Bonneville Power Administration (BPA) and the US Army Corps of Engineers.
Dam managers must now allow water to spill over the top of the Lower Granite, Little Goose and Lower Monumental dams on Snake river and the McNary dam on Columbia river.
BPA has warned that the loss of output will lead to a rise in electricity prices and cost the region an estimated US$67M.
There are no plans to challenge the spill order further, said BPA spokesman Ed Mosey.
Last month federal Judge James Redden declared the government's plan to protect endangered fish species as inadequate.
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