Salmon survived massive dams and fishing fleets, but now they’re feeling the heat of global warming — and it’s likely to hammer them as hard as anything they’ve faced.

Although the government has spent billions to save salmon, warming will probably force even more extreme measures in coming years at the expense of water and power for people.

Biologists who have spent their careers watching over the fish said temperatures expected to rise an average of 0.2 to 1 degree per decade over the next century will probably wipe out some fragile runs of salmon. Snow will  fall as rain instead, feeding floods that flush away their eggs. Heat waves  will multiply, leaving less refuge to which they can retreat.

The region’s signature fish needs cool water the way people need air. But temperatures in the Columbia River, their critical conduit to the sea, are rising toward lethal levels. The coolest years now are often warmer than the hottest years of the 1950s, according to temperature gauges near Bonneville Dam.

The climate is not the only thing driving that trend. Dams that slow water flow, allowing it to warm, and the loss of plants that once shaded tributary streams, keeping them cool, also play a part.

But climate is growing more dominant and is expected to push river temperatures about 2 degrees higher on average by 2040, according to the Independent Scientific Advisory Board, a panel of top fish and wildlife researchers who advise federal agencies.

Already, some steelhead going home to the Snake River divert into the cooler Deschutes River to escape the warm Columbia, said Bob Heinith, a biologist at the Columbia River Intertribal Fish Commission.

Days of unusually scorching heat and meager river flows killed more than 100 salmon last July in the Middle Fork of the John Day River, a tributary of the Columbia. That wiped out a large piece of the river’s remaining salmon run.

“We’ve had some fish kills, but they’ve never been this extensive,” said Tim Unterwegner, a state biologist who helped survey the dead fish. “It did not cool off at night like it usually does.”

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