In a letter to Northwest policymakers Tuesday, more than 50 scientists said breaching the four lower Snake River dams is the only action that can counteract warm summertime water in the lower Snake and Columbia rivers that often reaches temperatures that can be lethal to salmon and steelhead.
In the coming years, climate change will continue to worsen the problem of elevated water temperatures in the sunbaked slackwater reservoirs behind the dams, they said.
According to the letter, modeling by the Environmental Protection Agency shows federal dams on the two rivers elevate water temperatures. John Day Dam on the Columbia River causes a rise of as much as 6.3 degrees, and Ice Harbor, Lower Monumental, Little Goose and Lower Granite dams on the Snake River cause a rise of 12 degrees, compared to a free-flowing river.
“The impact of additional heating in lower Snake River reservoirs is clear, and it can drive water temperatures above 68°F for extended periods in late summer and early fall — dangerous for salmon and steelhead,” according to the letter signed by 55 scientists and addressed to the governors and congressional delegations representing Idaho, Oregon and Washington.
“Hot water kills cold-water fish. That is what is happening now, when adult salmon try to return home through reservoirs that are too warm for long periods of time,” said Dave Cannamela, a retired Idaho Fish and Game fisheries biologist who lives at Boise.
Rick Williams, an independent fisheries scientist who lives at Boise, said breaching the Snake River dams would lessen the problem even if water temperatures continued to sometimes exceed 68 degrees. Such incidences would be much shorter in nature, he said.
“We know these fish can be in sub-lethal conditions for limited amounts of time. When the length of time prolongs, the mortality starts to show up,” he said.
Summertime water temperatures have long been a concern for adult salmon and steelhead that must persist in the water as they surge upriver to spawn in cool mountain streams. The problem is most pronounced for endangered Snake River sockeye salmon that swim up the Columbia and Snake rivers starting in June when water temperatures are starting to climb. Steelhead and fall chinook must also contend with warm water on their return from the Pacific Ocean in July, August and September.
In 2015, a dry and warm winter left the mountains of the Pacific Northwest with meager snow packs that dissipated quickly in the spring. The arid winter was followed by a scorching summer that, when combined with low river flows, heated the water well above 68 degrees, considered a tipping point for salmon and steelhead. Temperatures into the 70s persisted for weeks on end and sockeye perished in unprecedented numbers. About 96 percent of the sockeye bound for Idaho’s Stanley Basin died in the Columbia and Snake rivers. More plentiful sockeye bound for the middle Columbia River died in similar numbers.
That summer’s low flows and high temperatures were well outside of normal conditions, but may not be in the future.