As air temperatures begin broiling southeastern Washington and western Idaho in late July and early August, river temperatures also start simmering.

This weekend, the Clearwater River near Orofino, about 40 miles east of Lewiston, Idaho, had 79.7-degree water temperatures on July 25. The Snake River at Anatone, Washington, about 25 miles southwest of Lewiston, was also hot. Water temperatures peaked at 74.4 degrees that same day.

The Clearwater and Snake rivers converge at Lewiston and the hot water from those two rivers mixing there is dangerous for salmon and steelhead that prefer cooler water temperatures. As dire as this can be, there is one tool the U.S. Army Corps of Engineers (USACE) has to help cool the river – Dworshak Dam, also near Orofino. Salmon are cold-water species and migrate most efficiently at 65 degrees, according to Chris Peery, fish biologist for Walla Walla District.

“Above about 68 degrees, they start showing signs of stress, migration delay, and, if temperatures go higher, mortality can start to occur,” said Peery. “Water that flows into the lower Snake River and the Lower Granite Dam reservoir comes from Hells Canyon and other tributaries and is always warmer than 68 in summer.”

Dworshak is a concrete dam standing 717 feet tall, which is more than 100 feet taller than the Space Needle in Seattle and is the third tallest dam in the United States. Dworshak’s spillway crest runs 3,287 feet, or more than nine football fields long. It is massive, with a storage capacity of more than 3.4 million acre-feet, or enough to cover the state of Connecticut with 1 foot of water. For comparison, 1 acre-foot of water would cover a football field with 1 foot of water.

USACE manages water releases from Dworshak to help address temperature concerns in the Snake and Columbia rivers, in consultation with the National Marine Fisheries Service and the U.S. Fish and Wildlife Service. By releasing cold water from Dworshak on July 25, it kept river temperatures behind Lower Granite Dam at 66.1 degrees or less.

“This is where the value of Dworshak’s reservoir comes in,” explained Peery. “Dworshak is 600 feet deep and water that flows out of the bottom of that reservoir is 42 degrees year-round,” he said. “If we manage it carefully, we can help cool the Snake River so it stays around 68 degrees in the tailrace of Lower Granite Dam, but there is a limited amount of water that we can release from Dworshak.”

Many factors influence river temperatures. Dams, agricultural or industrial water use, habitat quality, and climate conditions like drought, annual snowpack and precipitation all impact temperatures. In this case, upstream river temperatures can heavily influence temperatures in the lower Snake but Dworshak helps blunt this. Because those conditions vary throughout the season, Dworshak releases cannot eliminate high temperatures throughout the river system. They can, however, provide operational flexibility and help improve conditions in portions of the Clearwater and lower Snake rivers during periods of elevated summer temperatures, aiding salmon and steelhead.