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Why It Matters
When water temperatures in Idaho’s rivers exceed safe thresholds during summer heat waves, salmon and steelhead face stress, delayed migration, and potential death. The Army Corps of Engineers’ decision to release cold water from Dworshak Reservoir on July 25 represents a direct intervention to protect these fish populations during a period of dangerously warm conditions across western Idaho and southeastern Washington.
What Happened
Extreme heat pushed river temperatures to dangerous levels across the region. The Clearwater River near Orofino reached 79.7 degrees on July 25, while the Snake River 25 miles southwest of Lewiston hit 74.4 degrees. Both readings far exceeded the stress threshold for salmon and steelhead.
In response, the Army Corps of Engineers released cold water from Dworshak Dam, which pulls water from the bottom of the 600-foot-deep reservoir where temperatures remain near 42 degrees year-round. The release proved effective: water temperatures behind Lower Granite Dam, located downstream where the Clearwater and Snake rivers converge at Lewiston, remained at 66.1 degrees or cooler following the intervention.
Chris Peery, a fish biologist with the Corps of Engineers, explained that fish begin showing stress symptoms when water temperatures climb above about 68 degrees. “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,” Peery said.
The Corps consulted with the National Marine Fisheries Service and the U.S. Fish and Wildlife Service before executing the water release, following established protocols for managing the dam’s resources to benefit fish populations.
By the Numbers
- 79.7 degrees: Peak temperature recorded in the Clearwater River near Orofino on July 25
- 74.4 degrees: Peak temperature recorded in the Snake River 25 miles southwest of Lewiston on July 25
- 68 degrees: The temperature threshold above which salmon and steelhead begin showing stress symptoms
- 66.1 degrees: Maintained water temperature behind Lower Granite Dam following the cold-water release
- 3.4 million acre-feet: Total water storage capacity of Dworshak Reservoir
The Dam’s Complex Role
Dworshak Dam, which began construction in 1966 and stands 717 feet tall—making it the third tallest in the United States—was built for hydroelectric power generation and water management. However, the dam’s extreme height blocked salmon and steelhead from migrating to their natural spawning grounds upstream, and no fish ladder was constructed due to the structural challenges presented by the dam’s size.
To address the loss of fish passage, the Dworshak National Fish Hatchery was established and is now operated jointly by the U.S. Fish and Wildlife Service and the Nez Perce Tribe. The hatchery serves as a mitigation measure for the fish populations affected by the dam’s presence.
The cold-water release strategy represents one tool in a broader effort to manage the dam’s impact on fish health, though it carries limitations. According to a 2024 Hot Water Report from Save our Wild Salmon, while cold-water releases help during acute heat events, the benefit does not persist long and does not extend to cooling the three other major reservoirs downstream on the Snake River.
Looking Ahead
As climate patterns continue to produce summer heat waves, the Corps of Engineers will likely continue deploying strategic water releases from Dworshak Reservoir during peak temperature events. However, managing the river’s temperature remains a balancing act—the volume of cold water available for release is finite, and coordinating multiple competing water uses across the region requires ongoing consultation with federal fish and wildlife agencies.
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