The U.S. Army Corps of Engineers deployed a critical water-management strategy in late July to combat dangerously warm river temperatures threatening salmon and steelhead populations across Idaho’s Clearwater and Snake rivers. Federal managers released cold water from Dworshak Reservoir to cool downstream waters as a heat wave pushed surface temperatures above safe thresholds for the region’s fish species, a condition that poses significant challenges for fish migration and survival during spawning season.
On July 25, the Clearwater River near Orofino reached 79.7 degrees, while the Snake River 25 miles southwest of Lewiston hit 74.4 degrees—both readings well above the 68-degree threshold at which salmon and steelhead begin experiencing physiological stress. The Corps of Engineers coordinated the emergency water release from Dworshak Dam following consultation with the National Marine Fisheries Service and U.S. Fish and Wildlife Service, two federal agencies responsible for protecting endangered fish populations.
How Dworshak Reservoir Helps Cool Rivers
Dworshak Dam, one of the nation’s tallest structures at 717 feet and ranking third in U.S. dam height, holds more than 3.4 million acre-feet of water. The reservoir’s depth and size create a natural advantage for fish-protection efforts: the bottom layers of the reservoir maintain a stable temperature of approximately 42 degrees year-round, providing a source of cold water that managers can strategically release during summer heat events.
By opening spillways and outlets from the lower portions of the reservoir, the Corps delivered significantly cooler water downstream. Following the July 25 release, water temperatures behind Lower Granite Dam—another major facility on the Snake River system—remained at or below 66.1 degrees, providing temporary relief for migrating fish populations.
Chris Peery, a fish biologist with the Corps of Engineers, explained the biological stakes: “Above about 68 degrees, they start showing signs of stress, migration delay, and, if temperatures go higher, mortality can start to occur.” Peery also acknowledged the inherent limits of the strategy, noting that “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.”
Broader Challenges to River Temperature Management
The Clearwater and Snake rivers converge at Lewiston, creating a complex hydrological system where hot water from one tributary can mix with cooler flows, complicating efforts to maintain safe conditions throughout the system. Multiple factors influence river temperatures across the region, including drought conditions, agricultural and industrial water withdrawals, habitat quality, and climate conditions.
A 2024 report from Save Our Wild Salmon documented both the benefits and limitations of Dworshak cold-water releases. The analysis found that while releases from Dworshak do help cool adjacent waters, the benefit is temporary during extended summer heat and does not reach three downstream reservoirs on the lower Snake River—meaning the cooling effect diminishes as water moves further downstream.
The dam’s history also underscores the complex relationship between infrastructure and fish conservation. Construction of Dworshak Dam began in 1966 and ultimately blocked salmon and steelhead trout from migrating to their natural spawning grounds upstream. The reservoir’s 717-foot height made installation of a traditional fish ladder impossible, necessitating mitigation measures. The Dworshak National Fish Hatchery was subsequently constructed and is now operated jointly by U.S. Fish and Wildlife Service and the Nez Perce Tribe to offset losses caused by blocked migration.
Impact on Canyon County and Regional Fish Populations
For Canyon County residents and downstream communities relying on healthy fish populations, river temperature management during summer heat events remains a critical concern. Snake River health reports have documented progress on pollution reduction, though new environmental threats continue to emerge, and water temperature is one of several stressors affecting regional fisheries. Recent advisories issued by Idaho health officials regarding cyanobacteria in area reservoirs highlight the interconnected nature of water quality and aquatic ecosystem management.
What Comes Next
The Corps of Engineers will continue monitoring river temperatures and coordinating with federal fish-management agencies throughout the remainder of summer 2026. Managers indicated that additional cold-water releases from Dworshak may occur if river temperatures again exceed safe levels during subsequent heat waves. However, the limited volume of releasable water and the temporary nature of the cooling effect mean that long-term solutions to fish habitat challenges will likely require broader regional water-management strategies and continued discussion among federal agencies, state officials, tribal leaders, and stakeholders across the Pacific Northwest.