Climate change is causing warming ocean and river temperatures that can be harmful to fish. Damaging water temperatures can and do occur both in free-flowing rivers and rivers with dams. As an example, due to record high water temperatures, approximately 466,000 adult fish perished in the undammed Fraser River before reaching their spawning grounds in 1994.

The science is mixed on the direct effect that dams have on river temperatures. A 2003 EPA study indicated that dams may exacerbate temperature issues on the rivers, but a 2002 study performed by Pacific Northwest National Laboratory showed that dams within the Columbia and Snake river basins moderate extreme water temperatures.

The PNNL study concluded that “…the reservoirs decrease the water temperature variability. The reservoirs also create a thermal inertia effect that tends to keep water cooler later into the spring and warmer later into the fall compared to the unimpounded [i.e., free-flowing] river condition.”

A graph from the study, representing conditions at Ice Harbor Dam helps to illustrate the effect of dams on water temperatures.

Climate change effects on stream and river temperatures across the northwest U.S. from 1980–2009 and implications for salmonid fishes
2-28-2019 New York Times, “The World Is Losing Fish to Eat as Oceans Warm, Study Finds”
EPA – Climate change indicators
Foreman, M & B. James, C & C. Quick, M & Hollemans, Peter & Wiebe, Edward. (1997). Flow and Temperature Models for the Fraser and Thompson Rivers. Atmosphere-ocean
US Army Corps of Engineers – Lower Snake River Dams
Summary: Regional Scale Simulation of Water Temperature in the Columbia River Basin
Richmond, et al: Regional Scale Simulation of Water Temperature and Dissolved Gas Variations in the Columbia River Basin