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Key Findings

1. Using a 2003 EPA model, a letter signed by fish researchers and activists claimed the four Lower Snake River dams are increasing river temperatures by up to 6.8° C.
2. Examining the 16 years of temperature data on the Snake River since the study was released finds the model overstated the warming impact of the dams by at least 20%.
3. Temperature data also show the potential impact of the dams on temperatures is declining.
4. The greatest temperature variance between the dams has also moved away from the time when salmon are returning.
5. The letter cites 2015 as an example of the worst temperature impact. It is also the year of the largest returns at the Lower Granite Dam, which is the farthest dam upstream.
6. The temperature impact of the Lower Snake River dams is smaller than claimed, declining, and outside the time when it would have most impact on salmon and steelhead. Destroying the dams is unlikely to yield significant temperature improvements based on the data.

In October 2019, advocates for destroying the four Lower Snake River (LSR) dams released a letter arguing that the four dams were increasing water temperatures, harming populations of returning steelhead and salmon. The letter, signed by 55 “scientists,” was cited in several news articles and in Governor Inslee’s draft report on the dams.

The claim is based on a computer model that argues the dams slow down the flow of river water, which in turn increases the risk of heating from the sun. High water temperatures are harmful to salmon and steelhead, so an increase in temperature would add an additional stress to the fish as they move upstream to spawn. The letter claims, “When considered collectively, the four lower Snake Dams could affect temperatures up to a potential maximum of 6.8° C/12.2° F.”
 

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