Predicted Stream Temperatures Suggest Challenges for Pacific Salmon in Coming Decades

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Abstract
Stream temperature is a key performance driver for aquatic species and a direct metric of climate impacts. We adapted a stream temperature model to predict daily temperatures across the Pacific Northwest USA through 2100. Our results suggested that stream temperatures may rise by ~1°C by the 2050's and 2°C by the 2080's, with seasonal and geographic nuances. Snowmelt‐fed streams were especially likely to experience seasonal shifts in temperature patterns. We built local models for two watersheds to show how predictions can guide Pacific salmon conservation planning. In the North Santiam River, a Willamette River tributary in Oregon, we found that Chinook salmon and Steelhead spawners and eggs may face increasingly stressful conditions below large dams. Whereas above‐dam thermal habitat will remain or become suitable, suggesting the importance of maintaining dam passage. In the Wenatchee River, an upper Columbia River tributary in Washington, future temperatures may contribute to higher pre‐spawn mortality of Chinook salmon. Cooler thermal refuges may persist in glacial and snow‐influenced parts of the landscape, but reaching high‐elevation refuges will require migrating through warming rivers. It is essential to consider temperature patterns alongside other cumulative impacts throughout the salmon life cycle. Future stream temperature estimates can help anticipate climate‐driven habitat changes and support proactive conservation strategies for lotic species.

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