INCREASINGLY HOT AND DRY SUMMERS EXACERBATE LOW FLOWS AND THREATEN PACIFIC SALMON HABITAT THROUGHOUT NORTHWESTERN NORTH AMERICA

Excessively low stream flows in the late summer can disrupt aquatic life cycles, including those of ecologically and culturally significant species such as Pacific Salmon. Climate change is expected to drive hydrologic changes throughout northwestern North America, but the magnitude and direction of changes to low flows remain highly uncertain. Here we study 375 near-natural catchments, across rainfall-dominated, hybrid, snowmelt-dominated, and glacial regimes throughout the habitat range of Pacific Salmon from California to Alaska. Annual minimum summer discharge has decreased in most catchments; rainfall-dominated and hybrid catchments, which predominate in coastal watersheds and in the southern half of the range, have seen the most severe declines. We predict low flows using linear regression models which significantly outperform existing process-based models. We hindcast low flows back to 1900 and project changes to 2100 under four emissions scenarios. Low flows have historically been driven primarily by summer precipitation and moderately influenced by winter snow accumulation and summer temperature. However, we find that future changes will likely be driven by temperature because the magnitude of projected heating is large compared to the historical variability of temperature. Some further declines in low flows are probably inevitable in rainfall-dominated and hybrid catchments: under a low-emissions scenario, low flows will continue to decline to mid-century but then stabilize. Under a high-emissions scenario, 1-in-50-year low flows could occur almost every summer in rainfall and hybrid catchments. Bold climate action and mitigation strategies are urgently required to safeguard these habitats.