Rising temperatures will drive summer streamflow droughts and threaten Chinook salmon habitat throughout western North America

Chinook salmon (Oncorhynchus tshawytscha) are a keystone species for many ecosystems of western North America, are culturally and spiritually significant for many Indigenous Peoples, and underpin a multi-million dollar industry. However, in recent years extreme summer streamflow droughts have disrupted Chinook migration and rearing patterns. Climate change is driving hydrologic changes throughout the region, but future changes to summer low flows remain highly uncertain. Here we study 375 near-natural catchments throughout the habitat range of Chinook salmon from California to Alaska. The streams span rainfall-dominated, hybrid, snowmelt-dominated, and glacial regimes. Summer discharge has decreased in most catchments, with rainfall-dominated and hybrid catchments seeing the most severe declines.We develop linear regression models which outperform existing process-based models, and project changes to 2100 under four emissions scenarios. Summer low flows have historically been primarily driven by variability in summer precipitation and moderately influenced by winter snow accumulation and summer temperature. However, we find that future changes will probably be driven by rising temperatures because future summer temperatures could greatly exceed the historical envelope of variability. 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 many rainfall and hybrid catchments. In glacial catchments summer discharge has been relatively stable in recent years because increased glacial meltwater flows have compensated for increased evapotranspiration. However, many of these glaciers are projected to disappear within 20 to 30 years, and we project severe declines in summer streamflow when this does occur.Many populations of Chinook rear or migrate during the summer months for which we project extraordinary future streamflow droughts. It is unknown whether Chinook populations can shift their life stage timing or find alternate habitats quickly enough to avoid catastrophic impacts. Bold climate action and local mitigation strategies are urgently required to safeguard this ecologically, culturally, and economically vital species against future extreme events.