The Fate of Western Headwaters: Climate Controls on Base‐Flow Decline

Abstract Headwater streams comprise nearly 88% of the western U.S. river network and supply most of the region's surface water, making them especially sensitive to warming, snowpack loss, and drought. As surface‐water inputs decline, groundwater increasingly sustains streamflow, elevating the need to understand long‐term trends in base flow and their climatic drivers. We analyzed 75 years of streamflow records (1950–2024) from 115 headwater basins to quantify historical changes, climate controls, and future trajectories of base flow. Using statistical analyses and cluster‐specific Long Short‐Term Memory models trained on downscaled climate data, we assessed both historical behavior and projections under SSP2‐4.5 and SSP5‐8.5. Historical results show widespread base‐flow declines—most pronounced in early summer—driven by warming, reduced snowmelt, and declining antecedent moisture. Antecedent moisture emerged as the dominant positive driver, while snow and temperature exerted regime‐dependent effects. Future projections indicate continued declines of 45%–65% by late century, with earlier seasonal peaks and reduced summer flows. Snowmelt‐dominated and arid basins experience the largest relative reductions, whereas mixed‐regime systems contribute the greatest volumetric losses. These changes pose significant risks to municipal water supply, ecosystems, and wildfire resilience across the region. Collectively, our results highlight the vulnerability of groundwater‐supported streamflow to climate change and demonstrate the value of integrated statistical and machine‐learning approaches for regional hydrologic assessment.