Vegetation groundwater use drives streamflow declines in Colorado River headwaters

Snowmelt-driven regions provide water for billions of people, yet warming threatens to reduce streamflow from these watersheds1,2. The mechanics linking rising temperatures and streamflow decline remain poorly understood3, partly because snowfall takes months to become summer streamflow and steep terrain hinders direct observation. Here we show that vegetation in Colorado River headwaters draws on shallow stream-bound groundwater to sustain evapotranspiration during dry periods. Daily fluctuation patterns in soil moisture, groundwater, and streamflow indicate roots intercept groundwater to support transpiration when surface soils dry. Extending this analysis basin-wide across 18 headwater catchments, we demonstrate that summer temperature independently regulates streamflow with effects rivaling those of snowpack: warm summers reduce flows from high-snowpack years to average levels, while cool summers substantially elevate flows from moderate-snowpack years. These results reveal that growing-season temperature—not just winter precipitation—controls streamflow through groundwater-supported evapotranspiration. As warming continues and droughts intensify4, this unaccounted vegetation-groundwater pathway will deepen streamflow declines beyond those from reduced snowpack alone, with critical implications for water management in snowmelt-dependent regions.