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In the mountainous regions of western North America, snowmelt recharges groundwater and provides ecosystem-sustaining base flow during low-flow periods. Continued warming is expected to have large impacts on snowmelt hydrology and on low-flow regimes, but the relative impact of temperature and precipitation on low flows is unclear. To address this knowledge gap, the dominant climate controls on summer and winter season low flows in 63 near-natural catchments in mountainous ecoregions of western North America are identified with correlation analysis, and low-flow sensitivity to temperature and precipitation is quantified with multiple linear regression analysis. Results show that precipitation is the dominant control on the interannual variability of annual runoff and on the duration and severity of summer and winter low flows. The temperature sensitivity of low flows, however, can be as much as twice that of annual runoff. Warm winters correspond to significantly lower runoff; significantly longer, more severe summer low flows; and significantly shorter winter low flows. This highlights the importance of winter climate conditions for runoff and low flows in these mountain catchments and provides another line of evidence regarding the impacts of climate change on snowmelt hydrology.

Plain Language Summary In the mountains of western North American, snowmelt is an important component of streamflow. Precipitation that falls during the winter season is stored as snowpack, and melting of this snowpack generates streamflow. During spring when the snowpack is melting rapidly, streamflow peaks, but during the winter and summer months the contribution of snowmelt to streamflow diminishes, and streamflow is largely sourced from groundwater discharge. These periods correspond to low-flow periods. Continued warming is expected to have large impacts on the timing and magnitude of snowmelt and thus large impacts on the duration and severity of low-flow periods. The relative impact of precipitation and temperature on low flows is currently unclear. Analysis of streamflow and climate data from 63 mountain catchments shows that precipitation is the dominant control on total annual streamflow and on the duration and severity of low flows. Compared to annual streamflow, however, low flows are up to 2 times more sensitive to temperature, particularly winter temperatures above 0 degrees C. With no change in precipitation, continued warming in these mountain catchments will likely yield longer, more severe summer low flows, shorter winter low flows, and an overall decrease in annual streamflow.