Relationships Between Snowpack, Low Flows, and Stream Temperature in Maritime Western U.S. Mountains

Streamflow in mountain watersheds of the Western USA is highly seasonal and is regulated primarily by snowmelt rather than directly by precipitation. We investigate the sensitivity of annual minimum summer flows to potential snowpack reductions under a changing climate. Summer low flows have important influences both on human water supplies and on stream temperatures which impact the survival of fish such as salmon. Previous work has shown climate elasticity of low flows in response to snow water equivalent (SWE), precipitation (PPT), and potential evapotranspiration (PET) is higher in the Sierra Nevadas compared to the more northern Cascade mountain range. We use multiple linear regression to examine this elasticity in more detail, accounting for confounding variables (e.g., if SWE, PPT, and PET are not independent). In addition, we add predictor variables which capture the timing of water inputs and evaporative demand. We also investigate the role of storage carryover from previous years, since future climates may impact storage by altering the frequency of wet and dry years. This analysis includes watersheds in mountains of the Western U.S. that have over 35 years of both USGS streamflow data and snow water equivalent data. We only consider gages that are not affected by major upstream reservoirs. Where data is available, we also explore climate and storage predictors' impacts on high stream temperatures directly. We find that some watersheds are more sensitive to snowpack, while in others groundwater storage provides more of a buffer (which could also make the latter more sensitive to multi-year drought). More southern watersheds show greater climatic sensitivity than those in the Pacific Northwest. These results provide insight into what areas will likely experience lower flows and higher water temperatures in response to decreasing snowpack and longer drought periods.