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Summer streamflow predictions are critical for managing water resources; however, warming‐induced shifts from snow to rain regimes impact low‐flow predictive models. Additionally, reductions in snowpack drive earlier peak flows and lower summer flows across the western United States increasing reliance on groundwater for maintaining summer streamflow. However, it remains poorly understood how groundwater contributions vary interannually. We quantify recession limb groundwater (RLGW), defined as the proportional groundwater contribution to the stream during the period between peak stream flow and low flow, to predict summer low flows across three diverse western US watersheds. We ask (a) how do snow and rain dynamics influence interannual variations of RLGW contributions and summer low flows?; (b) which watershed attributes impact the effectiveness of RLGW as a predictor of summer low flows? Linear models reveal that RLGW is a strong predictor of low flows across all sites and drastically improves low‐flow prediction compared to snow metrics at a rain‐dominated site. Results suggest that strength of RLGW control on summer low flows may be mediated by subsurface storage. Subsurface storage can be divided into dynamic (i.e., variability saturated) and deep (i.e., permanently saturated) components, and we hypothesize that interannual variability in dynamic storage contribution to streamflow drives RLGW variability. In systems with a higher proportion of dynamic storage, RLGW is a better predictor of summer low flow because the stream is more responsive to dynamic storage contributions compared to deep‐storage‐dominated systems. Overall, including RLGW improved low‐flow prediction across diverse watersheds. Plain Language Summary: Water managers across the western United States depend on accurate streamflow prediction models for water planning and allocation during summer months. Historically, these models use snow metrics to predict summer flows, but increasing temperatures across the western US are decreasing snow input and accumulation and increasing early snow melt rates leading to changes in streamflow generation mechanisms. Here, we seek to understand how stream flows will respond under warmer climates in three watersheds with distinct climate and underlying bedrock in the western US. We expanded upon commonly used low‐flow model snow metrics to include snow and streamflow metrics from the previous year as well as groundwater dynamics during the annual recession curve. We found that including variables outside of commonly used snow parameters in the models produced more accurate predictions of summer low flows. We provide a framework for future analysis of streamflow response to warming across the western US. Key Points: Recession limb groundwater proportion is a strong predictor of summer low flowsRecession limb dynamics are especially important for improving low‐flow prediction at rain‐dominated sitesRecession limb groundwater may be a better predictor of low flow at sites where dynamic storage exceeds deep storage [ABSTRACT FROM AUTHOR]

High elevation mountains supply three fourth of the surface water in western United States.Although it is well ducumented that climate change can shift precipitation from snow towardrainfall and decrease snowmelt in these areas, its impact on water quality is much lessinvestigated and poorly understood. Here we examine the dynamics of water qualitychange using 4 decades of data on climate, water and water chemistry from CoalCreek, Colorado, a high elevation headwater catchment in central Rocky Mountainwith 100 years of mining in the past. The data show that although the annual meantemperature in the U.S. has increased by < 1˚ C since 1980s, it has increased by6.5˚ C in Coal Creek. Snow fraction has deceased from about 75% in 1980 to50% in 2018, paralleled by significant decrease in peak flow during snowmelt.Accompanying these are the changes in solute concentrations and their response todischarge variations. Although geogenic species (e.g., Ca, Mg, Si) have remainedrelatively similar, changes with heavy metals deriving out of mining tails are muchmore erratic. Some species (e.g., Cr, Co, Fe) see substantial increase in inter annualconcentrations whereas others (e.g., As, Cu) remain relatively constant. The mostdramatic alterations are observed for dissolved organic carbon (DOC): it remainedwithin a narrow range of 1-5 mg/L until in 2017 and 2018, when its concentrationsescalate to as high as 15-20 mg/L. This dramatic increase may be an early sign in ahigh-elevation mountain that is most vulnerable to climate change, and may indicate atipping point of climate change, above which the rates of soil carbon decompositionincrease substantially. The contrasting behavior of different solutes accentuate thecomplex nature of biogeochemistry and water quality response to climate change. [ABSTRACT FROM AUTHOR]