<p>Streamflow in the Upper Colorado River Basin, USA has decreased proportionally more than precipitation in the recent multi-decadal drought. The causes are debated. Understanding how precipitation, and seasonal temperature, vegetation, and evapotranspiration dynamics affect streamflow is essential. Here we use causal inference with historical data to identify surface runoff efficiency drivers. Runoff efficiency increases in years with higher precipitation and snow accumulation accompanied by cooler spring temperatures and delayed vegetation phenology, which generally attenuates biomass accumulation. Conversely, runoff efficiency decreases in years with lower precipitation and snow accumulation, or warmer springs, when vegetation activity and productivity are accelerated or amplified. Summer temperature, often identified as a driver of higher evaporation and aridity, does not emerge as statistically significant. Years with extreme phases of winter-spring precipitation have distinct atmospheric circulation patterns and associated sea surface temperatures, indicating the influence of larger-scale climate drivers on the Basin’s precipitation and runoff efficiency dynamics.</p>