Recent decades have seen strong declines in streamflow in many major river basins in the Western U.S., heightening concerns about the impacts of climate change on water resources in a continually developing region. While some drivers of streamflow trends and variability – namely, temperature and precipitation – have been investigated quite thoroughly, the importance of controls such as snowpack, soil moisture, and vegetation that are frequently reported as important both anecdotally and in the scientific literature remains unclear. As such, our expectations for future runoff efficiency, or the fraction of precipitation that winds up as streamflow, under continued radiative forcing are highly uncertain. Here, we assess the relative importance of rain versus snow partitioning, preseason soil moisture priming, vegetation phenology and productivity, and surface net radiation as drivers of runoff efficiency in unmanaged Western U.S. river basins using a suite of observational data products. We also evaluate these same relationships in the current generation of climate models, allowing us to assess the degree to which the primary tools we use to assess future hydroclimatic change accurately represent the underlying mechanisms. Together, this work elucidates the historical roles of several potential drivers of runoff efficiency across a wide variety of river basins and offers valuable constraints on expectations of future changes in water availability.