Most recent climate change impact studies are using Coupled Model Intercomparison Project Phase 5 (CMIP5) projections to replace older generation CMIP3 projections. Here we evaluate whether differences between projections based on comparable high emission pathways of a seven-member general circulation model CMIP3 versus CMIP5 ensemble change our understanding of the expected hydrologic impacts. This work focuses on the important snowmelt-dominated mountain runoff-generating regions of the western United States (WUS; Upper Colorado River Basin (UCRB), Columbia River Basin (CRB), and Sierra Nevada (SN) Basins). Significant declines in snowmelt, and shifts in streamflow timing owing to warmer, wetter CMIP5 projections match or exceed those based on CMIP3 throughout the WUS. CMIP3- and CMIP5-based projections, while generally in agreement about hydroclimatic changes, differ in some important aspects for key regions. The most important is the UCRB, where CMIP5-based projections suggest increases in future streamflows. Comparable hydrologic projections result from similar underlying climate signals in CMIP3 and CMIP5 output for the CRB and SN, suggesting that previous work completed in these basins based on CMIP3 projections is likely still useful. However, UCRB hydrologic projections based on CMIP5 output suggest that a re-evaluation of future impacts on water resources is warranted.