We used a revised version of the Snowmelt Runoff Model (SRMc21) to simulate the impact of progressively warming temperatures on snowmelt runoff in four headwaters catchments of the San Juan Mountains (Alamosa, Chama, Conejos and Del Norte) draining to the Rio Grande. MODIS snow cover was used to generate depletion curves, which are used in SRM, along with parameters adjustable to fit measured and simulated runoff. For each watershed, SRMc21 was first used to simulate daily streamflow with a high agreement between measured and computed runoff (R2=0.92 to 0.96).  SRMc21 was then used to simulate the impacts of sequentially increasing temperatures (1, 2, 3 and 4C). In each watershed total annual volume progressively declined with increasing temperature (mean decline of 5% with +1C, 8% with +2C, 10% with +3C and 13% with +4C temperature increase). For each watershed, the center of volume date shifted progressively earlier with each increase in temperature. Center of volume was one week earlier with a mean temperature increase of 1C and up to one month earlier with an increase of 4C. The ability to shift operational and institutional frameworks (such as water rights) to accommodate shifts in streamflow timing should be an area of future analysis. The simulated temperature increases are within CMIP5 multi-model mean projections for the region. By midcentury (2040-2069) the moderate scenarios (RCP 4.5) project a mean temperature increase of 2.4C whereas the high-emissions scenarios (RCP 8.5) project a mean temperature increase of 3.3C. End of century (2070-2099) moderate scenarios project an average increase of 3.0C whereas high-emissions scenarios project a 5.3C increase in mean temperature for this region.