Study Region- Upper Rio Grande, Colorado and New Mexico, USA: Climate change is predicted to further limit the water availability of the arid southwestern U.S. In this study, the Snowmelt Runoff Model is used to evaluate impacts of increased temperature and altered precipitation on snow covered area, streamflow timing and seasonal and total volume in the Upper Rio Grande.  Simulations investigate a fairly hot and dry future condition at the end of the 21st century using a regionally recommended general circulation model downscaled to existing climate stations. Twenty-four subbasins of the Upper Rio Grande containing appreciable snowmelt and a long-term gauging station are simulated.   New Hydrological Insights for the Region: Total basin snow covered area decreased by 55% due to future temperatures. SRM simulation shows that total annual 2099 volume for all basins was between 13% and 33% lower than 1999 total annual volume. Among the 24 basins there is considerable range in decrease in snow covered area (6-87%), total volume reduction (4-34%) and runoff timing shift (0-60 days early) indicating that climate change impacts are best evaluated at the sub-basin scale. Daily hydrographs show higher streamflow in March and April, but less from mid-May until the end of the water year.  The large decrease in volume in May, June and July will compound water management challenges in the Upper Rio Grande.