- California's water resources vary throughout the state owing to the regions varying topography, diverse climate, and the distribution of precipitation. Most of the state's precipitation falls over the northern coastal range and the western slopes of the Sierra Nevada Mountains. Winter snowpack that accumulates within these mountain basins serves as an efficient means of natural water storage. Moreover, the state's two massive water conveyance systems, the State Water Project (SWP) and the Central Valley Project (CVP), are integrally dependent upon winter snowpack accumulation, and subsequent spring snowmelt runoff. The SWP and CVP's extensive network of reservoirs, pipes, and aqueducts are engineered to collect and transport water from the snowcapped Sierra Nevada Mountains where it is plentiful, to farmland and urban communities where it is scarce but in greatest demand. However, increased warming within these mountain basins is causing a declined winter snowpack, altering the fraction of precipitation occurring as snow, and changing the timing of snowmelt derived streamflow. The loss of this immense amount of naturally occurring stored water, and its earlier arrival at the downstream reservoirs, has profound implications on the state's existing water management infrastructure. This work attempts to address these water management challenges that lie in the foreseeable future. Using a binary based deterministic approach, and a climatologically record of temperature and precipitation, "at-risk" snow dominated regions were identified throughout the Feather River Basin, and nested basins of the San Joaquin Watershed. These "at-risk" regions represent locations that would be the first to transition from a snow dominated, to a rain dominated precipitation regime under projected future warming scenarios. Future warming projections ranging from 1°C to 4°C were analyzed relative to the 1971-2000 base period. Results show that if warming trends considered by the IPCC 2007 report to be highly likely continue, nearly all snow dominated regions existing between 1500 and 2100 m in the San Joaquin Watershed would become rainfall dominated. Within the Feather River Basin, in the Sacramento Watershed, implications are even more alarming. A 3°C warming in February would result in approximately 87% of the regions previously snow covered area (SCA) becoming rainfall dominated; only 12% of the basin would remain snow covered. The decline of winter snowpack within all six study basins is closely correlated with elevation and average winter temperatures. Lower elevation, snow dominated regions near the rain to snow transition zone are highly sensitive to warmer temperatures relative to higher elevation, colder snow dominated regions. Furthermore, warming during high precipitation months, from December to February, would yield the largest reductions in loss of Snow Water Equivalent (or SWE). The loss of this immense amount of naturally occurring stored water, and its earlier arrival at the downstream reservoirs poses challenges and opportunities for California's water managers. For reservoir managers, adapting to a rapidly changing climate would require updating rigid flood control rule curves that were established based on hydrological trends during the first half of the twentieth century. Developing greater flexibility into flood-control rule curves could allow reservoir managers to store more water in the winter, thereby mitigating the consequences of snow loss from natural stored water sources. Faced with an expanding population and increased strains on water resources availability, sustaining future water demands hinges on developing adaptive water management strategies. By understanding basin and, at a finer scale, elevation specific vulnerability to snow loss due to warming, water managers can begin to guide effectual adaptation strategies.