The Sierra Nevada, the primary water source for California’s State Water Project and Central Valley Project—multi-purpose water storage and delivery systems that support agriculture, households, businesses, and the environment—is threatened by anthropogenic climate change. Using a high spatiotemporal resolution simulation from the variable-resolution Community Earth System Model-2 under the high-emissions (SSP5-8.5) scenario, we assess both historical and future hydroclimate changes across the three major basins encompassing the western slopes of the Sierra Nevada and California's Central Valley. This analysis explores +1.5°C, +2.0°C, and +3.0°C warming scenarios and examines their impacts on water resources. As the climate warms, California’s Sierra Nevada is projected to shift from a snow-dominated to a rain-dominated region, with 76% of precipitation expected to fall as rainfall in the Northern Sierra Nevada and 60% in the Central and Southern Sierra Nevada by +3°C. Our projections indicate a decline and earlier peak in snow water equivalent, an earlier onset of snowmelt, and significant reductions in runoff. These changes will have significant implications for California’s water resource management, impacting water storage, flood control, and the allocation of water to meet competing demands.