Rising temperatures reduce spring snowpack in the Sierra Nevada Mountains, with serious implications for summer water supply. Snowpack losses are expected to be quite large under most scenarios, but impacts on water supply are buffered by reservoir storage and management decisions. Warmer winter storms and earlier snowmelt runoff shift the timing of streamflow, forcing reservoir managers to choose between capturing winter runoff for summer water supply or maintaining space for winter flood control. Less reliable summer streamflows could disrupt California’s water rights system by reducing the value of rights to natural streamflow and increasing the value of rights to stored water. We can reduce the potential impacts of climate change on California’s water resources by planning for changes in the amount and timing of water supply, increasing cost-effective storage capacity, and implementing effective water conservation strategies.