One of the challenges facing California’s water planners is how to assess the possible effects of climate change, such as changes in rainfall and snowfall patterns, snowpack, runoff volume and timing, sea levels, and urban and agricultural water demands. This paper presents several advances in using future climate projection information in water resources planning, such as an improved understanding of how well selected climate models represent historical climate conditions and refined methodologies for representing streamflows, outdoor urban and agricultural water demands, and sea level rise in planning tools. Twelve climate projections were used to assess the future reliability of California’s main water supply projects. Midcentury and end‐of‐the‐century impacts were estimated for Sacramento‐San Joaquin Delta exports, reservoir carryover storage, groundwater pumping, power supply, and the Delta salinity standard known as X2. The vulnerability of the system to operational interruption was also examined. A sensitivity analysis was also conducted to examine the effects of air temperature on runoff in the Upper Feather River basin, the main inflow source to Lake Oroville. The range of impacts presented in this paper indicates a need to explore adaptation measures to improve the reliability of future water supplies in California.