The possible effects of climate change on the combined Central Valley Project-California State Water Project (CVP/SWP) were evaluated using a three-stage approach. In the first stage, runoff from four headwater 'study catchments' was simulated using rainfall/snowmelt-runoff models, with climatic input taken from CO2 doubling scenarios from three general circulation models (GCMs). In the second stage, long-term inflows to the CVP/SWP reservoir system were simulated, conditioned on the study catchment flows, using a stochastic disaggregation model. In the third stage, a system simulation model was used to evaluate the performance of the reservoir system. For all of the alternative climate scenarios, runoff would be shifted from the spring to the winter. Significantly lower water deliveries from the SWP would occur under the CO2 doubling scenarios. The reduced deliveries would occur because some of the increased winter runoff would be spilled from the reservoirs instead of being stored in the snowpact, even though the mean annual runoff increased slightly under some climate scenarios.