Some El Ni¿o events produce unusually large precipitation amounts in Northern and Central California. We use a high-resolution global model of the atmosphere coupled to a physically-based model of surface hydrology to investigate effects of increased atmospheric CO2 and this type of El Ni¿o, both individually and in combination, on monthly river flows in California. Increased CO2 changes the seasonal timing of river flows and increases their variability. SST anomalies typical of a strong El Ni¿o SST increase monthly-mean flows. The two perturbations together result in increased mean flows and increased variability, raising the possibility of both increased flood risk and water shortages. The river flow response to this strong El Ni¿o in an increased CO2 climate is significantly different from the sum of the responses to the individual perturbations.