Urban areas in Perris Valley, California, have more than tripled during the last 20 years, resulting in increased storm-runoff volumes and peak discharges. To quantify the effects of increased urbanization, rainfall-runoff models of the basin were developed to simulate runoff for 1970-75 and 1990-93 conditions. Hourly rainfall data for 1949-93 were used with the rainfall-runoff models to simulate a long-term record of storm runoff. The hydrologic effects of increased urbanization from 1970-75 to 1990-93 conditions were analyzed by comparing the frequency of annual peak discharges and runoff volumes, and a duration analysis of storm peak discharges. The maximum annual-peak discharge for the 1990-93 model simulation was 32 percent higher than the discharge for 1970-75 model simulation. However, the frequency analysis of each time series indicated the 100-year peak discharges for each study period were identical.