In 1988, the US Geological Survey began a study of the effects of potential climate change on the water resources of the Gunnison River basin. The Gunnison River, in southwestern Colorado, is an important tributary of the Colorado River, contributing approximately 40% of the flow of the Colorado River at the Colorado/Utah stateline. As part of the study, the sensitivity of annual and seasonal runoff in the East River basin, a sub-basin of the Gunnison River basin, to changes in temperature and precipitation was examined. To perform the sensitivity analyses, hypothetical climate changes were used to alter current time series of temperature and precipitation. The altered time series were then used as inputs to a hydrological model that translated these inputs into estimates of runoff. Mean annual and seasonal runoff resulting from a range of hypothetical climate changes were compared and evaluated. Results indicated that in general, changes in precipitation had a larger effect on changes in runoff than did changes in temperature. Changes in precipitation had significant effects on runoff during all seasons. Changes in temperature primarily affected the temporal distribution of runoff through the year. Changes in temperature affected the timing of snowmelt and the ratio of rain to snow, and therefore the effects of temperature were particularly significant during the spring and summer seasons. On an annual basis, increases in temperature led only to slight decreases in runoff. Results also indicated that the effects of an increase in mean annual temperature of 4°C on annual runoff could be offset by an increase in annual precipitation of between 4 and 5%, and that the magnitude of natural climatic variability was large and might mask the effects of long term climate changes.