Simulated effects of potential climate change on the surface-water resources in the Upper Midwest are compared to the region's historical variability in climate and streamflow, and implications of both climate change and variability on water supply are discussed. Hydrologic models developed for watersheds in Illinois were used to evaluate the response in simulated streamflow to various climate scenarios. The climate scenarios used in the hydrologic simulation are based on outputs from global climate models, and represent a range of potential future climatic conditions. The driest climate scenarios result in a considerable reduction in the simulated flows. The wetter and more moderate climate scenarios generally cause relatively small amounts of change in simulated streamflow amounts, with most estimated streamflow frequency values falling within 15 percent of the simulated record using the 1950-2000 historical climate data. For 5 of the 6 climate scenarios examined in this study, the potential change in average flow is less than 25%. In contrast, there has been considerable historical variability in climatic and hydrologic conditions in the Upper Midwest since the mid-1800s, with variability in the 30-year average precipitation of 10-15 percent and associated changes in average streamflow in excess of 40 percent. Midwestern precipitation and streamflow, in particular, have increased substantially over the last 30 years. It is not known whether the 1971-2000 increase in precipitation and streamflow is a regional manifestation of climate change, or is instead part of the natural variability in the climate. But for now, the recent increasing trends in precipitation may give more credence to the GCM predictions that suggest moderate increases in future precipitation and streamflows. Copyright ASCE 2005.