We used 20th century simulations by nine global climate models (GCMs) to provide input for a streamflow model to simulate baseline hydrologic conditions in the Upper Mississippi River Basin (UMRB). Statistical tests revealed that streamflow data produced by members of the GCM multi-model ensemble were serially uncorrelated at all lags and formed unimodal distributions and that GCM multi-model results may be used to assess annual streamflow in the UMRB. Although all low-resolution GCMs produced large differences from observations of streamflow and hydrological components simulated by the streamflow model, the nine-member ensemble performed quite well. Results of statistical tests indicate that, of all models used, the high-resolution GCM – the only high-resolution model tested – gives simulated streamflows much closer to observed values, despite the fact that its low-resolution sister model has no advantage over the other seven low-resolution models.