The Upper Mississippi River (UMRB) basin is an important source of water transportation, power generation and agriculture, among many other uses. It is also sensitive to extended precipitation extremes that can substantially alter the streamflow of the entire system. Projected streamflow changes are vital for managing the basin and its economic activities. We evaluate changes in streamflow extremes using RegCM4 downscaled climates driven by MPI and GFDL GCM simulations for historical and RCP8.5 scenarios. Bias correction to the data was applied as part of the NA-CORDEX program. Output from those simulations (daily precipitation, T-min and T-max) was then fed into the Soil and Water Assessment Tool (SWAT) model to simulate streamflow for the UMRB at Grafton, IL, for contemporary (1981-2010) and future scenario (2041-2070) climates. We diagnose monthly extremes of high flow (upper 5th percentile) and low flow (lower 5th percentile) in Grafton discharge and analyze how extremely wet and dry precipitation periods promote and sustain the streamflow extremes. The analysis includes examining precursor conditions in monthly precipitation, snowmelt, evapotranspiration (ET), and potential evapotranspiration (PET) in the months before the streamflow extremes for both the contemporary/scenario periods. We focus on high flows exceeding USGS flood stages and low flows that undermine river transportation and power generation.