Flood peak attenuation caused by storage of flood water on overbank surfaces effectively reduces the magnitude of peak discharges in some, but not all watersheds. Several geomorphic factors that affect the storage and conveyance of flood water were investigated to assess their quantitative influence on downstream peak discharges. The MIKE11 rainfall-runoff and hydrodynamic models were calibrated for the Grant River watershed, southwestern Wisconsin. Alternative geomorphic conditions were modeled and compared to the original case. Results indicate that channel-floodplain-terrace morphology, valley width, stream slope, and hydraulic roughness each influence peak discharges, especially for moderate magnitude (5- to 50-year recurrence interval) floods. Peak discharges varied by as much as 49% between simulations depending on geomorphic conditions. Watersheds that effectively attenuate produce peak discharges that are strongly correlated with total runoff. Watersheds that attenuate little produce peak discharges that exhibit greater variance due to storm intensity and duration. ¿ American Geophysical Union 1994