Geomorphology has long been engaged in characterizing the form and understanding the evolution of floodplains. This study primarily examines floodplain width in four tributaries to the Minnesota River in southern Minnesota, namely the Maple, Le Sueur, and Blue Earth Rivers and Seven Mile Creek. The tributary systems are relatively young, having formed after retreat of the Wisconsinan ice sheet, and are rapidly evolving in response to a base level fall of nearly 70 m–13,400 YBP. As a result, the lower reaches of all four river channels are aggressively incising. This study applies a new, freely available, open-code plug-in for ArcGIS to measure floodplain width as a function of elevation above the geomorphic top of bank. Systematic changes are observed in all four systems...