Floodplain width adjustments in response to rapid base level fall and knickpoint migration

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. Above the steep, incising knickzone reaches of each of the tributaries, relatively wide, and mostly unconfined, floodplains are observed. Within the knickzone, floodplains progressively narrow to between 25–60% of their width above the knickzone, despite the fact that sediment loads increase significantly in the downstream direction. A power law relationship is consistently observed between floodplain width and the ratio of local channel slope to contributing drainage area to about the negative one-third power in all four rivers. This same power law relationship is observed for two other rivers that differ significantly from our study systems in climatic and tectonic environment as well as base level history.