Floodplain inundation is believed to be the dominant physical driver of an array of ecosystem patterns and processes in the Upper Mississippi River System (UMRS). Here, we present the relative elevation of a slope-detrended floodplain terrain surface and river mile location used to map surface water depths derived from gaging locations along UMRS, as described in Van Appledorn et al. (2021; doi: 10.1002/rra.3628). We excluded areas permanently wetted (aquatic areas), surfaces in agricultural production, roads, and developed areas. The data are intended for use in geospatial analyses of UMRS floodplain ecosystem patterns and processes.