The U.S. Geological Survey Central Midwest Water Science Center completed a report (Over and others, 2023) documenting methods, results, and applications of an updated flood-frequency study for the State of Illinois. The study developed regional regression equations that relate the peak-flow quantiles and the basin characteristics of selected streamgages in Illinois, Indiana, and Wisconsin, based on data through water year 2017 (a water year is the period from October 1 to September 30 and is designated by the year in which it ends; for example, water year 2017 was from October 1, 2016, to September 30, 2017). The data provided through this data release are those digital datasets of basin characteristics that have been collected, tested, and ultimately selected for use in regional regression equations. These datasets consist of raster grid files for slope (slope100.zip), calculated from a published digital elevation model (DEM) (Schafer and Sharpe, 2023), soil slope (stats_slope100.zip) subsetted and resampled from Wolock (1997), a soil texture permeability index (texp_indx_rnd.zip) computed using data from Wolock (1997), land cover (nlcd16_22_23_24.zip) adapted from Yang and others (2018), a basin soil wetness measure (drclass1a.zip) computed from U.S. Department of Agriculture (USDA) (2013), and an urbanization fraction (urbthe2010.zip) computed from Theobald 2010 data (Theobald, 2005). Some basin characteristics are not included in this data release as they are easily derived from StreamStats basin delineations themselves, such as basin drainage area. The U.S. Geological Survey (USGS), in cooperation with the Illinois Center for Transportation (ICT) and the Illinois Department of Transportation (IDOT), prepared these digital datasets of basin characteristics for use in the Illinois StreamStats application (https://streamstats.usgs.gov/ss/). Two additional shapefiles are provided: a shapefile of the streamgages (IL_StreamStats_Gages.zip) and a shapefile of the associated delineated streamgage drainage basins (IL_StreamStats_DrainageBasins.zip) used in analysis (Over and others, 2023).