This digital spatial data set consists of the aquifer base elevation contours (50-foot contour interval) for part of the High Plains aquifer in the central United States. This subset of the High Plains aquifer covers the Republican River Basin in Nebraska, Kansas, and Colorado upstream from the streamflow station on the Republican River near Hardy, Nebraska, near the Kansas/Nebraska border. In Nebraska, the digitized contours extend to the South Platte, Platte, and Little Blue Rivers. In Colorado and Kansas, the digital contours extend to the edge of the High Plains aquifer. These boundaries were chosen to simplify boundary conditions for a computer simulation model being used for a hydrologic study of the Republican River Basin.