The High Plains aquifer in Kansas is part of a regional aquifer system that extends into Colorado, Nebraska, New Mexico, Oklahoma, South Dakota, Texas, and Wyoming. The aquifer in Kansas underlies an area of 31,000 square miles in the western and south-central part of the State. The aquifer is a hydraulically connected assemblage of unconsolidated water-bearing deposits. In western Kansas, the High Plains aquifer consists principally of the Ogallala Formation of the late Tertiary age and the overlying deposits of Quaternary age. In south-central Kansas, the aquifer consists of unconsolidated deposits principally of Quaternary age. Valley alluvium of Quaternary age also is included in the deposits in both areas. The High Plains aquifer is delimited on the east by outcrops of Permian or Cretaceous rocks and by unsaturated deposits of Quaternary age. The altitude and configuration of the water table during 1960 are shown for the High Plains aquifer in Kansas. The water table sloped generally from west to east at an average rate of 10 feet per mile. The altitude of the water table ranged from about 3,900 feet in the southwest corner of Sherman County, northwestern Kansas, to about 1,350 feet in northern Sedgwick County, southcentral Kansas. Groundwater moves perpendicular to the water table contours from higher altitudes in the western part of the High Plains to lower altitudes in the east. Contours that cross stream valleys without flexure may indicate that the water table was below the streambed. Upstream flexure of water table contours along streams indicates that groundwater flowed toward, and discharged into, streams. This is most evident along the South Fork Republican River in Cheyenne County, Northwestern Kansas, and the South Fork Ninnescah River in Pratt and Kingman Counties, south-central Kansas. (USGS)