This report describes the area in north-central Nebraska and south-central South Dakota drained by Ponca Creek and by the Niobrara River below Valentine, Nebr. The Niobrara River and Ponca Creek are neighboring eastward flowing tributaries of the Missouri River. The Dakota sandstone of Cretaceous age is the oldest formation tapped by wells; the water it yields to wells in small to moderate quantities is rather highly mineralized and very hard; it is unsuitable for irrigation and most domestic uses. Overlying the Dakota, in ascending order, are the following formations of Cretaceous age: the Graneros shale, Greenhorn limestone, Carlile shale, Niobrara formation, and Pierre shale. None of these is a source of water supply. The Niobrara is the oldest formation exposed, cropping out in only the deeper valleys at the eastern end of the area. The Pierre shale, which is exposed much more extensively, crops out in the deeper valleys throughout nearly all the area.

Except where the Niobrara River, its major tributaries, and Ponca Creek have cut their valleys into them, the Cretaceous rocks are overlain by semiconsolidated rocks of Tertiary age. Two Tertiary formations, the Brule and the Ogallala, are present in the area. The Brule formation underlies all the western part of the area and is exposed in the valleys of both the Niobrara and Keya Paha Rivers. The Ogallala formation, which overlaps the Brule, forms the upland on both sides of the river and is exposed in many places. The Brule is not a source of water supply, whereas the Ogallala yields small to moderately large quantities of water to many wells on the upland. The water in the Ogallala is of the calcium bicarbonate type and is moderately mineralized and hard.

Unconsolidated deposits of Quaternary age mantle the Tertiary rocks throughout nearly all the upland area south of the Niobrara River and in parts of the upland area north of the river. They also floor the Niobrara River valley. Where saturated, these sediments, which consist of stream-deposited sand and gravel and wind-deposited sand, yield small to large amounts of water to wells. The water in the Quaternary deposits is of the calcium bicarbonate type but is less mineralized and softer than that in the Ogallala.

The only significant source of recharge to the Dakota sandstone in the report area is underflow from the west. Except for waiter yielded to wells tapping the Dakota, water in the formation is discharged from the area by underflow to the east. In the upland part of the area, the Ogallala formation and the overlying deposits of Quaternary age constitute a single aquifer, water moving from one Into the other without apparent hindrance. This aquifer is recharged principally by the direct infiltration of precipitation but in part also by underflow from the west and south and by seepage from intermittent streams and ponds. Water is discharged from the upland aquifer by outflow through springs or seepage into streams, through the process of evapotranspiration, and by wells when they are pumped. Ground water leaves the report area by underflow where the Quaternary deposits in the valleys of the Niobrara River and Ponca Creek merge with the Quaternary deposits in the Missouri River valley.

In places where the Niobrara formation, the Pierre shale, or the Brule formation is at the surface or is mantled by thin deposits of the Ogallala or thin deposits of Quaternary age, only meager amounts of ground water can be obtained unless wells are deep enough to tap the Dakota sandstone. Elsewhere the Ogallala formation and the deposits of Quaternary age generally yield ample water for domestic and stock supplies, and in some places, notably in the vicinity of Ainsworth, they yield enough water for irrigation. Additional large supplies of ground water could be obtained on the upland in the southwestern and west-central parts of the area.

 The report contains an annotated bibliography of previous publications on the geology and ground-water resources of the area, brief descriptions of the Cretaceous, Tertiary, and Quaternary rocks, a map showing the contour of the water table, logs of test holes and wells not published elsewhere, results of analyses of ground- and surface-water samples, and records of all wells of large discharge and representative wells of small discharge.