Ground-water hydrology of prairie potholes in North Dakota

Prairie potholes (sloughs) are water-holding depressions of glacial origin in the prairies of the Northern United States and southern Canada. Water is supplied to the potholes by precipitation on the water surface, basin runoff, and seepage inflow of ground water. Depleticn of pothole water results from evapotranspiration, overflow, and seepage outflow. Since potholes generally do not overflow, seepage outflow is the principal way in which dissolved salts can be removed. Salinity of pothole water is therefore a good indication of the seepage balance. Net seepage outflow results in fresh to brackish waters that constitute ephemeral to semipermanent ponds, whereas net seepage inflow results in brackish to saline waters that constitute semipermanent to permanent ponds.

Because the water table in the glacial deposits is continuous with the water surface in prairie potholes, the watertable gradient is adjusted to the water-surface elevation of the potholes. In cross section the water table is represented by a nearly straight line which connects potholes. The configuration of the water table around a pothole determines the direction of ground-water flow with respect to the pothole. Ground water flows toward the pothole if the adjacent water table is higher than the pothole water surface and flows away from the pothole if the adjacent water table is lower than the pothole water surface.

Prairie potholes were studied in North Dakota on the Coteau du Missouri, an area of stagnation moraine. About 90 percent of the glacial drift on the Coteau du Missouri is glacial till, and the remainder is largely glacial outwash and lake sediments. Ground-water movement in glacial till is controlled by its lithology and structure. This till, being a poorly sorted, largely unstratified mixture of clay, silt, sand, and gravel, is not highly permeable, so ground water moves most readily along the joints. Because joints in glacial till are most numerous near the land surface (owing to weathering effects), the most active ground-water flow systems are shallow and localized in the vicinity of potholes. As a result, local ground-water flow systems have a noticeable effect on pothole hydrology, particularly the salinity