THE INTERACTION OF GROUND-WATER WITH PRAIRIE POTHOLE WETLANDS IN THE COTTONWOOD LAKE AREA, EAST-CENTRAL NORTH-DAKOTA, 1979-1990

The interaction of ground water with prairie wetlands in the Cottonwoo d Lake area has been the focus of research by the U.S. Geological Surv ey and the U.S. Fish and Wildlife Service since 1977. During this time , climatic conditions at the site ranged from near the driest to near the wettest of he century. Water levels in wetlands and in water-table wells throughout the study area responded to these changing climate c onditions in a variety of ways. The topographically highest wetlands r echarged ground water whenever they received water from precipitation. The wetland of principal interest, Wetland P1, which is at an interme diate altitude, received ground-water discharge much of the time, but it also had transpiration-induced seepage from it along parts of its p erimeter during all bur the wettest year. The large;fluctuations of th e water table in response to recharge and transpiration reflect the ea se with which water moves vertically through the fractured till. Later al movement of ground water is much slower; pore-water velocities are generally less than 3 m yr(-1). The water supply to the wetlands is la rgely from precipitation during fall, winter, and spring. During these periods, precipitation either falls directly on the wetland, or preci pitation that falls on the upland runs over frozen soils or saturated soils into the wetland. The average ratio of stage rise to total overw inter precipitation was 2.59 for the 12-year study period. After plant s leaf out, precipitation generally results in much lower rises of the wetland water level. The average ratio of stage rise to over-summer p recipitation was less than 1.0.