RECONSTRUCTION OF PAST LAKE LEVELS AND THEIR RELATION TO GROUNDWATER HYDROLOGY IN THE PARKERS PRAIRIE SANDPLAIN, WEST-CENTRAL MINNESOTA

The Parkers Prairie sandplain lies along the prairie-forest border in west-central Minnesota, an area with ample palynological evidence of significant environmental change during the late Pleistocene and Holocene. Past lake levels provide independent evidence of environmental change over the same temporal and spatial scales as pollen analysis. We used the method established by Digerfeldt (1986, 1988) to determine the lowest past levels of three closed-basin lakes in the sandplain. The method uses a combination of evidence from macrofossils, coarse minerogenic matter, and organic matter from a transect of cores taken in the near-shore region to reconstruct past lake levels lower than present. Carbonate content, magnetic susceptibility, and pollen content in the cores provided supplementary evidence and aided cross-correlation among the cores. Each of the three lakes had its lowest level between about 7200 and 6700 yr B.P.; lake levels ranged from 2.8 m to 6.2 m lower than modern levels. The greatest lake-level lowering occurred for the lake farthest from the river that drains the sandplain; the least lake-level lowering occurred for the lakes close to the river. This spatial pattern of lake-level lowering implies that a lowering of the regional water table caused by a reduction in groundwater recharge was the ultimate cause of the lower lake levels. The results demonstrate that surface-water hydrology is not the sole determinant of lake level, and that there are possible dangers in paleoclimatic interpretations of past lake levels based on the assumption of hydrologic isolation of basins.