CHARACTERIZATION OF THE HYDROLOGIC AND LANDSCAPE FACTORS CONTROLLING CALCAREOUS FEN OCCURRENCE ALONG GLACIAL LAKE AGASSIZ BEACH RIDGES, NORTHWESTERN MINNESOTA

Calcareous fens, peat-accumulating wetlands that are fed by calcium-rich groundwater and support several threatened species of calcium-loving plants, are found throughout Minnesota. Forty-six of the state’s nearly 300 identified calcareous fens are located in the Glacial Lake Agassiz beach ridge complex of northwestern Minnesota. Each is located adjacent to and immediately downslope of large sand/gravel beach ridges, where peat aprons have accumulated on the downslope seepage face. This setting is identical to that of previously studied calcareous fens at the base of valley walls along the more southerly Minnesota River. The beach ridge calcareous fens have not been as well-studied as those in the river valley setting. This investigation characterizes the hydrology and landscape setting of the beach ridge calcareous fens by detailed study of three calcareous fens and adjacent non-calcareous seepage wetlands distributed along the beach ridges. Three hypotheses about the hydrologic source for these fens are explored: whether they are primarily fed by groundwater from the surficial beach ridge aquifers, confined aquifers, or a combination of the two controlled by wet-dry periods. Water levels in piezometers in the confined aquifers, surficial beach ridge aquifers, and in and below the fens are compared with rainfall hydrographs to identify hydrologic connections. Water chemistry and stable isotopes are also used to characterize site hydrology.