129-51 Lakes, Rain, and Groundwater, Oh My! - Contributing Sources to the Hydrogeology of an Interdunal Wetland

Abstract:

We have studied the ecohydrology of a 1.25 ha interdunal wetland in a Lake Michigan coastal dune complex in Allegan County, Michigan, USA for 10 years. The wetland area is bounded by the Kalamazoo River to the north, Oxbow Lagoon, an offshoot of the Kalamazoo River to the east and south, and by Lake Michigan-Huron (MH) on the west. A sand layer ~22m thick underlies the wetlands between Lake MH and the river. The U.S. Drought monitor indicated abnormally dry conditions in spring 2025 transitioning to moderate drought, then returning to abnormally dry conditions until January 2026. Lake MH water levels were below the lakewide historic average from January 2025 until March 2026 when they rose above this average. In April 2025 we installed two Hobo data loggers in monitoring well 2 in the center of the wetlands. In June 2026 we installed two additional dataloggers in wells along a ~185m transect from the wetlands’ center (well 2) to the wetlands’ northeast corner (well 3) over a dune ridge to well 10 ~226 m west of Oxbow. Wells 2 and 3 are in areas dominantly vegetated with Cladium mariscoides while well 10 is vegetated with Ammophila breviligulata. Wells 2 and 3 show peakiness during rain events wherein water levels rise quickly due to temporary ponding during the event, then fall steeply afterwards. On days without rain, a strong diurnal pattern of falling groundwater levels during the day and rising levels during the night, indicating daytime evapotranspiration losses and nightly groundwater replenishment, is noted. Well 2 water levels also show a rise in elevation concurrent with that noted in the Lake MH 2025–2026 lake level curve, demonstrating the hydrologic connection between the aquifer underlying the wetlands and Lake MH. Well 10 water levels do not exhibit this same trend as they remain elevated for several days after the rain event before tapering off, mirroring the Kalamazoo River discharge curve. This suggests a strong river influence to this well’s hydrology as well as a hydrologic divide between wells 3 and 10. River water levels then show a diurnal fluctuation which is also noted in well 10’s water levels. Hence, our work indicates that the wetland’s hydrology is influenced by the interaction of multiple factors.

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