Linkages between groundwater and coastal wetlands of the Laurentian Great Lakes

Groundwater flow regimes adjacent to coastal wetlands of the Great Lakes are highly transient and vary among different types of coastal wetlands. Groundwater flow is controlled by (1) the physiography of the land adjacent to the wetland, (2) the relative elevations of the wetland and the lake, as they fluctuate over time, and (3) the amount of infiltration and evapotranspiration that occurs at the land and wetland. Groundwater from the mainland adjacent to a wetland will flow towards and discharge into the wetland throughout the year. In a spit that partially protects a wetland from a lake, the source of groundwater is precipitation and snowmelt; not water from the lake or wetland. Here, groundwater continually flows from either side of a central groundwater divide towards the lake or wetland, with the elevation of the lake only affecting the rate of groundwater drainage. Because barrier bars completely separate a lake from a wetland, the elevation of the lake and wetland are different. When the barrier bar is narrow, the resultant hydraulic gradient across the barrier bar causes groundwater flow to oscillate between flowing towards the lake during the fall and winter and towards the wetland during the spring and summer. But as the width of the barrier bar increases, the impact of the lake and wetland diminish relative to the amount of precipitation and snowmelt infiltrating into the barrier bar. Thus, the groundwater flow regime is characterized by a central groundwater divide with groundwater on either side continuously flowing towards the lake and wetland throughout the year. Intradunal wetlands are actually several small wetlands within a series of relic beach ridges and parabolic dunes. Groundwater flow regimes here are highly variable and transient with flow adjacent to different wetlands, and at different times of the year, exhibiting continuous flow to a wetland, oscillating direction of flow, and lateral migration of the groundwater divide. However, these groundwater flow patterns are caused by precipitation and evapotranspiration within the wetland complex and not by fluctuating lake levels.