Hydrology, Water Chemistry and Ecological Relations in the Raised Mound of Cowles Bog

Abstract

The Cowles Bog National Natural Landmark and the wetlands between the dunes near the south shore of Lake Michigan, in Indiana, contain plant species that are typical of circum-neutral fens. The distribution of eight, rather sharply delineated, vegetation types correlates most strongly with water level variations resulting from the presence of a 4.1-ha convex peat mound. A network of shallow ground-water wells installed in the wetland has identified an upwelling of water under artesian pressure at sites underlying the mound. The well-buffered water, containing high concentrations of inorganic solutes, is derived from an aquifer that is recharged on an upland moraine and is confined beneath a clay till sheet. A breach in this clay layer beneath the mound allows water to flow upward and radially outward as the hydraulic head is dissipated in the overlying marl and peat. The marl and organic lake sediments in the wetland were formed during the Nipissing level of ancestral Lake Michigan (4000-6000 years ago) when the wetland basin was probably a small bay of the lake. The peat mound developed when the lake level fell from the Algoma through to modern times. This increased the difference in hydraulic head and increased spring flows, which in turn induced peat formation.