GLACIAL TUNNEL VALLEYS: POTENTIAL CONDUITS AND CONTROLS ON GROUND-WATER FLOW TO THE GREAT LAKES

The ground-water flux to each of the Great Lakes and to their overall water balance is not well quantified, though various studies have indicated local, marked significance of ground-water discharge to the lakes and their ecology. The geologic framework and topography of the Great Lakes drainage basin are dominating factors that control ground-water flux rates and influence water quality in near-shore areas of the Great Lakes. The complex, heterogenous geology of the basin consists of unconsolidated Quaternary glacial deposits of highly variable thickness and character and a minor volume of nonglacial sediments over a bedrock framework that includes most major rock types, of Archean to Jurassic age. Potential conduits for significant ground-water flux include karst in Paleozoic limestones and evaporites, and glacial tunnel valleys. The latter, including glacially modified and buried bedrock valleys as well as subglacial channel networks, formed at the base of continental ice sheets to accommodate large discharges of meltwater from the ice sheets that advanced across the Great Lakes region to form its salient features. Some of the most prominent tunnel valley systems in Michigan are contiguous with the Great Lakes, and others overlie karst-prone bedrock that may provide ground-water conduits to the Great Lakes.