The lower Wisconsin River flows west across the Driftless Area of southwest Wisconsin to its confluence with the Mississippi River. However, a combination of coring, geomorphic analysis, and subsurface data indicates that the lower Wisconsin River valley was incised through the Cenozoic by an eastward-flowing river. This river, which we refer to as the ancestral Wyalusing River, originated in what is now the uppermost Mississippi River basin in Minnesota, flowed east and northeast across Wisconsin, and drained into the Atlantic Ocean via the Gulf of St. Lawrence. Blockage of the St. Lawrence drainage by middle (?) Quaternary glaciations impounded water in the valley until it spilled over and permanently rerouted the entire drainage toward the Gulf of Mexico, causing a reversal of flow direction along the lower Wisconsin River valley. It has long been known (e.g., Chamberlin and Leverett, 1894; Leverett, 1934) that a similar sequence of events rerouted northward-flowing rivers to become the modern Ohio River. Shifting these combined drainage basins from the St. Lawrence to the Mississippi basin implies significant impacts on hydrology of the two major basins, provenance and geochemistry of sediments delivered to the Gulf of St. Lawrence and Gulf of Mexico, and potentially North Atlantic oceanic circulation.
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