THE ROLE OF QUATERNARY GLACIATIONS IN REORGANIZING THE OHIO AND UPPER MISSISSIPPI RIVERS

Rivers respond to climate variations via numerous processes that operate over a range of timescales. In the eastern and midwestern United States an extreme example for response of rivers to Quaternary climate fluctuations has been the wholesale reorganization of the Ohio and upper Mississippi drainage basins.

Dating to the late 19^th Century, it has been understood that the modern Ohio River has experienced radical modifications during the Quaternary. Chamberlin and Leverett (1894) documented that the upper Ohio River (including the Allegheny and Monongahela Rivers) evolved prior to the Quaternary as a north-flowing tributary of the St. Lawrence River. Tight (1903) documented that the middle Ohio River was once similarly a northward-oriented drainage system. Both were reorganized as early to middle Quaternary glaciations blocked the lower portions of the St. Lawrence basin. This caused pro-glacial lakes to develop along the ice front; subsequent spill-over and stream piracy integrated the modern Ohio River into the greater Mississippi River basin. Now over a century later, emerging research (e.g., Carson et al., 2013) is demonstrating that the upper Mississippi River valley experienced a similar history of glacially driven reorganization that shifted drainage away from the Gulf of St. Lawrence and toward the Gulf of Mexico.

Thus, the net effect of Quaternary glaciations across the eastern and midwestern United States has been to shift the Laurentian continental drainage divide northward as the flow of water in the ancestral iterations of the Ohio and upper Mississippi Rivers has been diverted southward. The volume of runoff that has been diverted by this process away from the Gulf of St. Lawrence and toward the Gulf of Mexico is significant, as much as one quarter of the mean annual discharge of the modern Mississippi River. Because of the buoyancy forcing of deep water production in the North Atlantic, altering freshwater input to the North Atlantic via drainage rerouting could play an important role in both abrupt and long-term climate change. Current research is beginning to investigate the impact of permanently diverting that volume of freshwater away from the Gulf of St. Lawrence on North Atlantic thermohaline circulation and Quaternary climate.