QUATERNARY REORGANIZATION OF THE MISSISSIPPI AND ST. LAWRENCE DRAINAGE BASINS

Identification of ancestral drainage systems and understanding the events that lead to drainage reorganization in the North American mid-continent have been topics of research since the 19^thcentury. For example, the role of Quaternary glaciations in creating the course of the modern Ohio River has long been debated, as glaciations drove stream piracy events which converted north-flowing tributaries—the ancestral Teays and Pittsburgh Rivers—into the southwest-flowing Ohio River.

Deposits and geomorphic features along the lower Wisconsin River indicate that the upper Mississippi and Wisconsin Rivers have also experienced stream piracy and radical rerouting. Coring into the Bridgeport strath terrace along the lower Wisconsin River reveals that that bedrock surface dips to the east, indicating the valley was cut by an eastward-flowing river. This suggests that the upper Mississippi system (upstream of the confluence with the Mississippi and Wisconsin Rivers), referred to here as the Wyalusing River, evolved through the late Cenozoic as part of the St. Lawrence drainage system. Stream piracy immediately south of the modern confluence rerouted these streams, converting them to the headwaters of the greater Mississippi drainage.

Rerouting of the upper Mississippi and Ohio Rivers into entirely different drainage basins implies fundamental changes to the hydrology of the St. Lawrence and Mississippi Rivers. While it is unclear if the Teays River ever flowed into the St. Lawrence drainage or developed as a tributary to the buried Mahomet valley in Illinois, both the ancestral Pittsburgh and Wyalusing Rivers evolved as headwaters of the St. Lawrence basin before diversion to the Mississippi basin. The areas formerly drained by the Pittsburgh and Wyalusing Rivers comprise ~9% of the modern Mississippi River basin (~14% when including the Teays basin), and modern discharge from those areas represent ~12% of the mean annual discharge of the Mississippi River (~22% when including the Teays basin). The transfer of this drainage area and discharge to the Mississippi basin is mirrored by an equivalent loss from the St. Lawrence system during the Quaternary. The rerouting of this volume of freshwater from the Gulf of St. Lawrence to the Gulf of Mexico could have had significant impacts on oceanic circulation and global climate.