LATE CENOZOIC EVOLUTION OF THE UPPER MISSISSIPPI RIVER, STREAM PIRACY, AND IMPLICATIONS FOR NORTH AMERICAN MID-CONTINENT DRAINAGE SYSTEMS (Invited Presentation)

Remnant segments of a strath terrace in the lower Wisconsin River valley along its course across the unglaciated Driftless Area of southwestern Wisconsin provide a key for understanding the late Cenozoic evolution of the upper Mississippi River basin. Coring through unconsolidated sediment to the buried strath surface, combined with precise ground surface elevation information derived from high-resolution LiDAR, demonstrates that the strath surface dips to the east—in the opposite direction of all late Quaternary alluvial surfaces in the valley.

This indicates that the lower Wisconsin River valley was incised to the level of this strath by an eastward flowing river. Numerous features of regional geomorphology support this interpretation: (1) the lower Wisconsin River valley contains numerous ‘barbed tributaries’; (2) the lower Wisconsin River valley narrows in the downstream direction; (3) the curve of the valley wall at the confluence with the Mississippi River is inconsistent with that of two rivers joining in an alluvial system; and (4) the Mississippi River valley narrows markedly immediately downstream from the confluence with the Wisconsin River. Evaluation of over 115,000 logs of water wells in east-central Wisconsin confirms the presence of a buried bedrock valley with the proper elevation and slope to be the continuation of this ancestral ‘Wyalusing River’ behind the MIS 2 glacial margin.

We propose that this eastward-flowing river drained to the Gulf of St. Lawrence; it was pirated away and added to the upper Mississippi drainage system as a result of early or middle Quaternary glaciation. Ice that blocked the lower reaches of the St. Lawrence River would have back up a lake until it spilled over at the lowest drainage divide, causing reorganization of the drainage system. This mechanism has been recognized for over a century as the causative agent for pirating numerous northward-flowing streams and reorganizing them into the modern Ohio River.