How did early Great Lakes drain into the Mississippi River system?

During the retreat of the Wisconsin Glacial Stage approximately 14,000 years ago, the early Great Lakes were formed in basins scoured by massive ice sheets. Because the ice front blocked natural drainage outlets to the east, these early lakes were forced to drain southward into the Mississippi River system.12

Specific drainage patterns included:

• Lake Chicago and Lake Maumee: These early bodies of water, located in the basins of present-day Lake Michigan and Lake Erie, drained southward through the Illinois and Wabash river systems, respectively.1
• Lake Duluth: During the existence of Lake Algonquin, the early version of Lake Superior known as Lake Duluth also discharged its waters into the Mississippi.1
• General Drainage: More broadly, while the St. Lawrence outlet remained obstructed by ice, the early lakes utilized various southward paths, including the Mississippi-Ohio, Susquehanna, and Mohawk-Hudson systems.2

As the ice continued to retreat, these southward outlets were eventually abandoned in favor of eastward drainage toward the Atlantic Ocean.12