Lake Chicago

Lake Chicago was an ancient lake that existed in the southern Lake Michigan basin during the Pleistocene Epoch, approximately 14,000 years ago. As the ice sheet receded, this lake, along with others like Lake Maumee, began to form in the glacially scoured basins of what is now the Great Lakes region of North America.1

Initially, Lake Chicago drained southward into the Mississippi River via the Illinois and Wabash drainages. As the ice continued to melt and retreat, the drainage patterns shifted. Eventually, outlets to the east were established, leading to the Atlantic Ocean. The modern city of Chicago is built on the former bed of Lake Chicago, which is now the basin for Lake Michigan.23