Lake Chicago was a prehistoric proglacial lake formed about 13,000 years ago during the retreat of the Michigan Lobe in the Wisconsin Glacial Period, and it was the direct ancestor of modern Lake Michigan. Larger than Lake Michigan is today, it drained southwest through the Des Plaines and Illinois river systems and passed through three stages marked by falling shorelines as new outlets opened while the ice withdrew. Its changing waters shaped the southern Lake Michigan region, leaving beaches, spits, dunes, ridges, and former islands that still affect roads, settlement patterns, and topography. As drainage shifted north and east, it gave way to later proglacial lakes and the modern Great Lakes landscape.