The retreating glaciers left behind a large kettle - Lake Chicago
, called Lake Michigan today.
Over the course of thousands of years, water levels subsided as new outlets opened.
For example, the Chippewa stage saw a big drop in lake level as a new outlet opened in Ontario.
Temporary rising occurred in the Early Nipissing as the ice continued to melt.
The retreat of the heavy ice sheet also caused the earth's crust to rise; this "geostatic rebound"
continues to elevate the whole area, and Lake Michigan continues to shrink through its outlets.