RIVERS AND STREAMS
Rivers seem like such permanent parts of the landscape that it's hard to imagine that they come and go just as the glaciers did. In central Illinois there is a direct connection between the rivers we see today and the glaciers that scraped across the state. If we could time-travel back before the Great Ice Age began, we could visit ancient rivers that had been flowing across central Illinois for millions of years, draining the uplands and carrying water southward to the sea. The wide Mississippi River flowed from the north, then looped east into central Illinois; the even wider Mahomet-Teays River flowed west from the eastern part of the continent and met the Mississippi in what is now Mason County.
These rivers and their tributaries had carved out valleys in the bedrock and were flowing between tall limestone bluffs. Permanent as they seemed, they would not last forever. During the ice age the glaciers covered them, and when the last ice melted the old rivers were gone. The valleys of the Mahomet-Teays and the Mississippi were filled with sand, gravel, silt, and clay. The Mahomet-Teays would never reappear but the ancestral Mississippi had not been completely obliterated. The Illinoian glacier covered much of the northern part of the Mississippi River but the southern part was open and continued to carry rainwater, glacial meltwater and sediment to the sea. Once that glacier had melted the river returned to its old channel flowing near what is now Peoria, Havana and Beardstown.
As the Illinoian glacier was melting, some of its water and sediment probably drained away along a new channel, the ancestral Sangamon River. In the warm ice-free period that followed the Illinoian Glacial Episode, the Sangamon could easily have meandered back and forth, cutting its valley wider and deeper in the loose sediments left by the glaciers. When the climate turned cold once again, and the Wisconsin glacier advanced across northeastern Illinois, the upper end of the ancestral Mississippi River was again buried beneath the ice, as was the upper end of the Sangamon. The meltwater that had been flowing down the upper part of the ancestral Mississippi was diverted southwestward through a low spot in the bedrock near present-day Rock Island. This time, when the glacier melted back, the Mississippi River stayed in its present-day course along the western border of the state.