Illinois River - Geology

Geology

South of Hennepin, the Illinois River is following the ancient channel of the Mississippi River. The Illinoian Stage, about 300,000 to 132,000 years ago, blocked the Mississippi near Rock Island, diverting it into its present channel. After the glacier melted, the Illinois River flowed into the ancient channel. The Hennepin Canal roughly follows the ancient channel of the Mississippi upstream of Rock Island.

The modern channel of the Illinois River was shaped in a matter of days by the Kankakee Torrent. During the melting of the Wisconsin Glacier about 10,000 years ago, a lake formed in present-day Indiana, comparable to one of the modern Great Lakes. The lake formed behind the terminal moraine of a substage of that glacier. Melting ice to the north eventually raised the level of the lake so that it overflowed the moraine. The dam burst, and the entire volume of the lake was released in a very short time, perhaps a few days.

Because of the manner of its formation, the Illinois River runs through a deep canyon with many rock formations. It has an "underutilized channel," one far larger than would be needed to contain any conceivable flow in modern times.