1 Six Rivers , Five Glaciers , and an Outburst Flood

The waters of the modern Illinois River flow gently through looping meanders bordered by quiet backwater lakes and drop only a few inches in each river mile. Concealed beneath this gentle river is geologic evidence that the Illinois descended from ancient rivers with surprising and sometimes violent histories. The geologic story of the Illinois River is not only an account of an interesting chapter of Earth history, but it also reveals a rich geologic legacy of valuable and vulnerable resources that should be managed and used wisely. Modern, detailed, geologic field mapping has enabled new insights into the river’s history. Begun in 2000 by the Illinois State Geological Survey (ISGS), geologic mapping in the Middle Illinois River Valley area (Fig 1) was undertaken to aid planning for an expansion of Illinois Highway 29 between Chillicothe and I-180 west of Hennepin. Mapping was focused initially on the western bluff and valley bottom west of the river near the present highway but has since been expanded to more than 275 sq mi in Putnam, Marshall, and Peoria counties. Funding was provided by Illinois Department of Transportation (IDOT) and by ISGS. Several maps are scheduled to be published (McKay and others 2008a, 2008b, 2008c).