The Kankakee Torrent was a catastrophic flood that occurred between 14,000 and 18,000 years ago in the Midwestern United States. It resulted from a breach of a large glacial lake formed by the melting of the Wisconsin Glacier. While it is possible that the point of origin of the flood was from Lake Chicago, some authorities believe it actually came from further east, near what is today the center of the Lower Peninsula of the state of Michigan. The landscape south of Chicago still shows the effects of the torrent, particularly at Kankakee River State Park and on the Illinois River at Starved Rock State Park.
The Kankakee Torrent was responsible for the rapid creation of several geological features of Illinois. Both the Kankakee River and Illinois River largely follow paths carved out by the torrent, a process that is believed to have taken only days. Most notable today is a region in north-central Illinois known as Starved Rock; while most of Illinois is located on a low-lying plain with little variation in elevation, Starved Rock State Park features several canyons which were created in the Kankakee Torrent. Another, very different, geologic effect left over from the Kankakee Torrent is the existence of "sand prairies". Sand prairies exist where the massive flood waters stopped their movement and deposited large quantities of sand. When European settlers arrived, one remaining sign of these deposits were sand dunes located along parts of the Torrent's course.
The Kankakee River also bears several features directly resultant from the catastrophic Torrent, and Kankakee River State Park encompasses all the features that evolved as a result of the catastrophic flood event. Along much of its course, tributaries entering the Kankakee enter over waterfalls, a phenomenon known as "hanging tributaries". This is because the Torrent carved the Kankakee far deeper than would normal river erosion, and the erosion of the slow-moving tributaries into the bedrock has never caught up. This effect is most evident where Rock Creek joins the Kankakee. The Illinois State Geological Survey reports that Rock Creek's cutting through the bedrock (dolomites of the Joliet Formation) to the waterfall point, upstream of its confluence with the Kankakee River, is progressing at the rate of 3 inches per year.
The effects of the Kankakee torrent were not limited to northeast Illinois. The Ohio and Mississippi Rivers appear to have had their courses altered by the Kankakee Torrent, with the Ohio being pushed further south and the Mississippi further west.
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- Wiggers(1997), p.131