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. It is possible that the point of origin of the flood was from Lake Chicago. 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 floating callbacks leading to 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.
The area is underlain by bedrock of Silurian age. This rock is dolomite. Preglacial erosion the surface of the bedrock created an irregular surface, allows the glacial drift to vary in depth over short distances. Some areas are underlain by bedrock near the surface, in other areas it can be 100 feet (30 m) deep. Some hills are solely glacial drift; others may be entirely of limestone.
The water comes from the melting ‘Valparaiso glacier’, the glacier that deposited first the Manhattan moraine, part of the Valparaiso morainic system and the Valparaiso moraines to the northeast. The melt-water came from three glacial lobes of the Laurentide ice sheet. In addition to the melt-water from the Michigan lobe in the Michigan basin, melt-water came from the Huron-Erie lobe to the east in the Maumee valley in Ohio and from the Sturgis Lobe coming out of Michigan across northeast Indiana. The water backed up behind the Marseilles moraine forming a series of lakes. In the Morris basin was Lake Wauponsee, which reached east up the Kankakee valley into Indiana, It flowed south into the Iroquois River into Lake Watseka over the divide (Chatsworth ridge of the Marseilles morainic system, to the Vermillion River valley past Pontiac. The volume of water was blocked at the gap in the Farm Ridge moraine, a unit of the Bloomington morainic system near LaSalle. This blockage created Lake Ottawa between Farm Ridge and the Marseilles moraine in the Illinois Valley, and between the Lake Pontiac between the Cropsey (unit of the Bloomington morainic system, and Marseilles moraines in the Vermilion Valley.
The flood did not occur just once, or all at once. It was a repetitive event, over possibly hundreds of years. In the early years it was doubtless small, but as the years passed it became larger and larger until at some time it reached a maximum, and then, as the Valparaiso glacier receded, it gradually subsided. Before the torrent, the valley of the Kankakee River near the city of Kankakee was neither deep nor broad. It was wide plain of Marseilles drift, with a small river. During the early outflows, it spread across this plain. At its highest level it found channels in the Minooka ridge and flowed across the ridge to the drift plain in the west. As the outflows continued, the Marseilles drift plain in the east was eroded. This removed much of the drift, and even began eroding into the Silurian dolomite beneath. In the last stages channels were cut in the bedrock. The area along the southern bank of the Kankakee River, west of Kankakee, Illinois consist of long narrow ridges leading to the northwest. These are gravel bars from the Kankakee outwash. The force of outflow scoured some of the dolomite bedrock, which can be seen in the gravel bars. The heavy flow of water moved boulders on downstream and moved glacial erratics from the tills. The faster flow towards center created a channel, which the modern river still follows. Floodwaters were confined to the inter-morane channels, which moved it west, then to the south. Upon reaching the LaSalle, Illinois area, the flood entered the ancestral Mississippi valley, now abandoned. The area where the old Mississippi valley was joined is called the Big Bend of the Illinois River.
Outwash from the glacier was carried beyond the Kankakee area. The coarser outwash, of the coarser pieces of the local drift and with rubble ripped from the bedrock, became bars along the river. At the end of the outflow, the lighter sands and silts were deposited in bars and across the valley floor. The sand was picked up by the wind, creating sand dunes to the south and southeast. Sand bars can be found as far south as Sand Ridge State Forest in central Illinois.
- "Starved rock". Geology and Environmental Geosciences. Retrieved 1 January 2010.
- "The Kankakee Torrent". 2000-01-05.
- Moran, Janet (2009-11-16). "Kankakee River carved out its place in history". NWI.com.
- Ray Wiggers (1997). Geology underfoot in Illinois. Mountain Press Publishing. pp. 127–131. ISBN 978-0-87842-346-0. Retrieved 2 January 2011.
- "Vegetation of Hooper Branch Nature Preserve, Iroquois County, Illinois". Encyclopædia Britannica. Retrieved 2 January 2011.
- Wiggers(1997), p.131
- Earth Science Field Trip, Guide Leaflet, Kankakee Area, Kankakee High School; May 18, 1957; John C. Frye; State Geological Survey; Urbana, Illinois; 1957
- GeoThttp://geography.about.com/library/misc/uckankakee.htm; 2000, Jan 5