The Illinois River () is a principal tributary of the Mississippi River and is approximately long. Located in the U.S. state of Illinois, the river has a drainage basin of . The Illinois River begins with the confluence of the Des Plaines and Kankakee rivers in the Chicago metropolitan area, and it generally flows to the southwest across Illinois, until it empties into the Mississippi near Grafton, Illinois. Its drainage basin extends into southeastern Wisconsin, northwestern Indiana, and a very small area of southwestern Michigan in addition to central Illinois. Along its shores are several river ports, including Peoria, Illinois. Historic recreation areas on the river include Starved Rock and the internationally important wetlands of the Emiquon Complex and Dixon Waterfowl Refuge.
The river was important among Native Americans and early French traders as the principal water route connecting the Great Lakes with the Mississippi. The French colonial settlements along these rivers formed the heart of the area known as the Illinois Country in the 17th and 18th centuries. After the construction of the Illinois and Michigan Canal and the Hennepin Canal in the 19th century, the role of the river as link between Lake Michigan and the Mississippi was extended into the era of modern industrial shipping. The Illinois now forms the basis for the Illinois Waterway, extending the river's capabilities for navigation and commercial shipping.
Hydrography
The Illinois River is formed by the confluence of the Kankakee River and the Des Plaines River in eastern Grundy County, approximately southwest of Joliet. This river flows west across northern Illinois, passing Morris and Ottawa, where it is joined by the Mazon River and Fox River respectively. At LaSalle, the Illinois River is joined by the Vermilion River, and then it flows west past Peru and Spring Valley. In southeastern Bureau County it turns south at an area known as the "Great Bend", flowing southwest across western Illinois, past Lacon, Henry and downtown Peoria, the chief city on the river.
South of Peoria, the Illinois River goes by East Peoria and Creve Coeur and then Pekin in Tazewell County. It is then joined by the Mackinaw River, and then passes through the Chautauqua National Wildlife Refuge. Across from Havana, the Illinois is joined by the Spoon River coming from Fulton County, and across from Browning, it is joined by the Sangamon River, which passes through the state capital, Springfield, Illinois. The La Moine River flows into it approximately southwest of Beardstown, which is south of Peoria and Pekin and northwest of Lincoln and Springfield.
Near the confluence of the Illinois with the La Moine River, it turns south, flowing roughly parallel to the Mississippi across western Illinois. Macoupin Creek joins the Illinois on the border between Greene and Jersey counties, approximately upstream from the confluence with the Mississippi River.
For the last of its course, the Illinois is separated from the Mississippi River by only about , by a peninsula of land that makes up Calhoun County. The Illinois joins the Mississippi near Grafton, approximately northwest of downtown St. Louis and about upstream from the confluence of the Missouri River and the Mississippi.
Geology
South of Hennepin, the Illinois River follows 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 of the modern river.