The Illinois River is a principal tributary of the Mississippi River, approximately 273 miles (439 km) long, in the U.S. state of Illinois. The river drains a large section of central Illinois, with a drainage basin of 40,000 square miles (104,000 km²). The river was important among Native Americans and early French traders as the principal water route connecting the Great Lakes with the Mississippi. The colonial settlements along the river formed the heart of the area known as the Illinois Country. After the construction of the Illinois and Michigan Canal and the Hennepin Canal in the 19th century, the river's role as link between Lake Michigan and the Mississippi was extended into the era of modern industrial shipping.
South of Peoria it is joined by the Mackinaw River and then passes through the Chautauqua National Wildlife Refuge. Opposite Havana it is joined by the Spoon River, and across from Browning it is joined by the Sangamon River. The La Moine River flows into it approximately 5 miles (8 km) southwest of Beardstown.
Near its confluence with the La Moine River, it turns south, flowing roughly parallel with the Mississippi across southwestern Illinois. It is joined by Macoupin Creek on the border between Greene and Jersey counties, approximately 15 miles (24 km) upstream from its confluence with the Mississippi.
For the last 20 miles (32 km) of its course, it is separated from the Mississippi by only 5 miles (8 km). It joins the Mississippi near Grafton, approximately 25 miles (40 km) northwest of downtown St. Louis and about 20 miles (32 km) upstream from the confluence of the Missouri and the Mississippi.
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 presentday 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 overtopped 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 "underutilised channel," one far larger than would be needed to contain any conceivable flow in modern times.
The Illinois forms part of a modern waterway that connects the Great Lakes at Chicago to the Mississippi River. The waterway was originally established by the building of the Illinois and Michigan Canal that connected the Illinois River to the Chicago River. When the Chicago River was later reversed, the pollution and sewage of the city of Chicago flowed down into the Illinois River. The Illinois and Michigan Canal has since been replaced by the Illinois Waterway including the Chicago Sanitary and Ship Canal. The river is controlled by five locks and dams to facilitate river traffic. The waterway is heavily trafficked by barges transporting bulk goods such as grain and oil. It is used in the summer and early fall by pleasure boats cruising the Great Loop.
Peoria is currently a known polluter of the Illinois River from its sewage system, which dumps raw sewage into the river approximately 25 times each year. This violation of the Federal Clean Water Act has drawn the attention of the EPA, which has mandated that the city make improvements.