Its drainage basin extends into southeastern Wisconsin, northwestern Indiana, and a very small area of southwestern Michigan in addition to central Illinois.

Historic and 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.

Its other major tributaries include the Fox, Vermilion, Macoupin, Mackinaw, Spoon, Sangamon, and La Moine.

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.

The La Moine River flows into it approximately five miles (8 km) southwest of Beardstown, which is south of Peoria and Pekin and northwest of Lincoln and Springfield.

Macoupin Creek joins the Illinois on the border between Greene and Jersey counties, approximately 15 miles (24 km) upstream from the confluence with the Mississippi River.

After the I&M Canal was built, a string of cities, such as LaSalle, Peru, and Ottawa grew along the river, extending Chicago's influence into the Mississippi Valley.

Overfishing, habitat loss from heavy siltation, and water pollution have eliminated most commercial fishing except for a small mussel harvest to provide shells to seed pearl oysters overseas.

The Illinois forms part of a modern waterway that connects the Great Lakes at Chicago to the Mississippi River.

River traffic and flood control is managed by eight locks and dams operated by the U.S. Army Corps of Engineers.

The Illinois River is an important part of the Great Loop, the circumnavigation of Eastern North America by water.

During dry weather, sewage flows safely through the city's sewers to the Greater Peoria Sanitation District wastewater treatment plant.

However, about 28 times a year, melting snow or rainwater can overwhelm the sewers, causing untreated sewage to overflow into the Illinois River.

[17] The John Hartford song "Long Hot Summer Day" is written from the perspective of a barge worker on the Illinois River.