Illinois River Basics

Why is this river important?

The Illinois River is a major tributary of the Mississippi River. The Illinois River is the largest inland stream in Illinois. Its natural drainage basin covers an area of about 73,000 km3 (28,200 mi2) in Illinois, southeastern Wisconsin, and northwestern Indiana. Eight major tributaries (the LaMoine, Spoon, Fox, Des Plaines, Kankakee, Vermillion, Mackinaw, and Sangamon rivers) feed into the river.

The Illinois River has been called the "most studied" river in the world. Scientists have studied the geology and biology of the river for over 100 years. Around the turn of the century the river and its backwater lakes were among the most productive fisheries in the world. It still includes some of the richest habitat in the Midwestern United States for fish and waterfowl.

Where is it?

The Illinois River flows entirely within the state of Illinois. It originates at the confluence of the Des Plaines and Kankakee River valleys in northeastern Illinois and flows for 273 miles to its confluence with the Mississippi River in western Illinois. The river flows from east to west from its origins to Hennepin, where it makes a "great bend" and then flows southwestward toward its juncture with the Mississippi River.

Where does it meet the Mississippi?

The Illinois River meets the Mississippi River at Pere Marquette State Park near Grafton. Illinois. The confluence of the rivers is a stunning site along the Great River Road and offers vistas that seem endless.

How deep is it?

Like the Mississippi River, the Illinois River is relatively shallow. To make it navigable, it is dredged to create and maintain a nine foot deep channel.

What shape is it?

The river is relatively straight and parts of the river are lined with backwater lakes. The upper part of the valley is narrow. The valley from Hennepin south is very wide and once contained the Ancient Mississippi River. The Ancient Mississippi River was diverted westward into its current channel around 20,000 years ago by advancing glacial ice.

What else?

Like the Mississippi, the Illinois River is a "managed" river. In 1848, the Illinois and Canal opened to connect Lake Michigan with the Mississippi River through the Illinois River to promote commerce. In 1900, the flow of the Chicago River was reversed and the Sanitary and Ship Canal was constructed to connect the Des Plaines and Illinois Rivers with Lake Michigan and flush wastes from the city of Chicago down the Illinois River. From 1903-1920 floodplains were drained to create agricultural lands. From 1918-1921, many human-made levees were installed to protected floodplain agricultural lands from flooding. During the 1930s locks and dams were installed on the river (at Dresden Heights, Marseilles, Peoria, and La Grange) to enable commercial navigation. The "Illinois Waterway" as the system is called supports heavy barge traffic. The current integrated management plan for the Illinois River Watershed recognizes the importance of conservation and targets projects to restore wetlands and floodplain forests and reduce soil erosion.