Illinois River

Water type: River
Continent: North America
Climates: Continental, Temperate

The Illinois River is a principal tributary of the Mississippi River and is approximately 273 miles (439 km) long.

The Illinois River is formed by the confluence of the Kankakee River and the Des Plaines River in eastern Grundy County, approximately 10 miles (16 km) 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 five miles (8 km) 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 15 miles (24 km) upstream from the confluence with the Mississippi River.

For the last 20 miles (32 km) of its course, the Illinois is separated from the Mississippi River by only about five miles (8 km), by a peninsula of land that makes up Calhoun County. The Illinois 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 River and the Mississippi.

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