A river rising in northeast Illinois and flowing about 439 km (273 mi) generally southwest to the Mississippi River in west-central Illinois. River, northeastern Illinois, U.S. Formed by the junction of the Des Plaines River and Kankakee River in Illinois, it flows southwest across the state, joining the Mississippi River after a course of 273 mi (440 km). It drains an area of 25,000 sq mi (65,000 sq km) and occasionally broadens into wide expanses, such as Peoria Lake