Definitions for "Illinois River"
a river in Illinois that flows southwest to the Mississippi River
a river that runs diagonally through central Illinois, emptying into the Mississippi River near Alton.
The Illinois River is a principal tributary of the Mississippi River, approximately 273 miles (439 km) long, in the state of Illinois in the United States. 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.