The Illinois River is a principal tributary of the Mississippi River at approximately 273 miles (439 km) in length. Located in the U.S. state of Illinois, the river has a drainage basin of 28,756.6 square miles (74,479 km2). The Illinois River begins with the confluence of the Des Plaines and Kankakee rivers in the Chicago metropolitan area, and it generally flows to the southwest across Illinois, until it empties into the Mississippi near Grafton, Illinois. Its drainage basin extends into southeastern Wisconsin, northwestern Indiana, and a very small area of southwestern Michigan in addition to central Illinois. Along its shores are several river ports, including Peoria, 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 Mississippi River is the primary river, and second-longest river, of the largest drainage basin in the United States. From its traditional source of Lake Itasca in northern Minnesota, it flows generally south for 2,340 miles (3,766 km) to the Mississippi River Delta in the Gulf of Mexico. With its many tributaries, the Mississippi's watershed drains all or parts of 32 U.S. states and two Canadian provinces between the Rocky and Appalachian mountains. The main stem is entirely within the United States; the total drainage basin is 1,151,000 sq mi (2,980,000 km2), of which only about one percent is in Canada. The Mississippi ranks as the thirteenth-largest river by discharge in the world. The river either borders or passes through the states of Minnesota, Wisconsin, Iowa, Illinois, Missouri, Kentucky, Tennessee, Arkansas, Mississippi, and Louisiana.