Illinois River 20 miles (32 km) of its course, the Illinois is separated from the Mississippi River by only about five miles, 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 upstream from the confluence of the Missouri River and the Mississippi. South of Hennepin, the Illinois River is following the ancient channel of the Mississippi River. The Illinoian Stage, about 300,000 to 132,000 years ago, blocked the Mississippi near Rock Island, diverting it into its present channel. After the
Illinois River was relocated to the present site of Creve Coeur, near Peoria, where the Jesuits relocated. The Peoria Riverfront Museum contains a gallery, "Illinois River Encounter," that attempts to interpret the museum through an aquarium tank and displays of the river's geology, ecology, social history, engineering, and commercial use. From 1905 to 1915, more freshwater fish were harvested from the Illinois River than from any other river in the United States except for the Columbia River. The Illinois River was once a major source of mussels for the shell button industry. Overfishing, habitat loss from heavy siltation, and water pollution have