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
Southern Illinois River-to-River Conference approached by Benton, Herrin, Harrisburg, and West Frankfort for membership; these four schools were concerned over their small size relative to the other members of the South Seven Conference (where all four were then located) and sought out a conference more in line with their size. Massac County (then an independent school) was approached about becoming a member, and in 1993 the Southern Illinois River-to-River Conference was formed. At the start of the 2010-2011 school year Carterville left the Black Diamond Conference to join the Southern Illinois River-to-River Conference. Chester High School has taken their place in the Black Diamond