Southern Illinois Southern Illinois Southern Illinois (also known as "Little Egypt") is the southern third of the state of Illinois. The southern part of Illinois has a unique cultural and regional history. Part of downstate Illinois, the Southern Illinois region is bordered by the two most voluminous rivers in the United States: the Mississippi River and its connecting Missouri River to the west, and the Ohio River to the east and south with the Wabash as tributary. Southern Illinois' most populated city is Belleville at 44,478. Other principal cities include Alton, Centralia, Collinsville, Edwardsville, Effingham, O'Fallon, Harrisburg, Herrin, Mt. Vernon, Marion, and
Big Black River (Mississippi) northern border of Claiborne County. The Big Black River and most of its tributaries are silt-filled. The rivers carry large amounts of suspended sediment, resulting mostly from agricultural runoff. These tributaries are slow-flowing muddy streams. However, some are swift-flowing with sandy bottoms. The Battle of Big Black River Bridge, fought during the Battle of Vicksburg, was part of the Vicksburg Campaign in the American Civil War. The size of the Big Black River Basin drainage area is . Its elevation varies from 50 to 650 feet (14 to 198 meters) above sea level. It is long and averages 22 to