East St. Louis, Illinois, a city in St. Clair County. It lies on the Mississippi River opposite St. Louis, Missouri. East St. Louis is in a heavily industrialized area of steel mills, petroleum refineries, and chemical plants. The city's location makes it an important transportation center, served by numerous railways, highways, and barge lines. Near East St. Louis is Cahokia Mounds State Park, containing 25 prehistoric Indian mounds. A settlement called Illinoistown was founded here in 1818 and renamed East St. Louis in 1861. The city underwent a substantial economic decline beginning in the 1950's.