East St. Louis is a city in St. Clair County, Illinois, United States. It is directly across the Mississippi River from downtown St. Louis, Missouri, and the Gateway Arch National Park. East St. Louis is in the Metro East region of Southern Illinois. Once a bustling industrial center, like many cities in the Rust Belt, East St. Louis was severely affected by the loss of jobs due to the flight of the population to the suburbs during the riots of the late 1960s. In 1950, East St. Louis was the fourth-largest city in Illinois when its population peaked at 82,366.