The American Bottom is the flood plain of the Mississippi River in the Metro East region of Southern Illinois, extending from Alton, Illinois, south to the Kaskaskia River. It is also sometimes called "American Bottoms". The area is about 175 square miles (450 km), mostly protected from flooding in the 21st century by a levee and drainage canal system. Immediately across the river from St. Louis, Missouri, are industrial and urban areas, but nearby marshland, swamps, and the Horseshoe Lake (which was created by the river) are reminders of the Bottoms' riparian nature.
This plain with its rich alluvial soil, served as the center for the pre-Columbian Cahokia Mounds civilization, and later the French settlement of Illinois Country. Deforestation of the river banks in the 19th century to fuel steamboats had dramatic environmental effects in this region. The Mississippi River between St. Louis and the confluence with the Ohio River became wider and more shallow, as unstable banks collapsed into the water. This resulted in more severe flooding and lateral changes of the major channel, causing the destruction of several French colonial towns, such as Kaskaskia, which relocated; Cahokia, and St. Philippe, Illinois.
