American Bottom

The American Bottom is the flood plain of the Mississippi River in the Metro-East region of Southern Illinois, extending from Alton, Illinois, to the Kaskaskia River. It is also sometimes called "American Bottoms". The area is about 175 square miles (450 km2), mostly protected from flooding by a levee and drainage canal system. Immediately across the river from St. Louis, Missouri are industrial and urban areas, but many swamps and the major Horseshoe Lake are reminders of its riparian nature. Deforestation of the river banks in the 19th century to fuel steamboats had dramatic environmental effects in this region, leading to the Mississippi River between St. Louis and the confluence with the Ohio River becoming more wide and shallow, as unstable banks collapsed into the water. It resulted in more severe flooding and lateral changes of the major channel, causing the flooding and destruction of several French colonial towns, such Kaskaskia, Cahokia, and St. Philippe, Illinois.

The southern portion of the American Bottom is primarily agricultural, planted chiefly in corn, wheat, and soybean. The American Bottom is part of the Mississippi Flyway used by migrating birds and has the greatest concentration of bird species in Illinois. The flood plain is bounded on the east by a nearly continuous, 200–300 foot high, 80-mile (130 km) long bluff of limestone and dolomite, above which begins the great prairie that covers most of the state. The Mississippi River bounds the Bottom on its west; the river abuts the bluffline on the Missouri side. Portions of St. Clair, Madison, Monroe, and Randolph counties are in American Bottom. Its maximum width is about 9 miles (14 km) in the north, and it is about 2–3 miles in width throughout most of its southern extent.