The southern portion of the American Bottoms 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- to 300-foot high, 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, and the river abuts the bluffline on the Missouri side. Portions of St. Clair, Madison, Monroe, and Randolph counties are in the American Bottom. Its maximum width is about in the north, and it is about 2 to 3 miles in width throughout most of its southern extent.