The foothills of the Ozark Mountains begin in southwestern St. Louis County , with most of the rest of the county being a fairly level plateau . This western part of the county is the least developed , due to rugged topography . Bluffs along the Mississippi in the south of the county rise about 200 – 300 feet above the river . A major floodplain area is the Chesterfield Valley , in the western part of the county , along the Missouri River . It was formerly called " Gumbo Flats " after its rich , dark soil ; it was submerged by at least ten feet of water during the Great Flood of 1993 . The Corps of Engineers constructed a higher levee , and the county has permitted construction in the floodplain
Great Flood of 1993
Great Flood of 1993
St. Louis County, Missouri