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Saint Louis County, Missouri, United States (38.63987 -90.51881)
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.