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The Lower Mississippi Valley is a 780-km lowland in the south-central United States extending from near Cairo, Illinois, south to the Gulf of Mexico (Fig. 1). Valley width varies from about 40 to 200 km, and flood-plain elevations range from about 84 m at the Mississippi-Ohio River confluence to sea level where the deltaic plain meets the Gulf of Mexico. The lower Mississippi is North America’s largest river. Its chief tributaries are the upper Mississippi, Missouri, Ohio, Arkansas, Red, and Ouachita Rivers. The drainage basin encompasses over 3,200,000 km2, and average discharge to the Gulf of Mexico is approximately 12, 000 m3/s, with a recorded maximum and minimum of 56, 000 and 5, 600 m3/s.

The lower Mississippi Valley has developed since at least Cretaceous time in a terrain of igneous, metamorphic, and sedimentary rocks as old as Precambrian. The sediment delivered to the delta is chiefly silt and clay. Cycles of delta growth and deterioration have affected the amount of sediment delivered to the chenier plain west of the delta by longshore drift.

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