FLUVIAL GEOMORPHIC FEATURES OF THE LOWER MISSISSIPPI ALLUVIAL VALLEY

The Lower Mississippi Valley (LMV) has been one of the most intensivel y studied alluvial valleys in the world in terms of it's geological an d geomorphic framework and history. A brief outline of the history of the major geological and geomorphological investigations of the LMV is provided. The results of these investigations are discussed in terms of the fluvial geomorphic framework of the valley and the apparent sig nificant changes in the regime of the Mississippi River during the Lat e Wisconsinan and Holocene stages. The LMV occupies the broad deep syn clinal trough of the Mississippi Embayment which extends from Cairo, I llinois, to the Gulf of Mexico in a slightly sinuous north-south trend . The embayment is filled with a north to south thickening wedge of no n-marine and marine sediment ranging in age from Jurassic to Holocene. The major landscapes of the LMV may be considered in four regions: (1 ) a narrow active meander belt in a broad valley of Late Pleistocene v alley train in the northern third; (2) a wide mosaic of interwoven Hol ocene meander belts in the middle third; (3) a relatively narrow valle y of the Atchafalaya Basin bounded on each side by narrow meander belt s in the upper part of the lower third; and (4) the broad distributary wedge of the deltaic plain in the southernmost region of the valley. The valley trains vary in age and landform with the oldest occurring a s slightly dissected low ridges and the youngest as broad flats separa ted by shallow interwoven former braided channels. Meander belts forme d throughout the Holocene are comprised of low natural levee ridges fl anking abandoned courses and bordered by crescent-shaped oxbow lakes a nd ridge and swale topography. In the middle third of the valley, mean der belts are separated by expansive backswamps of very little relief. The deltaic plain is also exceptionally flat, interrupted by the low natural levee ridges of the abandoned deltaic distributaries. The floo dplain of the LMV is a complex mosaic of fluvial features and landscap es within the four landscape regions. Included in this mosaic are aban doned channels and courses, lateral accretion topography of ridges and swales, natural levees, crevasses and crevasse channels, distributary channels, backswamps and rimswamps, alluvial fans and aprons, valley trains (braided stream terraces), lakes and lacustrine deltas, terrace s, and the alluvial valley bluff. Changes in the hydraulic regime of t he Lower Mississippi River (LMR) since the Late Pleistocene have playe d a major role in the development of the landscape of the valley. The most important regime change was the diminishment of the influence of Wisconsinan glaciation in the upper Midwest and the resultant evolutio n of the Mississippi River from a broad braided outwash channel to a m ore narrow but sinuous meandering channel at the end of the Pleistocen e. During the Holocene, the Mississippi River undoubtedly responded to major climatic changes, rising sea level, tributary stream influence, and possibly tectonism, diapirism, and subsidence through the growth and evolution, and abandonment of it's meander belts and deltas.