Chapter 22
Ecological Subregions of the United States



Lower Mississippi Riverine Forest

One Section has been delineated in this Province: This Section is located in the south-central conterminous States, including parts of Missouri, Arkansas, Tennessee, Mississippi, and Louisiana. The area of this Section is about 44,300 mi2 (114,700 km2).

Section 234A--Mississippi Alluvial Basin

Geomorphology. This Section is in the Coastal Plains geomorphic province. The predominant landform consists of flat, weakly to moderately dissected alluvial plains. The plains were formed by deposition of continental sediments into a submerged, synclinal trough, which was later exposed by sea level subsidence. Elevation ranges from 0 to 660 ft (0 to 200 m). Local relief in most of the Section ranges from 0 to 100 ft (0 to 30 m).

Lithology and Stratigraphy. Rock units formed during the Cenozoic Era. About 20 percent consists of Tertiary marine deposits (lignitic sandy and argillaceous deposits). The remainder consists of about equal amounts of Quaternary marine deposits and glacial outwash. Recent alluvium consists of silt deposited during floods.

Soil Taxa. Soils are Aquepts, Aqualfs, Aquents, Udolls, and Udalfs. Haplaquepts, Ochraqualfs, Fluvaquents, Natraqualfs, Ochraqualfs, and Hapludalfs are found in back-swamp areas and older natural levees. Dystrochrepts, Udifluvents, and Fluvaquents occur in smaller areas. Hydraquents and Medisaprists are in southern Louisiana. These soils have a udic or aquic moisture regime, a thermic temperature regime, and montmorillonitic or mixed mineralogy. Soils, which formed in alluvium, are deep, medium textured, and have adequate or excessive moisture available for vegetation during the growing season.

Potential Natural Vegetation. K\"uchler classified vegetation as southern floodplain forest and oak-hickory forest. The predominant vegetation form is cold-deciduous, alluvial broadleaf forest, with small areas of cold-deciduous, broad-leaved forest on upland sites. The main cover type is oak-gum-cypress, where main species are Nuttall oak, water oak, laurel oak, cherrybark oak, cottonwood, sycamore, hackberry, red and silver maple, and baldcypress. The oak-hickory cover type consists of post oak, bur oak, northern red oak, black oak, and white oak.

Fauna. The elk, mountain lion, wolf, Carolina parakeet, and ivory-billed woodpecker once inhabited this Section. Presently, the fauna include white-tailed deer, black bear, bobcat, gray fox, raccoon, cottontail rabbit, gray squirrel, fox squirrel, striped skunk, swamp rabbit, and many small rodents and shrews. The turkey, bobwhite, and mourning dove are game birds in various parts of this Section. In wetlands, ibises, cormorants, herons, egrets, and kingfishers are common. Songbirds include the red-eyed vireo, cardinal, tufted titmouse, wood thrush, summer tanager, blue-gray gnatcatcher, hooded warbler, and Carolina wren. The herpetofauna include the box turtle, common garter snake, and timber rattlesnake.

Climate. Precipitation averages 45 to 65 in (1,150 to 1,650 mm) annually. Temperature averages 56 to 70 oF. (14 to 21 oC). The growing season lasts 200 to 340 days.

Surface Water Characteristics. This Section includes the lower part of the Mississippi River below its confluence with the Ohio River. The drainage pattern is varied, ranging from dendritic to trellis, and has developed without bedrock structural control. The principal river draining this and adjoining Sections is the Mississippi. The Red River enters the Mississippi near the southern edge of this Section. Many oxbow-type natural lakes have formed along the Mississippi River. Extensive wetlands and backswamps characterize this Section.

Disturbance Regimes. Periodic flooding has been the principal historical disturbance, but has been reduced by a series of levees and dams built for flood control.

Land Use. About 90 percent of the area is agricultural, cleared of natural vegetation and drained by a system of ditches.

Cultural Ecology. Reserved.

Compiled by Southern Region and Southern Research Station.