The Mississippi Alluvial Plain is a Level III ecoregion designated by the Environmental Protection Agency (EPA) in seven U.S. states, though predominantly in Arkansas, Louisiana, and Mississippi. It parallels the Mississippi River from the Midwestern United States to the Gulf of Mexico.

The Mississippi Alluvial Plain ecoregion has been subdivided into fifteen Level IV ecoregions.

Description The Mississippi Alluvial Plain extends along the Mississippi River from the confluence of the Ohio River and Mississippi River southward to the Gulf of Mexico; temperatures and annual average precipitation increase toward the south. It is a broad, nearly level, agriculturally-dominated alluvial plain. It is veneered by Quaternary alluvium, loess, glacial outwash, and lacustrine deposits. River terraces, Swales, and levees provide limited relief, but overall, it is flatter than neighboring ecoregions in Arkansas, including the South Central Plains. Nearly flat, clayey, poorly-drained soils are widespread and characteristic. Streams and rivers have very low gradients and fine-grained substrates. Many reaches have ill-defined stream channels.

The ecoregion provides important habitat for fish and wildlife, and includes the largest continuous system of wetlands in North America. It is also a major bird migration corridor used in fall and spring migrations, known as the Mississippi Flyway. Potential natural vegetation is largely southern floodplain forest and is unlike the oak–hickory and oak–hickory–pine forests that dominate uplands to the west in Ecoregions 35, 36, 37, 38, and 39; loblolly pine, so common in the South Central Plains (35), is not native to most forests in the Arkansas portion of Ecoregion 73. The Mississippi Alluvial Plain has been widely cleared and drained for cultivation; this widespread loss or degradation of forest and wetland habitat has impacted wildlife and reduced bird populations. Presently, most of the northern and central sections are in cropland and receive heavy treatments of insecticides and herbicides; soybeans, cotton, and rice are the major crops, and aquaculture is also important.

Agricultural runoff containing fertilizers, herbicides, pesticides, and livestock waste have degraded surficial water quality. Concentrations of total suspended solids, total dissolved solids, total phosphorus, ammonia nitrogen, sulfates, turbidity, biological oxygen demand, chlorophyll a, and fecal coliform are high in the rivers, streams, and ditches of the region; they are often much greater than elsewhere in Arkansas, increase with increasing watershed size, and are greatest during the spring, high-flow season. Fish communities in least altered streams typically have an insignificant proportion of sensitive species; sunfishes are dominant followed by minnows. Man-made flood control levees typically flank the Mississippi River and, in effect, separate the river and its adjoining habitat from the remainder of its natural hydrologic system; in so doing, they interfere with sediment transfer within the region and have reduced available habitat for many species.

Earthquakes in the early nineteenth century offset river courses in Ecoregion 73. Small to medium size earthquakes still occur frequently; their shocks are magnified by the alluvial plain's unconsolidated deposits, creating regional land management issues.

Batture Between the levees that parallel the Mississippi River is a corridor known as the batture lands. Batture lands are hydrologically linked to the Mississippi River, flood-prone, and contain remnant habitat for “big river” species (e.g., pallid sturgeon) as well as river-front plant communities; they are too narrow to map as a separate Level IV ecoregion.

Level IV ecoregions

Northern Holocene Meander Belts (73a)

The Northern Holocene Meander Belts ecoregion is a flat to nearly flat floodplain containing the meander belts of the present and past courses of the Mississippi River. Point bars, natural levees, swales, and abandoned channels marked by meander scars and oxbow lakes are common and characteristic. Ecoregion 73a tends to be slightly lower in elevation than adjacent ecoregions. Its abandoned channel network is more extensive than in the Southern Holocene Meander Belts (73k) of Louisiana. Ecoregion 73a is underlain by Holocene alluvium; it lacks the Pleistocene glacial outwash deposits of Ecoregion 73b. Soils on natural levees are relatively coarse-textured, well-drained, and higher than those on levee back slopes and point bars; they grade to very heavy, poorly-drained clays in abandoned channels and swales. Overall, soils are not as sandy as the Northern Pleistocene Valley Trains (73b) and are finer and have more organic matter than the Arkansas/Ouachita River Holocene Meander Belts (73h). Natural vegetation varies with site characteristics. Younger sandy soils have fewer oaks and more sugarberry, elm, ash, pecan, cottonwood, and sycamore than Ecoregion 73d. Widespread draining of wetlands and removal of bottomland forests for cropland has occurred. Soybeans, cotton, corn, sorghum, wheat, and rice are the main crops. Catfish farms are increasingly common and contribute to the already large agricultural base. The ecoregion covers .

Protected areas include Choctaw Island Wildlife Management Area (WMA), Lake Chicot State Park (SP), and Wapanocca National Wildlife Refuge (NWR) in Arkansas, Tensas River National Wildlife Refuge in Louisiana, Big Oak Tree SP in Missouri, Chickasaw NWR, Lake Isom NWR, T.O. Fuller SP, and Reelfoot Lake SP in Tennessee, and Reelfoot NWR in Tennessee and Kentucky. In Mississippi, the ecoregion is protected in the following areas: Dahomey NWR, Delta National Forest and Hillside NWR, Leroy Percy SP, Mathews Brake NWR, Morgan Brake NWR, Panther Swamp NWR, and the Yazoo NWR.

The area also includes four culturally important sites: Fort Defiance in Illinois, Columbus-Belmont State Park in Kentucky, Winterville Mounds in Mississippi and Towosahgy State Historic Site in Missouri.

Northern Pleistocene Valley Trains (73b)

The Northern Pleistocene Valley Trains ecoregion is a flat to irregular alluvial plain composed of sandy to gravelly glacial outwash overlain by alluvium; sand sheets, widespread in the St. Francis Lowlands (73c), are absent. The Pleistocene outwash deposits of Ecoregion 73b are usually coarser and better drained than the alluvial deposits of Ecoregions 73a, 73d, and 73f. They were transported to Arkansas by the Mississippi River and its tributaries and have been subsequently eroded, reduced in size, and fragmented by laterally migrating channels or buried by thick sediments. Ecoregion 73b has little local relief or stream incision. Elevations tend to be slightly higher than adjacent parts of Ecoregions 73a and 73d. Cropland is extensive and has largely replaced the original forests; soybeans are the main crop and cotton is also produced. The few remaining forests are dominated by species typical of higher bottomlands such as Nuttall oak, willow oak, swamp chestnut oak, sugarberry, and green ash. There are more lowland oaks in Ecoregion 73b than in Ecoregions 73a and 73d. The ecoregion covers within Arkansas, Mississippi, and Tennessee, with 78% in Mississippi, and the remainder roughly split between the remaining two states.

Protected areas within the region include the Tallahatchie NWR in Mississippi.

St. Francis Lowlands (73c)

The St. Francis Lowlands ecoregion is flat to irregular and has many relict channels. Ecoregion 73c is mainly composed of late-Wisconsinan age glacial outwash deposits and, in contrast to Ecoregion 73b, is partly covered by undulating sand sheets. Sand blows and sunk lands occur and have been attributed to the 1811–12 New Madrid earthquakes. Loess, which veneers older outwash deposits in Ecoregion 73g, is absent. Topography, lithology, and hydrology vary over short distances and natural vegetation varies with site characteristics. Cropland is extensive and has largely replaced the original forests; soybeans, corn, and cotton are the most common crops but wheat, sorghum, and rice are also produced. Although the streams of the St. Francis Lowlands have been extensively channelized, water quality tends to be better than in the less channelized areas of Ecoregion 73g because of a lack of loess veneer in Ecoregion 73c. The ecoregion covers within Arkansas and Missouri, with 64% in Missouri.

The lands immediately adjacent to the St. Francis River are preserved in the St. Francis Sunken Lands WMA in Arkansas. The Big Lake National Wildlife Refuge preserves flat flooded swamplands created by the New Madrid earthquakes.

Northern Backswamps (73d)

The Northern Backswamps ecoregion is made up of low-lying overflow areas on floodplains, and includes poorly drained flats and swales. Water often collects in its marshes, swamps, oxbow lakes, ponds, and low gradient streams. Soils developed from clayey alluvium including overbank and slack-water deposits; they commonly have a high shrink-swell potential and are locally rich in organic material. Water levels are seasonally variable. Native vegetation in the wettest areas is generally dominated by bald cypress–water tupelo forest; slightly higher and better drained sites have overcup oak–water hickory forest and the highest, best-drained areas support Nuttall oak forest. Today, bottomland forest, cropland, farmed wetlands, pastureland, and catfish farms occur. Backswamps are important areas for capturing excess nutrients from local waters and for storing water during heavy rain events. The ecoregion covers within Arkansas, Mississippi, and Louisiana, with 54% in Mississippi, 35% in Louisiana, and the balance in Arkansas.

Grand Prairie (73e)

The Grand Prairie ecoregion is a broad, loess-covered terrace formerly dominated by tall grass prairie and now primarily used as cropland. It is typically almost level. However, incised perennial and intermittent streams occur and a narrow belt of low hills is found in the east. Prior to the 19th century, flatter areas with slowly to very slowly permeable soils (often containing fragipans) supported Arkansas's largest prairie. They were generally bounded by open woodland or savanna. In all, about 400,000 acres of prairie grasses and forbs occurred in Ecoregion 73e, and were a sharp contrast to the bottomland forests that once dominated other parts of the Mississippi Alluvial Plain (73). Low hills were covered by upland deciduous forest containing white oak, black oak and southern red oak. Drier ridges were dominated by post oak. Narrow floodplains had bottomland hardwood forests. Cropland has now largely replaced the native vegetation. In the process, some prairie species have been extirpated from the ecoregion (e.g., greater prairie chicken); others have been sharply reduced in population and restricted to a few prairie remnants. Distinctively, rice is the main crop; soybeans, cotton, corn, and wheat are also grown. Rice fields provide habitat and forage for large numbers and many species of waterfowl; duck and goose hunting occurs. The ecoregion covers , entirely within Arkansas.

Western Lowlands Holocene Meander Belts (73f)