131A—Southern Mississippi River Alluvium
This MLRA (shown in red in the figure above) is in Louisiana (32 percent), Arkansas (26 percent), Mississippi (26 percent), Missouri (12 percent), Tennessee (3 percent), and Kentucky (1 percent). A small part of Illinois also is in the area. This MLRA makes up about 29,555 square miles (76,585 square kilometers). It includes the towns or cities of Lake Providence, Morgan City, and Houma, Louisiana; Greenville, Yazoo City, and Clarksville, Mississippi; Eudora, Helena, and West Memphis, Arkansas; Caruthersville, Kennett, and Sikeston, Missouri; and the west edge of Memphis, Tennessee. The cities of Baton Rouge and New Orleans, Louisiana, are just outside this area. From north to south, Interstates 57, 55, 40, 20, and 10 cross this area. The Delta National Forest is in the part of this area in Mississippi.
Numerous national wildlife refuges and State parks are throughout this area. Eaker Air Force Base and a small portion of the St. Francis National Forest is in the part of the area in Arkansas. This area is along a major flightpath for migratory waterfowl.
This area makes up most of the Mississippi Alluvial Plain Section of the Coastal Plain Province of the Atlantic Plain. It is on the alluvial plain along the lower Mississippi River, south of its confluence with the Ohio River. The landforms in the area are level or depressional to very gently undulating alluvial plains, backswamps, oxbows, natural levees, and terraces. The parts of the MLRA south of Baton Rouge, Louisiana, are on a deltaic plain. Landform shapes range from convex on natural levees and undulating terraces to concave in oxbows. These shapes differentiate water-shedding positions from water-receiving positions, both of which have a major role in soil formation and hydrology. Average elevations start at sea level in the southern part of the area and gradually rise to about 330 feet (100 meters) in the northwestern part. Maximum local relief is about 15 feet (5 meters), but relief is considerably lower in most of the area.
The extent of the major Hydrologic Unit Areas (identified by four-digit numbers) that make up this MLRA is as follows: Lower Mississippi-St. Francis (0802), 30 percent; Lower Mississippi-Yazoo (0803), 25 percent; Louisiana Coastal (0808), 8 percent; Boeuf-Tensas (0805), 7 percent; Lower Mississippi-Lake Maurepas (0807), 6 percent; Lower Mississippi (0809), 6 percent; Upper White (1101), 6 percent; Lower Red-Ouachita (0804), 5 percent; Lower Mississippi-Hatchie (0801), 5 percent; and Lower Mississippi-Big Black (0806), 2 percent. The lower Mississippi River and its tributaries drain nearly all of the MLRA, but the Atchafalaya River drains the extreme southwest part.
Bedrock in this area consists of Tertiary and Cretaceous sands formed as beach deposits during the retreat of the Cretaceous ocean from the midsection of the U.S. Alluvial deposits from flooding and lateral migration of the Mississippi River typically lie above the bedrock. These sediments are sandy to clayey fluvial deposits of Quaternary age and are many meters thick. The Yazoo, Tensas, and Atchafalaya Basins and the modern deltaic plain are in areas of Holocene deposits. The St. Francis Basin, in the northwestern part of the MLRA, and some surfaces surrounded by the Yazoo Basin, in the central part of the MLRA, are in areas of Wisconsin Stage deposits of Pleistocene age. Some small areas in the western part of the MLRA are covered by a thin mantle of pre-Wisconsin, Quaternary-age loess deposits.
The average annual precipitation in most of this area is 46 to 60 inches (1,170 to 1,525 millimeters). It can be as high as 65 inches (1,650 millimeters) in parts of the southern third of the MLRA. Most of the rainfall occurs as frontal storms during late fall, winter, and early spring, although an appreciable amount of precipitation also occurs as convective thunderstorms during the early part of the growing season. Hurricanes also can produce high amounts of rainfall. The total amount of the precipitation that occurs as snow ranges from less than 1 percent in the southern part of the MLRA to 28 percent in the northern part. The average annual temperature ranges from 56 to 69 degrees F (14 to 21 degrees C), increasing from north to south. The freeze-free period averages 285 days. It ranges from 210 days in the northern part of the area to 355 days in the southern part.
Following are the estimated withdrawals of freshwater by use in this MLRA:
Public supply—surface water, 1.3%; ground water, 1.9%
The total withdrawals average 7,965 million gallons per day (30,150 million liters per day). This is the sixth highest amount of water among all of the MLRAs. About 58 percent is from ground water sources, and 42 percent is from surface water sources. In most years the supply of moisture is adequate for maximum crop production. Surface water for public supply, industrial use, and some irrigation is available in quantity from the bayous, oxbow lakes, canals, and rivers throughout this area. The dominant use of the surface water in the area is for cooling thermoelectric power plants. Farms and small communities use treated surface or ground water for most purposes, except for irrigation. Numerous small, above-ground water impoundments are used for raising commercial catfish throughout the area. Most of the surface water is of good quality and is suitable for most uses with some treatment. High concentrations of suspended sediments, agricultural chemicals, and municipal and industrial wastewater discharges contribute to some local water-quality problems. Flooding is a major concern in most of the area.
The principal sources of ground water in this area are sandy and loamy materials in the Mississippi River alluvial deposits. For example, 74 percent of all the ground water used in Mississippi and almost all the irrigation water used in the “boot heel” area of Missouri are pumped from alluvial aquifers. Impermeable or very slowly permeable, smectitic clay layers many meters thick overlie these aquifers in many parts of the MLRA. Water moves through the clays via large desiccation cracks that open during dry periods and swell closed during wet periods. The ground water is used primarily for domestic purposes and irrigation, but it also is used for public supply and industry. It typically has levels of total dissolved solids that are less than the national secondary drinking water standard of 500 parts per million (milligrams per liter). At the extreme southern end of the area, in Louisiana, however, intrusion of seawater has raised the level of total dissolved solids enough that this water is not suitable for drinking or industrial use. Calcium, manganese, sodium, sulfate, and bicarbonate are the major ions in the ground water. Water in the river alluvium is generally hard or very hard. The iron content is extremely high in Arkansas but generally is not a significant problem in other parts of the area. Where the ground water in the alluvial aquifer is of poor quality, rural landowners obtain better quality drinking water from Tertiary and Cretaceous sands below the river alluvium.
The dominant soil orders in this MLRA are Alfisols, Vertisols, Inceptisols, and Entisols. The soil temperature regime is thermic in most of the MLRA. It is hyperthermic, however, south of Baton Rouge, Louisiana. The soils in the MLRA dominantly have an aquic soil moisture regime, smectitic clay mineralogy, and mixed sand and silt fraction mineralogy. The soils are very deep, dominantly poorly drained and somewhat poorly drained, and dominantly loamy or clayey. Nearly level Epiaquerts (Sharkey series), Vertic Epiaquepts (Tunica series), and Vertic Endoaquepts (Dowling series) dominate the alluvial flats and backswamps of Holocene to late Pleistocene age. Nearly level to gently sloping Endoaquepts (Commerce series), Udifluvents (Robinsonville series), and Fluvaquents (Convent series) dominate the natural levees of Holocene age. Nearly level to gently undulating, sandy Udifluvents (Bruno series) and Udipsamments (Crevasse series) dominate the levee splays and point bars of Holocene age. Nearly level to gently undulating Endoaqualfs (Dundee series), Hapludalfs (Dubbs series), and Epiaqualfs (Tensas series) dominate the terraces of Pleistocene age.
This area once consisted entirely of bottom-land hardwood deciduous forests and mixed hardwood and cypress swamps. The major tree species in the native plant communities in the areas of bottom-land hardwoods formerly were and currently are water oak, Nuttall oak, cherrybark oak, native pecan, red maple, sweetgum, eastern cottonwood, and hickory. The major tree species in the native plant communities in the swamps formerly were and currently are cypress, water tupelo, water oak, green ash, red maple, and black willow. The important native understory species are palmetto, greenbrier, wild grape, and poison ivy in the areas of bottom-land hardwoods and buttonbush, lizardtail, waterlily, water hyacinth, sedges, and rushes in the swamps.
Some of the major wildlife species in this area are white-tailed deer, feral hogs, red fox, coyote, rabbit, gray squirrel, American alligator, water turtles, water snakes, frogs, otters, beavers, armadillo, crawfish, wild turkey, mourning doves, ducks, and geese. Fishing is mainly in oxbow lakes, rivers, and bayous. The species of fish in the area include largemouth bass, smallmouth bass, catfish, drum, bluegill, gar, and yellow perch. Crawdads are a commercial species in the southern end of this MLRA.
Following are the various kinds of land use in this MLRA:
Most of this area is in farms, which produce mainly cash crops. Cotton, soybeans, milo, and corn are the main crops, and sugarcane is a major crop in the southernmost part of the area. Furrow irrigation is used in many areas during droughty parts of the growing season. Rice is grown in some land-leveled, flood-irrigated areas. Catfish and crawfish are produced commercially on farm ponds that are contained by levees. The catfish are produced throughout the MLRA, and the crawfish are produced in the southern part of the area. Migratory waterfowl are harvested throughout the area. Hardwood timber is harvested on most forested wetlands, and most of the forested areas are managed for wildlife. About 29 percent of this MLRA is not protected from flooding, and flooding occurs occasionally or frequently in these unprotected areas. Levees protect nearly all of the cropland, urban land, and grassland from flooding. Most areas of forested wetlands are not protected from flooding. Networks of drainage canals and ditches help to remove excess surface water from the cropland.
The major resource concerns are control of surface water, management of soil moisture, and maintenance of the content of organic matter and productivity of the soils. Conservation practices on cropland generally include nutrient management, crop residue management, and alternative tillage systems, especially no-till systems that reduce the cost of tillage. In many areas land leveling or shaping optimizes the control of surface water. Other major cropland management practices are control of competing vegetation and insects through aerial or ground spraying and fertility management programs that make use of chemical fertilizers.