131D—Southern Mississippi River Terraces

This MLRA (shown in red in the figure above) is in Arkansas (88 percent) and Louisiana (12 percent). It is in two separate areas that together make up about 2,945 square miles (7,635 square kilometers). The towns of Lonoke and Stuttgart, Arkansas, are in the northeastern part of the MLRA, which is called the Grand Prairie. Bastrop, Louisiana, is in the southwestern part of the MLRA. Interstate 40 crosses the northern tip of the Grand Prairie area. The Bayou Meto Wildlife Preserve and the White River and Overflow National Wildlife Refuges are in this MLRA. The MLRA is along a major flightpath of migratory waterfowl.

Physiography

This MLRA is in the Mississippi Alluvial Plain Section of the Coastal Plain Province of the Atlantic Plain. It consists dominantly of Pleistocene-age, level to gently sloping terraces along the Mississippi River. Slopes generally range from level to gently sloping but are steep along terrace escarpments. Channel scars are evident in some areas. Elevation is generally 50 to 250 feet (15 to 75 meters) on the terraces.

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), 56 percent; Lower Red-Ouachita (0804), 41 percent; and Boeuf-Tensas (805), 3 percent. Bayou Bartholomew and LaGrue Bayou are major tributaries in this area. La Grue Bayou drains the Grand Prairie area. Bayou Bartholomew, in the southwestern part of the MLRA, has been designated a National Wild and Scenic River.

Geology

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 rivers crossing this area typically lie above the bedrock. These sediments form Pleistocene-age alluvial terraces. Silty alluvium underlies most of the area. Clayey sediments are in old channel scars. The Pleistocene terraces are part of the Prairie Terrace complex. A minor portion of the area is in the Deweyville and Montgomery terrace formation. These terraces have a base of red alluvium capped by one to several meters of brownish alluvium.

Climate

The average annual precipitation in this area is 49 to 56 inches (1,245 to 1,420 millimeters), increasing from north to south. Most of the rainfall occurs as frontal storms in spring and early summer. Some high-intensity, convective thunderstorms occur in summer. The average seasonal snowfall is 5 inches (12 centimeters). The average annual temperature is 61 to 65 degrees F (16 to 18 degrees C), increasing from north to south. The freeze-free period averages 255 days. It ranges from 245 days in the northern part of the area to 270 days in the southern part.

Water

Following are the estimated withdrawals of freshwater by use in this MLRA:

Public supply—surface water, 1.4%; ground water, 1.2%
Livestock—surface water, 0.9%; ground water, 1.4%
Irrigation—surface water, 11.5%; ground water, 67.4%
Other—surface water, 15.0%; ground water, 1.1%

The total withdrawals average 750 million gallons per day (2,840 million liters per day). About 71 percent is from ground water sources, and 29 percent is from surface water sources. In most years the supply of moisture is not adequate for maximum crop production because of the distribution of rainfall during the growing season. Some surface water for industrial use and some limited irrigation is available from bayous, canals, and rivers throughout the area. Most of the surface water is of good quality and is suitable for most uses with some treatment. Some farms and small communities depend on treated surface water. High concentrations of suspended sediments, agricultural chemicals, and municipal and industrial wastewater discharges contribute to some local water-quality problems.

The principal sources of ground water in this area are sandy and silty materials within the alluvial terraces and the Cockfield and Sparta bedrock aquifers. The Sparta aquifer is a sole-source aquifer for most of the communities in the area. The ground water is used primarily for irrigation, but it is also used for domestic purposes and for public supply and industry. The median level of total dissolved solids in the alluvial aquifer is 330 parts per million (milligrams per liter), and the water in this aquifer is very hard. The iron content is generally high in the part of this MLRA in Arkansas, but it generally is not a significant problem in the part in Louisiana. The water in the Sparta and Cockfield bedrock aquifers typically has a lower content of salts than the water in the alluvial aquifer, and it is soft. The content of iron generally is below the national secondary standard for drinking water of 300 parts per billion (micrograms per million). These aquifers provide drinking water for rural landowners and small communities. They also provide water for public supply in the larger communities. More and more irrigation wells are being developed in the Sparta aquifer.

The aquifers in this area are recharged by the Arkansas and White Rivers. Pumping has exceeded the amount of recharge, so water levels have been dropping almost 1 foot per year. The historical decline in the elevation of the water table has prompted a major federally funded water resources project to be undertaken in the Grand Prairie part of this area. The focus of the project is on improving irrigation water management. The U.S. Army Corps of Engineers, in cooperation with the Natural Resources Conservation Service, other Federal agencies, and State agencies, is assisting farmers in the construction of canals, pipelines, and on-farm reservoirs for irrigation tailwater reclamation and reuse.

Soils

The dominant soils in this MLRA are Alfisols. They have a thermic soil temperature regime, an ustic or aquic soil moisture regime, and mixed mineralogy. They are very deep and formed dominantly in silty alluvium. They generally are moderately well drained to poorly drained. Gently sloping Hapludalfs (Goodwill series) are on natural levees and low terraces. Gently sloping to level Hapludalfs (Immanuel and Stuttgart series) and Fraglossudalfs (Grenada series) are on broad interfluves and along terrace escarpments. Level Endoaqualfs (Idee series), Albaqualfs (Dewitt series), Glossaqualfs (Ethel series), Epiaqualfs (Lagrue), and Fragiaqualfs (Henry series) are on low terraces and natural levees. Nearly level and level Endoaqualfs (Tichnor and Forestdale series) are on low terraces, natural levees, and flood plains. Nearly level Dystrudepts (Oaklimeter series) are along drainageways and on flood plains.

Biological Resources

This area supports hardwoods and pines. The Grand Prairie area, in Arkansas, originally supported tall prairie grasses interlaced with hardwood timber. Cherrybark and Shumard oak are widely distributed. Yellow-poplar, white ash, cottonwood, and black walnut are important species on the flood plains. Loblolly pine and shortleaf pine are on a wide variety of sites, mainly the eroded soils on uplands and ridges. Other hardwood species that commonly grow in this area are white oak, basswood, sweetgum, water oak, American elm, blackgum, sycamore, sassafras, southern red oak, chinkapin oak, American beech, and hickory.

Some of the major wildlife species in this area are white-tailed deer, coyote, bobcat, beaver, raccoon, skunk, armadillo, mink, cottontail, turkey, mourning dove, ducks, and geese. The species of fish in the area include channel catfish, largemouth black bass, crappie, and bluegill.

Land Use

Following are the various kinds of land use in this MLRA:

Cropland—private, 42%
Grassland—private, 4%
Forest—private, 46%; Federal, 1%
Urban development—private, 3%
Water—private, 3%
Other—private, 1%

Scattered tracts of forests and farms make up nearly all of this area. Rice, soybeans, and wheat are the main crops. In most areas furrow or flood irrigation is used throughout the growing season. Hardwood timber is harvested on some forested wetlands, and most forested areas are managed for wildlife. Bait fish are produced commercially in ponds that are contained by levees. Migratory waterfowl are harvested throughout the area.

The major soil resource concerns are management of soil moisture, erosion control, and maintenance of the content of organic matter and productivity of the soils. Depletion of ground water through excessive pumping is a major concern in the Grand Prairie area.

Conservation practices on cropland generally include nutrient management, crop residue management, and alternative tillage systems, especially no-till systems that reduce the need for 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 of herbicides and insecticides and fertility management programs that make use of chemical fertilizers.

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