131B—Arkansas River Alluvium

This MLRA (shown in red in the figure above) is in Arkansas (67 percent) and Louisiana (33 percent). It makes up about 3,955 square miles (10,245 square kilometers). The towns of Montrose, Dumas, and England, Arkansas, and Monroe, Louisiana, are in this MLRA. Interstate 20 passes through Monroe, Louisiana. Most parts of the Overflow National Wildlife Refuge, the Upper Ouachita National Wildlife Refuge, and the D’Arbonne National Wildlife Area are in this MLRA. The area is along a major flightpath of migratory waterfowl.

Physiography

This area is in the Mississippi Alluvial Plain Section of the Coastal Plain Province of the Atlantic Plain. It is on the alluvial plains along the lower Arkansas River in Arkansas and the Ouachita River in Louisiana and Arkansas. The landforms in the area are level or depressional to very gently undulating alluvial plains, backswamps, oxbows, natural levees, and terraces. Landform shapes range from convex on natural levees and undulating terraces to concave in oxbows. Landform shapes differentiate water-shedding positions from water-receiving positions, both of which affect soil formation and hydrology. Average elevations start at about 50 feet (15 meters) in the southern part of the area and gradually rise to about 250 feet (75 meters) in the northwestern part. Maximum local relief is about 10 feet (3 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: Boeuf-Tensas (0805), 48 percent; Lower Mississippi-St. Francis (0802), 26 percent; Lower Red-Ouachita (0804), 20 percent; and Lower Arkansas (1111), 6 percent. The lower reaches of the Arkansas River, the Ouachita River, and the tributaries of both these rivers drain the entire MLRA. A short reach of Bayou Bartholomew, a National Wild and Scenic River, is in the southern part of the area.

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 Arkansas and Ouachita Rivers typically lie above the bedrock. These sediments are sandy to clayey fluvial deposits of Holocene to late Pleistocene age and are many meters thick. The geologic surfaces are identified as the Arkansas Lowlands, which extend from the Yazoo Basin up the Arkansas River to the margin of the Coastal Plain, and the parts of the Tensas Basin west of Macon Ridge. The deposits on both of these surfaces are of Holocene age. In some areas late Pleistocene terrace deposits are within several meters of the present surfaces, but they do not crop out in the MLRA.

Climate

The average annual precipitation ranges from 49 to 58 inches (1,245 to 1,475 millimeters), increasing from north to south. 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. The total amount of the precipitation that occurs as snow ranges from less than 1 percent in the southern part of the MLRA to 5 percent in the northern part. The average annual temperature is 62 to 65 degrees F (16 to 18 degrees C). The freeze-free period averages 260 days and ranges from 250 to 275 days. It increases in length from north to south.

Water

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

Public supply—surface water, 2.1%; ground water, 1.2%
Livestock—surface water, 0.8%; ground water, 0.5%
Irrigation—surface water, 10.8%; ground water, 50.9%
Other—surface water, 31.7%; ground water, 1.9%

The total withdrawals average 1,570 million gallons per day (5,940 million liters per day). About 55 percent is from ground water sources, and 45 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 bayous, oxbow lakes, canals, and rivers throughout this area. The dominant use of surface water in the area is for cooling thermoelectric power plants. Numerous small, above-ground water impoundments are used for raising commercial catfish throughout the MLRA. 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 within the Arkansas and Mississippi River alluvial deposits. The ground water is used primarily for irrigation, but it also is used for public supply and industry. The median level of total dissolved solids is 330 parts per million (milligrams per liter), and the water is very hard. The iron content is extremely high in the part of this area in Arkansas but generally is not a significant problem in the part in Louisiana. The level of total dissolved solids can reach 4,000 parts per million (micrograms per liter) in the southeast corner of the part of the area in Arkansas, making the water from the alluvial aquifer unusable. The Sparta and Cockfield bedrock aquifers are used in the northern tip of this area. The ground water in these aquifers is lower in salts than the water in the alluvial aquifer, and it is soft. The iron content 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 larger communities and for industry. More and more irrigation wells are being developed in the Sparta aquifer.

Soils

The dominant soil orders in this MLRA are Vertisols, Alfisols, Inceptisols, and Entisols. The soils in the area have a thermic soil temperature regime. They dominantly have an aquic soil moisture regime, smectitic clay mineralogy, and mixed sand and silt fraction mineralogy. They are very deep and generally are poorly drained to well drained and loamy or clayey. Nearly level Epiaquerts (Perry series), Vertic Hapludolls (Desha series), and Vertic Epiaquepts (Portland series) dominate the Holocene-age alluvial flats and backswamps. Nearly level to gently sloping Eutrudepts (Coushatta series), Udifluvents (Roxana series), and Vertic Epiaquepts (Latanier series) dominate the recent Holocene-age natural levees. Nearly level to gently undulating, sandy Udifluvents (Bruno series) and Udipsamments (Crevasse series) dominate the recent Holocene-age levee splays and point bars. Nearly level to gently undulating Epiaqualfs (Hebert series), Hapludalfs (Rilla and Sterlington series), and Argiudolls (Caspiana series) dominate the Holocene-age natural levees along the older meander scars.

Biological Resources

This area once consisted entirely of bottom-land hardwood deciduous forest 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.

Land Use

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

Cropland—private, 70%
Grassland—private, 2%
Forest—private, 22%; Federal, 1%
Urban development—private, 1%
Water—private, 3%
Other—private, 1%

Farms and scattered tracts of forested wetlands make up nearly all of this area. The farms produce mainly cash crops. Cotton, soybeans, milo, and corn are the main crops. In many areas furrow irrigation is used during droughty parts of the growing season. Throughout the area, catfish are produced commercially on farm ponds that are contained by levees. Migratory waterfowl are harvested throughout the area. Hardwood timber is harvested on some forested wetlands, and most forested areas are managed for wildlife.

About 15 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 from flooding. Most of the 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. 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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