The Mississippi Alluvial Plain is a
Level III ecoregion designated by the
Environmental Protection Agency
A biophysical environment is a biotic and abiotic surrounding of an organism or population, and consequently includes the factors that have an influence in their survival, development, and evolution. A biophysical environment can vary in scale f ...
(EPA) in seven
U.S. states
In the United States, a state is a constituent political entity, of which there are 50. Bound together in a political union, each state holds governmental jurisdiction over a separate and defined geographic territory where it shares its sove ...
, though predominantly in
Arkansas
Arkansas ( ) is a landlocked state in the South Central United States. It is bordered by Missouri to the north, Tennessee and Mississippi to the east, Louisiana to the south, and Texas and Oklahoma to the west. Its name is from the Osage ...
,
Louisiana
Louisiana , group=pronunciation (French: ''La Louisiane'') is a state in the Deep South and South Central regions of the United States. It is the 20th-smallest by area and the 25th most populous of the 50 U.S. states. Louisiana is borde ...
, and
Mississippi
Mississippi () is a state in the Southeastern region of the United States, bordered to the north by Tennessee; to the east by Alabama; to the south by the Gulf of Mexico; to the southwest by Louisiana; and to the northwest by Arkansas. Miss ...
. It parallels the
Mississippi River
The Mississippi River is the second-longest river and chief river of the second-largest drainage system in North America, second only to the Hudson Bay drainage system. From its traditional source of Lake Itasca in northern Minnesota, it f ...
from the
Midwestern United States
The Midwestern United States, also referred to as the Midwest or the American Midwest, is one of four census regions of the United States Census Bureau (also known as "Region 2"). It occupies the northern central part of the United States. I ...
to the
Gulf of Mexico
The Gulf of Mexico ( es, Golfo de México) is an oceanic basin, ocean basin and a marginal sea of the Atlantic Ocean, largely surrounded by the North American continent. It is bounded on the northeast, north and northwest by the Gulf Coast of ...
.
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
The Ohio River is a long river in the United States. It is located at the boundary of the Midwestern and Southern United States, flowing southwesterly from western Pennsylvania to its mouth on the Mississippi River at the southern tip of Illino ...
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
An alluvial plain is a largely flat landform created by the deposition of sediment over a long period of time by one or more rivers coming from highland regions, from which alluvial soil forms. A floodplain is part of the process, being the sma ...
. It is veneered by
Quaternary
The Quaternary ( ) is the current and most recent of the three periods of the Cenozoic Era in the geologic time scale of the International Commission on Stratigraphy (ICS). It follows the Neogene Period and spans from 2.58 million years ...
alluvium,
loess
Loess (, ; from german: Löss ) is a clastic, predominantly silt-sized sediment that is formed by the accumulation of wind-blown dust. Ten percent of Earth's land area is covered by loess or similar deposits.
Loess is a periglacial or aeolian ...
,
glacial outwas
A glacial period (alternatively glacial or glaciation) is an interval of time (thousands of years) within an ice age that is marked by colder temperatures and glacier advances. Interglacials, on the other hand, are periods of warmer climate betwe ...
h, and
lacustrine
A lake is an area filled with water, localized in a basin, surrounded by land, and distinct from any river or other outlet that serves to feed or drain the lake. Lakes lie on land and are not part of the ocean, although, like the much larger ...
deposits. River terraces,
Swale
Swale or Swales may refer to:
Topography
* Swale (landform), a low tract of land
** Bioswale, landform designed to remove silt and pollution
** Swales, found in the formation of Hummocky cross-stratification
Geography
* River Swale, in North ...
s, and
levee
A levee (), dike (American English), dyke (English in the Commonwealth of Nations, Commonwealth English), embankment, floodbank, or stop bank is a structure that is usually soil, earthen and that often runs parallel (geometry), parallel to ...
s provide limited relief, but overall, it is flatter than neighboring ecoregions in Arkansas, including the
South Central Plains
The Piney Woods is a temperate coniferous forest terrestrial ecoregion in the Southern United States covering of East Texas, southern Arkansas, western Louisiana, and southeastern Oklahoma. These coniferous forests are dominated by several spec ...
. 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
wetland
A wetland is a distinct ecosystem that is flooded or saturated by water, either permanently (for years or decades) or seasonally (for weeks or months). Flooding results in oxygen-free (anoxic) processes prevailing, especially in the soils. The ...
s in North America. It is also a major bird migration corridor used in fall and spring migrations, known as the
Mississippi Flyway
The Mississippi Flyway is a bird migration route that generally follows the Mississippi, Missouri, and Lower Ohio Rivers in the United States across the western Great Lakes to the Mackenzie River and Hudson Bay in Canada. The main endpoints of t ...
. 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
Aquaculture (less commonly spelled aquiculture), also known as aquafarming, is the controlled cultivation ("farming") of aquatic organisms such as fish, crustaceans, mollusks, algae and other organisms of value such as aquatic plants (e.g. lot ...
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 dissolved solids (TDS) is a measure of the dissolved combined content of all inorganic and organic substances present in a liquid in molecular, ionized, or micro-granular ( colloidal sol) suspended form. TDS concentrations are often repor ...
, total phosphorus, ammonia nitrogen, sulfates, turbidity,
biological oxygen demand
Biochemical oxygen demand (BOD) is the amount of dissolved oxygen (DO) needed (i.e. demanded) by aerobic biological organisms to break down organic material present in a given water sample at a certain temperature over a specific time period. T ...
,
chlorophyll a, and
fecal coliform
A fecal coliform (British: faecal coliform) is a facultatively anaerobic, rod-shaped, gram-negative, non-sporulating bacterium. Coliform bacteria generally originate in the intestines of warm-blooded animals. Fecal coliforms are capable of growth ...
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. 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
The pallid sturgeon (''Scaphirhynchus albus'') is an endangered species of ray-finned fish, endemic to the waters of the Missouri and lower Mississippi river basins of the United States. It may have even reached the St. Croix River before co ...
) as well as river-front plant communities; they are too narrow to map as a separate Level IV ecoregion. 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.
Level IV ecoregions
Northern Holocene Meander Belts (73a)
![Reelfoot Lake State Park Cypress Boardwalk Obion County TN 2013-04-06 030](https://upload.wikimedia.org/wikipedia/commons/4/4a/Reelfoot_Lake_State_Park_Cypress_Boardwalk_Obion_County_TN_2013-04-06_030.jpg)
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
''Celtis laevigata'' is a medium-sized tree native to North America. Common names include sugarberry, Southern hackberry, or in the southern U.S. sugar hackberry or just hackberry.
Sugarberry is easily confused with common hackberry ('' C. occ ...
, elm, ash, pecan,
cottonwood, and
sycamore
Sycamore is a name which has been applied to several types of trees, but with somewhat similar leaf forms. The name derives from the ancient Greek ' (''sūkomoros'') meaning "fig-mulberry".
Species of trees known as sycamore:
* ''Acer pseudoplata ...
than Ecoregion 73d. Widespread draining of wetlands and removal of bottomland forests for cropland has occurred. Soybeans,
cotton
Cotton is a soft, fluffy staple fiber that grows in a boll, or protective case, around the seeds of the cotton plants of the genus ''Gossypium'' in the mallow family Malvaceae. The fiber is almost pure cellulose, and can contain minor perce ...
, 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
The Choctaw (in the Choctaw language, Chahta) are a Native American people originally based in the Southeastern Woodlands, in what is now Alabama and Mississippi. Their Choctaw language is a Western Muskogean language. Today, Choctaw people are ...
(WMA),
Lake Chicot State Park
Lake Chicot State Park is a Arkansas state park in Chicot County, Arkansas within the Arkansas delta. The park is located along Lake Chicot, a oxbow lake that was formerly the main channel of the Mississippi River. The lake, formerly a pollu ...
(SP), and
Wapanocca National Wildlife Refuge
The Wapanocca National Wildlife Refuge is a 5,484 acre (22 km2) wildlife refuge in Crittenden County, Arkansas, managed by the United States Fish and Wildlife Service.
Wapanocca National Wildlife Refuge was established in 1961 from land acqu ...
(NWR) in Arkansas,
Tensas River National Wildlife Refuge
The Tensas River National Wildlife Refuge is a protected wildlife area located west of the city of Tallulah, Louisiana, Tallulah in Madison Parish, Louisiana, Madison, Tensas Parish, Louisiana, Tensas and Franklin Parish, Louisiana, Franklin parish ...
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
Columbus-Belmont State Park, on the shores of the Mississippi River in Hickman County, near Columbus, Kentucky, is the site of a Confederate fortification built during the American Civil War. The site was considered by both North and South to ...
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)
![St-Francis-River-ar](https://upload.wikimedia.org/wikipedia/commons/1/16/St-Francis-River-ar.jpg)
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
''Sorghum'' () is a genus of about 25 species of flowering plants in the grass family (Poaceae). Some of these species are grown as cereals for human consumption and some in pastures for animals. One species is grown for grain, while many othe ...
, 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
''Quercus lyrata'', the overcup oak, is an oak in the white oak group (''Quercus'' sect. ''Quercus''). The common name, overcup oak, refers to its acorns that are mostly enclosed within the acorn cup. It is native to lowland wetlands in the east ...
–
water hickory
''Carya aquatica'', the bitter pecan or water hickory, is a large tree, that can grow over tall of the Juglandaceae or walnut family. In the American South it is a dominant plant species found on clay flats and backwater areas near streams and ...
forest and the highest, best-drained areas support
Nuttall oak
''Quercus texana'', commonly known as Nuttall's oak, is a fast-growing, large deciduous oak tree.
It is a tree growing up to 25 meters (83 feet) tall, with dark brown bark. It has leaves with sharp pointed lobes somewhat similar to those of t ...
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)
The Western Lowlands Holocene Meander Belts ecoregion is a flat to nearly flat floodplain containing the meander belts of the present and past courses of the
White River,
Black River, and
Cache River. Its meander belts are narrower than the Northern Holocene Meander Belts (73a), but point bars, natural levees, swales, and abandoned channels are common in both regions. Soils on natural levees are relatively coarse-textured, well-drained, and higher than those on levee back slopes and point bars; they grade to heavy, poorly-drained clays in abandoned channels and swales. Natural vegetation varies with site characteristics. Today, Ecoregion 73f contains some of the most extensive remaining tracts of native bottomland hardwood forest in the Mississippi Alluvial Plain. Cropland also occurs. Flood control levees are less developed and riverine processes are more natural and dynamic than in Ecoregion 73a. Backwater flooding in the White River occurs well upstream of its confluence with the higher Mississippi River; as a result, riparian and natural levee communities are less common and oak-dominated communities are more widespread than in Ecoregion 73a. Wetlands in the Cache-lower White River systems have been designated as one of only nineteen “Wetlands of International Importance” in the United States by the
Ramsar Convention on Wetlands
The Ramsar Convention on Wetlands of International Importance Especially as Waterfowl Habitat is an international treaty for the conservation and sustainable use of Ramsar sites (wetlands). It is also known as the Convention on Wetlands. It i ...
. Regulation of White River flow, in combination with the downcutting of the Mississippi River for navigation (and related wing levees and cutoffs), have altered flood regimes on the lower White River, thereby increasing stream bank instability and bottomland forest mortality in Ecoregion 73f. Most streams and rivers in Ecoregion 73f are fed by the Ozark Highlands and Boston Mountains; sediment load is generally less than in the Mississippi River. The ecoregion covers , with 99% within Arkansas.
Western Lowlands Pleistocene Valley Trains (73g)
The terraces of the Western Lowlands Pleistocene Valley Trains are largely composed of Pleistocene glacial outwash that was transported to Arkansas by the Mississippi River and deposited by braided streams. Physiography is widely muted by windblown silt deposits (loess), sand sheets, or sand dunes; loess and sand sheets are more widespread than in the Northern Pleistocene Valley Trains (73b) and St. Francis Lowlands (73c). Many interdunal depressions called “sandponds” occur and are either in contact with the water table or have a perched aquifer. Elevations are higher than adjacent parts of the Northern Holocene Meander Belts (73a) and Western Lowlands Holocene Meander Belts (73f); consequently, uplands are rarely if ever flooded. Native plant communities are different from more frequently inundated ecoregions; for example, post oak and loblolly pine are native to Ecoregion 73g but are absent from lower, overflow areas. Sandpond forest communities are generally dominated by overcup oak, water hickory, willow oak, and pin oak; understory in a few sandponds may include pondberry (Lindera melissifolia), federally listed as endangered. Today, cropland is extensive and the main crops are soybeans and cotton. Commercial crawfish, baitfish, and catfish farms are common. The Western Lowlands Pleistocene Valley Trains (73g) ecoregion is a wintering ground for water fowl. Duck hunting is widespread. The ecoregion covers between Arkansas and Missouri, with 86% in Arkansas.
Arkansas/Ouachita River Holocene Meander Belts (73h)
![Bayou Bartholomew near Pine Bluff, AR](https://upload.wikimedia.org/wikipedia/commons/3/34/Bayou_Bartholomew_near_Pine_Bluff%2C_AR.jpg)
The Arkansas/Ouachita River Holocene Meander Belts ecoregion is a flat to nearly flat floodplain containing the meander belts of the present and past courses of the lower
Arkansas River
The Arkansas River is a major tributary of the Mississippi River. It generally flows to the east and southeast as it traverses the U.S. states of Colorado, Kansas, Oklahoma, and Arkansas. The river's source basin lies in the western United Stat ...
and
Ouachita River
The Ouachita River ( ) is a river that runs south and east through the U.S. states of Arkansas and Louisiana, joining the Tensas River to form the Black River near Jonesville, Louisiana. It is the 25th-longest river in the United States ( ...
s. Point bars, natural levees, swales, and abandoned channels marked by meander scars and oxbow lakes are common and characteristic. Soils on natural levees are more coarse-textured, well-drained, and higher than those on levee back slopes and point bars. Soils grade to poorly-drained clays in abandoned channels and swales. Overall, soils have less organic matter than in the Northern Holocene Meander Belts (73a).
Bayou Bartholomew
Bayou Bartholomew is the longest bayou in the world, meandering approximately between the U.S. states of Arkansas and Louisiana.U.S. Geological Survey. National Hydrography Dataset high-resolution flowline dataThe National Map, accessed June 3, 20 ...
inhabits the longest section of abandoned channels. It flows against the edge of the
South Central Plains
The Piney Woods is a temperate coniferous forest terrestrial ecoregion in the Southern United States covering of East Texas, southern Arkansas, western Louisiana, and southeastern Oklahoma. These coniferous forests are dominated by several spec ...
, receives drainage from it, and has sufficient habitat diversity to be one of the most species-rich streams in North America. Bayou Bartholomew supports over half of all known mussel species found in Louisiana. Within an abandoned course,
bald cypress
''Taxodium distichum'' (bald cypress, swamp cypress; french: cyprès chauve;
''cipre'' in Louisiana) is a deciduous conifer in the family Cupressaceae. It is native to the southeastern United States. Hardy and tough, this tree adapts to a wide r ...
and/or
water tupelo
''Nyssa aquatica'', commonly called the water tupelo, cottongum, wild olive, large tupelo, tupelo-gum, or water-gum, is a large, long-lived tree in the tupelo genus ''(Nyssa)'' that grows in swamps and floodplains in the Southeastern United Sta ...
typically grow in the modern stream channel adjacent to a strip of wet bottomland hardwood forest dominated by
overcup oak
''Quercus lyrata'', the overcup oak, is an oak in the white oak group (''Quercus'' sect. ''Quercus''). The common name, overcup oak, refers to its acorns that are mostly enclosed within the acorn cup. It is native to lowland wetlands in the east ...
and
water hickory
''Carya aquatica'', the bitter pecan or water hickory, is a large tree, that can grow over tall of the Juglandaceae or walnut family. In the American South it is a dominant plant species found on clay flats and backwater areas near streams and ...
. The remainder of the native forest has largely been cleared and drained for cropland and pastureland. Corn, cotton, and soybeans are the main crops.
Arkansas/Ouachita River Backswamps (73i)
The flats, swales, and natural levees of the Arkansas/Ouachita River Backswamps ecoregion include the slackwater areas along the Arkansas and Ouachita rivers, where water often collects into swamps, oxbow lakes, ponds, and sloughs. In contrast to the Northern Backswamps (73d), this region is widely veneered with natural levee deposits. Soils derived from these deposits are Alfisols, Vertisols, and Inceptisols that are generally more loamy and better drained than the clayey soils of the Northern Backswamps (73d). As a result, willow oak and water oak are native instead of other species adapted to wetter overflow conditions. Drainage canals and ditches are common. This artificial drainage and the sandy veneer of natural levee deposits help explain why Ecoregion 73i is more easily and widely farmed than the Northern Backswamps (73d). Soybeans, corn, cotton, and rice are important crops but forests and forested wetlands also occur.
Macon Ridge (73j)
Macon Ridge is underlain almost entirely by Pleistocene glacial outwash deposits that were transported to Arkansas by the Mississippi River and deposited by braided streams. It is veneered by windblown silt deposits (i.e. loess) like Ecoregions 73e, 73g, and 74a. Soils are influenced by loess and contrast with the alluvial soils of Ecoregions 73a and 73h. Macon Ridge (73j) is a continuation of the Western Lowlands Pleistocene Valley Trains (73g) but is better drained, and supports drier plant communities. Its eastern edge is 20 to 30 feet above the adjacent, lithologically and physiographically distinct, Northern Holocene Meander Belts (73a). The western side of Macon Ridge (73j) is lower than the eastern side, and is about the same elevation as the lithologically and physiographically distinct Arkansas/Ouachita River Holocene Meander Belts (73h). Native forest types range from those of better drained bottomlands dominated by willow oak, water oak, and swamp chestnut oak to upland hardwood forests dominated by white oak, southern red oak, and post oak. Prairies and loblolly pinedominated areas may also have occurred on Macon Ridge (73j). Today, Ecoregion 73j is a mosaic of pastureland, forest, and cropland. Soybeans, cotton, and oats are major crops. The ecoregion covers between Arkansas and Louisiana, with 86% within Louisiana.
In Arkansas, most of Macon Ridge is within
Chicot County between
Lake Village and
Eudora.
Southern Holocene Meander Belts (73k)
The Southern Holocene Meander Belts ecoregion stretches from just north of
Natchez, Mississippi
Natchez ( ) is the county seat of and only city in Adams County, Mississippi, United States. Natchez has a total population of 14,520 (as of the 2020 census). Located on the Mississippi River across from Vidalia in Concordia Parish, Louisiana, N ...
south to
. Similar to the Northern Holocene Meander Belts (73a), point bars, oxbows, natural levees, and abandoned channels occur. This region, however, has a longer growing season, warmer annual temperatures, some hyperthermic soils, and more precipitation than its northern counterparts of 73a and 73h. Soils are somewhat poorly and poorly drained Inceptisols, Entisols, and Vertisols. The ecoregion contains minor species such as live oak, laurel oak, and Spanish moss that are generally not found in the more northerly regions. The bottomland forests have been cleared and the region has been extensively modified for agriculture, flood control, and navigation. The levee system is extensive throughout the region. Soybeans, sugarcane, cotton, corn, and pasture are the major crops, with crawfish aquaculture common. The ecoregion covers , almost entirely within Louisiana.
Southern Pleistocene Valley Trains (73l)
The Southern Pleistocene Valley Trains ecoregion is a continuation of the northern valley train regions in Mississippi, Arkansas, and Tennessee. It is composed of scattered small remnants of early-Wisconsin glacial outwash deposits, similar to those of Macon Ridge (73j). This ecoregion, however, has warmer annual temperatures, a longer growing season, and higher annual rainfall. Soils are somewhat poorly and poorly drained Alfisols, Inceptisols, and Vertisols with loamy and clayey surfaces. Some species occur here that are not present in the Macon Ridge (73j) or Western Lowlands Pleistocene Valley Trains (73g) ecoregions to the north in Arkansas. Overcup oak, Nuttall oak, honey locust, elm, water oak, sweetgum, blackgum, and hickory are the most common tree species. This region is generally higher than the adjacent Southern Backswamps (73m) ecoregion and soils are more sandy and better drained than the heavy clay soils of the backswamps. Cropland and pasture is common with corn, soybeans, and cotton as major crops. The ecoregion covers , entirely within Louisiana.
Southern Backswamps (73m)
The Southern Backswamps ecoregion is generally warmer, has a longer frost free period, and has more precipitation than the Northern Backswamps (73d). Similar to 73d, soils are mostly poorly drained, clayey Vertisols, rich in organic matter. Wetlands are common and flooding occurs frequently. Bottomland hardwood forests are more prevalent in this region than in the adjacent Southern Holocene Meander Belts (73k), where cropland is common. Channelization and flood control systems have modified this region and impacted many of the wetland habitats. The ecoregion covers , almost entirely within Louisiana.
Inland Swamps (73n)
The Inland Swamps ecoregion marks a transition, ranging from the fresh waters of the Southern Backswamps (73m) at the northern extent of the intratidal basins to the fresh, brackish, and saline waters of the deltaic marshes of Ecoregion 73o. It includes a large portion of the
Atchafalaya Basin
The Atchafalaya Basin, or Atchafalaya Swamp (; Louisiana French: ''L'Atchafalaya'', ), is the largest wetland and swamp in the United States. Located in south central Louisiana, it is a combination of wetlands and river delta area where the Atc ...
. Soils are mostly poorly or very poorly drained, clayey Entisols and Vertisols. Swamp forest communities are dominated by bald cypress and water tupelo, which are generally intolerant of brackish water except for short periods. In areas where freshwater flooding is more prolonged, the vegetative community is dominated by grasses, sedges, and rushes. This region contains one of the largest bottomland hardwood forest swamps in North America. Deposits include organic clays and peats up to thick, and inter-bedded fresh- and brackish-water carbonaceous clays. The levees in place on either side of the Mississippi River have diverted much of the river flow from its natural tendency to flow into the Atchafalaya Basin. Large concrete structures prevent diversion into the Atchafalaya River, and flow from the
Red River is also controlled. While this helps control flooding, it has also modified the region and contributed to the loss of wetland habitat. The ecoregion covers , entirely within Louisiana.
Deltaic Coastal Marshes and Barrier Islands (73o)
Brackish and saline marshes dominate the Deltaic Coastal Marshes and Barrier Islands ecoregion. The region supports vegetation tolerant of brackish or saline water including saltmarsh cordgrass, marshhay cordgrass, black needlerush, and coastal saltgrass. Black mangrove occurs in a few areas, and some
live oak
Live oak or evergreen oak is any of a number of oaks in several different sections of the genus ''Quercus'' that share the characteristic of evergreen foliage. These oaks are not more closely related to each other than they are to other oaks.
...
is found on
Grand Isle and along old natural levees. Extensive organic deposits lie mainly below sea level in permanently flooded settings resulting in the development of mucky surfaced Histosols. Sediments of silts, clays, and peats contain large amounts of methane, oil, and hydrogen sulfide gas. Inorganic sediments found within the ecoregion are soft and have high water contents. They will shrink dramatically upon draining. The wetlands and marshes act as a buffer to help moderate flooding and tidal inundation during storm events. Lack of sediment input, delta erosion, land subsidence, and rising sea levels threaten the region. The ecoregion covers , entirely within Louisiana.
See also
* Ecoregions defined by the EPA and the
Commission for Environmental Cooperation:
**
List of ecoregions in North America (CEC)
This list of ecoregions of North America provides an overview of North American ecoregions designated by the Commission for Environmental Cooperation (CEC) in its North American Environmental Atlas. It should not be confused with Wikipedia articl ...
**
List of ecoregions in the United States (EPA)
This list of ecoregions in the United States provides an overview of United States ecoregions designated by the U.S. Environmental Protection Agency (EPA) and the Commission for Environmental Cooperation (CEC). The CEC was established in 1994 by ...
**
List of ecoregions in Arkansas
This list of ecoregions in Arkansas provides an overview of ecoregions in the U.S. state of Arkansas designated by the U.S. Environmental Protection Agency (EPA) and the Commission for Environmental Cooperation (CEC). The Commission's 1997 rep ...
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List of ecoregions in Louisiana
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List of ecoregions in Mississippi
* The conservation group
World Wildlife Fund
The World Wide Fund for Nature Inc. (WWF) is an international non-governmental organization founded in 1961 that works in the field of wilderness preservation and the reduction of human impact on the environment. It was formerly named the Wo ...
maintains an alternate classification system:
**
List of ecoregions (WWF)
The World Wildlife Fund (WWF) identifies terrestrial, marine, freshwater, and ecoregions:
*Global 200
* List of terrestrial ecoregions (WWF) 867 terrestrial ecoregions.
*List of marine ecoregions (WWF)
The following is a list of marine ecoregions ...
**
List of ecoregions in the United States (WWF)
The following is a list of ecoregions in the United States as identified by the World Wide Fund for Nature (WWF). The United States is a megadiverse country with a high level of endemism across a wide variety of ecosystems.
Terrestrial ecoregi ...
References
*
{{Ecoregions in Arkansas
Ecoregions of the United States
Ecoregions of Arkansas
Ecoregions of Illinois
Regions of Arkansas
Regions of Illinois
Regions of Kentucky
Regions of Louisiana
Regions of Mississippi
Regions of Missouri
Regions of Tennessee