Gulf Coastal Plain

The Gulf Coastal Plain is bounded to the south by the Gulf of Mexico and to the north by the Ouachita Highlands and the southern Appalachian Mountains. Its northernmost extent reaches into southern Illinois along the Mississippi embayment. The topography is characterized as flat to rolling, interspersed with streams, river riparian areas, and marsh wetlands. It represents a westward extension of the Atlantic Coastal Plain.

The inner portion of the plain has been dissected into a complex of hills and valleys, increasing in altitude and relief inland. Distinctive features include the peninsular extension in Florida, the belted arrangement of relief and soils in Alabama and Texas, and the Mississippi embayment, extending inland to the junction of the Mississippi and Ohio Rivers at Cairo, Illinois.

The Florida peninsula is formed by a broad, low crustal arch. The visible portion of this arch constitutes the lowland peninsula, while a submerged portion extends westward under the Gulf of Mexico. Northern Florida consists largely of weak limestone, leading to underground drainage and the formation of sinkholes and lakes. The southern part of the state includes the Everglades, a large area of low, flat, marshy land overgrown with tall reedy grass. The eastern coast is fringed by sand reefs enclosing narrow lagoons, while the southern end features the Florida Keys, coral islands formed by the growth of lime-secreting organisms.

In Alabama and adjacent Mississippi, the plain is approximately 150 miles wide. A weak limestone base has been eroded to form a flat inner lowland with rich black soil, known as the “black belt.” This lowland is enclosed by the Chunnenuggee Ridge, a partly consolidated sandy escarpment. The ridge is not continuous, but a dissected escarpment descending gradually towards coastal prairies, which become low, flat, and marshy before reaching the Gulf.

The coastal plain extends 500 miles inland along the axis of the Mississippi embayment. This inland extension exhibits topographical discordance, sweeping northwestward across the piedmont belt, ridges, and valleys of the Appalachians. The embayment also borders the Ozark plateau of southern Missouri and northern Arkansas, and the Ouachita Mountains of Arkansas and Oklahoma.

The Mississippi River has eroded a broad valley across the embayment, approximately 40 to 50 miles wide in its northern reaches. The valley floor is covered with fine silt, sloping southward at half a foot per mile. The river’s length within the coastal plain is about 1,060 miles due to its windings. The river’s meanders are continually changing, with cut-offs occurring to maintain a relatively constant length. Floods, often occurring in spring or summer, can raise the river to 30, 40, or even 50 feet if two major tributaries flood simultaneously. The sediment carried by the river forms a delta at its mouth, with the Mississippi building forward its digitate delta.

The coastal plain constricts to 250 miles wide in western Louisiana before widening to 300 miles at the Texas-Mexico border. A belted arrangement of reliefs and soils characterizes the area, resulting from differential erosion. Most of the plain is treeless prairie, though sandier belts are forested. Inland, the Grand Prairie escarpment rises to altitudes of 1,200 or 1,300 feet, dipping gently seaward. The Black Prairie, supported by a chalk formation, features a gently undulating surface covered with marly strata and rich black soil, an important cotton district. The East Texas timber belt follows, transitioning to the Coast Prairie, a young plain with a seaward slope of less than 2 feet per mile.

Northern uplands are dominated by pine forests, while southern regions include tropical and subtropical moist broadleaf forests and western Gulf coastal grasslands. The area supports extensive freshwater wetlands, salt marshes, and coastal mangrove swamps, serving as important wintering grounds for waterfowl.

Recent satellite studies indicate groundwater resources are shrinking due to overuse, causing regional water security concerns. Surface water quality is declining due to increasing population, depleted streams, and land subsidence along certain coastlines. The Atlantic and Gulf Coastal Plain Aquifer is considered to have low to moderate stress, but trends suggest increasing human impact on water resources.