The ecoregion covers an area of 77,425 km2 (29,894 sq mi), extending along the shore of the Gulf of Mexico from southeastern Louisiana (west of the Mississippi Delta) through Texas and into the Mexican state of Tamaulipas as far as the Laguna Madre.

[5] This ecosystem, in part, has edaphic origins: the soils of this ecoregion are largely dominated by vertisol clays, the mechanical properties of which make it difficult for many tree species to grow and establish.

[7] From Southwest Louisiana west to the Upper Texas coast, the climate is wet humid subtropical, featuring significal annual precipitation.

The climate becomes more arid farther south along the Texas coast into northeastern Mexico, though precipitation totals still remain high enough for the humid subtropical classification.

The natural habitat of the area is a mix of tallgrass prairie similar to those found in inland Texas, with Indiangrass (Sorghastrum nutans), big bluestem (Andropogon gerardi), little bluestem (Schizachyrium scoparium), and switchgrass (Panicum virgatum) the primary tallgrass species that are typical of the coastal prairie, with several other shorter grasses and many herbaceous and woody species.

Farther south, from the lower one third of the Texas coast and through the Tamaulipan stretch, the drier climate supports shrubby areas of honey mesquite (Prosopis glandulosa), huisache (Vachellia farnesiana var.

Birds in the Mexican part of the region include Morelet’s seedeater (Sporophila morelleti), red-billed pigeon (Columba flavirostris), brown jay (Cyanocorax morio), Neotropic cormorant, white-winged dove (Leptotila verrequxi) and Audubon's oriole (Icterus graduacauda).

Mammals of the area include bobcats, collared peccary, white-tailed deer, eastern cottontails, with ocelot (Leopardus pardalis), Gulf Coast jaguarundi (Puma yagouaroundi cacomitli), southern yellow bat (Lasiurus ega), and Mexican spiny pocket mouse (Liomys irroratus) more abundant in Mexico.

Rancho Nuevo beach in Tamaulipas and along the Texas coast are the only nesting sites in the world for the Kemp's ridley sea turtle (Lepidochelys kempii) while other herpetofauna of the southern part of the ecoregion include Río Grande chirping frog (Eleutherodactylus cystignathoides) and Mexican white-lipped frog (Leptodactylus fragilis).

It is roughly coextensive with the Western Gulf coastal grasslands, with the important exception that the EPA-defined area terminates at the national boundary, while the natural ecoregion extends into northeastern Mexico.

In the EPA's definition, the principal distinguishing characteristics of the Western Gulf Coastal Plain are its relatively flat topography and mainly grassland potential natural vegetation.

Eragrostoid grasses, including the genera Bouteloua, Buchloe, Eragrostis, Hilaria, and Setaria increase in importance in Ecoregion 34b compared to 34a.

Most of the Coastal Sand Plain has been moderately to heavily grazed, and large areas have been converted to non-native range or pasture grasses.

[8] The Lower Rio Grande Valley ecoregion once supported dense, diverse grassland and shrub communities and low woodlands.

The region is underlain by a mix of Quaternary clays and sands with some Miocene-age sediments of the Goliad Formation at the western edge.

The soils, mostly Vertisols and Mollisols, are deep, loamy and clayey, and tend to be finer-textured than in the Lower Rio Grande Valley (34e) to the north.

The floodplain ridges once had abundant palm trees, and early Spanish explorers called the river "Rio de las Palmas."

It is the most subtropical climate of Texas, but hard freezes occasionally occur, affecting plants and animals that are at the northern limit of their range.

Both the Central and Mississippi flyways funnel through the southern tip of Texas and many species of birds reach their extreme northernmost range in this region.

The streams and rivers that supply nutrients and sediments to this region are primarily from the humid pine belt of the South Central Plains (ecoregion 35).

The estuarine and marsh complex supports marine life, supplies wintering grounds for ducks and geese, and provides habitat for small mammals and American alligators.

The region encompasses primarily Holocene deposits with saline, brackish, and freshwater marshes, barrier islands with minor washover fans, and tidal flat sands and clays.

Mollisols occur on tidal flats and coastal marshes, and Entisols form in sandy barrier islands and dunes.

Other native vegetation is mainly grassland composed of seacoast bluestem, sea-oats, common reed, gulfdune paspalum, and soilbind morning-glory.

[8] The Laguna Madre Barrier Islands and Coastal Marshes ecoregion is distinguished by its hypersaline lagoon system, vast seagrass meadows, wide tidal mud flats, large overwintering redhead duck population, numerous protected species, great fishery productivity, and a narrow barrier island with a number of washover fans.

The food web of the Laguna Madre is predominantly based on this submerged aquatic vegetation (seagrass and algae), rather than free-floating phytoplankton.