Loss of these forests has been caused by many activities, including water control structures, agriculture, logging, mining, petroleum extraction, development and pollution. The Gulf coast contains a diversity of saline, brackish, intermediate and fresh marsh wetlands , including wet prairies, forested wetlands, barrier islands, mud flats, estuarine bays, bayous and rivers. Saline and brackish marshes are most widely distributed south of the Galveston Bay, while intermediate marshes are the most extensive marsh type east of Galveston Bay. The existence and extent of specific plant species within these different wetland types depends on their specific tolerances to salt concentrations and variability in water depth. Some overlap of species can be found within the different wetland types on the Gulf coast.
Saline marsh is characterized by a monotypic stand of smooth cordgrass.
Other plants present may include blackrush, saline marsh aster, saltwort, glasswort and sea-lavender. Brackish marsh communities, the most extensive communities in the Galveston Bay system, are transitional between saline and intermediate marshes. Dominant species include marshhay cordgrass, saltgrass, saltmarsh bulrush, marsh hemp and hairypod cowpea. Intermediate marshes reflect greater plant diversity than the saline or brackish marsh assemblages. Species found in this environment include seashore paspalum, marshhay cordgrass, Olney bulrush, Colorado River hemp, common reedgrass, coastal water-hyssop, bearded sprangletop, and cattail.