Teche/Vermilion Basin Dynamics
The Teche/Vermilion Basin contains approximately 243,000 acres of wetlands in Vermilion, Iberia, and St. Mary parishes (figure 23). The basin extends westward from Point Chevreuil through East and West Cote Blanche bays, and includes Marsh Island and Vermilion Bay. The basin is bordered on the east by the West Atchafalaya Basin Protection Levee, on the west by Freshwater Bayou Canal, on the north by the Lafayette/Vermilion and St. Martin/Iberia parish lines, and on the south by the Gulf of Mexico. Portions of the marshes are privately owned, and over 50% of the marshes are managed as wildlife refuges by the state of Louisiana and the Audubon Society. Collectively, the Louisiana State Wildlife Refuge, Marsh Island Wildlife Refuge, and the National Audubon Society's Paul J. Rainey Wildlife Refuge encompass over 139,000 acres of marsh and associated bayous, ponds, and lakes. The basin is composed of 5% cypress/tupelo swamp, 14% fresh marsh, 11% intermediate marsh, 36% brackish marsh, and 3% salt marsh (LCWCRTF 1993). The marsh soils are classified as highly organic peats, which are continually forming on the marsh surface through plant production and, more importantly, below ground through root growth (Nyman et al. 1992).
The principal hydrologic features of the basin include the Vermilion River, Charenton Canal, the Gulf Intracoastal Waterway (GIWW), the natural levee ridges of the Vermilion River and Bayou Teche, East and West Cote Blanche bays, and Vermilion Bay. Unlike other basins in the chenier plain, the Teche/Vermilion Basin has direct riverine inputs. This basin is experiencing an increase in riverine conditions because of sediment-laden freshwater flow from the Atchafalaya River (DeLaune et al. 1987). Water and sediment from the Atchafalaya River enter the basin from the east, flow westward, and dominate hydrological conditions in East and West Cote Blanche bays, which are gradually filling with sediment.
Wetlands in the Teche/Vermilion Basin are stable relative to many other Louisiana coastal marshes, with only 50,540 acres of land loss (9% of land area) since 1932 (Dunbar et al. 1992). The current basin land loss is approximately 650 (Barras et al. 1994) to 900 (Dunbar et al. 1992) acres per year and accounts for less than 3% of Louisiana's total coastal land loss. This lower relative loss rate is attributable to the older, and therefore more compacted, soils of the chenier plain. Wetland loss in the basin tends to be localized and occurs primarily as either shoreline erosion or in isolated hot spots of interior marsh deterioration (figure 23). Alterations resulting in hot spots of loss include the construction of navigation channels, oil and gas access canals, spoil banks, and levees. Some of the basin's wetland loss may also be related to heavy grazing by nutria and possibly muskrat. At current land loss rates, the Teche/Vermilion Basin will lose between 12,840 and 17,860 acres of land during the next 20 without restorative action.
Marsh loss in the Teche/Vermilion Basin also occurs as a result of shoreline erosion along large bodies of water and navigation canals. Shoreline erosion on the large bays is mainly caused by natural wave energy. In contrast, erosion along the GIWW and other navigation channels is caused by boat wakes and water surges associated with the passage of large vessels. Some shorelines form natural barriers between dynamic water bodies, such as bays and navigation channels, and the relatively isolated marsh ponds and bayous of the marsh interior. When these shorelines erode, the high energies associated with the dynamic water bodies begin to erode the fragile interior marsh (figure 24).
Although the bays are relatively sediment rich, spoil banks, levees, and access canals have isolated some areas from sediment input, which can result in a net loss of sediment. Areas that become impounded are subject to excessive flooding, while areas that have existing hydrologic barriers removed may be subject to saltwater intrusion.