Mermentau Basin Dynamics
The Mermentau Basin lies in the eastern portion of the chenier plain in Cameron and Vermilion parishes. This 734,000-acre basin is bounded on the east by Freshwater Bayou Channel, on the south by the Gulf of Mexico, on the west by Louisiana Highway 27, and on the north by the GIWW (figure 27). The basin contains about 450,000 acres of wetlands, consisting predominantly of fresh (approximately 190,000 acres), intermediate (approximately 135,000 acres), and brackish marsh (approximately 101,000 acres). The basin is divided into two distinct subbasins by the Grand Chenier and Pecan Island ridge systems, which are linked by Louisiana Highway 82. The Lakes subbasin lies to the north, and includes Grand and White lakes and the GIWW. The Chenier subbasin lies to the south of Louisiana Highway 82 and includes Hog Bayou, Rockefeller Refuge, and other marsh areas south of Pecan Island. Wetlands within the Lakes Subbasin consist primarily of fresh marsh and submergent and floating aquatic vegetation. Vegetation types within the Chenier Subbasin range from fresh to saline, with fresh and intermediate marshes existing only in managed areas.
The dominant hydrologic features of the Lakes Subbasin are Grand and White lakes, with fresh water entering the subbasin through the Mermentau River, Lacassine Bayou, the Bell City Drainage Canal, the Gueydan Canal, the Warren Canal, and a number of other smaller drainage canals. Major outlets for discharge of water from the subbasin include the Catfish Point Control Structure, the Schooner Bayou Control Structure, the East End Control Structure, the Leland Bowman Lock, and the Freshwater Bayou Lock. A large number of water control structures have been constructed at sites where salt water could encroach into subbasin wetlands, such as in the Little Pecan Bayou area, east of the Mermentau River.
The hydrology of the Chenier Subbasin is dominated by the Lower Mermentau River and has been significantly altered through hydrologic management activities (e.g., for cattle pasture and waterfowl protection). The Mermentau River-Gulf of Mexico Navigation Channel has altered the hydrology of the river by connecting the river with the gulf near Grand Chenier. This connection allows high salinity water from the Gulf of Mexico to enter the Lower Mermentau River. Drainage for marshes located in the western portion of the subbasin occurs primarily via access canals and small bayous to the Gulf. The majority of marshes between Rollover Bayou and Freshwater Bayou Channel drain eastward via access canals into the Freshwater Bayou Channel.
The Gulf of Mexico beach is retreating across most of the Chenier Subbasin. However, mud deposits have resulted in a progradation of the eastern shoreline. The sediment source responsible for this progradation is likely a combination of reworked Atchafalaya River sediments and reworked spoil from maintenance dredging of the southern end of Freshwater Bayou Channel. In this prograding area, the shore consists of a very broad mud flat, colonized by smooth cordgrass on slightly elevated ridges.
A total of 117,825 acres of marsh have converted to open water since 1932, which accounts for 18% of the historical wetlands in the Mermentau Basin (Dunbar et al. 1992) and represents 9% of wetland loss in Louisiana. Current land loss rates are approximately 2,600 acres/year (Dunbar et al. 1992, Barras et al. 1994). At this rate, approximately 52,000 acres of wetland will be lost during the next 20 years (an additional 8.6% of the basin's wetlands) without restorative action (LCWCRTF 1993). This loss is expected to continue along the shorelines of the lakes and banks of the navigation channels in the Lakes Subbasin, and in the interior marshes of the Chenier Subbasin (figure 27). Erosion along the gulf shoreline is expected to continue at its present rate of 20-40 feet per year. Much of the Mermentau Basin's wetland loss is attributed to saltwater intrusion, ponding, and reductions in freshwater and nutrient inputs. Louisiana Highway 82 forms a north-south hydrologic barrier between the Lakes and Chenier subbasins from Oak Grove to Pecan Island, and an east-west hydrologic barrier between White Lake and Freshwater Bayou. The highway reduces sheet flow, starves the downstream chenier marshes of fresh water and sediment, and increases flooding in the Lakes Subbasin.
The most critical wetland problem in the Lakes Subbasin is excessive flooding. Prolonged high water leads to direct wetland loss and shifts in plant species composition. High water levels increase erosion rates along natural lake rims that protect more fragile interior marshes that are lower in elevation. Once the protective lake rims are lost, erosion rates accelerate. Erosion from vessel wakes is also a problem along the GIWW and the Freshwater Bayou Channel. Many areas within the subbasin have experienced marsh loss due to saltwater intrusion, which mainly impacts areas adjacent to human-made channels and dredged waterways. In these areas, salt-intolerant plants are destroyed, leaving marsh soils unprotected. Under these conditions, the subbasin's characteristic organic soils are easily eroded by tidal movement, resulting in the conversion of marsh to open water.
In the Chenier Subbasin, the combination of regional and localized hydrologic alterations associated with numerous access canals and board roads, plus the failure and abandonment of former forced drainage areas, resulted in extensive marsh loss. Although input of suspended sediment is currently rebuilding deteriorated marshes in the westernmost portion of the basin, marshes within the highly altered middle and upper portions of the basin are continuing to experience losses. Natural freshwater inputs from the Lakes Subbasin into marshes of the Chenier Subbasin were virtually eliminated with the construction of Louisiana highways 27 and 82. The problem is compounded by dredging projects that create additional connections between the Gulf and subbasin marshes, facilitating saltwater intrusion. The natural salinity and tidal regime of the subbasin was altered by the construction of the Freshwater Bayou Channel, Mermentau River-Gulf of Mexico Navigation Channel, and numerous access canals. Prior to these alterations, fresh and intermediate marshes were isolated from tidal exchange and associated high salinities. The introduction of high-salinity water destroyed much of the vegetation, exposing the underlying organic soils to tidal exchange, which resulted in extensive marsh loss.