Mermentau Basin
The basin contains about 450,000 acres of
wetlands, consisting of 190,000 acres of fresh marsh, 135,000 acres of
intermediate marsh, and 101,000 acres of brackish marsh. A total of 104,380
acres of marsh has converted to open water since 1932, a loss of 19 percent of
the historical wetlands in the basin.
Prior to human alterations, delta-building processes associated with the Mississippi River resulted in periodic building of marsh along the gulf coast of the Mermentau Basin. Construction of flood control and navigation projects on the Mississippi and Atchafalaya rivers restricted those natural processes to relatively small portions of the coast. Consequently, marsh-building now occurs on only the eastern-most portion of the Mermentau Basins coastline. This condition is further aggravated by continuing subsidence and sea level rise. In the Mermentau Basin, relative sea level rise results in an average water level rise of 0.25 inches per year. Although natural wetland building processes only occur along the eastern shore, natural marsh maintenance processes (e.g., plant deterioration and regeneration) can be fairly effective at keeping wetland loss rates low. However, these processes have been altered or interrupted and the ability of the system to maintain the marsh is jeopardized.
The two subbasins suffer from distinctly different hydrologic problems. The most critical wetland problem in the Lakes Subbasin is excessive flooding. A 5-mile-long segment of Louisiana Highway 27 almost totally blocks drainage from the western portion of the Lakes Subbasin into adjacent wetlands of the Calcasieu/Sabine Basin. Similarly, along the southern boundary of the Lakes Subbasin, Louisiana Highway 82 blocks drainage across 17 miles of marsh. The Freshwater Bayou navigation channel has altered the historic drainage pattern in the eastern portion of the Lakes Subbasin. These numerous blockages of drainage outlets significantly increase ponding in the subbasin.
The Catfish Point Control Structure, built to reduce saltwater intrusion into Grand Lake via the Mermentau River, controls the major drainage outlet from the Lakes Subbasin. High water levels in the gulf frequently prevent the drainage of the subbasin through the structure. Farther upstream, development and channelization of the Mermentau River watershed have increased the rate of run-off into the Lakes Subbasin. These factors, in combination with the loss of historic drainage outlets, result in periods of prolonged high water levels following heavy basin-wide precipitation. Because upland drainage improvements are continuing
Natural freshwater inputs from the Lakes Subbasin into the marshes of the Chenier Subbasin are reduced by the same highway embankments that impound water in the northern subbasin. The loss of those freshwater inputs is compounded by waterways and canals that create additional connections between the gulf and area marshes, facilitating saltwater intrusion.
Basin-Wide Land Loss Map for
Mermentau Basin. |
Basin-Wide Habitats Maps for Mermentau Basin. |
(Time lapsed animation of basin from 1956 to 1993.)
CWPPRA Restoration Sites for the Mermentau Basin
| PPL | Number | Agency | Project Name |
| 3 | ME-12 | NRCS | Southwest Shore White Lake Demonstration (Deauthorized) |
| 16 | ME-24 | COE | Southwest LA Gulf Shoreline Nourishment and Protection |
| 12 | ME-22 | COE | South White Lake Shoreline Protection |
| 15 | ME-23 | NMFS | South Pecan Island Freshwater Introduction |
| 11 | ME-20 | USFWS | South Grand Chenier Hydrologic Restoration Project |
| 10 | ME-18 | NMFS | Rockefeller Refuge Gulf Shoreline Stabilization |
| 7 | ME-14 | NMFS | Pecan Island Terracing |
| 9 | ME-17 | NRCS | Little Pecan Bayou Hydrologic Restoration |
| 8 | ME-11 | NRCS | Humble Canal Hydrologic Restoration |
| 10 | ME-19 | USFWS | Grand-White Lakes Landbridge Protection |
| 11 | ME-21a | COE | Grand Lake Shoreline Protection, Tebo Point |
| 11 | ME-21b | COE | Grand Lake Shoreline Protection, O&M Only [CIAP] |
| 9 | ME-16 | USFWS | Freshwater Introduction South of Highway 82 |
| 2 | ME-04 | NRCS | Freshwater Bayou Wetland Protection |
| 5 | ME-13 | NRCS | Freshwater Bayou Bank Stabilization |
| 1 | ME-08 | NRCS | Dewitt-Rollover Vegetative Plantings Demonstration (Deauthorized) |
| 1 | ME-09 | USFWS | Cameron Prairie National Wildlife Refuge Shoreline Protection |
17 projects listed
PPL stands for "Priority Project List"