Calcasieu / Sabine Basin Dynamics
The Calcasieu/Sabine Basin is located in southwest Louisiana and is the westernmost coastal basin not only in the chenier plain but also in the state of Louisiana. The approximate northern boundary of the basin is the GIWW, the eastern boundary follows the eastern leg of Louisiana Highway 27, the western boundary is the Sabine River and Sabine Lake, and the southern boundary is the Gulf of Mexico (figure 30).
Calcasieu and Sabine lakes dominate this basin's hydrology. Also influencing the basin's hydrology are the Sam Rayburn and Toledo Bend reservoirs and three major navigation channels-the Calcasieu Ship Channel, the Sabine-Neches Waterway, and the GIWW. The basin encompasses approximately 630,000 acres of land, 50% of which is classified as marsh. The marshes between Sabine and Calcasieu lakes range from saline to fresh with saline marshes located primarily near Calcasieu Lake and Sabine Pass. Brackish marshes are located adjacent to both lakes and extend inland, graduating into intermediate and fresh marshes.
On a geologic time scale, longshore sediment transport associated with delta-building and accumulation of organic material through vegetative growth resulted in marsh building within the Calcasieu/Sabine Basin. However, implementation of flood control and navigation projects on the Mississippi and Atchafalaya rivers interrupted those natural processes. Consequently, mineral marsh building within the basin has halted and the basin is experiencing net shoreline erosion and marsh loss. Subsidence and sea level rise also contribute to wetland deterioration and loss, and result in an average water level rise of 0.25 inches/year (Penland et al. 1989).
Almost all fresh marsh was converted to intermediate and brackish by the late 1970s as a result of saltwater intrusion and increased tidal influence. Hydrologic alterations within the basin also impact sediment introduction into affected marshes. Jetties on the Calcasieu Ship Channel and at Sabine Pass affect longshore sediment transport and sediment availability. Also, spoil banks and levees associated with navigation channels retard the flow of nutrients and fine suspended sediments into the basin's marshes, while limiting the export of organic material.
Erosion is a problem along the banks of the GIWW and the shores of Calcasieu Lake, Sabine Lake, and interior marsh water bodies. Erosion-related breaching of the shorelines and levees exposes the fragile interior marshes to increased water exchange and saltwater intrusion, resulting in loss of surface layers of organic material (USDA-NRCS {in print}). Along the Gulf of Mexico, shoreline erosion causes loss of back beach marshes and also threatens to alter the hydrology of nearby interior marshes.
A total of 116,791 acres (33% of the historic land area) of wetlands in the Calcasieu/Sabine Basin has converted to open water since 1932 (Dunbar et al. 1992, figure 30). Extensive channelization, increased energy levels, and saltwater intrusion caused much of the marsh loss in the basin. Current land loss rates range between approximately 1,000 (Dunbar et al. 1992) and 1,650 (Barras et al. 1994) acres per year. At these rates, up to 33,000 acres will be lost during the next 20 years if coastal restoration projects are not implemented.