Breton Sound Basin Dynamics
The Breton Sound Basin is located in southeast Louisiana and encompasses approximately 676,400 acres. It is bounded on the west by the Mississippi River, on the north by Bayou La Loutre, on the east by the south bank of the Mississippi River Gulf Outlet (MRGO), and on the south by Baptiste Collette Bayou and Breton Island. Wetlands make up 184,100 acres of the basin. Approximately 51,300 acres within the basin are public lands, equaling 28% of the total lands within the basin (LCWCRTF 1993).
Like the Pontchartain Basin, the Breton Sound Basin is a remnant of the Mississippi River delta lobe, the abandoned St. Bernard Delta. The principal hydrologic features of the Breton Sound Basin include the Mississippi River and its natural levee ridges, the flood protection levee, abandoned delta distributaries, and the freshwater diversions at Caernarvon, White's Ditch, Bohemia, and Bayou Lamoque. The barrier islands, which make up the Breton National Wildlife Refuge are far offshore and thus provide minimal protection. Elevations range from approximately +10 feet National Geodetic Vertical Datum (NGVD) along the natural levee ridges of the Mississippi River to +2 feet NGVD or less in the swamp and marsh areas within the basin.
Although the basin was historically flushed with large quantities of fresh water and sediment annually during the spring, this system has suffered a series of major human impacts, including the completion of flood-control levees along the Mississippi River in the 1920s, construction of the MRGO in 1963, and a labyrinth of smaller canals for oil extraction primarily from 1950 through 1980. Thus, the estuary has been decoupled from alluvial water, sediment, and nutrients. Subsidence rates in the basin range from 0.6 feet per century in the upper portion of the basin to 4 feet per century in the lower basin (LCWCRTF 1993).
The immediate impact of these alterations has been increased marine influence, including a landward shift in salinity. Much of the fresh and intermediate marsh that existed in the upper basin earlier in this century has either converted to more saline habitats or has become open water as a result of sediment and nutrient deprivation, erosional processes, or a combination of these occurrences. Secondary impacts include land loss, habitat degradation, loss of biodiversity, and an inland shift in oyster production. Along the shoreline of the outer marshes and around the perimeter of the larger bays, erosion rates of 5-10 feet/year are common.
Since 1932, 47,036 acres (almost 17%) of the wetland area in the Breton Sound Basin has converted to open water (Dunbar et al. 1992, figure 19). Without action, approximately 1,000 acres of marsh will continue to be lost each year (Dunbar et al. 1992, Barras et al. 1994). This loss amounts to approximately 20,000 acres during the next 20 years. If no action is taken to restore and protect the remaining wetlands, it is projected that an additional 18% will be lost by the year 2040.