Breton Sound Basin

The Breton Sound Basin is the remnant of a 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; the MRGO south disposal bank; Bayou Terre aux Boeufs and River aux Chenes (abandoned delta distributaries); and the freshwater diversions at Caernarvon, White's Ditch, Bohemia, and Bayou Lamoque.

The natural processes of subsidence, saltwater intrusion, and erosion of wetlands, and the human effects of river levee construction and the oil and gas industry, have caused major impacts to the Breton Sound Basin in recent decades. The two major wetland problems resulting from the natural processes and human intervention in this basin are sediment deprivation and saltwater intrusion.

Historically, the basin was flushed with large quantities of fresh water and sediments annually during the spring. Marine waters would then rise and enter the basin during the late summer and early fall months and would be flushed out the following spring. In the early 1930's, flood protection levees were raised along the Mississippi River as far south as Bohemia in the Breton Sound Basin. This prevented the annual input of fresh water, nutrients, and sediment that nourished the wetlands and combatted saltwater intrusion.

Between 1940 and 1970, 12.9 square miles (8,256 acres) of canals were dredged across and between the abandoned distributary ridges that run from the river to the outer fringes of the marsh (Gagliano et al., 1970). This has allowed channelized outflow of fresh water and increased tidal flux.

The combination of natural processes and human intervention has allowed salt water to enter close to the head of the basin. Much of the fresh and intermediate marsh that occurred 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 brought about by the construction of flood protection levees and saltwater intrusion caused by the dredging of oil and gas access canals through and between the natural distributary ridges.

Subsidence combined with sediment and nutrient deprivation has contributed greatly to the marsh loss in the upper and middle basin and even more greatly in the Bohemia Subbasin. The subsidence rate ranges from 0.6 feet per century in the upper portion of the basin to 4 feet per century in the lower portion. The effect of subsidence is very apparent in the area south of Bohemia, which was created by alluvial deposits of the Mississippi River less than 1,000 years ago. Large areas of wetlands flanking the Mississippi River in this area have subsided and are continuing to subside and convert to open water. Periodic overbank flows from the FIGURE BS1. BASIN AND SUBBASIN BOUNDARIES

Mississippi River occur in this area, and some wetlands immediately adjacent to the river are being maintained by this input of sediments and fresh water.

A significant cause of wetland loss in the Breton Sound Basin is erosion of shorelines by wind-wave action. Along the shoreline of the outer marshes and around the perimeter of the larger bays, erosion rates of 5 to 10 feet per year are common. These high rates occur in the fringe marshes because the Breton barrier islands are so far offshore that they offer little protection to the estuary behind them.

Breton Sound Basin Summary

Breton Sound Basin Discussion

Breton Sound Basin Dynamics

Basin-Wide Land Loss Map for Breton Sound Basin.

(Time lapsed animation of basin from 1956 to 1993.)

CWPPRA Restoration Sites for the Breton Sound Basin

PPL	Number	Agency	Project Name
15	BS-13	COE	Bayou Lamoque Freshwater Diversion
17	BS-15	EPA	Bohemia Mississippi River Reintroduction
2	BS-03a	NRCS	Caernarvon Diversion Outfall Management
17	BS-16	USFWS	Caernarvon Outfall Management/Lake Lery SR
10	BS-10	COE	Delta Building Diversion North of Fort St. Phillip
10	BS-11	USFWS	Delta Management at Fort St. Phillip
4	BS-07	COE	Grand Bay Crevasse (Deauthorized)
8	BS-09	NRCS	Upper Oak River Freshwater Siphon, Phase 1 (Deauthorized)
14	BS-12	NRCS	White Ditch Resurrection and Outfall Management
3	BS-04a	NRCS	White's Ditch Outfall Management (Deauthorized)

10 projects listed

PPL stands for "Priority Project List"