Terrebonne-Atchafalaya Basin Projects

The Terrebonne and Atchafalaya basins occupy the central coast of Louisiana. The Terrebonne-Atchafalaya Basin has the highest rate of land loss in Louisiana, making the need to rebuild these coastal wetlands even more urgent. The area is bordered to the east by Bayou Lafourche, a former outlet of the Mississippi River, and to the west by the Chenier Plain. The basin includes the growing Wax Lake and Atchafalaya deltas of the Atchafalaya River, which is the largest remaining natural distributary of the Mississippi River. However, in Terrebonne Bay, on the eastern side of the basin, wetlands are collapsing and becoming open water as the sediment-starved land sinks and salt water intrudes into freshwater wetlands. Habitats in this basin include bottomland hardwood forests, swamps, barrier islands and freshwater, brackish and saltwater marsh.

The priority projects selected for this basin focus on reestablishing a balance of fresh and salt water as well as sediment and nutrient distribution. To accomplish this goal, the Increase Atchafalaya Flow to Terrebonne Diversion project will take advantage of the Atchafalaya River and the Intracoastal Waterway to move fresh water and sediment eastward. This project will work in conjunction with the Houma Navigational Canal Lock, a successful 2012 priority project that is moving toward construction, to reintroduce and increase the retention of fresh water into Terrebonne’s marshes. Atchafalaya River Sediment Diversion will also direct sediment and fresh water to marshes in southwest Terrebonne, restoring freshwater flows and habitats. Both diversions will provide freshwater inputs and some sediment to the Terrebonne Ridge Restoration, which will create maritime forest, improve water quality and protect marsh from storm surge and wind energy. All three of these projects will help fight saltwater intrusion by restoring freshwater inputs and natural hydrologic patterns.

Our 2017 Coastal Master Plan Priority Projects

Projects in the Terrebonne-Atchafalaya

1. Atchafalaya River Sediment Diversion
   To be constructed off the Atchafalaya River, the Atchafalaya River Sediment Diversion will provide basin-wide benefits to marshes in southwest Terrebonne Parish. Sediment and fresh water diverted into the marshes will help build land and sustain other nearby projects planned for construction, like Mauvais Bois Ridge Restoration. This project will have the greatest benefits to freshwater habitats, such as forested areas, flotant and fresh and intermediate marsh, which are threatened by saltwater intrusion and sediment starvation.
2. Increase Atchafalaya Flow to Terrebonne Diversion
   This hydrologic diversion project stretches from the Atchafalaya River to the Houma Navigation Canal, which is part of the Gulf Intracoastal Waterway system. The marshes in the influence area are nearly an equal distance from the Mississippi and Atchafalaya rivers and are blocked from significant amounts of river water and sediment. These marshes have been rapidly converted to open water because of saltwater intrusion and sediment starvation. This project would dredge and deepen a portion of the Gulf Intracoastal Waterway to increase the flow of fresh water from the Atchafalaya River to help sustain Terrebonne marsh. Additional benefits from this project include using the dredged material to create marsh and increase flow capacity using two southern-flowing canals (Minor’s and Bayou Copasaw.)
3. Terrebonne Ridge Restoration: Mauvais Bois 
4. Terrebonne Ridge Restoration: Bayou DuLarge
   These projects will restore the historic Mauvais Bois and Bayou DuLarge Ridges located in Terrebonne Parish near Lake Mechant and Caillou Lake. The projects will restore and construct maritime forest in the lower Terrebonne marshes by using dredged sediment to increase the elevation of the relict ridges. Both ridges will help restore historic hydrologic patterns and reduce saltwater intrusion. They will also buffer storm surge and wind energy for inland communities. Two diversions, Atchafalaya River Diversion and Increase Atchafalaya Flow to Terrebonne, will work synergistically with these ridges to restore natural ecosystem processes in the area, including patterns of freshwater flows, salinities and land building.