The Pontchartrain-Maurepas Basin is dominated by three large estuarine lakes that are connected by tidal passes, with a fresh-to-salt gradient that runs from west to east. Coastal habitats in the basin include freshwater bottomland hardwood and swamp forest, and fresh, brackish and saltwater marshes. In the upper basin, the swamps are cut off from the nourishing fresh water, nutrients and sediment of the Mississippi River by levees installed for flood protection and navigation. Because of this and the Mississippi River Gulf Outlet (MRGO) shipping channel, these swamps have suffered from saltwater intrusion and are sinking and converting into marsh and open water. Although the MRGO was closed in 2009, restoration of the vast damage due to the MRGO has not been done, and nearby wetlands continue to suffer losses. The freshwater swamps once found in the lower basin have almost completely disappeared due to the lack of river water.

In the upper basin, Union Freshwater Diversion, East Maurepas Freshwater Diversion and Manchac Landbridge Diversion provide fresh water and sediment to the Maurepas Landbridge, which separates Lakes Maurepas and Pontchartrain and provides critical storm surge protection to surrounding communities, including Louisiana’s capital of Baton Rouge. In the lower basin, Central Wetlands Diversion will divert fresh water and sediment to the landbridge separating Lakes Pontchartrain and Borgne. These diversion projects help to prevent the three lakes from becoming a single arm of the Gulf and provide storm surge protection for millions of people. The diversions will also help strengthen and maintain the marsh creation projects, New Orleans East Landbridge Restoration and Golden Triangle Marsh Creation. These marshes will provide important habitat for birds, fish and wildlife and help buffer the new surge barrier on the east side of Lake Borgne.