The Terrebonne Basin, one of 12 major drainage basins in Louisiana, is part of the Lafourche Delta that formed between 800 and 2,500 years ago when the Mississippi River flowed through what is now Bayou Lafourche. After the river changed course, delta growth slowed and normal subsidence developed bays inside barrier islands that marked the delta’s farthest reach toward the Gulf of Mexico; Terrebonne Bay, Timbalier Bay, and the barrier islands of Timbalier and East Timbalier remain from that delta. Its French name means “good earth,” reflecting the basin’s fertility at the time of settlement. Today the basin includes about 729,000 acres of swamp and marsh, ranging from fresh marsh inland to brackish and saline marsh near the bays and Gulf, with major bayous including Terrebonne, Black, Blue, Dularge, Petit Calliou, and Grand Calliou. The area supports sugar cane production as well as tidal marshes and lagoons, including Lake Hatch in the Mandalay National Wildlife Refuge. Its two major urban complexes, Houma-Thibodaux and Plaquemine-Port Allen, are linked by waterways and other connections that support commerce and industry. The 1,300-mile Gulf Intracoastal Waterway, completed in the mid-1900s, passes through the middle of the basin and through the national wildlife refuge as it runs from Florida to Texas; designed mainly for barge traffic, it also serves recreational boaters, but it alters water flow in the rivers and bayous it meets, equalizing some waterways that natural distance and levees would otherwise keep separate. About 10 percent of the Atchafalaya flow turns east and west at the waterway and continues to the Gulf. The southern part of the basin faces major challenges, including coastal erosion, substantial subsidence, sediment deficit, marsh loss, and saltwater intrusion.