The Mississippi River Delta is the land the Mississippi has built where it meets the Gulf of Mexico in southeastern Louisiana, United States. It covers about 12,000 km² (3 million acres) from Vermilion Bay in the west to the Chandeleur Islands in the east, and holds more than 11,000 km² of coastal wetlands, 37 percent of the estuarine marsh in the lower 48 states. The US Geological Survey ranks it as the seventh-largest river delta on Earth. It is a river-dominated delta of marsh, swamp, bayous, natural levees and barrier islands, built over roughly 7,000 years in six overlapping delta complexes; the active mouth today is the bird's-foot Plaquemines–Balize lobe below New Orleans. There is no single park over the whole delta. Pieces are protected, such as the 190 km² Delta National Wildlife Refuge near Venice, bought in 1935, but most of the coast is private, state or industrial land, and, with net subsidence much faster than elsewhere in the United States, it is losing ground.
100 million years ago: the Mississippi Embayment opens a funnel
The delta's story starts long before there was a delta. In the late Cretaceous, about 100 million years ago, the crust of what is now the central United States sagged into a long trough, the Mississippi Embayment, pointing south toward the Gulf of Mexico. That low ground did what it still does: it gathered drainage from a huge interior and fed the sediment south. Today the Mississippi drains about 41 percent of the contiguous United States and delivers an average of about 13,000 m³ (470,000 cubic feet) of water per second to the Gulf, but the pattern of a continent emptying through one southern gate was set in the Cretaceous.
Paleogene to Miocene rivers: laying the deep foundation
Through the Paleogene, roughly 65.5 to 23 million years ago, several smaller regional rivers entered what is now southern Louisiana and spread their sediment over a wide front. In the Miocene, about 23 to 5.3 million years ago, the embayment became the main focus of deposition, and thick layers of sand, silt and clay piled up at the edge of the Gulf. These buried layers are the foundation the modern delta sits on. The same coast is now the heart of the Gulf oil and gas industry: Port Fourchon, at the southern tip of Lafourche Parish, services about 90 percent of the offshore oil rigs in the Gulf. The weight of all that sediment also matters for the future, because the young deposits on top compact and sink, and the delta only keeps its head above water while the river keeps adding more.
Maringouin and Teche: the first Holocene lobes, 7,500 to 3,500 years ago
The present delta plain dates from the Holocene. As the ice sheets melted, sea level rose quickly, and only when that rise slowed, about 7,500 to 8,000 years ago, could the river build land at the coast that the sea did not immediately drown. From then on, the Mississippi followed a delta cycle: it pushed a lobe of sediment out into the Gulf until its path became long and gentle, then broke through a levee upstream in an avulsion and took a shorter, steeper route to the sea. The abandoned lobe began to shrink as it subsided and waves reworked its edges into barrier islands. Before the river was leveed in the 20th century, this switching happened about every 1,000 to 1,500 years.
The first complex, the Maringouin, formed from about 7,500 to 5,500 years ago while sea level was still rising fast. The Teche delta followed from about 5,500 to 3,500 years ago, once the rise had slowed. Its old channel survives as Bayou Teche, a 201 km waterway from Port Barre to the lower Atchafalaya at Patterson, which now marks the western edge of the Atchafalaya Basin.
St. Bernard and Lafourche: avulsions east and west of New Orleans
About 4,000 years ago an avulsion sent the river east of where New Orleans now stands, building the St. Bernard delta between roughly 4,000 and 2,000 years ago. As that lobe was abandoned, its sandy edges were reworked into a curving chain of barrier islands; the Chandeleur Islands, the delta's eastern limit, are what remains of that shoreline. Then, about 2,500 years ago, a second avulsion swung the river west of the New Orleans site, and the Lafourche delta grew between roughly 2,500 and 500 years ago. Bayou Lafourche, a 171 km former distributary that left the Mississippi at Donaldsonville until it was dammed there in 1904, gave the lobe its name; its banks are lined so continuously with settlements that it is called the longest Main Street in the world.
The overlap of these dates is real, not a typo: one lobe was still active while the next was getting started, which is how the delta came to cover so wide a coast. Together the prehistoric and historic lobes built more than 16,000 km² (4 million acres) of coastal wetland, with bays, bayous and marsh in between them where each abandoned lobe subsided.
Plaquemines–Balize: the bird's-foot delta of the last 1,500 years
The modern lobe, the Plaquemines–Balize or bird's-foot delta, has formed over roughly the last 1,500 years between the old St. Bernard and Lafourche complexes. Below the Head of Passes the river splits three ways, into Southwest Pass, South Pass and Pass a Loutre, and each one pushes long, narrow fingers of levee and marsh out over deep water, giving the delta its claw-like outline. The Spanish navigator Alonso Álvarez de Pineda reached the river's mouth in 1519; La Salle claimed it for France in 1682, and in 1699 the French built a fort at La Balize, on a pass at the edge of the marsh, to control shipping. Today the Corps of Engineers keeps a 13.7 m (45 ft) shipping channel open from the mouth of Southwest Pass up to Baton Rouge, but the shifting mouth has always fought back: between 1700 and 1888 the main shipping channel was changed four times because of sandbars, mudflats and hurricanes.
The bird's-foot also shows how quickly a delta can make land when it is allowed to. Delta National Wildlife Refuge, about 16 km east of Venice, exists because the river broke through its natural levee in 1862 about 160 km below New Orleans, and the flood spread a new splay of sediment that grew into 190 km² of marsh, now reached only by boat.
Atchafalaya and Wax Lake: the youngest land, after 1942
About 500 years ago the river began its next switch. A westward bend near present-day Angola, Louisiana, cut into the Red River, and the lower Red became a distributary of the Mississippi: the Atchafalaya. It offered the Mississippi a much shorter, steeper path to the Gulf. Once a huge log jam called the Great Raft was cleared from the Atchafalaya around 1840, it began capturing more flow; between 1850 and 1950 its share of the combined Mississippi–Red flow rose from under 10 percent to about 30 percent. In 1953 the US Army Corps of Engineers concluded that the Mississippi could switch to the Atchafalaya by about 1990. The Corps stopped it with the Old River Control Structure, completed in 1963, which is now set to hold the Atchafalaya at 30 percent of the flow. The whole story of that capture is told in our guide to the Atchafalaya Basin.
Even held to 30 percent, the Atchafalaya carries enough sediment to build the youngest land on the Louisiana coast. In 1942 the Corps dug the Wax Lake Outlet to relieve flooding at Morgan City, and sediment soon filled Wax Lake and spilled into the bay. The outlet receives about 34 million tons of sediment a year; between 1941 and 2005 its delta pushed about 8 km into the sea, and by one estimate it covers about 29 km² (11.3 sq mi). Delta growth in the basin came in bursts, from 1952 to 1962 and again with the 1973 Mississippi flood. The Atchafalaya and Wax Lake deltas are the only parts of the Louisiana coast gaining ground rather than losing it, which is why scientists use them as a model for rebuilding the rest. For the textbook mechanics of how a lobe like this grows, see how deltas are formed.
Since 1930: about 4,900 km² handed back to the Gulf
The delta cycle always involved loss, but it was balanced by gain somewhere else. The 20th century broke that balance. After the Great Mississippi Flood of 1927, the Flood Control Act of 1928 launched the Mississippi River and Tributaries Project, and the river was walled in by levees that stopped it spilling sediment onto the marsh. Dams on the sediment-rich Missouri and other tributaries have cut the lower river's sediment load by almost half since the 1950s. Ten major navigation canals and more than 15,000 km (9,300 mi) of pipelines dredged through the marsh let saltwater into freshwater wetland, and subsidence, possibly sped up by fluid extraction, keeps lowering the ground. Since about 1930, Louisiana has lost more than 4,900 km² (1,900 sq mi) of land to open water. Hurricanes Katrina and Rita alone turned about 520 km² of wetland into open water in 2005.
The response is an attempt to restart the delta cycle by engineering. Louisiana's Coastal Protection and Restoration Authority wrote a $50 billion, 50-year master plan that the legislature approved unanimously in May 2012, with 109 projects ranging from barrier island rebuilding to marsh creation. Its flagship was the Mid-Barataria Sediment Diversion in Plaquemines Parish, a $2.3 billion gate meant to send river sediment into the Barataria Basin, which drew strong local opposition over its effects on fisheries and on communities such as Ironton. It broke ground on 10 August 2023 and was cancelled on 17 July 2025. Without new sediment, studies estimate that 10,000 to 13,000 km² of the delta could be under water by 2100, and the more than two million people who live on it, including the city discussed in our geography of New Orleans, depend on what is decided about that sediment in the next few decades.