The Mississippi Delta

After filling the gulf that had preceded it within 2,000 years, the Old River created several successive deltaic lobes which changed during the river’s changes of course, on a constant search for the shortest route to the sea. The current point which forms the “Missisippi River Delta” is just a 1,000 years old. Only a containment of the river and dams on the Old River prevented another diversion of the Mississippi towards the Atchafalaya during the 20th century.

René-Robert Cavelier de La Salle was the first French explorer to go down the Mississippi, coming from Canada as far as the delta in 1682. He baptized this land “Louisiana” and claimed it for the King of France. He was however then incapable of finding the exact position of the delta by sea. His attempt at colonization, which was launched in 1684, came to a stop on the Texan coasts where he lost his life in 1687. It was necessary to wait until 1699 for the Franco-Canadian expedition of Pierre Le Moyne d’Iberville to localize exactly this delta in the Gulf of Mexico. It was in 1717, that his younger brother by 20 years, Jean-Baptiste Le Moyne de Bienville convinced John Law, the royal financier, who had just bought up the rights over the Mississippi Company to set up a new capital of Louisiana, called New Orleans in honour of the ruler. Even though the Mississippi Company collapsed into utter bankruptcy in 1720, New Orleans had still been founded and became the capital of Louisiana in 1723, thanks to its strategic and defensive position in the marshes of the delta. However, Le Blond de la Tour, the engineer in charge of setting up the new capital, revealed in 1722 that “the lands of New Orleans are so full of loam that, as soon as it rains, you sink in them down to your knees.”

Among the candidates favourably impressed by the prestigious patronage, as claimed by the new colony, Antoine-Simon Le Page du Pratz, a young architect aged 23, took ship at La Rochelle in 1718. He lived in America managing plantations until 1734, and told of his experiences in his Mémoires sur la Louisiane which he published in a series of instalments in the Journal Économique de Paris from 1751 to 1753. These chronicles were finally brought together in three volumes as a Histoire de la Louisiane which was published in 1758. He observed that the lands of the region around New Orleans, where he managed a plantation from 1728 to 1734, were made up of mud deposed by the Mississippi during its annual spate, which generally start in the early spring and lasted about three months. These marshy lands were soon covered by grasses and reeds, which intercepted sediments during subsequent spates, stopping most of them at a small distance from the river. In this way, the banks of the Mississippi became higher than the surrounding territory.

Slave plantations of rice and indigo were set up along the river according to the Canadian ribbon system. The “Code Noir” was introduced into Louisiana in 1724 and each planter was responsible for setting up a dams protecting his lands on the banks of the river. The history of the occupation and settlement of the Mississippi Delta since the 18th century is a long series of floods, damming and increasingly extravagant diversions which cut off the river’s waters from the surrounding marches.

The Mississippi Delta is made up of uncompacted, water-soaked sediments which sink as they become more compact, in a phenomenon of subsidence, which speeds up when drawing nearer the ocean, from the action of fault-lines which make the most southern-lying lands slide slowly towards the abysses.  When a deltaic lobe stops being fed by one of the river’s main arms, it tends inevitably to crumble into the Mexico Golf, which regains ground.

Deprived of fresh sedimentary input by mankind, the delta was submerging progressively, then even more quickly as mining industries started, as of the 1940s, to extract millions of cubic metres of oil, natural gas and water from the undersoil, which sped up the subsidence of the delta. 5,000 km² of beach-land marshes gave way to stretches of water, in the last third of the 20th century. According to the models of the federal geological service, subsidence combined with the rising of the sea level could lead to an estimated loss of another 1,800 km² of wetlands by 2050. The State of Louisiana has spent 1 billion dollars a year to limit this submersion since 2007. This desperate struggle to save the delta has been made even more difficult by the accelerated rise of the sea level, created by global warming. With a delta of 75,000 km² at an altitude of under one metre, the entire Mississippi delta could well be under water by the end of the century.
 

Published in september 2020
Picture caption : Embouchure du fleuve St Louis. 18e