Definitions

delta plain

Mississippi River Delta

The Mississippi River Delta is the modern area of land (the river delta) built up by alluvium deposited by the Mississippi River as it slows down and enters the Gulf of Mexico. The deltaic process has, over the past 5,000 years, caused the coastline of south Louisiana to advance gulfward from 15 to 50 miles (24 to 80 km).

It is a biologically significant region, comprising 3 million acres (12,000 km²) of coastal wetlands and 40 % of the salt marsh in the contiguous United States. It is also a commercially significant region, supporting the economy of New Orleans with significant shipping traffic, providing 16 to 18 % of the US oil supply, and providing 16 % of the US's fisheries harvest, including shrimp, crabs, and crayfish.

Recent influences

About every thousand years, the Mississippi river has changed course. Each Mississippi River deltaic cycle was initiated by a gradual capture of the Mississippi River by a distributary which offered a shorter and steeper route to the Gulf of Mexico. After abandonment of an older delta lobe, which would cut off the primary supply of fresh water and sediment, an area would undergo compaction, subsidence, and erosion. The old delta lobe would begin to retreat as the gulf advanced, forming bayous, lakes, bays, and sounds.

In the last 100 years or so, the river has been diverting more of its flow to the Atchafalaya River, which branches off some 60 miles (95 km) northwest of New Orleans. In the mid-20th century, engineers observed that the Mississippi would soon abandon its current channel as the mainstream, and instead migrate to the Atchafalaya Basin.

Because there is extensive economic development along the current path of the Mississippi, and because extensive flooding and evacuation would occur in the new area, Congress instructed the Army Corps of Engineers to maintain the then-present 70% / 30% distribution of water between the Lower Mississippi and the Atchafalaya River channels, respectively. They did so by building the Old River Control Structure, which consisted of massive floodgates that could be opened and closed as needed at the entrance to the Old River.

Manmade changes to other parts of the Mississippi River have had a pronounced effect on the Delta region. Dams, artificial channeling, and land conservation measures have caused a decrease in sediment carried into the delta region, decreasing the rate of build up of the Delta. The currently active lobe is called the Birdfoot (after its shape) or Balize Delta (after the first French settlement La Balize at the mouth of the river). It has been active for 600-800 years.

At the same time, the rate of loss of the Delta has recently increased past the rate of build up, causing a net loss of wetlands in the Delta area. The rise in sea level has caused increased erosion, as the fresh water vegetation that previously protected against erosion dies due to the influx of salt water. Subsidence has increased.

History

(See La Balize, Louisiana)

Build-up of the Gulf of Mexico shoreline due to the outflow of the Mississippi River has been occurring in a periodic fashion since the late Jurassic period. This same process is responsible for build up of the larger Mississippi embayment; however, the delta region is the most recent and ecologically distinct portion.

The latest cycle of delta formation can be traced to the pleistocene epoch, when a large amount of ocean water was tied up in glaciers. The sea level was 300-400 feet (~100m) below present level, and causing the mouth of the Mississippi to be located SE-USshoreline.jpg into the Gulf of Mexico. 10,000 years ago, the glaciers began to melt, and the sea level began to rise. 5,000 - 6,000 years ago, the sea level stabilized, and formation of recognizable modern deltas began.

Lake Pontchartrain was formed during the evolution of two separate delta lobes. 4000-3800 years ago, the Cocodrie lobe * expanded over the area where New Orleans presently resides, forming the lake's southern shore. 2800-2600 years ago, the St. Bernard lobe * pushed forward and completed the lake's eastern shoreline. Lafourche delta lobe (shown in blue) was the youngest abandoned lobe. It reached just south of where Venice is today. The active Birdfoot or Balize delta lobe is shown in cream on the image.

In 1699 the French built their first rude fort at La Balize, on the Southeast Pass in Pass á Loutre, to control passage on the Mississippi. By 1721 they had built the wooden lighthouse-type structure (la balise means seamark in French) that gave the settlement its name. Built in the marshes, the village was vulnerable to hurricane damage. In addition, ships had to deal with the shifting conditions of tides, currents and mudflats through the mouth of the delta. From 1700 to 1888, the main shipping channel was changed four times in response to shifting sandbars, mudflats and hurricanes. La Balize was moved by 1853 to the Southwest Pass when that became the main shipping channel, then rebuilt at Pilottown about five miles upriver above Head of Passes after being destroyed by a hurricane. The Southwest Pass is currently the main shipping channel, with secondary uses at the South Pass and Pass á Loutre.

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