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. 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. While protecting from flooding and maintaining navigation interests, these controls have straitjacketed the Mississippi River within its banks. As a result, the sediment on which the delta complex depends is shunted out to sea -- the leading driver of the region's significance land loss rates.
Exacerbating the problem are other issues, such as the vast network of shipping channels – including the Mississippi River-Gulf Outlet, the Houma Navigational Canal and the Freshwater Bayou Canal. Thousands of miles of oil and gas canals have been dug to accommodate energy infrastructure and extraction. These channels and canals alter the natural hydrology and allow saltwater to penetrate deep into the wetlands, disrupting the salinity balance and killing the vegetation of freshwater wetlands, causing them to subside underwater. Additionally, 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. The sum of these problems mean that the Mississippi River Delta is losing its wetlands. Scientists estimate that the Mississippi River Delta loses a football field of land an hour -- over 1,900 square miles since the 1930s, roughly the size of Delaware.
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.
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