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.
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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
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