Delta

As the river that formed a delta inevitably goes through changes upstream, a particular lobe may be abandoned. This usually occurs because a crevasse forms upstream by avulsion, and provides a more direct route or a steeper slope by which water can reach the sea or lake. As a result, the crevasse evolves into a new distributary channel and builds up a new delta lobe, a process called delta switching. The old distributary channel downstream is filled in by fine-grained sediment and abandoned. Over the last 5,000 years, the Mississippi River has abandoned at least six major lobes by avulsion.

An even larger-scale redirection of flow threatens to trigger abandonment of the entire Mississippi River delta. The Atchafalaya River, which follows an old course of the Mississippi, has a steeper slope than the modern Mississippi. At a point where the Atchafalaya River flows directly adjacent to the Mississippi, it is possible for the Atchafalaya to capture, or pirate, the flow of the Mississippi. This could permanently redirect the Mississippi away from New Orleans, and into Atchafalya Bay to the west. Since the 1950s, the U.S. Army Corps of Engineers has controlled the flow of the Mississippi in this area. In the 1980s, when the Mississippi River had several seasons of unusually high water levels, additional efforts were necessary to avert this disaster, but it may threaten again in the future.