The Changing Courses of the Mississippi River
The Louisiana coast has been built over the last 7,000 years by the Mississippi River changing course and creating six different delta complexes. Before the extensive levee system that “trained” our river to stay in one place, the Mississippi changed course about once every 1,000 years.
The Mississippi River will always take the most direct path to the Gulf of Mexico—generally the route with the steepest gradient. As it occupies a course, it begins to deposit sediment along its route to the ocean. This decreases the slope of the river channel over time, and eventually, this path will no longer be the most efficient. At this point, the river will avulse (change course), occupying another path which has a steeper channel. The river will begin to divert increasingly more of its water and sediment down the other channel, until eventually the old channel is completely abandoned.
The modern Mississippi Delta is overdue for an avulsion. In a natural system, the Mississippi River would have overtaken the Atchafalaya River channel due to its more efficient gradient. Engineering efforts, through levees and diversion structures such as Old River Control Structure, enable us to prevent a course change. Despite our ability to control the river’s physical path, it is difficult to ensure that the amount of water and sediment traveling down the channel won’t decrease over time, since the system naturally began allowing more water and sediment to travel down the Atchafalaya and less down the Mississippi.
Our Bird’s Foot delta remains in the same physical location, but due to the natural “delta cycle” (tendency to abandon one delta and relocate to another), the Bird’s Foot is not the healthy prograding (building) delta that it once was. Instead, the delta is retreating over time.