By Alisha A. Renfro, Ph.D, Coastal Scientist, National Wildlife Federation
The Mississippi River has played an important role in the history, physical and economic growth of the United States. However, the Mississippi River and the delta region it built didn’t always look the way they do today. In an article by Michael Blum, Ph.D. and Harry Roberts, Ph.D. titled “The Mississippi Delta Region: Past, Present, and Future” published in The Annual Review of Earth and Planetary Sciences (vol. 40), researchers assemble the extensive scientific information that has been reported about the development of the Mississippi River Delta region, the state of the area today and what this means for the future of coastal Louisiana. This study will help scientists plan a sustainable future for the delta.
The modern landscape of the Mississippi River Delta region began to form around 7,000-8,000 years ago. As the river meandered back and forth across the delta plain, it deposited sediments that had been collected from throughout the river’s 31-state drainage basin. Five delta headlands were built as the river changed its course every 1,000-1,500 years across what is now coastal Louisiana. Once the river changes locations and is no longer actively building and maintaining a particular delta lobe, natural processes begin reworking the area, causing marsh to retreat landward and the sandy seaward extent of the lobe becomes a barrier-island arc. It was these processes that formed the Louisiana bayous and barrier islands that we know of today.
The modern day Bird’s Foot delta, also known as the Plaquemines/Balize delta, began forming just over 1,000 years ago. About 500 years ago, the Atchafalaya River, which branches off of the Mississippi River south of the Old River Control Structure, formed the Atchafalaya and Wax Lake deltas, which emerged in the 1970s. Today, active delta building is limited in the Mississippi River Delta to only the mouth of the Atchafalaya River and Wax Lake Outlet as well as a few controlled and uncontrolled diversions an along the Mississippi River. Without more land building opportunities, the delta will collapse.
Modifications along the river system, such as levees and dams, along with sea level rise and higher rates of land sinking have drastically changed conditions along the Mississippi River system that once built and maintained the delta region. Today, rather than migrating back and forth across the deltaic plain, the river is locked in place by an extensive levee system. Even if the river could still migrate, dams and reservoirs in the upper drainage basin states have reduced the amount of sediment carried by the river by half since the 1950s. In addition, sea level rise coupled with land sinking due to compaction of muddy river sediment and fluid withdrawal associated with the gas and oil industry has accelerated land loss in the abandoned delta headlands.
The changes in the river, sea level and sediment supply mean that the future delta landscape will not resemble that of the past. Restoring the delta-building processes of the river will require us to rethink how we manage the resources of the Mississippi River and re-imagine what the future coastline could look like. River diversions that reconnect the sediment and water resources in the Mississippi River to the marsh landscape that surrounds it is a crucial step toward restoring the delta-building and land-sustaining processes of the region.