By Alisha A. Renfro, Ph.D., National Wildlife Federation
As spring weather warms the Midwest, snow melts and drains from 31 states into the Mississippi River. In south Louisiana, the mighty Mississippi River is nearing its peak flow of nearly 900,000 cubic feet of water per second. Rolling down the river with the water is mud and sand, which are essential to building wetlands in the disappearing Mississippi River Delta.
Every hour in Louisiana, a football field of land becomes open water. This land loss crisis is caused in part by levees built for flood protection and navigation which severed the connection between the delta and the river, almost completely halting the land-building processes that once created this iconic landscape.
Sediment diversions are large-scale restoration projects that move sand and mud from the river through the levees into nearby wetlands during high river flows – such as this year’s spring high water flow – to restart the land-building process and help sustain existing wetlands. One important question to understand the full land-building potential of these projects is: Is the Mississippi River a long-term, sustainable source of mud and sand?
A recent Nature Geoscience paper by Jeffrey Nittrouer, Ph.D. of River University and Enrica Viparelli, Ph.D. of the University of South Carolina, “Sand as a stable and sustainable resource for nourishing the Mississippi River delta,” suggests that while the amount of mud carried by the Mississippi River has decreased since the 1970s, the sand it carries has remained steady and may do so for the next 600 years.
The Missouri River is an important source of sediment to the Mississippi, historically supplying about one-half of the total sediment moving down the Mississippi River. However, dams built in the 1950s along the Missouri have been hypothesized as the cause of the large reduction in the amount of total sediment (mud plus sand) that makes its way down to the Mississippi.
In this study, Nittrouer and Viparelli look at changes in the mud and sand carried by the river over a year since the 1970s and found that while the amount of mud that makes it to Tarbert Landing, Miss. (306 miles above the Bird’s Foot delta) has decreased over time, the amount of sand has been consistent. They point out that the likely source of sand is material that is being eroded out of the river channel.
Nittrouer and Enrica applied a model to simulate the response of a sudden reduction in sediment supply that likely occurred with dam construction along the Missouri River in the 1950s. The model indicates that erosion of sand in the river channel between Cairo, Ill. and Vicksburg, Miss. keeps the amount of sand available at Tarbert Landing, Miss. steady for the next 600 years.
Sand and mud are both needed to restore the Mississippi River Delta. Sand, which is 20 percent of the sediment carried by the river, is essential for building new platforms that can support marsh vegetation. Mud, which makes up the other 80 percent, is necessary for maintaining and increasing the resiliency of existing marsh to sea level rise and storm events.
This study suggests that there is a steady supply of sand to the lowermost part of the Mississippi River that can be put to work by constructing and using sediment diversions to mimic nature to build new land, help sustain existing wetlands and begin the restoration of the Mississippi River Delta. This is positive news for large-scale coastal restoration efforts.