By Alisha A. Renfro, Ph.D., National Wildlife Federation
The sediment and water transported by the Mississippi River built much of the ecologically-rich Mississippi River Delta and Louisiana coastline. But over the last decade, manmade modifications throughout the river basin to improve navigation and flood protection have disconnected the river from its delta. This has reduced the amount of sediment carried by the river and severed the connection between the river and the adjacent wetlands it naturally built. Sediment is a precious resource, and the ability to restore the Mississippi River Delta relies on a thorough understanding of how much sediment is moving in the river, where it is deposited and how much is lost to the Gulf of Mexico. Answering these questions will help scientists and coastal planners develop restoration projects, such as river diversions, that effectively reconnect the sediment in the river with the coastal wetlands that need it.
A recent study led by Mead Allison, Ph.D., “A water and sediment budget for the lower Mississippi-Atchafalaya River in flood years 2008-2010: Implications for sediment discharge to the oceans and coastal restoration in Louisiana” (Journal of Hydrology, Vol. 432-433), advances the understanding of resources transported through the Atchafalaya-Mississippi River system. Using data from monitoring stations, previous studies and boat-based measurements, the researchers measured and tracked the water and sediment as it moved through the Mississippi-Atchafalaya River system.
The Mississippi River discharge varied seasonally and annually during the study period (2008-2010). Averaged over the three-year study period, only 50 percent of the water measured at Baton Rouge, La. is still carried by the river by the time it reaches the Bird’s Foot Delta. Much of this loss occurs more than 30 miles below New Orleans and is due to natural and manmade breaks in the river levees. The fine mud and silt that comprise the bulk of the sediment carried by the Mississippi River followed a similar pattern as the water.
In contrast, sand, which is often considered crucial for coastal restoration, had a much different pattern. More than 50 percent of the suspended sand that was measured in the river was deposited either in the river channel or along the river bank between Tarbert Landing, Miss. and Baton Rouge, La. Down at the Bird’s Foot Delta, only around 2 percent of the suspended sand measured at Tarbert Landing, Miss. was transported through the southern passes and lost to the Gulf of Mexico The rest was either deposited in the river channel (approximately 30 percent) or transported out through the natural and manmade breaks in the river levee (approximately 15 percent).
The 2012 Louisiana Coastal Master Plan includes a suite of river diversions that are instrumental in diminishing the current rate of land loss in the region. The data from Allison’s study suggests that to use the limited amount of sand available to build land, diversions should be located above the rapidly sinking Bird’s Foot Delta and operate during rising river discharge to maximize the sediment transported through the diversion into the wetlands, while minimizing the sediment deposited in the river channel which can interfere with navigation. Strategically locating river diversions will both help rebuild land in the Mississippi River Delta as well as reduce the need to dredge the river for navigation.