New report studies river diversions as an important restoration tool

By Alisha Renfro, Ph.D., National Wildlife Federation

Since 1932, almost 1,900 square miles of ecologically and economically important land has been lost in coastal Louisiana. Historically, flooding from the Mississippi River built and maintained these coastal wetlands, but the construction of flood protection levees and upstream dams have cut off the connection between the river and its delta. River diversions direct water, nutrients and sediment back into the deteriorating wetlands and serve as an important restoration tool in the Mississippi River Delta.

A report prepared for the Coastal Protection and Restoration Authority of Louisiana titled “Assessment of ‘Lessons Learned’ from the Operations of Existing Freshwater Diversions in South Louisiana” explores almost 1,300 documents related to the subject of river diversions and their effects on soils, vegetation, wildlife and fisheries in coastal Louisiana. As an ecosystem-changing tool, river diversions have both benefits and drawbacks, and the goal of this effort was to investigate the current understanding of the diversions that are in place in order to use the best science and technology available to assist the planning, design and operation of future diversions.

The Wax Lake Delta (pictured) is proof that river diversions can build land in coastal Louisiana. (Photo credit: USGS)

The diversions currently in place in Louisiana have been implemented for a variety of reasons. The report evaluates siphons (West Point a la Hache, Violet), freshwater diversions (Caernarvon, David Pond), crevasses (Cubit’s Gap, West Bay) and floodway control structures (Bonnet Carré Spillway). The findings indicate that diversions that are modeled after natural crevasses with wide and deep channels are able to transport more sediment–particularly sand–and have been the most successful at building new land (e.g. Cubit’s Gap). Several studies also suggest that high river events–such as the one that occurred the spring of 2011–can account for much of the sediment that is deposited in the wetlands. Although mineral sediment input is important to rebuilding marshes, freshwater and nutrients also play an important role.

Freshwater and nutrient input from diversions have affected wetlands at some sites by increasing plant productivity, the abundance of freshwater and intermediate vegetation and submerged aquatic vegetation. High nutrients from the Mississippi River may also cause negative effects in the basins and wetlands by increasing phytoplankton growth, decreasing the quality of soil and the root production of vegetation. But increases in mineral sediment may help offset some of these factors.

Diversions can also influence important fishery species, and the report shows a slight increase in many of the important fishery species (e.g. crabs, white shrimp). The literature also indicated that populations of alligators, muskrats and waterfowl have increased in diversion areas.

Evaluation of the freshwater diversions currently in place indicates the importance of clear objectives, coordinated efforts and sharing of information between projects that will move towards maximizing the benefit for future projects development, design and implementation. Ecosystem-scale restoration projects such as river diversions that harness the power of the river to rebuild the delta have both benefits and drawbacks, but through increased understanding of these challenges, the most effective management decisions can be made to move towards a better future for the Mississippi River Delta.