New report studies river diversions as an important restoration tool
September 15, 2011 | Posted by Elizabeth Van Cleve in Diversions, Reports, Restoration Projects

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.

2 Responses to New report studies river diversions as an important restoration tool

  1. Please include the people of Plaquemines Parish in the decision making process of using river diversions versus suction dredge pipelining from the river. We are the true stake holders who make our livelyhoods in the marsh ecosystems in our area. The natural river crevasses on the east bank south of Ostrica, La. have been flowing for 35 years. The BS-11 project study shows that there is a 8.5% land loss rate in that area. The river cannot build land above sea level. The present dynamics of the river are not sufficient to accomplish what it once did many years ago. The passes in Venice are not building land, they are eroding and losing their banks and gulfward structure. It is nice to dream of the mighty Mississippi River saving southeast Louisiana, but the truth is the river diversions won't be able to compete with the present subsidence rate and sea level rise. Pipe lining sediment from the bottom of the river is the savior for the delta area. I was born there and lived through the coastal erosion process for the last 69 years. I know the river and I know the ecosystems. If the natural existing crevasses and passes are not presently forming deltas then new ones will not save the lower end of Plaquemines Parish. When that area is no longer there the protection which it now provides for New Orleans, etc. won't be there either.
    The citizens of lower Plaquemines Parish have not been adequately informed about the state master plan.

  2. Alisha Renfro says:

    Thank you for your comment.

    Louisiana’s 2012 Coastal Master Plan analyzed many different types of restoration tools, including marsh creation and sediment diversions. The Master Plan recognizes the importance of pipelining sediment for marsh creation, with more of the expected budget over the next 50 years going towards marsh creation than any other type of restoration. However, marsh creation is too costly to be done at a scale similar to current land loss rates. Marshes can be built to higher elevations that will extend their longevity as sea level rises, but without a sediment source these marshes are not sustainable over the long term. Therefore, sediment diversions are included to provide a source of sediment to the existing and created marshes that would help them maintain their elevation as sea level rises, as well as build new land. This process is observed in marshes throughout the world, but is currently limited in our sediment-starved delta system.

    To further answer the question about whether diversions are really necessary, the Coastal Master Plan compared estimates of land change over the next 50 years with and without diversions and found that excluding sediment diversions would reduce land-building by as much as 630 square miles. Please see the post “The Next 50 Years: Sediment diversions as a necessary restoration tool” for a more information.

    In order to maximize land building with sediment diversions, it is fundamental that lessons learned from the diversions and crevasses that currently exist along the river be applied. As you pointed out, despite the natural crevasse system on the east bank of the river, south of Ostrica, there is still a net land loss in this area. This reflects several different factors that affect land building in that region: 1) subsidence rates in that region are some of the highest in the world, ranging from 0.6 to 1.4 inches per year, 2) the amount of sand carried by the river decreases as it moves towards the Bird’s Foot delta, and 3) subdeltas (the land built by diversions) undergo a life cycle that includes a period of land growth followed by a period of deterioration that then repeats. Applying these lessons learned to maximize land building, we can locate future sediment diversions higher up along the river where subsidence rates are lower. We can identify locations where more sand is available to be captured and diverted. And we can apply our understanding of the subdelta life cycle to the operation and management of diversions over the long term.

    In the next year, significant funding will become available for restoration. As a long term resident of Lower Plaquemines Parish, you know the urgency of moving forward through your lifetime of experience. We welcome the opportunity to share your concerns, as well as the information that informs our goals for a sustainable coastal Louisiana. It is more critical than ever that we move forward together, with all the tools available to us, to restore our coast.

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