Can We Build Stable Land in the Mississippi River Delta with River Sediments?

The Cubit’s Gap Subdelta, which functions like a river diversion.

Does partially diverting the flow of the Mississippi River help build land, or can it contribute to wetland erosion? It’s a question that has plagued Louisiana scientists for over a decade. This controversy developed in the years after Hurricane Katrina, when scientists noticed that there were large areas of wetland loss near the Caernarvon Freshwater Diversion. Some looked at this situation and thought that the problem was that freshwater marshes, which are a major type of wetlands created by river diversions, are less structurally stable that salt marshes.

If this result held up elsewhere, it would bode poorly for large river diversions that are part of Louisiana’s Coastal Master Plan. But there was another school of thought that developed, which stated that the Caernarvon Diversion was simply too small to carry enough sediment to build land. This view suggests that larger river diversions would likely be successful land-building tools. Our research team looked at this issue and thought it would be a great opportunity to use the scientific method to rigorously sort through these competing ideas. The study, “Propensity for erosion and deposition in a deltaic wetland complex” was published in Remote Sensing of Environment and is available here.

The location of the Cubit’s Gap Subdelta, as seen from space on May 17, 2011. Image source: NASA/LSU Earthscan Lab.

We looked at the marshes in the Cubit’s Gap Subdelta, an active delta about 15 miles southeast of Venice, LA. This system is a good place to study large river diversions as it carries about 50,000-100,000 cubic feet of water per second (cfs). This flow is comparable to the Mid-Barataria Sediment Diversion in the state’s Master Plan (max capacity 75,000 cfs), and much larger than the Caernarvon Diversion (max capacity, 10,000 cfs, normal flow about 1,000 cfs). Reda Amer, a Tulane based GIS specialist led efforts to quantify the rate of land area change over the years 2000-2014, a period that included many hurricane strikes, including Hurricane Katrina. Annelise Muscietta, a Tulane graduate student, led efforts to measure the properties of the sediment in this delta and targeted areas that Reda had determined were either building land, eroding, or staying about the same.

What we found was exciting. The marshes that were closest to the river were often immune to erosion, and in some cases they were actively building. These marshes also had the strongest soils, the most mineral sediment, and the salinity of the marsh soils was almost completely fresh. The marshes furthest from the river were the most erosion-prone. These marshes also had the most organic sediments (largely decaying roots), the soils were relatively salty, and structurally these were the weakest soils we measured. Throughout this study, we measured the strength of a soil by inserting a vane, which looked something like the back part of an arrow, and carefully twisting this vane until the soil gave way.

Land loss and land bain in the Cubit’s Gap Subdelta. Note that Head of Passes along the Mississippi River is located in the lower left. Source: Amer et al., (2017)

These findings send a clear message: Large river diversions will likely build stable land because they can carry large amount of river sediment. The results of our work suggest that the problem with the Caernarvon marshes wasn’t so much that they were fresh, it was more that they were sediment poor. The Caernarvon diversion, at least in years leading up to Hurricane Katrina, was carrying too little sediment to build land, as it is relatively small and was rarely operated to capacity.  

This study isn’t the first to show that diverting the river is a great way to bring mineral sediments to wetlands, plenty of other great scientists have made similar points for years. We did show that these sediments don’t only build land, they help keep land there when other places erode. Having sediment-rich wetlands is critical to having wetlands that can withstand the forces of hurricanes and rising seas. As Louisiana looks to the future, it’s good to know that as far as large diversions are concerned, the Master Plan is working with the right toolkit.

Soil properties in the wetland sediments in the Cubit’s Gap Subdelta, with rates of land loss and land gain superimposed on top. Soil salinity is on the left pane, and bulk density in on the right pane. Source: Amer et al. (2017).

Amer, R., Kolker, A.S., Muscietta, A., (2017). Propensity for erosion and deposition in a deltaic wetland complex: Implications for river management and coastal restoration. Remote Sensing of Environment, 199:33-50.

Kearney, M.S., Riter, J.C.A., Turner, R.E., (2011). Freshwater river diversions for marsh restoration in Louisiana: Twenty-six years of changing vegetative cover and marsh areas. Geophysical Research Letters, 38: L16405, doi:10.1029/2011GL047847,