Fetch and distance from the bay control accretion and erosion patterns in Terrebonne marshes (Louisiana, USA)

Wetlands in the Mississippi River Delta are fast degrading. Sea level rise and low sediment supply are widely recognized as the two main factors contributing to land-to-water conversion. In order to determine what marsh areas are more resilient, it is fundamental to identify the drivers that regulate marsh accretion and degradation. In this study, a combination of field data and aerial images are used to determine these drivers in Terrebonne Bay, Louisiana, USA. We find that accretion and degradation patterns depend on whether the marsh is located inland in a sheltered area or facing open water. In the first case, the distance to the nearby channel is fundamental, since during flooding of the marsh platform more sediment is deposited in the proximity of channel banks. The accretion rates of marshes facing open water are high and correlate to fetch, a proxy for waves ability to resuspend bottom sediment. These areas are more resilient to sea level rise, but waves are also the main mechanism of degradation, as these marshes tend to degrade by edge erosion. Consequently, we define a bimodal evolution trajectory of the marshes in Terrebonne Bay: marshes close to the bay and facing open water accrete faster but show limited lateral erosion due to waves, whereas sheltered marshes accrete less and degrade in large swaths due to insufficient sediment supply.