St. Petersburg Coastal and Marine Science Center
Wetland Subsidence and Erosion
The two primary physical processes responsible for historical wetland loss in coastal Louisiana are land-surface subsidence and erosion. The magnitudes of subsidence and erosion at the wetland-loss core sites were estimated by comparing marsh-surface elevations, water depths, and vertical displacements of stratigraphic contacts that were correlated between short sediment cores. The amount of erosion at an open-water core site is equal to the difference in marsh-sediment thickness between the open-water core and the reference stratigraphic section at that site. Similarly, the amount of subsidence at an open-water core site is equal to the elevation difference of the correlated stratigraphic contact between the open-water core and the reference section at that site. The sum of subsidence and erosion at a core location describes the one-dimensional (1D) accommodation space created by the land-to-water change, which is the difference between the reference marsh-surface elevation and the existing water depth at that core site.
The predominance of subsidence or erosion at the study areas varied by physiographic and geologic setting. At the upper delta-plain interdistributary sites (Madison Bay, Pointe au Chien, Bully Camp, and DeLarge), subsidence greatly exceeded erosion; whereas at most of the lower delta-plain study areas (Caminada, Fourchon, and Leeville), erosion was about equal to or greater than subsidence. Along the channel margins at Bayou Perot, erosion generally exceeded subsidence, but magnitudes of subsidence were greater than or equal to the largest magnitudes of subsidence at all study areas. At all of the delta-plain study areas, organic-rich marsh deposits (peats) were preserved at all core sites where formerly emergent wetlands have converted to open water. In contrast, erosion exceeded subsidence at most of the Sabine National Wildlife Refuge study areas, and the thin chenier-plain peat deposits were mostly eroded at the open-water core sites.
Despite differences in geologic setting, similarities in temporal and spatial trends of wetland loss indicate that historical accommodation formation was likely initiated by similar processes in both the delta and western chenier plains. The importance of land-surface subsidence to initiating delta-plain wetland loss is underscored by the fact that erosion was totally contained within the peat section and did not penetrate into the underlying clastic sediments, even at core sites where erosion exceeded subsidence and extant water depths were greater than the thicknesses of the organic-rich marsh sediments. In addition, the expanses of wet marsh that were identified on the historical aerial photography in both the lower delta plain and the western chenier plain also indicates that subsidence was the process that initiated historical wetland loss in those areas. Wet marsh is an intermediate stage in the progression from emergent wetlands to open water and represents nearly uniform drowning of large sections of marsh. At SNWR in the western chenier plain and the Caminada headland sites (Leeville, Fourchon, and Caminada study areas) in the lower delta plain, initial subsidence likely lowered the emergent marshes to a position where they were more susceptible to erosion.