Trends and causes of historical wetland loss, Sabine National Wildlife Refuge, southwest Louisiana

Abstract

Prior U.S. Geological Survey studies (Open-File Reports 2005-1216 and 2009-1158) examined historical land- and water-area changes and estimated magnitudes of land subsidence and erosion at 10 wetland sites in the Mississippi River delta plain. The present study extends that work by analyzing interior wetland loss and relative magnitudes of subsidence and erosion at five additional wetland sites in Sabine National Wildlife Refuge (SNWR) in the western chenier plain. The study sites were selected because their geologic setting differed from that of the delta plain; also, although the refuge marshes had been managed partly to minimize wetland loss, interior wetland losses there were extensive. Historical aerial photography, datum-corrected marsh elevations and water depths, and sediment cores were integrated to evaluate historical land- and water-area changes at SNWR. The thickness of the uppermost Holocene sediments (peat and organic-rich mud) and the elevation of stratigraphic contacts were compared at marsh and open-water sites across areas of formerly continuous marsh to estimate magnitudes of recent elevation loss caused by vertical erosion and subsidence. Results of these analyses indicate that erosion greatly exceeded subsidence at most of the core sites, although both processes have contributed to historical wetland loss. Comparison of these results with results of our prior studies indicates that magnitudes of subsidence and total accommodation space that formed in the western chenier plain were less than those in the delta plain. Compared with the delta plain, where subsidence generally exceeded erosion and peat thicknesses were so great that peat was preserved even where erosion was greater than subsidence, the SNWR peats are thin and were absent (eroded) at most open-water sites. Although historical subsidence rates in the chenier plain are substantially lower than most of the same rates in the delta plain, the temporal and spatial trends of rapid wetland loss, highest rates of land-surface subsidence, and high rates of oil-and-gas production are similar, indicating that historical wetland loss was likely initiated by similar processes (deep-subsurface subsidence) in both regions.