CAUSES OF HISTORICAL WETLAND LOSS, SABINE NATIONAL WILDLIFE REFUGE, SOUTHWEST LOUISIANA

Prior studies (U.S. Geological Survey 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. This study extends that work, integrating aerial photography, marsh elevations, water depths, and sediment-core data to evaluate similar changes at four additional wetland sites in Sabine National Wildlife Refuge in the western chenier plain. The study sites were selected for their different geological setting compared with the delta plain; also, although the refuge marshes had been managed partly to minimize wetland loss, interior wetland losses there were extensive.

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 magnitudes of subsidence and 1-D (vertical) accommodation that formed in the western chenier plain were generally 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 chenier plain 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 in both the chenier and delta plains.

Table 1. Ranges of parameters (in cm) at wetland-loss sites in the chenier and delta plains.

	Chenier Plain	Delta Plain
Peat Thickness	13-75	89-365
Subsidence	0-26	0-149
Erosion	9-54	0-181
1-D Accommodation	13-75	35-202