Open-File Report 2009–1158
A prior study (U.S. Geological Survey Open-File Report 2005-1216) examined historical land- and water-area changes and estimated magnitudes of land subsidence and erosion at five wetland sites in the Terrebonne hydrologic basin of the Mississippi 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 the adjacent Barataria hydrologic basin. The Barataria basin sites were selected for their diverse physical settings and their recent (post-1978) conversion from marsh to open water. Historical aerial photography, datum-corrected marsh elevations and water depths, sediment cores, and radiocarbon dates were integrated to evaluate land-water changes in the Mississippi delta plain on both historical and geological time scales.
The thickness of the organic-rich sediments (peat) and the elevation of the stratigraphic contact between peat and underlying mud were compared at marsh and open-water sites across areas of formerly continuous marsh to estimate magnitudes of recent delta-plain elevation loss caused by vertical erosion and subsidence of the wetlands. Results of these analyses indicate that erosion exceeded subsidence at most of the study areas, although both processes have contributed to historical wetland loss. Comparison of these results with prior studies indicates that subsidence largely caused rapid interior wetland loss in the Terrebonne basin before 1978, whereas erosional processes primarily caused more gradual interior wetland loss in the Barataria basin after 1978.
Decadal variations in rates of relative sea-level rise at a National Ocean Service tide gage, elevation changes between repeat benchmark-leveling surveys, and GPS height monitoring at three National Geodetic Survey Continuously Operating Reference Stations indicate that subsidence rates since the early 1990s are substantially lower than those previously reported and are similar in magnitude to time-averaged subsidence rates at geological time scales. The historical decrease in land-loss rates across the Mississippi delta plain generally is consistent with the recent decrease in subsidence rates within the same region.
Posted September 2009
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Morton, R.A., Bernier, J.C., and Kelso, K.W., 2009, Recent subsidence and erosion at diverse wetland sites in the southeastern Mississippi delta plain: U.S. Geological Survey Open-File Report, 2009–1158, 39 p., plus app. (p. 41-221).
Field and Laboratory Analyses
Water-Level Measurements and Elevation Corrections
Sedimentary Facies and Depositional History
Mud Accumulation and Probable Causes
Historical Subsidence and Erosion of Delta-Plain Marshes
Subsurface-Fluid Production Near Coring Sites
Barataria and Bayou Perot Fields
Delta Farms Fields
Little Temple and Little Lake Fields
Geological Rates of Subsidence and Aggradation
Historical Land-Surface Subsidence Measurements
Continuously Operating Reference Stations
Conclusions and Implications
Appendix: Core Descriptions and Photographs