Five representative areas of the Mississippi River delta plain were investigated using remote images, marsh elevations, water depths, sediment cores, and radiocarbon dates to estimate the timing, magnitudes, and relative rates of marsh erosion and land subsidence at geological and historical time scales. In the Terrebonne-Lafourche region of rapid interior-wetland loss, former marshes are now submerged beneath water that averages 0.5 to 1.0 m deep. Most of the permanent historical flooding was caused by rapid subsidence and collapse of the delta plain that occurred during the late 1960s and 1970s. Subsequent erosion of the submerged delta-plain marsh was relatively minor at most of the coring sites.

Widespread nearly simultaneous collapse of marshes across the Mississippi delta plain appears to be unprecedented and not repeated in the geological record of the past 1,000 years. Surface and subsurface data strongly indicate that the rapid subsidence and associated wetland loss were largely induced by extraction of hydrocarbons and associated formation water. Average historical rates of subsidence between 1965 and 1993 were about 8 to 12 mm/yr, whereas average geological rates of subsidence for the past 5,000 years were about 1 to 5 mm/yr. Natural processes such as deep-seated salt migration and fault movement cannot be discounted entirely, but there is no compelling evidence that these processes were responsible for the observed historical changes. Results of this study provide a basis for determining the relative importance of subsidence and shoreline erosion as causes of past wetland loss and for predicting sites and probable mechanisms of future wetland loss. This information should improve the selection of project sites and designs for wetland-loss mitigation and coastal restoration in south Louisiana.