Subsidence, Sea-Level Rise, and Wetland Loss in the Lower Mississippi River Delta

Abstract

Since late Cretaceous, depocenters with oscillating deltas and migrating shorelines have provided a fundamental geologic rhythm to the coast of Louisiana. Sites of deltaic sedimentation have shifted, sea level has risen and fallen by more than 100 m, and sequences of preserved deltas have been vertically stacked in the geologic record. This paper summarizes, in the form of a case history, recent changes in the modern Mississippi River Delta with special emphasis on the causes for geometrically increasing rates of wetland loss that have been experienced since the turn of the century. Rates of relative sea-level rise and discharge of freshwater down the main stem of the Mississippi River (north of Louisiana) appear to have been constant throughout the 1900s, indicating that the demise of the Mississippi Delta is probably a result of an inadequate sediment supply and an inefficient sediment delivery network. The combined effects of levees that prevent overbank flooding and funnel sediments to deep water, upstream dams that trap sediments in the Missouri and Arkansas River basins, and formation of a new delta lobe 150 km to the west have had a profound effect on sediment supply. This loss of sediment load is occurring as the Mississippi Delta is nearing the end of its natural 1000-yr life cycle, and has overwhelmed the ability of fragile wetlands, already in a state of delicate balance, to survive the combined effects of global sea-level rise and subsidence. Mitigation through creation of an extensive network of artificial diversions will slow the rate of delta deterioration but will not be able to rejuvenate a dying delta lobe.