LOCALIZED EROSION AND PROGRADATION ALONG THE CHENIER PLAIN COAST, LOUISIANA

Rates of coastal land loss and wetland deterioration along the Louisiana shoreline are the highest in the United States. An estimated 40 square miles of marsh land are lost annually on the Mississippi delta in response to rising sea level, subsidence, and reduced sediment supply due to migrating depocenters in the Mississippi distributary system. Along an area of the Louisiana coast known as the chenier plain, west of Atchafalaya Bay, areas of mudflat progradation and accretion have been identified that contrast with the long-term trend of coastal retreat along the Gulf shoreline. This section of the coast is located downdrift of the Atchafalaya River discharge, which carries approximately 84 x 10⁶ metric tons of suspended sediment annually into the shallow shelf region.

Patterns of coastal morphology were assessed along 51 kilometers of this chenier plain during the spring and summer of 2001. Surveys extended from Little Constance Lake to northeast of Tigre Point. Areas experiencing erosion and ongoing land loss were identified by the presence of sand washover deposits encroaching upon well-established vegetation near the shoreline. Areas of accretion and progradation were characterized by the presence of mudflats containing recently established wetland vegetation at the waterline. The detailed coastal characterization survey identified alternating areas of erosion (submergence) and mudflat accretion along the western chenier plain. Pronounced accretion was inferred along the eastern chenier plain downdrift of the Freshwater Bayou shipping channel, and erosional processes were inferred east of Freshwater Bayou. Field observations, as well as initial results of isotopic activity analyses (⁷Be, ²¹⁰Pb, ¹³⁷Cs) from shallow-water cores, indicate that accretion and progradation on the eastern chenier plain, though probably fed by sediment discharge from the Atchafalaya river, are enhanced substantially by dredging activity in the Freshwater Bayou channel.