Coastal marsh formation and its relation to sediment exchange along the Chenier Plain in southwest Louisiana
It is well recognized that rivers are the primary pathway that delivers sediment to the ocean. However, the fate of these sediments is poorly understood, and is complicated by relative sea level rise and meteorological forcings. One coastal system to examine these issues is the Atchafalaya River-Chenier Plain (ARCP) of southwest Louisiana, which relies on Mississippi River sediment supply and is vulnerable to coastal erosion and land loss. Despite regional coastal degradation, some land gain and marsh growth have been observed here. Land gain in south Louisiana is generally observed at the mouths of the rivers — the Bird's Foot, and the Atchafalaya and Wax Lake deltas. However, satellite imagery and sedimentological analyses indicate that coastal lakes in southwest Louisiana have also filled in and converted into salt marshes in the last 40 years. To understand sediment delivery in these marshes, multiple short cores were collected in a central Chenier Plain tidal creek system, and analyzed for 210Pb, 137Cs, 7Be, δ13C, and grain size distribution. We propose that Chenier Plain reactivation processes are triggered by the increase in Atchafalaya River flow that began in the early 1900s. Fluvial sediments delivered through westward longshore transport and resuspended during energetic events become available to the sediment-starved Chenier coast, leading to deposition, infilling, mudflat progradation, and marsh growth.