Riverine Sediment Contribution to Distal Deltaic Wetlands: Fourleague Bay, LA

Abstract

To combat land loss along the Mississippi River Delta, Louisiana has launched a historic campaign to sustain and regrow coastal lands using, in part, sediment diversions. Previous research has focused primarily on sand-sized sediment load, which is usually deposited proximal to a river’s delta or a diversion’s outlet. Fine sediments constitute the majority of sediment load delivered by rivers, but are understudied with respect to dispersal processes, particularly in terms of sediment supply to distal deltaic bays and wetlands. The Atchafalaya River and associated wetlands serve as prime study areas for this purpose. Fourleague Bay has remained stable against the deteriorative effects of relative sea level rise, standing out along Louisiana’s declining coastline. Push cores were collected once every 2 months, from May 2015 to May 2016, along five central bay sites and five adjacent marsh sites within Fourleague Bay, Louisiana. All sites fall within ~ 10 to 30 km of the Atchafalaya Delta, extending south towards the Gulf of Mexico. Cores were extruded in 2-cm intervals, dried, ground, and analyzed via gamma spectrometry for the presence of 7Be. Inventories of 7Be were then calculated and used to determine daily apparent mass deposition rates (AMDR) over 12 months. Average AMDR values for the bay and the marshes are compared with Atchafalaya River discharge, wind data, and atmospheric pressure through the year of sampling. Peak marsh AMDR, 0.7 ± 0.2 kg m−2 d−1, occurred just after historically high river discharge. Peak bay AMDR, 1.2 ± 0.7 kg m−2 d−1, occurred during seasonal low river discharge and calm winds. Average bay and marsh AMDRs have a moderate negative correlation (r = − 0.51) when compared. Results indicate that, during periods of moderate to high river discharge, sediment bypasses the bay floor and enters the marshes directly when inundation occurs, a process enhanced by the passage of strong atmospheric fronts. During periods of low river discharge and relatively calm winds, riverine sediments aggregate directly onto the bay floor.