Vegetative and Riverine Sediment Source Controls on an Evolving Crevasse Splay in the Mississippi River Delta

We investigate controls on the evolution of an actively accreting wetland splay adjacent to the Fort Saint Philip (FSP) crevasse complex on the lowermost Mississippi River (MR) that is an analog to artificial sediment diversions under development by the State of Louisiana. The FSP complex has been active since 1973 when crevassing occurred during a large MR flood. To assess controls and feedbacks between flow, sediment, vegetation growth cycles, and wetland vegetation species composition, we map seasonal density and canopy properties and use cores to classify marsh substrates. Preliminary results suggest that patterns of sediment deposition and marsh plant colonization are controlled by discharge and elevation, and that flow paths and splay morphological evolution are influenced by floating and subaquatic vegetation (and vice versa). Elevation trends and historical imagery show the morphology of the splay is higher in the older section near the river, which has aggraded and prograded since 1973. Preliminary geochronology results using the radiotracer 7Be show up to 10 cm of rapid deposition of fine to very fine sand sourced from the MR during floods. There is a succession of freshwater marsh vegetation species which is controlled by elevation and hydroperiod, which ranges from permanently inundated at the distal part of the splay to flooded for 3 months annually at the head of the splay. Seasonal vegetation surveys of stem density, volume, and canopy height show that these characteristics change dramatically over seasonal growth cycles, suggesting that vegetation growth cycles combine with the timing of turbid river water flooding and the Gulf of Mexico water elevation to determine spatial and inter-annual sedimentation patterns. We aim to use field data about the specific plant species, stem density, and community distribution, and sediment characteristics to inform numerical models that are central to planning sediment diversions.