Vegetative and riverine sediment source controls on an evolving crevasse splay in the Mississippi River Delta
This study investigates 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. It is believed that 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. Results show that first-order controls on splay evolution are, (1) elevation (e.g., hydroperiod), (2) proximity to the MR and the crevasses that feed riverine sediment directly to the receiving area, and (3) species level differences in sediment trapping by vegetation. Elevation trends and historical imagery show the FSP splay is in-filling over inter-annual timescales (1998-2022 Google Earth historical imagery) and since 2006 has transitioned into an area of net deposition. Geochronology results using the radiotracer 7Be show up to 6.5 cm of rapid deposition of fine to very fine sand sourced from the MR during floods, while a remnant marsh boundary fronting Breton Sound enhances sediment retention within the FSP splay area. Communities of freshwater marsh vegetation species are similar to those found on other MR delta splays but follow complex patterns of zonation that are often patchy and overlapping. Seasonal vegetation surveys of stem density, volume, and canopy height show that these characteristics change dramatically over seasonal growth cycles, suggesting that vegetation combines 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 in coastal Louisiana.