Unraveling drivers of crevasse splay evolution: A case study of the Fort St. Philip crevasse on the Mississippi River Delta

Study region The Fort St. Philip (FSP) crevasse on the Mississippi River Delta, Louisiana, USA, represents a dynamic sediment diversion system connecting the river to the Breton Sound Basin. This region is characterized by rapid subsidence, tidal influence, and vegetation-hydrology interactions that shape deltaic land building. Study focus This study quantifies the morphodynamic evolution of the FSP crevasse splay between 2013 and 2019 and evaluates the relative roles of vegetation, tides, waves, and diverted sediment in controlling land building. We applied a Delft3D-FM morphodynamic model validated using satellite-derived elevations generated through a Random Forest machine-learning approach. The model incorporates site-specific sediment properties, subsidence rates, and seasonal vegetation dynamics. New Hydrological Insights for the Region Results show that vegetation and diverted riverine sediment were the dominant positive drivers of splay growth, while tides and waves acted as erosional forces. Vegetation increased splay volume above 0.00 m NAVD88 by 77 % in comparison to the simulation without vegetation, whereas tidal and wave processes produced substantial net erosion. Hydroperiod analysis reveals persistent inundation in distal and medial zones, enhancing sediment receiving and retention, while proximal areas remain strongly controlled by seasonal river discharge. The integration of machine-learning-derived topography with process-based modeling provides a transferable framework for assessing sediment diversion performance and wetland resilience in river-dominated deltas worldwide.