Identifying the role of tides, rainfall and seasonality in marsh sedimentation using long-term suspended sediment concentration data

Abstract

This study utilizes data on suspended sediment concentrations obtained daily from a terminal tidal creek over a period of a decade as part of the Long-Term Ecological Research Program (LTER) to examine the mechanisms contributing to salt marsh sedimentation. A month long time-series of tidal velocity and amplitude measurements are used to predict tidal currents, elevation and phase during the sediment data collection periods. Tidal variation of suspended sediment load and fluxes are estimated and are used to evaluate the role of spring–neap tidal cycles, rainfall and seasonality in salt marsh sedimentation. The results indicate that spring tides correspond to periods of higher sediment concentrations and sediment import while neap tides correspond to periods of lower concentrations and slight sediment export. This is due to higher current velocities during spring tide, which increase the availability of sediments within the marsh system, in combination with an increase in the duration and area of marsh inundation during spring tides. Low water rainfall is responsible for elevated suspended sediment concentration in the channels due to increased erosion of the exposed marsh surface and creek banks. These low-tide rainfall events are not strictly erosional periods but rather appear to be associated with periods of sediment redistribution within the marsh system, as sediments eroded from the marsh surface are advected into the back marsh basin and deposited during subsequent high waters. In the back marsh, sediment import rates are increased by approximately 50% during periods of rainfall as compared to sediment import rates corresponding to periods without local rainfall, suggesting increased sediment accumulation on the marsh surface during these times. Finally, a seasonal signal of suspended sediment concentrations is observed with highest average concentrations found in the summer when water temperature and biological activity are at a maximum. Despite this increased sediment availability in summer, the fall season is responsible for the majority of sediment imported annually (55%). This occurs because mean sea level is greatest during these times and leads to increased periods of marsh inundation. Therefore, the frequency and duration of marsh inundation is more important than sediment availability in controlling surface accumulation within this marsh system.