A STUDY OF THE ROLE OF TIDES, RAINFALL AND SEASONALITY IN MARSH SEDIMENTATION FROM LONG-TERM SUSPENDED SEDIMENT CONCENTRATION DATA: NORTH INLET, SOUTH CAROLINA

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. Modern hydrodynamic 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 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 the increased erosion of the exposed marsh surface and creekbanks. These low-tide rainfall events are not erosional periods but rather appear to be associated with periods of sediment redistribution within the marsh system. Sediment import during periods of rainfall is increased by approximately 50% when compared to sediment import corresponding to periods without local rainfall. Finally, a seasonal signal of suspended sediment concentrations is observed with highest average concentrations found in the summer when water temperature and biological activity is 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 times 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.