CHANGES IN SEDIMENT PROPERTIES OF THE SAVANNAH RIVER ESTUARY (SAVANNAH, GA) FOLLOWING THE 2012 DROUGHT

Sedimentation in an estuarine environment is influenced by river discharge and stratification at the fresh/saltwater interface. In a partially mixed estuarine environment, the salt wedge is weakened by tidal mixing. Change in rainfall and subsequent river discharge shifts the position of the fresh/saltwater interface upstream or downstream. Drought reduces the downstream flow of freshwater, allowing tidal currents to move the salt wedge and corresponding sediments upstream. Heavy rainfall and increased discharge result in erosion and greater sediment transport downstream. In this study, sixteen sediment samples taken from the Savannah River estuary were used to characterize sediment deposits during drought conditions (February 28, 2012) and the beginning of the wet season (June 4, 2012). All samples were collected using a Van Veen grab sampler aboard the R/V Savannah. Grain size distributions were determined by wet sieving; and loss-on-ignition was used to estimate organic content. Samples taken in February from the inner estuary were 93% silt/clay. By June, the silt/clay fraction of inner estuary sediments had decreased to 37% (a 56% decrease). There was a shift to a bimodal distribution of silt and sand at these sites in June. The middle estuary was dominated by sands and exhibited little change in grain size distribution over the course of the study. In the outer estuary, there was a 35% increase in silt/clay content from February (6%) to June (41%). Outer estuarine samples had little sediment coarser than fine sands in June, although coarse sands and shell hash had been present in February. The organic content of estuarine sediments fluctuated with the distribution of fine sediments. Inner estuary sedimentary organic contents decreased from 11% to 3%, while the organic content of outer estuarine samples increased from <1% to 6% between February and June. The data suggest that fine fluvial sediment accumulated in the inner estuary during the drought, but was transported and deposited in the outer estuary after significant rain events.