SOUTHEASTERN ESTUARIES: PATTERNS AND PROCESSES OF DEPOSITION AND ACCUMULATION

Although estuaries are typically long-term repositories for fine-grained particles, within the estuarine environment these particles are dynamically remobilized and redistributed. Sediment distribution and accumulation patterns exhibit spatial and temporal heterogeneity on a variety of scales. Work during the past decade in the fluvial and saltmarsh estuaries of the Georgia and South Carolina coasts documents the effects of sedimentary processes on the estuarine stratigraphic record. In South Carolina, both Winyah Bay and the Ashepoo River in the ACE Basin National Estuarine Research Reserve exhibit extremely rapid fine-grained sediment deposition at rates approaching 5 cm/month as documented by the distribution of the radiotracers Th-234 and Be-7. Deposits are dominated by muddy sediments with thin sand stringers. These deposits are removed each year and widely distributed into the extensive salt marshes. The residual deposits, a black, low-porosity mud overlain by a few cms of sand, represents the stratigraphic signature of this massive deposition. In Winyah Bay, harbor dredging annually creates accommodation space that is quickly filled by muddy deposits within at most a few months. In the Satilla River estuary, sediment distribution and accumulation patterns exhibit variability on tidal to decadal timescales and on broad spatial scales as well. Redistribution of mobile sediments as evidenced by high-concentration mud suspensions (up to 5 g/L) occurs with each tide but sediment accumulation as documented with Pb-210 accumulation rates is discontinuous on decadal timescales. Sidescan surveys of the Satilla River document that sediment is discontinuously distributed along the estuary, although the upper mesotidal setting (3 m spring tidal range) would argue for widespread and homogenous distribution of material.