ECOPHYSICALLY DRIVEN TIDAL CREEK EXPANSION IN THE SANTEE DELTA, SC

Recent research indicates that local relative sea-level rise (RSLR) manifests within saltmarshes by expansion of tidal creek systems to accommodate increasing tidal prism, as seen in coastal marshes along the southeast seaboard. In the Santee Delta, South Carolina, creeks are headward eroding onto marsh platforms, suggesting disequilibrium with RSLR. While long-term (1922-2009) tide gage records in Charleston show a rate of 3.2 mm/yr, this rate can be dramatically higher during decadal timescales. In addition, RSLR is increased due to compaction of the fine-grained deltaic sediment that comprises the marsh platform, particularly after damming of the Santee River in 1939. Creek morphology remains self-similar in time, and historical aerial photographs establish that these channels began forming in the early 1940’s and headward eroding at a rate of 1.9 m/yr. Analysis of long-term sediment accumulation rates using Pb-210 radioisotopes suggest the marsh platform is able to maintain elevation with respect to these high rates of RSLR. It is hypothesized that continued headward incision of tidal creeks is facilitated by biologically driven subsidence at the creek heads. Intense bioturbation from burrowing crabs causes plant dieback and enhanced decomposition of organic material from infiltration of oxygenated water. Measured infiltration rates are three orders of magnitude higher in the burrowed creek heads than in control areas (1000 ml/min and 0.6 ml/min respectively), and root biomass is reduced within creek heads (marsh=4.3 kg/m3, head=0.6 kg/m3). In addition, burrowing activity increases sediment resuspension. These processes combine to form bare, topographically-depressed regions, which focuses tidal flow. Strong ebb dominance leads to a net export of sediment and extension of the tidal channel into the denuded creek head region.