RAPID EVOLUTION OF A MARSH TIDAL CREEK NETWORK IN RESPONSE TO SEA LEVEL RISE

Tidal creeks in Cape Romain, South Carolina, are extending rapidly onto the established marsh platform producing an unusual morphology, which remains self-similar in time. A time-series of aerial photographs establishes that these channels began forming in the early 1940's and are headward eroding at an approximate rate of 1.9 m/yr.

The rapid rate of headward erosion indicates that the marsh platform is in disequilibrium with local relative sea level rise (RSLR); responding to an increased tidal prism through the extension of the drainage network. Long-term (1922-2007) tide gage records in Charleston suggest a rate of 3.2 mm/yr, however this rate may be dramatically higher during observed decadal variations. RSLR in the SRD is likely even higher still as sediment cores reveal that the marsh is predominantly composed of fine-grained sediment, making it highly susceptible to compaction and subsidence. Furthermore, loss in elevation will have been exacerbated by the decrease in sediment supply due to the damming of the Santee River in 1939.

The observed tidal creeks exhibit a distinctive morphology associated with biological mediation during their evolution. Biological feedbacks play a strong role in the morphological development of the creeks. Dieback of vegetation coupled with intense burrowing by crabs produces a bare and topographically depressed region beyond the creek head toward which the channel extends. Measured infiltration rates are three orders of magnitude higher in the burrowed creek head than in a control area (1000 ml/min and 0.6 ml/min respectively). Infiltration of oxygenated water enhances decomposition of organic matter and root biomass is reduced within the creek head (marsh=4.3 kg/m^3, head=0.6 kg/m^3). Burrowing activity also increases resuspension of sediment. The developing depression focuses ebb tidal flow into the channel leading to strong ebb dominance throughout the creek and a net discharge of suspended sediment. Thus the headward incision of tidal creeks is initiated by biologically driven subsidence at the creek heads.

The results of this study provide an alternative scenario to marsh submergence as a response to increasing SLR and clear evidence of the importance of biological feedback in the evolving morphology of marsh tidal creeks.