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 are headward eroding at an approximate rate of 1.9 m/yr. The rapid rate of headward erosion suggests that the marsh platform is in disequilibrium and unable to keep pace with high local relative sea level rise (RSLR >3.2mm/yr) through accretionary processes. 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 channel head toward which the channel head extends. We examine the mechanisms producing this headward extension and pinnate channel morphology, and report a new pattern of creek incision in a regime of rapid RSLR.