The evolution of a fringing salt marsh is influenced by the interaction of many factors, including relative sea-level rise, marsh aggradation, nearshore sedimentation, wave climate, and tidal range. Rising relative sea levels generally cause the marsh/upland border to transgress the uplands, thereby potentially increasing the areal extent of a marsh. In contrast, the marsh shoreline may either retreat or prograde in response to the interaction between marsh and lagoonal processes. Consequently, the direction and rate of marsh shoreline movement are critical factors in governing the areal extent of a marsh. Therefore, based on the geomorphic history of a fringing salt marsh in Rehoboth Bay, Delaware, a model for the response of a marsh shoreline is presented. Depending upon the relative rates of marsh and lagoonal processes, a marsh shoreline can either (1) retreat by erosion, (2) prograde, or (3) drown in response to local relative sea-level rise. Specifically, if the rate of marsh aggradation is equal to or greater than the rate of relative sea-level rise (RSLR), then the marsh shoreline either progrades or erodes if the nearshore sedimentation rate is greater than or less than the rate of RSLR, respectively. If the marsh aggradation rate is less than the rate of RSLR, then the marsh drowns.