A one-dimensional numerical model for the coupled long-term evolution of salt marshes and tidal flats is presented. The model framework includes tidal currents, wind waves, sediment erosion, and deposition, as well as the effect of vegetation on sediment dynamics. The model is used to explore the evolution of the marsh boundary under different scenarios of sediment supply and sea level rise. Numerical results show that vegetation determines the rate of marsh progradation and regression and plays a critical role in the redistribution of sediments within the intertidal area. Simulations indicate that the scarp between salt marsh and tidal flat is a distinctive feature of marsh retreat. For a given sediment supply the marsh can prograde or erode as a function of sea level rise. A low rate of sea level rise reduces the depth of the tidal flat increasing wave dissipation. Sediment deposition is thus favored, and the marsh boundary progrades. A high rate of sea level rise leads to a deeper tidal flat and therefore higher waves that erode the marsh boundary, leading to erosion. When the rate of sea level rise is too high the entire marsh drowns and is transformed into a tidal flat.