A mathematical model is presented to investigate the physical processes which shape tidal creek cross sections, from the initial channel formation within youthful salt marshes to its long-term morphodynamic evolution. Values of bottom shear stresses due to tidal currents are used to calculate erosion and deposition rates of cohesive sediments. The model accounts for the possible growth of salt-marsh macrophytes on the marsh platform, and how these macrophytes affect flow resistance and the rate of sedimentation. The results show that large temporal gradients of leading flow rates induce relevant temporal variabilities of channel geometry in time. In particular, as long as the marsh platform lies below an elevation which allows significant discharges to occur, the channel deepens and narrows down. On the contrary, as soon as the marsh-platform elevation gets close to or higher than mean sea level, the reduction in the discharge flowing through the cross section, leads to channel silting and narrowing.