Salt marshes are dynamic systems able to laterally expand, contract, and vertically accrete in response to sea level rise. Here, we present the grand challenges that need to be addressed to fully characterize marsh morphodynamics. The review focuses on physical processes and quantitative models. Without predictive models, it is impossible to determine the future marsh evolution under accelerated sea level rise. In these models, one of the challenges is to resolve both horizontal and vertical dynamics within the same framework. Vertically, the marsh has to accumulate enough material to contrast rising water levels. Horizontally, marsh erosion at the ocean side must be compensated by landward expansion in forests, lawns, and agricultural fields. The dynamics of the marsh‐upland boundary are still not fully understood and will require more research in the upcoming years. The complexity of marsh vegetation is seldom captured in predictive models of marsh evolution. More research is needed to understand the effects of each species or species assemblages on hydrodynamics and sediment transport. Here, we further advocate that a sediment budget resolving all sediment fluxes in a marsh complex is the most important metric of marsh resilience. Characterization of these fluxes will enable to connect salt marshes to other landforms and to unravel feedbacks controlling the evolution of the entire coastal system. Current models of marsh evolution rely on sparse data sets collected at few locations. Novel remote sensing techniques will provide high‐resolution spatial data that will inform a new generation of computer models.

Ponds are unvegetated rounded depressions commonly present on marsh platforms. The role of ponds on the long‐term morphological evolution of tidal marshes is unclear—at times ponds expand but eventually recover the marsh platform, at other times ponds never recover and lead to permanent marsh loss. Existing field observations indicate that episodic disturbances of the marsh vegetation cause the formation of small (1–10 m) isolated ponds, even if the vegetated platform keeps pace with relative sea level rise (RSLR) and that isolated ponds tend to deepen and enlarge until they eventually connect to the channel network. Here I implement a simple model to study the vertical and planform evolution of a single connected pond. A newly connected pond recovers if its bed lies above the limit for marsh plant growth or if the inorganic deposition rate is larger than the RSLR rate. A pond that cannot accrete faster than RSLR will deepen and enlarge, eventually entering a runaway erosion by wave edge retreat. A large tidal range, a large sediment supply, and a low rate of RSLR favor pond recovery. The model suggests that inorganic sediment deposition alone controls pond recovery, even in marshes where organic matter dominates accretion of the vegetated platform. As such, halting permanent marsh loss by pond collapse requires to increase inorganic sediment deposition. Because pond collapse is possible even if the vegetated platform keeps pace with RSLR, I conclude that marsh resilience to RSLR is less than previously quantified.

The long‐term (3000 years) morphodynamics of an idealized muddy backbarrier tidal basins is studied using a shallow‐water hydrodynamics and wind‐wave model (Deltf3D‐FLOW‐WAVE), modified to include fully coupled marsh organogenic accretion, biostabilization, drag increase, and wave‐induced marsh edge erosion. The latter process is implemented with a novel probabilistic algorithm. Starting from an initially empty basin with a uniform bed slope, a network of channels incise the mudflat, sediment is released, and marshes establish at the basin landward margin. If enough mud is supplied to the basin from the shelf, marsh progradation counteracts marsh edge erosion and marshes expand. Marsh expansion does not completely fill the basin but leaves open a few km‐wide channels, large enough for waves to resuspend sediment. Starting from a basin (almost) filled with marshes, a drop in the external mud supply or an increase in the rate of relative sea level rise causes the basin to empty out by marsh edge erosion, while the marsh platform, aided by reworking of the sediment released by marsh retreat and mudflat deepening, keeps pace even with fast rates (10 mm/yr) of relative sea level rise. Even if the marsh does not drown, the marsh retreats faster if the rate of sea level rise increases, because more sediment is sequestered to fill the newly created accommodation space and is thus not available for marsh progradation. This study suggests that prediction of marsh erosion requires a basin‐scale sediment budget, and that edge erosion, not platform drowning, is likely to dominate marsh loss.

The relationship between lateral erosion of salt marshes and wind waves is studied in Hog Island Bay, Virginia USA, with high-resolution field measurements and aerial photographs. Marsh retreat is compared to wave climate calculated in the bay using the spectral wave-model Simulating Waves Nearshore (SWAN). We confirm the existence of a linear relationship between long-term salt marsh erosion and wave energy, and show that wave power can serve as a good proxy for average salt-marsh erosion rates. At each site, erosion rates are consistent across several temporal scales, ranging from months to decades, and are strongly related to wave power. On the contrary, erosion rates vary in space and weakly depend on the spatial distribution of wave energy. We ascribe this variability to spatial variations in geotechnical, biological, and morphological marsh attributes. Our detailed field measurements indicate that at a small spatial scale (tens of meters), a positive feedback between salt marsh geometry and wave action causes erosion rates to increase with boundary sinuosity. However, at the scale of the entire marsh boundary (hundreds of meters), this relationship is reversed: those sites that are more rapidly eroding have a marsh boundary which is significantly smoother than the marsh boundary of sheltered and slowly eroding marshes.