Salt marshes are valued for their ecosystem services, and their vulnerability is typically assessed through biotic and abiotic measurements at individual points on the landscape. However, lateral erosion can lead to rapid marsh loss as marshes build vertically. Marsh sediment budgets represent a spatially integrated measure of competing constructive and destructive forces: a sediment surplus may result in vertical growth and/or lateral expansion, while a sediment deficit may result in drowning and/or lateral contraction. Here we show that sediment budgets of eight microtidal marsh complexes consistently scale with areal unvegetated/vegetated marsh ratios (UVVR) suggesting these metrics are broadly applicable indicators of microtidal marsh vulnerability. All sites are exhibiting a sediment deficit, with half the sites having projected lifespans of less than 350 years at current rates of sea-level rise and sediment availability. These results demonstrate that open-water conversion and sediment deficits are holistic and sensitive indicators of salt marsh vulnerability.

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Herein, we investigate the relationship between wind waves, salt marsh erosion rates, and the planar shape of marsh boundaries by using aerial images and the numerical model Coupled‐Ocean‐Atmosphere‐Wave‐Sediment‐Transport Modeling System (COAWST). Using Barnegat Bay, New Jersey, as a test site, we found that salt marsh erosion rates maintain a similar trend in time. We also found a significant relationship between salt marsh erosion rates and the shape of marsh boundaries which could be used as a geomorphic indicator of the degradation level of the marsh. Slowly eroding salt marshes are irregularly shaped with fractal dimension higher than rapidly deteriorating marshes. Moreover, for low‐wave energy conditions, there is a high probability of isolated and significantly larger than average failures of marsh portions causing a long‐tailed distribution of localized erosion rates. Finally, we confirm the existence of a significant relationship between salt marsh erosion rate and wind waves exposure. Results suggest that variations in time in the morphology of salt marsh boundaries could be used to infer changes in frequency and magnitude of external agents.