Salt marshes are valuable ecosystems that must trap sediments and accrete in order to counteract the deleterious effect of sea‐level rise. Previous studies have shown that the capacity of marshes to build up vertically depends on both autogenous and exogenous processes including eco‐geomorphic feedbacks and sediment supply from in‐land and coastal ocean. There have been numerous efforts to quantify the role played by the sediments coming from marsh edge erosion on the resistance of salt marshes to sea‐level rise. However, the majority of existing studies investigating the interplay between lateral and vertical dynamics use simplified modelling approaches and they do not consider that marsh retreat can affect the regional scale hydrodynamics and sediment retention in back‐barrier basins.

In this study, we evaluated the fate of the sediments originating from marsh lateral loss by using high‐resolution numerical model simulations of Jamaica Bay, a small lagoonal estuary located in New York City. Our findings show that up to 42% of the sediments released during marsh edge erosion deposits on the shallow areas of the basin and over the vegetated marsh platforms, contributing positively to the sediment budget of the remaining salt marshes. Furthermore, we demonstrate that with the present‐day sediment supply from the ocean the system cannot keep pace with sea‐level rise even accounting for the sediment liberated in the bay through marsh degradation. Our study highlights the relevance of multiple sediment sources for the maintenance of the marsh complex.