Salt marsh assessments focus on vertical metrics such as accretion or lateral metrics such as open-water conversion, without exploration of how the dimensions are related. We exploited a novel geospatial data set to explore how elevation is related to the unvegetated-vegetated marsh ratio (UVVR), a lateral metric, across individual marsh “units” within four estuarine-marsh systems. We find that elevation scales consistently with the UVVR across systems, with lower elevation units demonstrating more open-water conversion and higher UVVRs. A normalized elevation-UVVR relationship converges across systems near the system-mean elevation and a UVVR of 0.1, a critical threshold identified by prior studies. This indicates that open-water conversion becomes a dominant lateral instability process at a relatively conservative elevation threshold. We then integrate the UVVR and elevation to yield lifespan estimates, which demonstrate that higher elevation marshes are more resilient to internal deterioration, with an order-of-magnitude longer lifespan than predicted for lower elevation marshes.