Tidal-scale observations reveal the event structure of marsh-edge retreat

Saltmarshes are undergoing rapid retreat worldwide, typically manifested as lateral erosion of marsh edges driven by a combination of continuous erosion and episodic mass failure. Observations resolved at weekly to seasonal intervals mask episodic failures, producing deceptively smooth retreat trends. Here, we present a non-contact Marsh Edge Monitor (MEM) observational framework that uses air-based ultrasonic distance sensing to provide the tidal-scale chronology of marsh-edge retreat. A stable–abrupt–stable distance signature in the time series provides an operationally robust indicator of irreversible failure events. During periods of rapid marsh-edge retreat at the study site, cumulative retreat is dominated (>70%) by discrete failure events, indicating that marsh-edge retreat is fundamentally organized as a sequence of discrete geomorphic events rather than a continuous process. Resolving failure timing at tidal scales enables these dominant failures to be aligned with their near-term, potentially triggering hydrodynamic conditions, highlighting the potential importance of short-term forcing (1–3 tides) relative to longer-term averages (7 tides). These results demonstrate that key marsh-edge retreat dynamics can only be resolved with observations of sufficient temporal resolution and continuity.