Coastal marshes and their valuable ecosystem services are feared to be lost by sea level rise, yet the mechanisms of marsh degradation into ponds and potential recovery are poorly understood. We quantified and analyzed elevations of marsh surfaces and pond bottoms along a marsh loss gradient (Blackwater River, Maryland, USA). Our analyses show that ponds deepen with increasing tidal channel width connecting the ponds to the river, indicating a new feedback mechanism where channels lead to enhanced tidal export of pond bottom material. Pond elevations also decrease with increasing pond size, consistent with previous work identifying a positive feedback between wind wave erosion and pond size. These two positive feedbacks, combined with bimodal elevation distributions and sharp topographic boundaries between interior ponds and the marsh platform, indicate alternative elevation states and imply that marsh loss by pond formation is nearly irreversible once pond deepening exceeds a critical level. Plain Language Summary Coastal marshes are highly valued ecosystems, but in some areas with increased sea level rise these vegetated marshes disappear and convert into large ponds. Currently, we do not fully understand how these ponds are formed and why marsh vegetation is not recovering in these areas. In this study we measured the soil elevation of marshes and ponds in an area where large marsh surfaces have converted to ponds (Blackwater River, Maryland, USA). We found that ponds are generally deeper when the connection of the ponds with the tidal channels is wider. This indicates that pond sediments can be exported through these channels, and the wider the channel, the easier sediment is exported, leading to deeper ponds. Larger ponds are also deeper because larger waves can develop there, resulting in more wave erosion. These two processes both lead to deeper ponds. Furthermore, we found that there is a sharp elevation drop from the marsh platform into ponds, and that intermediate elevations rarely occur. These all suggest that ponds, once they are formed, are very difficult to recover into marsh vegetation.

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