Rethinking Tidal Meanders: Dynamic Evolution and Eco-Morphodynamic Impacts

Highly sinuous meandering channels are a defining feature of tidal coastal wetlands, yet their morphodynamic behavior remains far less understood than that of their fluvial counterparts. For decades, tidal meanders have been considered fundamentally different from river meanders, largely because coastal landscapes appear to lack the classic morphological signatures of active meandering, such as cutoffs, oxbow lakes, and scroll bars. This apparent absence has reinforced the idea that bidirectional tidal flows and strong eco-geomorphic feedbacks suppress fluvial-style bend evolution in intertidal environments. Recent observations of rapid channel migration in tidal wetlands, however, challenge this view and suggest that tidal meanders may be far more dynamically similar to rivers than traditionally assumed.

Here we combine field observations, remote sensing, and numerical modeling to investigate the planform evolution of meandering tidal channels across global coastal wetland biomes. Our goal is to resolve a long-standing paradox: if tidal meanders are dynamically migrating features, why do their landscapes appear to lack the geomorphic footprints of active meandering?

A global-scale analysis shows that tidal meander cutoffs are widespread, but are rarely preserved in forms that are easily recognizable in the field or in aerial imagery. Besides being generally small in size, tidal cutoffs are – unlike river meanders - seldom fully disconnected from the parent channel: high channel density and strong hydrological connectivity promote frequent reattachment and reworking of cutoff bends. Second, cutoff meanders are typically rapidly filled, due to high suspended sediment concentrations and intense biological activity. Vertical accretion rates in these former channels exceed those of surrounding marshes by an order of magnitude, leading to swift burial of cutoff morphologies. As a result, the crescent-shaped oxbow lakes so typical of fluvial plains are replaced in tidal wetlands by ephemeral, sediment-filled depressions that are rapidly colonized by vegetation and hence difficult to identify as formerly active streams.

Despite these differences in preservation, the geometric properties of tidal and fluvial cutoffs are remarkably similar, indicating that the same curvature-driven instabilities govern bend growth and cutoff in both environments. This conclusion is supported by a global analysis of tidal meander migration, which shows that vegetation strongly modulates migration rates—but not the underlying mechanism. Channels in unvegetated tidal flats migrate much faster than those in marshes and mangroves, yet their planform evolution follows the same fluvial-like rules.

These findings have important implications for coastal wetland evolution, restoration, and carbon cycling. Because abandoned tidal channels act as hotspots of sediment and organic matter accumulation, they represent previously unrecognized sinks of blue carbon. Moreover, the recognition that tidal meanders obey the same physical laws as river meanders allows established river morphodynamic models to be extended to coastal wetlands, enabling more robust predictions of coastal wetland eco-morphodynamic evolution.

Overall, our findings overturn the long-standing view that tidal meanders are morphodynamically distinct from fluvial ones, revealing them instead as fully dynamic, migrating geomorphic features whose signatures are masked by the unique eco-geomorphic processes of tidal landscapes.