Meander cutoffs in tidal coastal landscapes: rare of everywhere?

Highly sinuous meandering channels are common landforms in fluvial and coastal environments. As meanders migrate laterally, driven by sediment erosion and deposition along their outer and inner banks, respectively, they eventually cut off, leaving behind the characteristic crescent-shaped morphologies of scroll-bars and oxbow lakes. Oxbows are particularly important not only from ecological perspectives, for the diverse habitats they provide, but also because they retain signatures of the flow characteristics that shaped them, thus allowing for paleoflow reconstruction.

While alluvial plains carved by meandering rivers are littered with scars of meander cutoffs, tidal coastal settings have been perceived by geomorphologists for much of the past century as lacking morphological evidence of active meandering – even though both environments exhibit similar meander-planform dynamics and width-adjusted migration rates.

Here we analyze the planform characteristics and evolution of meander cutoffs from a variety of fluvial and tidal landscapes around the world. We combine field observations and remotely sensed data to track the abandonment of individual meander bends and the subsequent progressive infill and vegetation colonization of the meander cutoffs.

We show that tidal-meander cutoffs tend to be symmetric in planform, seldom disconnected from their parent channel, and fill up as much as 10 times more rapidly than neck cutoffs formed by meandering rivers.

We suggest that cutoffs in tidal meanders are far more widespread than previously thought, and that their supposed paucity is explained by several processes typical of tidal landscapes that collectively militate against the formation and preservation of meander oxbows after cutoff.

These results have important implications for the conservation and restoration of critically endangered coastal environments, as well as for better assessing the capacity of tidal wetlands to store large amounts of blue carbon.