A puzzling aspect of marsh morphodynamics is the occurrence of channel bank slumping despite the absence of channel widening and lateral migration. An apparently unrelated conundrum is why the vertical accretion rate on the low marsh adjacent to channels is higher than the rate of relative sea-level rise; this sedimentation excess should not occur at steady state or in a regime of accelerated sea-level rise. Here we suggest that lateral erosion and vertical sedimentation on channel banks are linked by creep, the process by which soil is moved downslope by gravity. The model presented herein predicts a dynamic equilibrium configuration for the cross section such that sedimentation excess on the channel banks is transferred by creep toward the channel, where tidal flow erosion and suspended sediment transport directed toward the banks close the sediment budget. This model predicts that bank slumping is an equivocal indicator of ongoing marsh loss and that high sedimentation rates on channel banks do not necessarily constitute a margin of resilience to accelerated sea-level rise.

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