Vegetation and the morphodynamics of tidal channels: Insights from Plum Island Sound, Massachusetts

Abstract

The sinuous channels of tidal marshes exhibit marked similarities to their upland counterparts, but their tendency to rapidly narrow into the distal platform may indicate that the processes governing the evolution of form may differ. Additionally, while marsh vegetation facilitates platform aggradation, the role of vegetation in temporally stabilizing the spatially varying channel form remains unclear. Here, we leveraged topographic surveys, historical aerial photos, and sediment cores to evaluate the physical controls on channel migration dynamics within tidal marshes of Plum Island Sound, Massachusetts, USA, a U.S. National Science Foundation Long-Term Ecological Research site. We found that width-normalized rates of channel migration were highest in the distal platform, decreasing exponentially towards the platform edge. This trend coincided with observations of bank instability, in which the frequency of bank slumping progressively increased away from the platform edge. The distal platform, characterized by heightened channel mobility, was also where measured belowground biomass was greatest. The normalized difference vegetation index, reflecting aboveground biomass, was also highest within the distal marsh. We hypothesize that while surface root biomass is denser in the distal marsh, shallow rooting depths may limit the stabilizing influence of vegetation on channel banks, contributing, alongside other factors such as tidal shear stress and seepage-induced erosion, to the enhanced width-normalized rates of channel migration observed in these reaches. Our findings underscore the complex interplay between vegetation and channel morphology in tidal marshes, highlighting the nuanced role of vegetation in platform stability. The work offers insights for the development of adaptive management strategies aimed at preserving these vital coastal ecosystems in the face of sea-level rise.