Influence of Inundation and Suspended-Sediment Concentrations on Spatiotemporal Sedimentation Patterns in a Tidal Freshwater Marsh

Abstract

Long-term marsh sustainability is threatened by environmental changes, especially accelerated rates of sea-level rise (SLR) and changing fluvial sediment supplies. Although direct observations of long-term marsh responses to these changes are challenging, this study examines shorter-term responses via bimonthly sedimentation measurements over 5 years in Dyke Marsh Preserve (DMP), a tidal freshwater marsh in the Potomac River (Chesapeake Bay tributary, USA). Observed patterns are evaluated with physical drivers (winds, river discharge, water-level changes, suspended-sediment concentrations (SSC)) that influence spatiotemporal variability within the marsh. Linear regression models reveal that water-level changes exert more influence on this variability than SSC in a system with plentiful sediment supply. Sedimentation rates increase linearly with inundation time, but a quadratic regression model explains more of the variability, especially for the inorganic fraction of sediment. This result indicates that fluvial sediment supply is key for marsh accretion and suggests an optimal response of sedimentation to increased inundation that non-linearly affects marsh vulnerability to future SLR. Ultimately, the SSC in adjacent waters is not available to nourish the marsh unless it is transported and retained on the platform. Self-sustaining feedbacks among inundation, sedimentation, elevation, and plants likely aid in maintaining equilibrium elevations over time and thus also marsh sustainability.