Sources, Mechanisms, and Timescales of Sediment Delivery to a New England Salt Marsh

Abstract

The availability and delivery of an external clastic sediment source is a key factor in determining salt marsh resilience to future sea level rise. However, information on sources, mechanisms, and timescales of sediment delivery are lacking, particularly for wave-protected mesotidal estuaries. Here we show that marine sediment mobilized and delivered during coastal storms is a primary source to the North and South Rivers, a mesotidal bar-built estuary in a small river system impacted by frequent, moderate-intensity storms that is typical to New England (United States). On the marsh platform, deposition rates, clastic content, and dilution of fluvially-sourced contaminated sediment by marine material all increase down-estuary toward the inlet, consistent with a predominantly marine-derived sediment source. Marsh clastic deposition rates are also highest in the storm season. We observe that periods of elevated turbidity in channels and over the marsh are concurrent with storm surge and high wave activity offshore, rather than with high river discharge. Flood tide turbidity also exceeds ebb tide turbidity during storm events. Timescales of storm-driven marine sediment delivery range from 2.5 days to 2 weeks, depending on location within the estuary; therefore the phasing of storm surge and waves with the spring-neap cycle determines how effectively post-event suspended sediment is delivered to the marsh platform. This study reveals that sediment supply and the associated resilience of New England mesotidal salt marshes involves the interplay of coastal and estuarine processes, underscoring the importance of looking both up- and downstream to identify key drivers of environmental change.