Storm Impacts on Estuarine Marshes

Sediment supply and tidal range are two of the most important factors governing the rate of salt marsh accretion. Here we highlight the importance of coastal storms in governing these two primary controls on marsh resilience. Moored observations are combined with measurements of marsh sedimentation from the North and South River estuaries, two bar-built estuaries that share a barrier-beach inlet connecting them directly to Massachusetts Bay. A significant storm in November of 1898 shifted the common inlet for these systems 6 km to the north. As a result the North River estuary was shortened by over 25% and the South River lengthened by an even larger fraction. Sedimentation rates in the North River since the switch in inlet location are roughly double that observed in the South River, and are as great as 4 times the rate of sea level rise over the last century. Moored observations collected during March and April of 2018 support enhanced sediment trapping in the North River following the formation of the 1898 inlet due to an increase in both high tide elevation and tidal energy.

Our data also indicate that a majority of the sediment accumulating in North and South River marshes are derived from marine sources mobilized during storms. Periods of elevated turbidity in the estuary are concurrent with high wave activity off-shore and turbidity peaks in the North River estuary are substantially higher during flood tide than ebb. The primary fresh water tributaries are ruled out as the dominant source of sediment trapped in study marshes because river discharge lags period of high turbidity in the estuary, and estimates of sediment input from fluvial sources can only account for a small fraction of the total amount of inorganic sediment that has accumulated in the North and South River marshes over the last century. Results highlight coastal erosion by storms as an important source of sediment to marshes, as well as the lasting impact of storm-induced changes to barrier-inlet morphology on marsh accretion.