Salt Marsh Response to Inlet Switch-Induced Increases in Tidal Inundation

There is widespread concern that rapidly rising sea levels may drown salt marshes by exceeding the rate at which these important ecosystems can build elevation. A significant fraction of marshes reside within backbarrier estuaries, yet little attention has been paid to how changes in inlet geometry influences estuarine tides and marshes. In 1898, a coastal storm eroded a new inlet through the barrier beach that fronts the North-South Rivers Estuary in Massachusetts, USA. The new inlet shortened the North River by 5.6 km and lengthened the South River channel by the same amount. Modern measurements of tidal attenuation suggest that channel shortening abruptly increased mean high tide along the North River by at least 30 cm. Foraminifera communities within North River marsh sediments indicated an environmental change from infrequent to frequent inundation at the time of the 1898 switch in inlet location, which supports this hypothesis. Increased mineral sediment deposition after the inlet switch played a dominant role in allowing marshes along the North River channel to adjust to greater inundation. Following the inlet switch, sediment accreted in North River marshes at 2-5 times the rate of sea level rise (SLR). The North River channel widened by an average of 18% relative to pre-1898 conditions to accommodate the increased tidal prism. The role of mineral sediment accretion in making this marsh resilient to an abrupt increase in inundation depth highlights the importance of maintaining adequate sediment supplies in coastal regions as SLR accelerates. Salt marshes form along sheltered coastlines, incorporating mud and dead roots atop a grassy platform at about the elevation of high tide. It is unclear whether marshes will build their platforms fast enough to keep up with expected increases in sea level. In 1898, a coastal storm eroded a new opening in a beach, behind which a salt marsh extends inland along two tidal channels. The new inlet shortened the distance from the open ocean to parts of the salt marsh, causing high tides that flood the salt marsh to increase more than 30 cm. We considered this "natural experiment" in increased tidal flooding to understand how other marshes might survive rapid sea level rise (SLR). Sediment core samples from the marsh showed that higher tides after the inlet switch resulted in lots of mineral sediment deposited on the marsh platform. This shows that mineral sediment is very important to salt marsh survival when sea level is rising fast. Increased tidal flow after the inlet switch eroded and widened channels. Some of that eroded channel bank sediment deposited atop the marsh, but most of the sediment that allowed the marsh to survive rapid SLR came from the ocean.