RAPID WETLAND EXPANSION DURING EUROPEAN SETTLEMENT AND ITS IMPLICATION FOR MARSH SURVIVAL UNDER MODERN SEDIMENT DELIVERY RATES (Invited Presentation)

Fluctuations in sea level rise rates are thought to dominate the evolution of coastal wetlands. Here, we consider a contrasting scenario in which land-use related changes in sediment delivery rates drive the formation of expansive marshland, and vegetation feedbacks maintain the morphology of marshes despite recent sediment supply reduction and sea level acceleration. Historical maps and stratigraphic analysis suggest that parts of the Plum Island Estuary (MA) formed during the 1700s and 1800s when salt marshes rapidly prograded across a shallow subtidal bay. We attribute this marsh expansion to increased rates of sediment delivery associated with regional deforestation associated with European settlement. Expansive marshland exits along the North American coast today despite 20^th century sea level acceleration and sediment supply reduction associated with dam construction and reforestation. Numerical modeling suggests that these factors lead to deepening of marsh elevations relative to sea level, but that ecogeomorphic feedbacks that enhance accretion and limit channel erosion allow marshes to persist in a metastable equilibrium even under conditions in which they could not develop. If true, expansive marshland along the North American coast is a relict feature of high 19^th century sediment delivery rates, and marshland lost today will not be recovered in the future, even if rates of sea level rise and sediment delivery were to stabilize.