INFLUENCE OF GEOMORPHOLOGY OF COASTAL WETLANDS IN CASCO BAY ON PREDICTED RESPONSE TO SEA LEVEL RISE

The geomorphology of the Casco Bay shoreline has a profound effect on the size, characteristics and spatial distribution of tidal marshes in the region. Wetlands along Casco Bay’s steep shorelines and narrow, glacial cut embayments develop predominately in three geomorphic contexts: head of valley wetland complexes, sheltered fringing marshes along larger tidal channels, and exposed fringing wetlands on more open shorelines. We used high resolution elevation data derived from LIDAR to model responses of wetlands in these three settings to moderate sea level rise (<3 ft; 0.91 m). Since little is known about the sediment dynamics of these wetlands, we developed a simple model to test how sediment accretion rates may affect wetland response. For many, but not all, Casco Bay wetlands predicted tidal wetland gains and losses are similar, leading to small net change in wetland area. Inundation, however, is projected to lead to widespread shift from high marsh to low marsh. In the sea level rise scenarios we examined, sediment accretion rates have limited effect on total wetland gains and losses, but strongly influence the proportion of high marsh to low marsh. Tidal wetland gains at many sites occur primarily via encroachment into adjacent non-tidal wetlands. This suggest our region will see less frequent conflict between marsh migration and development than will communities to our south, but it will be difficult to assess net changes in ecosystem services as a result of marsh migration.