Salt Marsh Restoration at Cape Cod National Seashore, Massachusetts

Abstract

Cape Cod has a 350-year history of coastal wetland loss due to tide restrictions, including 1400 hectares of original salt marsh estuaries that are still diked today (Justus 2001). Salt marshes were diked for various reasons: to ease foot, wagon, and later automobile and train passage across the many salt marshes that wove throughout the outer Cape’s upland farms and villages; to favor salt hay farming by reducing tidal flooding and thereby encouraging the growth of high-marsh grasses such as Spartina patens (salt meadow cordgrass); to allow the cultivation of salt-sensitive crops in the organic-rich wetland soils; and to eliminate, through drainage, habitat for floodwater-breeding mosquitoes. Typically an earthen dike was built across an inlet or narrow reach of tidal creek or marsh to an elevation that blocked all but the highest storm tides. This structure effectively blocked seawater flow into upstream wetlands but also tended to impound freshwater that normally discharged to the sea during low tides. Thus, to allow discharge, all dikes were fitted with culverts, albeit with one-way valves on their seaward ends to prevent saltwater inflow during flood tides. To further wetland drainage, diked salt marshes were subsequently ditched and creeks channelized to expedite freshwater discharge. Virtually all of the Cape’s diked marshes have been treated in this way.