Restoring Tidal Flow to a New England Salt Marsh

Abstract

Salt marshes have experienced the brunt of human civilization for eons as they were diked for pasture or producing salt hay and less saltwater-dependent crops, filled for port, commercial, and residential development, used as landfills and to dispose of dredged material, ditched in efforts to reduce mosquito populations in coastal communities, or have had their connection to estuaries simply reduced or severed by roads and railroads. This was largely done because they were viewed as unproductive wastelands, public health hazards, or because their location was important for accessing deep water or connecting two points of land, or simply providing a desirable location for homes. In the 1960s scientists studying coastal habitats started writing about the ecological significance of these wetlands in the United States in terms the public could understand (e.g., Goodwin 1961, Odum 1961, and Teal and Teal 1969). Consequently the public was becoming more informed of the importance of these wetlands to coastal fisheries as well as to migratory birds as they witnessed accelerating destruction of salt marshes for residential and other development. In the 1960s, state legislatures began passing laws to restrict development of these wetlands, first in New England states then elsewhere (see Tiner 2013 for a comprehensive review of the history of tidal wetlands). Today salt marshes are among America’s most valued natural resources and government agencies and non-government organizations (NGOs) are both actively involved in restoring these wetlands. Most cases of this restoration involve bringing back tidal flow and more saline conditions in one way or another. Where the marshes have been crossed by a road or railroad, tidal flow has either been eliminated or restricted to varying degrees that has greatly affected soil salinities and promoted growth of brackish and freshwater species. In many cases in the northeastern U.S., these crossings have led to a drastic change in plant composition and vegetation structure – from a diverse salt marsh community dominated by low-growing halophytic plants to a virtual monoculture of common reed (Phragmites australis) – a non-native2 that favors less saline habitats and grows to 3.7 m (12 feet) or more in height under the best circumstances. Some options for restoring tidal flow in these situations include: 1) reconnecting the marsh to the adjacent estuary (where tidal flow was eliminated), 2) removing tidal gates, and 3) expanding the size of the existing culverts. These may be some of the simplest restoration projects from a construction standpoint, although concerns about increased flooding on private property surrounding the marsh is often the major hurdle to overcome. A small restoration project in Massachusetts serves as one example of the effectiveness of simply restoring tidal flow can bring about a return of salt marsh vegetation to an area that had been colonized by common reed. While some restoration projects are initiated as mitigation for destruction of wetland elsewhere, this project was a “pro-active project” – simply done for the benefit of the environment - to restore native halophytic vegetation and reduce the extent of non-native common reed.