Salt Marsh Tidal Restrictions Fact Sheet

Mapping of Tidally Restricted Salt Marshes in Buzzards Bay

(This is a webpage version of a fact sheet introduced in 1999. It has been updated to reflect the June 2002 printing of the Atlas)

What is a Tidally Restricted Salt marsh?

Many salt and brackish marshes are crossed by highways, roads, and railroads of various dimensions. These transportation routes can pass through tidal marshes and over tidal creeks and rivers. Usually bridges are required to span rivers and broad creeks but many roadways leading to bridges are built on fill deposited in wetlands. Roads crossing small creeks may have culverts installed to allow tidal water to pass beneath them. In many cases, these culverts are too small to pass enough tidal water to maintain natural salt marsh vegetation upstream.


Some culverts have tide gates or flapper valves allowing fresh water to leave but not allowing tidal waters to enter the upstream marsh. At some road crossings, a culvert has not been provided at all, and tidal flow has been eliminated altogether. Bridges may have similar restricting effects if the openings are not wide enough to maintain salt and brackish marshes upstream.

Tidal restrictions have many detrimental affects to healthy salt marsh systems. They result in hydrologic changes that significantly alter the chemical integrity of the upstream salt marsh ecosystem. The once strongly saline environment changes to one that is brackish or fresh water. Over time, the freshening of salt marshes causes a transformation in the vegetation. Salt marsh grasses and rushes may die off and be replaced by common reed. Common reed often forms a monoculture of tall reeds (up to 12 feet or more), leading to both lowered plant diversity and a change in vegetative structure. What once was a low grassy meadow, is turned into a tall reedy thicket.

Common reed blocks tidal flows even further, reducing estuarine organisms’ access to the salt marsh. The export of organic matter is also reduced or eliminated where tidal exchange is restricted or cut off by a tide gate, flapper valve, or dike. This is a problem because the exportation of salt marsh organic matter is a vital life-support function for the detrital-based food web. Together changes in vegetation and the reduction of the detrital food base cause a major shift in wildlife use and a decrease in species diversity.

A reduction of tidal flow also causes a dramatic change in soil chemistry. Sea water contains large quantities of sulfur. In the normally anoxic marsh soil conditions, it is reduced by soil bacteria into sulfide. The sulfide forms insoluble pyrites with iron. When restricted areas receive insufficient tidal flushing, the soil sulfides become exposed to air where they become reoxidized to sulfate. The result is the formation of sulfuric acid. Under these conditions, the soil acidity is so extreme, with pH values as low as 1.5 the vegetation can no longer survive.

Changes in saturated conditions causes another problem too. It allows aerobic digestion of accumulated organic matter, eventually causing the marsh to “slump.” This slumping occurs as the structure of the marsh begins to break down. The peaty layers of fibric organic matter which were formed over many years of healthy salt marsh activity are broken down by microbial digestion causing the edges of the marsh to fall in.

Creation of: “The Atlas of Tidally Restricted Salt Marsh in Buzzards Bay Massachusetts”

The Buzzards Bay NEP completed a draft Atlas in 1999. The document underwent several drafts, and the final document’s printing was delayed until June 2002. The atlas was based on interpretation of infra red aerial and black and white orthographic photographs of the Buzzards Bay coastline to identify potential restriction sites. These sites were field checked to verify the existence of a restriction, and to collect information about the type of restriction and the affected salt marsh. At some sites, photographs were taken with a digital camera to catalog the variety and condition of the structures. The data collected in the field visits was transferred to a GIS database to create a series of GIS maps. GPS was used at some sites.

How does the Atlas Help?

The Atlas documents the extent of salt marsh presently impacted by human activities, especially transportation facilities. It is hoped that the atlas will be used by municipal agencies, state agencies, and others to initiate salt marsh restoration activities for these sites when appropriate. Municipal public works departments are particularly encouraged to check the Atlas when road or bridge work is contemplated. The information will highlight locations where something as simple as redesigning a culvert could restore tidal flow and a salt marsh. The Atlas may also serve as a source for projects which could be included in the Regional Transportation Plan for state and federal transportation funding.

Written by Mary E. Johnson – DEP Wetlands Intern

For more information or a copy of the atlas contact:

The Buzzards Bay NEP, 2870 Cranberry Hwy, E. Wareham, MA 02538

Tel. (508) 291-3625