The salt marshes we see today began forming in coves and protected harbors along the Narragansett Bay shoreline approximately 3,500 years ago (Orson et al, 1987). An exact figure of the extent of salt marsh destruction since European settlement is not known, but consider that much of downtown Providence and Quonset Point are built on filled coastal wetlands.Furthermore, between 1955 and 1964, ten percent of coastal wetlands in Rhode Island larger than 40 acres in size were filled for property development. 1996 aerial photographs of coastal resource areas in Narragansett Bay found that there are 3738.8 acres of salt marsh remaining in Narragansett Bay (RIDEM, Narragansett Bay Estuary Program and Save the Bay, 1999). It is estimated that RI has lost 50% of the coastal wetlands since European colonization.

Filling of salt marshes
It is estimated that over half of Rhode Island’s salt marshes have been filled such as the Great Salt Cove that is now downtown Providence.  Many marshes were filled with sand and mud from dredging activities. Filling in a salt marsh  can eliminate a marsh all together, or lead to changes in elevation that prevent the ebb and flow of tides on the marsh surface, inhibiting the growth of specially adapted salt marsh plants.   Even small changes in elevation can drastically affect the drainage of water and allow growth of invasive plants such as Phragmites australis. Phragmites is a tall reed normally found along the upland edge of the marsh. This plant can tolerate sites disturbed by filling, polluted runoff, or increased freshwater drainage. Filling leads to an overall loss of estuarine habitat.

Dikes, roads, or railroad crossings with inappropriately sized drainage pipes
Inadequate culverts and pipes can frequently cause a number of problems in salt marshes. Restriction of tidal flow results in a direct reduction in estuarine habitat area. Lowering of salinity, caused by the reduction of tidal input, can cause major changes in the composition of salt marsh vegetation, wetland chemistry and other wetland processes. A common symptom of lowered salinity is the invasion of Phragmites australis in the marsh. Phragmites can become invasive in an altered salt marsh and can outcompete other salt marsh plants, thereby decreasing habitat diversity.  Reducing tidal flow into a salt marsh results in lower production of salt marsh dependent species, including juvenile finfish, shellfish and other species that depend on salt marsh habitat for part or all of their life cycle.
 
Mosquito Ditches
Mosquito ditches are straight ditches that are dug to drain water from the upland part of a salt marsh directly into open tidal waterways.  In the 1930’s, during the New Deal, many of Rhode Island’s salt marshes were ditched to control mosquito breeding.  At the time, ditching was thought to control populations of mosquitoes by eliminating the standing water in upper marsh areas where mosquitoes breed.  However, such ditching drains pools of water which support mosquito larvae-eating fish, thus leading to increased mosquito populations.  Furthermore, this water drainage results in alteration of normal wetland hydrology. Ditching is known to disrupt waterfowl, songbird, and wading bird habitat preferences and can affect the health of salt /brackish marsh plant communities.

Runoff from surrounding land-use
Runoff originating from stormwater from roads, fertilized landscapes, and failing septic systems can alter the vegetation in the salt marsh.  Increased nutrient-rich freshwater can promote invasive plant species, such as Phragmites australis. 

Sea level rise
Healthy salt marshes are able to keep up with sea level rise due to accretion or build up of the sediments with organic material.  Impounded and unhealthy marshes are unable to trap enough sediments to stay above the rising waters and are drowned.  As sea levels rise, salt marshes tend to retreat backwards towards the land to stay above water.  Donnelly and Bertness (2001) studied this pattern  of migration of the border between low marsh and high marsh in relation to sea level rise.  They found that low marsh Spartina alterniflora had been advancing landward and outcompeting high marsh Spartina patens since the mid-1800’s.  The rate of accretion in the marsh more than doubled since the mid-1800’s for high marsh and up to 5 fold for low marsh.  Donnelly and Bertness conclude that more frequent flooding from regional change in sea level is responsible for the landward migration. Eventually in Narragansett Bay low marsh will completely outcompete high marsh.  If the rate of sea level rise continues to increase, even low marsh could be drowned.  Heavily developed shorelines, such as those of Rhode Island offer no place for the marshes to retreat.  Thus Rhode Island will eventually lose even its healthiest marshes as waters rise.