Two-thirds Of Studied Salt Marshes Could Be Lost Based On Current Sea Level Rise Projections

CHATHAM – In less than 25 years, as much as 40 percent of four salt marshes in West and South Chatham could be lost, according to a recent study.

 The situation becomes even dire looking farther into the future. By 2100, some 62 percent of the marshes investigated in the study could be gone.

 Those losses are based on current sea level rise and climate change projections, according to the study by the Center for Coastal Studies (CCS). Of the 205.5 acres of salt marsh in the study, 124.2 acres will remain in 2050, 83.4 acres in 2070, and 78.5 acres in 2100.

 The study, funded by a community preservation appropriation, establishes a baseline of current conditions in the marshes, Gerry Stahl of the energy and climate action committee told the select board Dec. 2. Nine more salt marshes are set to be studied by CCS with funds approved at last May’s annual town meeting.

Sea level is rising faster in New England than anywhere else in the world, Stahl said, threatening the ability of salt marshes to respond to rapidly changing conditions. Salt marshes not only sequester carbon and provide habitat for a plethora of flora and fauna but are also a critical buffer against storms. 

Under ideal circumstances, salt marshes respond to rising waters by migrating landward. However, when houses and other man-made impediments, such as revetments, stand in the way, the result is the loss of the marsh.

In the study area — which covers marshes in Forest Beach, Cockle Cove, Buck’s Creek and Oyster River — there are 418 residential properties valued at $571 million, according to Stahl. Some are developed, some are vacant lots, some are town property. Further study is needed to determine which properties would be impacted by marsh retreat as well as whether there is sufficient space for the marshes to migrate landward. In some cases, “managed retreat” — moving infrastructure back from the shore or marsh — might need to be investigated.

 “If there’s a house there, then that’s a problem,” Stahl said. “I think this is going to be an issue for the town in the coming years, as sea level rises and some of these houses start to get flooded.”

Protecting wetlands and habitats bordering salt marshes are among the options for preserving them, along with the more drastic possibilities of relocating man-made structures. 

 “Such options would, of course, require careful investigation of individual properties and extensive community discussion in the coming years — including prioritizing compared to properties around the nine other marshes to be studied in 2026,” Stahl wrote in an analysis of the study, which he said provide “very fine-grained elevation data” that can inform future decisions.

The next study will look at salt marsh systems at Champlain Creek, Cotchpinicut, Frost Fish Creek, Minister’s Point, Morris Island, Muddy Creek, Nickerson Neck, Red River and Tom’s Neck. 

 “Then we’ll have a picture of marshes throughout Chatham and all the data we need to do analysis and to be able to prioritize where we should talk about doing some interventions to help preserve the marshes,” Stahl said.

One surprise the study found was that there’s something else destroying the town’s salt marshes: purple marsh crabs, which “have become a real problem for Cape Cod,” said Natural Resources Director Greg Berman. The crabs are native to New England and not invasive, but their population has exploded in recent years, according to the CCS study, possibly due to climate change, pushing them northward and reducing natural predators. The nocturnal crabs feast on salt grasses and roots, thus killing the marsh.

 “There’s no clear solution,” Berman said. 

Protecting salt marshes is important for the town’s coastal environment because they protect upland from waves and storms, reduce wind and draw water away after a flood, Stahl said.

 “If you don’t have marshes there, if they’re all dead, then you lose a major protection for homes along those areas,” he said.