NARRAGANSETT – On a spongy spit of marshland on the Narrow River, excavators sit by a pile of sand and silt while workers tromp through the wet mix that sucks at their boots.

Seagulls pick out crabs and clams from the muddy stew that has been deposited from the river bed onto the salt marsh. The pungent smell of low tide hangs in the air.

“It’s a little too soupy to work with,” says Jennifer White, a wildlife biologist with the U.S. Fish and Wildlife Service, explaining the idle machinery.

Here, on the banks of this scenic river that meanders from North Kingstown to Narragansett Bay, construction crews are pumping thousands of cubic yards of dredged material on top of the marsh in an effort to elevate it above rising seas. A similar scene is playing out on a marsh along Ninigret Pond in Charlestown.

Both projects will shed light on how best to protect marshes as polar ice caps melt, ocean waters warm and expand, and the pace of sea level rise picks up.

It’s a vexing question. Salt marshes offer some of the most productive wildlife habitat in the world, serving as nurseries for fish, feeding grounds for wading birds and nesting areas for migratory birds.

But they are fragile. Through a natural process known as accretion, marshes grow higher as waterborne sediments get trapped in their plants and as vegetation breaks down to add to the underlying layer of peat.

The rate of accretion in Rhode Island, however, is no longer keeping up with the pace of rising seas. A landmark study by the Narragansett Bay National Estuarine Research Reserve, on Prudence Island, found that waters are now rising in the state at more than three times the rate that marshes are growing.

As a result, water is pooling on marshes and drowning plants like smooth cord grass, which have adapted to being flooded by the tides but not to being submerged over long periods. As those plants die off, they leave ever-expanding patches of bare ground.

State agencies and environmental groups are working to identify places for marshes to shift inland to higher ground. The first project to clear a path for a marsh to migrate was implemented by the Rhode Island Department of Environmental Management at Sapowet Marsh, in Tiverton, last fall. But there aren’t many similar pieces of open space along the state’s 400 miles of coastline, much of which has been heavily developed.

So Fish and Wildlife, The Nature Conservancy and the state Coastal Resources Management Council are exploring ways to boost the elevation of the most threatened marshes.

The first project to use a technique known as “thin layer deposition” was carried out on 11 acres of Sachuest Marsh, in Middletown, a year ago. That patch is slowly revegetating.

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