The salt marsh might be the defining feature of the Lowcountry coast, mile after mile of sweeping grasses. It's the nursery of countless marine creatures, including shrimp. It's a filter that helps keep the waters clean.

And it's drowning, right in front of our eyes, being flooded faster than it can move inland as the sea rises. A Boston University study of tidal creeks in Cape Romain National Wildlife Refuge offers some of the first, see-it-for-yourself evidence.

The loss of marsh compounds the damage from the overwash of barrier islands, jeopardizing the future of the 64,000-acre island seascape refuge northeast of Charleston that is a singular habitat for a host of species. For example, the refuge is maybe the best nesting area in the Southeast for sea turtles.

What the marsh loss suggests for the rest of the coast is even more disturbing. The study is the latest in a series of alarming research that shows the coastal grasses are being literally eaten away.

There's an estimated 400,000 acres of coastal marshes in South Carolina. A federal Environmental Protection Agency study in 1998 indicated sea level is rising about a foot per century on the East Coast. Conservationists warn the climate warming is exacerbating that.

The study focuses on a stretch of Horsehead Creek behind remote Cape Island east of McClellanville. On the south bank of Horsehead, smaller tidal creeks work their way back into the marsh in the classic, winding pattern. But on the north bank, creeks are forming that plunge straight into the marsh, to pits where crabs have devoured the grasses.

It's called rapid headward erosion, and it's trouble. The sediment is disappearing, the grasses aren't replenishing, and the bare spots are filling in with creek, not marsh.

"There's no doubt it's happening there," said Dennis Allen, lab director at the Belle W. Baruch Institute for Marine and Coastal Sciences in North Inlet, just outside Cape Romain, who has spent 30 years studying tidal creeks in the Lowcountry. In North Inlet, researchers have found low-lying marshes flooding more frequently, the marsh grasses thinning and dying off.

"There are signs for sure the marsh is having a hard time keeping up with sea level rise in this day and age," Allen said. Research by Baruch Institute director Jim Morris suggests 20 to 50 percent of what we see today as salt marsh will become sand flats and open water lagoon systems in the next 20 to 30 years, Allen said.

For Sarah Dawsey, U.S. Fish and Wildlife Service biologist at Cape Romain, the study is documentation of what she has been seeing. Cape Island alone holds 1,041 of a total 3,129 sea turtle nests in South Carolina this year. Nearly 25 percent of the island has been overwashed since 1954. There is, as Dawsey says, "not much we can do about it."

On Bull Island, south of Cape Island, 20 to 25 feet of shoreline is disappearing per year. A levee has been rebuilt twice back in the dunes for a brackish water impoundment on the island's edge because the impoundment is habitat for hundreds of wading birds, waterfowl and alligators. There's no longer any dune shoulder left to attach the levee.

One big storm likely will destroy the impoundment, said Raye Nilius, Fish and Wildlife Service refuge project leader.

Allen points out that the study area, Horsehead Creek, is itself a straight-away creek that's rare to the coast marshes. He's not sure how broadly the study's findings apply to the coast as a whole.

But, "it's good, solid science that demonstrates these particular systems are struggling," he said.

Reach Bo Petersen at 937-5744 or bpetersen@postandcourier.com.