In 2019, Maryland pumped 60,000 cubic yards of dredged mud onto eroding Swan Island; the material rebuilt about 12 acres of marsh and dunes with 200,000 plants

Shipping channels in the Chesapeake Bay require periodic dredging, and all that gray, heavy mud has to go somewhere. According to Jenny Davis and her co-authors in their 2024 peer-reviewed paper titled “Beneficial use of sediments to restore a Chesapeake Bay marsh island,” published in the journal Frontiers in Sustainability, it has previously been dumped in upland containment or the open ocean. Both have removed sediment from coastal systems that need it to keep pace with rising seas. At Swan Island, a 25-acre parcel of marsh in the Bay, federal engineers took an alternative approach by using this material in the environment.

The same study reports that the U.S. Army Corps of Engineers' Baltimore District placed about 60,000 cubic yards of sediment on 12 acres of the island in 2019. Scientists monitored this plot for three years to see how things went. On paper, it sounds like a tidy win, but in reality the situation is more complicated, and more interesting.

A tiny island with a big job


Swan Island is uninhabited but nevertheless works daily. According to Davis et al., the island acts as a natural wave break for the town of Ewell on the nearby Smith Island. Before the restoration, it had been eroding and subsiding for years, and its interior marsh had broken into fragments, threatening the loss of the whole island. NOAA's National Centers for Coastal Ocean Science documented the restoration as it happened: from October 2018 to April 2019, the Baltimore District dredged the navigation channel serving Ewell and placed roughly 60,000 cubic yards of the dredged material onto Swan Island, then, in June and July of 2019, planted approximately 200,000 marsh and dune plugs by hand, the same figures later reported by Davis et al. In recent years, the alternative of dumping dredged material in upland containment or the open ocean has proved increasingly costly, on top of the sediment loss it causes downstream.

The part you may have skipped

Some of this worked, as high-marsh grass took root, and most monitoring plots in this area had over 50% cover by September 2020. However, after three years, 22 of 30 low-marsh plots were still bare, meaning only about 8 of the 30 had established meaningfully, and a concrete sill along the northeast shore was too low to serve its purpose. Sediment spilled past that barrier and buried more than two acres of underwater grass habitat nearby: a clear, negative side effect of the project, not merely a hypothetical one.

The shoreline was eroding at about 3.3 meters, or roughly 11 feet, per year, similar to pre-restoration trends, according to the Frontiers in Sustainability paper. And here's the catch: the 12 acres cited up top is the footprint the Corps physically rebuilt with sediment and planted in 2019, not a permanent measurement of success. Three years later, researchers found the island's actual vegetated area was about 8 acres, essentially unchanged from before the project. The land was rebuilt; the plant cover on it, overall, was roughly back where it started.

Why it's still worth doing

Doing nothing is not neutral; according to Erwin and colleagues in a 2010 study titled “Islands at bay: rising seas, eroding islands, and waterbird habitat loss in Chesapeake Bay (USA),” published in the Journal of Coastal Conservation, a group of 15 islands in Tangier Sound lost 21% of their area between the early 1990s and 2007-08. During the same period, nesting common terns in this complex fell by 96%. Erwin's team claims that stabilizing or restoring some of the fastest-eroding islands is essential to maintain waterbird communities.

It should be noted that, while the team at Swan Island does not describe this as a failure, they describe an island that is still evolving, where rapid plant establishment matters most. For instance, when they put 22,000 plugs in 2021, about 20% of the target area was laid thick within a year, the plants having taken root in more sheltered spots.

The takeaway from Swan Island is that coastal repair works less like a one-time purchase and more like a subscription: a plan for adaptive replanting, paired with a funding source that can cover it, meaningfully improves the odds of success, though it can't guarantee them. It's a useful question to carry into the next coastal restoration announcement, not just what got built, but who's paying for the maintenance in year four.