Salt marshes connect coastal ecology with the history of working waterfronts, fishing communities, roads, and settlements. These statistics describe their distribution, habitat value, carbon storage, restoration, and modeled flood-protection effects across the United States and the Northeast. The figures come from NOAA, EPA, and the U.S. Fish and Wildlife Service; the legacy research record supplying these source labels was not independently verified here.
Contents
- Where salt marshes occur
- Wetland area and loss
- Habitat and blue carbon
- Modeled protection at Waquoit Bay
- A comparison of modeled road inundation
- Buzzards Bay costs
- Restoration at Parker River
Where salt marshes occur
NOAA’s What is a salt marsh? describes salt marshes as a worldwide coastal habitat, especially common at middle and high latitudes. In the United States, they occur on every coast. The Gulf Coast contains about half of the nation’s salt marshes, making it the largest regional concentration identified in these figures.
Their geographic reach matters for local-history readers because a marsh is not only an open area beside the water. It is part of a connected coastal landscape that can shape where people build roads, locate homes, fish, and move goods. The national distribution also provides context for Rhode Island and southern New England: local marshes are part of a habitat type that extends across every U.S. coast.
NOAA reports that salt marshes provide food, refuge, or nursery habitat for more than 75% of fisheries species. That percentage describes the share of fisheries species receiving one or more of those functions, not the proportion of a particular catch that comes from a single marsh. It nevertheless shows why marsh condition can matter beyond the edge of a tidal creek.
Wetland area and loss
The EPA’s About Coastal Wetlands reports about 40 million acres of coastal wetlands in the conterminous United States. Coastal wetlands account for 38% of total wetland acreage there, while 81% of coastal wetlands are in the Southeast. A separate EPA Wetlands summary states that wetlands covered 5.5% of the land area in the 48 contiguous states and that about 95% of those wetlands were freshwater wetlands.
Those percentages use different denominators, so they should not be added together. The 38% figure concerns the share of wetland acreage represented by coastal wetlands; the 81% figure describes the regional distribution of coastal wetlands; and the 5.5% and 95% figures describe wetlands across the 48 contiguous states and the freshwater share of that broader category.
EPA reported an average coastal-wetland loss rate of about 80,000 acres per year from 2004 to 2009 across the Atlantic, Pacific, Gulf of America, and Great Lakes coastal watersheds. In the earlier 1998–2004 study period, the average was about 59,000 acres per year. These are study-period averages, not a current annual estimate.
For 2004–2009, EPA recorded loss or conversion of 265,723 acres of freshwater wetlands and 94,999 acres of saltwater wetlands. The figures describe loss or conversion over that study period; they do not establish that all affected acreage was salt marsh, since the categories are freshwater and saltwater wetlands rather than a salt-marsh-only inventory.
Habitat and blue carbon
The ecological importance of coastal wetlands is also visible in fisheries data. EPA reports that more than 50% of commercial fish and shellfish species in the Southeastern United States rely on coastal wetlands. Combined with NOAA’s national figure of more than 75% of fisheries species receiving food, refuge, or nursery habitat from salt marshes, the statistics point to two different scopes: one is a national salt-marsh habitat measure, and the other is a regional commercial-species measure.
EPA’s Blue Carbon Reservoirs from Maine to Long Island NY identifies a New England blue-carbon target area containing 218,222 acres of eelgrass meadows and salt marsh. That area stores 7,523,568 megagrams of blue carbon. A megagram is a metric tonne, but the source’s reported unit is retained here because it is the basis of the comparison figures below.
EPA expresses that same stock through several equivalences. It equals the annual emissions of 5,994,024 passenger vehicles, the burning of more than 30,521,000,000 pounds of coal, the annual energy use of 3,474,000 homes, or the annual operation of 7,498 wind turbines. EPA also compares it with one year of carbon accumulation in 32,646,000 acres of upland forest.
These equivalences are different ways of communicating the same reported blue-carbon stock. They are not separate additions to the stock, and they do not mean that the New England target area has the same physical characteristics as vehicles, coal, homes, turbines, or upland forest. For a coastal community, the practical point is that the salt-marsh and eelgrass landscape has a measurable carbon reservoir as well as habitat value.
Modeled protection at Waquoit Bay
EPA’s Estimating the Monetary and Nonmonetary Benefits of Salt Marsh in the SNEP Region describes modeled outcomes for study areas in the Southeast New England coastal region. Across the SNEP region, the study marshes totaled 39,430 acres, representing 17% of the region’s salt-marsh area.
At Waquoit Bay, the study area included 5,009 tidally influenced marsh acres, a nearby population of 43,817 people, and 27,765 housing units. The report modeled conditions with marsh present and without marsh, using 2040 and 2070 horizons. Those are scenario results, not measurements of future inundation already observed.
With marsh present, Waquoit Bay avoids about $66,000 in inundation costs by 2040 and about $532,000 by 2070. The figures are modeled avoided costs for the stated scenario and years. They should not be read as a general price for every acre of marsh or as a present-day estimate for Pettaquamscutt or another local shoreline.
The road results show how the modeled difference changes by event frequency:
| Waquoit Bay scenario | 2040 with marsh | 2040 without marsh | 2070 with marsh | 2070 without marsh |
|---|---|---|---|---|
| Always inundated | 0.20 mi | 1.25 mi | 0.21 mi | 1.25 mi |
| Inundated every 30 days | 0.66 mi | 1.27 mi | 0.83 mi | 1.27 mi |
| 10-year storm | 22.34 mi | 22.77 mi | 22.51 mi | 22.77 mi |
| 100-year storm | 92.07 mi | 92.44 mi | 92.24 mi | 92.44 mi |
In the always-inundated scenario, the modeled road length is 1.05 miles lower with marsh than without marsh in 2040, and 1.04 miles lower in 2070. The every-30-days scenario shows a smaller modeled difference: 0.61 miles in 2040 and 0.44 miles in 2070. The storm scenarios show narrower differences in the supplied results, with 0.43 miles separating the 2040 10-year scenarios and 0.37 miles separating the 2040 100-year scenarios.
The frequency labels are important. “Always inundated,” “every 30 days,” “10-year storm,” and “100-year storm” describe different modeled exposure conditions. They are not interchangeable measures of the same event, and a 100-year storm label does not mean that flooding can occur only once in a century.
A comparison of modeled road inundation
At Waquoit Bay, the supplied figures show marsh-presence differences in each listed scenario, but the scale varies considerably. The largest absolute separation appears in the always-inundated category, while the storm categories show smaller differences against a much larger total road length.
| Event category | 2040 difference, with vs. without marsh | 2070 difference, with vs. without marsh |
|---|---|---|
| Always inundated | 1.05 miles | 1.04 miles |
| Every 30 days | 0.61 miles | 0.44 miles |
| 10-year storm | 0.43 miles | 0.26 miles |
| 100-year storm | 0.37 miles | 0.20 miles |
The comparison is a direct subtraction of paired values in the EPA table and is used only to make the supplied scenarios easier to read. It does not estimate a trend between 2040 and 2070, and it does not predict conditions outside the modeled Waquoit Bay study area.
Buzzards Bay costs
The same EPA SNEP study reports separate modeled road results for Buzzards Bay. In 2040, 1.01 miles of road are always inundated with marsh present, compared with 2.21 miles without marsh. For roads inundated every 30 days, the reported values are 1.31 miles with marsh and 2.21 miles without marsh.
The report also gives road-inundation costs for the marsh-present scenario at Buzzards Bay: $64.3 million in 2040 and $573.4 million in 2070. These values are modeled costs, and the supplied facts do not provide a matching without-marsh cost for those two years. They therefore should not be turned into an unsupported savings estimate.
Waquoit Bay and Buzzards Bay illustrate why place names, years, and scenario conditions belong beside every statistic. A dollar figure from Buzzards Bay cannot be transferred to a Rhode Island shoreline without a separate local model, and a road-mile result from Waquoit Bay cannot be treated as a measurement of all SNEP marshes.
Restoration at Parker River
USFWS reports that Parker River National Wildlife Refuge protects 4,662 acres of barrier-island habitat and includes 2,660 acres of salt marsh. The Great Marsh around Parker River covers 20,000 acres. Refuge staff restored 100 acres of salt marsh in 2021, a project figure that describes restoration activity rather than the total marsh area.
The restoration record includes several measurable interventions. In 2015, two runnels were dug to drain a 3.52-hectare pool. By 2019, about 60% of the pool had drained after the runnels were dug. Staff created 23 additional runnels between 2020 and 2021.
Another approach used native hay in 17 ditches from 2014 to 2017. The treated ditches rose by 15 centimeters on average by 2017. At 12 sites, ditch-plug removal reduced inundation from an average of 94% to 18%.
The Parker River figures describe specific restoration actions and observed or reported outcomes at a named refuge. They do not establish that every salt marsh responds in the same way, but they show how marsh management can be documented through acreage, number of structures or ditches, water depth, drainage, and inundation percentage. For local history, that level of measurement helps connect long-term changes in coastal landscapes with the practical work used to maintain them.