The Chesapeake Bay wetlands are the tidal marshes, swamps and underwater grass beds that line the largest estuary in the United States, along the shores of Maryland and Virginia. They fringe a bay of 4,479 sq mi (11,601 km²) whose shoreline, counting its tributaries, runs for 11,684 miles (18,804 km), and they drain a watershed of 64,299 sq mi (166,534 km²) spread over six states and Washington, D.C. The landscape grades from salt marsh of spartina grass near the Atlantic mouth to brackish marsh in the middle bay and freshwater tidal marsh up the rivers. Their core protection is the Chesapeake Bay Estuarine Complex, a Ramsar wetland of international importance of about 450 km² (174 sq mi) designated on 4 June 1987, alongside Blackwater National Wildlife Refuge (founded in 1933) and National Estuarine Research Reserves in both states. Many of those protections were responses to specific threats, and the story of the wetlands is best told that way: what has been lost, and what was done about it, with dates.
Blackwater, 1933 to 2022: nutria, rising water and a sinking marsh
Blackwater National Wildlife Refuge, on Maryland's Eastern Shore about 12 miles (19 km) south of Cambridge in Dorchester County, was set up in 1933 as a sanctuary for waterfowl on the Atlantic Flyway. It now covers more than 28,000 acres (110 km²) of brackish tidal marsh, freshwater impoundments, fields and forest, fed by the tea-coloured Blackwater and Little Blackwater rivers. More than 250 bird species have been recorded, and in winter the refuge holds up to 15,000 geese and 10,000 ducks. The refuge also shelters three recovered species, the Delmarva fox squirrel, the peregrine falcon and the bald eagle, the last two of which had been hit hard by DDT. About half of it is also part of the Harriet Tubman Underground Railroad National Monument, and a portion passed to the National Park Service in 2014.
The nutria
The refuge's best-known threat arrived on purpose. Nutria, large South American rodents raised for fur, were introduced to the area in the 1930s and 1940s. With the fur trade gone they multiplied into the tens of thousands across the Delmarva Peninsula, and by eating the roots of marsh plants they destroyed an estimated 7,000 to 8,000 acres (2,800 to 3,200 ha) of marsh at Blackwater alone. Intensive trapping began in 2002, and by 2003 a multimillion-dollar eradication programme was underway. Its methods included trained dogs and "Judas" nutria, sterilised animals fitted with radio collars that lead trappers back to their colonies. By 2012 the project had nearly succeeded, and in September 2022 officials announced that nutria had been completely eradicated from Maryland's Eastern Shore. California copied the approach when nutria reappeared there in 2017.
Rising water
Removing the nutria did not stop the water. Water levels in the bay have already risen by about one foot (0.3 m), and projections range from a further 1.3 to 5.2 ft (0.4 to 1.6 m) over the next 100 years. For the wetlands that means coastal marshes destroyed and salt water pushing into otherwise brackish parts of the bay. A vivid example lies in the bay off Dorchester County. Holland Island had about 360 residents in 1910, with a school, church, post office and a fleet of 41 skipjacks. Erosion on its west side began seriously in 1914, the last family left in 1918, and in October 2010 the last house on the island, built in 1888, collapsed.
Conowingo Dam and the Susquehanna since 1928
The Susquehanna supplies the head of the bay, and since 1928 it has passed through the Conowingo Dam, about 9.9 miles (16 km) above its mouth in Maryland, 5 miles (8 km) south of the Pennsylvania line. It is the largest dam in Maryland, holds back a 9,000-acre reservoir, and on completion was the second-largest hydroelectric project in the United States after Niagara Falls; four extra turbines added in 1978 raised its capacity from 252 to 548 MW.
For the wetlands, Conowingo and the Holtwood and Safe Harbor dams upstream matter in two ways. First, they cut off migratory fish. American shad could no longer swim up the Susquehanna to spawn, so from 1984 to 1999 shad were caught at Conowingo and trucked above all three dams. A fish lift opened at Conowingo in 1991, all three dams had lifts by the 2000 season, and 153,000 shad passed the Conowingo lift that year, though only 30 to 50% then made it to Holtwood. Second, the reservoirs trap sediment and nutrients that would otherwise reach the bay. A 1998 USGS study suggested they could fill before 2020 and stop doing so. In big floods they release it: in July 2018, 20 of the dam's 53 floodgates were opened, sending debris and sediment down the bay. Storms add to the load: in June 1972 the remnants of Hurricane Agnes severely flooded the Susquehanna, and Agnes became the costliest hurricane to hit the United States up to that time.
Smaller tributary dams are now coming out. Reopening fish passages was a goal of the 2014 Chesapeake Bay Watershed Agreement, and in 2022 and 2023 alone, 13 dams were removed, reopening 303 miles (488 km) of streams to migrating fish.
Farm runoff, dead zones and invasive Phragmites since the 1970s
The bay's most persistent threat starts far inland. Nitrogen and phosphorus from farms, lawns, sewage and air pollution feed algal blooms that block sunlight and then rot, using up oxygen. In the 1970s the Chesapeake was found to contain one of the first marine dead zones identified anywhere. By 2010, dead zones were estimated to kill 75,000 tons of bottom-dwelling clams and worms each year. About half of the bay's nutrient load comes from manure and poultry litter, much of it from farms in Pennsylvania such as those in Lancaster County. In the late 1990s, blooms of Pfiesteria piscicida killed large numbers of fish and were blamed on runoff from chicken farms.
The underwater meadows that provide food and habitat for the bay's wildlife have suffered too. In the southern bay, beds of eelgrass have shrunk by more than half since the early 1970s. The oyster reefs that once filtered the water have collapsed too: Maryland had roughly 200,000 acres (810 km²) of oyster reefs, and by 2008 about 36,000 acres (150 km²). In pre-colonial times oysters could filter the bay's entire volume in about 3.3 days; by 1988 it took 325 days. Overharvesting, pollution and two diseases, MSX and Dermo, all played a part.
On the marsh surface, invasive plants compete with native ones. The European subspecies of the common reed, Phragmites australis subsp. australis, forms dense stands that crowd out wild rice and cattails and offer poor habitat to native animals. Purple loosestrife and Japanese stiltgrass are also well established in Chesapeake wetlands, and Brazilian waterweed, spread by aquarium dumping, can clog the low-salinity tidal waters.
From the 1983 Chesapeake Bay Agreement to the 1987 Ramsar listing and the 2010 TMDL
Protection came in layers. In September 1983, after seven years of research, the US Environmental Protection Agency reported that the bay was "an ecosystem in decline". That December the governors of Maryland, Virginia and Pennsylvania, the mayor of the District of Columbia and the EPA administrator signed the first Chesapeake Bay Agreement, creating the Chesapeake Bay Program. The 1987 agreement set a target of cutting the nitrogen and phosphorus entering the bay by 40% by 2000. The Chesapeake 2000 agreement, adopted in June 2000, brought in Delaware, New York and West Virginia and recommitted to protecting 114,000 acres (460 km²) of underwater grasses.
Specific wetlands gained their own status. Eastern Neck National Wildlife Refuge, a 2,286-acre island at the mouth of the Chester River and a staging site for tundra swans, dates from 1962. The Ramsar listing of the Chesapeake Bay Estuarine Complex followed on 4 June 1987. Maryland's National Estuarine Research Reserve protects three contrasting marshes: the 3,426-acre salt marsh at Monie Bay, designated in 1985, and the freshwater tidal marshes of Jug Bay (2,087 acres) and Otter Point Creek (726 acres), both designated in 1990. Virginia's reserve, designated in 1991 and managed by the Virginia Institute of Marine Science, covers Sweet Hall Marsh, Taskinas Creek and the Catlett and Goodwin islands along the York River.
The strongest legal tool arrived later. After the Chesapeake Bay Foundation sued in 2009, the EPA issued a total maximum daily load (TMDL) for nitrogen, phosphorus and sediment on 29 December 2010, the largest such document it had produced, and the courts upheld it against challenges from agriculture and construction groups. It obliges every state in the watershed to write detailed pollution-reduction plans. A second lawsuit in 2020 over Pennsylvania's and New York's progress was settled in 2023, and in May 2025 Maryland's governor Wes Moore signed the Chesapeake Bay Legacy Act, which funds certification for farmers who cut runoff. Results remain mixed: the foundation reported in 2022 no improvement in nitrogen or dissolved oxygen, though oysters and rockfish were recovering.
To see where the bay's fresh water and sediment come from, follow the Susquehanna River upstream; for the open water these marshes protect, read our Chesapeake Bay guide, or place Blackwater and the Eastern Shore in the wider geography of Maryland.