A wetland is land that is saturated or covered by shallow water for long enough each year that its soils run short of oxygen and only plants adapted to waterlogging can dominate. Marshes, swamps, bogs, fens and mangroves are all wetlands; what unites them is not a particular look but that combination of water, distinctive soil and water-tolerant vegetation.
How Scientists and Lawmakers Define a Wetland
The international definition comes from a treaty signed on 2 February 1971 in the Iranian Caspian resort of Ramsar. The Convention on Wetlands, usually just called the Ramsar Convention, describes wetlands as "areas of marsh, fen, peatland or water, whether natural or artificial, permanent or temporary, with water that is static or flowing, fresh, brackish or salt, including areas of marine water the depth of which at low tide does not exceed six metres." That is deliberately broad. It sweeps in rivers, lakes, coral reefs, estuaries and even rice paddies, because the treaty was written to protect waterbird habitat wherever it occurs. It came into force in 1975, and as of early 2025 it had 172 member countries and 2,531 listed sites covering more than 2.6 million square kilometres.
Regulators who have to decide whether a particular field can be filled need something narrower. In the United States, the Army Corps of Engineers applies three tests, set out in its 1987 Wetlands Delineation Manual, and a site normally has to pass all three:
- Hydrology: the ground is flooded or saturated near the surface for a significant part of the growing season, typically at least two weeks in most regional guidance.
- Hydric soils: soils that formed under saturation and lost their oxygen. They show it in grey or bluish "gleyed" colours, rusty mottles where iron has moved, thick dark organic layers, and sometimes the rotten-egg smell of hydrogen sulfide.
- Hydrophytic vegetation: a plant community dominated by species adapted to wet, oxygen-poor roots, such as cattails, sedges, bulrushes, willows or bald cypress.
The legal stakes are high. Section 404 of the Clean Water Act requires a permit to dredge or fill wetlands that count as "waters of the United States", and in Sackett v. EPA (2023) the Supreme Court limited that protection to wetlands with a continuous surface connection to other covered waters, removing federal coverage from a large share of isolated wetlands.
Marsh, Swamp, Bog and Fen
Ecologists divide wetlands mainly by two questions: what grows there, and where the water comes from.
Marsh
A marsh is a wetland dominated by soft-stemmed, non-woody plants: grasses, reeds, rushes and sedges. Freshwater marshes fringe lakes and rivers; tidal salt marshes, built of cordgrasses such as Spartina, line sheltered coasts and are flooded twice a day. The Meadowlands in the photo above is a brackish marsh where the common reed Phragmites has spread over ground that Dutch settlers once mowed for salt hay.
Swamp
A swamp is a forested or shrub-covered wetland. In the American South that means bald cypress and tupelo standing in water, as in the Great Dismal Swamp of Virginia and North Carolina or Louisiana's Atchafalaya Basin. On tropical coasts the swamp trees are mangroves, which tolerate salt and anchor themselves on stilt roots; they are explained in more depth in our guide to what a mangrove is.
Bog
A bog is a peatland fed almost entirely by rain and snow. Because rainwater carries few minerals, bogs are acidic and starved of nutrients, and they are dominated by sphagnum moss, which acidifies its surroundings further. Dead plant matter barely rots in those conditions, so it piles up as peat, commonly at around a millimetre a year. Over thousands of years a raised bog can swell into a shallow dome several metres above the surrounding land, like Viru Bog in Estonia. Carnivorous sundews and pitcher plants thrive because they take their nitrogen from insects rather than the soil.
Fen
A fen is also a peatland, but it is fed by groundwater or streams that have run through mineral soil. That makes it less acidic, richer in nutrients and usually more diverse, with sedges, grasses and wildflowers rather than a carpet of moss. Fens and bogs are often stages of one process: as peat in a fen builds up above the reach of groundwater, the surface becomes rain-fed and the fen turns into a bog. Our article on what a bog is follows that transition further.
Wetland, Lake or Floodplain? Where the Lines Fall
The Ramsar definition counts lakes as wetlands, but most ecologists and regulators separate them by depth and light. A lake is deep enough that open water dominates and rooted plants cannot grow across the middle; a wetland is shallow enough that emergent plants can colonise most of it, or its surface is soil rather than water for much of the year. The two often grade into each other, with a marsh fringe around a lake that slowly creeps inward as sediment and dead vegetation fill the basin.
A floodplain is a landform, not an ecosystem: the flat ground beside a river built up by its floods. Parts of it may be wetland, such as backswamps, oxbow lakes and seasonally flooded forests, while higher ground on the same floodplain may be dry farmland that floods only once a decade. The test is how long water stays. Land that floods for a few days a year rarely develops hydric soils; land saturated for weeks of every growing season does.
What Wetlands Do for Water, Wildlife and Climate
Wetlands cover only about 6 percent of Earth's land surface, but they do several jobs that are expensive to replace.
Flood storage. A wetland acts as a sponge and a slow drain, holding storm water and releasing it over days or weeks. Draining that storage sends the same water downstream faster and higher, which is why floodplain wetland restoration now features in flood plans from the Mississippi to the Rhine.
Water cleaning. As water slows through dense vegetation, sediment settles out, microbes in oxygen-poor soils convert nitrate to nitrogen gas, and plants take up phosphorus.
Carbon storage. Peatlands occupy only about 3 to 4 percent of the world's land, yet according to the UN Environment Programme's 2022 Global Peatlands Assessment they hold close to a third of all soil carbon, roughly twice the carbon stored in the biomass of all the world's forests. When they are drained, the peat oxidises: degraded peatlands release about 4 percent of human-caused greenhouse gas emissions each year.
Habitat. About 40 percent of the world's species live or breed in wetlands, the figure the Ramsar Convention uses. That counts every species that depends on a wetland at some stage of its life, not species found only there. Coastal marshes and mangroves are nurseries for many commercial fish and shellfish.
How Much Has Been Lost
The Ramsar Convention's first Global Wetland Outlook, published in 2018, estimated that about 35 percent of the world's natural wetlands disappeared between 1970 and 2015. The second edition, released in July 2025 with improved data, put the loss since 1970 at about 22 percent, around 411 million hectares, and the current rate at roughly 0.5 percent a year. Long-run estimates have also been revised. A widely quoted 2014 figure of up to 87 percent lost since 1700 has been challenged by a 2023 study in Nature that reconstructed land use and found closer to a fifth of the 1700 extent gone, about 3.4 million square kilometres, with losses concentrated in Europe, the United States and China.
The causes are the same everywhere: drainage for farming, filling for cities and ports, dams that change river flows, and pollution. The Meadowlands show all of them. Most of the original marsh was diked, dumped on or built over in the twentieth century; the Hackensack Meadowlands Development Commission was created in 1969 partly to control the landfills, and today about 8,400 acres of open space remain.
Five Wetlands That Show the Range
Pantanal (Brazil, Bolivia, Paraguay). The world's largest tropical wetland covers an estimated 140,000 to 195,000 km² of the upper Paraguay River basin. About 80 percent of its floodplain goes under water in the rainy season, which concentrates fish, caiman and jaguar. Fires in 2020 burned roughly a quarter of it.
Sudd (South Sudan). Where the White Nile's Bahr al-Jabal spreads across a flat clay plain, it forms a swamp of papyrus and reeds of usually more than 30,000 km², swelling past 130,000 km² in wet years. About half the river's inflow evaporates there. The Jonglei Canal, begun in 1978 to bypass the swamp and deliver that water to Sudan and Egypt, stopped in 1984 with 240 of its 360 km dug. The Sudd became a Ramsar site in 2006; see our profile of the Sudd wetlands.
Okavango Delta (Botswana). An inland delta that never reaches the sea. Rain falling on the Angolan highlands in January and February takes months to travel downriver, so the flood spreads over 6,000 to 15,000 km² and peaks between June and August, in Botswana's dry winter, when wildlife from the surrounding Kalahari crowds in. Some 96 to 98 percent of the water is lost to evaporation, transpiration and groundwater. In 2014 it became the 1,000th site on the World Heritage List.
Sundarbans (Bangladesh and India). The largest mangrove forest on Earth, about 10,000 km² on the seaward edge of the Ganges–Brahmaputra delta, roughly 60 percent of it in Bangladesh. It is home to Bengal tigers that swim between islands, and its mangroves blunt the storm surges of cyclones coming off the Bay of Bengal.
Everglades (Florida). Marjory Stoneman Douglas called it the "River of Grass" in 1947, the year Everglades National Park was established: a sheet of water a few centimetres deep moving slowly south through sawgrass from Lake Okeechobee to Florida Bay. Drainage canals have cut the original wetland roughly in half, and the Comprehensive Everglades Restoration Plan, authorised by Congress in 2000, is trying to restore some of that flow. The Everglades and the other sites above all appear in our ranking of the largest swamps and wetlands in the world.