Wetlands are defined by water—and by what water does to the land
A wetland can look like a marsh filled with reeds, a peat bog, a seasonally flooded grassland, a mangrove forest, a river delta or a shallow lake edge.
That variety makes wetlands easy to misunderstand.
They are not simply places that are permanently underwater, and they are not all “swamps”. What unites them is the persistent or recurring influence of water on soils, vegetation and ecological processes.
The Convention on Wetlands uses an intentionally broad definition. It includes lakes and rivers, marshes and swamps, peatlands, wet grasslands, oases, estuaries, deltas, tidal flats, mangroves, coral reefs and even some human-made systems such as reservoirs, rice paddies and salt pans.
This breadth reflects the central fact about wetlands: they sit at the boundary between terrestrial and aquatic systems and often perform functions of both.
Why wetlands were historically treated as wasteland
For centuries, many societies viewed wetlands mainly as obstacles.
They were drained to create farmland, filled for construction, channelised to speed water away from settlements or converted for roads and industry. Some posed genuine challenges: mosquitoes, difficult transport, seasonal flooding and soils unsuitable for conventional buildings.
But the idea that a drained wetland is automatically “improved” ignores what the original ecosystem was doing.
Wetlands temporarily store water. They trap sediment. Their vegetation can remove nutrients and pollutants. Peatlands accumulate large carbon stores. Floodplains spread high river flows across wide areas. Coastal wetlands help buffer waves and storm surges. Many wetlands are nurseries and feeding grounds for fish and birds.
Removing the wetland does not remove the need for these functions. It often transfers the cost to drainage systems, water-treatment plants, flood defences and downstream communities.
The major wetland types
Wetland classification can become technical, but several broad groups help explain their diversity.
Inland wetlands include marshes, swamps, peatlands, floodplains, lakes, rivers, springs and wet grasslands. Some hold water permanently; others flood seasonally.
Coastal wetlands include salt marshes, mangroves, estuaries, tidal flats and seagrass-related systems. They are influenced by tides, salinity and the mixing of freshwater and seawater.
Human-made wetlands include reservoirs, fish ponds, rice paddies, wastewater-treatment wetlands and salt pans. They can provide habitat and water-management functions, though they are not necessarily ecological substitutes for natural wetlands.
The key point is that “wetland” describes a family of ecosystems, not one landscape type.
Wetlands and flood regulation
One of the most important wetland services is water storage.
When rivers rise, connected floodplains and marshes can spread and temporarily hold part of the flow. Vegetation slows water movement. Depressions store runoff. This can reduce or delay downstream flood peaks under some conditions.
Wetlands do not prevent every flood. Their capacity can be overwhelmed, and the effect depends on their size, location and hydrological connection.
But removing floodplain storage and forcing rivers into narrow channels can move water faster toward downstream settlements. In cities, filling natural low-lying areas may eliminate places where stormwater once collected safely.
The Convention on Wetlands therefore lists flood control among the ecosystem services wetlands provide.
Wetlands and drought
The relationship also works in the opposite direction.
Wetlands can store water during wet periods and release it slowly, supporting streamflow, soil moisture or groundwater connections during drier periods. Some wetlands are important recharge zones; others are fed by groundwater and indicate the presence of shallow aquifers.
Not every wetland increases groundwater recharge, and hydrology differs greatly among systems. Peatlands, marshes and floodplains can behave very differently.
What they share is a role in regulating the movement and storage of water through landscapes.
That makes wetland loss part of the broader story of water scarcity and climate adaptation.
Natural water treatment
Wetland plants, soils and microorganisms can trap sediments and transform or retain nutrients.
As water slows, suspended particles settle. Plants take up nutrients. Microbial processes can transform nitrogen compounds. Organic soils can bind some contaminants.
This is why constructed wetlands are sometimes deliberately used to help treat wastewater or agricultural runoff.
Natural wetlands should not be treated as unlimited pollution filters, however. Excess nutrients, toxic chemicals, plastics and sewage can overwhelm ecological processes and damage the wetland itself.
A wetland that removes some pollution is providing a service—not a licence to pollute it.
Biodiversity concentrated around water
Wetlands support exceptionally rich biological communities.
Fish use floodplains and marshes as breeding or nursery habitat. Migratory birds depend on chains of wetlands as feeding and resting sites. Amphibians need aquatic environments for part of their life cycle. Invertebrates, aquatic plants and microorganisms create complex food webs.
Some species live almost entirely within wetlands; others depend on them seasonally.
The loss of one wetland can therefore matter far beyond its boundary. A migratory bird may travel thousands of kilometres but still fail if one critical stopover site disappears.
This is one reason the Ramsar system focuses on networks of internationally important wetlands rather than isolated sites alone.
Wetlands as carbon stores
Wetlands influence climate in different ways.
Peatlands are especially important because partially decomposed plant material can accumulate over thousands of years, storing very large quantities of carbon in waterlogged soils. When peatlands are drained or burned, that stored carbon can be released.
Coastal wetlands such as mangroves, salt marshes and seagrasses also accumulate carbon in vegetation and sediments and are commonly described as “blue carbon” ecosystems.
Some wetlands emit methane, a potent greenhouse gas, as part of natural anaerobic decomposition. This does not make their protection climatically irrelevant. Climate accounting must consider long-term carbon storage, methane, carbon dioxide emissions from drainage and the consequences of land conversion.
The Global Wetland Outlook 2025 identifies wetlands as important allies in both climate mitigation and adaptation.
How much wetland has been lost?
The latest global assessment is sobering.
The Convention on Wetlands’ Global Wetland Outlook 2025 estimates that about 411 million hectares of wetlands—roughly 22 percent of the global total—have been lost since 1970. It also estimates that around a quarter of remaining wetlands are in poor ecological condition.
The report identifies agricultural expansion, pollution, infrastructure development, hydrological disruption and climate change among interacting pressures.
These numbers are estimates assembled from global datasets rather than a perfect census of every wetland. Wetlands are difficult to map consistently because some are seasonal, remote or changing rapidly.
Even with that uncertainty, the direction is clear: wetland area and ecological condition continue to decline.
Why drainage changes more than the water level
Draining a wetland can fundamentally alter its chemistry and biology.
Peat exposed to air begins to decompose more rapidly. Soil can subside. Fire risk may increase. Species adapted to waterlogged conditions disappear. Rivers may become more disconnected from floodplains.
Restoring a wetland therefore involves more than putting water back into a hole.
Hydrology must often be repaired: drains blocked, river connections restored, tidal exchange reopened or groundwater conditions recovered. Native vegetation may return naturally once water regimes improve, or it may need assistance.
This is why successful wetland restoration begins with understanding how water originally moved through the site.
Wetlands are valuable because they are dynamic
People often prefer landscapes with fixed boundaries: land here, water there.
Wetlands violate that preference.
Their boundaries expand and contract. A floodplain may be dry for much of the year and still be ecologically a floodplain. A seasonal marsh may disappear during drought and return when rains arrive. A mangrove forest advances and retreats with sediment and tides.
Trying to freeze these systems into permanent water or permanent dry land can destroy the processes that make them useful.
The economic value is enormous, but not the whole argument
The Global Wetland Outlook 2025 estimates that the world’s remaining wetlands provide ecosystem services worth up to tens of trillions of dollars annually.
Such estimates help policymakers compare wetland protection with the costs of engineered infrastructure and land conversion.
But many wetland values cannot be reduced neatly to a market price: cultural identity, biodiversity, spiritual importance, future scientific value and the existence of species that have no commercial use.
Economic valuation is therefore one argument for conservation, not the only one.
Protecting wetlands means giving water somewhere to go
Wetlands appear messy because they are places of movement and transition.
That apparent disorder is exactly what allows them to store floodwater, filter runoff, accumulate carbon, create habitat and link rivers, soils, aquifers and coasts.
When societies eliminate wetlands, they often replace flexible natural systems with rigid infrastructure—and then spend heavily managing the consequences.
The more useful approach is to recognise wetlands as part of water infrastructure from the beginning.
They are not leftover land waiting for a “better” use.
They are working ecosystems.
Sources / Further Reading
Convention on Wetlands — The importance of wetlands — https://www.ramsar.org/about/our-mission/importance-wetlands
Convention on Wetlands — Global Wetland Outlook 2025 — https://www.ramsar.org/document/global-wetland-outlook-2025-valuing-conserving-restoring-financing-wetlands
Global Wetland Outlook 2025 — interactive overview — https://www.global-wetland-outlook.ramsar.org/
Convention on Wetlands — Global Wetland Outlook publications — https://www.ramsar.org/resources/publications/global-wetland-outlooks
Suggested Internal Links
Understanding Mangroves and Coastal Protection — Planned internal link
Understanding Biodiversity Loss — Planned internal link
Understanding Water Pollution — Planned internal link
What Is Water Scarcity — Planned internal link
Understanding the Importance of Lakes and Ponds — Planned internal link
What Is Groundwater Recharge — Planned internal link
