Wetlands 101: Everything You Need to Know

Quick Key Facts

• Wetlands exist on every continent except Antarctica. 
• Wetlands are home to hydrophytes, literally water plants.
• Freshwater marshes are the most common and widespread wetlands in North America.
• The Sundarbans in India and Bangladesh is a saltwater swamp that boasts the world’s biggest uninterrupted mangrove forest at 140,000 hectares.
• Wetlands provide habitat or breeding grounds for 40% of all plant and animal species. 
• Peatlands cover only around 3% of the Earth’s surface but store around 30% of its soil carbon.
• According to one estimate, wetlands are disappearing at three times the rate of forests and are considered the world’s most imperiled ecosystem. 
• Nearly half of the tidal wetlands along the U.S. coast are vulnerable to sea-level rise by 2100.
• In Sackett v. EPA, the U.S. Supreme Court ruled that a wetland only had federal protection if its surface waters touched the waters of a navigable body of water.
• In the Lower 48 U.S. states, 75% of remaining wetlands are on private land.

What Are ‘Wetlands’?

A wetland is exactly what it sounds like: ground that is covered by or saturated with water for all or part of the year. The water that makes a wetland can come from a variety of different sources, including ocean tides; freshwater sources like lakes, rivers, or ponds; underground aquifers, or rain.

Wetlands are found on every continent except Antarctica. Some famous examples are the Everglades in Florida, the Pantanal in Brazil and Monadh Mor in Scotland. There is an amazing diversity of wetlands based on how frequently their soils are saturated, the surrounding climate and the source of the water, among other factors. All wetlands have moistened soils that are described as hydric, and these soils nurture water-loving plants, called hydrophytes. Hydrophyte literally comes from the Greek words for “water” and “plant.” These plants can either spend their lives under the water, floating on top of it or submerged in moist soils and range from mangrove trees to duckweed, but they are all adapted to a watery, low-oxygen environment. 

In the past, wetlands were often dismissed as waste areas or sources of mosquitoes and disease. Because of this, human societies have often not treated them with the respect they deserve, either filling them in to build cities, draining them for farmland or using them as garbage dumps. However, wetlands are actually vitally important ecosystems that perform essential functions, from purifying water to storing carbon. As human activities like the burning of fossil fuels push the climate toward a tipping point, wetlands are more important than ever.

What Are the Main Types of Wetlands?

There are many different types of wetlands and means of classifying them. Broadly, wetlands can be divided into coastal or tidal wetlands and inland or non-tidal wetlands. In the U.S., different agencies have different systems for classifying wetlands based on factors like geography, vegetation, water source and how the water flows through the landscape. However, according to National Geographic, there are three basic types of wetlands recognized by scientists: marshes, swamps and bogs. The U.S. Environmental Protection Agency also includes a fourth type: the fen. Other names for wetlands you may be familiar with include mires, muskegs, sloughs, playa lakes, vernal pools, wet meadows, pocosins, lagoons and deltas.

Marshes

A marsh is a wetland that is frequently or always flooded and where long-stem plants grow in water-saturated soils. They are most common in mid latitudes, and they are divided into tidal or saltwater marshes and non-tidal or freshwater marshes. Tidal or saltwater marshes are often found near estuaries — a biodiverse habitat where a river opens out into the ocean. Typical plants in saltwater marshes include smooth cordgrass close to the tide and short smooth cordgrass, spike grass and saltmeadow rush farther from the shore. Examples are the saltwater marshes of northern Australia — home to the saltwater crocodile — and the salt marshes surrounding Maryland’s Chesapeake Bay.

^{The Transquaking River feeds into the Fishing Bay salt marsh near Blackwater National Wildlife Refuge in Dorchester County, Maryland, on June 5, 2018. Will Parson / Chesapeake Bay Program}

Freshwater or non-tidal marshes are typically found by rivers, streams or lakes, or in depressions where water can gather. Typical vegetation includes bulrushes, reeds, lilly pads and cattails. Freshwater marshes are the most common and widespread types of wetlands in North America. Examples include much of the Everglades and prairie potholes — depressions left in the ground by melting chunks of glacier that created wetlands as they melted. These are important resting stops for migratory birds.

Swamps

A swamp is a wetland characterized by woody plants. Swamps can be divided into either freshwater and saltwater swamps or forested swamps and shrub swamps. All swamps are home to water-tolerant trees or shrubs, but the species depend on the climate and location. There are many freshwater swamps in the tropics, where it is hot and humid all year, but there are swamps as far north as the boreal forests of Russia, Alaska and Canada. Freshwater swamp plants can range from red maples in the northeastern U.S. to cypress in the U.S. south. Swampy shrubs include willows, button bush and smooth alder. One famous example of a freshwater swamp is the Louisiana bayou. 

Saltwater swamps are mostly found in the tropics along coastal sand flats or mudflats.The water in saltwater swamps is brackish, meaning it is a mix of salt and freshwater. The trees that typically live in saltwater swamps are called mangroves, which have tall roots that hold their short trunks above water level. Mangroves both attract soil, keep it in place and help create it when they decay. They also provide habitat for marine animals and birds. One example of a saltwater swamp is the Sundarbans in India and Bangladesh, which boasts the world’s biggest uninterrupted mangrove forest at 140,000 hectares. The swamp is home to 260 bird species, the estuarine crocodile, the Indian python and the Bengal tiger.

Bogs

A bog is a type of wetland that is formed when sphagnum moss and other plants accumulate, either trapping the water from a pre-existing lake or pond or collecting and trapping rain water. Most of the current water in a bog comes from rainwater, not groundwater or another source. Because of the water source and the presence of mosses, bog soil and water is acidic and has less nutrients. This means that bogs are not as fertile and only nurture certain types of plants that therefore only attract certain animals. Carnivorous plants like pitcher plants and sundew are some of the unique plants that favor bogs, while cranberries and blueberries are some of the few crops that can be grown in them. One large animal that does spend time in bogs is the moose, who feeds on aquatic plants. Examples of bogs include the cranberry bogs of Massachusetts and the Great Kemeri Bog in Latvia’s Kemeri National Park.

Over time, the plant matter that builds up at the bottom of a bog turns to peat. That means a bog is a type of peatland, or an area where there are 20 to 40 centimeters of peat in the top 80 centimeters of soil. Between 50 and 70% of all wetlands are peatlands. Peat is a fuel in its own right, and can turn into coal with more time and pressure. In addition, bogs are very good at preserving things because of their lack of oxygen and the presence of natural tannins, which are used in preserving leather. Several “bog bodies” — human corpses with hair and clothing — have been found in bogs dating from thousands of years ago, mostly in northern Europe. The most well-preserved “bog body” is Tollund Man, who was found in Denmark’s Bjældskovdal bog in 1950 but lived around 280 B.C.E. He was found wearing a sheepskin hat, an ox-skin belt and skin covering all but his arms and hands.

Fens

Fens are similar to bogs in that they are wetlands that create peat over time. They are also more common in the Northern Hemisphere. However, they are distinct in that they do not rely on rain or snow for their nutrients, instead accessing them via water flowing downhill or through groundwater below. Because of this, they have a broader diversity of plants and animals, including rushes, wildflowers, sedges and grasses. 

What Are the Benefits of Wetlands?

Wetlands only cover 6% of the Earth’s land area, but they are vitally important for plant and animal life, nutrient and water cycles and human well-being.

Providing Habitat

Wetlands are very important for biodiversity. About 40% of the Earth’s plants and animals either live or breed in wetlands. In fact, wetlands are on par with coral reefs and tropical rainforests for being some of the world’s most productive ecosystems. This is because they are “biological supermarkets.” Plant matter decays in the water to become detritus, which provides food for insects, small fish and shellfish that then attract larger fish, birds, reptiles, amphibians and mammals. Wetlands play an important role in the life cycles of several species, including salmon, trout, oysters and blue crabs. Some species live only in wetlands, including more than one-third of those on the U.S. endangered and threatened species list. Many birds rely on wetlands for at least part of the year. They are important pit stops for migrating birds, and some migratory species would go extinct without certain wetlands. 

Improving Water Quality

Wetlands act as a natural water filtration system, keeping larger bodies of water like rivers and oceans clean. When water enters a wetland, its pace is slowed by the many plants, allowing any sediments within it to become suspended in the wetlands. These sediments, which are often attached to toxic chemicals or nutrient pollution like excess nitrogen or phosphorus from agriculture or wastewater, are then absorbed by plants or settle at the bottom of the wetland. Natural processes can absorb excess nutrients into the wetlands’ nutrient cycle or turn toxic chemicals into less toxic forms. If water passes through a wetland, up to 90% of the sediment within it may be trapped by the wetland, allowing the water to run clear. 

Managing Water Flow

Wetlands can also help prevent or reduce the impact of flooding by absorbing excess water and then letting it enter the surrounding landscape or groundwater at a slower pace. Indeed, one acre of wetland can store up to 1.5 million gallons of excess water. The vegetation in wetlands can slow flood waters as well. While this water storage reduces the severity of floods, it also delays the beginning of droughts during drier periods. On the coasts, wetlands can help protect inland areas from storm surges by absorbing both water and wave energy. Mangroves, for example, can act as a natural storm break. Along the U.S. Atlantic and Gulf coasts, where hurricanes are a yearly hazard, wetlands prevent almost $700,000 in storm damage per square mile each year on average.

Sequestering Carbon

Another thing that scientists are learning about wetlands in the context of the climate crisis is that they are natural sinks for carbon dioxide, drawing down around 8.1 million tons of carbon dioxide from the air annually. “All ecosystems store carbon, but wetlands store a lot more than all the rest,” said Michigan Tech School of Forest Resources and Environmental Science professor Rod Chimner. Coastal wetlands may sequester carbon as much as 55 times faster than tropical rainforests. Peatlands are also important carbon sinks — while they only take up around 3% of the Earth’s land area, they store around 30% of its soil carbon. 

Supporting Local Economies

Because they are such productive ecosystems, wetlands can also provide many economic benefits to humans if managed sustainably. In fact, more than one billion people derive their livelihoods from wetlands. For example, many fisheries depend on wetlands to keep their population numbers high, including more than half of the U.S. commercial seafood harvest. Several crops are cultivated in wetlands, such as cranberries, blueberries and wild rice. In addition, wetlands provide opportunities for ecotourism through kayaking, birdwatching and other water-based activities. 

Cultural Benefits

While some human societies have dismissed or disparaged wetlands, others have developed close cultural ties with them. The Cajun culture in Louisiana, for example, is intimately tied to life on the bayou. Wetlands have inspired cultural traditions and mythology from all over the world, from crane festivals in Japan to the legend of the kelpie, the Celtic water horse that is sometimes drawn with a mane of bulrushes. Many Indigenous peoples around the world have developed their cultures based on wetlands, and these ecosystems remain important for their spirituality and livelihoods. 

What Are the Main Threats Facing Wetlands? 

Despite their irreplaceable value, wetlands have historically been undervalued by settler or industrial societies, to devastating consequences. In the U.S., more than half of all of the wetlands in the lower 48 states were drained between the 1600s and today. Globally, the Ramsan Convention on Wetlands calculated that 35% of all wetlands have been lost since 1970, and that they are now disappearing at three times the rate of forests, making them the world’s most imperiled type of ecosystem. A 2023 study published in Nature put the extent of global loss at a lower but still significant 21% between 1700 and 2020, with the U.S. taking the lead for greatest wetland loss of any country. While the rate of wetland loss has declined in the U.S. since the 1970s, it still loses around 60,000 acres a year.

Agriculture

Land-use change is the greatest threat to wetlands both historically and currently, with conversion of wild lands to agricultural lands being the leading cause of wetland loss. Agriculture has degraded more than half of the Wetlands of International Importance. In some cases, wetlands will be drained to be converted to cropland. This can be counterproductive, as wetlands can actually support agriculture if left alone by providing soil nutrients, water for grazing animals and drought prevention, among other benefits. However, livestock grazing can harm wetlands when not properly managed, as the animals may devour plants that help prevent erosion of streambanks and sedimentation of the water.

Development

Wetlands have also been drained and filled to be turned into human developments. Major cities including London, Venice, New Orleans and New York were all built on wetlands. In the U.S., pressure from development is overtaking agriculture as the leading cause of wetland loss. Converting wetlands into urban areas can worsen flooding because the water-absorbing vegetation is replaced with impervious materials like concrete.

Dams

The damming of rivers for agriculture or energy can harm wetlands by diverting water away from them. By altering the flow of rivers, these dams can reduce seasonal flooding, causing certain wetlands to shrink and harming the species that depend on them for habitat. One example is the Colorado River Delta, which was once the largest wetland in North America but dwindled over the 20th century as almost 90% of the Colorado River was diverted for households, farms and energy in the U.S. before even reaching Mexico. While the delta has largely dried up, there are now efforts in place to restore it.

Pollution

While wetlands are adept at filtering pollutants, sometimes the amount of contaminants that human activities dump into the environment can become too much for them. In addition, as wetlands disappear, this puts more pressure on the remaining wetlands to filter more toxins. Major pollutants that harm wetlands include sediment, fertilizer, human and animal waste, pesticides and heavy metals. These pollutants can then harm the plants and animals in the wetlands. For example, fertilizer pollution can encourage too many plants to grow, which then deprive the remaining wetland plants and animals of oxygen as they die and decay. The rise in plastic pollution is also impacting wetlands, killing birds and turtles. Another recent study found that tire particles are harming organisms in estuaries. 

Climate Crisis

The climate crisis threatens coastal wetlands through sea-level rise and coastal erosion, while it exposes others to drought and desertification. For example, 43 to 48% of the tidal wetlands along the U.S. coast are vulnerable to sea-level rise by 2100. Globally, 20 to 90% of coastal wetlands may be swallowed by higher ocean levels. Wetlands in the Arctic and in mountain ecosystems are also especially at risk from climate change as rapid ice melt alters these environments. 

What Is Sackett v. EPA and How Does It Impact Wetlands?

One recent impediment to the protection of wetlands in the U.S. is the Supreme Court decision Sackett v. EPA. This decision, ruled in 2023, involved what counts as part of the “waters of the United States” that the federal government is authorized to protect under the Clean Water Act of 1972. For most of the act’s history, waters of the United States, or WOTUS, have been interpreted to mean any body of water in a given watershed. In Sackett v. EPA, however, the court ruled that only navigable waters, streams leading directly to navigable waters or wetlands directly connected via surface water to navigable waters merit federal protections. This puts many wetlands that may be connected to a larger river via groundwater, vegetation or downhill flow at risk from development. 

How Can We Protect Wetlands?

Despite the many threats to wetlands, there is growing awareness of their ecological importance as well as a growing movement to both save existing wetlands and restore degraded ones. 

Preservation

There are many ways to preserve wetlands on the global, national, local and individual level. The Convention on Wetlands is an international treaty aimed at the conservation and “wise use” of wetlands. It was adopted in Ramsar, Iran, in 1971, and today nearly 90% of UN member states are “contracting parties.” The internationally recognized goal of protecting 30% of lands and waters by 2030 can also be an opportunity to protect wetland ecosystems. One recent example is the protection in 2022 of Argentina’s Mitre Peninsula, which included one of South America’s largest peatlands.

In the Lower 48 U.S, nearly 75% of wetlands are on private land, so it is vital that people with wetlands on their property act as stewards. Individuals who care about wetlands can lobby for national, state, or local laws that protect wetlands; volunteer or donate money to organizations that preserve wetlands; oppose developments that would harm or fill wetlands; and reduce the runoff of pollution into wetlands by installing rain barrels and permeable surfaces in their yards and avoiding pesticides and nutrient-rich fertilizers.

^{USDA’s Natural Resources Conservation Service, in partnership with the Oregon Department of Fish and Wildlife, worked with private landowners on the habitat for the Oregon chub on a Wetlands Reserve Program site known as the Dunn Pond. The program allows landowners to voluntarily restore and protect wetlands. NRCS Oregon / Flickr}

Restoration

It is also possible to restore wetlands that have been damaged or degraded. This can happen through both re-establishing the wetland environment and rehabilitating the function of the ecosystem. In the U.S., wetland restoration is sometimes required by law to offset the permitted destruction of or damage to a wetland for a particular project. 

There are many examples of wetland restoration projects. One example is the concept of “sponge cities” in China. Urban planners are replacing concrete flood walls or river banks with plants and parks to help prevent floods and restore urban wetlands. Another example is the effort to bring water flow back to the Colorado River Delta. A 2018 study of these efforts found that restored areas attracted 74% more birds and 20% more bird species than areas that were not.

Takeaway

Wetlands are unique and valuable ecosystems that provide tremendous benefits for both non-human and human life — if humans can learn to appreciate them. In the past few centuries of industrial development, human societies have often worked against wetlands, seeing them as obstacles to growing food or building settlements. This attitude has harmed both wetlands and humans, leading, for example, to dwindling fisheries and cities that flood more regularly and intensely. 

But if human societies can learn to once again work with wetlands, these amazing marshes, swamps, bogs and fens can help to protect against storms, reduce flooding, restore biodiversity, fight the climate crisis and make the world a little more extraordinary.