Wetlands are found on every continent except Antarctica, covering an estimated 12 to 16 million square kilometers of the Earth’s surface. The largest concentrations sit in Asia, North America, and Latin America, though wetlands exist in nearly every climate zone, from Arctic tundra to tropical river basins. They form wherever water meets land in a lasting way: along coastlines, beside rivers, in low-lying plains, and across vast northern landscapes where the ground stays waterlogged for much of the year.
What Makes a Place a Wetland
Three things must be present for an area to qualify as a wetland. First, the ground has to be saturated with water or flooded at some point during the growing season, with the water table sitting within inches of the surface for at least a week. Second, that prolonged wetness creates oxygen-poor soil, a distinct type that develops only under waterlogged conditions. Third, the plants growing there have to be species adapted to life in saturated ground. All three criteria must be met simultaneously. This is why a field that floods briefly after a heavy rain isn’t a wetland, but a marsh that stays soggy through spring and summer is.
About 91% of the world’s wetlands are inland, found along rivers, lakes, and in poorly drained lowlands. The remaining 9% are coastal wetlands: salt marshes, mangrove forests, and tidal flats where freshwater systems meet the ocean.
The World’s Largest Wetland Systems
The single largest wetland on Earth is the West Siberian Lowland in Russia, a sprawling peatland covering roughly 2.7 million square kilometers. It stretches across western Siberia between the Ural Mountains and the Yenisei River, a flat expanse of waterlogged ground, bogs, and shallow lakes that freezes in winter and thaws into a vast, spongy landscape each summer.
The Amazon River Basin ranks second. The Amazon’s floodplain wetlands extend across Brazil, Peru, Colombia, and Bolivia, inundating enormous areas of forest during the wet season. These seasonally flooded forests, known locally as várzea and igapó, are some of the most biologically productive ecosystems on the planet.
Rounding out the top five are the Hudson Bay Lowland in northern Canada, the Congo River Basin in Central Africa, and the Mackenzie River Basin, also in Canada. Other major systems include the Pantanal (straddling Brazil, Bolivia, and Paraguay), the Mississippi River Basin, the Lake Chad Basin in West Africa, the Nile River Basin, and the Prairie Potholes scattered across the northern Great Plains of the United States and southern Canada.
Northern Peatlands
Some of the most extensive wetlands on Earth are peatlands in the high latitudes of the Northern Hemisphere. These thick, carbon-rich bogs and fens dominate large stretches of Canada, Scandinavia, Russia, and Alaska, where cold temperatures slow the decomposition of plant material and allow organic matter to accumulate over thousands of years. Northern peatlands expanded rapidly after the last ice age, particularly across Europe and Western Siberia between 12,000 and 10,000 years ago, and continued growing as glaciers retreated across North America.
Today, these northern peatlands store enormous quantities of carbon in their waterlogged soils. Their total area grew from roughly 2.2 million square kilometers during the early post-glacial period to over 4 million square kilometers, making them one of the largest wetland types by total coverage. They’re concentrated above roughly 50°N latitude, forming a near-continuous band across the boreal and subarctic zones of three continents.
Tropical Wetlands
Tropical wetlands look nothing like the bogs of Siberia or Canada. In the Amazon Basin, vast lowland forests flood to depths of several meters each year as rivers swell during the rainy season. These flooded forests support species found nowhere else, from river dolphins to fish that eat fruit falling from submerged trees.
The Congo River Basin, spanning over 3.7 million square kilometers across Central Africa, contains another globally significant wetland complex. The Cuvette Centrale, a massive peatland in the heart of the Congo Basin (primarily in the Republic of Congo and the Democratic Republic of Congo), stores huge amounts of carbon belowground. Unlike northern peatlands that formed in cold conditions, these tropical peatlands developed under year-round warmth and humidity, buried beneath swamp forests.
The Pantanal, often called the world’s largest tropical wetland by continuous area, floods seasonally across a low-lying plain in South America. During the wet season (roughly November through March), water covers up to 80% of its surface, creating a mosaic of marshes, lagoons, and flooded grasslands that supports one of the densest wildlife populations in the Western Hemisphere.
Mangrove Forests Along Tropical Coasts
Mangroves are coastal wetlands that grow where salt water and fresh water mix in tropical and subtropical regions. They occupy roughly 137,760 square kilometers across 118 countries, with the heaviest concentration between 5°N and 5°S latitude, essentially hugging the equator. The largest mangrove forests grow along the coasts of Indonesia, Brazil, Nigeria, Australia, and Mexico.
These salt-tolerant trees and shrubs anchor sediment with their tangled root systems, protecting shorelines from erosion and storm surge. They also serve as nursery habitat for commercially important fish and shrimp species. Despite their importance, mangroves have been declining for decades due to coastal development, aquaculture, and rising sea levels.
Wetlands in the United States
The U.S. contains several globally significant wetland systems. The Prairie Potholes, millions of small depressions left by retreating glaciers, dot the landscape of the Dakotas, Montana, Minnesota, and Iowa. These shallow, seasonal wetlands are the primary breeding ground for most of North America’s ducks and other waterfowl.
The Mississippi River Basin includes vast floodplain wetlands stretching from Minnesota to the Gulf of Mexico. The Everglades in southern Florida form a unique “river of grass,” a slow-moving sheet of water flowing across a shallow limestone plain. Alaska’s North Slope and interior lowlands contain enormous expanses of permafrost wetlands and tundra bogs. Louisiana’s coastal marshes, the bottomland hardwood swamps of the Southeast, and the salt marshes along the Atlantic seaboard round out a diverse national wetland portfolio.
How Many Wetlands Are Formally Protected
The Ramsar Convention, an international treaty established in 1971, designates Wetlands of International Importance. Currently, over 2,500 Ramsar Sites exist across 172 countries, covering almost 2.5 million square kilometers. These range from small urban marshes to massive wilderness areas. Designation doesn’t guarantee full legal protection in every country, but it commits governments to maintaining the ecological character of these sites and managing them sustainably.
Despite these protections, wetland loss continues globally. Drainage for agriculture, urban expansion, and water diversion have eliminated large percentages of the wetlands that existed just a few centuries ago. Some estimates suggest that over a third of the world’s wetlands have been lost since the 1970s alone, making them one of the most rapidly declining ecosystem types on Earth.