The Gator Hook Strand of South Florida’s Big Cypress. (Photo: Sarah West)

What’s the planet’s mightiest swamp?

It’s a surprisingly tricky question, given the terminology. Ecologists define wetlands by different criteria, and the classification and naming schemes aren’t universal. Marshes, fens, bogs, mires, moors, wet prairies—these words all have technical definitions that can align, overlap, or flat-out contradict their vernacular meanings. In casual use, “swamp”—maybe because it’s such a resonant, evocative term—can serve for nearly any sodden lowland.

 Bibon Swamp, NW Wisconsin. (Photo: Ethan Shaw)

In North America, though, “swamp” typically refers to a wetland dominated by trees or shrubs (and not peat-choked, like a bog). By this definition, then, many of the world’s waterlogged ecosystems aren’t swamps. By far the vastest wetlands, for example, are the bogs of the pancake-flat West Siberian Lowlands, which collectively sprawl across more than a million square miles between the Urals and the Yenesey Valley. The Florida Everglades is often conceived of as swampland, but—though it does support scattered baldcypress domes and strands—it is, at heart, a great rain-fed sheetflow marsh. Not a swamp.

The Everglades proper is a huge freshwater marsh complex (photo by Ethan Shaw)

For sheer ambience, though, a forested wetland—a true swamp—is hard to beat among global ecosystems: Even a pocket-sized example is fiercely wild. Swamps may arise where rivers regularly leap their banks in rainy-season or snowmelt pulses to inundate their timbered floodplains; or where trees invade the basins of dead or dying lakes and marshes; or where the waves and the seabed and the mix of fresh- and saltwater along a seashore are conducive to trees called mangroves.

What we’ll survey here—in the spirit of all the countless morasses that have been drained, logged, and burnt off—are some of the very greatest tracts of swampland left. From the Florida tidewater to the depths of the Congo, these wilderness bottomlands are still big enough to get lost in, big enough to embody those primal swamps of our imagination.

The sort of disturbing/wonderful sight that makes some folks fret about swamps: an American alligator hauls off a white-tailed deer in Georgia’s Harris Neck NWR.(Photo: Terri Jenkins, U.S. Fish & Wildlife Service.)

The Flooded Forest: Amazon Basin

In the great drowned forests of Amazonia, the pink river dolphin, or boto, cruises among the stilt roots of Cecropia trees in the murk; parrots flash in the aguajale palm swales; heavy-jawed tambaqui fish munch floating tree fruits while immense catfish spawn. As heavy rains swell the Amazon system, rivers and tributaries overspill and mingle their floodwaters across grand reaches of bottomland rainforest. Reckoned (by Paul A. Keddy and Lauchlan H. Fraser in The World’s Largest Wetlands) at some 670,000 square miles, these seasonal floodplain wetlands—which prevail to varying degree from December to June—may constitute the planet’s biggest swamp mosaic. 

The boto, or pink river dolphin, roams the Amazonian flooded forest; it also favors the confluences of black- and whitewater rivers. (Photo: Jorge Andrade/via Wikimedia Commons.)

The inundations are divided between the várzea floodplains of so-called “whitewater” rivers (silt- and nutrient-rich flows out of the Andes) and the less fertile igapó bottoms of “blackwater” and “clearwater” rivers. Amazonia’s flooding regime conjures a striking diversity of wetland habitats, from dense canebrakes to waterlily-strung oxbows, and establishes the foundational ecological schedule below the higher terra firme rainforest occupying most of the basin.

The Gorilla Swamps of the Cuvette Centrale

Some 73,000 square miles of swamps and associated wetlands fortress the Congo Basin, second only to the Amazon in overall size. The broadest expanses of swamps lie southwest of Mbandaka, cloaking tributaries like the Oubangui, Likouala-aux-Herbes, and Sangha as they feed the Congo River. Across the Congo, impressive morasses also shroud lakes Tumba and Mai-Ndombe. This sparsely populated region has long harbored legends of a semi-aquatic dinosaur-like beast called the Mokele-mbembe.

Western lowland gorilla, Congo Basin swamp-walker. (Photo: Trisha M Shears/via Wikimedia Commons.)

Hardwoods dominate the seasonally flooded bottomlands, while the wettest reaches are often bristling jungles of raffia palm and pandanus. These Congolian swamp forests, critical refuge for forest elephants, are also some of the world’s most important remaining homes for great apes: western lowland gorillas, chimpanzees, and bonobos. Gorillas—which survive here in some of the greatest numbers and densities left anywhere in Africa—wade through the quagmires, gnawing on raffia pith and constructing high-and-dry nests out of palm fronds.

Bottomland Swamps of America

Baldcypresses in the Okefenokee Swamp. (Photo: Ryan Hagerty, U.S. Fish & Wildlife Service/via Wikimedia Commons.)

The swamps of the Atlantic-Gulf Coastal Plain and especially the lower Mississippi River valley constitute one of the planet’s great concentrations of forested wetlands. Prior to Euro-American settlement, the Mississippi alluvial plain may have nourished some 81,250 square miles of bottomland swamps and forests. Today, drainage, dams, and levees have greatly diminished this extent, but outstanding remnants—the dominion of alligators, water moccasins, and black bears—remain. Baldcypress (which may live 1,000 years or more) and tupelo/gum wall in deepwater sloughs, while slightly higher floodplain ground supports richer canopies of silver maple, sweetgum, sycamore, and other hardwoods.

Strictly speaking, the Atchafalaya and the Okefenokee may be America’s two single-largest swamps. The former comprises nearly a million acres of bottomland hardwood forest and baldcypress-tupelo swamp blanketing the basin of Louisiana’s 135-mile Atchafalaya River—a floodway distributary of the Mississippi that offers that river a shorter, steeper course to the Gulf of Mexico. The 700-square-mile Okefenokee Swamp on the Georgia-Florida line occupies the sand- and peat-filled depression of a former seaway, where baldcypress, tupelo, and bay swamps intergrade with marshes, wet prairies, tree hammocks, and sprawling lakes.

The Atchafalaya Swamp. (Photo: U.S. Army Corps of Engineers/via Wikimedia Commons.)

Out on South Carolina’s coastal plain, meanwhile, the great Congaree Swamp is America’s biggest old-growth bottomland hardwood forest and brandishes one of the loftiest canopies of any broadleaf forest on the planet: Many of its vine-draped trees, including persimmon, sweetgum, and baldcypress, exceed 130 feet in height, and there are cherrybark oaks and loblolly pines here nearly 170 feet tall. Congaree National Park protects the ancient floodplain woods—laced with tributary sloughs locally called “guts”—on the north side of the Congaree River.

New Guinea’s Giant River Swamps

The Fly River, Papua New Guinea. (Photo:National Geospatial-Intelligence Agency/via Wikimedia Commons.)

By any measure, the island of New Guinea—divided between Indonesian Papua in the west and Papua New Guinea in the east—is one of the planet’s swamp hotspots. Mammoth alluvial forests occupy the floodplains of the Mamberamo and Sepik rivers in the north and the Digul, Lorentz, Fly, Purari, and others on the more extensive coastal plain in the south, all fed by voluminous rainfall in the Central Highlands. Tall hardwoods such as Campnosperma as well as pandanus and sago palm define these mires, many virtually trackless to outsiders. The Asmat Swamp (part of the homeland of the Asmat people) draining to the Arafura Sea in southern Papua covers close to 12,000 square miles. It has been called the biggest river swamp in the world.

The Mangal: Great Coastal Swamps of the Tropics

Mangrove swamps—also called mangal—flourish along tropical and subtropical seashores and estuaries; they’re the low-latitude counterpart of the temperate salt marsh. “Mangrove” isn’t a taxonomic but an ecological grouping, referring to a diverse array of evergreen hardwoods (and even a palm—the nipa) adapted to brackish conditions. Mangal tends to be naturally patchy in distribution, but some big tracts exist, particularly along well-watered deltas.

Satellite view of the tiger-haunted Sundarbans mangal on the Bay of Bengal. (Photo: Jesse Allen, NASA/via Wikimedia Commons.)

Encompassing some 2,510 square miles along the convoluted mouths of the Ganges and Brahmaputra rivers in West Bengal and Bangladesh, the Sundarbans is often said to be Earth’s biggest mangrove swamp, though actually several others are comparable in extent. Size aside, this mangal—named for the swamp’s dominant mangrove, the sundri—is unquestionably magnificent: a salty, cyclone-battered jungle prowled by Indo-Pacific crocodiles, freshwater dolphins, sharks, and—most notorious of all—several hundred Bengal tigers. Roughly 10 people a year, mostly woodcutters, honey-gatherers, and fishermen, are killed by these half-amphibious big cats.

Other great mangals (identified in the exhaustive World Atlas of Mangroves) include two huge West African expanses, in the Niger Delta and on the seacoast between Senegal and Sierra Leone (a tract that, at 3,045 square miles, may be the largest mangrove complex on Earth); and the tidal tangles draping western and southern New Guinea’s coastal lowlands. The mangal with the greatest biomass carpets about 2,516 square miles of the Brazilian coast between Belém and Sao Luis.

Mangroves in Papua New Guinea. (Photo: Natalieragan/via Wikimedia Commons)

One of the largest contiguous mangals in the Western Hemisphere lies along the southwest Florida coast, where the Big Cypress Swamp and the two main conduits of the Everglades—the Shark River and Taylor sloughs—drain into the Gulf of Mexico and Florida Bay. The northwestern portion is the Ten Thousand Islands, a dizzying maze of mangrove keys; to their southeast, brackish jungle enfolds the Chatham, Lostmans, Broad, Harney, and Shark rivers.

Red mangrove (the “walking tree”), South Florida. (Photo: Ethan Shaw.)