A marsh is a wetland, an area of land where water covers ground for long periods of time


6 - 12+


Biology, Earth Science, Geography, Physical Geography

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A marsh is a type of wetland, an area of land where water covers ground for long periods of time. Unlike swamps, which are dominated by trees, marshes are usually treeless and dominated by grasses and other herbaceous plants.

Herbaceous plants have no woody stem above ground, and they grow and die back on a regular cycle. Herbaceous plants can be annuals (which grow anew every year), biennials (which take two years to complete their life cycle), or perennials (which take more than two years to complete their life cycle.)

Marsh grasses and other herbaceous plants grow in the waterlogged but rich soil deposited by rivers. The plants' roots bind to the muddy soil and slow the water flow, encouraging the spread of the marsh. These watery pastures are rich in biodiversity.

There are three types of marshes: tidal salt marshes, tidal freshwater marshes, and inland freshwater marshes. Marshes are also common in deltas, where rivers empty into a larger body of water. Although all are waterlogged and dominated by herbaceous plants, they each have a unique ecosystem.

Tidal Marshes

Both saltwater and freshwater tidal marshes serve many important functions: They buffer stormy seas, slow shoreline erosion, offer shelter and nesting sites for migratory waterbirds, and absorb excess nutrients that would lower oxygen levels in the sea and harm wildlife.

The marshes along the Gulf Coast in the U.S., for instance, help protect communities in the states of Louisiana, Alabama, Texas, and Florida from hurricanes. Marshes cannot stop hurricanes, of course, but the wetland slows the progress of the storm and absorbs much of the surging water from the Gulf of Mexico.

As marshes are drained for industrial and agricultural development, this layer of protection is diminished. Storm surges have no marshy "sponge" to absorb the water and wind of the hurricane, and coastal communities are especially vulnerable.

The fisheries of the Gulf Coast are also reduced as marshes are drained for development. The reduced habitat for fish decreases their population as more animals compete for fewer resources.

Draining marshes also increases saltwater intrusion. Saltwater intrusion is the process by which saltwater seeps into wetlands and even the water tables beneath them. This reduces the amount of freshwater for hygiene, drinking, industry, and irrigation. Saltwater intrusion also changes the chemistry of the tidal marsh, making it much more saline. Some species, such as cordgrass, can adapt to these changes. More delicate species are unable to adapt quickly and may become endangered.

Finally, draining marshes increases the direct runoff flowing to the ocean. Marshes are able to absorb toxic chemicals that leach into waterways from agricultural pesticides and industrial pollutants. Without the marshy sponge, runoff flows directly to the ocean, often creating coastal "dead zones" in which there is little life below the water's surface. The dead zones that regularly develop in and around the Chesapeake Bay and Gulf of Mexico are not only the result of agricultural and industrial runoff but also a lack of marshland to process such runoff.

Tidal Salt Marshes
Tidal salt marshes form a grassy fringe near river mouths and bays and along coastlines protected from the open ocean. Ocean tides fill the marsh with salty water and cause the water level to rise and fall twice a day. The marsh is deeper at high tide and shallower at low tide.

Plants such as sawgrass and pickleweed can tolerate fluctuating tidal waters, which are too salty for most trees and bushes.

Like all marshes, tidal salt marshes are home to a wide variety of bird species. Small birds such as terns feed on fish, insects, and crustacean species found in the marsh. Ducks and cormorants are aquatic birds that rely on the grassy marsh for nesting sites as well as food such as fish, shrimp, and crabs. Even large raptors such as osprey are supported by tidal salt marshes.

Commercially valuable fish and shellfish find food and shelter in salt marshes. The extensive tidal salt marshes along the coast of the U.S. state of Georgia, for instance, feature a large number of these species, including cordgrass (sometimes used as fodder for livestock), shrimp, and crab.

Tidal Freshwater Marshes
Tidal freshwater marshes lie farther inland than salt marshes, but are close enough to the coast to be affected by tidal fluctuations. Like in salt marshes, the water level rises and falls twice every day along with the tides.

Tidal freshwater marshes, however, are fed by freshwater streams and do not have a large salt content. They are common boundaries between forests and rivers.

Herbaceous plants called sedges dominate the tidal freshwater marsh ecosystem. Sedges include water chestnut and papyrus. Marshy papyrus is one of the most important plants in the development of civilization: Papyrus growing in the marshy delta of the Nile River was dried, treated, and used as an early form of paper by ancient Egyptians.

The abundant insects of freshwater tidal marshes provide food for birds such as wrens and blackbirds. Waterbirds, such as ducks and herons, are also common in freshwater tidal marshes.

Freshwater tidal marshes also provide spawning grounds for fish such as shad and herring. These fish are anadromous. Anadromous fish hatch in freshwater but migrate and live most of their lives in the ocean. They return to freshwater rivers, streams, and marshes to spawn.

Inland Marshes

Inland freshwater marshes are found along the fringes of lakes and rivers where the water table, the upper surface of underground water, is very high. They vary in size from bowl-shaped depressions called prairie potholes to the vast, watery grasslands of the Florida Everglades.

Vegetation in freshwater marshes depends on the presence of water. Wet meadows, for instance, do not have standing water for most of the year. They do not support aquatic plants. Plants establish seeds on a yearly basis and only bloom with annual or biannual flooding of the meadow.

Insects, especially butterflies, flourish in wet meadows. The ecosystem supported by these primary consumers includes frogs, snakes, and even apex predators such as bears.

Other freshwater marshes are much more aquatic. The Everglades, the largest freshwater marsh in the United States, is drowned in a shallow layer of water all year. In fact, the Everglades form a wide, slow-moving river draining out of Lake Okeechobee.

The Everglades are rich in biodiversity. This so-called "River of Grass" supports such plants as sawgrass, cypress, and mangrove forests. It is home to animals such as ducks, geese, raccoons, turtles, and frogs. Predators such as alligators and panthers are also indigenous to the Everglades.

The Okavango Delta in Botswana is probably the largest freshwater marsh in the world. The Okavango River empties into the Kalahari Desert, forming a delta in an arid region instead of near an ocean or lake. The Okavango Delta is a series of marshes totaling about 15,000 square kilometers (5,800 square miles).

Okavango marshes are made up of dense beds of papyrus, water lilies, and underwater plants such as bladderworts. The Okavango Delta is a haven for diverse animal species. Some animals live directly in and around the marshes, such as hippopotamuses and crocodiles. Other animals, such as giraffes and elephants, use the marshes as a source of freshwater in the middle of the dry Kalahari Desert.

Marshes and People

A number of human activities pose a threat to marsh ecosystems. Development along the Gulf Coast of the U.S. has reduced the marsh habitats in Texas, Louisiana, Mississippi, Alabama, and Florida. Massive development in South Florida has reduced the amount of water flowing through the Everglades. Wildlife such as the Florida panther are endangered because of the reduction of habitat.

The marshes of Doana National Park, in Andalusia, Spain, have been greatly affected by human activity along the Guadalquivir and Guadiamar Rivers. The rivers have been drained and diverted to expand agricultural production, salt extraction, and tourist facilities. With less water feeding into their ecosystems, the marshes at Doana have been reduced from 150,000 hectares (370,600 acres) to only 30,000 hectares (74,100 acres).

As a result, plant and animal species have diminished. The World Wildlife Fund and the Spanish government are now working to increase the water flow that enters the ecosystem. Their approach, as with most marsh restoration programs, requires the cooperation of government officials, environmental regulators, agricultural producers, and the public.

Fast Fact

Marsh Arabs
The Madan, or Basra Marsh dwellers of southern Iraq, are thought by some historians to be descendants of the ancient Sumerian civilization. These so-called "Marsh Arabs" have lived for millennia by fishing and grazing buffalo in the lush delta of the Tigris and Euphrates Rivers. Adapting to the ebb and flow of the tide, the marsh dwellers reside in floating reed houses and transport themselves in boats and canoes.

Unfortunately, the marshes were reduced drastically during the presidency of Saddam Hussein. The number of Marsh Arabs in Iraq shrunk from about 400,000 to as few as 20,000. Today, the Madan find it difficult to maintain a livelihood as the polluted, drained, and saline waters of the marshes cannot support enough commercially viable wildlife.

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Jeannie Evers, Emdash Editing, Emdash Editing
National Geographic Society
Last Updated

November 2, 2023

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