The three main types of wetlands are swamps, marshes, and peatlands. They can be identified by size, the type of soil, and the plants that live in them, as well as by the type and amount of water they hold. Some located inland are fresh water, while coastal wetlands may be fresh or salt water. Many areas can contain several different types of wetlands. The Great Dismal Swamp in Virginia, which contains both peat bogs and freshwater swamps, is an example.
A swamp is a wetland that is characterized by poorly drained soil and plant life dominated by trees. Swamp soil is usually saturated for most of the year. The water can range from 1 inch (2.5 centimeters) to more than 1 foot (30 centimeters) deep.
Because swamp ground is constantly waterlogged, plant life is kept to a minimum. The trees and plants that grow there must be able to tolerate having their root systems wet for long periods of time. The water is stagnant (without movement) and the dead plant matter settles on the bottom and receives little oxygen. Without oxygen, the dead matter cannot fully decay. This gives the swamp its characteristic brown water and unpleasant smell.
Swamps are usually found in low-lying areas near rivers or along coastal areas. They get water from the overflow of the river or from ocean tides. Inland swamps always contain fresh water, but coastal swamps may contain either fresh or salt water.
Some swamps, called pocosins, form in upland areas. The term"pocosin" comes from the Algonquin phrase meaning "swamp on a hill." Rain is usually their only source of water, meaning the soil is low in minerals and nutrients which come from ground water. Drainage is poor in pocosins and the saturated soil is acidic (high in acids). In this environment, decomposition (breaking down) of waste is slow, and decaying matter accumulates over time. Sometimes pocosins are flooded by slow-moving streams.
Freshwater swamp: Freshwater swamps develop near the edges of lakes and next to rivers that overflow their banks. Partly decayed plant matter stagnant water soon create swamp conditions. The Everglades in Florida. Okefenokee Swamp in Georgia, and the Great Dismal Swamp in North Carolina and Virginia, are examples.
Saltwater swamp: A saltwater swamp is formed by the ebbing (receding) and flowing of the ocean tides. (The tides are a rhythmic rising and lowering — of the oceans caused by the gravitational pull of the Sun and Moon.) When the tide is low, the flat areas of the swamp are not under water. Saltwater swamps can be found in New Guinea, Indonesia, Kenya, Zaire, along the Mekong River in Vietnam, and along the Ganges River in India.
In some low-lying coastal areas regularly flooded with sea water mangrove swamps develop. A mangrove swamp is a coastal, saltwater wetland that is found in tropical and subtropical climates such as southern Florida and Puerto Rico. (Tropical and subtropical climates are found in areas close to the equator and are characterized by warm weather.) The mangrove swamp supports salt-loving shrubs and trees such as the mangrove, from which it takes its name. Because of the high salt content, few other plants grow there.
About 90 percent of all wetlands are marshes. Marshes are dominated by nonwoody plants such as grasses, reeds, rushes, sedges, and rice. Some marshes are so thickly covered with vegetation that they look like fields of grass.
Marshes are usually found in temperate climates, where summers are hot and winters are cold, and they often form at the mouths of rivers (the point where a river enters into a larger river or ocean), especially those having large deposits of soil and sand, called deltas. They also form in flat areas that are frequently covered by shallow fresh or salt water.
Freshwater marsh: Freshwater marshes are the most common of the world's temperate wetlands. Some receive no rain, and most are found along the edges of lakes and rivers or where groundwater, streams, or springs cause flooding. The Camargue in the Rhone Delta in southern France is an example. The different types of freshwater marshes include prairie potholes, riparian areas, wet meadows, and washes.
Prairie potholes are small marshes that can be found by the millions throughout the north central area of the United States and south central Canada. Usually no more than a few feet deep, they may cover as much as 10,000 acres (4,000 hectares). Prairie potholes are important to migratory waterfowl as places to rest, feed, and nest. In Africa, these small marshes are called dambos. In North America it is estimated that 50 to 70 percent of all waterfowl are hatched in prairie potholes.
Wet meadows are freshwater marshes that frequently become dry. They look like grasslands but the soil is saturated. They are common in temperate and tropical regions around the world, including the midwestern and southeastern United States. Their dominant plant is the sedge, a flowering herb that resembles grass.
Riparian wetlands are marshes found along rivers and streams. They range in width from a few feet (meters) to as much as 12 miles (20 kilometers). Smaller riparian wetlands are common in the western United States. Larger ones are located along large rivers, such as the Amazon in South America. Riparian wetlands are unique from others in that the vegetation and life forms found immediately adjacent to the river or streams differ from those of the surrounding area, usually a forest. This marked contrast produces a diversity and enhances the benefits for wildlife, both in food (grasses and sedges) and in shelter (shrubs and trees) in a relatively small place.
A wash is a dry streambed that becomes a wetland only after a rain. Washes are found in dry plains and deserts. The plant life they support usually has a short growing season and disappears during dry periods. In North Africa and Saudi Arabia, washes are called wadis. In the American west they are called arroyos.
Saltwater marsh: Saltwater marshes, also called salt marshes, are found in low, flat, poorly drained coastal areas. They are often flooded by salt water or brackish water (a mixture of both fresh and salt water). Saltwater marshes are especially common in deltas, along low seacoasts, and in estuaries (arms or inlets of the sea where the salty tide washes in and meets the freshwater current of a river). They can be found in New Zealand, in the Arctic, and along the Atlantic, Pacific, Alaskan, and Gulf coasts.
Saltwater marshes are greatly affected by the tides, which raise and lower the water level on a daily basis. A saltwater marsh may have tidal creeks, tidal pools, and mud flats, each of which has its own ecosystem (a network of organisms that have adapted to a particular environment).
The high salt content of the sea water in the marsh makes it hard for plants to adapt. However, grasses such as marsh grass, cord grass, salt hay grass, and needlerush thrive here.
Peatlands are wetlands in which peat has formed. Peat is a type of soil made up of the partially decayed remains of dead plants, such as sphagnum moss and even trees. In peatlands, dead plant matter is produced and deposited at a greater rate than it decomposes. Over time, sometimes over thousands of years, a layer of this plant matter is formed that may be as deep as 40 feet (12 meters). Several conditions are necessary for peat to form, however. The soil must be acidic, waterlogged from frequent rains, and low in oxygen and nutrients. Because the bacteria responsible for decomposing plant matter cannot thrive under these conditions, the layers of partially decayed plant matter accumulate.
There are about 1,500,000 square miles (4,000,000 square kilometers) of peatland around the world. Peatlands are located in colder, northern climates as well as in the tropics and can be found in Russia, Scandinavia, northern Europe, England, Ireland, Canada, and the northern United States.
The types of peatlands are temperate bogs, fens, and tropical tree bogs.
Temperate bog: Ninety percent of peatlands are found in northern temperate climates where they are called bogs. Bogs have a soft, spongy, acidic soil that retains moisture, which comes primarily from rainwater. Some bogs have taken as long as 9,000 years to develop and are sources of peat that can he burned as fuel.
Temperate bogs have been described as soft, floating carpets. The carpet is made mostly of sphagnum moss, which can be red, orange, brown, or green and may grow as long as 12 inches (30 centimeters). Sphagnum moss can grow in an acidic environment, and it holds fresh rainwater in which other bog plants can grow. Because of the high concentration of sphagnum moss, other bog plants have developed unusual adaptations to help them get nutrients. For example, the bog myrtle forms a partnership with the bacteria in its roots to obtain extra nitrogen. Besides the bog myrtle, other plants that grow in bogs include grasses, small shrubs such as leatherleaf, flowering plants such as heather, and poison sumac. Some rare wildflowers, such as the lady slipper orchid and the Venus' flytrap, are also found in bogs.
Most bogs lie in depressed areas of ground. Some, however, called raised bogs, grow upward and are an average of 10 feet (3 meters) higher than the surrounding area.
Blanket bogs, which are shallow and spread out like a blanket, form in areas with relatively high levels of annual rainfall. Their average depth is 8 feet (2.6 meters). Blanket bogs are found mainly on lowlands in the west of Ireland and in mountain areas. After a heavy rainfall, bogs that are located on steep slopes can wash down like a huge landslide of jelly and cover cattle, farms, and even villages.
Fen: When a peatland lies at or below sea level and is fed by mineral-rich groundwater, it is called a fen. The characteristic plants in a fen are grasses, sedges, and reeds rather than sphagnum moss, and the soil does not become as acidic as in a bog. Therefore, less peat accumulates in a fen and it becomes only about 6 feet (2 meters) thick. As plant matter accumulates in a fen over time, it will form a raised bog.
Tropical tree bog: Bogs found in tropical climates are called tree bogs. These bogs produce peat from decaying trees rather than from sphagnum moss. The tree most commonly found in these bogs is the broad-leaved evergreen. Temperatures are warmer than in temperate regions, causing decay to occur more rapidly. As a result, not as much peat develops. The only source of water is rain.
Tropical tree bogs can be found in South America, Malaysia, and Indonesia.
Source: "Wetland." U·X·L Encyclopedia of Biomes, Vol. 3. U·X·L, Detroit: 2000.