Wetlands are areas that are covered or soaked by ground or surface water often enough and long enough to support special types of plants that have adapted for life under such conditions. Wetlands occur where the water table (the level of groundwater) is at or near the surface of the land, or where the land is covered by shallow surface water (usually no deeper than about 6 feet [1.8 meters]). They form the area between places that are always wet, such as ponds, and places that are always dry, like forests and grasslands.
Many wetlands are not constantly wet, experiencing what is called a wet/dry cycle. Some are wet for only part of the year, like those that are drenched during heavy seasonal rains. Some have no standing water but, because they are near the water table, their soil remains saturated (soaked with water). Others may dry out completely for long periods, sometimes even for years.
Wetlands are among the world's most productive and biodiverse environments (environments that support a large variety of life forms). Only rain forests and coral reefs are more biodiverse.
Wetlands are found all over the world, in every climate — from the frozen landscape of Alaska to the hot zones near the equator — except Antarctica. They make up about 6 percent of the world's landmass. In the United States there are about 240,000,000 acres (96,000,000 hectares) of wetland, and some type can be found in every state. Alaska alone contains about 70,000,000 acres (68,000,000 hectares) of wetlands.
Wetlands have a life cycle that begins with their formation and may involve many changes over time.
Many wetlands were formed when glaciers retreated after the last Ice Age, about 10,000 years ago. Some of the glaciers left depressions in the ground, called kettles, which were perfect places for water to gather. In some places, buried ice melted to form kettle lakes that eventually turned into wetlands.
Wetlands are formed by the overflowing of river banks and changes in sea level (the average height of the sea), which can leave waterlogged areas behind. Some wetlands are formed with help from beavers making dams that cause rivers or streams to back up and flood the surrounding area. Landslides or centuries of heavy winds may also change the terrain or carve out depressions in the ground where water then collects.
Climate, too, plays a key role in the formation of wetlands. Lots of rain and little drainage, for example, can cause the ground to become waterlogged.
Source: "Wetland." U·X·L Encyclopedia of Biomes, Vol. 3. U·X·L, Detroit: 2000.