A wetland is any location where the land is covered by mainly shallow water, either seasonally or long term, and which contains characteristic vegetation of aquatic plants that are adapted to the low oxygen soil. Wetlands can occur as a result of any source of fresh, brackish (partially salty) or saltwater and any topography that allows the water to be retained.
There is great variety in wetlands, including size, shape, source of water, and biota, but they can be classified into five broad categories according to the type of location and water source: riverine (river), estuarine (estuary), palustrine (marsh), lacustrine (lake), and marine (ocean). Among the many common names for wetlands in general and for some specific types are swamps, marshes, bogs, estuaries, floodplains, mudflats, mangroves, ponds, fens, deltas, billabongs, coral reefs, shallow seas and lagoons.
Formerly thought of as useless swamps to be used as dumping grounds or to be drained and filled in for other uses, it is now increasingly understood that wetlands have several very important functions. One is providing critical habitat for fish, amphibians, waterfowl and other wildlife, including many endangered species. Others include providing foodstuffs for a large part of humanity, carbon sequestration, flood control, and waste treatment. Unfortunately, roughly half of the world’s wetlands have been destroyed by human activity since since the start of the 20th century. Efforts are now being made in some countries to protect and restore wetlands, but the results so far have been minimal.