swamp, shallow body of water in a low-lying, poorly drained depression, usually containing abundant plant growth dominated by trees, such as cypress, and high shrubs. Swamps develop in moist climates, generally in such places as low-lying coastal plains, floodplains of rivers, and old lake basins or in areas where normal drainage has been disrupted by glacial deposits. In the United States, swamps cover approximately 100,000 sq mi (260,000 sq km), most of them occurring as small swamps in northeastern states that were covered with glaciers in the past. The most extensive swamps are found along the Atlantic and Gulf coastal plains, notable examples being the Everglades of S Florida, Dismal Swamp of Virginia, and Okefenokee Swamp of Georgia and N Florida. Because the bottom of a swamp is at or below the water table, swamps serve to channel runoff into the groundwater supply, thus helping to stabilize the water table. During periods of very heavy rains, a swamp can act as a natural flood control device, as excess runoff can be temporarily stored in its basin. Swamp vegetation varies with climate. Grasses, rushes, and sphagnum moss predominate in temperate climates; cypress and mangrove predominate in more tropical regions. Lush vegetation provides great protection for nesting waterfowl and fish as well as a hospitable habitat for many types of small mammal such as beaver and otter. Swamps that are drained make excellent agricultural land because of the high organic content of the bottom sediments. In addition, rising land values and demand have encouraged the drainage of many swamplands, such as coastal Florida, for home development. However, a problem associated with recently drained swamps is oxidation of the thick peat deposits forming the soil, which can result in subsidence of the land and such problems as cracked walls, broken underground pipes, and buckled roadways. The increased use of drained swampland for urban construction, with its associated acres of blacktop paving and storm sewers, results in greater runoff and increases the probability of flooding and pollution in these regions. Swamp drainage also destroys the nesting areas of many wildlife species. Thus, environmentalists have urged, with increasing success, the slowing down of swamp drainage. There are a variety of local terms for swamps, including bog, marsh, fen, and moor. However, bog usually refers to a swampy depression with a thick mat of living and dead organic matter floating on the water surface and a low level of oxygen in the water below. Marsh implies a large area of wet land where the dominant vegetation consists of low-lying grasses, rushes, and sedges.