Wetlands are areas, other than lakes or rivers, whose soils are saturated with water for indefinite or prolonged periods of time. Often these wetlands result from the surface exposure of groundwater--that is, the water that has percolated down through the soil and accumulated and built up on a layer of rock or other impervious material. The upper boundary of the groundwater, called the water table, rises or falls as the amount of groundwater varies. Swamps, marshes, and bogs are wetlands in which the water table is at, near, or above the surface of the land. Plant growth in these areas is limited to species that can with stand having their roots submerged for long periods of time. The water in these areas may be tea-colored or dark brown because of the presence of organic acids, such as tannic acid, derived from decaying vegetation.
Depending on many factors, including climate, mineral content, and the permanency of surface water, wetlands may be mossy, grassy, shrubby, or wooded. Conventionally, a swamp is a wetland whose soil is permanently or protractedly saturated with water and whose vegetation is dominated by woody plants, such as trees. A marsh is similar, but its dominant vegetation is herbaceous, consisting of such plants as grasses or sedges. A bog is an acidic wetland that appears relatively dry, with only small amounts of shallow water visible, but its ground surface is spongy and wet to the touch: the surface material is largely sphagnum moss or other organic matter rather than mineral soil.
This terminology is not precise: what one locality might call a bog, another might refer to as a swamp or marsh. Additionally, all three types of wetlands may be present and closely associated in one area, further confusing the use of these names. Intergradations also occur. A northern boggy area may support trees--such wetlands are known as muskegs. In Britain, boggy areas may be referred to as moors or heaths, and a less acid sedgy peatland may be called a fen. Consequently, in an effort to avoid the confusion of these popular names, most modern scientific classifications of wetlands no longer use these terms.