That seemingly simple question has sparked a heated debate in environmental and political circles. The determination of how often wetlands are covered with water and to what depth the soils are saturated frames a 1991 controversy about wetlands regulation that is discussed below. Wetlands may contain fresh or ocean water. Fresh water saturates about 94 percent of remaining U.S. wetlands, commonly known as marshes, swamps, fens, and prairie potholes. Many inland wetlands are connected to streams, lakes, or rivers, and may help absorb floodwaters.
Beaver dams help create small inland wetlands.
These inland areas receive water from upwelling groundwater and from rain and surface runoff, which may carry pollutants. Inland wetlands often serve as natural flood controls, and help filter out toxic chemicals such as heavy metals. Coastal wetlands comprise about 5 percent of the wetlands remaining in the Continental United States. These wetlands, found primarily along the Atlantic Coast and the Gulf of Mexico, may consist of saltwater marshes dominated by grasses, shrub-covered river floodplains, or wooded bogs. Many of these wetlands contain a mixture of fresh water from rivers and streams, and salt water from the ocean.