Swamp, area of poorly drained, low-lying land saturated with water. Swamps normally covered by water are called marshes. The obstruction of drainage that causes swamps to form may result from the flatness of the land, the presence of impermeable rock beneath the surface, or the growth of dense vegetation. Lakes that are filled in by sediment may develop into swamps. When drained, swamps usually make fertile farmland. Large swamps in the United States include the Everglades (Florida), the Dismal (North Carolina), and the Okefenokee (Georgia and Florida).