An everglade is a natural region of flooded grasslands in the southern portion of the U.S. state of Florida, comprising the southern half of a large watershed. It is a subtropical wetland ecosystem spanning two million acres across central and south Florida. During the wet season, Lake Okeechobee overflows, releasing water into a very slow-moving river that is 100 miles long and 60 miles wide, which spans from Lake Okeechobee to Florida Bay. The Everglades are sustained primarily by the atmosphere, and evapotranspiration is the key mechanism by which water leaves the region. The Everglades are a complex system of interdependent ecosystems, and several ecosystems are present in the Everglades, with boundaries between them being subtle or absent. The primary feature of the Everglades is the sawgrass marsh, and it is known for its many wading birds, such as white and glossy ibises, roseate spoonbills, egrets, herons, and wood storks. The Everglades is also home to a high diversity of beautiful orchids, some of which spend their entire life up in the trees, getting their nutrients through aerial roots from the air, rain, and organic matter around them. The Everglades is being threatened by numerous plants and animals that were introduced both on purpose and by accident.