Basin is a region drained by a river and its tributaries. Tributaries are streams or other channels that carry water to a river. The Mississippi River and its tributaries drain a basin that covers about two-fifths of the United States (excluding Alaska and Hawaii). Other large basins are the Congo in Africa, the Amazon in South America, and the Yangtze in Asia. The boundary between basins is called a drainage divide.
A closed basin is an area where streams have no outlet to the sea. Water can escape only by evaporating or seeping into the ground. The lowest part is generally occupied by a lake or, in deserts, by a playa (dry lake bed). Some closed basins in desert areas have salt lakes, such as the Great Salt Lake in Utah.
