Ouachita, river, c.600 mi (970 km) long, rising in the Ouachita Mts., W Ark. It flows east, southeast, and south through a cotton-producing region of S Arkansas and NE Louisiana and into the Red River system. It is joined by the Tensas River at Jonesville, La., below which it is called the Black River. Hot Springs, Ark., and Monroe, La., are the largest cities on the river. The river is navigable for shallow-draft vessels below Arkadelphia. Three dams in the river near Hot Springs—Remmel (completed 1925), Carpenter (1931), and Blakeley Mountain (1955)—impound respectively Lake Catherine, Lake Hamilton, and Lake Ouachita (63 sq mi/101 sq km, Arkansas' largest). There is a hydroelectric power plant at Blakeley Mountain Dam. The lakes, part of a federal flood-control project, are the center of a popular recreation area.