The Yellowstone River (Assiniboine: ȟeȟága wakpá, įǧų́ǧa wakpá, į́yąǧi wákpa ) is a tributary of the Missouri River, approximately long, in the western United States. Considered the principal tributary of the upper Missouri, the river and its tributaries drain a wide area stretching from the Rocky Mountains in the vicinity of the Yellowstone National Park across the mountains and high plains of southern Montana and northern Wyoming.