The Yellowstone River is a tributary of the Missouri River, approximately 671 miles (1,080 km) long, in the Western United States. Considered the principal tributary of the upper Missouri, via its own tributaries it drains an area with headwaters across the mountains and high plains of southern Montana and northern Wyoming, and stretching east from the Rocky Mountains in the vicinity of Yellowstone National Park. It flows northeast to its confluence with the Missouri River on the North Dakota side of the border, about 25 miles (40 km) west of Williston.