The Yellowstone River is a tributary of the Missouri River, approximately 692 miles (1,114 km) long, in the Western United States. Considered the principal tributary of 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 west of present-day Williston.

The Missouri River is a river in the Central and Mountain West regions of the United States. The nation's longest, it rises in the eastern Centennial Mountains of the Bitterroot Range of the Rocky Mountains of southwestern Montana, then flows east and south for 2,341 miles (3,767 km) before entering the Mississippi River north of St. Louis, Missouri. The river drains semi-arid watershed of more than 500,000 square miles (1,300,000 km2), which includes parts of ten U.S. states and two Canadian provinces. Although a tributary of the Mississippi, the Missouri River is slightly longer and carries a comparable volume of water. When combined with the lower Mississippi River, it forms the world's fourth-longest river system.