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 Yellowstone River watershed is a river basin spanning 37,167 square miles (96,260 km2) across Montana, with minor extensions into Wyoming and North Dakota, toward headwaters and terminus, respectively. The Yellowstone Basin watershed contains a system of rivers, including the Yellowstone River, and four tributary basins: the Clarks Fork Yellowstone, Wind River and Bighorn River, Tongue River, and Powder River. These rivers form tributaries to the Missouri River.
The mainstem of the Yellowstone River is more than 700 miles (1,100 km) long. At the headwaters, elevations exceed 12,800 feet (3,900 m) above sea level and descends to 1,850 feet (560 m) at the confluence with the Missouri River in far western North Dakota. The watershed spans 34,167 square miles (88,490 km2). The area contains many lakes, including Yellowstone Lake. There are no storage dams located on the mainstem of the Yellowstone River. However, the watershed contains five major reservoirs built on tributary rivers: Bull Lake, Boysen, Buffalo Bill, Big Horn, Tongue River, and Lake De Smet reservoirs.