|States||Wyoming, Montana, North Dakota|
|- left||Gardner River, Shields River|
|- right||Lamar River, Boulder River, Stillwater River, Big Horn River, Tongue River, Powder River|
|Source||Confluence of North and South Forks|
|- location||Absaroka Range, Wyoming|
|- location||McKenzie County, near Buford, North Dakota|
YELLOWSTONE RIVER Latitude and Longitude:
|Length||692 mi (1,114 km) |
|Basin||70,000 sq mi (181,299 km2) |
|Discharge||for Sidney, MT|
|- average||13,773 cu ft/s (390 m3/s) |
|- max||159,000 cu ft/s (4,502 m3/s) |
|- min||3,108 cu ft/s (88 m3/s)|
Map of the Yellowstone River watershed
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 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.
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 North Dakota. The watershed spans 34,167 square miles.  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: Bull Lake, Boyson, Buffalo, Bighorn, Tongue River, and Lake De Smet reservoirs. 
The river rises in northwestern Wyoming in the Absaroka Range, on the Continental Divide in southwestern Park County. The river starts where the North Fork and the South Fork of the Yellowstone River converge. The North Fork, the larger of the two forks, flows from Younts Peak. The South Fork flows from the southern slopes of Thorofare Mountain. The Yellowstone River flows northward through Yellowstone National Park, feeding and draining Yellowstone Lake, then dropping over the Upper and Lower Yellowstone Falls at the head of the Grand Canyon of the Yellowstone within the confines of the park. After passing through the Black Canyon of the Yellowstone downstream of the Grand Canyon, the river flows northward into Montana between the northern Absaroka Range and the Gallatin Range in Paradise Valley. The river emerges from the mountains near the town of Livingston, where it turns eastward and northeastward, flowing across the northern Great Plains past the city of Billings.
East of Billings, it is joined by the Bighorn River. Further downriver, it is joined by the Tongue near Miles City, and then by the Powder in eastern Montana. It flows into the Missouri River near Buford, North Dakota just upstream from Lake Sakakawea.
In Montana the river has been used extensively for irrigation since the 1860s. In its upper reaches, within Yellowstone Park and the mountains of Montana, it is a popular destination for fly fishing. The Yellowstone is a Class I river from the Yellowstone National Park boundary to the North Dakota border for the purposes of stream access for recreational purposes. 
The division of water rights to the entire Yellowstone River Basin among Wyoming, Montana and North Dakota, governed by a 1950 compact, was disputed in a 2010 lawsuit brought directly in the U.S. Supreme Court by Montana against Wyoming. Oral argument took place in January 2011. On May 2, 2011, the Court held 7-2 (by Justice Thomas, with Justice Scalia dissenting) that Montana had no valid claim for diminution of its water, since Wyoming was irrig