River, northwestern Wyoming and southern and eastern Montana, U.S. The river rises in Wyoming.
It enters Yellowstone National Park and feeds into Yellowstone Lake , below which it plunges 422 feet (129 m) in two spectacular waterfalls and enters the Grand Canyon of the Yellowstone River. It then continues north into Montana. There it flows northeast into the Missouri River on the MontanaNorth Dakota boundary. It is 692 mi (1,114 km) long and has been developed extensively for irrigation. Its principal tributaries are the Bighorn , Tongue, and Powder rivers. It was first explored in 1806 during the Lewis and Clark Expedition .