Yellowstone River, a river in the northwestern United States. It is 671 miles (1,080 km) long. The river's source is in the Absaroka Range of the Rocky Mountains in northwestern Wyoming. The river flows north through Yellowstone Lake and Yellowstone National Park and then veers northeastward through Montana into North Dakota. Near the Montana-North Dakota border it joins the Missouri. The Yellowstone's tributaries include the Bighorn and Powder rivers. The river is navigable only by vessels of extremely shallow draft.