Yellowstone, river, 671 mi (1,080 km) long, rising in NW Wyo., and flowing NE through Mont. to enter the Missouri River near the N.Dak. line; it drains c.70,400 sq mi (182,340 sq km). The Yellowstone receives the Bighorn, Powder, Tongue, and many smaller rivers. The most scenic aspects of the river are found in Yellowstone National Park in NW Wyoming. There, the river feeds and drains Yellowstone Lake, 139 sq mi (360 sq km), the largest high-altitude (alt. 7,331 ft/2,234 m) lake in North America. After leaving the lake, the river drops 109 ft (33 m) at Upper Falls, then 308 ft (94 m) at Lower Falls, before entering the deep and spectacular Grand Canyon of the Yellowstone (19 mi/31 km long); Tower Falls, 132 ft (40 m) high, is at the northern end of the canyon. The river's waters have been used for irrigation since the late 1860s. The U.S. Bureau of Reclamation operates several projects on the Yellowstone that are used for irrigation, flood control, power production, and recreation. These include the Huntley project near Billings, Mont., the Buffalo Rapids project near Glendive, Mont., and the Savage unit of the Missouri River basin project.