The Missouri River is a river in the Central and Mountain West regions of the United States. The nation's longest, it rises in the eastern Centennial Mountains of the Bitterroot Range of the Rocky Mountains of southwestern Montana, then flows east and south for 2,341 mi (3,767 km) before entering the Mississippi River north of St. Louis, Missouri. The river drains a semi-arid watershed of more than 500,000 sq mi (1,300,000 km2), which includes parts of ten U.S. states and two Canadian provinces. Although a tributary of the Mississippi, the Missouri River is slightly longer and carries a comparable volume of water. A fellow tributary, the Ohio River, carries more water. When combined with the lower Mississippi River, it forms the world's fourth-longest river system.