The Missouri River
is the longest river in North America. Rising in the Rocky Mountains of western Montana, the Missouri flows east and south for 2,341 miles (3,767 km) before entering the Mississippi River north of St. Louis, Missouri. The river drains a sparsely populated, semi-arid watershed of more than 500,000 square miles (1,300,000 km2
), which includes parts of ten U.S. states and two Canadian provinces. Although nominally considered a tributary of the Mississippi, the Missouri River above the confluence is much longer and carries a comparable volume of water. When combined with the lower Mississippi River, it forms the world's fourth longest river system.