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 before entering the
Mississippi River north of
St. Louis, Missouri. The river takes drainage from a sparsely populated,
semi-arid watershed of more than half a million square miles (1,300,000 km
2), which includes parts of ten U.S. states and two Canadian provinces. When combined with the lower Mississippi River, it forms the
world's fourth longest river system.