 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 takes drainage from a sparsely populated , semi @-@ arid watershed of more than half a million square miles ( 1 @,@ 300 @,@ 000 km2 ) , 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 . 
