 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 . 
 The Missouri River <a href='drop_aux' title='1'><mark>is</mark></a>the longest river in North America . Rising in the Rocky Mountains of western Montana , the Missouri <a href='uninflect' title='2'><mark>flows</mark></a> east and south for 2 @,@ 341 miles ( 3 @,@ 767 km ) before entering the Mississippi River north of St. Louis , Missouri . The river <a href='uninflect' title='3'><mark>takes</mark></a> drainage from a sparsely populated , semi @-@ arid watershed of more than half a million square miles ( 1 @,@ 300 @,@ 000 km2 ) , which <a href='uninflect' title='4'><mark>includes</mark></a> parts of ten U.S. states and two Canadian provinces . When combined with the lower Mississippi River , it <a href='uninflect' title='5'><mark>forms</mark></a> the world <a href='null_genetive' title='6'><mark>'s</mark></a>fourth longest river system . 
The Missouri River da longest river in North America . Rising in tdaRocky Mountains of western Montana , the Missouri flow east and south for 2 @,@ 341 miles ( 3 @,@ 767 km ) b4 entering the Mississippi River north of St. Louis , Missouri . The river take drainage from a sparsely populated , semi @-@ arid watershed of more den half a million square miles ( 1 @,@ 300 @,@ 000 km2 ) , which include parts of ten U.S. states and 2 Canadian provinces . When combined wit thdaower Mississippi River , it form thedarld  fourth longest river system .