Missouri river longest tributary of the mississippi river and second longest river in north america it is formed by the confluence of the jefferson madison and gallatin rivers in the rocky mountains area of southwestern montana gallatin county us about 4000 feet 1200 metres above sea level. Missouri americas longest river map of the missouri river and its tributaries innorth america just west of bozeman montana where the gallatin jefferson and madison rivers converge at three forks americas longest river is born. The missouri river in north dakota with a distance of more than 2341 miles the missouri river is among the longest rivers in north america the river also ranks among the largest on the continent in terms of volume discharge with a maximum discharge of 075 million cubic feet per second