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 will travel more than 2300 miles before it joins the mississippi in its namesake state at st louis forming the worlds fourth longest river system as it rolls south to the gulf of mexico. 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 the watershed of the missouri river is also one of the largest in north america as it drains an area of over 0529 million square miles which is equivalent in size to the canadian province of quebec or one sixth of the entire united . 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