 With a drainage basin spanning 529 @,@ 350 square miles ( 1 @,@ <unk> @,@ 000 km2 ) , the Missouri River 's catchment encompasses nearly one @-@ sixth of the area of the United States or just over five percent of the continent of North America . <unk> to the size of the Canadian province of Quebec , the watershed encompasses most of the central Great Plains , stretching from the Rocky Mountains in the west to the Mississippi River Valley in the east and from the southern extreme of western Canada to the border of the Arkansas River watershed . Compared with the Mississippi River above their confluence , the Missouri is twice as long and drains an area three times as large . The Missouri accounts for 45 percent of the annual flow of the Mississippi past St. Louis , and as much as 70 percent in certain droughts . 
