The Missouri River's drainage basin spans , encompassing nearly one-sixth of the area of the United States or just over five percent of the continent of North America. Comparable to the size of the Canadian province of Quebec, the watershed covers 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