Just as a tree has a main trunk and many branches, a river system is made up of a main river and all its tributaries, or smaller streams. The point where two rivers join is called a confluence. In the Mississippi river system, for example, the confluence of the Mississippi and the Ohio, one of the principal tributaries, is near Cairo, Illinois. The entire land area drained by a river system is called a river basin, or sometimes a watershed.
Divide and water parting are terms used to designate the boundary along high land separating rivers and river basins. In the United States the Continental Divide in the Rocky Mountains separates the waters that eventually flow to the Atlantic and Pacific oceans.