The land area that contributes water to a river system is called a drainage basin(Figure 5.3). The drainage basin of one stream is separated from the drainage basin of another by an imaginary line called a divide. Divides range in scale from a ridge separating two small gullies on a hillside to a continental divide, which splits whole continents into enormous drainage basins. The Mississippi River has the largest drainage basin in North America(Figure 5.4). Extending between the Rocky Mountains in the west and the Appalachian Mountains in the east, the Mississippi River and its tributaries collect water from more than 3.2 million square kilometers(1.2 million square miles) of the continent.