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 North America (Figure 5.4). Extending between the Rocky Mountains in the west and the Appalachian Mountain 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.