The region drained by a stream and its tributaries is
called a drainage basin. Mountain ranges or other raised
areas called drainage divides separate adjacent drainage
basins. For example, streams on the western slope of the
Rocky Mountains are parts of the Colorado and Columbia
River drainage basins, which ultimately empty into the
Pacific Ocean. Tributaries on the eastern slope of the
Rockies are parts of the Mississippi and Rio Grande
basins, which flow into the Gulf of Mexico. A drainage
basin can be large, like the Mississippi basin, or as small
as a single mountain valley.