Drainage Basin / Watershed
The precipitation that falls into a valley, and on surrounding interfluves flows downward usually creating a stream or river. The area of land that contributes water to a stream or river is called a watershed, or drainage basin. Small drainage basins generally contribute to streams, while the water from larger drainage basins come together to form large rivers.
Often, small drainage basins or watersheds combine with one another, creating larger and larger networks of drainage basins. All of these combined drainage basins are together referred to simply as a drainage basin, or as one watershed.
The area between two drainage basins is known as a drainage divide. In North America a massive drainage divide known as the Continental Divide separates the water that flows towards the Pacific Ocean, with water that drains towards the Gulf of Mexico, and towards the Atlantic Ocean.