How valleys are formed. Systems of valleys extend through plains, hills, and mountains. Rivers and streams flowing through valleys drain interior land regions to the ocean. The bottoms of many valleys have fertile soil, which makes excellent farmland.
All valleys are similar in shape.
The bottom of a valley is called its floor. Most floors slope downstream. Mountain valleys usually have narrow floors. But in low-lying plains, a floor may be several miles or kilometers wide. The part of the floor along riverbanks is called the flood plain.