The large river eventually reaches the ocean, carrying along the way excess water drained from land by its watershed. Water not absorbed by roots enters the ground water supply by infiltration. Most water runs off the surface, down a hill and eventually into a river. Ditches and small streams, which may be dry when it does not rain, can quickly fill up with water when it does rains. If you have ever studied a map in detail, you will notice how rivers and streams form a network of waterways across the countryside.
Little streams come together to form small rivers.
Small rivers join together and become medium-sized rivers. All these rivers may be tributaries of a large river. Collectively, the network of rivers and streams form a watershed which drains the land of excess water. This type of patern is repeated many places in nature. In the human circulatory system, for example, or in human lungs, like those represented to the left.