As rain falls on the Earth, some of the water soaks into the soil. Some makes puddles on the ground. Some of it runs downhill and makes a stream. Streams often start high in the mountains. Streams may run together and make a small river. Other streams may empty into this river. They are known as tributaries of the river. A river and all its tributaries make up a river system.
The place where the water starts flowing is called the source of the river. Rivers run until they empty into a lake or an ocean. The end of the river is called its mouth. .....