Trees are very important factors in creating a river because water that they emit gently through their roots forms the initial body of the river. Therefore, rivers begin in mountains or hills, where rain water or snowmelt collects and forms tiny streams called gullies.

Gullies either grow larger when they collect more water from more roots and become streams themselves or meet streams and add to the water already in the stream. When one stream meets another and they merge together, the smaller stream is known as a tributary. The two streams meet at a junction. It takes many tributary streams to form a river. A river grows larger as it collects water from more tributaries. Streams usually form rivers in the higher elevations of mountains and hills.

Meanwhile, the areas of depression between hills or mountains are known as valleys.

A river in the mountains or hills will usually have a deep and steep V-shaped valley as the continuous current of water cuts away at the rock as it flows downhill. The downstream flow of river picks up pieces of rock and carries them along, breaking them into smaller and smaller sedimentary pieces.

By cutting, slicing,carving and moving rocks, running water changes the earth’s surface even more than catastrophic events such as earthquakes or volcanoes.