Here's why:

* Erosion: The main river erodes its channel, creating a slope. This slope encourages water flow, which in turn erodes smaller channels branching off the main river. These smaller channels are the tributaries.

* Deposition: Deposition usually occurs in the lower reaches of a river, where the water slows down and carries less sediment. While deposition can reshape tributaries, it doesn't create them.

In essence, tributaries are formed as smaller streams are carved out by the erosive power of water flowing towards a larger river.