* Erosion: The main river's flow erodes the surrounding land, creating channels and valleys. These channels can be carved out by the river's flow, or by smaller streams that are feeding into the main river.

* Deposition: While deposition plays a role in shaping the landscape, it doesn't directly create tributaries. Deposition occurs when a river slows down and drops sediments, building up landforms like deltas and floodplains.

How Tributaries Form:

1. Headward Erosion: The main river's flow erodes the land upstream, causing its source to move further back. This process can create smaller channels that eventually connect to the main river, forming tributaries.

2. Lateral Erosion: The main river erodes the land on its sides, carving out wider valleys. This can lead to the development of smaller streams that flow into the main river, also forming tributaries.

In summary: Tributaries are formed by the erosive power of water, carving out channels and valleys that feed into a main river.