1. Stream Gradient Change:

* Steep gradient in mountains: Streams flowing down steep mountain slopes carry large amounts of sediment, like sand, gravel, and boulders.

* Abrupt decrease in gradient: As the stream exits the mountains and enters a flatter plain, its velocity decreases significantly. This sudden decrease in velocity causes the stream to lose its ability to carry the sediment load.

2. Sediment Deposition:

* Sediment accumulation: The slowed stream deposits the sediment load at the base of the mountains, forming a cone-shaped deposit.

* Fan shape: The deposited sediment spreads outwards from the stream's mouth, creating the fan shape.

3. Repeated Flooding and Deposition:

* Flooding events: Repeated flooding events contribute to the growth of the alluvial fan.

* Layer upon layer: Each flood deposits a new layer of sediment on top of the previous one, building up the fan's height and size.

4. Braided Channels:

* Multiple channels: Alluvial fans are often characterized by multiple channels that braid across the fan surface.

* Changing flow paths: These channels constantly shift as sediment deposition changes the flow path of the stream.

5. Climate and Weather:

* Arid and semi-arid environments: Alluvial fans are common in arid and semi-arid regions where rainfall is sporadic but intense.

* Flash floods: Intense rainfall leads to flash floods, which transport large amounts of sediment and contribute to the fan's formation.

In summary: Alluvial fans form when a fast-flowing stream carrying a large sediment load abruptly decreases its velocity upon entering a flat plain, causing the sediment to deposit and form a cone-shaped deposit that grows over time through repeated flooding events.