Why can a mountain stream leave a fan of stones and sand?

A mountain stream can rush through a narrow valley carrying stones, sand and finer sediment. Where the channel reaches flatter, open ground, its flow can spread sideways and become shallower or slower. With less ability to push and keep carrying its load, the water begins leaving some of that material behind.

Larger pieces often stop nearer the channel opening, while smaller particles can travel farther. The deposited material changes the ground beneath later flows. A stream that has partly filled its old channel may spill into another route during the next flood.

Repeated spreading, deposition and changes of route build a broad deposit that can resemble an opened hand fan. Geologists call it an alluvial fan. The fan records many separate journeys of water and sediment, not one stream branching permanently into all the lines at once. Actual deposits depend on the slope, flood strength and mixture of material.

An alluvial fan can form on a dry valley floor without entering a lake or sea. That distinguishes its setting from a river delta, where a river enters a body of water. In both cases water leaves sediment behind when transport conditions change. Here, the change from a confined mountain channel to open land explains why the deposit spreads from a narrow point into a wide fan.