An alluvial fan is an accumulation of sediments that fans outwards from a concentrated source of sediments, such as a narrow canyon emerging from an escarpment. They are characteristic of mountainous terrain in arid to semiarid climates, but are also found in more humid environments subject to intense rainfall and in areas of modern glaciation. They range in area from less than 1 square kilometer (0.4 sq mi) to almost 20,000 square kilometers (7,700 sq mi).

Alluvial fans typically form where a flow of sediment or rocks emerge from a confined channel and are suddenly free to spread out in many directions. For example, many alluvial fans form when steep mountain valleys meet a flat plain. The transition from a narrow channel to a wide open area reduces the carrying capacity of flow and results in deposition of sediments. The flow can take the form of infrequent debris flows like in a landslide, or can be carried by an intermittent stream or creek.

The reduction of flow is key to the formation of alluvial fans. If a river exits a mountain valley without any reduction in flow, it is more common to see the formation of an alluvial plain. The steepness of an alluvia formation depends on how much flow decreases when entering flat ground as sediment will be deposited further away from its source if river flow is high.

Alluvial fans are most commonly found at the foot of desert mountains, such as in the Great Basin of western North America, in the New Red Sandstone of south Devon, or all across the major population centers of Xinjiang in the Taklamakan Desert and Junggar Basin.

Alluvial fans are not unique to Earth, as they are simply a result of gravity and geometry, and thus have also been found abundantly on Mars and Titan, showing that fluvial processes have occurred on other worlds.

Some of the largest alluvial fans are found along the Himalaya mountain front on the Indo-Gangetic plain. A shift of the feeder channel (a nodal avulsion) can lead to catastrophic flooding, as occurred on the Kosi River fan in 2008.

An alluvial fan is an accumulation of sediments that fans out from a concentrated source of sediments, such as a narrow canyon emerging from an escarpment. This accumulation is shaped like a section of a shallow cone, with its apex at the source of sediments.

Alluvial fans vary greatly in size, from only a few meters across at the base to as much as 150 kilometers across, with a slope of 1.5 to 25 degrees. Some giant alluvial fans have areas of almost 20,000 square kilometres (7,700 sq mi). The slope measured from the apex is generally concave, with the steepest slope near the apex (the proximal fan or fanhead) and becoming less steep further out (the medial fan or midfan) and shallowing at the edges of the fan (the distal fan or outer fan). Sieve deposits, which are lobes of coarse gravel, may be present on the proximal fan. The sediments in an alluvial fan are usually coarse and poorly sorted, with the coarsest sediments found on the proximal fan.