An alluvial fan is a fan- or cone-shaped deposit of sediment crossed and built up by streams. If a fan is built up by debris flows it is properly called a debris cone or colluvial fan. These flows come from a single point source at the apex of the fan, and over time move to occupy many positions on the fan surface. Fans are typically found where a canyon draining from mountainous terrain emerges out onto a flatter plain, and especially along fault-bounded mountain fronts.
A convergence of neighboring alluvial fans into a single apron of deposits against a slope is called a bajada, or compound alluvial fan.
As a stream's gradient decreases, it drops coarse-grained material. This reduces the capacity of the channel and forces it to change direction and gradually build up a slightly mounded or shallow conical fan shape. The deposits are usually poorly sorted. This fan shape can also be explained with a thermodynamic justification: the system of sediment introduced at the apex of the fan will tend to a state which minimizes the sum of the transport energy involved in moving the sediment and the gravitational potential of material in the fan. There will be iso-transport energy lines forming concentric arcs about the discharge point at the apex of the fan. Thus the material will tend to be deposited equally about these lines, forming the characteristic fan shape.
Alluvial fans are often found in desert areas subject to periodic flash floods from nearby thunderstorms in local hills. The typical watercourse in an arid climate has a large, funnel-shaped basin at the top, leading to a narrow defile, which opens out into an alluvial fan at the bottom. Multiple braided streams are usually present and active during water flows.
Phreatophytes are plants that are often concentrated at the base of alluvial fans. They have long tap roots 30 to 50 feet (9.1 to 15.2 m) to reach water that has seeped through the fan and hit an impermeable layer, sometimes collecting in springs and seeps. These stands of shrubs cling to the soil at their bases and often form islands of habitat for many animals as the wind blows the sand around the bushes away.
Alluvial fans also develop in wetter climates. In Nepal the Koshi River has built a megafan covering some 15,000 km2 (5,800 sq mi) below its exit from Himalayan foothills onto the nearly level plains where the river traverses into India before joining the Ganges. Along the upper Koshi tributaries, tectonic forces elevate the Himalayas several millimeters annually. Uplift is approximately in equilibrium with erosion, so the river annually carries some 100 million cubic meters (3.5 billion cu ft) of sediment as it exits the mountains. Deposition of this magnitude over millions of years is more than sufficient to account for the megafan.
All along the interface between the Indo-Gangetic Plain and the Himalaya in India, Pakistan, Nepal and Bhutan the outermost, lowest Siwalik foothills are built of poorly consolidated sedimentary rocks that have eroded into a wide, continuous alluvial apron called Bhabar in Hindi and Nepali. Despite overpopulation on the plains, this bhabar zone is highly malarial and has remained largely uninhabited.
In North America, streams flowing into California's Central Valley have deposited smaller but still extensive alluvial fans. Such as that of the Kings River flowing out of the Sierra Nevada creates a low divide, turning the south end of the San Joaquin Valley into an endorheic basin without a connection to the ocean.
Alluvial fans are subject to flooding and can be even more dangerous than the upstream canyons that feed them. Their slightly convex perpendicular surfaces cause water to spread widely until there is no zone of refuge. If the gradient is steep, active transport of materials down the fan creates a moving substrate that is inhospitable to travel on foot or wheels. But as the gradient diminishes downslope, water comes down from above faster than it can flow away downstream, and may pond to hazardous depths.
In the case of the Koshi River, the huge sediment load and megafan's slightly convex transverse surface conspire against engineering efforts to contain peak flows inside manmade embankments. In August 2008 high monsoon flows breached the embankment, diverting most of the river into an unprotected ancient channel and across surrounding lands with high population density. Over a million people were rendered homeless, about a thousand lost their lives and thousands of hectares of crops were destroyed. The Koshi is known as the Sorrow of Bihar for contributing disproportionately to India's death tolls in flooding, which exceed those of all countries except Bangladesh.
Alluvial fans are also found on Mars descending from some crater rims over their flatter floors. Three alluvial fans have been found in Saheki Crater on Mars. These fans confirmed fluvial flow on Mars and further supported the theory that liquid water was once present in some form on the Martian surface. In addition, observations of fans in Gale crater made by satellites from orbit have now been confirmed by the discovery of fluvial sediments by the Curiosity rover.
Alluvial fans have been observed by the Cassini-Huygens mission on Titan using the Cassini orbiter's synthetic aperture radar (SAR) instrument. These fans are more common in the drier mid-latitudes at the end of methane/ethane rivers where it is thought that frequent wetting and drying occur due to precipitation, much like arid fans on Earth. Radar imaging suggests that fan material is most likely composed of round grains of water ice or solid organic compounds about two centimetres in diameter.