An alluvial fan is an accumulation of sediments shaped like a section of a shallow cone, with its apex at a point 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 kilometre (0.39 sq mi) to almost 20,000 square kilometres (7,700 sq mi).
Alluvial fans typically form where flow emerges from a confined channel and is free to spread out and infiltrate the surface. This reduces the carrying capacity of the flow and results in deposition of sediments. The flow can take the form of infrequent debris flows or one or more ephemeral or perennial streams.
Alluvial fans are common in the geologic record, such as in the Triassic basins of eastern North America and the New Red Sandstone of south Devon. Such fan deposits likely contain the largest accumulations of gravel in the geologic record.
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.
Alluvial fans can exist on a wide spectrum of size scales, 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. 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 sediments becoming less coarse toward the distal fan.
When there is enough space in the alluvial plain for all of the sediment deposits to fan out without contacting other valleys walls or rivers, an unconfined alluvial fan develops. Unconfined alluvial fans allow sediments to naturally fan out and the shape of the fan is not influenced by other topological features. When the alluvial plain is narrow or short parallel to depositional flow, the fan shape is ultimately affected. Wave or channel erosion of the edge of the fan sometimes produces a "toe-trimmed" fan.
When numerous rivers and streams exit a mountain front onto a plain, the fans can combine to form a continuous apron. In arid to semi-arid environments, this is referred to as a bajada and in humid climates the continuous fan apron is called a piedmont alluvial fan.
Alluvial fans usually form where a confined feeder channel exits a mountain front or a glacier margin. As the flow exits the feeder channel onto the fan surface, it is able to spread out into wide, shallow channels or to infiltrate the surface. This reduces the carrying power of the flow and results in deposition of sediments.
Flow in the proximal fan, where the slope is steepest, is usually confined to a single channel (a fanhead trench), which may be up to 30 meters (98 ft) deep. This channel is subject to blockage by accumulated sediments or debris flows, which causes flow to periodically break out of its old channel (nodal avulsion) and shift to a part of the fan with a steeper gradient, where deposition resumes. As a result, normally only part of the fan is active at any particular time, and the bypassed areas may undergo soil formation or erosion.
Alluvial fans can be debris-flow-dominated or stream-flow-dominated. Which kind of fan is formed is controlled by climate, tectonics, and the bedrock lithology in the area feeding the flow onto the fan.
Debris flows are a type of landslide that takes the form of a continuous, rapidly moving mass of water and material that is composed mainly of coarse debris. Typically, 20 to 80 percent of the particles in a debris flow are greater than 2 mm in diameter.
Debris-flow-dominated alluvial fans occur in all climates but are more common where the source rock is mudstone or matrix-rich saprolite rather than coarser, more permeable regolith. The abundance of fine-grained sediments encourages the initial hillslope failure and subsequent cohesive flow of debris. Saturation of clay-rich colluvium by locally intense thunderstorms initiates slope failure. The resulting debris flow travels down the feeder channel and onto the surface of the fan.
Debris-flow-dominated alluvial fans are found to consist of a network of mostly inactive distributary channels in the upper fan that gives way to mid- to lower-level lobes. The channels tend to be filled by subsequent cohesive debris flows. Usually only one lobe is active at a time, and inactive lobes may develop desert varnish or develop a soil profile from eolian dust deposition, on time scales of 1,000 to 10,000 years. Because of their high viscosity, debris flows tend to be confined to the proximal and medial fan even in a debris-flow-dominated alluvial fan, and streamfloods dominate the distal fan. However, some debris-flow-dominated fans in arid climates consist almost entirely of debris flows and lag gravels from eolian winnowing of debris flows, with no evidence of sheetflood or sieve deposits. Debris-flow-dominated fans tend to be steep and poorly vegetated.
Stream flow processes take place on all alluvial fans but are the main process for sediment transport on stream-flow-dominated alluvial fans.
Stream-flow-dominated alluvial fans occur where there is perennial, seasonal, or ephemeral stream flow that feeds a system of distributary channels on the fan. In arid or semiarid climates, deposition is dominated by infrequent but intense rainfall that produces flash floods in the feeder channel. This results in sheetfloods on the alluvial fan, where sediment-laden water leaves its channel confines and spreads across the fan surface. These may include hyperconcentrated flows containing 20% to 45% sediments. As the flood recedes, it often leaves a lag of gravel deposits that have the appearance of a network of braided streams.
Where the flow is more continuous, as with spring snow melt, incised-channel flow in channels 1-4 meters (3.3-13.1 ft) high takes place in a true network of braided streams. Such stream-flow-dominated alluvial fans tend to have a shallower slope but can become enormous, and include the Kosi and other fans along the Himalaya mountain front in the Indo-Gangetic plain. Here, continued movement on the Main Boundary Thrust over the last ten million years has focused the drainage of 750 kilometres (470 mi) of mountain frontage into just three enormous fans.
An example of an active stream-flow-dominated alluvial fan is found in the semi-arid region between the Kunlun and Altun mountain ranges that form the southern border of the Taklamakan Desert in northwest China. This particular fan is 60 kilometres (37 miles) in total length. One lobe of the fan has flowing streams that are continually depositing sediment so that the fan is still prograding into the alluvial plain. The feeder channels consist of straight channels as well as instances of braided channels because of the large volume of sediment sourced from the local uplands.
Alluvial fans are common in the geologic record, but may have been particularly important before the evolution of land plants in the mid-Paleozoic. They are characteristic of fault-bounded basins and can be 5,000 meters (16,000 ft) or more thick due to tectonic subsidence of the basin and uplift of the mountain front. Most are red from hematite produced by diagenetic alteration of iron-rich minerals in a shallow, oxidizing environment. Examples of paleofans include the Triassic basins of eastern North America and the New Red Sandstone of south Devon, the Devonian Hornelen Basin of Norway, and the Devonian-Carboniferous in the Gaspé Peninsula of Canada. Such fan deposit likely contain the largest accumulations of gravel in the geologic record.
Alluvial fans are characterized by coarse sedimentation, though with an overall proximal to distal fining. Gravels show well-developed imbrication with the pebbles dipping towards the apex. Fan deposits typically show well-developed reverse grading caused by outbuilding of the fan. However, a few fans show normal grading indicating inactivity or even fan retreat. Normal or reverse grading sequences can be hundreds to thousands of meters in thickness. Depositional facies that have been reported for alluvial fans include debris flows, sheet floods and upper regime stream floods, sieve deposits, and braided stream flows.
Debris flow deposits are common in the proximal and medial fan. These consist of coarse-grained massive gravel and blocks which contain relatively large portions of fine-grained matrix. Debris flow deposits lack sedimentary structure, other than occasional reverse-graded bedding towards the base, and they are poorly sorted. The proximal fan may also include gravel lobes that have been interpreted as sieve deposits, where runoff rapidly infiltrates and leaves behind only the coarse material. However, the gravel lobes have also been interpreted as debris flow deposits.Conglomerate originating as debris flows on alluvial fans is described as fanglomerate.
Stream flow deposits tend to be sheetlike, better sorted, and sometimes show well-developed sedimentary structures such as cross-bedding. These are more prevalent in the medial and distal fan. In the distal fan, where channels are very shallow and braided, stream flow deposits consist of sandy interbeds with planar and trough slanted stratification. The medial fan of a streamflow-dominated alluvial fan shows nearly the same depositional facies as ordinary fluvial environments, so that identification of ancient alluvial fans must be based on radial paleomorphology in a piedmont setting.
Where alluvial fans are overlain by clay or marl sediments, they can be a potential trap for hydrocarbons and a possible exploration target.
Alluvial fans are built in response to erosion induced by tectonic uplift, and upwards coarsening of beds reflects cycles of erosion in the highlands feeding sediments to the fan. However, climate and changes in base level may be as important. Alluvial fans in the Himalayas show older fans entrenched and overlain by younger fans, which in turn are cut by deep incised valleys showing two terrace levels. Dating via optical stimulated thermoluminescence (OSL) suggests a hiatus of 70 to 80 thousand years between the old and new fans, with evidence of tectonic tilting at 45 thousand years ago and an end to fan deposition 20 thousand years ago. Both the hiatus and the more recent end to fan deposition are thought to be connected to periods of enhanced southwest monsoon precipitation. Dating of beds in Death Valley suggest that peaks of fan deposition during the last 25 thousand years occurred during times of rapid climate change, both from wet to dry and from dry to wet.
Alluvial fans are often found in desert areas often subjected 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 (plants with long tap roots capable of reaching a deep water table) characteristically form fan-toe phreatophyte strips. The phreatophytes may form sinuous lines radiating from the fan toe. These trace buried channels of coarse sediments from the fan that have interfingered with impermeable playa sediments.
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.
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 which creates a low divide, turning the south end of the San Joaquin Valley into an endorheic basin without a connection to the ocean.
The biggest natural hazard on alluvial fans are floods, hyperconcentrated flows, and debris flows, typically resulting from heavy and prolonged rainfall. Floods commonly take the form of short (several hours) but energetic flash floods that occur with little or no warning. These are characterized by high velocities and capacity for sediment transport. Debris flows resemble freshly poured concrete, consisting mostly of coarse debris. Hyperconcentrated flows are intermediate between floods and debris flows, with a water content between 40 and 80 weight percent. Floods may transition to hyperconcentrated flows as they entrain sediments, while debris flows may become hyperconcentrated flows if they are diluted by water. Because flooding on alluvial fans carries large quantities of sediment, channels can rapidly become blocked, creating great uncertainty about flow paths that magnifies the dangers.
In August 2008 high monsoon flows breached the embankment of the Koshi River, diverting most of the river into an unprotected ancient channel and across surrounding lands with high population density that had been stable for over 200 years. 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. These fans confirmed past fluvial flow on the planet 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.