Alluvial fan
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.
Alluvial fans typically form where flow emerges from a confined channel and is free to spread out and infiltrate the surface.
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.
A shift of the feeder channel (a nodal avulsion) can lead to catastrophic flooding, as occurred on the Kosi River fan in 2008.
[11] When numerous rivers and streams exit a mountain front onto a plain, the fans can combine to form a continuous apron.
[12][13] Alluvial fans usually form where a confined feeder channel exits a mountain front[14][15] or a glacier margin.
[15] Flow in the proximal fan, where the slope is steepest, is usually confined to a single channel[6] (a fanhead trench[3]), which may be up to 30 meters (100 ft) deep.
[15] 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.
Debris flows are slurry-like mixtures of water and particles of all sizes, from clay to boulders, that resemble wet concrete.
[19] Debris flow 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.
In arid or semiarid climates, deposition is dominated by infrequent but intense rainfall that produces flash floods in the feeder channel.
[26] Here, continued movement on the Main Boundary Thrust over the last ten million years has focused the drainage of 750 kilometres (470 miles) of mountain frontage into just three enormous fans.
[30] 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.
[25] In the distal fan, where channels are very shallow and braided, stream flow deposits consist of sandy interbeds with planar and trough slanted stratification.
[33] Alluvial fans are characteristic of mountainous terrain in arid to semiarid climates,[34][6] but are also found in more humid environments subject to intense rainfall[7] and in areas of modern glaciation.
Both the hiatus and the more recent end to fan deposition are thought to be connected to periods of enhanced southwest monsoon precipitation.
[38] Alluvial fans are often found in desert areas, which are 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.
[34] Phreatophytes (plants with long tap roots capable of reaching a deep water table) are sometimes found in sinuous lines radiating from arid climate fan toes.
Uplift is approximately in equilibrium with erosion, so the river annually carries some 100 million cubic meters (3.5×10^9 cu ft) of sediment as it exits the mountains.
Unlike alluvial fans on Earth, those on Mars are rarely associated with tectonic processes, but are much more common on crater rims.
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.
[46] 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.
[48] Alluvial fans have been observed by the Cassini-Huygens mission on Titan using the Cassini orbiter's synthetic aperture radar instrument.
Radar imaging suggests that fan material is most likely composed of round grains of water ice or solid organic compounds about two centimeters in diameter.
Many urban, industrial, and agricultural areas are located on alluvial fans,[50] including the conurbations of Los Angeles, California; Salt Lake City, Utah; and Denver, Colorado, in the western United States, and in many other parts of the world.
At a minimum, major structural flood control measures are required to mitigate risk, and in some cases, the only alternative is to restrict development on the fan surface.
[60] 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.
[61] On January 1, 1934, record rainfall in a recently burned area of the San Gabriel Mountains, California, caused severe flooding of the alluvial fan on which the towns of Montrose and Glendale were built.
The river has a history of frequently and capriciously changing its course, so that it has been called the Sorrow of Bihar for contributing disproportionately to India's death tolls in flooding.
