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alluvial fan
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What is an alluvial fan?
An alluvial fan is a fan-shaped deposit of sediment that accumulates at the mouth of a mountain canyon where a stream's sediment transport diminishes or stops. These fan-shaped deposits can occur in various climates but are typically larger in semiarid regions. Alluvial fans border mountain fronts, with the apex of each fan located just within a canyon mouth that serves as the outlet for a mountain drainage system.
These fans consist of a wide range of sediment sizes sorted from apex to base. The rivers that deposit alluvial fans are typically fast-flowing, so the first material laid down is usually coarse.
Alluvial fans are built up due to tectonic uplift, climatic change, and variations in the balance between stream discharge, debris load, and surface gradient. In desert or semiarid areas, alluvial fans can act as groundwater reservoirs when stream water infiltrates the deposit.
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How does an alluvial fan form?
Alluvial fans form as unconsolidated sediment accumulates at the mouth of a mountain canyon due to a decrease in sediment transport by the stream. These fan-shaped deposits are common in arid and semiarid regions, where seasonal rainfall or snowmelt drives sporadic flash floods and mudflows, moving sediment from eroding mountains to adjacent basins.
The process begins with fast-flowing rivers depositing coarse material near the fan's apex, while finer sediments settle towards the base. Infiltration of surface water further encourages the deposition of finer materials. Tectonic uplift, climatic changes, and internal factors like stream discharge and surface gradient also influence the creation of alluvial fans.
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What materials are found in alluvial fans?
Alluvial fans typically consist of a wide assortment of sediment sizes, which are sorted from the fan's apex to its base. The initial material deposited tends to be coarse due to the fast-flowing rivers that create alluvial fans. Closer to the apex, coarser sedimentary fractions are more common, while finer materials like silts and sands are found toward the base.
Debris flows or mudflows are capable of transporting large boulders across considerable distances. Deposits stemming from debris flows are characteristically unsorted, with clast sizes ranging from clay to boulders.
Infiltration of surface water into the early coarse debris encourages the deposition of finer materials. Additionally, the braided distributary channels perform some sorting, laying shallow sheets of sand and silt over the fan's surface, while coarser sands and gravels are laid down in the main channels where water tends to flow more quickly.
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Why do rivers or streams often create alluvial fans at the base of mountains?
Alluvial fans form at the base of mountains where a river or stream exits a narrow canyon onto a plain. The sudden change in slope causes a decrease in stream velocity, reducing the water's capacity to carry sediment. As a result, the river deposits its load of sediments, ranging from coarse boulders and gravel near the apex to finer silts and sands towards the base, forming a fan-shaped deposit. This depositional process is especially prominent in areas with seasonal rainfall or snowmelt, where sporadic flash floods transport large amounts of sediment from the mountains to the adjacent basin.
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How do floods affect the shape and size of alluvial fans?
Floods significantly influence the shape and size of alluvial fans. Alluvial fans are created by the deposition of sediment at the mouth of a mountain canyon due to a decrease in the stream's ability to transport sediment.
Floods, especially flash floods and mudflows, are often associated with sediment transfer in arid and semiarid regions. The fast-flowing rivers that deposit alluvial fans typically lay down coarse material first, with a range of sediment sizes sorted from the apex to the base of the fan.
Climatic changes and the balance between stream discharge, debris load, and surface gradient also affect how alluvial fans are built up. The transfer of sediment from source basins to depositional sites on a fan surface involves various types of flow, from high-viscosity debris or mudflows to normal water flows.
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Why are alluvial fans important for ecosystems and human activities?
Alluvial fans are vital to both ecosystems and human activities, especially in drier climates, because they can act as important groundwater reservoirs when stream water infiltrates the deposit and percolates downward. This groundwater can be tapped by wells for irrigation and water supply.
Wet alluvial fans can also concentrate heavy mineral particles, potentially forming economically significant placer deposits such as gold.
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alluvial fan, unconsolidated sedimentary deposit that accumulates at the mouth of a mountain canyon because of a diminution or cessation of sediment transport by the issuing stream. The deposits, which are generally fan-shaped in plan view, can develop under a wide range of climatic conditions and have been studied in the Canadian Arctic, Swedish Lappland, Japan, the Alps, the Himalayas, and other areas. They tend to be larger and more prominent in arid and semiarid regions, however, and generally are regarded as characteristic desert landforms. This is particularly true in the basin-and-range type of areas of parts of Iran, Afghanistan, Pakistan, the western United States, Chile and Peru, Sinai and western Arabia, and Central Asia, where the basic landscape configuration consists of mountains set against adjacent basins.
A brief treatment of alluvial fans follows. For full treatment, see river: Alluvial fans.
Alluvial fans are of practical and economic importance to society, particularly in arid and semiarid areas where they may be the principal groundwater source for irrigation farming and the sustenance of life. In some instances, entire cities, such as Los Angeles, have been built on alluvial fans.
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Alluvial fans border the mountain fronts with the apex of each fan just within a canyon mouth that serves as the outlet for a mountain drainage system. Sediment from erosion within the mountains is moved by these drainage systems to the adjacent basin. In arid and semiarid regions, this is either an irregular or seasonal process driven by strongly seasonal rainfall or rapid snowmelt. Sediment transfer is thus frequently associated with sporadic flash floods that may include mudflows. The fans, the main sites of deposition, are therefore an intrinsic part of an erosional-depositional system in which mountains tend slowly to wear away and basins to fill with sediment through geologic time.
Since the rivers that deposit alluvial fans tend to be fast-flowing, the first material to be laid down is usually coarse. However, fans consist of a wide range of sediment sizes and a high degree of sorting from apex to base. The initial formation of a fan is often furthered by the infiltration of surface water into the early deposit of coarse debris. This infiltration encourages the deposition of finer material. Usually the coarser sedimentary fraction forms towards the apex, with fine sands and silts toward the base. In addition, the braided distributary channels also perform some sorting, laying shallow sheets of sand and silt over the surface of the fan, while coarser sands and gravels are laid down in the main channels where water tends to flow more quickly.
Alluvial fans are built up in response to tectonic uplift, climatic change, and variations in the internal (autocyclic) balance between stream discharge, debris load, and surface gradient. Even in tectonically active mountains, climate can be a strong influence in alluvial fan development, as has been demonstrated in parts of the Himalayas.
Alluvial fans in desert or semiarid areas may act as groundwater reservoirs when stream water infiltrates the deposit and gradually percolates downward toward the base. If the water becomes trapped between impermeable layers within the fan, it may be tapped by wells along the base, and these may exhibit artesian flow as the groundwater builds up pressure within the fan. The use of desert fans as permanent sources of water is limited, however, because periodic rainfall or snowmelt provides only a very slow rate of recharge.
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An alluvial fan can be large and may occupy a wide area, ranging from only a few metres in radius at its base to more than 150 km (95 miles). When a number of rivers discharge onto a plain, their fans may combine to create a piedmont alluvial fan.
Many fans in humid areas are actually fossil features created during earlier periods of intense erosion and deposition. The Plateau de Lannemezan on the northern side of the Pyrenees in France, for example, is a large piedmont alluvial fan that is still being built up by the tributaries of the Garonne and Adour rivers. This fan, though, is much too large to have been constructed by present-day rivers. It was formed during the late Cenozoic Era and is made up of coarse Pliocene gravels derived from the Pyrenees.