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Have you ever thought about how streams coming down from the mountain can create a fan-shaped area?
Well, it's a phenomenon called the alluvial fan, where the stream channels end up depositing sediment, called alluvium, in the shape of a fan or cone. Once the river channels dry up, you can see the triangle-shaped alluvial fan and the traces of the water bodies.
The alluvium is necessary sediment for areas as it helps form fresh fertile soil by river banks. An alluvial fan is often created when a river, stream, or even a small trickle interacts with mountain fronts or even a steep wall of a canyon. The expanse of an alluvial fan typically depends on the height of the place from where the stream is descending to the flat landform and the rate of deposition.
If you want to discover more facts about the alluvial fan, continue reading.
The simplest meaning of an alluvial fan is a fan-shaped formation caused by the deposition of sand, gravel, or silt known as alluvial, and water-borne sediment, rather than erosion. The sediment of alluvium gives the fan its name.
When a water body, like a river, travels through an area that has gravel, or silt, the debris flows with it and finally gets deposited in the base of a mountain or a canyon wall where the streams or rivers end. As the stream branches out, it takes the shape of a cone or a fan, therefore creating the alluvial fan. If the rivers flood, the alluvial fan can end up being even larger. Alluvial fans are often associated with arid and semi-arid areas, where the streams are often formed after sudden rainfall.
On having a closer look at the alluvial fan, you will notice the feature of a narrow mouth, which is called its apex. In contrast, another feature, the spread-out fan-shaped area, is called the apron. You may find the upper fan will have larger sediment particles like gravel and rocks, while the more fine sediment or alluvium gets deposited in the lower fan area. In some areas, the alluvial fan flooding and flash floods can become a yearly phenomenon, giving rise to vegetated surfaces with deeper stream channels.
The different types of alluvial fans include debris-flow-dominated alluvial fans, stream-flow-dominated alluvial fans, bajada, underwater fans, alluvial fans formed without water, debris cone, and extraterrestrial alluvial fans.
In the debris-flow-dominated alluvial fans, the sediment deposited by the water bodies is usually greater than 0.07 in (2 mm) in diameter. So, in such an alluvial fan you can mainly find rocks, gravel, or other coarse sediments. These alluvial fans are said to form in all kinds of climates and are typically steep with little to no vegetation.
As you can understand, in the stream-flow-dominated alluvial fans, the flow is dominated by streams. These fans are usually caused when there is a perennial, seasonal, or ephemeral presence of streamflow to feed the different water channels. Those occurring in the arid or semi-arid areas often face flash floods due to sudden rainfall, and the sediment deposited can give rise to braided streams.
Bajadas are a type of alluvial fan common in dry climates, such as in the American southwest. The underwater alluvial fans can be caused by underwater currents where the alluvium is deposited under a glacier or submarine hill.
By far, the strangest phenomenon has to be the creation of an alluvial fan without water. It's known as colluvial fans and is caused by mass wasting, a phenomenon where the soil and sediments move in a downward movement to create the fan. Landslides are known to commonly cause colluvial fans.
Similarly, when an alluvial fan is slowly deposited with sediments and debris, over time, it can turn into a debris cone. This structure has a steeper apex than a flat alluvial fan. The steep debris cones can also be formed by mass wasting instead of water.
When it comes to extraterrestrial alluvial fans, the structures have been spotted on the planet Mars as well as on Titan, the largest moon of Saturn.
Alluvial fan structures are found all over the world, especially in areas that are prone to sudden flooding. So, we cannot call them rare. Even though it's commonly associated with semi-arid and arid-areas, alluvial fans are surely found in humid places too.
In the United States, the most famous of alluvial fans have to be the ones present in the Death Valley, California, present within the Death Valley National Park. As you may know, Death Valley is regarded as the hottest and driest spot in the U.S., making it the perfect location for the formation of an alluvial fan. The alluvial fan at the end of the Copper Canyon in Death Valley was formed by a river that's dry right now. A fascinating fact about Death Valley is that you can find different forms of alluvial fans in the same area. The alluvial fans of the east side are said to be smaller compared to those present on the west side in the south Death Valley. On the east side, the small fans can be seen right beside the Funeral Mountains.
The alluvial fan by the Koshi River of Nepal is an example of alluvial fans formed in humid climates. It's categorized as a mega-fan, stretching over an area of 5,800 sq mi (15,000 sq km). Unfortunately, the area witnesses frequent alluvial fan flooding, a common natural disaster for alluvial fans, especially due to sudden climate change and torrential rainfall.
Having said that, the Taklimakan desert alluvial fan is said to be the largest in the world. It's present between the Kunlun and Altun mountain ranges of Xinjiang, China.
As alluvial fans are a natural phenomenon caused by the flow of streams and silt deposition, it is difficult to understand the purpose behind it.
However, what's interesting about alluvial fans is that in arid and semi-arid areas, these structures can be an indicator of exceptional principle groundwater availability. Hence, cities like Los Angeles have been formed on alluvial fans as the water lying under it is enough to sustain the life of human beings, and it can also be used for farming.
As we have discussed already, the size of an alluvial fan may vary based on the steepness of the slope, as well as how the water is flowing and even the rate of deposition.
Commonly, you can find the expanse of the fan to be anywhere around 3.3 ft (1 m) to about 93.2 mi (150 km). When it comes to the feature of the slope, it typically has a steepness ranging between 1.5-25°. At canyon mouths, the alluvial fans tend to be quite steep due to the deposition of coarse materials. It would be wise to remember that these are only a few estimates, and there are a number of expansive alluvial fans that cover thousands of miles.
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