Alluvial fan

Alluvial fan 01

Alluvial Fan

Gigantic Alluvial Fan Being Uplift by New Fault

Alluvial Fan of Rawa Danau

Alluvial fan, Taklimakan Desert, XinJiang Province, China, NASA, ASTER

Alluvial fan is a geomorphological feature formed by sediment deposits created as water flows onto a broader surface from a narrow canyon exit, typically at the base of mountains. These fan-shaped deposits occur where the velocity of a stream decreases rapidly, causing sediments carried from the higher elevations to be deposited. Alluvial fans are found in a variety of environments across the world and can vary greatly in size, composition, and the processes by which they are formed.

Formation

The formation of an alluvial fan begins when water flowing from a narrow valley spreads out and slows down upon reaching a plain, causing the sediment it carries to be deposited. The process is influenced by factors such as the gradient of the terrain, the load and composition of the sediment, and the flow velocity of the water. Over time, these deposits build up to form a fan-shaped feature. Alluvial fans can be formed by a single, catastrophic flooding event or through less dramatic but more continuous sediment deposition processes.

Characteristics

Alluvial fans typically have a convex shape and are characterized by a steep, apex near the mountain base, which gradually flattens towards the distal edge. The surface of an alluvial fan can be quite uneven, with active channels that migrate over time, depositing new layers of sediment. The composition of the sediment can vary widely, from fine silt and sand to larger boulders and gravel, depending on the material available in the upstream catchment area and the energy of the water flow.

Ecological and Human Importance

Alluvial fans can play a critical role in local ecosystems, providing habitats for a variety of plant and animal species. In arid and semi-arid regions, the presence of an alluvial fan can significantly affect local biodiversity by supplying water and fertile soil.

Humans have also utilized alluvial fans for agricultural purposes, taking advantage of the fertile soil for crop cultivation. However, settlements on or near alluvial fans can be at risk from flash floods, especially in areas prone to sudden, heavy rains.

Examples

Famous examples of alluvial fans include the Kosi River fan in India and Nepal, one of the largest in the world, and the alluvial fans in the Death Valley National Park, USA, which are well-documented due to their stark beauty and the contrast they provide to the surrounding desert landscape.

See Also

• Floodplain
• Delta
• Sediment transport
• Geomorphology