alluvial fan

A vast alluvial fan blossoms across the desolate landscape between the Kunlun and Altun mountain ranges that form the southern border of the Taklamakan Desert in Xinjiang. The left side is the active part of the fan, and appears blue from water flowing in the many small streams Credit: NASA/GSFC/METI/ERSDAC/JAROS/ASTER

Alluvial fan in Death Valley

Alluvial fan in the French Pyrenees

An alluvial fan is a fan-shaped deposit formed where a fast flowing stream flattens, slows, and spreads typically at the exit of a canyon onto a flatter plain. A convergence of neighboring alluvial fans into a single apron of deposits against a slope is called a bajada, or compound alluvial fan.^[1]

Formation

Owing to the flow as stream gradient decreases, coarse-grained solid material carried by the water is dropped. As this reduces the capacity of the channel, the channel will change direction over time, gradually building up a slightly mounded or shallow conical fan shape. The deposits are usually poorly-sorted.^{[1] [2]} This fan shape can also be explained with a thermodynamic justification: the system of sediment introduced at the apex of the fan will tend to a state which minimizes the sum of the transport energy involved in moving the sediment and the gravitational potential of material in the cone. There will be iso-transport energy lines forming concentric arcs about the discharge point at the apex of the fan. Thus the material will tend to be deposited equally about these lines, forming the characteristic cone shape.

In arid climates

Alluvial fans are often found in desert areas subject to periodic flash floods from nearby thunderstorms in local hills. They are common around the margins of the sedimentary basins of the Basin and Range province of southwestern North America. 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 are plants that are often concentrated at the base of alluvial fans, which have long tap roots 30 to 50 feet (9.1 to 15 m) to reach water. The water at this level is derived from water that has seeped through the fan and hit an impermeable layer that funneled the water to the base of the fan where it is concentrated and sometimes forms springs and seeps if the water is close enough to the surface. These stands of shrubs cling onto the soil at their bases and over time wind action often blows away sand around the bushes which form islands of habitat for many animals.

In humid climates

Alluvial fans also develop in wetter climates. In Nepal the Koshi River has built a megafan covering some 150,000 km² (58,000 sq mi) below its exit from Himalayan foothills onto the nearly level plains 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.^[3]

In North America, streams flowing into California's Central Valley have deposited smaller but still extensive alluvial fans. That of the Kings River flowing out of the Sierra Nevada creates a low divide, turning the south end of the San Joaquin Valley into an Endorheic basin without a connection to the ocean.

Flood hazards

Alluvial fans are subject to flooding and can be even more dangerous than the upstream canyons that feed them. Their slightly convex perpendicular surfaces cause water to spread widely until there is no zone of refuge. If the gradient is steep, active transport of materials down the fan creates a moving substrate that is inhospitable to travel on foot or wheels. But as the gradient diminishes downslope, water comes down from above faster than it can flow away downstream, and may pond to hazardous depths.

In the case of the Koshi River, the huge sediment load and megafan's slightly convex transverse surface conspire against engineering efforts to contain peak flows inside manmade embankments. In August 2008 high monsoon flows breached the embankment, diverting most of the river into an unprotected ancient channel and across surrounding lands with high population density. Over a million people were rendered homeless 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.

Gallery

Fault scarp cuts alluvial fan, Death Valley	Mini alluvial fan, Death Valley
Alluvial Fan in Southern Iran. Image from NASA's Terra satellite

References and notes

1. ^ ^{a b} American Geological Institute. Dictionary of Geological Terms. New York: Dolphin Books, 1962.
2. ^ To clarify, solids are sorted as usual, with coarse sediment dropped out first -- but the sorting of an individual flood event is then "jumbled" by the next flood, leaving the overall fan sediment package poorly-sorted.
3. ^ National Aeronautics and Space Administration. "Geomorphology from Space; Fluvial Landforms, Chapter 4: Plate F-19". http://daac.gsfc.nasa.gov/geomorphology/GEO_4/GEO_PLATE_F-19.shtml. Retrieved on 2009-04-18. 

See also

• Alluvium
• Alluvial plain
• Floodplain
• Placer deposit
• River delta
• Subaqueous fan

External links

• Moore, J and Howard, A (2005). "Large Alluvial Fans on Mars". Journal of Geophysical Research 110: E04005. doi:10.1029/2004JE002352. http://erode.evsc.virginia.edu/papers/moore_fans_2004JE002352.pdf.