- Alluvial fan
An alluvial fan is a fan-shaped deposit formed where a fast flowing
streamflattens, slows, and spreads typically at the exit of a canyononto 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.American Geological Institute. "Dictionary of Geological Terms". New York: Dolphin Books, 1962.]
Owing to the slowing of
flowas stream gradient decreases, coarse-grained solidmaterial carried by the wateris 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. [ 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. ] This fan shape can also be explained with a thermodynamicjustification: the system of sedimentintroduced at the apex of the fan will tend to a state which minimizes the sum of the transport energyinvolved in moving the sediment and the gravitational potentialof 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
desertareas subject to periodic flash floods from nearby thunderstormsin local hills. They are common around the margins of the sedimentary basins of the Basin and Rangeprovince of southwestern North America. The typical watercoursein an arid climatehas 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 (30–50 feet) 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 soilat their bases and over time windaction often blows away sandaround the bushes which form islands of habitat for many animals.
In humid climates
Alluvial fans also develop in wetter climates. In Nepal the
Koshi Riverhas built a megafan covering some 150,000 km2 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 Himalayasseveral millimeters annually. Uplift is approximately in equilibrium with erosion, so the river annually carries some 100 million cubic meters of sediment as it exits the mountains. Deposition of this magnitude over millions of years is more than sufficient to account for the megafan. [http://daac.gsfc.nasa.gov/geomorphology/GEO_4/GEO_PLATE_F-19.shtml] .
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 Valleyinto an Endorheic basinwithout connection to the ocean.
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 that exceed all countries' except Bangladesh
Fault scarpcuts alluvial fan, Death Valley Death Valley
References and notes
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Look at other dictionaries:
alluvial fan — n. a gradually sloping mass of alluvium that widens out like a fan from the place where a stream begins to slow down … English World dictionary
alluvial fan — Physical Geog. a fan shaped alluvial deposit formed by a stream where its velocity is abruptly decreased, as at the mouth of a ravine or at the foot of a mountain. Also called alluvial cone. [1870 75] * * * ▪ geological feature unconsolidated… … Universalium
alluvial fan — /əˌluviəl ˈfæn/ (say uh.loohveeuhl fan) noun a fan shaped alluvial deposit formed by a stream where its velocity is abruptly decreased, as at the mouth of a ravine or at the foot of a mountain. Also, alluvial cone … Australian English dictionary
alluvial fan — allu′vial fan′ n. geo gel a fan shaped alluvial deposit formed by a stream where its velocity is abruptly decreased, as at the mouth of a ravine … From formal English to slang
alluvial fan — noun a fan shaped deposit where a fast flowing stream flattens out • Syn: ↑alluvial cone • Hypernyms: ↑geological phenomenon … Useful english dictionary
alluvial fan — noun Date: 1873 the alluvial deposit of a stream where it issues from a gorge upon a plain or of a tributary stream at its junction with the main stream … New Collegiate Dictionary
alluvial fan — a fan shaped deposit of river load where energy has been lost due to the river exiting abruptly from a narrow upland valley to a lowland plain … Geography glossary
alluvial fan — A fan like deposit of detrital material from steep mountain slopes  … Lexicon of Cave and Karst Terminology
alluvial fan — A low, outspread mass of loose materials and/or rock material, commonly with gentle slopes, shaped like an open fan or a segment of a cone, deposited by a stream (best expressed in semiarid regions) at the place where it issues from a narrow… … Glossary of landform and geologic terms
alluvial fan — noun A cone shaped heap of alluvium deposited by a river … Wiktionary