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Formation
Owing to the slowing of flow, 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.
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 (30–50 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 soil at their bases and over time wind action often blows away sand around the bushes which form islands of habitat for many animals.
Gallery
 Fault scarp cuts alluvial fan, Death Valley |
 Mini alluvial fan, Death Valley |
References and notes
- ^ a b American Geological Institute. Dictionary of Geological Terms. New York: Dolphin Books, 1962.
- ^ 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.
See also
