Alluvial Fans

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Alluvial Fans By Tim Baker Tom Coburn
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Basic Definition

• An alluvial fan is a fan-shaped deposit formed
  where a fast flowing stream flattens following a
  steep gradient, slows, and spreads out
  typically at the exit of a canyon onto a flatter
  plain.
• A convergence of neighbouring alluvial fans
  that cover a huge area at the bottom of a slope
  is called a bajada, or compound alluvial fan.

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Formation

• Alluvial fans are fan-shaped landforms that
  form in areas where there is a sudden change in
  landscape, from a steep gradient (such as a
  mountainous region) to a flat plain. They are
  similar to deltas, but are deposited on land,
  rather than in the sea.
• A river flows down the slope, with gravity
  creating plenty of energy and meaning it can
  carry a large load.

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Formation (continued)
The river then flows out onto the flatter land
and all the energy it had in its previous stage
is suddenly diminished substantially. This
dramatic loss of energy, results in deposition.
At the apex of the fan, the larger, coarser
material is deposited as this is the part of the
load that requires the most energy. As you move
further away from the apex, the deposited
material is the finer sands and gravel.

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Formation (continued)

• The deposition of material in the river channel
  chokes the flow and causes the water to look for
  another way to reach its destination. The
  resulting streams are called distributaries. This
  creates the fan shape, as the river continues to
  find a different route, and then the deposition
  of more material blocks it up, creating the
  cone-shaped fan that becomes broader and
  shallower further away from the apex.

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An example of an alluvial fan forming in the
mountains of Spitsberg, Norway. These are perfect
conditions for the creation of alluvial fans.
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Features

• The shape of the fan is related to the grain
  size.
• Fans built of boulders and cobbles have a high
  pronounced arch, whose pavements may contain
  fragments a metre or more across that protrude
  well above the average surface
• Fans built of silt, sand, and fine gravel have
  broad, flattened profiles

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The End