Alluvial Fans
By
Tim Baker
&
Tom Coburn
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.
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.
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.
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.
An example of an alluvial fan forming in the mountains of Spitsberg, Norway.
These are perfect conditions for the creation of alluvial fans.
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
The End…