alluvial fans

Fan- or cone-shaped deposits of fluvial gravel,
sand, and other material radiating away from a single point source on a mountainside. They represent erosionaldepositional
systems in which rock material is eroded from
mountains and carried by rivers to the foot of the mountains,
where it is deposited in the alluvial fans. The apex of an alluvial
fan is the point source from which the river system
emerges from the mountains and typically breaks into several
smaller distributaries forming a braided stream network that
frequently shifts in position on the fan, evenly distributing
alluvial gravels across the fan with time. The shape of alluvial
fans depends on many factors, including tectonic uplift and
subsidence, and climatic influences that change the relative
river load-discharge balance. If the discharge decreases with
time, the river may downcut through part of the fan and
emerge partway through the fan surface as a point-source for
a new cone. This type of morphology also develops in places
where the basin is being uplifted relative to the mountains. In
places where the mountains are being uplifted relative to the
basin containing the fan, the alluvial fan typically displays
several, progressively steeper surfaces toward the fan apex. In
many places, several alluvial fans merge together at the foot
of a mountain and form a continuous depositional surface
known as a bajada, alluvial apron, or alluvial slope.
The surface slope of alluvial fans may be as steep as 10°
near the fan apex and typically decreases in the down-fan
direction toward the toe of the fan. Most fans have a concave
upward profile. The slope of the fan at the apex is typically
the same as that of the river emerging from the mountains,
showing that deposition on the fans is not controlled by a
sudden decrease in gradient along the river profile.
Alluvial fans that form at the outlets of large drainage
basins are larger than alluvial fans that form at the outlets of
smaller drainage basins. The exact relationships between fan
size and drainage basin size is dependent on time, climate,
type of rocks in the source terrain in the drainage basin,
structure, slope, tectonic setting, and the space available for
the fan to grow into.
Alluvial fans are common sights along mountain fronts
in arid environments but also form in all other types of climatic
conditions. Flow on the fans is typically confined to a
single or a few active channels on one part of the fan, and
shifts to other parts of the fan in flood events in humid environments
or in response to the rare flow events in arid environments.
Deposition on the fans is initiated when the flow
leaves the confines of the channel, and the flow velocity and
depth decrease dramatically. Deposition on the fans may also
be induced by water seeping into the porous gravel and sand
on the fan surface, which has the effect of decreasing the flow
discharge, initiating deposition. In arid environments it is
common for the entire flow to seep into the porous fan
before it reaches the toe of the fan.
The sedimentary deposits on alluvial fans include fluvial
gravels, sands, and overbank muds, as well as debris flow
and mudflow deposits on many fans. The debris flows are
characterized by large boulders embedded in a fine-grained,
typically mud-dominated matrix. These deposits shift laterally
across the fan, although the debris and mudflow deposits
tend to be confined to channels. The fan surface may exhibit
a microtopography related to the different sedimentary facies
and deposit types.
The development of fan morphology, the slope, relative
aggradation versus downcutting of channels, and the growth
or retreat of the toe and apex of the fan are complex phenomena
dependent on a number of variables. Foremost
among these are the climate, the relative uplift and subsidence
of the mountains and valleys, base level in the valleys,
and the sediment supply.
Further Reading
Bull, William B. “Alluvial Fans.” Journal of Geologic Education 16
(1968): 101–106.
Ritter, Dale F., R. Craig Kochel, and Jerry R. Miller. Process Geomorphology,
3rd ed. Boston: WCB-McGraw Hill, 1995.
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