Geoscience Reference
In-Depth Information
(a)
TOPOGRAPHIC RELATIONSHIPS
Simple mountain-front
unconfined fan
Backfilled
mountain-front fan
Tributary-junction
fan
Inter-montane
confined fan
vv
vv
v
v
(b)
FAN/CHANNEL RELATIONSHIPS
(c)
SUCCESSIVE DEPOSITION OF FAN
SEGMENTS BY CHANNEL SWITCHING
Mountain slope
Divides
vv
Mountain front
Alluvial fan
vv
vv
Marginal
channel
Axial
channel
Channels
(i)
Fanhead trench
Fan
surface
wash
(ii)
Successive
fan segments
(iii)
(i) (ii) (iii)
Abandoned
axial channel
Distributary
channel
Interfan
channel
Figure 14.1
Topographic and locational characteristics of alluvial fans: (a) topographic relationships; (b) fan/channel relation-
ships; (c) channel switching and surface age of fan segments (modified from Denny, 1967).
14.1.2
Global occurrence and distribution of
dryland alluvial fans
Chiverrell, Harvey and Foster, 2007) and even in the hu-
mid tropics (e.g. Kesel and Spicer, 1985).
The conditions that favour alluvial fan deposition are,
however, particularly well developed in arid and semi-arid
mountain regions. Several conditions seem to be impor-
tant. As the result of sparse vegetation cover, intense storm
rainfall and the dominance of overland flow processes,
desert mountains have high rates of sediment production
and steep desert streams have high sediment concentra-
tions (Laronne and Reid, 1993). The domination of fluvial
environments by flash floods, with associated high unit
stream power (Baker, 1977: Wolman and Gerson, 1978),
is also important. The tendency for discharge to diminish
downstream by transmission losses through the channel
Alluvial fans are not restricted to dry regions. Indeed they
can occur in any mountain region where there is juxtapo-
sition of a high rate of sediment supply from mountain
catchments and a sudden drop in unit stream power at
mountain-front or tributary-junction locations. They have
been described in many environments (for examples see
Rachocki and Church, 1990; Harvey, Mather and Stokes,
2005), including arctic environments (e.g. Leggett, Brown
and Johnston, 1966; Ritter and Ten Brink, 1986), alpine
environments (e.g. Kostaschuk, Macdonald and Putnam,
1986; Derbyshire and Owen, 1990), humid temperate re-
