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the older fan surfaces are indurated by a calcrete crust,
reducing the erodibility of the older sediments and re-
stricting the width of the fan channels (see above). This
factor when combined with the increase in gradient at the
intersection point, where the channel gradient gives way to
an inherited fan surface gradient, resulted in an increase in
unit stream power sufficient to allow erosion, rather than
the more normal decrease in unit stream power associated
with a reduction in confinement and associated intersec-
tion point deposition (Wasson, 1974). Intersection-point
headcut development appears to be favoured by calcrete-
crusted fan surfaces, on which there is a large increase in
channel gradient at the intersection point and where chan-
nels are restricted in width (Figure 14.14). Successive
headcuts may form downfan to create throughfan dissec-
tion (e.g. Figure 14.15(c)), although this type of dissec-
tion can also result from tectonically or base-level induced
distal dissection that propagates headwards. These rela-
tionships can be illustrated by the fan profiles for selected
individual fans (Figure 14.14, with summary gradient and
channel width data given in Table 14.1). For the exam-
ple fans, note that on all fans except La Sierra (subjected
to tectonically induced dissection; see Section 14.3.2.1
above) distal fan gradients are greater than channel gradi-
ents within the fanhead trench. Note also that Corachos is
a strongly crusted fan and has the greatest
G
/
S
ratio and a
relatively narrow channel, all properties conducive to the
formation of intersection-point headcuts.
A second, simpler, response to climatic change is illus-
trated by the response of the Nevada and Zzyzx, Califor-
nia, fans to the Pleistocene to Holocene climatic change
(Harvey, Wigand and Wells, 1999). In both cases large
volumes of alluvial fan sediments pre-date the Late Pleis-
tocene pluvial Lakes Lahontan and Mojave. The Zzyzx
fans were active during the Pleistocene-Holocene climatic
transition (Harvey and Wells, 1994, 2003; see also Fig-
ure 14.6), undergoing some hillslope debris-flow activity,
limited fanhead trenching and distal progradation. On the
other hand, the Stillwater fans in northern Nevada were
inactive at that time (Harvey, Wigand and Wells, 1999).
Lake levels fell and there was some base-level induced dis-
tal incision, but it was not until climatic aridification in the
Mid Holocene that significant fan sedimentation resumed.
The reasons for these differences are partly due to vegeta-
tion differences (themselves climatically controlled) and
partly due to climate itself. Even during the Late Pleis-
tocene the lower elevation, more southerly Zzyzx area
was under desert scrub vegetation, whereas at the same
time the Stillwater fans were under juniper woodland and
the mountan catchments were under coniferous forest.
Only in the Mid Holocene did desert scrub vegetation
and Wells, 1999), allowing a reduction of erosional thresh-
olds. Climatically the Early Holocene saw the penetration
of unstable 'monsoonal' air masses, with their associated
intense storm rainfall, into southeastern California. It was
only in the Mid Holocene that there appeared to be pene-
tration of these air masses further north (Harvey, Wigand
and Wells, 1999).
A final example deals with the alluvial fans of the Mu-
sandam peninsular in northern UAE and Oman (Al Farraj
and Harvey, 2000, 2005). Here three main periods of Late
Quaternary alluvial fan sedimentation are identified. The
first, of an uncertain date, involved substantial sedimen-
tation of relatively small gravels throughout the region,
but especially in the high mountains, suggesting a colder
climatic regime with mechanical weathering important in
the mountains. The second phase, in the Late Pleistocene,
involved much coarser sediments and in many places can
be directly linked to slope failures, suggesting a wetter
climate. The third phase, in the Early Holocene, saw re-
working of older deposits and was probably associated
with a wetter climate than today.
14.3.2.3 Base-level change
Most fans toe out to stable base levels, so over time fans
would extend or prograde distally (Figure 14.16). Base-
level change is irrelevant for fan dynamics on these fans.
Even for fans on the margins of basins of interior drainage
where base levels may slowly rise as sedimentation takes
place in the basin centre, the rate and amount of change is
likely to have little or no effect on fan dynamics. However,
for fans that toe out into main river valleys or on coastlines,
both marine and lacustrine, base-level change can be a
major control over alluvial fan dynamics.
Conventionally a base-level fall would trigger incision
of fan toe zones and a base-level rise would promote
sedimentation in fan toe zones. In reality, the situation is
more complex. Even fans toeing out on to a stable valley-
floor base level may be subject to 'toe trimming' by lateral
migration of the main river (Leeder and Mack, 2001) or in
coastal areas by wave erosion of the distal fan areas. Such
erosion could effectively shorten and steepen the distal
fan profile, causing a wave of incision to work headwards
up the fan (Figure 14.16).
Distal incision can, of course, be produced by a fall
in base level, in the fluvial case by a fall in local base
level caused by the incision of a main stream, a situation
documented on Wadi Al Bih, UAE (Al Farraj and Har-
vey, 2000, 2005; Harvey, 2002b). Where fans toe out at
a marine or lacustrine margin, such incision will
only
oc-
cur if the newly exposed sea or lake floor has a steeper
