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port is more predictable, probably reflecting the
lack of armouring in arid-zone rivers, as the high
supply of slope sediment ensures that fines are
not preferably removed from the bed (Dietrich
et al. 1989; Laronne et al. 1994). Large clasts
(pebble to cobble) can be moved as much as
3 km in one flood event (Leopold et al. 1966).
General bedforms associated with rivers are
discussed in Chapter 3. Here the characteristics
more commonly associated with the deposits
of ephemeral rivers are highlighted. In these
rivers tractional bedload tends to be dominated
by transverse and longitudinal mid-channel
barforms. These comprise imbricated, sorted
sediments associated with upstream dipping
low-angle beds representing the bar top. If water
depth is sufficient an avalanche face and associ-
ated cross-strata may develop. As a barform is
typically associated with a coarser, upstream bar
head and a finer downstream bar tail, migration
of the bar head over the bar tail will commonly
lead to a coarsening upwards within the bar
deposits. The finer part of the bedload and
coarser element of the suspended load tend to be
deposited as horizontal lamination in sand beds.
These may develop from upper flow regime plane
beds or from pulses of sediment-rich water super-
imposed on the overall flood wave. The rapidly
waning flood stage and abundant fines (typically
muds) that make up the extremely concentrated
water:sediment ratio tend to deposit thick (up
to 10 cm) clay drapes. Once exposed and baked,
these clay drapes have a tendency to desiccate
and curl and be reworked in ensuing flows as
mud clasts.
part of arid geomorphological systems (Harvey
1997). Alluvial fans, as long as they remain
untrenched, tend to trap the deposits of each
successive flood event originating from the
associated fan catchment. Once alluvial fans
become incised (trenched) throughout they will
begin supplying coarse sediment to adjacent
environments such as playa and river systems.
Thus untrenched fans provide a useful record
of processes operative within mountain areas
bordering basins in arid regions.
In general (i.e. in the absence of any major
external influences such as climate) alluvial
fans tend to be dominated by debris flows in the
younger, more proximal parts of the alluvial
fan. As the fan system matures, supplies of fine
material become exhausted, the drainage network
expands and the conditions for the generation
of fluvial processes are increasingly met, leading
to the dominance of fluvial deposition in the fan
dispersal area. As the fluvial processes require
lower gradients than the debris flow processes
to maintain transport, trenching of the proximal
fan surface is often a common feature associated
with mature fans (Harvey 1990).
5.3.3.1 Streamflow processes
Within alluvial fan accumulation areas the trans-
port and deposition of sediment by streamflow
may occur in (i) clearly defined channels such
as the fan head trench (Fig. 5.12), (ii) in wide,
ill-defined channels or (iii) as unconfined flows
(sheetfloods). The last of these is a relatively rare
phenomenon (section 5.4.3). In arid regions,
where sediment movement is typically transport-
limited, sediments are deposited from flows that
are hyperconcentrated with sediment (40 -70%
by weight, Costa 1988) as well as more normal
streamflow events (less than 40% sediment by
weight, Costa 1988; Fig. 5.11).
'Normal' streamflow leads to the deposition
of imbricated, sorted sediments (see e.g. Nemec
& Steel 1984; Costa 1988). Where sediment is of
a suitable calibre and water depths are sufficient
(i.e. in confined areas of flow such as the fan-
head trench), bars with an avalanche face may
develop. Elsewhere, particularly in less confined
5.3.3 Alluvial fans
Alluvial fans are fan-shaped bodies dominated
by coarse sediment that has been transported
from steep upland catchments (Fig. 5.12). They
require the juxtaposition of an upland source
area and a lowland sediment accumulation area,
where the fan will develop. Their other main
requirement is a source area capable of produc-
ing course material, and high, flashy, peak dis-
charges. These conditions are commonly met in
arid environments and so fans form an integral
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