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In-Depth Information
benches that can greatly influence river and floodplain
forms and processes, including the formation of small
gorges and waterfalls in the indurated or lithified allu-
vium (Shaw and Nash, 1998; Makaske, 1998; Nanson,
Tooth and Knighton, 2002; Nash and Smith, 2003). In
limestone gorges in Crete, for example, recent floods have
been unable to erode the calcite-cemented lower parts of
coarse-grained terraces (see Box 12.1) and lateral channel
activity has been constrained. Within lower energy flood-
plain wetlands in southern Africa, vegetation transpiration
promotes subsurface chemical precipitation, and the sub-
sequent soil volumetric increases can lead to the initiation
and growth of distinctive, near-circular vegetated islands
that become raised above the general level of flooding (e.g.
McCarthy, Ellery and Ellery, 1993; McCarthy and Ellery,
1995; Tooth
et al.
, 2002b) (Figure 12.11(b) and (c)). While
induration and lithification of alluvium can occur in other
(particularly tropical) environments, it appears to be com-
mon in some dryland fluvial settings because the char-
acteristically strong net moisture deficits readily promote
reprecipitation of minerals from solution.
Allan and Baker, 1988), and in some cases this pro-
motes the formation of extensive anabranching (Figure
12.2(c) and Box 12.3). In contrast, as Fielding, Alexander
and Newman-Sutherland (1997) have noted, vegetation in
perennial rivers tends to respond to sedimentation, typi-
cally only colonising bars or islands that are sufficiently
elevated to be protected from regular inundation (Nan-
son and Beach, 1977; Hupp, 1990; Hupp and Osterkamp,
1996).
12.4.4
Fluvial-aeolian interactions
A final distinctive aspect of some dryland rivers is the
tendency for widespread interaction with surrounding
aeolian dunefields (Figure 12.8(b)). Over the last few
decades, numerous studies have documented a diverse
range of fluvial-aeolian interactions over various tempo-
ral and spatial scales from many drylands worldwide (for
overviews, see Bullard and Livingstone, 2002, and Bullard
and McTainsh, 2003). In some settings, channel activity
controls aeolian processes and landforms, for instance by
eroding dune flanks, by breaching dune cordons or by
supplying the material for winds to rework into dunes
(e.g. Nanson, Chen and Price, 1995; Clarke and Rendell,
1998; Page
et al.
, 2001; Maroulis
et al.
, 2007; Cohen
et al.
, 2010). In other settings, aeolian activity influences
channel processes and landforms by controlling chan-
nel alignment or by ponding floodwaters (e.g. Lancaster
and Teller, 1988; Teller, Rutter and Lancaster, 1990; Yang
et al.
, 2002). Clearly, while the strength and direction
of these fluvial-aeolian interactions can vary greatly de-
pending on local circumstances, the scale and spatial ex-
tent of these interactions sets many drylands rivers apart
from rivers in more humid environments, where a greater
degree of soil development and a higher percentage of
vegetation cover tend to limit aeolian activity.
12.4.3
Channel-vegetation interactions
Another distinctive aspect of some dryland channels is
the fact that a variety of native and invasive vegetation
types tends to grow on the lower banks and channel beds.
Along ephemeral channels, vegetation types may include
large tree and shrub species (Figure 12.2(c)), while wet-
land channels may be characterised by various grasses
and sedges rooted in peat (Figure 12.6(b)). As Chapter 13
also highlights, such riparian vegetation has many diverse
influences on channel process, form and behaviour, in-
cluding increasing flow resistance, promoting localised
bed scour and/or aggradation, contributing to bank
stabilisation/destabilisation, and enabling channel recov-
ery following destructive floods (e.g. Osterkamp and
Costa, 1987; Graeme and Dunkerley, 1993; Dunkerley,
2008a; Tooth, 2000b; Rountree, Rogers and Heritage,
2000; Knighton and Nanson, 2002; Hooke and Mant,
2002; Sandercock, Hooke and Mant, 2007). Some as-
pects of dryland river channel-vegetation interactions are
similar to rivers in more humid environments, but in other
cases the interactions differ between rivers with ephemeral
or intermittent flow and those with perennial flow (Tooth
and Nanson, 2000a; Nanson, Tooth and Knighton, 2002).
In channels that are dry year-round or seasonally, for in-
stance, vegetation can establish on channel beds and can
directly initiate the formation of depositional features such
as bars, benches, ridges or islands (e.g. Hadley, 1961;
12.5
Conclusions
Much of the older literature contains statements regarding
the distinctiveness of dryland rivers, as expressed by par-
ticular flow and sediment processes, channel morpholo-
gies or behaviours, and in some respects these percep-
tions still persist. Supporting empirical evidence can be
found, but has been derived mainly from research con-
ducted in a limited range of dryland settings, namely
small, steep rivers that commonly drain tectonically ac-
tive catchments. Consequently, such statements need to
be tempered by taking account of the findings from
