Geoscience Reference
In-Depth Information
12.4
Reassessing distinctiveness and
diversity
firmer geomorphological basis for contextualising find-
ings and thus for more rigorously comparing rivers be-
tween and within different drylands. For instance, while
broad generalisations about dryland rivers are essential
for making sense of highly variable natural phenom-
ena, statements that Mediterranean dryland rivers tend to
have different characteristics to Australian dryland rivers
(see above) downplay or ignore the fact that some small,
steep Mediterranean rivers have similarities with the char-
acteristics of the upland or piedmont reaches of
some
Australian rivers (e.g. low-order, bedrock or coarse-
grained tributaries; see Jansen and Brierley, 2004; Jansen,
2006). The key point is that while there are upland and
piedmont river styles in both regions, because of the differ-
ent gross physiographies in each (a function of different
tectonic and geological frameworks), the spatial extent
and relative proportions of the upland, piedmont, lowland
and floodout zones varies dramatically. In many Mediter-
ranean catchments with a coastal outlet, the upland and
piedmont zones commonly possess high relative relief, are
tectonically active and are relatively extensive, whereas
the lowland zone is more restricted in extent and the flood-
out zone is typically nonexistent, with most deposition
occurring in deltas, estuaries or offshore (Figure 12.4(a)).
Consequently, river styles are dominated by those most
commonly associated with the relatively high-energy
upland and piedmont settings (e.g. gorge-bound rivers,
braided rivers) and the styles associated with moderate or
lower energy lowland and floodout settings are more lim-
ited or nonexistent. By contrast, in many Australian catch-
ments, the upland and piedmont zones typically have lim-
ited relative relief, are tectonically stable and are relatively
restricted in spatial extent by comparison with the lowland
and floodout zones (Figure 12.8(a) and (b)). Hence, river
styles associated with lower energy settings (e.g. single-
thread straight, anabranching, anastomosing and distribu-
tary rivers, and floodouts) are most prominent. Southern
Africa possesses more of a balance between the spatial ex-
tent of tectonically-quiescent, upland, piedmont, lowland
and floodout zones, so river styles tend to be associated
with a wide variety of energy settings, including various
bedrock, alluvial and mixed bedrock-alluvial styles (e.g.
braided, single-thread straight, meandering, anabranch-
ing/anastomosing and distributary rivers, and floodouts)
(Figures 12.2(a) and (b), 12.6 and 12.7).
Against this backdrop of global diversity, one can
choose either to emphasise the differences between
river characteristics across drylands or to search for
commonalities between dryland rivers. Similarly, one
can choose to highlight the differences between the
characteristics of dryland rivers and rivers in more humid
The three regions highlighted above cover a range of dif-
ferent local climatic, tectonic, structural, lithological and
vegetative settings, and illustrate how dryland river char-
acteristics vary accordingly. A full global assessment that
includes the drylands of other parts of Europe, Africa,
North and South America, the Middle East and Asia un-
doubtedly would reveal a variety of dryland river styles,
owing to the different local climates, degrees of tectonic
activity, geological controls and riparian vegetation as-
semblages, as well as their different histories of environ-
mental change (Chapters 1 to 4). Although information is
sparse for some of these regions, descriptions of bedrock
gorges and coarse-grained alluvial rivers in the American
southwest, Middle East and South America (e.g. Graf,
1988; Wells, 1990; McLaren
et al.
, 2004; Hoke
et al.
,
2004; Unkel
et al.
, 2007; Magilligan
et al.
, 2008) re-
veal morphological and sedimentological similarities with
some Mediterranean and southern African rivers. Inland
deltas in north Africa (e.g. McCarthy, 1993; Makaske,
1998) have some similarities with the Okavango Delta
in southern Africa. Sand-bed rivers that decrease in size
downstream and terminate on broad alluvial plains or
among aeolian dunefields have been described from dry-
lands in the American southwest (e.g. Langford, 1989;
Clarke and Rendell, 1998), east and north Africa (e.g.
Vanney, 1960; Mabbutt, 1977; Billi, 2007) and China (e.g.
Yang
et al.
, 2002; Yang, 2006), many of which have sim-
ilarities with some Namibian or central Australian rivers.
It is notable, however, that many parts of the Australian
drylands possess river styles (e.g. alluvial anabranch-
ing/anastomosing rivers, reticulate channels, waterholes)
that are either unusual in global terms or that occur on
a frequency and across spatial scales unknown in other
drylands.
The foregoing global assessment - albeit partial - re-
veals that not only is there great diversity in river charac-
teristics
between
different drylands but that there is also
great diversity
within
drylands, including between neigh-
bouring rivers and along individual rivers (see Boxes 12.1
to 12.3). Schumm's (1977) concept of the production,
transfer and deposition zones in an idealised fluvial system
can be adapted by using physiographic terms to subdivide
catchments, which highlights the fact that the character-
istics of an individual dryland river reach vary according
to whether it is located in an upland, piedmont or lowland
setting and whether it ends in an endoreic basin (floodout,
playa or aeolian dunefield setting) or is through-flowing
to the coast (Figure 12.10). This approach provides a
