Geoscience Reference
In-Depth Information
Figure 12.10
Zones of an 'idealised' fluvial system (after Schumm, 1977, and Gordon, McMahon and Finlayson, 1992), also
illustrating the subdivision into upland, piedmont, lowland, and floodout or deltaic/estuarine zones. These zones are not specific
to dryland rivers, but help to provide the geomorphological context for comparing findings from rivers across different drylands.
characteristics of dryland rivers and rivers in more humid
regions is evidenced by exogenous rivers, many of which
possess perennial flow, commonly exhibit well-developed
meanders and in many cases display a greater tendency
towards equilibrium behaviour, precisely the sorts of char-
acteristics that are commonly ascribed to many humid re-
gion rivers. Examples of such rivers include the Okavango
River, Botswana (McCarthy and Ellery, 1998), and the
Rio Grande, southern Colorado, USA (Jones and Harper,
1998). Even discounting exogenous rivers, however, over-
lap with humid region river characteristics is also demon-
strated by the fact that no endogenous river styles or be-
haviours are necessarily unique to drylands, as they may
also be found in more humid settings. Gorge-bound rivers
in Crete (Box 12.1), for example, have many similari-
ties with humid rivers in similarly confined settings (e.g.
Wohl, 1992). Meandering rivers in South Africa (Box
12.2) are morphometrically similar to meandering rivers
in humid regions (e.g. Hooke, 2004). Anabranching rivers
in central Australia (Box 12.3) are initiated and develop
in response to particular sets of processes that in large
part are related to the dryland climate, but nevertheless
rivers in humid regions (Nanson and Knighton, 1996).
Even floodout formation - once thought to be a key ex-
pression of dryland climatic influence on river behaviour
owing to downstream discharge losses (Sullivan, 1976;
Mabbutt, 1977) - has been described from fluvial settings
in more humid regions (e.g. Melville and Erskine, 1986;
Fryirs and Brierley, 1998; Grenfell, Ellery and Grenfell,
2009), where instead it may be promoted by lithologically
controlled downstream decreases in gradient and con-
comitant declines in sediment transport capacity (Tooth,
2004).
If this is the case, can we therefore make any general-
isations regarding dryland rivers? In particular, are there
any characteristics that distinguish dryland rivers from
rivers in other climatic settings? While there appear to
be few (if any) river styles that could only occur in dry-
lands and, similarly, few (if any) river styles that could
not occur in drylands, some processes, forms and be-
haviours are nevertheless more common to dryland rivers
as compared to rivers in other climatic settings. In this
general sense, dryland rivers
as a group
do exhibit some
distinctive characteristics, as outlined in the following
