Geoscience Reference
In-Depth Information
drylands. Globally, drylands encompass a wide variety
of climatic, physiographic, geological and vegetative set-
tings. Rivers are a product of their environments; there-
fore, as environmental settings change within and between
drylands, so do river characteristics. At the most basic
level, the diversity in dryland river characteristics is re-
flected by subdivisions into exogenous and endogenous
river types or into exoreic and endoreic drainage. Diver-
sity can also be demonstrated by highlighting how dry-
land rivers operate across a spectrum of conditions, e.g.
relating to flow regime (ephemeral, intermittent, peren-
nial), sediment calibre (coarse-grained, fine-grained) or
boundary characteristics (bedrock, alluvial, indurated or
lithified alluvial), long-term behavioural tendencies (in-
cisional, aggradational, migratory, avulsive) and riparian
vegetation associations (nonvegetated or vegetated with
trees, shrubs and/or grasses). Different combinations of
natural environmental factors give rise to different river
styles, and river patterns in particular can vary across a
wide spectrum (braiding, single-thread straight, meander-
ing, anabranching/anastomosing, distributary), with pat-
tern changes commonly occurring in response to subtle
variations in the controlling factors. Knighton and Nanson
(1997) highlighted that dryland rivers form part of a con-
tinuum of hydrological conditions from hyperarid to very
humid, and so many dryland river styles have morpholog-
ical and sedimentological overlap with the characteristics
of rivers in wetter regions. Climate is just one factor in-
fluencing dryland river characteristics and it is commonly
mediated or overridden by other influencing factors. The
key message is that when assessing dryland river charac-
teristics, rather than focusing on only limited ranges of
drylands and interpreting the findings as representing a
much larger reality, there is a need to step back and look
at the 'big picture' across drylands as a whole. Many pre-
vious statements regarding dryland river characteristics
either can no longer be sustained as generalisations or,
at the very least, the geographical and geomorphological
context for those generalisations needs to be clarified. In-
deed, today's understanding of dryland rivers undoubtedly
will change as more examples are investigated.
Arguments for the distinctiveness and diversity of dry-
land rivers are not mutually exclusive.
As a group
, dryland
rivers do display some common forms, processes and be-
haviours, some (but not all) of which are distinctive in
comparison with most humid region rivers (e.g. transmis-
sion losses, aeolian-fluvial interactions). Equally, how-
ever, recognition must be given to the fact that not all
dryland rivers conform to these generalisations. In any
case, it is a moot point whether it is more important
to stress the distinctiveness or to acknowledge the di-
both are important for developing a comprehensive un-
derstanding of complex natural phenomena. From a prac-
tical (applied) perspective, both have implications for de-
veloping environmentally sensitive, sustainable manage-
ment practices for dryland rivers, particularly as there is
now greater awareness of the importance of fluvial ge-
omorphological processes for issues such as assessing
the stability of engineering structures, estimating reser-
voir lifespans, predicting bank erosion and water quality,
and maintaining riverine ecological health. The history of
river management in drylands is riddled with examples
where mismanagement has occurred because of the appli-
cation of inappropriate practices that commonly have been
based on experience with the operation of very different
types of humid rivers (e.g. Graf, 1985; Beaumont, 1989;
Powell, 2008). Of equal danger, however, is the wholesale
replacement of these inappropriate practices with other in-
appropriate management practices derived from a limited
and unrepresentative subset of dryland river types. Recog-
nition of both distinctiveness and diversity are important
if fluvial geomorphologists are to facilitate the design of
management practices that are tailored to the characteris-
tics of individual dryland rivers.
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