Geoscience Reference
In-Depth Information
12
Distinctiveness and diversity of arid zone
river systems
Stephen Tooth and Gerald C. Nanson
12.1
Introduction
cipitation derived from local convective thunderstorms,
frontal systems or tropical storms, possibly in combi-
nation with smaller contributions from snowmelt and
groundwater.
Except in areas of groundwater resurgence, these en-
dogenous rivers typically have intermittent flows (sea-
sonal floods followed by little or no flow) or ephemeral
flows (occasional floods being interspersed with longer
periods of no flow). Many endogenous rivers fail to reach
the ocean, instead terminating within drylands on low-
land alluvial plains, in playa basins or among aeolian
dunefields as part of endoreic systems (interior draining).
Endogenous rivers tend to be much shorter than exoge-
nous rivers but in certain physiographic settings nonethe-
less can still be major landscape features. Large areas of
dryland Australia, Africa, South America and Asia, for
example, have many endogenous river systems that arise
in arid and semi-arid ranges and traverse extensive pied-
mont and lowland settings over hundreds of kilometres.
Most research into dryland rivers has focused on the
smaller endogenous rivers, and it is commonly claimed
that the dryland climate sets these rivers apart from their
counterparts in wetter and/or cooler climatic settings.
Much empirical evidence demonstrates, for example, how
some endogenous dryland rivers are characterised by flow
and sediment transport processes, channel forms or spa-
tial and temporal patterns of channel change that ap-
pear highly distinctive in comparison with humid region
rivers (e.g. Graf, 1988; Bull and Kirkby, 2002). At the
same time, however, there is growing recognition that
even endogenous dryland rivers are inherently diverse
and difficult to typify and that in some instances there is
considerable overlap with the characteristics of rivers in
To those unfamiliar with arid zones, synonymous terms
such as 'arid zone river', 'desert river' or 'dryland river'
almost sound illogical, for aridity would not seem to be
conducive to sustaining river systems. In reality, most dry-
lands support numerous river systems, many of which play
a central role in landscape change and exert a strong influ-
ence on human use of these marginal environments. Some
rivers may be sourced largely from outside dryland set-
tings (exogenous or allogenic rivers), having headwaters
in more humid uplands but with sections of their lower
courses located in much drier settings (Figure 12.1(a)).
Moisture is derived from snowmelt and rainfall in the
headwaters, possibly along with small contributions from
groundwater, with relatively little runoff being received
from within the dryland. These rivers typically have peren-
nial (albeit variable) flow and tend to be major landscape
features, many being hundreds or thousands of kilometres
long and commonly traversing the dryland as part of ex-
oreic systems (draining to the ocean). Exogenous dryland
rivers include some of the largest and best-known rivers
in the world, including the Colorado (Figure 12.1(a)) in
the American southwest (sourced in the Rocky Moun-
tains), the Nile in north Africa (sourced in the Ethiopian
Highlands and East African plateau), the Orange River
in South Africa (sourced in the mountains in Lesotho)
and the Tigris/Euphrates system in Iraq (sourced in the
mountains of eastern Turkey).
The majority of rivers, however, are sourced entirely
from within dryland settings (endogenous or endogenic
rivers) (Figure 12.1(b)). Moisture sources can be variable
in composition, scale and seasonality, but include pre-
