Geoscience Reference
In-Depth Information
Australia's low relief and elevation results from a lack
of significant uplift since Gondwana break-up, with Ceno-
zoic tectonic activity having been limited to broad, crustal
warping and local, minor faulting. This tectonic and phys-
iographic framework has facilitated the development of
many large, low-gradient, dryland river systems that arise
in ancient, stable headwater ranges and follow predomi-
nantly alluvial courses across extensive plains towards the
centre of topographic depressions, with the Finke River
in central Australia providing a prime example (Pickup,
Allan and Baker, 1988). Palaeoclimatic changes have also
been important; many Australian dryland rivers originated
under wetter climates in the Late Mesozoic or Early Ceno-
zoic and have since undergone many changes in erosional
and depositional behaviour in response to a long-term
trend to drier and more variable climates, including the
breakdown of many formerly better integrated drainage
networks (e.g. van de Graaff
et al.
, 1977; Salama
et al.
,
1993; Morgan, 1993). Quaternary palaeoclimatic fluctu-
ations involving conditions both drier and wetter than
present have also impacted on Australia's dryland rivers,
with the continent progressively becoming drier with each
glacial cycle during at least the Mid- to Late Quater-
nary (Nanson
et al.
, 2008), but direct glacial, periglacial
and eustatic influences have been minimal. Relative tec-
tonic stability, typically low denudation rates and lim-
ited slope-channel coupling have tended to restrict sedi-
ment supply to many Australian dryland rivers, so that in
comparison with many Mediterranean and some southern
African rivers, long-term vertical channel activity (pro-
nounced aggradation and incision) tends to have been
muted, except in localised areas (e.g. Croke, Magee and
Price, 1996). Lateral channel activity (migration and avul-
sion) has been more pronounced, however, leading to the
formation of many extensive low-gradient, 'fan-shaped'
plains. In combination with unusually well-developed ri-
parian vegetation assemblages and variations in sediment
calibre, diverse river styles have developed across the Aus-
tralian drylands, including numerous sandy, single-thread,
planar-bed, straight rivers and sandy anabranching rivers
(Figure 12.2(c) and see Box 12.3), muddy anastomos-
ing rivers with extensive floodplains marked by water-
holes, 'braided' channel and reticulate channel networks
(Figure 12.2(d)), distributary rivers (Figure 12.8(a)) and
rivers that decrease in size downstream and disappear
in floodouts on alluvial plains, in playas or among aeo-
lian dunefields (Figure 12.8(a) and (b)) (Woodyer, Tay-
lor and Crook, 1979; Nanson, Rust and Taylor, 1986;
Nanson
et al.
, 1988; Nanson, Tooth and Knighton, 2002;
Dunkerley, 1992, 2008a; Knighton and Nanson, 1994a,
1997, 2000; Bourke and Pickup, 1999; Tooth, 1999, 2005;
Tooth and Nanson, 1999, 2000a, 2000b, 2004; Fagan and
Nanson, 2004; Wakelin-King and Webb, 2007; Fisher
et al.
, 2008). In some instances, 'reforming channels' oc-
cur farther downvalley and either join a larger river sys-
tem or disappear in another floodout (Tooth, 1999). Aus-
tralia has an extensive Quaternary record of widespread
fluvial-aeolian interactions (e.g. Nanson, Chen and Price,
1995; Nanson
et al.
, 2008; Page
et al.
, 2001; Maroulis
et al.
, 2007; Cohen
et al.
, 2010), and across parts of
the interior, river systems have interacted with aeolian
dunefields to create complex landform assemblages of
anabranching/anastomosing and distributary channel net-
works, floodplains and floodplain wetlands, floodouts and
reforming channels, waterholes and pan and playa com-
plexes (Figure 12.8(b)).
Box 12.3
Alluvial rivers on the Northern Plains of central Australia
On the Northern Plains in arid central Australia, ephemeral rivers are commonly low sinuosity ('straight'), but vary
from single thread to anabranching, both along individual rivers and between neighbouring rivers (Figure 12.9(a)).
The Marshall River exemplifies how the dryland climate exerts a strong control on the degree of anabranching
through its direct and indirect influence on flow regime, sediment supply and riparian vegetation growth strategies
(Tooth and Nanson, 2004). The Marshall follows a roughly west-east course across the Plains and is joined at
various points along its length by large tributaries arising in ranges to the north (Figure 12.9(a)). Localized rainfall
in these tributary catchments sometimes results in the Marshall flowing for short distances downstream of the
tributary junctions, while the reaches upstream remain dry, and these inflows also provide additional gravelly sand.
Over time, the more frequent provision of water to sections of the Marshall downstream of the tributary junctions
encourages greater numbers and/or denser growth of river red gums (
Eucalyptus camaldulensis
) on the channel
bed. These trees are deep-rooted, long-lived species and, once established, are able to withstand high flood flow
velocities. By acting as obstacles to flow, these trees commonly initiate ridges as leeside accumulations of gravelly
sand with minor cohesive fines. Subsequent vegetation colonisation in the intervals between floods, together with
further deposition of sediment, helps these incipient ridges to stabilise and to grow longitudinally, laterally and
