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similar channel-bed gradient (Figure 12.9(c)), discharge and bank strength to the Marshall, but because it is joined
by few tributaries in its middle reaches (Figure 12.9(a)), bed material is composed of medium to coarse sand and
the channel remains relatively free of in-channel trees. Consequently, and in strong contrast to the anabranching
Marshall (Figure 12.9(d)), the Plenty remains predominantly single thread (Figure 12.9(e)) but is variably wide
(
100-1200 m), and in places appears transitional to braiding (Tooth and Nanson, 2004). These findings show
that dryland river process, form and behaviour in central Australia is finely tuned to local environmental factors,
particularly those related to the dryland climate. Variations along and between dryland rivers do not result from
any significant changes in discharge, gradient, bed material, bank strength or human land use but instead seem to
depend on complex and subtle sets of processes involving adjustments between flow hydraulics, sediment transport,
vegetation density, number and geometry of channels, and boundary roughness.
∼
As in southern Africa, human impact on dryland rivers
was probably minimal prior to the start of European
colonisation of the continental interior in the early to
mid nineteenth century, but has intensified over the last
100-150 years. Vegetation clearance and the introduction
of cattle and sheep have led to river degradation in some
dryland catchments (Pickup, 1991; Fanning, 1999), but
outside of the heavily regulated Murray-Darling system,
there are very few dryland rivers that have been dammed
or subjected to other forms of extensive flow regulation
or direct channel modification, so Australia retains an
unusual number of near-pristine, large dryland river sys-
tems. Flow regimes are highly variable, with long peri-
ods of below-average flows alternating with wetter peri-
ods with occasional very large floods. Late Pleistocene
and Holocene palaeoflood records from headwater gorges
and piedmont settings provide evidence for significant
geomorphic impacts during irregular events, including
bedrock erosion, aeolian dune reworking and gravel bed-
form deposition (Pickup, 1991; Patton, Pickup and Price,
1993; Bourke, 1998; Hollands
et al.
, 2006; Jansen and
Brierley, 2004; Jansen, 2006). By contrast, the impacts
of even large historical floods have tended to be much
more muted, with low-energy, slow-moving floodwaves
typically accomplishing little widespread or substantial
geomorphic work, particularly where boundary resistance
is high as a result of indurated alluvial terraces, cohesive
muds or riparian vegetation. Along Cooper Creek, east-
ern central Australia, very few substantial morphologi-
cal changes can be detected on aerial photographs dating
from the last 50-60 years despite regular (seasonal) floods
that commonly inundate floodplains up to 60 km wide
(Knighton and Nanson, 1997; Tooth and Nanson, 2000b;
Fagan and Nanson, 2004). On the Northern Plains, cen-
tral Australia, channels have remained highly stable over
the last few decades despite several large floods, although
some changes have occurred in the lower reaches, includ-
ing splay formation (Figure 12.8(a)) and channel aban-
Overall, many Australian rivers do not readily fit
the 'textbook' generalisations regarding dryland rivers.
Across the interior, a variety of upland, bedrock-confined
dryland rivers can be found with some morphological
and sedimentological similarities to upland rivers in other
drylands, but rates of fluvial geomorphic activity tend
to slow in comparison to dryland rivers draining more
tectonically active dryland catchments (Jansen, 2006).
Australia's numerous sandy, single-thread, planar-bed,
straight rivers have morphological similarities to those
described from other drylands (e.g. Wolman and Ger-
son, 1978; see Chapter 13) but occur in unusually large
numbers across the interior, while the sandy anabranch-
ing (Figure 12.2(c)) and muddy anastomosing rivers are
not commonly associated with other drylands. In partic-
ular, the Channel Country's extensive muddy floodplains
with abundant waterholes (Knighton and Nanson, 1994a,
2000) and 'braided' and reticulate (Figure 12.2(d)) chan-
nel networks (Nanson, Rust and Taylor, 1986; Fagan and
Nanson, 2004) are highly unusual features, and to date
have not been described from other drylands. Character-
istics of many other Australian dryland rivers, such as the
pronounced downstream decreases in channel size, flood-
outs, reforming channels, floodplain wetlands and ex-
tensive aeolian-fluvial interactions, have been described
from other continents, but Australia exhibits these charac-
teristics on a frequency and over spatial scales unknown
elsewhere. Although many upland gorges and piedmont
settings are subject to periodic, high-magnitude flood
events, Australia is better known for its extensive, rela-
tively low-energy, lowland rivers, which are characterised
by slow-moving, long-duration floods that represent hy-
draulic conditions very different from the flash floods typ-
ical of many other dryland rivers (Knighton and Nanson,
2001, 2002). Many of these lowland rivers appear to be
insensitive even to recent large flood events, having under-
gone little substantial or long-lasting change, and in many
instances are characterised by long-term equilibrium con-
