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vertically. These depositional mechanisms eventually form extensive subparallel ridges or large islands that separate
anabranches (Figure 12.2(c)). Alternative erosional mechanisms for anabranch formation can occur where high
flows in the trunk channel cause local ponding of tributary inflows, ultimately resulting in scour of the floodplain
surfaces adjacent to the vegetated banklines along the trunk channel and, over time, to the excision of ridges or
large islands from the floodplain (Tooth and Nanson, 1999). The Marshall clearly operates close to a transitional
condition, changing downstream of tributary junctions from single-thread or weakly anabranching reaches with
relatively wide (60-150 m), straight channels to strongly anabranching reaches with numerous narrower (typically
<
60 m wide), straight channels (Figure 12.9(b)). Field observations and theoretical model results suggest that the
formation of these narrow anabranches minimises boundary roughness and helps to maintain sediment throughput
across these low-gradient plains (Tooth and Nanson, 2004).
Figure 12.9
(a) Location of the Marshall and Plenty Rivers, central Australia (inset). The two rivers follow closely adjacent,
subparallel courses for
∼
70 km and an anabranch of the Marshall River crosses the low-relief divide near Thring Bore to join
the Plenty River (from Tooth and Nanson, 2004). (b) Relationship between tributary inflows (downward-pointing arrows)
and the degree of anabranching along the Marshall River, based on assessment of the number of channels at 1 km intervals
downstream (determined from aerial photographs and field surveys). Horizontal bars indicate where large islands are present
in addition to narrower ridges, with the number of anabranches typically increasing for a short distance downstream of the
tributary junctions (from Tooth and Nanson, 2004). (c) Longitudinal profiles of the Plenty and Marshall Rivers, demonstrating
the similarity in channel-bed gradient through the study reach (constructed from contour crossings and spot heights on 1 :
100 000 topographic maps) (from Tooth and Nanson, 2004). (d) Examples of surveyed cross-sections from the Marshall and
Plenty Rivers, illustrating the contrasting morphological characteristics, and (e) the Plenty River, illustrating the contrasting
morphological characteristics. The distribution of trees is schematic only (after Tooth and Nanson, 2004). Reproduced by
permission of the Geological Society of America.
The closely adjacent middle reaches of the Plenty River (Figure 12.9(a)) provides a further illustration of the
importance of these tributary inputs and the riparian vegetation to anabranching development. The Plenty has a
