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Macquarie University ResearchOnline
This is the author version of an article published as: Ralph, T. J. and Hesse, P.P. (2010) Downstream hydrogeomorphic changes along the Macquarie River, southeastern Australia, leading to channel breakdown and floodplain wetlands. Geomorphology 118(1-2): 48-64.
Access to the published version: http://dx.doi.org/10.1016/j.geomorph.2009.12.007 Copyright: 2010 Elsevier B.V.
http://dx.doi.org/ http://dx.doi.org/10.1016/j.geomorph.2009.12.007
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Downstream hydrogeomorphic changes along the Macquarie River, southeast- ern Australia, leading to channel breakdown and floodplain wetlands
Timothy J. Ralph, Paul P. Hesse
PII: S0169-555X(09)00523-6 DOI: doi: 10.1016/j.geomorph.2009.12.007 Reference: GEOMOR 3167
To appear in: Geomorphology
Received date: 1 May 2008 Revised date: 3 December 2009 Accepted date: 7 December 2009
Please cite this article as: Ralph, Timothy J., Hesse, Paul P., Downstream hy- drogeomorphic changes along the Macquarie River, southeastern Australia, lead- ing to channel breakdown and floodplain wetlands, Geomorphology (2009), doi: 10.1016/j.geomorph.2009.12.007
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http://dx.doi.org/10.1016/j.geomorph.2009.12.007 http://dx.doi.org/10.1016/j.geomorph.2009.12.007
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Downstream hydrogeomorphic changes along the Macquarie River,
southeastern Australia, leading to channel breakdown and floodplain
wetlands
Timothy J Ralph1 and Paul P Hesse*
Department of Environment and Geography, Macquarie University, N.S.W. 2109,
Australia
* Corresponding author. Email: Paul.Hesse@mq.edu.au; Telephone: +61 2 9850 8384;
Facsimile: +61 2 9850 8420 1 Present address: Rivers and Wetlands Unit, Department of Environment, Climate
Change and Water N.S.W., PO Box A290, Sydney South, N.S.W. 1232, Australia.
Email: Tim.Ralph@environment.nsw.gov.au
Abstract
Floodplain wetlands and floodouts in the Macquarie Marshes are fed by numerous
anastomosing and distributary channels on the lower reaches of the Macquarie River,
southeastern Australia. River discharge is seasonally and annually variable and is
affected by both interannual and interdecadal climatic trends, related to ENSO and IPO.
A downstream comparison of hydrologic data shows that flows decrease significantly in
magnitude and stream power along the allogenic lower Macquarie River as distributary
outflows are not matched by tributary inputs. Intrinsic hydrologic and geomorphic
thresholds along the lower Macquarie River lead to the breakdown of the trunk stream
into smaller distributaries, with disintegration of channelised flows and development of
extensive wetlands on the low gradient, dryland alluvial floodplain-fan. Cross-sectional
analysis shows that the morphology and morphometry of the trunk stream change
accordingly, with adjustments in planform and cross-sectional shape and marked
downstream reductions in channel capacity and bed width. Despite monotonic declines
in river discharge, gross stream power and channel cross-sectional area, there is a
threshold change in the geomorphic response. Channel width-to-depth ratio decreases in
the middle reaches and then increases, while channel sinuosity increases until
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immediately upstream of the core wetlands and then rapidly declines as straight
channels enter areas of extensive channel breakdown. Floodplain width is greatest on
the alluvial plain, allowing a broader area of floodplain wetlands to develop, that are
characterised by fine cohesive sediments. Floodplain connection is greatest where
floodouts and wetlands form in the Marshes. Channel breakdown and floodplain
wetland formation along the lower Macquarie River and in the Macquarie Marshes are
distinct examples of a nonequilibrium response to downstream declining discharge and
stream power, which is an inherent condition in many dryland Australian rivers during
the Holocene.
Keywords: allogenic river hydrology; El Niño Southern Oscillation; avulsion; channel
breakdown; floodplain wetlands; Macquarie Marshes
1. Introduction
Channel breakdown and extensive floodplain wetlands are typical features of the lower
reaches of many rivers in the Murray-Darling Basin of southeastern Australia. The
wetlands are important waterbird and wildlife habitats whose health is much debated
and a subject of contention in resource management and nature conservation today.
However, the floodplain wetlands are a fluvial landscape that is both poorly
documented and poorly understood, and the debates over land and water management
proceed largely in ignorance of the geomorphic factors leading to the formation and
maintenance of the wetlands or their natural dynamics. For the most part these fluvial
systems and their landforms have escaped detailed geomorphological investigation
despite their large extent and common occurrence. The floodplain wetlands and their
associated channel breakdown features are similar in many respects to floodouts,
described in more arid areas of Australia, as ‘a site where channelized flow ceases and
floodwaters spill across adjacent alluvial surfaces’ (Tooth, 1999a, p222). But the
features described here also differ in many ways, including the size of the inflowing
streams and their perennial flows. In this paper we examine the hydrological conditions
and channel morphology of the Macquarie River that feeds the largest example of the
floodplain wetlands in the Murray-Darling Basin, the Macquarie Marshes. Our
hypothesis is that the floodplain wetlands and trunk stream channel breakdown occur
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because of the particular catchment and channel hydrological and sediment transport
conditions.
The size and shape of alluvial river channels reflect the type, amount, and rate of
discharge and sediment supply that comes from the upstream catchment area, as
transmitted through the cross-sectional profile (Leopold et al., 1964; Knighton, 1998).
Similarly, the morphology of an alluvial floodplain is also genetically related to the
channel that supplies it, and both the channels and floodplain may be affected by
inherited features that are imposed by the landscape in which they develop. In addition,
regional hydrology, topography, and geomorphology tend to vary in the downstream
direction; and so downstream adjustments occur in channel and floodplain forms as
alluvial rivers, and their processes respond to changes in these environmental controls.
However, unlike temperate zone rivers, where discharge tends to increase downstream
and channel capacity increases accordingly, many dryland rivers exhibit the reverse
pattern: a decrease in discharge downstream with associated reductions in channel size
and adjustments of form. These downstream declines are particularly pronounced on the
lower reaches of dryland rivers that receive variable floods and have significant
transmission losses, distributary outflows and few tributary contributions in their lower
catchments (Tooth, 1999b, 2000a, b). This is the case for the lower Macquarie River, a
tributary of the Barwon-Darling River in southeastern Australia (Fig. 1), which
undergoes a series of hydrological and morphological changes as it flows across a low,
broad alluvial plain, a floodplain-fan, before it breaks down at a range of scales to form
the Macquarie Marshes.
This paper investigates the modern hydrology of the Macquarie River, the longitudinal
variation of flows and the longitudinal morphological changes of the channel and
floodplain from the mid-upper catchment to the lower reaches where breakdown occurs.
We consider whether the hydrological character and morphological response provide
coherent explanations for channel breakdown and marsh formation and also consider
alternative explanations. Neotectonic activity has been proposed as the cause of the
formation of the Macquarie Marshes (Watkins and Meakin, 1996), and an example of
tectonic damming occurs on the Murray River elsewhere in the Murray-Darling Basin
(see Bowler, 1967). Alluvial trunk stream damming has also led to the formation of
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terminal wetlands on another large river in the