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Timing and causes of gully erosion in the riparian zone of the semi-arid tropical Victoria River, Australia: Management implications
Timing and causes of gully erosion in the riparian zone of the semi-arid tropical Victoria River, Australia: Management implications G.L. McCloskey, R.J. Wasson, G.S. Boggs, M. Douglas PII: DOI: Reference:
S0169-555X(16)30311-7 doi: 10.1016/j.geomorph.2016.05.009 GEOMOR 5603
To appear in:
Geomorphology
Received date: Revised date: Accepted date:
16 November 2015 3 May 2016 3 May 2016
Please cite this article as: McCloskey, G.L., Wasson, R.J., Boggs, G.S., Douglas, M., Timing and causes of gully erosion in the riparian zone of the semi-arid tropical Victoria River, Australia: Management implications, Geomorphology (2016), doi: 10.1016/j.geomorph.2016.05.009
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Timing and causes of gully erosion in the riparian zone of the semi-arid tropical Victoria River, Australia: management implications
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McCloskey, G.L.a,, Wasson, R.J.b,c, Boggs, G.S.c,d, and Douglas, M.c,e
Department of Natural Resources and Mines, PO Box 937, Cairns, Qld 4870, Australia
b
Institute of Water Policy, Lee Kuan Yew School of Public Policy, National University of Singapore,
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a
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Singapore
Charles Darwin University, Darwin, NT 0909, Australia
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Wheatbelt NRM, Northam, WA 6401, Australia
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School of Earth and Environment, University of Western Australia, Perth, WA 6009, Australia
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c
Abstract
Gully erosion in the seasonally wet tropics of Australia is a major source of sediment in rivers. Stabilization of gullies to reduce impacts on aquatic ecosystems and water storages is a focus for management. However the cause of the gully erosion is poorly understood and so a critical context for soil conservation is missing. It is uncertain if they are the result of post-European cattle grazing or are they much older and related to non-human factors. The causes of riparian gully erosion along a reach of the Victoria River in the semi-arid tropics of Australia were investigated using several methods. Gully complexes were described and characterised by two major components: a Flood
Corresponding Author: Tel +61 7 4222 5447 Fax +61 7 4222 5493 email: [email protected]
ACCEPTED MANUSCRIPT Drainage Channel (FDC) and upslope of this an Outer Erosion Feature (OEF) characterised by badlands set within an amphitheatre. The OEF is likely to be a major source of sediment that appears
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to be of recent origin. A review of historical records, combined with Optically Stimulated
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Luminescence (OSL) dating, showed that the FDCs were well established prior to the introduction of
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domestic stock. It also showed that the badlands began to develop about 90 years ago; that is, about 40 years after the arrival of domestic stock. In addition, an analysis of aerial photos coupled with an on-ground survey and analysis of fallout radionuclides revealed that erosion processes are still active
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within the gully complexes. While the FDCs are natural drainage channels, cattle grazing probably triggered the badland formation, with the expansion aided by increased rainfall in the past 40 years.
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Therefore the OEFs are of human origin and protection from grazing of the riparian zone should slow
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badland erosion and reduce sediment input to the river.
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Key Words
caesium-137
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Victoria River, gully erosion, age of gullies, land use, Optically Stimulated Luminescence (OSL),
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1 Introduction
The triggers and controls of gully erosion are poorly understood in tropical and sub-tropical environments, partly because there are few studies of this question. For the same reason, these processes, and the impacts of land use on these processes, are not as well understood in northern Australia as they are in a temperate zone. Recent interest in northern Australia as a potential ‘foodbowl’ (Anon, 2013), as well as investments by land managers and funding bodies in improved land management practices, have resulted in a renewed research focus in the region. Riparian erosion is a phenomenon that warrants further investigation, since impacts from future land use development could have significant consequences for river systems, such as habitat loss and reduced water storage capacity in existing and future reservoirs. The research in this paper focuses on riparian
ACCEPTED MANUSCRIPT gully erosion along a reach of the Victoria River, northern Australia. The causes of the current erosion features, as well as processes driving continued erosion, are determined to provide an input
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to improved catchment management.
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Gully erosion has long been considered a major source of sediment to streams and receiving waters
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in both temperate and tropical Australian rivers (Bartley et al., 2007; Olley and Wasson, 2003; Rutherfurd, 2000; Wasson et al., 2002). Furthermore, an increase in erosion/sediment yield since European settlement is widely reported, although most research has been done in southern
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Australia (Rustomji and Pietsch, 2007), with an expanding body of work in the Great Barrier Reef
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catchments (McCloskey et al., 2014; Tindall et al., 2014; Waters et al., 2014). While research into erosion processes in northern Australia is not uncommon (Brooks et al., 2009; Rustomji et al., 2010;
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Shellberg, 2011; Shellberg et al., 2013; Wilkinson et al., 2015), the links to land use have not been
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fully investigated. It is important for future planning and management in the region to determine the processes of gully initiation and development in northern Australia, and what impact, if any, land use
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has had on these processes.
A range of gully forms has been described in the literature, most of which are ‘hillslope gullies’.
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More generally, gullies are defined as larger than rills, but smaller than river channels or streams (Knighton, 1998). Common features of hillslope gullies include steep sides and a steeply sloping, or near vertical, head scarp (Knighton, 1998). In addition, hillslope gullies are typically longer than they are wide. Hillslope gully erosion is a threshold phenomenon as a function of slope and catchment area (Knighton, 1998), and is initiated when the characteristics of flow (shear stress) exceed a critical value (Sidorchuk, 2006). Gullies identified in tropical climates of northern Australia do not fit the description of hillslope gullies, and have alternatively been called ‘alluvial gullies’ (Brooks et al., 2009; Shellberg, 2011), or, as referred to in this paper, ‘gully complexes’ (McCloskey, 2010). Riparian gully erosion has been described in other areas of northern Australia, including the Fitzroy and Ord River catchments in north-Western Australia (Wasson et al., 2002), rangelands along the
ACCEPTED MANUSCRIPT mid-Western Australian coast (Blandford, 1979; Branch, 1981; Leys, 1980; Medcalf, 1944), and more recently, the Daly River catchment in the Northern Territory (Furlonger, 2004; Wasson et al., 2010).
The early Western Australian studies (Blandford,
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this identified riparian gullies (Condon, 1986).
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A reconnaissance erosion survey was also conducted in the Victoria River District in the 1980s and
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1979; Branch, 1981; Leys, 1980; Medcalf, 1944) clearly suggest that overstocking with cattle, which reduces the resistance to erosion by removing vegetation and disturbing the soil, and rainfall-runoff drives erosion. More recent research suggests that groundwater seepage should be added as a
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formative process (Poesen et al., 2003). Overstocking has also been considered to be a cause of gully development in Kenya and Mediterranean Europe (Vandekerckhove et al., 2000; Zucca et al.,
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2006).
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In this paper, we evaluate the role of land use by cattle grazing in riparian gully complex erosion
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along a reach of the Victoria River, northern Australia. Gully complex erosion processes are also
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inferred, and recommendations for management are made. This research will contribute to filling the current gap in northern Australian erosion studies, particularly in the semi-arid tropics, and will
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add to international understanding of this important phenomenon.
2 The Victoria River catchment The main stem of the Victoria River is ~700 km long, and is located approximately 500 km southwest of Darwin, the capital city of the Northern Territory. It has a catchment area of 125,000 km2 (Fig. 1). The focus of this study is a ~20 km reach in the mid-catchment (Fig. 1). The Victoria River District (VRD) is in the semi-arid tropics, with a seasonal rainfall pattern which has 90–95% of annual precipitation between November and April during the wet season (Roth et al., 1999). Mean annual rainfall ranges from 500 mm in the south of the catchment, to over 1,000 mm in the north. The northern VRD experiences tropical cyclones, while the southern VRD may only experience the remnants of tropical cyclones as heavy rain associated with tropical lows that have decayed from
ACCEPTED MANUSCRIPT cyclones. At the Victoria River Downs rainfall station, the long-term (over 120 years of recorded data) annual average rainfall is 658 ± 19 mm, while the average for the past 40 years at the same
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location is 787 ± 5 mm (Fig. 2). Texture contrast soils occur along channel banks and levees of major
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drainage lines, and these are susceptible to erosion (Karfs, 2000). Deeper loamy and clay-rich soils
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are present on the poorly drained flatter areas, and shallow skeletal soils are present on the steep hilly areas with rocky outcrops (Karfs, 2000).
Cattle were introduced to the VRD in the late 1800s and cattle grazing has been the primary land use
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for over 100 years. There are presently 30 operating cattle stations ranging in size from 1,100 to
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12,000 km2. Generally the stocking rate is low, averaging eight head of cattle per km2. However, in the past stock numbers were difficult to control and monitor, and were almost certainly
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considerably higher. Similarly, there is spatial heterogeneity of stocking rates, both in the past and
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present, whereby the greatest stock numbers are in the ‘best’ paddocks, generally including the
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riparian zone of the Victoria River.
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Fig. 1. Location of the Victoria River Catchment and the study reach (in oval).
The study reach was chosen because it is representative of gully complexes along the river, and accessible for fieldwork. The reach contains eight gully complexes at varying stages of development, evident in aerial photo analysis. The gully complexes occur on grey and brown clay soils associated with the Cambrian Antrim Plateau Volcanics. All of the gully complexes are located on the eastern (right) bank of the Victoria River, and continue to be grazed by cattle.
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Flood Drainage Channel (FDC)
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Alluvial apron
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Badlands
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Fig. 2. Long-term annual rainfall totals (blue) at Victoria River Downs station and the 20 year moving average (black). The average annual rainfall has increased over the past 40 years.
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ACCEPTED MANUSCRIPT Fig. 3. Aerial view of site VRD01. The amphitheatre and badlands are visible upslope of the vegetated gully complex drainage network.
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3 Gully complexes
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Gully complexes have two components. The first is a large sinuous and steep-sided channel that
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begins on the floodplain and terminates at the river, hereafter termed a Flood Drainage Channel (FDC), which drains overbank floodwater back to the river. The second is a dendritic gully network and badlands that have developed upslope from the head of an FDC, forming an amphitheatre with
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a distinct head scarp, and hereafter called the outer erosion feature (OEF) (McCloskey, 2010). Typically, the OEFs are wider than they are long, unlike hillslope gullies. Fig. 3 is an aerial view of site
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VRD01, a well-developed gully complex, while Fig. 4 provides a diagrammatic representation of a typical gully complex found in the VRD. It could be argued that the only gullies in these complexes
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are in the OEF and to include the FDCs in the description is confusing. The FDCs are included,
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however, because of the apparent link with the badlands and gullies upslope, a relationship
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described in more detail below, and their gully likeness.
Gully complexes, or similar features from northern Australia, have only recently been described in
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the literature. The most recent example is that of alluvial gullies in the Mitchell River catchment, Queensland, by Brooks et al. (2009). They define alluvial gullies in that catchment as “young incisional features entrenched into alluvium not previously incised since initial deposition”. Characteristic features of alluvial gullies identified by Brooks et al. (2009) include: a distinct head scarp, the height of which increases with local relief; a dense network of rills and gullies, nested within a macro-gully network; and width often larger than length. They propose a continuum of gully form, from hillslope (or colluvial) gullies to alluvial gullies. It is likely that the gully complexes identified along the Victoria River lie in the mid-range of the continuum tending towards the alluvial gully end of the spectrum, as they demonstrate some similarities with hillslope gullies.
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Terrace Margin
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Gully/Rill Network
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Outer Erosion Feature (OEF)
Interfluves
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Headwall
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Floodplain
Victoria River
Fig. 4. Diagrammatic representation of a typical gully complex.
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Fig.5. Field photographs. a) Interfluves and rills/small gullies making up the badlands. b) View across the alluvial apron. c) View within the Flood Drainage Channel (FDC) at site VRD01.
Badlands are contained within the gully complex, though the entire complex cannot be called a badlands. Characteristic features of badlands are steep slopes, high drainage density, a lack of vegetation, thin and highly dispersive regolith, and rapid erosion rates (Howard, 1994; Piccarreta et al., 2006). The badland slopes within the gully complexes have convex gentle slopes, similar to the badlands described for the Mancos Shale, Utah (Howard, 1994), or the biancane form described c throughout Italy (Piccarreta et al., 2006). The Victoria River gully complexes therefore are morphologically badlands. Another characteristic feature of badlands is the alluvial surface (alluvial apron in Fig. 4), at the downslope edge of the badland areas. Howard (1994) describes this surface
ACCEPTED MANUSCRIPT as one of transportation (and some deposition), with the gradient determined by the relationship between sediment load and discharge. Runoff from the badlands at the study site flows through a
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series of rills and small gullies located within the alluvial apron, and then into the FDC. This
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description fits more closely with that of Engelen (1973), who notes that the lower part of the
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pediment (alluvial apron) is either dissected by headward cutting channels of a larger ephemeral stream downstream, or it is of low gradient without pronounced channels.
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deposited sediments on the alluvial apron are likely temporary, and will be partly transported during
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the next major runoff event (Engelen, 1973). Brooks et al. (2009) oppose the use of the term ‘badlands’ in describing the alluvial gullies found in the Mitchell River catchment. They argue that
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‘badlands’ is not an appropriate term for describing alluvial gullies, as the processes inferred by the term badlands (involving uplift or re-exposure due to base level change, or formation in soft rock
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terrain) do not accurately describe the processes occurring in the Mitchell River alluvial gullies.
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Further, the alluvial gullies described by (Brooks et al., 2009) form exclusively on alluvial surfaces
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while badlands can occur away from a channel and on colluvial surfaces. We argue that the term badlands is a morphological concept and does not denote specific processes of formation or regolith
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type.
4 Methods
Determining the age of the outer erosion features is critical to understanding what caused them. If they existed before the introduction of cattle then overgrazing by domestic stock cannot be the cause. Four methods were used in this study, two to determine the age of gully complexes and two to calculate recent changes. The first step (I) in assessing gully complex age was sourcing historical documentation, in the form of the diaries of the first European explorers to the region, and these were analysed for references to gully erosion (Gregory and Gregory, 1884). The second method (II) used Optically Stimulated Luminescence (OSL) to date sediments within the FDCs that are believed to be coeval with the development of the outer erosion feature. The dates also provide minimum
ACCEPTED MANUSCRIPT ages for the FDCs. Methods to determine recent changes to gully complexes included (III) aerial photo analysis coupled with on-ground survey (III), as well as the use of fallout radionuclides (137Cs
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and 210Pb) (IV).
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4.1 Age of gully complexes
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4.1.1. Historical documentation
The diary of the first European explorer of the region, A.C. Gregory, provides the primary source of historical documentation. Several other members of the Gregory expedition, which occurred from
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1854–1856, also kept records, or wrote letters home, and these also proved a valuable source of
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information, especially those of J.R. Elsey, the surgeon and naturalist appointed to the expedition. The documentation was reviewed for references to gully erosion, as well as descriptions of the main
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river channel, including vegetation, bank steepness, and bank composition.
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4.1.2. Sediment dating using OSL
Inset alluvial terraces were identified in many of the FDCs, and several were identified within the
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FDC at VRD08, of which two were selected for OSL dating. The inset terraces were deposited after the FDC formed, and can therefore provide a minimum age for the FDC. The terraces consist of
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material derived mostly from upslope rather than from the river and therefore represent a sediment input from the outer erosion feature, followed by incision. Three samples were taken from two terraces, located at S 16°12.887’ E 131°09.216’ and S 16°12.903’ E 131°09.190’ respectively (Fig. 6). A more comprehensive sampling regime could not be undertaken due to budget constraints. The excavated face of one site is presented in Fig. 7 and sections of the sampling sites and the lithostratigraphy of both sites are presented in Fig. 8. Each sample was taken by inserting a 30 mm diameter light safe steel pipe into finely laminated sand layers. The pipes were excavated and each end of the pipes was secured by using a PVC cap and tape. An additional 1 kg sample of surrounding sediment was also taken for dosimetry.
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Head scarp Alluvial apron
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Elevation (m)
FDC
Sample Site 1 Sample Site 2
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100 600
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Distance (m)
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Fig. 6. Location of OSL sampling sites on a longitudinal profile of the VRD08 gully complex.
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Fig. 7. VRD08 OSL Site 2. (a) Excavated face of sample location. Depth of face is 60 cm, and sample VRD08OSL6 was taken at a depth of 55 cm (b). Clearly visible stratification.
Sample preparation was designed to isolate pure extract of 180-212 µm light safe quartz grains following standard procedures (Aitken, 1998). Treatments were applied to remove contaminant carbonates, feldspars, organics, heavy minerals and acid soluble fluorides. The outer ~10 m alphairradiated rind of each grain was removed by double etching each sample in 48 % Hydrofluoric Acid.
ACCEPTED MANUSCRIPT Burial doses were determined from measurement of the OSL signals emitted by single grains of quartz. Equivalent doses (De) were determined using a modified SAR protocol (Olley et al., 2004). A
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dose-response curve was constructed for each grain. Burial doses are calculated as the weighted
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mean of all grains comprising the lowest dose sub-population, identified by inspection of radial plots
Canberra, Australia, in February 2007.
4.2.1. Air photo analysis, on-ground survey
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4.2 Recent changes to the gully complex
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and kernel density plots. The OSL analysis was undertaken at the CSIRO Black Mountain Laboratory,
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Aerial photos of the study reach in 1991 (1:48,500) were scanned and georeferenced in ArcGIS, using the 1:250,000 topographic map (MGA projection, GDA94 datum) and consistent reference points
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from overlapping areas. At least 10 ground control points per image were used, with a second order
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polynomial transformation and RMSE of less than 1m. The same process was undertaken for the 1948 (1:50,000) aerial photos, using more control points based on the previously georeferenced
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1991 aerial photos. This process was able to be completed using distinct landforms and man-made features as control points due to the absence of surveyed benchmarks. While absolute accuracies
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using this method are low, relative accuracies are high between the photography and geometric properties of mapped features making them suitable for change analysis. After individual photos had been georeferenced, they were combined into mosaics to provide complete coverage of each erosion feature.
Morphological variables which were easily identified and, therefore, could be mapped at each site were selected for detailed GIS analyses. A new GIS layer was created for each variable at each site per year (for example, VRD01 1948 badlands and 1991 badlands). The lengths of headward retreat of the scarp boundary and change in badlands and alluvial aprons were recorded at 20 points on the borders of each erosion feature, between the 1948 and 1991 layers. A ground survey was carried out during the 2006 dry season using a DGPS (5 mm vertical and horizontal accuracy). The DGPS
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that the headward retreat of the upslope boundary could be determined from 1991 to 2006.
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4.2.2. Fallout radionuclides
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If the slopes within the gully complexes have been stable for several decades they should contain an inventory of the fallout radionuclides 137Cs and 210Pb(ex). Eighteen soil samples were collected from three gully complexes (six samples per site at VRD02, VRD07, and VRD08), and two reference
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samples from the apparently undisturbed surface of the river terrace above the headcut (Ref 2 and
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Ref 7/8). Two 50 m transects were laid out at each gully complex site, with the transects crossing the scarp boundary, such that three samples were taken from within the badlands, and three upslope of
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the margin of the complex on the top of the scarp. Samples were collected from the upper 20 cm
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using steel tubes 110 mm in diameter. Samples were taken at 0, 20 and 50 m on each transect. Two reference samples were taken at VRD02 and only one sample each from VRD07 and VRD08 given
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their close proximity to one another. Radionuclide analysis was carried out at the ERISS
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environmental radioactivity laboratory, Darwin, using high resolution gamma counting.
5 Results
5.1 Analysis of historical documentation Examination of historical records shows that steep-sided channels joining the river were well established throughout the VRD prior to European settlement. These channels were called gullies by the explorers; a term with a looser application than is now the case. Descriptions indicate that the ‘gullies’ ranged in size, and often caused difficulties for the exploration party. In particular, Gregory mentions “bivouacking in a small gully” on the 24th January 1856 (Gregory and Gregory, 1884).
ACCEPTED MANUSCRIPT The extracts below are typical of the diary entries where ‘gullies’ were noted:
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“…examine the country to the southward, and followed the river through a fine grassy plain til 10.00,
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when it entered the sandstone ranges, and the valley contracted to half a mile; the hills were steep,
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but the level ground in the valley, except where intersected by gullies, was good traveling and well grassed.”
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16th January 1856 (Gregory and Gregory, 1884)
“The aneroid barometer was completely put out of adjustment by the principal lever having been
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moved from its position by a violent shake in crossing one of the deep gullies.”
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13th April 1856 (Gregory and Gregory, 1884)
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“Several tributary gullies having passed, but none worthy of special notice.”
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29th November 1855 (Gregory and Gregory, 1884)
The first two extracts are from an area near the present day Victoria River Downs homestead, while
area.
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the third excerpt is from the present day Pigeon Hole station, both south/upstream of the study
In addition to the diary kept by A.C. Gregory, several other members of the expedition made observations of their surroundings, as noted earlier. Lewis (2002) quotes from an undated letter written by J.R. Elsey to his parents while he was stationed at the Depot Camp:
“The bank on wh.(sic) we are, sloped rather Steeply to the River (sic), which is here lined with a dense thicket of mangroves, except at our landing place. This bank is traversed at frequent intervals by gullies, which carry down the waters during the rainy Season (sic), but are usually dry, & their beds
ACCEPTED MANUSCRIPT occupied by dense masses of hard tall reeds, among wh. (sic) at our first arriving, wallabies were abundant.”
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(The mention of mangroves is clearly a misidentification as the explorers were a long way from the
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sea) (Lewis, 2002).
The above descriptions clearly describe riparian channels, not major tributary streams, given that some are described as small. In the historical records, there is no mention of features that would
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now be described as badlands, even though they are likely to have received the attention of the explorers. It seems likely that the explorers were describing the FDCs as gullies rather than the outer
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erosion features. It is concluded from these sources that the FDCs predate European settlement but
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the badlands do not.
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The historical documentation provided an insight not only into whether the FDCs and badlands were
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present in the 1880s, but it also contributes to an understanding of river channel processes in the region. For example, in the final excerpt in their report, Gregory and Gregory (1884) note that the beds of the FDCs were occupied by dense masses of hard, tall reeds. This is not the case in the
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present day FDCs along the Victoria River with implications for hydrological and sediment transport/deposition processes for both the main channel and the FDCs. The walls of many of the FDCs are actively eroding near their outlets by slumping as the river widens and undercuts the walls. This has also been observed in the nearby Daly River catchment where the widening is attributed to increased discharge as rainfall has increased since the 1970s (Wasson et al., 2010). In the Victoria River this process has been aided by the loss of the dense riverbank vegetation reported by the explorers.
ACCEPTED MANUSCRIPT 5.2 Sediment dating using OSL Fig. 8 shows sections in two inset alluvial terraces, with the litho-stratigraphy of each of the sample
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sites, and the results of the OSL analyses. The deepest sample at Site 1 has an age of 81 ± 7 yrs.
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Extrapolating this value, at a constant rate of deposition between the upper and lower dates, the
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base of the inset terrace is ~90 years old. Given that the location of the sampling site is 300 m upstream of the FDC outlet, the FDC must have been well developed by the time the inset terrace
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began to accumulate ~90 years ago.
The inset alluvial terraces also indicate processes occurring upslope of the FDC ~90 years ago. Most
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of the inset terrace sediments contain re-worked carbonate nodules, the source of which is the OEF where these nodules densely cover the surface of the badlands, occurring as lag gravel derived from
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the B horizon by erosion. Therefore, the incorporation of these nodules in the Site 1 sediments
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alluvial terrace sediments.
Depth (cm)
50 40 30 20
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indicates that the OEF was well established ~90 years ago and that the OEF is the source of the inset
OSL Sampling Site 1
VRD08OSL1 71±11a
VRD08OSL3 81±7a Sand Gravel Clay
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OSL Sampling Site 2 70
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Sand Gravel River sediments
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VRD08OSL6 19 ± 5a
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Fig. 8. Litho-stratigraphy and OSL dates of inset alluvial terraces located within the FDC at site VRD08. River sediments denote the stratigraphic layer derived from the river (via overbank flooding) as opposed to upslope
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sediments.
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The inset terrace at Site 2 is only 19 ± 5 years old based on a sample taken at a depth of 58 cm (Fig.
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8), and is within the same FDC as Site 1. Terrace 2 therefore formed after the incision of the terrace at Site 1. At Site 1, the top 47.5 cm is made up of alternating layers of finely laminated sand and carbonate nodules. Sample VRD08OSL1 is from the top layer of finely laminated sand, which is 4.5
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cm thick. Sample VRD08OSL3 is from the deepest laminated sand layer at approximately 30 cm. Also at Site 2, an additional layer occurs of dark clay that is unlikely to have been derived from upslope and presumably came from the river during backing up of floodwaters (labelled ‘river sediments’ in Fig. 8). But the overwhelming proportion of the sediment in the inset terraces has the same characteristics as the soils in the OEF.
5.3 Rate of erosion – air photo analysis, on-ground survey Aerial photo analysis and on-ground survey results indicate that the OEFs are expanding upslope while the FDCs are largely stable. There has been a minor increase in FDC length (upslope) and an increase in the number of large trees within the lower parts of the FDCs since 1948. The total areas of the gully complexes at the four sites, classified as gully complexes on both 1948 and 1991 aerial
ACCEPTED MANUSCRIPT photographs, have increased over the 43-year period. The greatest increase occurred at VRD02, while VRD07 is presently the largest gully complex with an area of 1.3 km2. All of the OEFs have migrated upslope during the past 58 years (determined from air photo analysis, coupled with a 2006
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ground survey), at an average rate of approximately 1.3 m yr−1 (minimum 0.3 m yr−1 at VRD04 and
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maximum 1.5 m yr−1 at VRD07). Interestingly the four sites that have evolved from FDCs to gully complexes have migrated during the past 15 years at a rate similar to the gully complexes that were mapped as evolving between 1948 and 1991. At sites VRD01, VRD02, VRD03, VRD04 and VRD05, this
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rate has been slower during the past 15 years compared to the previous 43 years. This indicates that stabilization of the outer erosion features is underway, probably by reaching a threshold of
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catchment area and/or gradient at which upslope migration is no longer possible.
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The calculated rate of upslope retreat could be higher than the actual rate of upslope retreat in an
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easterly direction (perpendicular to the river), as measurements were taken along the entire
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complex margin, and therefore retreat may be occurring at a greater rate in a northerly or southerly direction, i.e., parallel to the river, and not truly upslope. Nonetheless each of the OEFs is continuing
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to erode into upslope river terrace surfaces, be it parallel or perpendicular to the main channel.
5.4 Fallout radionuclides The reference sample for site VRD02 (on the upper floodplain) has a
137
Cs activity of 5.6x10-5 ±
3.2x10-5 mBq cm-2, and the reference value representing both sites VRD07 andVRD08 has an activity of 7.48x10-5 ± 3.3x10-5 mBq cm-2. Both reference values are effectively zero, results that obliged a further examination of the reference sites that had been considered largely undisturbed. It was found that floods have reached ~2 m over the site, as shown by flood debris in trees, indicating that they are sites of deposition by sediment presumably unlabelled by the fallout radionuclides. Therefore the reference sites of Elliott et al. (2002) with 45.5 mBq cm-2 on hilltops that are not subject to erosion, have been adopted as the input values for this study. All samples from the OEF,
ACCEPTED MANUSCRIPT excluding one, show a near zero activity for
137
Cs, implying that erosion occurred either after or
during the 137Cs fallout period since about 1960, the earliest date for which the nuclide can now be 210
Pb(ex), all values are also
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detected. Although there is no reliable input reference value for
Cs values. Exposed tree
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roots also provide evidence that erosion is still occurring.
137
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essentially zero, a result that supports the conclusions drawn from the
6 Discussion
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The formation of gully complexes appears to have occurred by two main processes. Firstly, the FDC was probably formed during floodplain development to drain large overbank flows back to the
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channel. FDC co-evolution with the floodplain is a natural formative process. Secondly, the OEF, containing badlands and an alluvial apron, is most likely a result of runoff processes from upslope.
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Headward migration of erosion from the FDC led to the formation of the outer erosion feature,
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probably initiated from cattle tracks. It is likely that cattle favour, or use, the FDCs for gaining access
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to the river, and this was observed in the field at these field sites and others. The role of cattle as a ‘geomorphic agent’ is well documented (Trimble and Mendel, 1995). It is this hypothesised link
complex’.
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between the FDCs and the badlands that justifies the inclusion of both components in the term ‘gully
The two methods of determining the age of gully complexes produced the same result. Firstly, the analysis of historical documentation revealed numerous references to riparian gullies. The main difficulty in interpreting the historical records is how the word ‘gully’ was used, and whether the gullies referred to in the historical records are tributary channels or FDCs. It can, however, be inferred that badlands formed at a date after the first exploration of the region, and therefore after the arrival of domestic stock, given these distinctive features are not mentioned in the historical records. It is likely that the gullies described by Gregory (1884) are the FDCs, and were present prior to the arrival of stock, while the badlands formed at a later date, following the arrival of domestic stock.
ACCEPTED MANUSCRIPT OSL dating was also used to determine the age of gully complexes and the dating of inset alluvial terraces provides us with both a minimum age for the FDCs and the OEFs. The FDCs were well
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developed at the time of the inset alluvial terrace accumulation, and the outer erosion feature was
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established some 90 years ago. The FDC inset terraces may therefore be considered a pulse signal of
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increased erosion, probably as a result of cattle grazing, upslope of the FDC. The estimated basal age of the terrace of ~90 years is approximately 40 years after the introduction of cattle, indicating a significant lag before the impact of cattle became obvious, as represented in Fig. 9. This could
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indicate a period of resistance to erosion by the vegetation and soil surface, but more sample sites are needed to test for additional pulses. The incision of the alluvium within the FDC to create the
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inset terrace at Site 1 is likely to be a response to reduced sediment availability as the development of the outer erosion feature slowed, and occurred before the deposition of the terrace at Site 2
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some 19 years ago. This suggests at least two pulses of erosion and sediment delivery. The roles of variations in rainfall and grazing pressure in these pulses, and perhaps the lag between
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the arrival of cattle and the development of the badlands, are yet to be investigated. Sediment pulses after European settlement have been reported elsewhere, both within Australia (Bartley and
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Rutherfurd, 2005; Rustomji and Pietsch, 2007; Wasson et al., 1998), and internationally (Burkard and Kostaschuk, 1995). Australian sediment pulses have been linked to periods and combinations of drought, pasture degradation, and episodic rainfall events (McKeon et al., 2004).
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First pulse of sediment; OEF starts to develop
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Start of increased rainfall period
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Introduction of cattle to the VRD
Second sediment pulse 19±5 years ago
1940
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1880
2007
Fig. 9. Diagrammatic representations of sediment pulses from OEFs preserved as inset terraces in FDCs.
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The results of the radionuclide analyses, specifically near zero values for
137
Cs and
210
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confirm that erosion of the badlands has continued during the past century. The
Pb (excess),
137
Cs results
indicate that ongoing badland erosion has removed the nuclide during the fallout period, or all
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surfaces have eroded, or are eroding, after chosen, that also had near zero values for
137
Cs deposition. It is likely that the reference sites
137
Cs and
210
Pb (excess), are receiving sediment with no
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fallout radionuclide label during flood events. This conclusion implies that flood sediment that goes overbank are derived almost entirely from materials unlabelled or weakly labelled by fallout radionuclides, namely from gullies and riverbanks (cf. Wallbrink and Murray (1993)), suggesting the need for a greater understanding of gullies as a river sediment source.
On-ground and aerial photo analysis also clearly demonstrated that erosion of gully complexes is continuing. While the FDCs appear to be stable, the outer erosion features continue to develop, but at a rate that is producing less sediment as shown by comparison of aerial photographs. This conclusion supports the proposition that incision to create the inset terraces is the result of waning sediment supply. At sites where the gully complex has reached its maximum upslope threshold (area-slope threshold as per hillslope gullies; Knighton, 1998), the complex expands laterally and
ACCEPTED MANUSCRIPT deepens with the badlands increasing in local relief, with the head scarp increasing in height. Overbank flows are known to inundate and deliver sediment to the badlands, while runoff from
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rainfall events and enhanced runoff due to cattle grazing, due to the formation of tracks which
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become preferential flow paths, and denudation of vegetation, are all factors driving erosion
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processes. Groundwater seepage is also a key driver of OEF expansion following its initiation, whereby saturated sub-surface soil slumps, leaving topsoil unsupported and therefore collapsing.
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This can occur at the gully walls and heads, as shown in Fig. 10.
Fig. 10. Blowout above headscarp as a result of groundwater seepage.
It is likely that the riparian gully complexes, particularly the OEFs, along a reach of the Victoria River have been, and continue to be, a major source of sediment for the Victoria River over the past 150 years. This is consistent with sediment budgets described in other catchments in various parts of Australia (Shellberg, 2011; Wilkinson et al., 2015). While the FDCs are naturally formed, the badlands
ACCEPTED MANUSCRIPT are more recent features, the development of which is most likely the result of grazing by domestic stock with temporal variations a result of hydroclimatic fluctuations. Further investigation could
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more specifically determine the contribution of each of these factors to gully complex development,
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through field-based experiments by excluding cattle from some of the gully complexes and
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monitoring changes in erosion as rainfall changes occur, with and without cattle grazing.
7 Conclusions
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Riparian gully erosion along the Victoria River does not conform to either the morphology or documented processes driving the more commonly described hillslope gullies (Brooks et al., 2009;
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McCloskey, 2010). The gullies identified in this research have been termed ‘gully complexes’ in order to distinguish them from hillslope gullies, and they consist of a large flood drainage channel
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(FDC) and an Outer Erosion Feature (OEF). Dating of inset alluvial terraces within the FDCs provided
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a minimum age of 81 ± 7 years, which is inferred to be approximately when the OEF began to form.
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The FDCs developed in conjunction with the floodplain and pre-date the arrival of cattle. In contrast, the OEF developed in more recent times, and is linked to both cattle grazing, increased rainfall and
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groundwater seepage. There is evidence that headward expansion of the gully complexes has been slowing as they consume their catchments despite increased rainfall in the past 40 years, thereby reducing the input of sediment to the river and leading to incision of the alluvial fills within the FDCs and producing inset terraces. The search for stable reference sites for the radionuclide tracer analysis produced evidence that overbank sediments derived from the river are mostly the products of erosion of subsoils, which are likely to be exposed in gullies and channel banks. This conclusion adds weight to the importance of gullies as river sediment sources, although their contribution relative to channel banks has yet to be determined.
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gully complex, or simply excluding cattle from these areas. Further interventions may be required to
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enhance revegetation on these unstable landforms. However, as noted above, this reduction should
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be quantified in relation to the whole sediment budget, in order to prioritise management action.
Acknowledgments
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Charles Darwin University, CSIRO, Tropical Savannas Cooperative Research Centre, and the Victoria River District Conservation Association are acknowledged for their operational funding
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contributions. We thank Chunky Schultz and Scott King for field assistance, Therese Fox of ERISS for assistance with radionuclide laboratory analysis, and Dr. Tim Pietsch of CSIRO for undertaking the
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Engelen, G.B., 1973. Runoff processes and slope development in Badlands National Monument, South Dakota. Journal of Hydrology, 18(1), 55-79. Furlonger, L., 2004. Sources of Sediment in the Daly River, Northern Territory. Honours Thesis, Charles Darwin University. Gregory, A.C., Gregory, F.T., 1884. Journal of Australian Explorers. Goverment Printer, Brisbane. Howard, A.D., 1994. Badlands. In: A.D. Abrahams, A.J. Parsons (Eds.), Geomorphology of Desert Environments. Chapman and Hall, London, pp. 213-242. Karfs, R.L., 2000. Geology, Geomorphology and Soils. In: M. Kraatz (Ed.), Managing for healthy country in the VRD. Tropical Savannas CRC, Darwin, NT, pp. 9-10. Knighton, D., 1998. Fluvial Forms and Processes A New Perspective. Arnold, London, UK. Lewis, D., 2002. Slower than the Eye can See. Tropical Savannas CRC, Darwin, N.T. Leys, J.F., 1980. An Evaluation of the Effectivementss of Revegetation in Controlling Runoff Rates and Suspended Sediment Load in the Hardman Basin (NT) of the Ord River Catchments, Land Conservation Unit, Conservation Commission of the Northern Territory, Armidale, NSW. McCloskey, G.L., 2010. Riparian Erosion Morphology, Processes and Causes along the Victoria River, Northern Territory, Australia. PhD Thesis, Charles Darwin University, Darwin. McCloskey, G.L., Ellis, R., Waters, D.K., Carroll, C., 2014. Modelling reductions of pollutant loads due to improved management practices in the Great Barrier Reef Catchments: Cape York NRM region, Technical Report, Volume 2. 978-0-7345-0440-1, Queensland Department of Natural Resources and Mines, Cairns. McKeon, G.M., Hall, W.B., Henry, B.K., Stone, G.S., Watson, I.W., 2004. Pasture degredation and recovery in Australia's rangelands: Learning from History, Queensland Department of Natural REsources, Mines and Energy. Medcalf, F.G., 1944. Soil Erosion Reconnaissance of the Ord River Valley and Watershed, Western Australian Department of Land Surveys, Perth, WA. Olley, J.M., Pietsch, T., Roberts, R.G., 2004. Optical dating of Holocene sediments from a variety of geomorphic settings using single grains of quartz. Geomorphology, 60(3-4), 337-358. Olley, J.M., Wasson, R.J., 2003. Changes in the flux of sediment in the Upper Murrumbidgee catchment, Southeastern Australia, since European settlement. Hydrological Processes, 17(16), 3307-3320. Piccarreta, M., Faulkner, H., Bentivenga, M., Capolongo, D., 2006. The influence of physico-chemical material properties on erosion processes in the badlands of Basilicata, Southern Italy. Geomorphology, 81(3-4), 235-251. Poesen, J., Nachtergaele, J., Verstraeten, G., Valentin, C., 2003. Gully erosion and environmental change: Importance and research needs. Catena, 50(2-4), 91-133. Roth, C., Water, C.L.a., Australia, M.a.L., 1999. Catchment management, water quality & nutrient flows as they relate to the northern beef industry : scoping study to identify appropriate catchments for NAP3-funded research. CSIRO Land and Water?]. Rustomji, P., Pietsch, T., 2007. Alluvial sedimentation rates from southeastern Australia indicate post-European settlement landscape recovery. Geomorphology, 90(1-2), 73-90. Rustomji, P., Shellberg, J., Brooks, A., Spencer, J., Caitcheon, G., 2010. A catchment sediment and nutrient budget for the Mitchell River, Queensland. A report to the Tropical Rivers and Coastal Knowledge (TRaCK) Research Program, CSIRO Water for a Healthy Country National Research Flagship. Rutherfurd, I.D., 2000. Some Human Impacts on Australian Stream Morphology. In: S. Brizga, G.L. Finlayson (Eds.), River Management: The Australasian Experience. John Wiley and Sons Ltd, Chichester, pp. 11-49. Shellberg, J.G., 2011. Alluvial Gully Erosion Rates and Processes Across the Mitchell River Fluvial Megafan in Northern Queensland, Australia. PhD, Griffith University. Shellberg, J.G., Brooks, A.P., Rose, C.W., 2013. Sediment production and yield from an alluvial gully in Northern Queensland, Australia. Earth Surface Processes and Landforms, 38(15), 1765-1778.
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Sidorchuk, A., 2006. Stages in gully evolution and self-organized criticality. Earth Surface Processes and Landforms, 31(11), 1329-1344. Tindall, D., Marchand, B., Gilad, U., Goodwin, N., Byer, S., Denham, R., 2014. Gully mapping and drivers in the grazing lands of the Burdekin catchment. RP66G Summary Report., Queensland Department of Science, Information Technology, Innovation and the Arts, Brisbane, Australia. Trimble, S.W., Mendel, A.C., 1995. The cow as a geomorphic agent - A critical review. Geomorphology, 13(1-4), 233-253. Vandekerckhove, L., Poesen, J., Wijdenes, D.O., Nachtergaele, J., Kosmas, C., Roxo, M.J., De Figueiredo, T., 2000. Thresholds for gully initiation and sedimentation in Mediterranean Europe. Earth Surface Processes and Landforms, 25(11), 1201-1220. Wallbrink, P.J., Murray, A.S., 1993. Use of fallout radionuclides as indicators of erosion processes. Hydrological Processes, 7(3), 297-304. Wasson, R.J., Caitcheon, G., Murray, A.S., McCulloch, M., Quade, J., 2002. Sourcing sediment using multiple tracers in the catchment of Lake Argyle, Northwestern Australia. Environmental management, 29(5), 634-646. Wasson, R.J., Furlonger, L., Parry, D., Pietsch, T., Valentine, E., Williams, D., 2010. Sediment sources and channel dynamics, Daly River, Northern Australia. Geomorphology, 114(3), 161-174. Wasson, R.J., Mazari, R.K., Starr, B., Clifton, G., 1998. The recent history of erosion and sedimentation on the Southern Tablelands of southeastern Australia: sediment flux dominated by channel incision. Geomorphology, 24(4), 291-308. Waters, D.K., Carroll, C., Ellis, R., Hateley, L.R., McCloskey, G.L., Packett, R., Dougall, C., Fentie, B., 2014. Modelling reductions of pollutant loads due to improved management practices in the Great Barrier Reef Catchments: Whole of GBR, Technical Report, Volume 1. 978-1-74230999, Queensland Department of Natural Resources and Mines, Brisbane. Wilkinson, S.N., Bartley, R., Hairsine, P.B., Bui, E.N., Gregory, L., Henderson, A.E., 2015. Managing gully erosion as an efficient approach to improving water quality in the Great Barrier Reef Lagoon. Report to the Department of Environment., Austalia. Zucca, C., Canu, A., Della Peruta, R., 2006. Spatial distribution and morphological features of gullies in an agropastoral area in Sardinia, Italy. International Journal of Sediment Research, 21(2), 132-144.
Highlights A previously undescribed erosion feature is identified and characterised. The Outer Erosion Feature is of post-settlement origin, containing badlands. The Flood Drainage Channel component is a natural feature. Managing stock will slow badland erosion and sediment input to the river.
S0169-555X(16)30311-7 doi: 10.1016/j.geomorph.2016.05.009 GEOMOR 5603
To appear in:
Geomorphology
Received date: Revised date: Accepted date:
16 November 2015 3 May 2016 3 May 2016
Please cite this article as: McCloskey, G.L., Wasson, R.J., Boggs, G.S., Douglas, M., Timing and causes of gully erosion in the riparian zone of the semi-arid tropical Victoria River, Australia: Management implications, Geomorphology (2016), doi: 10.1016/j.geomorph.2016.05.009
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Timing and causes of gully erosion in the riparian zone of the semi-arid tropical Victoria River, Australia: management implications
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McCloskey, G.L.a,, Wasson, R.J.b,c, Boggs, G.S.c,d, and Douglas, M.c,e
Department of Natural Resources and Mines, PO Box 937, Cairns, Qld 4870, Australia
b
Institute of Water Policy, Lee Kuan Yew School of Public Policy, National University of Singapore,
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a
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Singapore
Charles Darwin University, Darwin, NT 0909, Australia
d
Wheatbelt NRM, Northam, WA 6401, Australia
e
School of Earth and Environment, University of Western Australia, Perth, WA 6009, Australia
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c
Abstract
Gully erosion in the seasonally wet tropics of Australia is a major source of sediment in rivers. Stabilization of gullies to reduce impacts on aquatic ecosystems and water storages is a focus for management. However the cause of the gully erosion is poorly understood and so a critical context for soil conservation is missing. It is uncertain if they are the result of post-European cattle grazing or are they much older and related to non-human factors. The causes of riparian gully erosion along a reach of the Victoria River in the semi-arid tropics of Australia were investigated using several methods. Gully complexes were described and characterised by two major components: a Flood
Corresponding Author: Tel +61 7 4222 5447 Fax +61 7 4222 5493 email: [email protected]
ACCEPTED MANUSCRIPT Drainage Channel (FDC) and upslope of this an Outer Erosion Feature (OEF) characterised by badlands set within an amphitheatre. The OEF is likely to be a major source of sediment that appears
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to be of recent origin. A review of historical records, combined with Optically Stimulated
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Luminescence (OSL) dating, showed that the FDCs were well established prior to the introduction of
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domestic stock. It also showed that the badlands began to develop about 90 years ago; that is, about 40 years after the arrival of domestic stock. In addition, an analysis of aerial photos coupled with an on-ground survey and analysis of fallout radionuclides revealed that erosion processes are still active
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within the gully complexes. While the FDCs are natural drainage channels, cattle grazing probably triggered the badland formation, with the expansion aided by increased rainfall in the past 40 years.
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Therefore the OEFs are of human origin and protection from grazing of the riparian zone should slow
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badland erosion and reduce sediment input to the river.
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Key Words
caesium-137
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Victoria River, gully erosion, age of gullies, land use, Optically Stimulated Luminescence (OSL),
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1 Introduction
The triggers and controls of gully erosion are poorly understood in tropical and sub-tropical environments, partly because there are few studies of this question. For the same reason, these processes, and the impacts of land use on these processes, are not as well understood in northern Australia as they are in a temperate zone. Recent interest in northern Australia as a potential ‘foodbowl’ (Anon, 2013), as well as investments by land managers and funding bodies in improved land management practices, have resulted in a renewed research focus in the region. Riparian erosion is a phenomenon that warrants further investigation, since impacts from future land use development could have significant consequences for river systems, such as habitat loss and reduced water storage capacity in existing and future reservoirs. The research in this paper focuses on riparian
ACCEPTED MANUSCRIPT gully erosion along a reach of the Victoria River, northern Australia. The causes of the current erosion features, as well as processes driving continued erosion, are determined to provide an input
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to improved catchment management.
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Gully erosion has long been considered a major source of sediment to streams and receiving waters
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in both temperate and tropical Australian rivers (Bartley et al., 2007; Olley and Wasson, 2003; Rutherfurd, 2000; Wasson et al., 2002). Furthermore, an increase in erosion/sediment yield since European settlement is widely reported, although most research has been done in southern
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Australia (Rustomji and Pietsch, 2007), with an expanding body of work in the Great Barrier Reef
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catchments (McCloskey et al., 2014; Tindall et al., 2014; Waters et al., 2014). While research into erosion processes in northern Australia is not uncommon (Brooks et al., 2009; Rustomji et al., 2010;
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Shellberg, 2011; Shellberg et al., 2013; Wilkinson et al., 2015), the links to land use have not been
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fully investigated. It is important for future planning and management in the region to determine the processes of gully initiation and development in northern Australia, and what impact, if any, land use
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has had on these processes.
A range of gully forms has been described in the literature, most of which are ‘hillslope gullies’.
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More generally, gullies are defined as larger than rills, but smaller than river channels or streams (Knighton, 1998). Common features of hillslope gullies include steep sides and a steeply sloping, or near vertical, head scarp (Knighton, 1998). In addition, hillslope gullies are typically longer than they are wide. Hillslope gully erosion is a threshold phenomenon as a function of slope and catchment area (Knighton, 1998), and is initiated when the characteristics of flow (shear stress) exceed a critical value (Sidorchuk, 2006). Gullies identified in tropical climates of northern Australia do not fit the description of hillslope gullies, and have alternatively been called ‘alluvial gullies’ (Brooks et al., 2009; Shellberg, 2011), or, as referred to in this paper, ‘gully complexes’ (McCloskey, 2010). Riparian gully erosion has been described in other areas of northern Australia, including the Fitzroy and Ord River catchments in north-Western Australia (Wasson et al., 2002), rangelands along the
ACCEPTED MANUSCRIPT mid-Western Australian coast (Blandford, 1979; Branch, 1981; Leys, 1980; Medcalf, 1944), and more recently, the Daly River catchment in the Northern Territory (Furlonger, 2004; Wasson et al., 2010).
The early Western Australian studies (Blandford,
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this identified riparian gullies (Condon, 1986).
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A reconnaissance erosion survey was also conducted in the Victoria River District in the 1980s and
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1979; Branch, 1981; Leys, 1980; Medcalf, 1944) clearly suggest that overstocking with cattle, which reduces the resistance to erosion by removing vegetation and disturbing the soil, and rainfall-runoff drives erosion. More recent research suggests that groundwater seepage should be added as a
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formative process (Poesen et al., 2003). Overstocking has also been considered to be a cause of gully development in Kenya and Mediterranean Europe (Vandekerckhove et al., 2000; Zucca et al.,
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2006).
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In this paper, we evaluate the role of land use by cattle grazing in riparian gully complex erosion
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along a reach of the Victoria River, northern Australia. Gully complex erosion processes are also
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inferred, and recommendations for management are made. This research will contribute to filling the current gap in northern Australian erosion studies, particularly in the semi-arid tropics, and will
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add to international understanding of this important phenomenon.
2 The Victoria River catchment The main stem of the Victoria River is ~700 km long, and is located approximately 500 km southwest of Darwin, the capital city of the Northern Territory. It has a catchment area of 125,000 km2 (Fig. 1). The focus of this study is a ~20 km reach in the mid-catchment (Fig. 1). The Victoria River District (VRD) is in the semi-arid tropics, with a seasonal rainfall pattern which has 90–95% of annual precipitation between November and April during the wet season (Roth et al., 1999). Mean annual rainfall ranges from 500 mm in the south of the catchment, to over 1,000 mm in the north. The northern VRD experiences tropical cyclones, while the southern VRD may only experience the remnants of tropical cyclones as heavy rain associated with tropical lows that have decayed from
ACCEPTED MANUSCRIPT cyclones. At the Victoria River Downs rainfall station, the long-term (over 120 years of recorded data) annual average rainfall is 658 ± 19 mm, while the average for the past 40 years at the same
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location is 787 ± 5 mm (Fig. 2). Texture contrast soils occur along channel banks and levees of major
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drainage lines, and these are susceptible to erosion (Karfs, 2000). Deeper loamy and clay-rich soils
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are present on the poorly drained flatter areas, and shallow skeletal soils are present on the steep hilly areas with rocky outcrops (Karfs, 2000).
Cattle were introduced to the VRD in the late 1800s and cattle grazing has been the primary land use
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for over 100 years. There are presently 30 operating cattle stations ranging in size from 1,100 to
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12,000 km2. Generally the stocking rate is low, averaging eight head of cattle per km2. However, in the past stock numbers were difficult to control and monitor, and were almost certainly
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considerably higher. Similarly, there is spatial heterogeneity of stocking rates, both in the past and
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present, whereby the greatest stock numbers are in the ‘best’ paddocks, generally including the
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riparian zone of the Victoria River.
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Fig. 1. Location of the Victoria River Catchment and the study reach (in oval).
The study reach was chosen because it is representative of gully complexes along the river, and accessible for fieldwork. The reach contains eight gully complexes at varying stages of development, evident in aerial photo analysis. The gully complexes occur on grey and brown clay soils associated with the Cambrian Antrim Plateau Volcanics. All of the gully complexes are located on the eastern (right) bank of the Victoria River, and continue to be grazed by cattle.
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Flood Drainage Channel (FDC)
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Alluvial apron
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Badlands
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Fig. 2. Long-term annual rainfall totals (blue) at Victoria River Downs station and the 20 year moving average (black). The average annual rainfall has increased over the past 40 years.
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3 Gully complexes
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Gully complexes have two components. The first is a large sinuous and steep-sided channel that
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begins on the floodplain and terminates at the river, hereafter termed a Flood Drainage Channel (FDC), which drains overbank floodwater back to the river. The second is a dendritic gully network and badlands that have developed upslope from the head of an FDC, forming an amphitheatre with
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a distinct head scarp, and hereafter called the outer erosion feature (OEF) (McCloskey, 2010). Typically, the OEFs are wider than they are long, unlike hillslope gullies. Fig. 3 is an aerial view of site
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VRD01, a well-developed gully complex, while Fig. 4 provides a diagrammatic representation of a typical gully complex found in the VRD. It could be argued that the only gullies in these complexes
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are in the OEF and to include the FDCs in the description is confusing. The FDCs are included,
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however, because of the apparent link with the badlands and gullies upslope, a relationship
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described in more detail below, and their gully likeness.
Gully complexes, or similar features from northern Australia, have only recently been described in
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the literature. The most recent example is that of alluvial gullies in the Mitchell River catchment, Queensland, by Brooks et al. (2009). They define alluvial gullies in that catchment as “young incisional features entrenched into alluvium not previously incised since initial deposition”. Characteristic features of alluvial gullies identified by Brooks et al. (2009) include: a distinct head scarp, the height of which increases with local relief; a dense network of rills and gullies, nested within a macro-gully network; and width often larger than length. They propose a continuum of gully form, from hillslope (or colluvial) gullies to alluvial gullies. It is likely that the gully complexes identified along the Victoria River lie in the mid-range of the continuum tending towards the alluvial gully end of the spectrum, as they demonstrate some similarities with hillslope gullies.
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Terrace Margin
X
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Gully/Rill Network
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Outer Erosion Feature (OEF)
Interfluves
Alluvial apron
Flood Drainage Channel (FDC)
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Headwall
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Floodplain
Victoria River
Fig. 4. Diagrammatic representation of a typical gully complex.
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Fig.5. Field photographs. a) Interfluves and rills/small gullies making up the badlands. b) View across the alluvial apron. c) View within the Flood Drainage Channel (FDC) at site VRD01.
Badlands are contained within the gully complex, though the entire complex cannot be called a badlands. Characteristic features of badlands are steep slopes, high drainage density, a lack of vegetation, thin and highly dispersive regolith, and rapid erosion rates (Howard, 1994; Piccarreta et al., 2006). The badland slopes within the gully complexes have convex gentle slopes, similar to the badlands described for the Mancos Shale, Utah (Howard, 1994), or the biancane form described c throughout Italy (Piccarreta et al., 2006). The Victoria River gully complexes therefore are morphologically badlands. Another characteristic feature of badlands is the alluvial surface (alluvial apron in Fig. 4), at the downslope edge of the badland areas. Howard (1994) describes this surface
ACCEPTED MANUSCRIPT as one of transportation (and some deposition), with the gradient determined by the relationship between sediment load and discharge. Runoff from the badlands at the study site flows through a
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series of rills and small gullies located within the alluvial apron, and then into the FDC. This
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description fits more closely with that of Engelen (1973), who notes that the lower part of the
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pediment (alluvial apron) is either dissected by headward cutting channels of a larger ephemeral stream downstream, or it is of low gradient without pronounced channels.
In addition, the
deposited sediments on the alluvial apron are likely temporary, and will be partly transported during
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the next major runoff event (Engelen, 1973). Brooks et al. (2009) oppose the use of the term ‘badlands’ in describing the alluvial gullies found in the Mitchell River catchment. They argue that
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‘badlands’ is not an appropriate term for describing alluvial gullies, as the processes inferred by the term badlands (involving uplift or re-exposure due to base level change, or formation in soft rock
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terrain) do not accurately describe the processes occurring in the Mitchell River alluvial gullies.
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Further, the alluvial gullies described by (Brooks et al., 2009) form exclusively on alluvial surfaces
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while badlands can occur away from a channel and on colluvial surfaces. We argue that the term badlands is a morphological concept and does not denote specific processes of formation or regolith
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type.
4 Methods
Determining the age of the outer erosion features is critical to understanding what caused them. If they existed before the introduction of cattle then overgrazing by domestic stock cannot be the cause. Four methods were used in this study, two to determine the age of gully complexes and two to calculate recent changes. The first step (I) in assessing gully complex age was sourcing historical documentation, in the form of the diaries of the first European explorers to the region, and these were analysed for references to gully erosion (Gregory and Gregory, 1884). The second method (II) used Optically Stimulated Luminescence (OSL) to date sediments within the FDCs that are believed to be coeval with the development of the outer erosion feature. The dates also provide minimum
ACCEPTED MANUSCRIPT ages for the FDCs. Methods to determine recent changes to gully complexes included (III) aerial photo analysis coupled with on-ground survey (III), as well as the use of fallout radionuclides (137Cs
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and 210Pb) (IV).
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4.1 Age of gully complexes
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4.1.1. Historical documentation
The diary of the first European explorer of the region, A.C. Gregory, provides the primary source of historical documentation. Several other members of the Gregory expedition, which occurred from
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1854–1856, also kept records, or wrote letters home, and these also proved a valuable source of
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information, especially those of J.R. Elsey, the surgeon and naturalist appointed to the expedition. The documentation was reviewed for references to gully erosion, as well as descriptions of the main
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river channel, including vegetation, bank steepness, and bank composition.
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4.1.2. Sediment dating using OSL
Inset alluvial terraces were identified in many of the FDCs, and several were identified within the
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FDC at VRD08, of which two were selected for OSL dating. The inset terraces were deposited after the FDC formed, and can therefore provide a minimum age for the FDC. The terraces consist of
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material derived mostly from upslope rather than from the river and therefore represent a sediment input from the outer erosion feature, followed by incision. Three samples were taken from two terraces, located at S 16°12.887’ E 131°09.216’ and S 16°12.903’ E 131°09.190’ respectively (Fig. 6). A more comprehensive sampling regime could not be undertaken due to budget constraints. The excavated face of one site is presented in Fig. 7 and sections of the sampling sites and the lithostratigraphy of both sites are presented in Fig. 8. Each sample was taken by inserting a 30 mm diameter light safe steel pipe into finely laminated sand layers. The pipes were excavated and each end of the pipes was secured by using a PVC cap and tape. An additional 1 kg sample of surrounding sediment was also taken for dosimetry.
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Head scarp Alluvial apron
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Elevation (m)
FDC
Sample Site 1 Sample Site 2
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110 105
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200
400
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100 600
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1000
1200
River
1400
Distance (m)
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Fig. 6. Location of OSL sampling sites on a longitudinal profile of the VRD08 gully complex.
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Fig. 7. VRD08 OSL Site 2. (a) Excavated face of sample location. Depth of face is 60 cm, and sample VRD08OSL6 was taken at a depth of 55 cm (b). Clearly visible stratification.
Sample preparation was designed to isolate pure extract of 180-212 µm light safe quartz grains following standard procedures (Aitken, 1998). Treatments were applied to remove contaminant carbonates, feldspars, organics, heavy minerals and acid soluble fluorides. The outer ~10 m alphairradiated rind of each grain was removed by double etching each sample in 48 % Hydrofluoric Acid.
ACCEPTED MANUSCRIPT Burial doses were determined from measurement of the OSL signals emitted by single grains of quartz. Equivalent doses (De) were determined using a modified SAR protocol (Olley et al., 2004). A
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dose-response curve was constructed for each grain. Burial doses are calculated as the weighted
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mean of all grains comprising the lowest dose sub-population, identified by inspection of radial plots
Canberra, Australia, in February 2007.
4.2.1. Air photo analysis, on-ground survey
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4.2 Recent changes to the gully complex
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and kernel density plots. The OSL analysis was undertaken at the CSIRO Black Mountain Laboratory,
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Aerial photos of the study reach in 1991 (1:48,500) were scanned and georeferenced in ArcGIS, using the 1:250,000 topographic map (MGA projection, GDA94 datum) and consistent reference points
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from overlapping areas. At least 10 ground control points per image were used, with a second order
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polynomial transformation and RMSE of less than 1m. The same process was undertaken for the 1948 (1:50,000) aerial photos, using more control points based on the previously georeferenced
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1991 aerial photos. This process was able to be completed using distinct landforms and man-made features as control points due to the absence of surveyed benchmarks. While absolute accuracies
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using this method are low, relative accuracies are high between the photography and geometric properties of mapped features making them suitable for change analysis. After individual photos had been georeferenced, they were combined into mosaics to provide complete coverage of each erosion feature.
Morphological variables which were easily identified and, therefore, could be mapped at each site were selected for detailed GIS analyses. A new GIS layer was created for each variable at each site per year (for example, VRD01 1948 badlands and 1991 badlands). The lengths of headward retreat of the scarp boundary and change in badlands and alluvial aprons were recorded at 20 points on the borders of each erosion feature, between the 1948 and 1991 layers. A ground survey was carried out during the 2006 dry season using a DGPS (5 mm vertical and horizontal accuracy). The DGPS
ACCEPTED MANUSCRIPT survey was conducted around both the boundary of the gully complexes and the main FDC to its outlet at the river. The results were incorporated into the GIS aerial photo-mapping environment, so
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that the headward retreat of the upslope boundary could be determined from 1991 to 2006.
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4.2.2. Fallout radionuclides
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If the slopes within the gully complexes have been stable for several decades they should contain an inventory of the fallout radionuclides 137Cs and 210Pb(ex). Eighteen soil samples were collected from three gully complexes (six samples per site at VRD02, VRD07, and VRD08), and two reference
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samples from the apparently undisturbed surface of the river terrace above the headcut (Ref 2 and
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Ref 7/8). Two 50 m transects were laid out at each gully complex site, with the transects crossing the scarp boundary, such that three samples were taken from within the badlands, and three upslope of
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the margin of the complex on the top of the scarp. Samples were collected from the upper 20 cm
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using steel tubes 110 mm in diameter. Samples were taken at 0, 20 and 50 m on each transect. Two reference samples were taken at VRD02 and only one sample each from VRD07 and VRD08 given
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their close proximity to one another. Radionuclide analysis was carried out at the ERISS
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environmental radioactivity laboratory, Darwin, using high resolution gamma counting.
5 Results
5.1 Analysis of historical documentation Examination of historical records shows that steep-sided channels joining the river were well established throughout the VRD prior to European settlement. These channels were called gullies by the explorers; a term with a looser application than is now the case. Descriptions indicate that the ‘gullies’ ranged in size, and often caused difficulties for the exploration party. In particular, Gregory mentions “bivouacking in a small gully” on the 24th January 1856 (Gregory and Gregory, 1884).
ACCEPTED MANUSCRIPT The extracts below are typical of the diary entries where ‘gullies’ were noted:
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“…examine the country to the southward, and followed the river through a fine grassy plain til 10.00,
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when it entered the sandstone ranges, and the valley contracted to half a mile; the hills were steep,
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but the level ground in the valley, except where intersected by gullies, was good traveling and well grassed.”
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16th January 1856 (Gregory and Gregory, 1884)
“The aneroid barometer was completely put out of adjustment by the principal lever having been
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moved from its position by a violent shake in crossing one of the deep gullies.”
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13th April 1856 (Gregory and Gregory, 1884)
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“Several tributary gullies having passed, but none worthy of special notice.”
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29th November 1855 (Gregory and Gregory, 1884)
The first two extracts are from an area near the present day Victoria River Downs homestead, while
area.
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the third excerpt is from the present day Pigeon Hole station, both south/upstream of the study
In addition to the diary kept by A.C. Gregory, several other members of the expedition made observations of their surroundings, as noted earlier. Lewis (2002) quotes from an undated letter written by J.R. Elsey to his parents while he was stationed at the Depot Camp:
“The bank on wh.(sic) we are, sloped rather Steeply to the River (sic), which is here lined with a dense thicket of mangroves, except at our landing place. This bank is traversed at frequent intervals by gullies, which carry down the waters during the rainy Season (sic), but are usually dry, & their beds
ACCEPTED MANUSCRIPT occupied by dense masses of hard tall reeds, among wh. (sic) at our first arriving, wallabies were abundant.”
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(The mention of mangroves is clearly a misidentification as the explorers were a long way from the
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sea) (Lewis, 2002).
The above descriptions clearly describe riparian channels, not major tributary streams, given that some are described as small. In the historical records, there is no mention of features that would
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now be described as badlands, even though they are likely to have received the attention of the explorers. It seems likely that the explorers were describing the FDCs as gullies rather than the outer
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erosion features. It is concluded from these sources that the FDCs predate European settlement but
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the badlands do not.
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The historical documentation provided an insight not only into whether the FDCs and badlands were
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present in the 1880s, but it also contributes to an understanding of river channel processes in the region. For example, in the final excerpt in their report, Gregory and Gregory (1884) note that the beds of the FDCs were occupied by dense masses of hard, tall reeds. This is not the case in the
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present day FDCs along the Victoria River with implications for hydrological and sediment transport/deposition processes for both the main channel and the FDCs. The walls of many of the FDCs are actively eroding near their outlets by slumping as the river widens and undercuts the walls. This has also been observed in the nearby Daly River catchment where the widening is attributed to increased discharge as rainfall has increased since the 1970s (Wasson et al., 2010). In the Victoria River this process has been aided by the loss of the dense riverbank vegetation reported by the explorers.
ACCEPTED MANUSCRIPT 5.2 Sediment dating using OSL Fig. 8 shows sections in two inset alluvial terraces, with the litho-stratigraphy of each of the sample
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sites, and the results of the OSL analyses. The deepest sample at Site 1 has an age of 81 ± 7 yrs.
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Extrapolating this value, at a constant rate of deposition between the upper and lower dates, the
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base of the inset terrace is ~90 years old. Given that the location of the sampling site is 300 m upstream of the FDC outlet, the FDC must have been well developed by the time the inset terrace
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began to accumulate ~90 years ago.
The inset alluvial terraces also indicate processes occurring upslope of the FDC ~90 years ago. Most
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of the inset terrace sediments contain re-worked carbonate nodules, the source of which is the OEF where these nodules densely cover the surface of the badlands, occurring as lag gravel derived from
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the B horizon by erosion. Therefore, the incorporation of these nodules in the Site 1 sediments
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alluvial terrace sediments.
Depth (cm)
50 40 30 20
10 0
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indicates that the OEF was well established ~90 years ago and that the OEF is the source of the inset
OSL Sampling Site 1
VRD08OSL1 71±11a
VRD08OSL3 81±7a Sand Gravel Clay
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OSL Sampling Site 2 70
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Sand Gravel River sediments
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40 30
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Depth (cm)
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20 10
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VRD08OSL6 19 ± 5a
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Fig. 8. Litho-stratigraphy and OSL dates of inset alluvial terraces located within the FDC at site VRD08. River sediments denote the stratigraphic layer derived from the river (via overbank flooding) as opposed to upslope
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sediments.
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The inset terrace at Site 2 is only 19 ± 5 years old based on a sample taken at a depth of 58 cm (Fig.
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8), and is within the same FDC as Site 1. Terrace 2 therefore formed after the incision of the terrace at Site 1. At Site 1, the top 47.5 cm is made up of alternating layers of finely laminated sand and carbonate nodules. Sample VRD08OSL1 is from the top layer of finely laminated sand, which is 4.5
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cm thick. Sample VRD08OSL3 is from the deepest laminated sand layer at approximately 30 cm. Also at Site 2, an additional layer occurs of dark clay that is unlikely to have been derived from upslope and presumably came from the river during backing up of floodwaters (labelled ‘river sediments’ in Fig. 8). But the overwhelming proportion of the sediment in the inset terraces has the same characteristics as the soils in the OEF.
5.3 Rate of erosion – air photo analysis, on-ground survey Aerial photo analysis and on-ground survey results indicate that the OEFs are expanding upslope while the FDCs are largely stable. There has been a minor increase in FDC length (upslope) and an increase in the number of large trees within the lower parts of the FDCs since 1948. The total areas of the gully complexes at the four sites, classified as gully complexes on both 1948 and 1991 aerial
ACCEPTED MANUSCRIPT photographs, have increased over the 43-year period. The greatest increase occurred at VRD02, while VRD07 is presently the largest gully complex with an area of 1.3 km2. All of the OEFs have migrated upslope during the past 58 years (determined from air photo analysis, coupled with a 2006
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ground survey), at an average rate of approximately 1.3 m yr−1 (minimum 0.3 m yr−1 at VRD04 and
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maximum 1.5 m yr−1 at VRD07). Interestingly the four sites that have evolved from FDCs to gully complexes have migrated during the past 15 years at a rate similar to the gully complexes that were mapped as evolving between 1948 and 1991. At sites VRD01, VRD02, VRD03, VRD04 and VRD05, this
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rate has been slower during the past 15 years compared to the previous 43 years. This indicates that stabilization of the outer erosion features is underway, probably by reaching a threshold of
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catchment area and/or gradient at which upslope migration is no longer possible.
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The calculated rate of upslope retreat could be higher than the actual rate of upslope retreat in an
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easterly direction (perpendicular to the river), as measurements were taken along the entire
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complex margin, and therefore retreat may be occurring at a greater rate in a northerly or southerly direction, i.e., parallel to the river, and not truly upslope. Nonetheless each of the OEFs is continuing
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to erode into upslope river terrace surfaces, be it parallel or perpendicular to the main channel.
5.4 Fallout radionuclides The reference sample for site VRD02 (on the upper floodplain) has a
137
Cs activity of 5.6x10-5 ±
3.2x10-5 mBq cm-2, and the reference value representing both sites VRD07 andVRD08 has an activity of 7.48x10-5 ± 3.3x10-5 mBq cm-2. Both reference values are effectively zero, results that obliged a further examination of the reference sites that had been considered largely undisturbed. It was found that floods have reached ~2 m over the site, as shown by flood debris in trees, indicating that they are sites of deposition by sediment presumably unlabelled by the fallout radionuclides. Therefore the reference sites of Elliott et al. (2002) with 45.5 mBq cm-2 on hilltops that are not subject to erosion, have been adopted as the input values for this study. All samples from the OEF,
ACCEPTED MANUSCRIPT excluding one, show a near zero activity for
137
Cs, implying that erosion occurred either after or
during the 137Cs fallout period since about 1960, the earliest date for which the nuclide can now be 210
Pb(ex), all values are also
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detected. Although there is no reliable input reference value for
Cs values. Exposed tree
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roots also provide evidence that erosion is still occurring.
137
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essentially zero, a result that supports the conclusions drawn from the
6 Discussion
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The formation of gully complexes appears to have occurred by two main processes. Firstly, the FDC was probably formed during floodplain development to drain large overbank flows back to the
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channel. FDC co-evolution with the floodplain is a natural formative process. Secondly, the OEF, containing badlands and an alluvial apron, is most likely a result of runoff processes from upslope.
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Headward migration of erosion from the FDC led to the formation of the outer erosion feature,
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probably initiated from cattle tracks. It is likely that cattle favour, or use, the FDCs for gaining access
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to the river, and this was observed in the field at these field sites and others. The role of cattle as a ‘geomorphic agent’ is well documented (Trimble and Mendel, 1995). It is this hypothesised link
complex’.
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between the FDCs and the badlands that justifies the inclusion of both components in the term ‘gully
The two methods of determining the age of gully complexes produced the same result. Firstly, the analysis of historical documentation revealed numerous references to riparian gullies. The main difficulty in interpreting the historical records is how the word ‘gully’ was used, and whether the gullies referred to in the historical records are tributary channels or FDCs. It can, however, be inferred that badlands formed at a date after the first exploration of the region, and therefore after the arrival of domestic stock, given these distinctive features are not mentioned in the historical records. It is likely that the gullies described by Gregory (1884) are the FDCs, and were present prior to the arrival of stock, while the badlands formed at a later date, following the arrival of domestic stock.
ACCEPTED MANUSCRIPT OSL dating was also used to determine the age of gully complexes and the dating of inset alluvial terraces provides us with both a minimum age for the FDCs and the OEFs. The FDCs were well
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developed at the time of the inset alluvial terrace accumulation, and the outer erosion feature was
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established some 90 years ago. The FDC inset terraces may therefore be considered a pulse signal of
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increased erosion, probably as a result of cattle grazing, upslope of the FDC. The estimated basal age of the terrace of ~90 years is approximately 40 years after the introduction of cattle, indicating a significant lag before the impact of cattle became obvious, as represented in Fig. 9. This could
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indicate a period of resistance to erosion by the vegetation and soil surface, but more sample sites are needed to test for additional pulses. The incision of the alluvium within the FDC to create the
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inset terrace at Site 1 is likely to be a response to reduced sediment availability as the development of the outer erosion feature slowed, and occurred before the deposition of the terrace at Site 2
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some 19 years ago. This suggests at least two pulses of erosion and sediment delivery. The roles of variations in rainfall and grazing pressure in these pulses, and perhaps the lag between
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the arrival of cattle and the development of the badlands, are yet to be investigated. Sediment pulses after European settlement have been reported elsewhere, both within Australia (Bartley and
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Rutherfurd, 2005; Rustomji and Pietsch, 2007; Wasson et al., 1998), and internationally (Burkard and Kostaschuk, 1995). Australian sediment pulses have been linked to periods and combinations of drought, pasture degradation, and episodic rainfall events (McKeon et al., 2004).
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First pulse of sediment; OEF starts to develop
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Start of increased rainfall period
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Introduction of cattle to the VRD
Second sediment pulse 19±5 years ago
1940
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1880
2007
Fig. 9. Diagrammatic representations of sediment pulses from OEFs preserved as inset terraces in FDCs.
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The results of the radionuclide analyses, specifically near zero values for
137
Cs and
210
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confirm that erosion of the badlands has continued during the past century. The
Pb (excess),
137
Cs results
indicate that ongoing badland erosion has removed the nuclide during the fallout period, or all
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surfaces have eroded, or are eroding, after chosen, that also had near zero values for
137
Cs deposition. It is likely that the reference sites
137
Cs and
210
Pb (excess), are receiving sediment with no
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fallout radionuclide label during flood events. This conclusion implies that flood sediment that goes overbank are derived almost entirely from materials unlabelled or weakly labelled by fallout radionuclides, namely from gullies and riverbanks (cf. Wallbrink and Murray (1993)), suggesting the need for a greater understanding of gullies as a river sediment source.
On-ground and aerial photo analysis also clearly demonstrated that erosion of gully complexes is continuing. While the FDCs appear to be stable, the outer erosion features continue to develop, but at a rate that is producing less sediment as shown by comparison of aerial photographs. This conclusion supports the proposition that incision to create the inset terraces is the result of waning sediment supply. At sites where the gully complex has reached its maximum upslope threshold (area-slope threshold as per hillslope gullies; Knighton, 1998), the complex expands laterally and
ACCEPTED MANUSCRIPT deepens with the badlands increasing in local relief, with the head scarp increasing in height. Overbank flows are known to inundate and deliver sediment to the badlands, while runoff from
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rainfall events and enhanced runoff due to cattle grazing, due to the formation of tracks which
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become preferential flow paths, and denudation of vegetation, are all factors driving erosion
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processes. Groundwater seepage is also a key driver of OEF expansion following its initiation, whereby saturated sub-surface soil slumps, leaving topsoil unsupported and therefore collapsing.
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This can occur at the gully walls and heads, as shown in Fig. 10.
Fig. 10. Blowout above headscarp as a result of groundwater seepage.
It is likely that the riparian gully complexes, particularly the OEFs, along a reach of the Victoria River have been, and continue to be, a major source of sediment for the Victoria River over the past 150 years. This is consistent with sediment budgets described in other catchments in various parts of Australia (Shellberg, 2011; Wilkinson et al., 2015). While the FDCs are naturally formed, the badlands
ACCEPTED MANUSCRIPT are more recent features, the development of which is most likely the result of grazing by domestic stock with temporal variations a result of hydroclimatic fluctuations. Further investigation could
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more specifically determine the contribution of each of these factors to gully complex development,
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through field-based experiments by excluding cattle from some of the gully complexes and
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monitoring changes in erosion as rainfall changes occur, with and without cattle grazing.
7 Conclusions
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Riparian gully erosion along the Victoria River does not conform to either the morphology or documented processes driving the more commonly described hillslope gullies (Brooks et al., 2009;
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McCloskey, 2010). The gullies identified in this research have been termed ‘gully complexes’ in order to distinguish them from hillslope gullies, and they consist of a large flood drainage channel
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(FDC) and an Outer Erosion Feature (OEF). Dating of inset alluvial terraces within the FDCs provided
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a minimum age of 81 ± 7 years, which is inferred to be approximately when the OEF began to form.
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The FDCs developed in conjunction with the floodplain and pre-date the arrival of cattle. In contrast, the OEF developed in more recent times, and is linked to both cattle grazing, increased rainfall and
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groundwater seepage. There is evidence that headward expansion of the gully complexes has been slowing as they consume their catchments despite increased rainfall in the past 40 years, thereby reducing the input of sediment to the river and leading to incision of the alluvial fills within the FDCs and producing inset terraces. The search for stable reference sites for the radionuclide tracer analysis produced evidence that overbank sediments derived from the river are mostly the products of erosion of subsoils, which are likely to be exposed in gullies and channel banks. This conclusion adds weight to the importance of gullies as river sediment sources, although their contribution relative to channel banks has yet to be determined.
ACCEPTED MANUSCRIPT Key management recommendations that stem from this investigation include the establishment of off-stream watering points for cattle, and reducing the amount of time cattle are grazed within the
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gully complex, or simply excluding cattle from these areas. Further interventions may be required to
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enhance revegetation on these unstable landforms. However, as noted above, this reduction should
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be quantified in relation to the whole sediment budget, in order to prioritise management action.
Acknowledgments
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Charles Darwin University, CSIRO, Tropical Savannas Cooperative Research Centre, and the Victoria River District Conservation Association are acknowledged for their operational funding
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contributions. We thank Chunky Schultz and Scott King for field assistance, Therese Fox of ERISS for assistance with radionuclide laboratory analysis, and Dr. Tim Pietsch of CSIRO for undertaking the
References
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drafts of this manuscript.
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OSL laboratory analysis. We thank Dr. Chris Carroll and Dr. Peter Hairsine for comments on earlier
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Highlights A previously undescribed erosion feature is identified and characterised. The Outer Erosion Feature is of post-settlement origin, containing badlands. The Flood Drainage Channel component is a natural feature. Managing stock will slow badland erosion and sediment input to the river.