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Geomorphology
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The dominant erosion processes supplying fine sediment to three major rivers intropical Australia, the Daly (NT), Mitchell (Qld) and Flinders (Qld) Rivers
Gary G. Caitcheon a, Jon M. Olley b,⁎, Francis Pantus b, Gary Hancock a, Christopher Leslie a
a CSIRO Land and Water, Canberra, ACT 2601, Australiab Australian Rivers Institute, Griffith University, Nathan, QLD 4111, Australia
Article history:Received 27 April 2011Received in revised form 13 January 2012Accepted 1 February 2012Available online 9 February 2012
Keywords:137CsProbability density functionSediment sourcesChannel erosionTropicsLanduse management
The tropics of northern Australia have received relatively little attention with regard to the impact of soil ero-sion on the many large river systems that are an important part of Australia's water resource, especially giventhe high potential for erosion when long dry seasons are followed by intense wet season rain. Here we use137Cs concentrations to determine the erosion processes supplying sediment to two major northern Austra-lian Rivers; the Daly River (Northern Territory), and the Mitchell River (Queensland). We also present datafrom five sediment samples collected from a 100 km reach of the Cloncurry River, a major tributary of theFlinders River (Queensland). Concentrations of 137Cs in the surface soil and subsurface (channel banks andgully) samples were used to derive ‘best fit’ probability density functions describing their distributions.These modelled distributions are then used to estimate the relative contribution of these two componentsto the river sediments. Our results are consistent with channel and gully erosion being the dominant sourceof sediment, with more than 90% of sediment transported along the main stem of these rivers originatingfrom subsoil. We summarize the findings of similar studies on tropical Australian rivers and conclude thatthe primary source of sediment delivered to these systems is gully and channel bank erosion. Previously,as a result of catchment scale modelling, sheet-wash and rill erosion was considered to be the major sedimentsource in these rivers. Identifying the relative importance of sediment sources, as shown in this paper, will pro-vide valuable information for landmanagement planning in the region. This study also reinforces the importanceof testing model predictions before they are used to target investment in remedial action.
In south-eastern Australia numerous studies have demonstratedthe dominance of gully and channel bank erosion in supplying sedi-ment to river systems (Eyles, 1977; Olley et al., 1993; Fryirs andBrierley, 1998; Wallbrink et al., 1998; Wasson et al., 1998; Fanning,1999; Prosser et al., 2001a; Olley and Wasson, 2003; Rustomji et al.,2008). This gully and channel erosion was triggered around180 years ago by changes in land-use associated with European set-tlement. The clearing of forests, the introduction of grazing stock,the drainage of valley bottoms and the clearing of riparian vegetationinitiated gully and channel incision. In less than one hundred yearsnearly every valley in south-eastern Australia was affected and largevolumes of sediment were delivered to rivers draining these valleys(Olley and Wasson, 2003).
The major erosion process supplying sediment to northern tropicalrivers is yet to be fully assessed. In the 2001 National Land and WaterResources Audit (Prosser et al., 2001b) hillslope surface erosion was
l rights reserved.
considered to dominate the supply of sediment in savanna landscapesof northern Australia. This was largely based on the assumptions thatthe open woodland vegetation that dominates the savannah, coupledwith the intense tropical rainfall and seasonal burning regimes, wouldresult in high hillslope sediment yields and that gully erosion in thesetropical landscapes was limited because the channel networks were attheir fullest extent. However, as more field based research is conductedon tropical systems the relative contributions of subsurface gully andchannel erosion appear to be more akin to the situation in south-eastern Australia, where subsoil erosion dominates (Wasson et al.,2002; Bartley et al., 2007; Brooks et al., 2008; Hughes et al., 2009a,2009b; Wasson et al., 2010).
In this paper we use 137Cs concentrations to estimate the erosionprocesses supplying sediment along the main channel of two majornorthern Australian Rivers; the Daly River catchment south of Darwin(Fig. 1A) in Australia's Northern Territory, and the Mitchell Rivercatchment on the Cape York Peninsula in northern Queensland(Fig. 1B). We analyse 210Pb and 137Cs concentrations in surface soilsamples, samples collected from channel and gully source areas, andriver sediments samples. We also present data from five sedimentsamples collected along the Cloncurry River, a major tributary of theFlinders River in Northern Queensland that flows into the southern
189G.G. Caitcheon et al. / Geomorphology 151–152 (2012) 188–195
part of the Gulf of Carpentaria approximately 250 km south of theMitchell River (Fig. 1C). We then summarize the results from otherrecent studies and draw conclusions as to the dominance of channeland gully erosion in northern tropical systems in Australia. Differentsediment sources need different management actions to reducetheir contributions. Identifying the relative importance of sedimentsources, as shown in this paper, will provide valuable informationfor land management planning in the region.
Fallout radionuclides (137Cs and 210Pb) have been widely used todetermine the relative contributions of hillslope and channel erosionto stream sediments (Walling and Woodward, 1992; Olley et al.,1993; Wallbrink et al., 1994, 1998; Everett et al., 2008). Fallout210Pb is a naturally occurring radionuclide, formed through the radio-active decay of 222Rn gas. The parent of 222Rn is 226Ra, part of the 238Udecay series. These radionuclides are present in all soils. Some 222Rngas escapes from the soil into the atmosphere where it decays to210Pb. This 210Pb is then deposited on the soil surface, primarilywith rain. The maximum concentrations of fallout 210Pb in soils arefound at the surface. Concentrations then decrease over depth to de-tection limits at about 100 mm depth.
137Cs is a product of atmospheric nuclear weapons testing that oc-curred during the 1950–70s. Initially the distribution of this nuclide inthe soil decreased exponentially with depth, with the maximum con-centration at the surface. However, due to processes of diffusion themaximum concentration is now generally found just below the surfacein undisturbed soils. The bulk of the activity of this nuclide is retainedwithin the top 100 mm of the soil profile. In subsoils recently exposedby erosion 137Cs is virtually absent (Wallbrink and Murray, 1993).
As both fallout radionuclides are concentrated in the surface soil,sediments derived from sheet and rill erosionwill have high concentra-tions of both nuclides, while sediment eroded from gullies or channelshave little or no fallout nuclides present. By measuring the concentra-tion in suspended sediments moving down the river, and comparingthem with concentrations in sediments produced by the different ero-sion processes, the relative contributions of each process can bedetermined.
2. The Daly, Mitchell and Flinders catchments
The Daly River catchment (53,000 km2) is situated south of Dar-win in the Northern Territory of Australia (Fig. 1A), and remains rel-atively undisturbed. Elevations range from sea level where theestuary discharges into the Timor Sea to around 400 m on the Arn-hem Land sandstone plateau in the headwaters of the KatherineRiver. The Katherine River is the largest tributary, and becomes theDaly River downstreamof the Flora River confluence. The Flora, Fergussonand Douglas Rivers are the other major tributaries. Mean annual rainfallgrades from about 600 mm in the south to over 1300mm in northernparts of the catchment, with most of the rain falling during the summermonsoon season from November to March. Dry season flows are main-tained by discharge from limestone aquifers in the central part of thebasin (Wasson et al., 2010). Over 90% of the catchment area is vegetatedby open Eucalyptus savannah woodland with a grassy understory, grow-ing on infertile soils that support low-intensity cattle grazing. About 5% ofthe catchment has been cleared for agriculture and pastures (Townsendand Padovan, 2005).
The Mitchell River catchment (72,000 km2) has its headwaters inthe Atherton Tablelands region immediately west of Cairns(Fig. 1B). River headwaters flow west across the Cape York Peninsulain northern Queensland into the Mitchell River, which discharges onthe eastern side of the Gulf of Carpentaria. The main tributaries arethe Alice, Palmer, Lynd and Walsh Rivers, with headwater elevationsrising to about 1200 m. The rugged hill terrain in much of the head-water areas grades to Australia's largest fluvial megafan that domi-nates the western part of the catchment (Brooks et al., 2009). Meanannual rainfall of around 600 mm in the south increases to over
1200 mm in the northeast and northwest, mainly falling between De-cember andMarch. The dominant vegetation is open Eucalyptus savan-nah woodland with a grassy understory used for low-intensity grazing.Shellberg et al. (2010) propose that intensive grazing of riparian zoneson the megafan has contributed to widespread alluvial gully erosion.
The headwaters of the Flinders River flow west from the Great Di-viding Range in a region of tropical Queensland between the eastcoast city of Townsville and the inland mining town of Mt Isa(Fig. 1C). The river then flows north and drains into the southernend of the Gulf of Carpentaria near the town of Normanton, wherethe mean annual rainfall is 920 mm. Most of the rain falls betweenDecember and March. The Flinders’ main tributary is the CloncurryRiver that drains the south-western part of the catchment. Riverflows mainly occur during the wet season, with little or no flow duringthe dry season (April–November). The total catchment area is approxi-mately 109,000 km2,most of which is vegetated by open Eucalyptus sa-vannah woodland with a grassy understory used for low-intensitygrazing.
3. Methods
3.1. Sampling
Sampling locations are shown in Fig. 1A–C. In the Daly and MitchellRiver catchments, surface soil samples were collected to characterisesediment derived from hillslope, gully and channel sources. River sedi-ment samples were collected from locations along the main channelfrom the headwaters to the catchment outlets, and from each of themajor tributaries. In the Flinders catchment no source soil sampleswere collected. River sediment samples were collected from five loca-tions along the lower section of the Cloncurry River, one of the FlindersRiver's major tributaries. Locations at all sample sites were recorded ona GPS receiver. Approximately 0.5–1 kg of dry soil or sediment was col-lected and stored in plastic bags.
3.1.1. Surface soil sourcesIn order to characterise the 210Pb and 137Cs activity concentrations
of surface soils eroded from hillslopes, samples were collected fromhillslope drainage lines in small drainage basins where there was nodefined channel and no evidence of subsoil erosion. This alluviumsampling strategy was aimed at collecting material that is, or has re-cently been, in-transit and therefore more likely to be transportedto streams than in-situ hillslope soil. Most surface soil samples werecollected from the steeper hillslopes as it is expected that this iswhere soil mobilisation and transport is greatest.
3.1.2. Channel bank and gully sourcesProfiles of channel bank material were collected by taking many
small subsamples down the actively eroding bank face. In the MitchellRiver catchment banks consisted of well consolidated ‘old’ alluvial de-posits, while most of the sites sampled in the Daly River catchmentconsisted of relatively unconsolidated, younger looking deposits. Sub-soil sources were represented by samples of alluvium collected fromimmediately below gullies and from sheet-eroded areas where thesoil B-horizon (a weakly coherent and porous subsoil layer) is ex-posed. For consistency, material from zero to approximately 2 cmdepth was collected at all soil sampling sites.
3.1.3. Channel sedimentsFine sediment that appeared to have been recently deposited (e.g.
mud drapes on channel-bed sand) was sampled during two consecu-tive dry seasons from the main channels and major tributaries in boththe Daly and Mitchell River catchments. To ensure representativesamples of the material present in each sampled reach were collected,numerous subsamples were collected and physically combined intoindividual samples. This process was repeated at several locations
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along sampled reaches. At the Cloncurry River sampling sites grabsamples were obtained from mud at the bottom of permanentwater-holes along the river channel.
3.2. Sample treatment and measurement
Upon deposition onto the soil surface, fallout 210Pb and 137Cs bindstrongly to soil particles, mostly in the upper 10–15 cm of soil pro-files. Since these radionuclides bind preferentially to fine-grainedparticles, it is necessary to fractionate soils and sediment to minimisevariations in concentrations due to differences in particle size distri-butions within samples (Wallbrink et al., 1999; Walling, 2005). Inthis study we only analysed the clay and fine silt fraction (b10 μm)of soils and sediments to minimize particle size effects. We also cor-rected for variations in organic matter and interstitial water contentby using “mineral” concentrations determined from loss-on-ignitionmeasurements.
Analysis of the samples for gamma radionuclides was undertakenat the CSIRO radionuclide laboratory. All samples were individuallyslurried with water and settled to the point where the fine fraction,less than 10 μm, was decanted, dried and pressed into sealed perspexcontainers for radionuclide analysis following the procedures de-scribed in Leslie (2009).
3.3. Data analysis
To determine the relative contributions of the surface and subsoilsediment sources to each of the river systems, the following proce-dure was developed. The 137Cs activity concentration data in the sur-face soil and subsurface soil samples were used to derive ‘best fit’probability density functions describing their distributions. The prob-ability density plots were created by summing multiple Gaussianfunctions, each with means and standard deviations equal to each in-dividual sample 137Cs activity concentration and its uncertainty:
f 137Cs� �
¼ ∑j
1ffiffiffiffiffiffiffiffiffiffiffiffi2πσ2
j
q e
137Cs−μjð Þ22σ2
j
264
375 ð1Þ
Where μj is the 137Cs activity concentration of the j-th individualsample, and σj its uncertainty. These distributions were then fittedusing standard probability functions.
The modelled surface soil and subsoil distributions are then usedto estimate the relative contribution of these two components tothe river sediment samples:
Axþ B 1−xð Þ ¼ C; ð2Þ
where A is the modelled activity concentration distribution in the sur-face soil samples, B is the modelled distribution in the subsoil sam-ples, C is the resultant distribution with a mean of μc and a standarddeviation of σc, and x is the relative proportion of surface soil contrib-uting to river sediment—x is modelled as a truncated normal distribu-tion such that 0≤x≤1. The two distribution functions were sampled10,000 times with varying mixing proportions, thus producing a new‘mix’ distribution per proportion. The best estimate of the relative con-tribution from each source in the river sample is obtained when theabsolute difference (summed as squares) in the means |μc−μm| andthe standard deviations |σc−σm| of the generated ‘mix’ distributionsand the river sample distributions are at a minimum, where μm andσm are the mean and standard deviation of the activity concentrationsin the samples collected from the river.
The traditional approach of using 137Cs activity concentrations toestimate the contribution of surface soil to river sediments (e.g.Olley et al., 1993; Wallbrink et al., 1998) uses the means and standarderrors of the data from each of the sources and the resulting mix to
calculate the relative contribution. Inherent in this approach is the as-sumption that the concentration data in the sources and the resultingmixture are all normally distributed. As shown below this assumptionis incorrect for the data sets examined here, and to the best of ourknowledge has not been tested for in previous studies. The new ap-proach described here does not make this assumption; the measureddistributions are fitted using the best match from a range of standarddistributions. The selected distribution is then used to generate fitteddistributions that are used in the mixing model.
4. Results
4.1. Mitchell River catchment
Activity concentrations of 137Cs and 210Pb in the source samplescollected from the Mitchell catchment are highly correlated (r2=0.90,Fig. 2). Surface soil activity concentrations range from 2.2±0.4 to19.0±1.0 Bq kg−1 for 137Cs and 68±9 to 450±14 Bq kg−1 for 210Pb;in the channel bank and gully samples they range from −0.2±0.2 to2.3±0.4 Bq kg−1 for 137Cs and −10±4 to 89±8 Bq kg−1 for 210Pb;the river sediment samples range from0.6±0.1 to 4.6±0.3 Bq kg−1for137Cs and−4±5 to 94±6 Bq kg−1 for 210Pb with the majority of con-centrations falling within the concentration range of the channel bankand gully samples. The correlation between 210Pb and 137Cs concentra-tions in the source samples means that they cannot be used indepen-dently to estimate the relative contribution of the sources to the riversediments andwe have used the 137Cs data to estimate the relative con-tribution of surface soil to the sediment samples collected from alongthe main stem of the Mitchell River and from each of the majortributaries.
The 137Cs activity concentration data are shown in Fig. 3 togetherwith probability density plots for each of the source data sets. Thespread in the 137Cs activity concentration of the surface soil samplesmay be due to variation in factors such as rainfall (Basher, 2000), ero-sion depth (Sutherland, 1991) and bioturbation (Hughes et al.,2009a). The distribution is strongly asymmetric (7.5 Bq kg−1 mean,6.4 Bq kg−1 median, and 4.2 Bq kg−1 standard deviation) and isbest approximated (r2=0.87) by a standard gamma-distribution(Fig. 3). The data from the channel bank and gully sources is moresymmetrically distributed (0.87 Bq kg−1mean, 0.82 Bq kg−1median,and 0.8 Bq kg−1 standard deviation) and is best approximated by anormal distribution (r2=0.91) (Fig. 3).
Fig. 4 shows resulting probability density plots for each of the bestestimates together with the relevant 137Cs activity concentrationsfrom the tributaries and main channel of the Mitchell River. The rele-vant mean 137Cs concentrations and associated standard deviationsfor each of the groups of samples are presented in Table 1 togetherwith the best estimates of the relative contribution of surface soil tothe river sediments. Inherent in this approach is the assumptionthat the sampled 137Cs concentrations are part of a continuous distri-bution resulting from the mixing of the two source distributions.
4.2. Daly River catchment
Activity concentrations of 137Cs and 210Pb in samples collectedfrom the Daly catchment are also correlated (r2=0.44, Fig. 5). Surfacesoil activity concentrations range from 2.6±0.4 to 17.5±1.0 Bq kg−1
for 137Cs and 32±8 to 500±22 Bq kg−1 for 210Pb; in the channelbank and gully samples they range from 0.9±0.2 to 3.4±0.2 Bq kg−1
for 137Cs and−25±8 to 46±10 Bq kg−1 for 210Pb; the river sedimentsamples range from 0.1±0.5 to 12.6±0.6 Bq kg−1for 137Cs and 3±7to 236±11 Bq kg−1 for 210Pb with the majority of concentrations fall-ing within the concentration range of the channel bank and gully sam-ples. We have used the same approach as described above to estimatethe relative contribution of surface soil to the sediment samples
Fig. 1.Maps of the catchments of Daly (A), Mitchell (B) and Flinders (C) Rivers showing the locations of sediment (closed circles) and soil sampling (closed triangles). The inset mapshows the locations of the three catchments in northern Australia.
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Fig. 3. 137Cs data from samples collected to characterize the surface soil (grey closedcircles) and channel bank and gully (open circles) sources in the Mitchell Catchmentshown in ranked order. The error bars are equivalent to one standard error on themean and are derived from the analytical uncertainties. The thin solid line and thedashed line are the respective probability density plots and the grey solid line andthe thick black line are themodelled fits to each of these data sets. The data from samplescollected from the river and its tributaries are shown for comparison (closed triangles).
Fig. 2. Activity concentrations of 137Cs and 210Pb in surface soil (grey circles), channelbank and gully samples (open circles) and river sediment samples (closed black circles)collected from the Mitchell catchment. The error bars are equivalent to one standarderror on the mean and are derived from the analytical uncertainties. The solid and dashedlines show the line of best fit and the 95% confidence limits respectively fitted through thesource sample data.
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collected from along the main stem of the Daly River and from each ofthe major tributaries.
The 137Cs data from samples collected to characterize the surface soilsources in the Daly catchment are consistent with the distribution for thesurface soil sources from the Mitchell catchment (Fig. 6) and we haveused the same probability density function to model the surface soilsource data fromboth catchments, as described in Section 3. The 137Cs ac-tivity concentrations in the samples collected to characterise the channelbank and gully sources in the Daly tend to be higher than those from sim-ilar sources in the Mitchell (2.1 Bq kg−1mean, and 0.8 Bq kg−1 standarddeviation for samples from the Daly, compared to 0.87 Bq kg−1mean,and 0.8 Bq kg−1 standard deviation for samples from the Mitchell) andwe have adjusted the model distribution accordingly.
Fig. 6 shows resulting probability density plots for each of the bestestimates together with the relevant 137Cs activity concentrationsfrom the tributaries and main channel of the Daly River. The relevantmean 137Cs concentrations and associated standard deviations foreach of the groups of samples are presented in Table 2 togetherwith the best estimates of the proportion of surface soil contributingto the river sediments.
4.3. Flinders River catchment
Activity concentrations of 137Cs and 210Pb in the five samples col-lected from along the Cloncurry River, a major tributary of the FlindersRiver that flows into the southern part of the Gulf of Carpentaria ap-proximately 250 km south of the Mitchell River range from 0.1±0.2to 0.5±0.2 Bq kg−1 for 137Cs and 2±2 to 12±3 Bq kg−1 for 210Pb,with mean 0.4±0.1 and 6.0±1.8 mean values respectively. These de-posited channel mud samples collected after the 2008–09 wet seasonalong a c. 100 km reach of the river. Using the model we developedfor the Mitchell catchment, because of its close proximity, we estimate0% surface soil contribution to these sediments.
5. Discussion
The results presented in Tables 1 and 2, and the results for theCloncurry River samples are consistent with channel and gully ero-sion being the dominant source of sediment at all of the river-channel sites sampled in this study. The estimated surface soil
contributions along the Mitchell River are consistently low, with esti-mates in the lower reaches of less than 5%. For the Daly–KatherineRiver we plotted the relative surface soil contributions with distanceupstream of the catchment outlet (Fig. 7A). The graph shows that sur-face soil contributions are highest in the upper catchment but stilldominated by subsoil sources, and they decrease rapidly to less than5% along most of the main channel. Wasson et al. (2010) found thatin the mid to lower Daly River >89% of the fine sediment originatedfrom subsoil sources. Their study noted that channel bank erosion isan important sediment source due to recent increases in annual rain-fall and discharge, particularly since 1990. Observations by the au-thors indicate that bank erosion is common along the Daly River, aswell as some of its major tributaries like the lower Katherine River.We have not found documented evidence about gully erosion in theDaly catchment, and from observation it is not as widespread or ac-tive as it is in the Mitchell catchment (Brooks et al., 2009).
In the Mitchell catchment observations indicate that channel bankerosion is also widespread, particularly along the main channel of theMitchell River, but there is considerable evidence that alluvial gully ero-sion is also a significant sediment source. Very extensive, active alluvialgully erosion has been documented by Brooks et al. (2009) in the lowerMitchell catchment, with most of the erosion occurring along the mainand lower tributary channels where gullies feed directly into the rivers.
Relative surface soil contributions estimated from samples collectedon theDaly–Katherine River and theMitchell River and theirmajor trib-utaries are plotted against upstreamcontributing area in Fig. 7B. There isnot a simple relationship between upstream catchment area and the es-timated surface soil contribution; below ~20,000 km2 the surface soilcontribution is variable (0–40%); above ~20,000 km2 surface soil contri-butions are consistently low (b10%). In the b20,000 km2 catchmentsthe variable contribution of surface soils is likely to be related to factorssuch as cover, channel condition, extent of gully erosion, and topogra-phy. We are currently examining the causes of this variability.
There are two potential explanations for the low surface soil con-tribution in the lower reaches (>20,000 km2):
1. It is well known that for most large river systems most of the sed-iment which enters the system does not get transported to theoutlet of the river. Sediment yield per unit area generally declineswith increasing catchment area (Walling, 1983; Wasson, 1994).
Fig. 4. Probability density plots for each of the best estimates together with the relevant137Cs activity concentrations from sediment samples collected from the tributaries andmain channel of the Mitchell River. In each case the distribution represent data from10,000 iterations of themodel, and the area under each curve equals a summedprobabilityof 1. The error bars on the 137Cs data are equivalent to one standard error on the mean andare derived from the analytical uncertainties. The numbers under each tributary and rivername represent the estimated topsoil contribution.
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The most probable cause of the widely observed decline in unitsediment yield with catchment area is sediment deposition onfoot-slopes, floodplains and within channels (Prosser et al.,
Table 1Mean 137Cs activity concentrations and associated standard deviations for each of theriver sampling sites in the Mitchell Catchment. The column labelled “Surface soil %”gives the best estimates of the relative % contribution of surface soils to river sediment.
Mitchell Rivercatchment reach
137CsBq kg−1
StdevBq kg−1
Surfacesoil %
Stdev
Mitchell R (upstream Walsh R) 1.47 0.10 8 1Walsh R 2.01 0.74 14 2Mitchell R (upstream Lynd R) 1.05 0.21 3 1Lynd R 2.19 1.63 20 10Mitchell R (upstream Palmer R) 1.49 0.36 8 1Palmer R 2.23 0.54 17 2Mitchell R (downstream Palmer R) 1.03 0.36 3 1Mitchell R (upstream Alice R) 1.11 0.21 3 1Alice R 3.62 0.59 30 1Mitchell R (downstream Alice R) 1.21 0.25 3 1
2001a). The further a sediment source is from the outlet of theriver the lower the probability of sediment derived from thatsource reaching the outlet. In both the Daly and the Mitchell catch-ments as catchment size increases the extent of floodplains in-creases significantly and the slope of the land surface adjacent tothe channel and of the channel itself decreases. These factors arelikely to increase the opportunity for sediment deposition and de-crease the contribution of surface soil erosion along the river. Sub-soil erosion is evident along both the main channels. Channel bankderived sediment and material derived from alluvial gully erosionalong the lower reaches would have a higher probability of beingtransported to the catchment outlet. Hillslope derived materialfrom the upper catchment would have a higher probability ofbeing stored within the system. This provides one possible inter-pretation of the pattern observed in Fig. 7. Hillslope derived mate-rial from the upper catchment is being deposited along the systemand local proximal subsoil sources, with low 137Cs concentrations,dominate in the lower reaches.
2. Another possibility is that the surface soil is transported along thesystem but the total sediment load increases downstream and thesupply of subsoil material increases until the subsoil signaldominates.
To distinguish between these two possibilities we require sedi-ment load data from locations along both river systems; these dataare not available.
There are few published estimates of surface soil contributions toAustralian tropical rivers using 210Pbex and 137Cs. As discussed above,Wasson et al. (2010) used 137Cs to show that in the mid to lower DalyRiver less than 11% of the fine sediment originated from surface soilsources (Table 3). Wasson et al. (2002) also found using 137Cs in astudy of the Ord River catchment in north-western Western Australiathat about 10% of the sediment originated from surface soils. A studyby Hughes et al. (2009b) of a headwater catchment of the FitzroyRiver in the dry tropics of central Queensland, surface soil erosionwas found to produce less than 20% of the river sediment in non-cultivated parts of the catchment. In the wet/dry tropical HerbertRiver catchment in central Queensland, Bartley et al. (2004) used137Cs to determine that about 50% of the sediment in the lower riveroriginated from surface soils. This estimate was not corroborated byTims et al. (2010) who did a follow up study in the same catchmentusing 239Pu. Like 137Cs, 239Pu is a product of atmospheric testing of
Fig. 5. Activity concentrations of 137Cs and 210Pb in surface soil (grey circles), channelbank and gully (open circles) and river sediment (closed black circles) samples collectedfrom the Daly catchment. The error bars are equivalent to one standard error on themean and are derived from the analytical uncertainties. The solid and dashed lines showthe line of best fit and the 95% confidence limits respectively for the source sample data.
Fig. 7. Surface soil contributions along the main channels of the Daly and Mitchell Rivers.Relative surface soil contributions on the Daly–Katherine River (closed circles) and fromits tributaries (open circles) against distance from the catchment outlet (A) and againstupstream contributing area (B). Data from the main channel of the Mitchell River (closedsquares) and its tributaries (open squares) are also shown in (B). The error bars are equiv-alent to one standard deviation on the mean and are derived from the mixing model.
Table 2Mean 137Cs activity concentrations and associated standard deviations for each of theriver sampling sites in the Daly Catchment together with the best estimates of the relativecontribution of surface soils source. The column labelled “Surface soil %” gives the bestestimates of the relative % contribution of surface soils to river sediment.
Daly River catchment reach 137CsBq kg−1
StdevBq kg−1
Surfacesoil %
Stdev
Katherine R (downstream of the town ofKatherine)
4.85 3.96 41 10
King R 3.17 2.33 36 1Katherine R (upstream Flora R) 2.92 1.20 13 9Flora R 1.04 0.44 1 2Daly R (upstream Fergusson R) 2.88 3.36 2 19Fergusson R 1.71 0.68 1 1Daly R (upstream Douglas R) 1.05 0.37 3 1Douglas R 2.03 1.49 1 1Daly R (downstream Douglas R) 2.34 1.35 2 9Daly R at Nancar 0.89 0.53 1 1
Fig. 6. Probability density plots for each of the best estimates together with the relevant137Cs activity concentrations from sediment samples collected from the tributaries andmain channel of the Daly River. In each case the distribution represent data from 10,000iterations of the model, and the area under each curve equals a summed probability of 1.The error bars on the 137Cs data are equivalent to one standard error on the mean andare derived from the analytical uncertainties. The numbers under each tributary andriver name represent the estimated topsoil contribution.
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nuclear weapons, but it can be measured with greater sensitivity. Theysampled the Herbert River catchment after a greater than one-in-fiveyears average recurrence interval flood, and their results showed thatsurface soils were the minor contributor to river sediment everywhereexcept in some sugar cane cultivation and forested areas. SimilarlyWilkinson et al. (in press) showed that subsoils were also the dominantsource of the sediments in themain channels of twomajor tributaries inBurdekin River basin, the Bowen and Upper Burdekin Rivers (Table 3).
These results indicate that surface soils are a minor component ofthe sediment being transported in large river systems in tropical Aus-tralia. It is widely accepted that subsoils are the dominant sedimentsource in southern temperate grazing lands (Wasson, 1994; OlleyandWasson, 2003). It is clear that this source is also dominant in Aus-tralia's wet/dry tropics, where previously, as a result of catchmentscale modelling, sheet-wash and rill erosion were considered to be
Table 3Tropical Australian studies that have used radionuclide tracers to estimate relative sur-face soil contributions to the lower catchment.
Catchment Mean surface soilcontribution %
Tracer Reference
Daly 11 137Cs Wasson et al. (2010)Ord 10 137Cs Wasson et al. (2002)UpperFitzroy
20 137Cs and210Pbex
Hughes et al. (2009b)
Herbert 50 137Cs Bartley et al. (2004)a
Herbert 20 239Pu Tims et al. (2010)a
Burdekin 17 137Cs, 210Pbex,C
Wilkinson et al. (inpress)
Mitchell 3 137Cs This studyDaly 1 137Cs This studyCloncurry 0 137Cs This study
a Note these two studies were carried out pre and post cyclone Larry.
195G.G. Caitcheon et al. / Geomorphology 151–152 (2012) 188–195
the major sediment source (NLWRA Audit, 2001). This study rein-forces the importance of testing model predictions before they areused to target investment in remedial action.
6. Conclusion
The results presented are consistent with channel and gully erosionbeing the dominant source of sediment at all of the river-channel sitessampled in this study and they show that most (90% plus) of the sedi-ment being transported along the main stem of the Mitchell, Daly andCloncurry rivers originates from subsoil erosion of gullies and streambanks. This result, together with the evidence from similar studies ontropical Australian rivers indicates the primary source of sediment totropical rivers is gully and channel bank erosion.
Acknowledgements
This study was jointly funded by the Tropical Rivers and CoastalKnowledge (TRaCK) program, Griffith University and CSIRO Landand Water. We are grateful to Danny Hunt, Jim Brophy (CSIRO), andDavid Williams and Errol Kerle (Northern Territory Government,NRETAS) for assistance with fieldwork. Stephen Faggotter providedsamples from the Cloncurry River. Colin McLachlan assisted withsample preparation. We are grateful to Dr Rebecca Bartley (CSIRO)and an anonymous reviewer for comments which improved thismanuscript.
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