Research Article 2014 WRR Burdekin sediment budget | Page 9

Water Resources Research
10.1002/2013WR014386
stream profiles has been incorporated into the LRE model, and contributes to the uncertainty in each load
estimate. Belperio [1979] showed the clay and fine silt fractions were well mixed through the water column
during flood flows in the Burdekin River, which further confirm the robustness of our sampling approach for
the clay and silt fractions; however, we acknowledge that the grab sampling technique may result in the
sand-sized fraction being significantly underestimated.
4. Results
4.1. Catchment-Wide Discharge and Sediment Load Budgets
4.1.1. Subcatchment Contributions to Total Catchment Discharge
Two of the largest discharge years on the 91 year record at the end-of-river stream gauge occurred during
this study, including the 2007/2008 (27.5 million ML, sixth largest) and 2008/2009 (29.4 million ML, fourth
largest) water years. In both water years catchment discharge exceeded three times the mean annual dis-
charge (see Figure 3a). During the 2007/2008 water year, streamflow in all major subcatchments far
exceeded mean annual discharge, including 6.2 million ML and 5.9 million ML from the Upper Burdekin and
Suttor subcatchments above the BFD, respectively, and an estimated 9.5 million ML from the ungauged
Lower Burdekin (Figure 3b). Overflow from the BFD (18 million ML) dominated end-of-river discharge, with
minimal retention of water from the subcatchments above the BFD.
Streamflow from the Upper Burdekin subcatchment dominated total Burdekin River discharge volume for the
2008/2009 water year, with a near-record 20 million ML (Figure 3b). Approximately 35% of the total annual
discharge during 2008/2009 occurred in the 6 days following Tropical Cyclone Ellie’s path through the upper
catchment, and 90% of all discharge in the 2008/2009 water year occurred during the two wet season months
January and February, 2009 (Bureau of Meteorology, www.bom.gov.au/cyclone/history/index.shtm). The Cape
(2.30 million ML) and Bowen (1.38 million ML) Rivers also experienced above average discharge in 2008/2009
(Figure 3b). Similarly to 2007/2008, end-of-river discharge was dominated by the catchments above the BFD.
Discharge in the 2006/2007 and 2009/2010 water years were comparable to average annual discharge vol-
umes (Figure 3a). The 2005/2006 water year was well below average across the entire Burdekin catchment,
with an annual discharge of just 2.2 million ML. The BFD was well below capacity at the start of this wet sea-
son due to drought, allowing around 40% of inflow from upstream subcatchments to be captured (Figure 3a).
4.1.2. Subcatchment Contributions to Total Catchment Sediment Export
Application of the LRE model indicates that the Upper Burdekin subcatchment was the source of between
76 and 95% of suspended sediment influx to the BFD over each water year from 2005/2006 to 2009/2010
(Figure 3). In comparison, the Cape, Belyando, and Suttor subcatchments each contributed between just 1
and 11% of the suspended sediment loads delivered to the dam during each of the monitored water years.
Suspended sediment trapping within the BFD ranged from 50 to 85% over the five water years, with the
highest trapping occurring in 2005/2006 (85%) and 2009/2010 (82%) [Lewis et al., 2013]. In both of these
years, similar sediment load inputs and export from the dam occurred (Figure 3). During the 5 year study
period, the Lower Burdekin subcatchment area contributed 55–82% to the end-of-river suspended sedi-
ment export. The bulk of this sediment was derived from the Bowen River (7110 km 2 at Myuna gauge),
which includes approximately 50% of the total Lower Burdekin subcatchment area (Figure 3).
4.2. Subcatchment Annual Sediment Yields
The Bowen River had the largest annual sediment yield of all Burdekin subcatchments when sediment loads
were normalized to catchment area, with a mean annual yield of 530 t km 22 yr 21 over the five water years
(Table 2). The mean annual sediment yield from the Upper Burdekin subcatchment, five times the size of
the Bowen subcatchment, was 147 t km 22 yr 21 ; with the highest yield (415 t km 22 yr 21 ) occurring during
the above average 2008/2009 water year. Sediment yields from the Cape, Belyando, and Suttor subcatch-
ments were markedly lower, with the study period means ranging between 5 and 23 t km 22 yr 21 (Table 2).
An exception occurred in the Suttor subcatchment during the wet 2007/2008 water year which resulted in
a sediment yield of 65 t km 22 yr 21 .
4.3. Minor Tributary Hot-Spot Sources
Site-averaged TSS concentrations over the study period ranged from 115 to 4075 mg L 21 across the minor
tributary volunteer network sites, providing a reliable indication of sediment source or hot spot areas to
BAINBRIDGE ET AL.
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