Research Article 2014 WRR Burdekin sediment budget | Page 12

Water Resources Research
10.1002/2013WR014386
Figure 4. Four year (2005–2009) mean suspended sediment load contributions from each of the major subcatchments (Upper Burdekin5
6.22 million tonnes; Cape5 0.30 Mt; Belyando5 0.16 Mt; Suttor5 0.26 Mt, Bowen5 3.76 Mt), Burdekin Falls Dam (2.52 Mt), and end-of-river
export (Inkerman5 8.44 Mt), with volume (million tonnes) represented by circle area. The proportions of clay, fine silt, and coarse sediment
fractions contributed by each of these sites are also shown, within each circle. A triangle denotes an ‘‘uncaptured’’ sediment load (2.16 Mt)
contributed to the end-of-river from the ungauged component of the Lower Burdekin subcatchment.
average mean annual concentration (MAC) for the Upper Burdekin (735 mg L 21 ) was below the Burdekin-
wide average of 980 mg L 21 .
In comparison, the Belyando, Suttor, and Cape River subcatchments all had lower end-of-catchment MAC’s
(335, 220, 245 mg L 21 , respectively), despite elevated TSS concentrations in some tributaries of the
Belyando and Suttor subcatchments, including the Carmichael (1100 mg L 21 ), upper Belyando (925 mg
L 21 ), and upper Suttor (850 mg L 21 ) Rivers. All sites in the Bowen River tributary had elevated concentra-
tions compared to the Burdekin-wide average, except for the small (36 km 2 ) rainforest headwater site on
the upper Broken River (115 mg L 21 ). The Little Bowen River had the highest average TSS concentration
(3270 mg L 21 ) within the Bowen. The gauged site (Myuna) had the highest 5 year average MAC of the five
Burdekin subcatchments (2880 mg L 21 ; Figure 5). The Bogie River, the second largest tributary of the Lower
Burdekin had below average TSS concentrations at both upper (305 mg L 21 ) and lower (510 mg L 21 ) loca-
tions (Figure 5).
4.4. Clay, Fine Silt, and Coarse Sediment Load Budget
The clay and fine silt sediment fractions (<16 mm) dominated (>70%) suspended sediment at all Burdekin
subcatchment sites over the four water years (2005–2009) where sediment particle-size data were available
(Figure 4). The Upper Burdekin subcatchment was the dominant (90%) source of all clay, fine silt, and coarse
sediment fraction loads into the BFD (Figure 4). Minor sediment load contributions into the dam from the
other three upstream subcatchments were dominated (78–91%) by the clay and fine silt fractions; with the
clay-only component dominating the sediment fraction contributed by the Belyando (50%) and Suttor
(61%) subcatchments (Figure 4). The efficiency with which different particle-size fractions are trapped within
the BFD was considered by Lewis et al. [2013], but they did not directly report the specific trapping of the
clay and fine silt-sized fractions. Our reanalysis of these data averaged over the four water years show that
31% of the clay, 66% of the fine silt, and 92% of the coarse sediment fractions were trapped by the BFD,
with an overall average trapping efficiency of 66% (Figures 3 and 4). The BFD overflow and Bowen River
subcatchment sites contributed a similar clay load of 1.32 and 1.03 million tonnes, respectively, to the end-
of-river over the 4 year average (Figure 4). Export at the end-of-river was dominated (81%) by the clay and
fine silt sediment fractions (Figure 4).
BAINBRIDGE ET AL.
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