Research Article 2014 WRR Burdekin sediment budget | Page 16

Water Resources Research
10.1002/2013WR014386
between these wet tropical climates and the more temperate semiarid or ‘‘dryland’’ regions [see Tooth,
2000]. Indeed, sediment yields from the Bowen River reflect its wet coastal location, particularly in 2006/
2007, 2007/2008 and 2008/2009 where mean annual discharge (0.80 million ML) was well exceeded (1.03,
2.49, and 1.38 million ML, respectively). Bowen sediment yields (370–1035 t km 22 yr 21 ) in these years are
comparable with rates generated in much wetter tropical rainforest studies of the Andes [Restrepo and
Kjerfve, 2000] and NW Amazon basin [Laraque et al., 2009] in South America, and Borneo [Chappell et al.,
2004] and northern Vietnam [Ha Dang et al., 2010] in South-East Asia (Table 3). In contrast, sediment yields
generated from all other Burdekin subcatchments and the end-of-river are more comparable to those gen-
erated in north-eastern Africa and India [Dunne, 1979; Nyssen et al., 2009; Panda et al., 2011], where wet-dry
tropical conditions also prevail (i.e., high variability, rare but extreme runoff events). Thus data generated in
this study and the sediment budget approach utilized might be most applicable for use in such climatic
regions, where limited sediment sourcing and yield studies have been conducted.
5.3. Minor Tributary Hot-Spot Sources
Across the Burdekin subcatchments variations in sediment load contributions are driven by their varied
topography, geology, rainfall, and vegetation. The Upper Burdekin and Bowen subcatchments have steep
terrain, with highly incised river channels (>18 m channel depths) that are highly efficient in streamflow
and sediment transport [Fielding and Alexander, 1996; Roth et al., 2002]. The tributaries with the highest
average TSS concentrations are located within these two subcatchments, including the north-western
region of the Upper Burdekin, a relatively steep landscape hosting old sedimentary rock deposits prone to
erosion. Tributaries monitored in this region include the Dry and Clarke Rivers, and Camel and Grey Creeks
(Figure 5). The Little Bowen River was also identified as a hot-spot within the Bowen River subcatchment,
with large areas of exposed soils and gullying, also containing old sedimentary rock deposits, and TSS con-
centrations peaking >10, 000 mg L 21 in both 2006/2007 and 2008/2009. Recent sediment tracing by Wilkin-
son et al. [2013] also identifies the Little Bowen River as a major sediment source, together with large areas
of gully erosion immediately upstream of the Myuna gauge. Tributaries within both the Bowen (Broken
River) and Upper Burdekin (Star and Running Rivers) subcatchments with coastal rainforest headwaters con-
tribute considerable streamflow to the end of each subcatchment, but have low sediment concentrations
compared to other tributaries within these subcatchments (Figure 5). For example, the Star and Running
Rivers contributed 30% of Upper Burdekin discharge in the 2005/2006 water year, but only 3% of the
total sediment load exported by this subcatchment. The tributaries draining these wetter coastal catch-
ments are naturally forested with different geology types to the western tributaries of the Upper Burdekin
and Bowen subcatchments which are less densely vegetated and widely composed of weathered and erod-
ible lithologies (Figure 2).
In contrast, the south-western Cape, Belyando, and Suttor subcatchments have low relief, expansive anasto-
mosing floodplains (overbank flooding at gauged site depths of 8, 8, and 4.5 m, respectively), less stream
power for entraining coarser material, and greater opportunity for sediment deposition before it is exported
from these subcatchments. Thus, although the steeper headwater tributaries within these western sub-
catchments produce high sediment concentrations (e.g., Carmichael and upper Suttor Rivers), mean end of
subcatchment yields remain low (<23 t km 22 yr 21 ) compared to the Upper Burdekin (147 t km 22 yr 21 ) and
Bowen (530 t km 22 yr 21 ) subcatchments (Table 2), both of which have greater sediment availability and
transportability.
5.4. Clay, Fine Silt, and Coarse Sediment Load Budget
Suspended sediment loads exported by all Burdekin subcatchment sites were dominated by the clay (<4
mm) and fine silt (4–16 mm) sediment fractions over the four water years 2005/2006 to 2008/2009 (Figure 4).
As noted in the methods, the coarser sand fraction may be underestimated in our results. Clay, fine silt, and
coarse sediment loads into the BFD were all dominated by the Upper Burdekin subcatchment, including
4.35 million tonnes of clay and fine silt per year on average over this 4 year period. In comparison, the Cape,
Belyando, and Suttor subcatchments combined contributed an average of 0.60 million tonnes of clay and
fine silt per year (Figure 4). Although the BFD traps an average of 66% of incoming sediment and consider-
ably reduces sediment delivery to the end-of-river from these four upstream subcatchments, it is the
coarser sediment fraction that is preferentially trapped. As a result, the clay-sized fraction dominates all
sediment carried over the dam spillway [Figure 4; Lewis et al., 2013]. While the Bowen River was a much
BAINBRIDGE ET AL.
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