Research Article 2014 WRR Burdekin sediment budget

Water Resources Research 10.1002/2013WR014386 The inﬂuence of drought breaking years and sediment supply availability on Burdekin River annual sus- pended sediment export has also been highlighted in this study. For instance, end-of-river export was 30% greater in 2007/2008 than 2008/2009, despite both water years having discharges of similar volumes; 27.50 and 29.35 million ML, respectively. The earlier year had a larger sediment contribution from the catch- ment area below dam, with higher sediment yields per unit area (Table 2). The Burdekin has been described as a supply-limited catchment [Amos et al., 2004] and given that above average discharge occurred across the entire catchment in 2007/2008 (Figure 3), it is also likely there was a depletion in available sediment supply for runoff in the subsequent year. Previous studies have highlighted the increased sediment loads delivered during drought-breaking ﬂoods [Mitchell and Furnas, 1996; McCulloch et al., 2003; Amos et al., 2004], which was also observed in this study with a drought breaking ﬂood year in 2006/2007 which fol- lowed a series of relatively dry years, including 2005/2006 (Figure 3). Total discharge in the 2006/2007 and 2009/2010 water years were similar to average annual discharge, however, the sediment load exported in 2006/2007 (7.2 million tonnes) was double the annual average and three times greater than the sediment load exported in 2009/2010 (2.49 million tonnes; Figure 3). The 2009/2010 sediment load also reﬂects the depleted sediment supply after the two record ﬂood years, and improved ground cover across the entire catchment resulting from this wetter period, which results in decreased soil loss [Bartley et al., 2014]. Indeed Kuhnert et al. [2012] found a signiﬁcant decrease in sediment loads at the end-of-river site as ground cover increases. 5.6. Implications for Great Barrier Reef Management The Upper Burdekin and Bowen River subcatchments have the highest suspended sediment yields of all Burdekin subcatchments and were the major sediment sources during this study. Their wetter coastal locations, steeper topography, and weathered geology result in high streamﬂow and sediment transport efﬁciency. The Upper Burdekin is the major source of discharge to both the BFD and end-of-river, and the dominant source of all sediment fractions (i.e., clay, ﬁne silt and coarse sediment) into the BFD. The BFD reservoir is an efﬁcient sediment trap, and has reduced the suspended sediment load supplied from the large upstream catchment area (88% of the entire catchment) to end-of-river export, including the Upper Burdekin source. The reservoir has also inﬂuenced the sediment-size fractions transported from this upstream catchment area, with the ﬁner clay fraction now dominating all sediment exported over the dam spillway to the river mouth and adjacent GBR lagoon. This study identiﬁed the Bowen River as the major source of end-of-river suspended sediment export. This catchment has a comparatively small upstream area and the highest sediment yields (mean of 530 t km 22 yr 21 ) across the Burdekin, providing a clear focus area for management efforts aimed at reducing the export of all sediment-size fractions. However, our ﬁndings show that similar load contributions of both the clay and ﬁne silt fractions were delivered from the two major source areas: the Bowen River and the BFD overﬂow. Targeted source area remediation of the clay and ﬁne silt sediment fractions of increased ecological importance should ﬁrst be conﬁned to the Bowen River tributary if assessed on a per unit area contribution; however, we cau- tion further investigation into the geochemical and clay mineralogy characteristics of these different clay/ﬁne silt sources, and their subsequent transport in and likely impact on the marine environment is required. The sediment sourcing, reservoir inﬂuence on sediment-size transport, and yield data gener- ated across the Burdekin has broader application in other dry tropical river catchments, particularly those located in wet-dry tropical savannah climates. This study also highlights the importance of incor- porating sediment particle size into catchment sediment budget studies where management goals are aimed at reducing downstream turbidity and sedimentation on marine ecosystems such as seagrass and coral reef ecosystems. The inﬂuence of this terrigenous ﬁne sediment within the GBR has been recently highlighted by Fabricius et al. [2014], who correlated increased inshore turbidity with rainfall and runoff events from GBR Rivers such as the Burdekin. Finer sediment particles, often with an attached organic component once in the marine environment, are easily resuspended and transported along the GBR shelf (Orpin et al., 1999; Wolanski et al., 2008; Webster and Ford, 2010; Brodie et al., 2012] and are the most harmful sediment type to GBR receiving ecosystems such as corals [Fabricius and Wolanski, 2000; Weber et al., 2006; Humphrey et al., 2008], seagrass and other associated communities such as reef ﬁsh [Wenger and McCormick, 2013]. The combined inﬂuence of increased ﬁne sediment particles with decreased salinity (i.e., synergistic effects on coral fertil- ization; see Humphrey et al., 2008] during extended ﬂood plume conditions in above average Burdekin BAINBRIDGE ET AL. C 2014. 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