Research Article 2014 WRR Burdekin sediment budget | Page 2

Water Resources Research 10.1002/2013WR014386 GBR have remained constant over thousands of years due to the availability of abundant terrigenous sedi- ment along the GBR’s inner shelf [Larcombe et al., 1995; Orpin and Ridd, 2012]. In contrast, recent evidence suggests a strong link between increased inshore turbidity and higher sediment yields to the GBR from streams draining coastal catchments that have been modified by European settlement [Fabricius et al., 2013, 2014]. Increased turbidity associated with river plumes and subsequent dry season resuspension events may directly impact GBR coral and seagrass communities by reducing light available for photosyn- thesis [Fabricius, 2005; Collier et al., 2012]. When accompanied by high sedimentation rates, smothering may also occur [Weber et al., 2006]. Reduced vigor of coral communities affected by elevated turbidity and sedi- mentation can also result in increased macroalgal cover [De’ath and Fabricius, 2010] and more frequent coral disease outbreaks [Haapkyla et al., 2011]. Further, the clay and fine silt-sized sediment particles are eas- ily resuspended [Browne et al., 2012; Davies-Colley and Smith, 2001], and have the greatest effect on corals in the form of increased and persistent turbidity regimes and sedimentation of organic-rich flocs [Bainbridge et al., 2012; Fabricius, 2005; Humphrey et al., 2008; Weber et al., 2006]. The focus of this study, the Burdekin River catchment (130, 400 km 2 ) has an annual average discharge of 9.18 million ML (range: 0.25–54.03 million ML) over a 91 year gauge record to 2012 (1921–2012) [Department of Environment and Resource Management, 2012]. The Burdekin contributes the highest suspended sediment load to the GBR (30% of total) of all the coastal catchments, exporting an average of 3.93 million tonnes of suspended sediment annually, corresponding to an average area yield of 30 t km 22 yr 21 (1986–2010) [Kuhnert et al., 2012]. Historical records from inshore coral cores influenced by Burdekin River discharge and recent catchment modeling efforts suggest that annual sediment export is five to eight times higher than pre- European loads [McCulloch et al., 2003; Kroon et al., 2012]. Although low compared to tropical rivers globally (see discussion), this marked increase in export since European settlement (1850) threatens the sensitive ecosystems of the GBR, making efforts to reduce sediment runoff from the Burdekin catchment a manage- ment priority [Bartley et al., 2014]. To inform targeted and effective management of sediment erosion within the Burdekin, catchment-wide sediment source and transport annual budgets were constructed using empiri- cal field data collected at key river network locations between 2005 and 2010. The contributions of clay (<4 mm), fine silt (4–16 mm), and coarse (>16 mm) sediment fractions were quantified to isolate sediment sources at a relatively coarse ‘‘sub-catchment’’ scale before ‘‘hot-spot’’ tributaries were identified and specific environ- mental drivers for erosion were investigated. This study builds on sediment trapping estimates of a large res- ervoir within the catchment reported in Lewis et al. [2013], and quantifies the significant influence this impoundment has on downstream sediment transport and end-of-river export. This study reveals that the highest loads of the finer sediment fraction (i.e., clay and fine silt), which are of most interest from a manage- ment perspective are not necessarily derived from areas yielding the highest total suspended sediment load, and highlights how climate variability influences sediment loads; for example, elevated loads are typically transported by run-off events following prolonged drought. This study demonstrates that sediment budgets incorporating sediment particle-size fractions are far more useful to managers seeking to reduce fine sedi- ment export and inshore turbidity than the traditional ‘‘yield-only’’ approach. 2. Study Area The Burdekin River catchment is located within the seasonally dry tropics of north-eastern Australia (Figure 1). It is the second largest catchment draining into the GBR lagoon. The Burdekin catchment includes five major subcatchments: the Upper Burdekin River; the Cape River; the Belyando River; the Suttor River; and the Lower Burdekin (Figure 1). All but the Lower Burdekin subcatchment drain into Lake Dalrym- ple—an artificial lake impounded behind the Burdekin Falls Dam (BFD). Although Lake Dalrymple has a capacity of 1.86 million ML, the dam has overflowed every wet season but one since its construction was completed in 1987 [Faithful and Griffiths, 2000], indicating the enormous run-off from this large catchment (capacity to inflow ratio 5 0.24). The Bowen River is the only major tributary that discharges directly into the Burdekin River downstream of the BFD, comprising 50% of the Lower Burdekin subcatchment area. In this study, we focused on the gauged Bowen River subcatchment where streamflow can be gauged relatively accurately, which is not possible for the broader (ungauged) Lower Burdekin area. The coastal mountain ranges that enclose the eastern margins of the Bowen and Upper Burdekin Rivers have peaks rising to 750–1070 m and are steeply sloped, vegetated with rainforest, and receive the highest mean annual rainfall (up to 2370 mm yr 21 ) across the Burdekin (Figure 2). Steep mountain ranges reaching BAINBRIDGE ET AL. C 2014. American Geophysical Union. All Rights Reserved. V 9068