Research Article 2014 WRR Burdekin sediment budget

Water Resources Research 10.1002/2013WR014386 larger source of suspended sediment loads to the end-of-river when compared to the BFD source, the mean particle size speciﬁc loads over this period reﬂect the increasing proportional importance of the BFD source with respect to the contribution of ﬁnes to the end-of-river. Indeed the proportional contribution from the Bowen River to BFD source reduces from 1.5:1.0 for the bulk sediment fraction (3.76 and 2.52 mil- lion tonnes from the Bowen River and BFD sources, respectively), to 1.2:1.0 when the combined clay and ﬁne silt fractions are considered (i.e., 2.86 and 2.36 million tonnes, respectively) and further to 0.8:1.0 when the clay-only fraction is considered (1.03 and 1.32 million tonnes, respectively). However, the clay-only sedi- ment yield from the smaller Bowen River tributary (145 t km 22 yr 21 ) is 10-fold higher compared to the BFD overﬂow source (11 t km 2 yr 21 ). Despite the inﬂuence of the BFD in reducing sediment export from the sizable upstream catchment area, and the increased importance of the Lower Burdekin subcatchment area as the major sediment source, management efforts targeting the ﬁner sediment fractions still need to consider this large source area above the BFD. Further geochemical and clay mineralogy tracing analyses may also highlight the relative importance of apparent minor sediment sources such as the Belyando and Suttor Rivers, which contribute almost exclusively the clay and ﬁne silt fractions (86% and 91%, respectively, Figure 4). Waterholes within these two subcatchments are constantly turbid [Burrows et al., 2007], and ﬁne dispersible clay particles are known to be contributed to the BFD by the Suttor arm [Fleming and Loofs, 1991], with the reservoir often remaining turbid long after ﬂood conditions recede (Z. T. Bainbridge, personal observation, 2005-2010; Flem- ing and Loofs, 1991; Grifﬁths and Faithful, 1996]. Such tracing may discriminate these dispersive clay types from other potential clay sources across the Burdekin and determine which clay mineral types are preferen- tially transported through the catchment and further into the adjacent marine environment. A further research gap is the quantiﬁcation of the relative contributions of suspended clays washed as surface runoff into tributaries compared to those yielded from lower in the soil proﬁle by gully and stream bank erosion; this quantiﬁcation will help to further target erosion management efforts. Recent research has identiﬁed these subsurface erosion processes as major sediment sources in the larger Australian tropical catchments, including the Burdekin, under current climatic and land management conditions (see reviews by Caitcheon et al. [2012] and Bartley et al. [2014]). 5.5. Burdekin River Discharge and Sediment Export to the GBR Lagoon Above average discharge across the Burdekin subcatchments in the 2007/2008 and 2008/2009 water years resulted in total Burdekin discharge to the GBR lagoon that were three times the mean annual discharge, and are ranked as the sixth (2007/2008) and fourth (2008/2009) largest years on record (Figure 3b). These wetter years were followed by the third largest discharge year on record in 2010/2011 (34.8 million ML), which saw an extended period of river discharge into the GBR lagoon for 200 days [Bainbridge et al., 2012]. This study has fallen within a ‘‘wet cycle’’ in the longer term interdecadal cycling of wet and dry con- ditions in the Burdekin, where rainfall and streamﬂow trends coincide with the Paciﬁc Decadal Oscillation [Lough, 2007]. The current wet conditions followed a period of drought in the mid late 1990’s/early 2000’s and preceding wetter cycles in the 1950’s, 1970’s and the late 1980’s/early1990’s. Reconstructed streamﬂow using coral luminescence showed an increase in the cyclic variability of rainfall and streamﬂow in the 20th century, as well as the extent of both wet and dry conditions [Lough, 2007]. The tight cluster of very wet years highlighted in this study are projected to occur more regularly as climate change progresses [Lough, 2007], increasing the frequency and volume of terrestrial sediment discharged to the inshore GBR. As part of a broader research effort focused on managing Burdekin River export to the GBR lagoon, Kuhnert et al. [2012] calculated annual Burdekin suspended sediment export using the Loads Regression Estimator (LRE) on 24 years of available suspended sediment data (1986–2010). This data analysis incorporated key controlling features of Burdekin sediment export including covariates representing (1) ratio of streamﬂow sourced from above the BFD, and (2) annual dry season vegetation ground cover ﬁgures, representing the inﬂuence of cover on sediment erosion [Kuhnert et al., 2012]. Using these explanatory terms, they were able to produce an average load of 3.93 (80% CI53.41–4.45) million tonnes, with tight uncertainty bounds repre- senting errors associated with the input data, thus providing resource managers with our current best esti- mate of present day Burdekin sediment export. When compared to this study period, three of the ﬁve water years far exceeded this long-term average, including 7.2 million tonnes in 2006/2007, 14.81 million tonnes in 2007/2008, and 10.86 million tonnes in 2008/2009 (Figure 3), illustrating the variability of suspended sedi- ment export from this river catchment and the inﬂuence of wetter climatic cycles. BAINBRIDGE ET AL. C 2014. 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