4.2 Bowen Broken Bogie catchment
4.2.1 Sub-catchment sources Current catchment modelling of the BBB sub-catchments indicate that the rate of erosion is particularly high in the Bowen River , Little Bowen , Bogie and Pelican Creek subcatchments ( C . Dougall presented in Waterhouse et al ., 2016 ). The estimates for the Broken River sub catchment are considered to be an overestimate in the model due to limitations in estimating the cover factor in forested areas ( C . Dougall , pers . comm .) and should not be ranked as a priority sub-catchment for sediment management .
Suspended sediment monitoring and sediment tracing within the Bowen catchment have primarily focused on the sediment contributions from the Little Bowen and Broken River sub-catchments to the downstream Bowen River . Sediment tracing by Wilkinson et al . ( 2013 ) found the Little Bowen to contribute 85 % of the sediment load exported at this Little Bowen-Broken River confluence . Suspended sediment data collected at the outlet of this tributary during high flow events supports this finding , highlighting the Little Bowen subcatchment as a high sediment contributor , with an average TSS concentration of 3,270 mg L-1 , and TSS concentrations exceeding 10,000 mg L-1 during multiple flood events over the monitored period ( Table 3 ; Bainbridge et al . 2014 ; 2016 ). These concentrations , the highest known measured at a tributary scale across the GBR catchment area , are then diluted by Broken River waters . This high flow generating sub-catchment has headwaters originating within the steep granitic coastal range , and typically receives the highest rainfall across the Burdekin region ( see Figure 9 ).
Suspended sediment concentration data collected at the Lower Bogie River over three wet seasons ( 2006-07 to 2008-09 ) does not highlight this sub catchment as an above average sediment contributor , as identified by current modelling efforts ( Table 3 . Total suspended sediment ( TSS ) concentrations ( mg L-1 ) and average ( mean ± standard deviation , SD ) sediment particle size for tributary monitoring sites located across the BBB ( modified from Bainbridge et al . 2016 ). Suspended sediment samples collected over rising and peak stages of the flood hydrograph . Table 3 ). This is especially in comparison to concentrations generated by the Little Bowen sub catchment , or the lower reach of the Bowen River itself . Given the disparity between the latest model outputs and our current understanding from monitoring and research efforts , the relative contribution of the Bogie River sub-catchment to the end-of-Burdekin River is a key knowledge gap . Quantification of the relative contribution of the large alluvial gully networks along the main Bowen River channel downstream of the Collinsville Weir ( Bowen catchment ) is also required ( i . e . lower reaches of Parrot , Rosella and Glenmore Creek sub-catchments ).
Suspended sediments collected at the Bowen River ( Myuna gauge ) and Lower Bogie River sites are dominated (~ 60-65 %) by the finer particle size fraction (< 10 µ m ), with a slight increase in the contribution of the clay-only size fraction moving upstream within the Bowen River i . e . Bowen River immediately downstream of the Little Bowen-Broken River confluence ( Table 3 ). Geochemical and clay mineral tracing within the Bowen catchment found basaltic and granitic source areas to contribute most per hectare , and overall to this finer fraction (< 10 µ m ) exported by the Bowen River ( Bainbridge et al . 2016 ; Wilkinson et al . 2013 ). Recent research that included sites within the Bowen catchment has also highlighted this finer sediment fraction to be enriched in bioavailable nitrogen and phosphorus , especially of surface soil origin ( Burton et al . 2015 ). However , given the dominance of subsurface soil erosion within the Bowen catchment ( see next section ), sub-surface sediments are likely to be a significant source / load contribution of bioavailable nutrients exported by this catchment .
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