Bowen Broken Bogie River Basin
Bowen Broken Bogie subcatchments
The Bowen Broken Bogie Basin is comprised of 7 sub-catchments, as follows:
SedNet Modelling of Water Quality
Model results for the Bowen Broken Bogie Basin are summarized as follows:
- Basin modelled area: 11,580 sq. km.
- Source contributions: Hillslope = 74%; Gully = 16%; Streambank = 10%
- Total suspended sediment (flow weighted) supply: 1,036 kt/yr
- Total suspended sediment supply (flow weighted; normalized to area): 895 kg/ha/yr
- Suspended sediment end-of-basin (flow weighted) yield: 746 kt/yr (for Bowen-Broken, excluding Bogie)
- Event Mean Concentration (flow weighted): 461 mg/L (for Bowen-Broken, excluding Bogie)
- Mean Annual Flow: 1,618,299 ML (for Bowen-Broken River, excluding Bogie)
Overall, hillslope erosion is identified as the major source of sediment and particulate nutrients affecting water quality in the Bowen Broken Bogie basin (74%), although gully (16%) and streambank (10%) erosion are also predicted to be significant contributors. Loss of sediment and associated particulate nutrients from all sources (supply) is only moderate (1,036 kt/yr) when compared to other, larger basins. However, this loss equates to 895 kg/ha/yr, which is high when compared to all sub-catchments and other basins.
Modelled suspended sediment loads for the Bowen River and sub-catchments above the gauging station on the Bowen River at Myuna were estimated to permit a more accurate and explicit comparison between modelled estimates and monitoring data. These loads are as follows:
- Sub-basin modelled area: 9,353 sq. km.
- Total suspended sediment (flow weighted) supply: 869 kt/yr
- Total suspended sediment (flow weighted) yield: 801 kt/yr (exported from sub-basin i.e. 68 kt deposited on floodplain)
All Bowen Broken Bogie subcatchments have relatively high rates of sediment loss when compared to sub-catchments across the Burdekin Dry Tropics region. The Bowen River subcatchment, followed by Pelican Creek and and Broken River are predicted to have very high rates of soil erosion (1,414, 1,012 & 1,023 kg/ha/yr respectively). While these losses are largely a consequence of high predicted rates of hillslope erosion (74%), both gully (16%) and streambank (10%) erosion are also predicted to be significant. Estimated sediment loss from the Little Bowen River sub-catchment (832 kg/ha/yr) may be substantially underestimated on the basis of monitoring data.
Total sediment loss from all sources is predicted to be greatest in the Broken River sub-catchment (224 kt/y) followed by the Bogie River (188 kt/yr), Bowen (174 kt/yr), Pelican Creek (147 kt/yr) & Little Bowen River (121 kt/yr) sub-catchments. Gully and streambank erosion are predicted to be significant sources of sediment in the Bowen River sub-catchment (29% & 24% respectively).
Overall, comparison between modelled estimates and monitoring data for the Bowen River at Myuna suggest that the modelled predictions may be significantly underestimating the total suspended sediment load from this basin.
Water Quality Monitoring
Bainbridge et al. (2007b) report that the Bowen River sub-basin is a high contributor of sediment and nutrient loads to the end-of-the Burdekin River due to its close proximity to the Burdekin River mouth, and its location below the BFD means there is little opportunity for trapping of these materials. The 5-year mean TSS concentration (3,340 mg/L) for the Bowen River is higher than all sites monitored in the Burdekin Region, except for two subcatchments in the Upper Burdekin Basin (Camel Creek & Dry River) which are considerably smaller in size (260 and 680 km2 respectively) compared to the Bowen catchment at Myuna (~7,200 km2). Although the wetter Broken River subcatchment is often the dominant water supply for the Bowen River catchment through rainforest supplemented flows, the monitoring conducted in the Bowen sub-basin suggests that the majority of sediment supply is sourced from small, highly eroded tributaries located within close proximity to the Myuna autosampler. The Little Bowen tributary which has large areas of exposed soils and gullying is also likely to be a considerable source of sediment to the end of the Bowen River. Sampling conducted at monitoring sites established this wet season on the Bowen River immediately downstream of the Broken River and Little Bowen River junction, and on the Little Bowen River immediately upstream of this junction have shown that high TSS concentrations can be generated in the Little Bowen tributary (mean of 4,000 mg/L for the 2006/07 wet season), which are then diluted by the considerable flows from the Broken River. Particulate nitrogen and phosphorus concentrations, as well as NOx concentrations were elevated in the Bowen River subcatchment, particularly at the Myuna site. Elevated NOx concentrations (mean of 340 µg N/L) were also measured in the upper Broken River site, however further monitoring of this site is recommended as only one wet season was monitored due to the lack of flow events in this upper catchment.
The Bogie River subcatchment was monitored over the 2005-5 & 2006-7 wet seasons, with a mean TSS concentration of 605 mg/L at the end of this catchment. Nutrient specialisation and concentrations are also typical of grazed subcatchments. Although this catchment has not yielded high concentrations, it should still be considered for management purposes due to its close proximity to the Burdekin River mouth, with little opportunity for trapping of material exported from this subcatchment.
Total suspended sediment loads calculated from monitoring data at Myuna during the 2003-4, 2004-5, 2005-6 & 2006-7 wet seasons were reported to be 368 kt, 340 kt, 180.5 kt & 2,240 kt. When adjusted to the mean annual flow, these loads become 1,930 kt, 1,630 kt, 1,600 & 3,360 kt respectively. All years had below average flows. The average monitored sediment load of 2,130 kt is well above that predicted by SedNet (801 kt exported) and suggest that the modelled predictions may be significantly underestimating the total suspended sediment load from this basin.
Results of a Rapid Land Condition Assessment (adopted from Hassett et al. 2000) are presented below. The assessment has been devised to subjectively characterise condition while traversing the BDT region by vehicle. The data are based on a total of 4666 observations across the Burdekin region between 2004 and 2007.
The data were collected to provide independent information on land condition and provide a regional perspective. Resource assessment data are most useful when interpreted with other sources of data e.g. time-series remote sensing, modelling and water quality monitoring.
The estimated condition of the Bowen Broken Bogie Basin is proportioned as follows:
- A Condition: 10%
- B Condition: 36%
- C Condition: 44%
- D Condition: 10%
Data from the Bowen Broken Bogie Basin is based on 290 observations.
On the basis of the rapid assessment, the Bowen Broken Bogie Basin is estimated to have the largest proportion of land in poor (C) condition (44%), followed by fair (B) condition land(36%). 10% of observed land was in very poor (D) condition and good (A) condition.
Resource Condition Summary
The Bowen Broken Bogie Basin is relatively small (~ 11,730 sq. km.) and covers around 9% of the BWQIP region. Common to most of the BQWIP basins, land use is dominated by grazing on natural or modified pastures. However, approximately 24% of the land area is is set aside for conservation and minimal use, most of which is within the Broken River subcatchment. While the condition of riparian habitat varies markedly between subcatchments, from good (A) to very poor (D), there has been a general decline in condition over the last 30 years, principally due to clearing along streams and floodplains. Similarly, there is great diversity in the value of aquatic habitats, and knowledge of their condition and ecology between subcatchments. Waterways vary between largely sandy, dry ephemeral creek systems to permanently flowing clear-water rivers and creeks that originate in mountain rainforest.
Hillslope erosion is identified by models as the major source of sediment and particulate nutrients affecting water quality within the Bowen Broken Bogie Basin, while gully and streambank erosion are also identified as a significant contributors. The rate of soil erosion for the Basin overall is predicted to be high and well above the BWQIP region average, with individual subcatchments losing up to almost three times the BWQIP region average. The Bowen, Little Bowen, Broken, Pelican Creek and Bogie River subcatchments are all predicted to have high rates of soil erosion and to contribute substantially to the total sediment load at end-of-catchment. Rapid assessment of grazing land condition rates most of land area to be in poor (C) or fair (B) condition. However, analyses of ground cover from satellite imagery (reference) identify extensive areas of highly vulnerable and marginal 'D' condition land, while gully erosion is reported to be extensive in some areas.
Water quality in the Bowen Broken Bogie Basin is predicted by models to have moderately to highly elevated loads and concentrations of suspended sediment at the end-of-basin during wet season flow events. However, water quality monitoring from the Bowen River over 5 years have recorded still much higher sediment concentrations and loads than predicted by models. In contrast, sediment concentrations and loads from the Broken River subcatchment have been calculated to be lower than predicted by models.
Bowen River sub-basin is evidently a high contributor of sediment and nutrient loads to the Burdekin River mouth due to its close proximity and its location below the BFD, which means there is little opportunity for trapping of these materials. Furthermore, modelled predictions are thought to be significantly underestimating the total suspended sediment load from this basin.
Water Quality Targets
The following water quality target was developed based on Best Management Practice Guidelines for Water Quality Improvement, extensive modelling of a range of management scenarios, preparation of a discussion paper (reference) and then, finally, a series of workshops. These preparatory activities were undertaken in collaboration with landholders (graziers and cane farmers), industry representatives, Government, the scientific community and BDTNRM staff.
- Attain a minimum 40% reduction in mean annual sediment load from the Bowen-Broken River sub-basin (measured at Myuna) from current (2008) by 2058 (i.e. reduction from approximately 1,355 kt/yr in 2008 to 813 kt/yr by 2058)
- Improving Water Quality Through On-Ground Land Management Changes in the Bowen-Broken River Catchment: A Framework for Action - November 2004 (Hardcopy at NQ Dry Tropics (137))
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