Suttor River Basin
Suttor Basin is comprised of 6 subcatchments, as follows:
- Diamond Creek Subcatchment
- Logan Creek Subcatchment
- Lower Suttor River Subcatchment
- Rosetta Creek Subcatchment
- Sellheim Creek Subcatchment
- Upper Suttor River Subcatchment
SedNet Modelling of Water Quality
Model results for the Upper Burdekin Basin are summarized as follows:
- Basin modelled area: 17,852 sq. km.
- Source contributions: Hillslope = 60%; Gully = 31%; Streambank = 9%
- Total suspended sediment (flow weighted) supply: 735.7 kt/yr
- Total suspended sediment supply (flow weighted; normalized to area): 412 kg/ha/yr
- Suspended sediment end-of-basin (flow weighted) yield: 1,272.6 kt/yr
- Event Mean Concentration (flow weighted): 578 mg/L
- Mean Annual Flow: 2,198,560 ML
Hillslope erosion is identified as the major source of sediment and particulate nutrients affecting water quality in the Suttor Basin (60%), although gully erosion (31%), in particular, is also predicted to be significant contributor. Loss of sediment and associated particulate nutrients from all sources (supply) is relatively low (735.7 kt/yr) compared to the contributions of other basins. This loss equates to 412 kg/ha/yr, which is quite low when compared to all subcatchments and other basins.
Modelled suspended sediment loads for the Suttor Basin above the gauging station on the Suttor River at Mt. Coolon Road 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: 17,203 sq. km.
- Total suspended sediment (flow weighted) supply: 1,509 kt/yr
- Total suspended sediment (flow weighted) yield: 1,355 kt/yr
Hillslope erosion is identified as the major source of sediment and particulate nutrients affecting water quality in the Suttor Basin (60%), although gully erosion (31%), in particular, is also predicted to be significant contributor. The Sellheim River subcatchment is predicted to have a substantially higher rate of soil erosion (per unit area) than other Suttor Basin subcatchments (869 kg/ha/yr), which is also high when compared to all subcatchments and other basins. The Lower Suttor subcatchment is also predicted to have a moderately high rate of soil erosion (675 kg/ha/yr). Both subcatchments are predicted to have substantially higher rates of soil erosion than the Basin average. The Rosetta Creek subcatchment is predicted to have a rate of soil erosion (383 kg/ha/yr) that is close to the Basin average, while the Upper Suttor River and Logan Creek subcatchments are below the Basin Average (328 & 277 kg/ha/yr). Diamond Creek subcatchment is predicted to have the lowest rate of soil erosion (229 kg/ha/yr), which is well below the Basin average and low when compared to all subcatchments and other basins. The average rate of soil erosion over the entire Basin (412 kg/ha/yr) is comparatively low when compared to all subcatchments and other Basins.
Total soil loss from all sources is predicted to be greatest in the Lower Suttor subcatchment (198 kt/yr), followed by the Upper Suttor River subcatchment (171 kt/yr). The Upper Suttor River subcatchment is very large (5,155 sq. km.) and thus produces a disproportionately large contribution of sediment to waterways when considering its below Basin average rate of soil erosion. The Sellheim River, Rosetta Creek, Logan Creek and Diamond Creek subcatchments had progressively lower total soil loss (122, 96, 93 & 55 kt/yr respectively). The total soil loss from all sources in the Suttor Basin (736 kt/yr) is relatively low when compared to all other Basins.
Water Quality Monitoring
Bainbridge et al. (2007b) report that the lack of significant flow events in the Suttor Basin during the monitoring period has resulted in less confidence in the sediment and nutrient data from the Basin. Although end-of-basin mean TSS concentrations were relatively low (440 mg/L), these concentrations are likely to be higher during larger flow events. Higher mean TSS concentrations were consistently measured at the upper Suttor River (940 mg/L) over four years of monitoring. The apparent lower settling potential of the suspended sediment transported in these subcatchments poses an additional risk as this finer material may not be trapped effectively in the BFD, and is likely to travel further in the marine environment (Lewis et al., 2006). Nutrient concentrations are generally typical of the grazed areas for all the Suttor Basin subcatchments, with DON generally dominating N, and P dominated by the particulate form. Exceptions to this were the upper Suttor River where NOx concentrations were consistently elevated over the four years of monitoring.
Total suspended sediment loads calculated from Suttor River monitoring data at Mt. Coolon Road during the 2005-6 & 2006-7 wet seasons are reported to be 135 kt & 97 kt. When adjusted to the mean annual flow, these loads are 1,150 kt & 125 kt respectively. The 2005-6 & 2006-7 wet season flows were both well below average. While the 2005-6 & 2006-7 monitored loads were lower than model predictions (220 kt), flow adjusted loads for both years were in even poorer agreement.
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 Suttor Basin is proportioned as follows:
- A Condition: 16%
- B Condition: 37%
- C Condition: 40%
- D Condition: 7%
Data from the Suttor Basin is based on 656 observations.
On the basis of the rapid assessment, the Suttor Basin is estimated to have the largest proportion of land in poor (C) condition (40%), followed by fair (B) condition (37%) and good (A) condition land (16%). 7% of observed land was in very poor (D) condition.
Resource Condition Summary
The Suttor Basin is intermediate in size (~ 18,000 sq. km.) and covers around 13% of the BWQIP region. Common to most of the BQWIP basins, land use is dominated by grazing on natural or modified pastures. Approximately 6% of the land area is used for dryland cropping of cereals, while less than 1% is set aside for conservation and minimal use. Riparian habitat throughout the basin has declined in condition over the last 30 years, principally due to clearing along headwater streams and floodplains, and is mostly assessed to be in poor (C) condition. Suttor Basin waterways are poorly known ecologically, but numerous persistently and highly turbid water bodies are reported to be widespread.
Hillslope erosion is identified by models as the major source of sediment and particulate nutrients affecting water quality within the Suttor Basin, while gully erosion is also identified as a significant contributor. The overall rate of soil erosion is predicted to be comparatively low, but with some major differences between subcatchments. The Sellheim River and Lower Suttor River subcatchments are predicted to have the highest rates of soil erosion within the basin, while the Lower and Upper Suttor River subcatchments are predicted to contribute the most sediment to the end-of-basin load. 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 high density 'D' condition land, and highly vulnerable and marginal 'D' condition land, particularly in the Logan Creek, Diamond Creek and Upper Suttor River subcatchments.
Water quality in the Suttor Basin is predicted by models to have moderately elevated loads and concentrations of suspended sediment at the end-of-basin. However, modelled and monitored sediment concentrations and loads display little consistency. The lack of significant flow events in the Suttor Basin during the monitoring period resulted in less confidence in the sediment and nutrient data from the Basin. In light of the particular risk posed by very fine suspended sediment from the Suttor Basin, which is more likely to reach to coast and travel further in the marine environment, and the reported extent of degraded land, further water quality monitoring of this basin would be useful in order to resolve apparent discrepancies between monitoring, modelling and land condition analyses.
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 Suttor River Basin (measured at Mt. Coolum) from current (2008) by 2058 (i.e. reduction from approximately 175 kt/yr in 2008 to 105 kt/yr by 2058)
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