The Carbunup River catchment lies on the western side of the Geographe catchment with its headwaters on the the Whicher Scarp. The Carbunup River (originally named the Lennox River by John Molloy in 1835) is characterised by thick vegetation through much of the riparian zone. Riparian vegetation provides habitat for a diversity of native fauna including small mammals, birds, reptiles and amphibians. Recent surveys in summer 2016 by the Department of Water and Environmental Regulation found a rich and valuable aquatic fauna which is reliant on a number of permanent pools through the dry season.
The river was identified as an important nursery for the Western pygmy perch and home to a large population of Carter’s freshwater mussel (currently listed as vulnerable on the 2014 IUCN Redlist of threatened species). The Carbunup River is of cultural and historic significance to the Wadandi people and several sites of significance occur within the catchment. A River Action Plan developed for GeoCatch in 2000 outlines the characteristics, values, condition and management issues of the Carbunup River.
The Carbunup River catchment supports a thriving agricultural industry dominated by beef and dairy grazing. Vineyards are scattered throughout the catchment and horticultural land-use occurs in the lower catchment. Native vegetation occurs within the Treeton state forest in the upper catchment and in much of the riparian zone.
Image: The catchment supports beef grazing, dairy grazing, vineyards and horticulture.
The headwaters of the Carbunup River begin on the Whicher Scarp within the Treeton State forest, the river then flows in a northerly direction through agricultural land before discharging into Geographe Bay near Siesta Park. The river is largely unmodified until its lower reaches where there it is maintained as a drain (Lennox River Drain) by the Water Corporation. In 1940 a weir was constructed at the junction of the Lennox Drain and the Carbunup River to prevent saltwater ingress and provide larger areas of arable land. The water upstream of the weir is currently used by farmers for irrigation. On stream dams have been constructed in the lower catchment. Since 2000, the mean annual flow for the Carbunup River is 29GL/yr of a total of 203GL/yr for the waterways of the Geographe catchment.
The Carbunup River has very good water quality and contributes comparatively low nutrient loads to Geographe Bay relative to other sub-catchments. The high phosphorus retention index (PRI) of the soils in the catchment combined with vegetation in the riparian zone contribute to this good water quality.
Fertiliser from beef grazing contributes the largest proportion of the phosphorus and nitrogen loads to the Carbunup River. Dairy shed effluent, horticulture and rural living are also significant contributors of both nitrogen and phosphorus.
The nutrient loads in the Carbunup River are sourced from a diversity of agricultural land uses.
The Department of Water and Environmental Regulation has been monitoring the water quality of the Carbunup River fortnightly since 2006. Data from 2008-2021 is presented in the table below. The data shows total nitrogen and phosphorous concentrations to be consistently below water quality targets. The Carbunup River catchment is categorised as a ‘protection’ catchment as waterways are currently meeting both the nitrogen and phosphorus targets established in the Water Quality Improvement Plan.
Total nitrogen and total phosphorus trends for the Carbunup River catchment from 2011 to 2021.
Since 2009 programs led by GeoCatch and partners have focussed on reducing protecting and enhancing native vegetation along the river. Key achievements between 2009 and 2015 include:
Thirteen landholders undertook soil testing and nutrient mapping to inform fertiliser decisions, assisting farmers to use only the phosphorous needed for the pasture growth required.
One dairy effluent management plan was developed to support the re -use of dairy effluent, reducing nutrients entering the waterways.
Eleven landholders undertook 5.85 km of riparian fencing and 3.8ha of revegetation with the aim to restore and protect riparian vegetation, prevent stock access, increase the absorption of nutrients and to reduce sedimentation and pollution.
The Department of Water carried out a review of the river condition in 2016 as part of the Healthy Rivers Program. This included an assessment of ecological condition, interpret water quality data, highlight values and threats and provide guidance for management.
Management programs will continue to focus on maintaining the good water quality of the waterways of the Carbunup River catchment under the Revitalising Geographe Waterways program. Promoting best-practice agricultural fertiliser and dairy effluent management in the catchment and ongoing management of riparian vegetation are key goals for this ’protection’ catchment. Other priority goals include raising community awareness to recognise the values of the Carbunup River and water quality monitoring to assess changes in nutrient status.
Farmers will be supported in the implementation of best practice fertiliser management programs through the Fertiliser Management project where soil testing, whole farm nutrient mapping, agronomic advice and workshops will be completed on at least three farms in the catchment. Revegetation and fencing are proposed in the riparian zone of the river on at least six properties.
Monitoring the water quality of the Carbunup River will continue in the Catchment Water Quality Monitoring Program and the Water Quality Improvement Plan will be reviewed and updated. GeoCatch will continue to work with community groups and partners to raise community awareness and promote best practice agricultural management.
Image: Actions to reduce nutrients entering the Carbunup River will continue as part of Revitalising Geographe waterways program.
