Five Mile Brook is the most northern waterway in the Geographe Catchment, located just south of Bunbury. It is a seasonal waterway, and its catchment lies entirely on the Swan Coastal Plain. The Creek flows directly into Geographe Bay at Minninup Beach. Water sampling by local school groups have found evidence of macroinvertebrates, tadpoles and long-necked turtles.
The Five Mile Brook catchment supports a thriving agricultural industry dominated by beef grazing. One dairy is located in the central catchment. Rural lifestyle lots and urban residential areas associated with the southern extension of Dalyellup Estate are key land uses in the lower catchment.
Image: Beef grazing is the primary land-use in the upper catchment.
Five Mile Brook flows only in the winter months, discharging to Geographe Bay at Minninup Beach. The waterway is modified into an artificial drain for several kilometres before reaching the Bay. In the summer months the mouth of the creek closes over due to lack of flow. When the flow intensifies in the winter months the drain fills, breaks the sandbar, and water flows into Geographe Bay. Since 2000, the mean annual flow for Five Mile Creek is 5GL/yr of a total of 203GL/yr for the waterways of the Geographe catchment.
Five Mile Brook has poor water quality, likely resulting from a combination of poor soils which do not retain nutrients and intensive land uses in the catchment.
The nutrient loads in Five Mile Brook are driven by intensive agricultural land-uses, with a high proportion of beef grazing in the catchment. Smaller contributions of nutrient loads are from dairy sheds, dairy grazing (fertiliser), septics, rural lifestyle and urban residential properties.
In 2014 a site on Five Mile Brook was added to the fortnightly water quality monitoring program to assess trends in water quality. Due to the seasonal nature of this waterway there is limited data on which to assess water quality. Limited data shows phosphorus and nitrogen concentrations to be above water quality targets.
The Five Mile Brook catchment is categorised as a ‘recovery’ catchment as waterways do not meet the nitrogen or phosphorus target established in the Water Quality Improvement Plan.
The Department of Water and Environmental Regulation has been monitoring the water quality of Five Mile Brook catchment fortnightly since 2018. Data from 2018-2021 is presented in the table below. The data shows total nitrogen and total phosphorus concentrations to be consistently above water quality targets.
The Vasse Diversion Drain catchment is categorised as a ‘recovery’ catchment as this waterway does not meet the nitrogen or phosphorus target established in the Water Quality Improvement Plan.
Between 2009 and 2015 three landholders carried out soil testing and nutrient mapping to inform fertiliser decisions, assisting farmers to use only the phosphorous needed for the pasture growth required. From 2015-2017 0.4ha of revegetation and 0.75km of fencing was carried out in the riparian zone with the aim to prevent stock access, increase the absorption of nutrients and to reduce sedimentation and pollution.
In Spring 2017 the Water Corporation and South West Catchments Council carried out revegetation, rock installation and fencing to stabilise the sand dune adjacent to the Brook.
Images: Improving the riparian zone of Five Mile Brook to reduce nutrients entering the waterway.
Management programs will continue to focus on reducing nutrients entering the Five Mile Brook catchment under the Revitalising Geographe Waterways program. Monitoring water quality and revisiting of modelling is a priority for this ‘recovery’ catchment.
Monitoring the water quality of Five Mile Brook will continue in the Catchment Water Quality Monitoring Program and the Water Quality Improvement Plan will be reviewed and updated.
Implementation of best practice agricultural fertiliser management and dairy effluent management, targeted riparian management, targeted perennial pastures and soil amendment are also key goals for this catchment.