The river begins near Mount Torrens in the Mount Lofty Ranges, at around 650 m elevation, and flows south westerly to the river’s mouth at Port Noarlunga. The catchment area for the entire basin is approximately 564 km2 and consists of the Onkaparinga Catchment, McLaren Vale, and the Adelaide plains south of Flag Staff Hills and north of Myponga Beach. The Onkaparinga Basin has an annual average rainfall ranging from 450mm on the Adelaide plains to 1000mm in the northern end of the catchment (at Lenswood) and 1200mm along the western edge (at Stirling).
The hydrology of the Onkaparinga River catchment is influenced by the operation of the Metropolitan Adelaide Water Supply System. The Murray Bridge-Onkaparinga Pipeline is used to supplement the natural runoff from the Onkaparinga River catchment. Water from the pipeline is discharged into the Onkaparinga River at Hahndorf and then flows into Mt Bold Reservoir (storage capacity 46,372 ML). Water from Mt Bold Reservoir travels to Clarendon Weir where it is diverted to Happy Valley Reservoir (prior to drinking water treatment) or flows to the Coast.
The flow characteristics of the Onkaparinga River (downstream of Hahndorf) are highly modified by the inflows from the pipeline. For example, the Onkaparinga River downstream from Hahndorf has undergone a seasonal reversal of flow, that is high flows in the summer (due pumping) and drier in the winter months, while he Onkaparinga River below Clarendon Weir has suffered a dramatic reduction in flows due to impoundment of natural flows with Mr Bold Reservoir. A study to determine how much flow the Onkaparinga should have to meet the needs of the environment has been conducted. Visit the Onkaparinga River Water Requirements page for more information. A draft Water Allocation Plan for the Western Mount Lofty Ranges has been developed. This Plan will guide the management of the water resources of the Western Mount Lofty Ranges, to achieve a sustainable long-term productive future for our region.
Under the Natural Resources Management Act, the groundwater resources of the McLaren Vale have been prescribed, with the McLaren Vale Prescribed Wells Area - Water Allocation Plan developed in 2007.
There are 37,000 hectares in the catchment area above Clarendon Weir, consisting of 13 sub-catchments above Mt Bold Reservoir and 2 between Mt Bold Reservoir Clarendon Weir. The Upper Onkaparinga Catchment contains 18 townships, and numerous small communities and settlements, with a total population of around 30,000 people.
Extensive irrigation in the catchment is predominantly for horticulture and viticulture while less intensive irrigation is associated with dairy farming and grazing. Irrigation of orchards, grapevines and pasture increased substantially in the Central Hills region between 2000 and 2004, with an increase in demand of 818, 1,640 and 1,300 hectares, respectively. In 2005, there were approximately 2,800 farm dams in the catchment.
Include landuse map and description
Water quality is influenced by many factors, both natural and human-induced, including soil and vegetation types, climate, land use and management practices. Water quality may be particularly threatened by land uses operating in close proximity to watercourses and reservoirs, where there is little opportunity to buffer potential contaminants entering the system by overland flow or groundwater baseflow.
Total nitrogen and total phosphorus concentrations generally exceed guideline values for aquatic ecosystems and are sufficient to promote algae growth within reservoirs, with trends in concentration appearing to increase broadly across the area. Nutrient and sediment loads are higher where land uses are intensive (such as market gardening, orchards, and urban development). The export of nutrient loads is related to both current land use and management and also practices that occurred in the past. Excessive fertiliser applications in some areas (for example Cox Creek) are still being exported from this sub-catchment, even though land use management practices have now significantly changed
Land uses may generate point or diffuse source pollution. Point source pollution is generally associated with industrial activity or specific facilities such as wastewater treatment plants. Diffuse source pollution is a major contributor to poor water quality in the Region and is created when many small sources of water pollution across a catchment combine to create a significant water pollution impact. Urban development, agriculture and forestry can all create significant diffuse source pollution. Work in the Torrens and Onkaparinga catchments indicates that nutrients in surface water generally arise from catchments (diffuse sources). For example, 55% and 90% of the total phosphorus at Millbrook Reservoir and Kangaroo Creek respectively is from catchment sources (Water Science 2005). Diffuse catchment sources also supply significant amounts of suspended particulate matter, although bed and bank erosion also contributes a significant proportion of the suspended particulate matter.
When rainfall occurs, producing catchment run-off, the water quality of rivers and streams can be reduced significantly. During two weeks of intense runoff in 1996, 81% of the annual suspended sediment load and 67% of the total phosphorus load were exported from the Sixth Creek sub-catchment. During this period, the suspended sediment concentration went from 62 mg/L to 2,010 mg/L (AWQC 2001).
The upper Cox Creek sub-catchment exports very high nutrients loads. A wetland and sedimentation pond have been established to retain these pollutants and improve downstream water quality.
The Board provides a Land and Biodiversity Services which is directed at protecting biodiversity, native vegetation, habitats (including inland waters) and primary production, and managing interactions between primary production, and cleared rural land and ecosystems. These services contribute to improving water quality within the region. The Board has a Watercourse Restoration Program, which improves water quality and biodiversity within the region.
The EPA has assessed River Health in the Mt Lofty Ranges, stating that region is biologically diverse, with more than 500 types of aquatic macroinvertebrates being recorded from 1994–1999. The most common members include oligochaetes (worms), hydrobiid snails, amphipod crustaceans (Austrochiltonia australis), chironomid midge larvae (Cricotopus and Chironomus species), blackfly larvae (Simulium ornatipes) and hypogastrurid springtails. A number of rare types of macroinvertebrates are found in the region. Streams in good condition were generally from catchments with large proportions of native vegetation such as Scott, Bakers Gully, Echunga, Brownhill creeks and most of the main channel of the Onkaparinga River. Transfers of River Murray water impact on the Torrens River from Mt Pleasant to the junction with Sixth Creek, and also in the Onkaparinga River from downstream of the inflow to Mt Bold Reservoir. Horticultural and agricultural impacts are evident in Cox, Lenswood, Aldgate and Inverbrackie creeks. Wastewater treatment plant discharges impact sections of the Sturt River and Hahndorf Creek.