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 urban basin comprises, the Field River Catchment, Christies Creek Catchment, the Lower Onkaparinga Catchment, Pedler Creek, and the catchments of the Willunga Hills. Landuse is predominantly urban, with industry in the Port Stanvac Lonsdale area (including Adelaide’s Desalination Plant, which is due for commissioning in April 2011).

Regional stormwater (capture from the upper catchment and the Adelaide plains) is considered an asset, and is being used to supplement traditional water resources.  Significant re-use projects are occurring including the irrigation of many reserves and ovals within the basin.

Water Proofing the South is a localised integrated water resource management strategy based entirely in the City of Onkaparinga. It will provide alternative water sources such as reclaimed water and stormwater to replace the use of traditional water sources such as mains water and groundwater so that overall water use in the region is sustainable—economically, environmentally and socially.

Water Proofing the South Stage 1 consists of 8 key elements listed below:

1. Christies Beach Waste Water Treatment Plant (CBWWTP) quality upgrade
2. Christies Creek Upgrade
   a. Morrow Road Sedimentation Pond
   b. Wilfred Taylor Reserve
3. Environmental Flows Study
4. McLaren Flat Reuse
5. Southern Urban Reuse Project
6. Willunga Basin Water Company distribution network expansion
7. Willunga Effluent Reuse
8. Willunga Balancing Storage and Trade Waste

The focus for Water Proofing the South Stage 2 is the capture, storage, treatment and reuse of stormwater to service reuse opportunities through the creation of an integrated system of Managed Aquifer Recovery (MAR) Schemes across the city. The objective is to offset as much as possible of the city's mains water demand with fit-for-purpose water.

Christies wastewater treatment plant is located within the sub-basin.  Work is currently under way to upgrade the plant and increase re-use options (through the Water Proofing the South Program).  Recycled water from the Plant is provided for agricultural use (primarily viticulture) in the Willunga Basin area. To increase the amount of water reused, SA Water has partnered with the City of Onkaparinga in the Water Proofing the South Project. Christies Wastewater Treatment Plant, this will be utilised in developments at Seaford Meadows for urban uses including garden watering and toilet flushing. Upgrades will also reduce nutrient exports to the marine environment and in doing so provide outcomes towards the Adelaide Coastal Waters Study.

Whilst there were once many natural wetlands within the area, the only remaining significant natural wetland is the Washpool Lagoon at Sellicks Beach. The City of Onkaparinga has prepared a management plan for the Washpool Lagoon, in partnership with other state agencies with various levels of responsibility over the site. The Onkaparinga Estuary is also major coastal estuary and wetland area, although it has been significantly affected by the construction of the Mount Bold Reservoir and encroachment of urban settlement. Other small coastal estuaries exist near the coastal outlets of Port Willunga Creek, Inglewood Creek and Pedler Creek. 

There is approximately 32 kilometres of unique and varied coastline within the urban Onkaparinga basin, featuring open beaches, reefs, dunes and cliff formations. The marine boundary of the Adelaide Mount Lofty Ranges Natural Resources Management Board (AMLR NRMB) encompass the sheltered waters of eastern Gulf St Vincent, from Middle Beach in the north, to more exposed coastline of southern Fleurieu Peninsula at Middleton Beach.

The City of Onkaparinga state that the coast is under pressure from:

• pollution from the 24 major stormwater outfalls, creeks and rivers from our nine catchments, and effluent from the Christies Beach Wastewater Treatment Plant
• the effects of farm run-off, recreation, fishing and tourism
• the results of sand dredging off Port Stanvac and land use changes and development along the coast

While the combination of these factors has lead to pollution and degradation of the coastline, catchment and water resource programs operated by the Board, Council and state agencies seek to minimise these impacts and improve marine health.

The Adelaide Coastal Waters Study (ACWS) has shown that pollution of coastal waters commonly reduces the attractiveness of the water, can cause algal blooms and fish kills and has been found b to contribute to the loss of seagrass. Implementation of some of the recommendations from the ACWS will come under the Adelaide Coastal Water Quality Improvement Plan (ACWQIP) - an initiative led by the SA EPA with funding support from the Australian Government Coastal Catchment Initiative - being developed in 2008. The ACWQIP will include the targets set for the Port Waterways in the Port River Water Quality Improvement Plan and Appendices.