Onkaparinga River, near Hahndorf
2008 Aquatic Ecosystem Condition Report
- Affected by nutrient enrichment, fine sediments and water pumped from the River Murray.
- Macroinvertebrate community dominated by species tolerant of pollution.
- Riparian zone invaded by weeds.
About the location
The Onkaparinga rises near Mount Torrens in the southern Mount Lofty Ranges and flows southwest, eventually entering Gulf St Vincent at Port Noarlunga. Its upper reaches flow mainly through agricultural land used for livestock grazing (47%), horticulture (14%) and dairying (14%). The river also carries River Murray water delivered via a pipeline from Murray Bridge as part of the supply network for Adelaide. This water is discharged into the Onkaparinga just to the south of Hahndorf. The Hahndorf Wastewater Treatment Plant discharges chlorinated effluent into Hahndorf Creek, which also flows into the river east of the township.
The site selected for monitoring was located about 100 metres downstream from the Murray Bridge to Onkaparinga pipeline discharge point, off River Road.
The river was given a Fair rating at this site because the ecosystem showed moderate changes to animal and plant life, and some changes to the way the ecosystem functions. The river was moderately disturbed with the transfer of River Murray water creating turbid, artificial flows for several kilometres downstream. Evidence confirmed nutrient enrichment and fine sediment was affecting the stream.
The river was a slow-flowing stream about 11 metres wide and more than a metre deep when sampled in spring 2008. No shallow riffle habitats existed because they were swamped with imported water, reducing the diversity of animal species and potentially impacting lower and middle reaches of the river. A moderately diverse community of 38 macroinvertebrate species was collected from both shallow and deep pool habitats in the channel. The community was dominated by species tolerant of high nutrient levels, including water scuds (small crustaceans) from the families Ceinidae and Eusiridae. No highly sensitive species were found.
The water was fresh (salinity of 240 mg/L) and well oxygenated (89% saturation). It was also alkaline (pH 8.6), cloudy, or turbid, and carried moderate to high levels of nutrients such as nitrogen (0.65 mg/L) and phosphorus (0.06 mg/L). The water contained large amounts of phytoplankton, and green filamentous algae (Cladophora) floated on the surface.
The sediments were mostly made up of silt and detritus; and appeared to be well aerated.
Several types of aquatic plants were identified in the channel, including Water Ribbons (Triglochin), watermilfoil (Myriophyllum), knotweed (Persicaria), rush (Juncus) and sedge (Cyperus).
River Red gums and acacias grew in the riparian zone, as well as weeds and introduced pasture grasses. Cereal crops and rural gardens covered the surrounding area.
Special environmental features
The Onkaparinga River at this site provides habitat for two uncommonly collected types of mites (Arrenurus and Peza ops).
Pressures and management responses
|Livestock have direct access at the site and upstream, causing sediment erosion and adding excessive nutrients (which leads to habitat disturbance, algal growth and aquatic weeds).||The Adelaide and Mount Lofty Ranges NRM Board's land management program encourages and promotes managing land to improve water quality. This includes incentives for waterway and wetland fencing to exclude or limit stock from entering riparian zones.|
|Limited riparian zone vegetation at the site and upstream, providing minimal buffer protection from catchment landuses (reducing habitat quality).||The Adelaide and Mount Lofty Ranges NRM Board’s land management program encourages and promotes managing land to improve water quality. This includes incentives for revegetation programs around waterways and wetlands and stock exclusion as well as educating landholders about the importance of riparian vegetation in managing soil erosion.|
|Large decrease in natural water flows (reducing ecological integrity).||Through water allocation planning the Adelaide and Mount Lofty Ranges NRM Board seeks to manage a sustainable water supply for the region so that there is enough water available for everyone (including the environment) even in drought conditions.|
|Extensive weed growth in the riparian zone at the site and upstream (reducing habitat quality).||The Adelaide and Mount Lofty Ranges NRM Board has several pest plant (weed) mitigation and control programs. They work closely with landholders to control weeds on their property and to help stop the spread to other properties and waterways.|
This aquatic ecosystem condition report is based on monitoring data collected by the EPA and prepared in conjunction with the Adelaide and Mount Lofty Ranges NRM Board.