Onkaparinga River, south from Mylor
2011 Aquatic Ecosystem Condition Report
- Permanently wet, flowing, freshwater river in autumn and spring 2011
- Moderately diverse macroinvertebrate community with several rare, sensitive and flow-dependent species
- Obvious signs of moderate to gross nutrient enrichment
- Riparian vegetation dominated by woody weeds and introduced grasses.
About the location
Onkaparinga River is a large stream in the Southern Mount Lofty Ranges that rises south from Mount Torrens and flows in a south-westerly direction, discharges into Mount Bold Reservoir, and ultimately flows into Gulf St Vincent at Port Noarlunga South. The monitoring site was located off Smith Road, about 2.5 km south from Mylor. The major land uses in the 31,522 hectare catchment are stock grazing, rural and urban residential living, irrigated cropping and remnant native vegetation.
The river was given a Fair rating because the site sampled showed evidence of moderate changes in ecosystem structure, and some changes to the way the ecosystem functions. There was evidence of human disturbance including nutrient enrichment, degraded riparian vegetation and fine sediment deposition in the channel but the stream still provided habitat for some rare and sensitive macroinvertebrate species.
A moderately diverse community of at least 30 species of macroinvertebrates was collected from the flowing river, 1.6-18 m wide and over 1 m deep in places, in autumn and spring 2011. The river consisted of a slow-flowing channel and small areas of fast-flowing riffle habitats. The community was dominated by generalists and species tolerant to poor water quality such as amphipods, worms and isopods. It also included smaller numbers of native and introduced snails, limpets, yabbies, springtails, beetles, blackflies, chironomids, mayflies, waterbugs, stoneflies and caddisflies. Several rare and sensitive species were collected, including an amphipod (Family Perthiidae), chironomid (Riethia), blackfly (Austrosimulium furiosum), mayflies (Offadens and Atalophlebia australasica), caddisflies (Taschorema evansi, Ulmerochorema membrum, Triplectides similis and Anisocentropus) and a stonefly (Dinotoperla evansi). The site also provided habitat for several species normally associated with flowing water, including a caddisfly (Cheumatopsyche sp. 2) and many of the above-listed rare and sensitive species. No fish were seen at the site in 2011.
The water was fresh (salinity ranged from 247-283 mg/L), well oxygenated (88-90% saturated), generally clear, and with moderate to high concentrations of nutrients such as nitrogen (0.78-0.97 mg/L) and phosphorus (0.09 mg/L).
The sediments were dominated by detritus, boulder, silt, algae, sand and clay in the slow-flowing channel and boulder, bedrock and algae in the riffles. Samples taken from below the surface were silt grey in appearance and sulfidic, indicating that the sediments were anaerobic or lacked oxygen. A large amount of silt, 1-5 cm deep in places, covered the channel and about 10 m of bank showed evidence of erosion due to past flood damage.
A moderate amount of phytoplankton occurred in the river and over 35% of the channel was covered by filamentous algae (mostly Cladophora) in autumn. Aquatic plants covered around 10% of the river and included both submerged (Myriophyllum) and emergent species (Juncus, Cyperus, Isolepis and Rumex). The riparian vegetation was dominated by woody weeds (willows and ash trees) over introduced grasses and other weeds, with only a few River Red Gums present along the banks of the river. The surrounding vegetation was eucalypt woodland with a weedy understorey dominated by introduced grasses.
Special environmental features
The Onkaparinga River provides permanent, flowing habitats that support a wide range of aquatic species, including several rare, sensitive and flow-dependent species.
Pressures and management responses
|Widespread introduced weeds 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.
|Livestock having direct access at the site and upstream (causing sediment erosion and adding excessive nutrients).
|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 creek and upstream (reducing habitat quality, increasing sediment erosion).
|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.
This aquatic ecosystem condition report is based on monitoring data collected by the EPA. It was prepared with and co-funded by the Adelaide and Mount Lofty Ranges NRM Board.