Onkaparinga River, west from Blewitt Springs
2011 Aquatic Ecosystem Condition Report
- Permanently wet, flowing, freshwater river in autumn and spring 2011
- Diverse macroinvertebrate community with many rare, sensitive and flow-dependent species
- Obvious signs of moderate nutrient enrichment
- Riparian vegetation dominated by gums and weeds.
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 in the Onkaparinga Gorge near a track off Chapel Hill Road, about 1.5 km west from Blewitt Springs. The major land uses in the 51,236 hectare catchment are stock grazing, water supply and remnant native vegetation, with smaller areas used for irrigated cropping, rural and urban residential living, and forestry.
The creek was given a Good rating because the site sampled showed evidence of relatively minor changes in ecosystem structure and function. There was evidence of human disturbance including nutrient enrichment and degraded riparian vegetation dominated by weeds but the river still provides significant habitat for a number of rare and sensitive macroinvertebrates.
A diverse community of at least 52 species of macroinvertebrates was collected from the flowing creek, 3-18 m wide and over 1 m deep in places, in autumn and spring 2011. Large areas of still to slowly flowing pools were connected by extensive areas of fast-flowing riffle habitats in both seasons sampled. The community was dominated by generalists and species tolerant to poor water quality such as chironomids, blackflies, mites and biting midges. It also included smaller numbers of flatworms, native and introduced snails, worms, amphipods, freshwater shrimp, springtails, beetles, mayflies, waterbugs, odonates, stoneflies and caddisflies. Numerous rare and sensitive species were collected, including mites (Limnesia, Coaustraliobates, Koenikea and Albia), blackflies (Austrosimulium furiosum and Simulium melatum), mayflies (Offadens, Atalophlebia australasica and Atalophlebia auratum), stoneflies (Dinotoperla evansi and Illiesoperla mayi) and caddisflies (Taschorema evansi, Ulmerochorema membrum and Anisocentropus). A number of species normally associated with flowing habitats were also recorded, including a beetle (Platynectes decempunctatus), blackfly (Simulium ornatipes) 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 944 mg/L in autumn to 556mg/L in spring), well oxygenated (90-102% saturated), slightly turbid and coloured, and with moderate to high concentrations of nutrients such as nitrogen (0.46-0.76 mg/L) and phosphorus (0.01-0.04 mg/L).
The sediments were dominated by detritus, boulder, cobble, pebble and algae in the pools and bedrock, boulder and algae in the riffles. Samples taken from below the surface were mostly composed of grey sands and showed no evidence that the sediments were anaerobic or lacked oxygen. Only small amounts of fine silt covered the streambed in places and no significant areas of bank erosion were recorded at the site in 2011.
Filamentous algae (mostly Cladophora) covered over 10% of the site and aquatic plants extended over 35% of the channel, including both submerged (Myriophyllum) and emergent species (Cyperus, Cotula, Isolepis, Juncus, Rumex, Phragmites and Typha). The riparian vegetation comprised River Red Gums and wattles over woody weeds (blackberries and wild rose), introduced grasses, weeds (iris, fennel and thistles) and patches of rushes and sedges. The surrounding vegetation was mostly eucalypt woodland over introduced grasses, olives and assorted scattered native shrubs.
Special environmental features
The Onkaparinga River near Blewitt Springs provides permanently flowing habitats that support a wide range of aquatic species, including many rare, sensitive and flow-dependent macroinvertebrates.
Pressures and management responses
|Insufficient natural water flows in the creek resulting from water extraction and climate variability (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.|
|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.