Onkaparinga River, Oakbank
2011 Aquatic Ecosystem Condition Report
- Permanently wet, flowing, freshwater stream in autumn and spring 2011
- Moderately diverse macroinvertebrate community with a few rare and sensitive species
- Obvious signs of nutrient enrichment and sedimentation
- Riparian vegetation dominated by gums and woody 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 off Size Road, about 1 km north-east from Oakbank. The major land uses in the 9,479 hectare catchment are stock grazing, irrigated cropping, rural residential living and defence facilities.
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. Therewas evidence of human disturbance including nutrient enrichment, degraded riparian habitat and fine sediment deposition but the stream still provided habitat for a few rare and sensitive macroinvertebrate species.
A moderately diverse community of at least 38 species of macroinvertebrates was collected from the flowing river, 1.2-7 m wide and over 1 m deep in places, in autumn and spring 2011. The river comprised large sections of slow to non-flowing pools connected by small, fast-flowing riffle habitats. The community was dominated by generalists and species tolerant to poor water quality such as amphipods (Austrochiltonia), blackflies and worms. It also included smaller numbers of flatworms, introduced and native snails, limpets, bivalves, mites, freshwater shrimp, springtails, beetles, craneflies, biting midges, sciomyzids, chironomids, waterbugs, odonates, stoneflies and caddisflies. Three rare and sensitive species were collected, including a blackfly (Austrosimulium furiosum), stonefly (Riekoperla naso) and caddisfly (Taschorema evansi). The site also provided habitat for several species normally associated with flowing water, including a beetle (Platynectes decempunctatus), chironomid (Rheotanytarsus), blackfly (Simulium ornatipes), and some of the above-listed rare and sensitive species; most were only collected from the riffle habitat in spring. No fish were seen at the site in 2011.
The water was fresh (salinity ranged from 452-830 mg/L), well oxygenated (62-77% saturated), clear but slightly coloured, and with moderate to high concentrations of nutrients such as nitrogen (0.56-0.94 mg/L) and phosphorus (0.08-0.11 mg/L).
The sediments were dominated by detritus and silt in the pools and gravel, pebble and detritus in the riffles. Samples taken from below the surface were composed of grey silts and sands, and were sulfidic in spring, indicating that the sediments occasionally lacked oxygen. A large deposit of fine silt covered the streambed to a depth of 5 cm in autumn but only about 1 cm was noted in spring, indicating that winter floods presumably transported most of it downstream. No significant area of bank erosion was noted at the site in 2011.
About 10% of the channel was covered by filamentous green algae (including Cladophora) and up to 65% of the river was covered by large growths of aquatic plants that included floating (Azolla and Spirodela), submerged (Crassula) and emergent species (Persicaria, Typha, Rumex, Juncus, Cotula, Isolepis and introduced Rorippa). The riparian vegetation consisted of River Red Gums, woody weeds (ash, willows and blackberries) and bracken. The surrounding vegetation comprised a woodlot on one bank and cleared paddocks and rural gardens on the other.
Special environmental features
The Onkaparinga River at Oakbank provides permanently flowing habitats that support a diverse aquatic community which includes several rare, sensitive and flow-dependent macroinvertebrate species.
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.