Onkaparinga River, downstream from Mount Bold Reservoir
2011 Aquatic Ecosystem Condition Report
- Permanently wet, freshwater river in autumn and spring 2011
- Sparse macroinvertebrate community with several rare and sensitive species
- Obvious signs of moderate nutrient enrichment
- Riparian vegetation dominated by River Red 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 near a track off Razor Creek Road, about 1 km south-west from Mount Bold Reservoir. The major land uses in the 38,329 hectare catchment are water supply and stock grazing, with smaller areas used for irrigated cropping, rural and urban residential living, remnant native vegetation and hay production.
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 and degraded riparian vegetation but the stream still provided habitat for some rare and sensitive macroinvertebrate species.
A sparse community of at least 25 species of macroinvertebrates was collected from the river, 15-30 m wide and over 1 m deep in places, in autumn and spring 2011. The river consisted of a series of non-flowing but connected pool habitats when it was sampled. The community was dominated by generalists and species tolerant to poor water quality such as amphipods (Austrochiltonia) and mites. It also included smaller numbers of flatworms, limpets, introduced snails (Physa), freshwater shrimp, beetles, mayflies, waterbugs, stoneflies and mayflies. Several rare and sensitive species were collected, including mites (Family Oxidae, Limnesia and Recifella), a mayfly (Thraulophlebia inconspicua), stonefly (Dinotoperla evansi) and caddisflies (Lingora and Triplectides similis). Most of these species are also typically found in flowing habitats, which indicates that this section of river regularly flows or includes very small areas of flowing habitat that were not very conspicuous when the site was sampled. No fish were seen at the site in 2011.
The water was fresh (salinity ranged from 221-272 mg/L), well oxygenated (101% saturated), turbid and slightly coloured, and with moderate to high concentrations of nutrients such as nitrogen (1.07-1.13 mg/L) and phosphorus (0.08-0.09 mg/L). Note that the water quality was influenced by the release of water from the Mount Bold Reservoir into the river reach where this site was sampled, before being diverted to Happy Valley Reservoir via Clarendon weir further downstream; the water therefore comprised local catchment and River Murray water from the Adelaide to Murray Bridge pipeline.
The sediments were dominated by detritus, clay and algae. Samples taken from below the surface were silt grey in appearance and showed no evidence that the sediments were anaerobic or lacked oxygen. Over 1 cm of silt covered the riverbed in places and about 10 m of bank showed evidence of erosion in spring, due to recent flood damage.
About 10% of the site was covered by filamentous algae (mostly Cladophora) in spring and aquatic plants such as dock (Rumex), rush (Juncus) and reeds (Phragmites) covered over 10% of the river. The narrow riparian zone was dominated by River Red Gums over blackberries and introduced grasses. The surrounding vegetation was low woodland comprising gums and wattles over introduced grasses and weeds.
Special environmental features
The Onkaparinga River downstream from the reservoir supports a number of rare and sensitive species of macroinvertebrates that survive in the permanently wet, freshwater habitats of the largest river catchment in the region.
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.|
|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.