Onkaparinga River, Woodside
2011 Aquatic Ecosystem Condition Report
- Permanently wet, slow-flowing, freshwater river in autumn and spring 2011
- Moderately diverse macroinvertebrate community with no rare or sensitive species
- Obvious signs of moderate to gross nutrient enrichment
- Riparian vegetation dominated by introduced trees, grasses 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 opposite Post Office Road in Woodside. The major land uses in the 4,833 hectare catchment are stock grazing and hay production, with smaller areas used for irrigated cropping, remnant native vegetation, and rural and urban residential living.
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 habitats.
A moderately diverse community of at least 30 species of macroinvertebrates was collected from the slow-flowing creek, 3.3-4.5 m wide and over 150 cm deep, in autumn and spring 2011. The river consisted of a slow-flowing, deep channel in autumn but was shallower in spring when a small area of faster-flowing riffle habitat was also present. The community was dominated by generalists and species tolerant to poor water quality such as mites, amphipods (Austrochiltonia), waterbugs (Micronecta) and chironomids. It also included smaller numbers of hydrozoans (Hydra), flatworms, native and introduced snails, leeches, freshwater shrimp, yabbies, beetles, mosquitoes, blackflies, mayflies, damselflies and caddisflies. Two species normally associated with flowing water were collected in spring, including a blackfly (Simulium ornatipes) and dytiscid beetle (Platynectes decempunctatus). No rare or sensitive species were detected, and no fish were seen at the site in 2011.
The water was fresh (salinity ranged from 517-815 mg/L), well oxygenated (72-77% saturated), clear in autumn but turbid in spring, and with high concentrations of nutrients such as nitrogen (0.74-1.33 mg/L) and phosphorus (0.11-0.18 mg/L).
The sediments were dominated by detritus, sand and silt, with cobble, pebble, gravel and clay also present. Samples taken from below the surface were sandy grey and showed no evidence that the sediments were anaerobic or lacked oxygen. Over 1 cm of silt covered the streambed in spring but no evidence of any significant areas of bank erosion was recorded at the site in 2011.
No sign of any filamentous algae was seen at the site but small growths of phytoplankton were recorded during the autumn and spring surveys. A few patches of aquatic plants covered less than 10% of the channel, including submerged (Crassula and Callitriche) and emergent species (Juncus, Isolepis and Rumex). The narrow riparian zone was dominated by introduced grasses and weeds with patches of willow, ash and a few scattered River Red Gums lining the banks of the river. The surrounding vegetation was horse grazing paddocks with a few isolated gum trees remaining in the local landscape.
Special environmental features
The Onkaparinga River at Woodside provides permanently wet, flowing, freshwater habitats that support at least two flow-dependent 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.