Torrens River, 3.5 km north-east from Birdwood
2011 Aquatic Ecosystem Condition Report
- Permanently wet, flowing, freshwater stream in autumn and spring 2011
- Moderately diverse macroinvertebrate community with a few rare, sensitive and flow-dependent species
- Obvious signs of nutrient enrichment and the effects from the water transfer from the River Murray (Mannum to Adelaide pipeline)
- Riparian vegetation dominated by introduced grasses and weeds
About the location
The Torrens River is a large stream in the Southern Mount Lofty Ranges that rises a few kilometres north from Mount Pleasant and drains in a south-westerly direction where it discharges into Gulf St Vincent at Breakout Creek. The monitoring site was located near ‘Narcoonah’ off the Adelaide-Mannum Road, about 3.5 km north-east from Birdwood. The major land use in the 4,750 hectare catchment is stock grazing, with smaller areas used for remnant native vegetation, irrigated cropping, rural and urban residential living, and forestry.
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, turbid River Murray water from the Adelaide to Mannum pipeline, degraded riparian vegetation and fine sediment deposition in the channel. Despite this, the stream still provided habitat for some rare and sensitive macroinvertebrate species.
A moderately diverse community of at least 33 species of macroinvertebrates was collected from the flowing river, 2.8-9 m wide and over 1 m deep in places, in autumn and spring 2011. Fast-flowing riffle habitats comprised over 80% of the site in autumn, characterised by flows of 2 m/sec; in spring the riffle habitat extended over 15% of the site and flow speeds ranged up to 0.5 m/sec. Still and slow-flowing habitats along the edge of the channel comprised the remainder of the riverine environment that was sampled. The community was dominated by generalists and species tolerant to poor water quality such as chironomids, amphipods, blackflies and worms. It also included smaller numbers of flatworms, native and introduced snails, isopods, freshwater shrimp, beetles, mosquitoes, biting midges, mayflies, waterbugs, dragonflies and caddisflies. No sensitive or rare species were recorded in autumn when turbid, River Murray flows were being transferred to downstream reservoirs, whereas two significant species (a mayfly Atalophlebia and caddisfly Taschorema) were collected in spring when the stream was much clearer. Species normally associated with flowing water were also recorded, including blackfly larvae (Simulium ornatipes), dytiscid beetles (Platynectes decempunctatus) and the above-mentioned rare and sensitive species. The only fish collected from the site was the introduced Mosquitofish (Gambusia) in autumn.
The water was fresh (salinity ranged from 195 mg/L in autumn to 971 mg/L in spring), well oxygenated (80-93% saturated), turbid in autumn but clear in spring, and with high concentrations of nutrients such as nitrogen (1.02-1.38 mg/L) and phosphorus (0.07-0.18 mg/L).
The sediments were dominated by cobble, boulder and bedrock in the riffles and algae and detritus in the slow-flowing edge habitats. Samples taken from below the surface were slightly blackened and anaerobic in autumn, indicating the sediments lacked oxygen but they appeared to be well-aerated in spring. A large 1-5 cm deposit of fine silt covered the slow-flowing channel in spring when about 10 m of bank showed evidence of erosion due to recent flood damage.
A large amount of phytoplankton was present in autumn and contributed towards the high turbidity of the water discharged from the Mannum to Adelaide pipeline from further upstream near Mount Pleasant (supplying the Kangaroo Creek and Millbrook Reservoirs). About 10% of the channel was covered by filamentous algae (including Cladophora) in spring but it was not detected earlier in the year. Aquatic plants covered about 35% of the stream in autumn as a result of the extensive growth of cumbungi (Typha) at the site; in spring, only 10% of the channel was covered by a wider range of emergent species (Typha, Cyperus, Juncus and Rumex). The riparian vegetation was dominated by introduced grasses and weeds (including wild rose and agapanthus) under a few River Red Gums and wattles. The surrounding vegetation was stock grazing land with a few scattered gum trees in the local landscape.
Special environmental features
The Torrens River near Birdwood provides habitat for at least two rare and sensitive macroinvertebrate species, and another two flow-dependent species. The upper Torrens River also supports state-listed threatened native fish species, including the Mountain Galaxias and Climbing Galaxias (M. Hammer, S. Wedderburn & J. van Weenen, 2009, Action Plan for South Australian Freshwater Fishes 2007-2012. Native Fish Australia (SA) Inc.).
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.|
|River Murray water discharges, introducing water of different quality and flow dynamics to the natural runoff (differences in flow timing, temperature, chemistry, nutrient and biological status).||SA Water will continue to work with the Adelaide and Mount Lofty Ranges NRM Board , Department for Environment, Water and Natural Resources and the EPA on catchment management programs to improve the water quality and environmental conservation status of the Torrens River. SA Water will continue best management practices on its landholdings in the Torrens Catchment. SA Water will continue to monitor water quality in the Torrens catchment and manage its water transfers to minimise impacts on river bank stability.|
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.