Torrens River, 1 km west from Birdwood
2011 Aquatic Ecosystem Condition Report
- Permanently wet, flowing, freshwater stream in autumn and spring 2011
- Moderately diverse macroinvertebrate community with some rare, sensitive and flow-dependent species
- Obvious signs of gross nutrient enrichment and effects from the water transfer from the Adelaide to Mannum pipeline
- Riparian vegetation dominated by introduced grasses and weeds under River Red Gums.
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 opposite the junction of the Adelaide-Mannum and Angas Creek roads, about 1 km west from Birdwood. The major land uses in the 11,873 hectare catchment are stock grazing, remnant native vegetation and irrigated cropping, with smaller areas used for rural and urban residential living, forestry, hay production and quarries.
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, high turbidity from the River Murray water transfer to the catchment, and degraded riparian habitats. Despite this, the stream still provided habitat for some rare and sensitive macroinvertebrate species.
A moderately diverse community of at least 37 species of macroinvertebrates was collected from the flowing river, 2.5-7.5 m wide and up to 47 cm deep, in autumn and spring 2011. About 40% of the stream consisted of fast-flowing riffles in autumn when large volumes of turbid River Murray water was being discharged to the Torrens River further upstream, as part of the water supply network for Adelaide; only about 20% of the river comprised riffle habitat in spring, when clearer, catchment flows were passing downstream. The community was dominated by generalists and species tolerant to poor water quality such as waterbugs (Micronecta), blackflies and amphipods. It also included smaller numbers of native and introduced snails, worms, freshwater shrimp, yabbies, beetles, mosquitoes, soldierflies, chironomids, mayflies, damselflies and caddisflies. A number of rare and sensitive species were collected including a blackfly (Austrosimulium furiosum) and several mayflies (Offadens, Atalophlebia australasica, Atalophlebia australis and Thraulophlebia inconspicua). A number of flow-dependent species were also recorded, including a blackfly (Simulium ornatipes), chironomid (Rheotanytarsus), hydropyschid caddisfly (Cheumatopsyche sp. 2) and the above-listed rare and sensitive species. The only fish seen at the site was an introduced carp in spring. .
The water quality of the site varied in response to the inflows of turbid River Murray water via the Mannum to Adelaide pipeline further upstream in autumn, compared with what appeared to be natural catchment flows in spring. The water was fresh to moderately fresh (salinity ranged from 261 mg/L in autumn to 1,262 mg/L in spring), well oxygenated (78-105% saturated), turbid in autumn but clear in spring, and with high concentrations of nutrients such as nitrogen (0.87-1.8 mg/L) and phosphorus (0.07-0.42 mg/L).
The sediments were dominated by cobble, boulder, detritus, sand and algae. Samples taken from below the surface were sandy grey in colour and showed no evidence that the sediments were anaerobic or lacked oxygen. Only a small deposit of silt, about 1 cm deep, covered the streambed and about 10 m of banks showed evidence of erosion caused by past flood damage.
A large amount of phytoplankton was recorded in autumn, presumably associated with the River Murray water discharged into the Torrens near Mount Pleasant. Small amounts of filamentous algae (mostly Cladophora) were seen in autumn but larger growths covered over 65% of the channel in spring. A range of aquatic plants was also present and covered about 35% of the channel, and included both submerged (Crassula, Vallisneria, Myriophyllum and Stuckenia) and emergent species (Cyperus, Eleocharis, Isolepis, Juncus, Phragmites, Rumex and Typha). The riparian vegetation consisted of scattered River Red Gums over introduced grasses, weeds and some patches of sedges and rushes. The surrounding vegetation was cattle grazing paddocks with a few remnant gum trees remaining in the local landscape.
Special environmental features
The Torrens River near Birdwood provides permanently flowing habitats that support a range of rare, sensitive and flow-dependent macroinvertebrates. It also supports patches of a rare plant for the region called eelgrass (Vallisneria) that has presumably colonised the Torrens River as a result of the water transfer; this plant is commonly found in the lower River Murray. The upper Torrens River also supports some 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.