Eastern branch of Western River, u/s from estuary
2013 Aquatic Ecosystem Condition Report
- Permanently flowing stream in autumn and spring 2013
- Diverse macroinvertebrate community with several sensitive and flow-dependent species present
- Water was moderately fresh to saline, clear and high in nitrogen only in autumn
- Riparian vegetation comprised native vegetation
About the location
Western River is located on the north-western end of Kangaroo Island. The main eastern branch rises at an elevation of about 235 m and flows in a northerly direction for about 10 km until it discharges into Investigator Strait in Western River Cove. The major land uses in the 8,814 hectare catchment upstream from the site included minimal uses (29%), nature conservation (25%), plantation forestry (24%) and grazing modified pastures (20%), and included smaller areas of roads, cropping, intensive animal production, wetlands and dams. The site sampled was located in the lower reaches of the eastern branch upstream from Western River Road, about 1.5 km south-east from Western River Cove.
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 of the riparian and terrestrial vegetation surrounding the river and also the presence of marron in 2013.
A diverse community of at least 30 species of macroinvertebrates was collected or seen from the river (17 species in autumn and 24 in spring), 0.8-2.5 m wide and up to 28 cm deep, in autumn and spring 2013. The river consisted of large areas of flowing riffle habitats connecting slower flowing pools in autumn, whereas the slow-flowing pools were more extensive in spring. The community was dominated by large numbers of blackfly larvae (Simulium ornatipes and Austrosimulium furiosum) in the riffles and included low numbers of introduced snails (Potamopyrgus), mites, amphipods, sminthurid springtails, beetles, mosquitoes, biting midges, danceflies, chironomids (including Eukiefferiella, Parakiefferiella and Thienemanniella), waterbugs, odonates, stoneflies (Dinotoperla evansi and Illiesoperla mayii) and caddisflies (including Cheumatopsyche). Marron and yabby burrows were recorded from the banks, indicating that they commonly occurred at the site in 2013. The stoneflies, blackflies and caddisfly were significant records for the river, since these sensitive species are typically found in clear, freshwater flowing habitats. The other species recorded were more generalist species that are tolerant to poorer water quality and habitat conditions.
The water was moderately fresh to saline (salinity ranged from 2,316-3,331 mg/L), well oxygenated (108-132% saturation), clear and with low concentrations of phosphorus (0.01 mg/L) and high nitrogen concentrations only in autumn (0.30-0.75 mg/L).
The sediments were dominated by cobble and pebble in the riffles and finer silts, detritus and sand in the slower-flowing pool habitats. Samples taken from below the surface were well-aerated grey sands in autumn but the presence of sulphide and blackened rocks in spring, indicates that the sediments turn anaerobic and lack oxygen for at least part of the year. There was some minor bank erosion extending over about 10% of the site in spring, which appeared to have been caused by recent flood damage during winter. There was no sign of any animal droppings in the vicinity of the river, despite the presence of cattle in surrounding paddocks. This suggests that the exclusion fencing adjacent to the river had been successful in keeping stock from accessing the site.
There was a small to moderate growth of phytoplankton recorded during the year (chlorophyll a ranged from 0.7-4.2 μg/L) but there was no sign of any significant filamentous algal growths seen in 2013. Over 10% of the channel was covered by sedges (Cyperus) and rushes (Juncus). The riparian zone extended over 5m wide in places and was dominated by gums over sedges and bracken. The surrounding vegetation near the river comprised cleared cattle grazing land with a few scattered gums in the local landscape.
Special environmental features
Western River provides a permanently wet, fresh to brackish habitat that supports several sensitive, flow-dependent species and a wide range of tolerant, generalist macroinvertebrates.
Pressures and management responses
|Limited riparian zone vegetation adjacent to the creek, providing minimal buffer protection from catchment landuses (reducing habitat quality).||The Kangaroo Island NRM Board has funded the fencing of significant areas of riparian vegetation in the catchment and continues to work with landowners to increase the fencing of watercourses.|