Tod River, near the Tod Reservoir
2010 Aquatic Ecosystem Condition Report
- Permanent saline river with slow-flowing habitats present in both autumn and spring.
- Sparse macroinvertebrate community with no rare or sensitive species.
- Obvious signs of moderate to gross nutrient enrichment.
- Riparian vegetation was mostly introduced grasses and weeds.
- Large silt deposits in the channel and some bank erosion evident.
About the location
Tod River is the major surface water supply catchment in Eyre Peninsula. It rises north of Yallunda Flat and flows in a southerly direction before discharging into Spencer Gulf at the southern end of Louth Bay. The major land uses are sheep grazing, cropping, and minor areas of native vegetation and urban settlement. The Tod Reservoir, located just downstream from the site, is a large off-stream dam that receives water via concrete aqueducts from Pillaworta Creek and the Tod River.
The monitoring site was located off Growdens Road, about two kilometres north of the reservoir.
The river was given a Poor rating because the site sampled showed evidence of major changes in ecosystem structure and moderate changes to the way the ecosystem functions. There was considerable evidence of human disturbance, including nutrient enrichment, fine sediment deposition, bank erosion and poor riparian habitat.
A sparse community of about 21 species of macroinvertebrates was collected from the slow-flowing pool habitats, 4–6 metres wide and 40 centimetres deep, in autumn and spring 2010. The community was dominated by species tolerant to poor water quality and high salinity such as springtails in autumn and amphipods (Austrochiltonia australis) in spring. It also included moderate numbers of dytiscid beetles (Necterosoma penicillatus), blackfly larvae (Simulium ornatipes), chironomids (Procladius, Dicrotendipes and Chironomus) and waterbugs (Anisops and Micronecta). Smaller numbers of mosquitoes, biting midges, soldierflies, damselflies, dragonflies and caddisflies were also recorded. No sensitive or rare species were found. The only flow-dependent species collected was the blackfly in spring; it was not recorded in autumn despite the presence of some small flowing riffle habitats. The only fish recorded from the site was the introduced mosquitofish (Gambusia) that was collected in both seasons sampled.
The water was saline (salinity of 7,000 mg/L in autumn and 5,749 mg/L in spring), well oxygenated (98–110% saturation) and clear, with moderate to high concentrations of nutrients such as nitrogen (1.03–1.25 mg/L) and phosphorus (0.03–0.06 mg/L).
The sediments were dominated by detritus, gravel, sand and clay; samples taken from below the surface were occasionally sulfidic and anaerobic, or lacking in oxygen. Heavy sedimentation was noticed in autumn when more than 10 centimetres of silt covered the riverbed in places. In spring, less than one centimetre of silt was recorded from the channel, indicating that scouring flows had moved much of the fine sediment further downstream during the winter period. About 10 metres of bank erosion was noted in autumn, at a time when stock had been grazing on the edges of the water.
Little evidence of the presence of algae was noted in autumn but during the spring sampling period, moderate amounts of phytoplankton and filamentous algae (including Spirogyra) were recorded; the latter covered over 10% of the channel. Small growths of emergent aquatic plants (Bolboschoenus and Rumex) covered less than 10% of the channel.
The very narrow riparian zone consisted of introduced grasses, thistles, sedges (Baumea), rushes (Juncus) and acacias. The surrounding vegetation at the site was cropping land and grazed grassland, with only a little remnant native vegetation near the river.
Special environmental features
Pressures and management responses
|Livestock have direct access at the site and upstream in the catchment, causing sediment erosion and adding excessive nutrients (which leads to habitat disturbance, algal growth and aquatic weeds).||
The Eyre Peninsula NRM Board promotes managing land to improve water quality. This includes incentives for:
|Limited natural riparian vegetation at the site and upstream in the catchment, providing minimal buffer protection from catchment landuses (reducing habitat quality).|
|Saline groundwater inflow (reducing ecological integrity).||The Eyre Peninsula NRM Board promotes the revegetation of recharge areas known to contribute to dryland salinity and encourages the adoption of perennial pastures as an alternative to annual cropping in these areas.|