Inman River, off Inman Valley Road, near Victor Harbor
2008 Aquatic Ecosystem Condition Report
- Affected by salinity, nutrient enrichment and fine sediment.
- Macroinvertebrate community dominated by species tolerant of pollution and brackish conditions.
- Riverbanks eroded and riparian zone invaded by weeds.
- Catchment provides habitat to threatened fish species.
About the location
The Inman River rises west of the Inman Valley on the Fleurieu Peninsula and flows southeasterly into Encounter Bay, south of Victor Harbor.
The site selected for monitoring was located off the Inman Valley Road, about two kilometres northwest of Victor Harbor. Livestock grazing (55%) and dairying (29%) are the main land uses in this area.
The river was given a Fair rating at this site because the ecosystem showed evidence of moderate changes to animal and plant life, and some changes to the way the ecosystem functions. The stream is being affected by moderately high nutrient levels, salinity and fine sediment.
The river consisted of a series of connected pools, ranging from areas with shallow water to over one metre deep, at the time of sampling in December 2008.
The pools provided habitat for a moderately diverse community of 37 macroinvertebrate species. Species tolerant of high nutrient levels and salinity levels well over the typical threshold for many freshwater organisms (salinity of about 1,000 mg/L) were the most commonly collected, including tiny crustaceans called water scuds (Austrochiltonia australis), waterbugs (Micronecta) and chironomids (Procladius and Tanytarsus). One fly larva from the family Dixidae was the only sensitive species collected at the site.
The water was moderately fresh (salinity of 2,400 mg/L) and well oxygenated (93% saturation). It was clear and only slightly coloured, however, there were moderate to high levels of nutrients such as nitrogen (0.77 mg/L) and phosphorus (0.05 mg/L).
Sand made up half the sediments on the riverbed, which also included detritus and silt. The sediment was anaerobic and sulfidic, indicating too much organic material had entered the river in the past. More than half the river’s banks were eroded by the combined impacts of cattle trampling, slumping and high flows after heavy rainfall.
Phytoplankton and two types of green filamentous algae (Cladophora and Spirogyra) covered more than 10% of the river. Up to 65% of the channel was also covered by aquatic plants such as Narrow-leafed Cumbungi (Typha domingensis), Sharp Clubrush(Schoenoplectus pungens), Sea Rush (Juncus kraussii) and stonewort (Nitella).
River Red Gums and acacias grew sparsely along the river’s edge. Woody and herbaceous weeds and introduced grasses made up most of the understorey, including gorse, thistles, melons and kikuyu. Limited native vegetation remained in the surrounding area which was covered mainly with cereal crops.
Special environmental features
The mid to lower reaches of the Inman River and Back Valley Creek provides habitat for a range of threatened and more common native fish species, including the Southern Pygmy Perch, Climbing Galaxias, Murray–Darling Carp Gudgeon, Common Galaxias and Western Bluespot Goby . However, recent sampling during 2007–09 has failed to detect the continued presence of some species in the Inman catchment (M Hammer, Aquasave Consultants, 2009).
Pressures and management responses
|Livestock have direct access at the site and upstream, causing sediment erosion and adding excessive nutrients (which leads to habitat disturbance, algal growth and aquatic weeds).||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 site and upstream, providing minimal buffer protection from catchment landuses (reducing habitat quality).||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.|
|Saline groundwater inflow (reducing ecological integrity).||The Adelaide and Mount Lofty Ranges NRM Board has installed telemetered groundwater monitoring stations at key locations within the region. These are monitored for level and salinity; unusual results (such as high salinity influxes) are investigated.|
This aquatic ecosystem condition report is based on monitoring data collected by the EPA and prepared in conjunction with the Adelaide and Mount Lofty Ranges NRM Board.