Light River, near Hamilton
2011 Aquatic Ecosystem Condition Report
- Permanently wet, mostly saline stream in autumn and spring 2011
- Moderately diverse macroinvertebrate community with no rare or sensitive species but includes two flow-dependent species
- Obvious signs of gross nutrient enrichment
- Riparian vegetation dominated by introduced grasses, rushes and native trees.
About the location
The Light River is a large river in the Northern Mount Lofty Ranges that rises south from Waterloo in the Mid North and flows in a southerly direction past Kapunda, and then heads south-west where it eventually discharges into Gulf St Vincent near Middle Beach. The monitoring site was located off Priors Road, west from Hamilton. The major land uses in the 26,602 hectare catchment are cereal cropping and stock grazing, with smaller areas used for oil seed production, remnant native vegetation, and other agricultural and residential uses.
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, salinisation, degraded riparian vegetation and fine sediment deposited in the channel.
A moderately diverse community of at least 33 species of macroinvertebrates was collected from the river, 1-2-4.3 m wide and up to 82 cm deep, in autumn and spring 2011. The river consisted of large areas of still to slow-flowing pool habitats connected by faster-flowing riffle habitats in autumn but comprised a non-flowing channel in spring. The community was dominated by generalists and species tolerant to high salinity and poor water quality such as amphipods (Austrochiltonia), blackflies and chironomids. It also included smaller numbers of nematodes, native and introduced snails, mites, freshwater shrimp, yabbies, springtails, beetles, biting midges, soldierflies, waterbugs, odonates and caddisflies. No rare or sensitive species were collected and only one uncommon species, a nepid waterbug (Laccotrophes), was observed. Two flow-dependent species were collected, including a beetle (Platynectes decempunctatus) and blackfly (Simulium ornatipes). Introduced pest fish called Mosquitofish (Gambusia) were the only fish recorded from the site in 2011.
The water was fresh when the river was flowing but saline when flow ceased (salinity ranged from 440-4,863 mg/L), and it was also generally well oxygenated (56-73% saturated), clear, and with moderate to high concentrations of nutrients such as nitrogen (0.75-1.64 mg/L) and phosphorus (0.04-0.06 mg/L).
The sediments were dominated by detritus, silt, boulder, cobble and algae in the non-flowing sections and detritus, silt, algae, sand and clay in the riffles. Samples taken from below the surface were silt grey in appearance and showed no evidence that the sediments were anaerobic or lacked oxygen. A large silt deposit, 5-10 cm deep, covered the streambed in spring and about 10 m of bank showed evidence of erosion caused by stock damage in autumn but was not evident later in the year.
A large amount of phytoplankton occurred in the stream in spring and around 10% of the channel was covered by filamentous green algae (mostly Cladophora) during both sampling periods. Over 35% of the river was covered by several types of emergent plants (Phragmites, Rumex, Cyperus and Isolepis). The riparian vegetation was dominated by introduced grasses, rushes (Juncus) and some paperbark trees (Melaleuca) and gums. The surrounding vegetation was cleared grazing land with only a few patches of gum trees remaining in the local landscape.
Special environmental features
None detected apart from the presence of two flow-dependent species of macroinvertebrates.
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.|
|Saline groundwater inflows to the creek (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. It was prepared with and co-funded by the Adelaide and Mount Lofty Ranges NRM Board.