Avon River - Gwambygine Pool

River condition of the Gwambygine Pool site (site code: SA614AVON1, site reference: 6150023) in the Avon River catchment between York and Beverley was assessed between 13 and 14 November 2023. Results are compared with previous assessments.

Healthy Rivers assessments are conducted using standard methods from the South West Index of River Condition (SWIRC), which incorporates field and desktop data from the site and from the broader catchment. Field data collected include the following indicators, assessed over about a 100 m length of stream:

• Aquatic biota: fish and crayfish community information (abundance of native and exotic species across size classes, general reproductive and physical condition)
• Water quality: dissolved oxygen, temperature, specific conductivity, and pH (logged in situ over 24 hours) as well as laboratory samples for colour, alkalinity, turbidity and nutrients
• Aquatic habitat: e.g. water depth, substrate type, presence of woody debris and detritus, type and cover of macrophytes and draping vegetation
• Physical form: channel morphology, bank slope and shape, bioconnectivity (barriers to migration of aquatic species), erosion and sedimentation
• Fringing zone: width and length of vegetation cover within the river corridor and lands immediately adjacent, structural intactness of riparian and streamside vegetation
• Hydrology: measures of flow (velocity) at representative locations (compared against data from stream gauging stations within the system)
• Local land use: descriptions of local land use types and activities (compared against land use mapping information for the catchment)

The site was previously assessed during September 2010.

This site was one of four assessed on the Avon and Dale rivers during November 2023; the other sites were Avon Pelham Brook (in Toodyay), Avon York and Reserve Pool.

Other departmental data: The Gwambygine Pool site is about 12 km upstream of the Department of Water and Environmental Regulation’s (the department) flow gauging station on the Avon River known as Ballardong Street York (site reference: 615024), which has been in operation since 1995.

Search the site code or site reference in the department's Water Information Reporting (WIR) system to find data for this site and nearby sampling points (flows, surface water quality, groundwater monitoring, the department's meteorological data). See also the Bureau of Meteorology website for additional meteorological data for the area.

The Swan-Avon catchment is the largest catchment in south-west Western Australia, covering over 120,000 km². It is climatically and geologically diverse. The mainstem of the Avon River has its headwaters at Lake Yealering, 40 km east of Pingelly. It travels northwards, joins the Dale River and passes through Beverley, York, Northam and Toodyay before turning east and then south-east as the river traverses the Darling Scarp before it joins Wooroloo Brook, where the Avon River becomes the Swan River.

Gwambygine Pool is on the Avon River about 12 km upstream of the Wheatbelt town of York, which is about 90 km east of Perth. Prior to European settlement, the Avon River in Noongar Ballardong country was a braided river, with many small channels amongst vegetated islands and with deep, well-shaded pools.

Historically, the Avon River has been disturbed by two activities: clearance of woodland for agriculture, and ‘training’ of the river channel to abate flooding. These two activities mobilised the salt that was long established deep in the soil, and the coarse riverbed sediments, respectively.

The cumulative effects of land clearing, eutrophication, salinisation, sedimentation and the river training scheme resulted in a highly degraded catchment and river system. However, as awareness has increased over recent decades, significant investment and progress has been made to fence, revegetate and protect the Avon River channel. It is estimated at least 95 per cent of the Avon River has been fenced and that significant recovery of riparian vegetation has occurred as a result (NRM Wheatbelt 2013). However, fencing needs constant maintenance, and the current condition of the fencing is unknown.

The Gwambygine Pool assessment site is at the upstream end of Gwambygine Pool on the Avon River, between Beverley and York. This pool is one of the largest and ecologically most highly regarded pools on the Avon River, largely because it is less saline than other Avon pools. This is because of the influence of the Dale River (which is relatively fresh) which discharges to the Avon River about 10 km upstream.

The pool is about 1 km long and 30 m wide. At the time of sampling the average pool width and depth at the site was 15–30 m and 1.5–2 m, respectively, although the centre of the pool was more than 2 m deep. Bankfull width was about 40 m.

The banks were concaved, with some sections of undercutting or with wide lower benches. Banks were steep and high on the right bank (with evident erosion), and moderate to low on the left bank, and the low-flow channel (i.e. banks below the current waterline) was U-shaped.

Note: the image above is looking upstream, and bank sides are named left and right when looking downstream.

Gwambygine Pool was too deep in the centre for a comprehensive assessment of aquatic habitat. The following descriptions relate to habitat through the accessible margins closer to the riverbanks.

The site was entirely pool habitat. The margins of the pool were relatively well shaded with tree overhang providing stream shading along 85–95 per cent of both banks and extended for about 4 m from the banks. This means much of the centre of the pool was unshaded, but this was offset by the fact the centre of the pool was relatively deep, where cooler water would offer fauna refuge from warmer surface waters.

Aquatic habitat was near optimal except for a slight amount of sediment deposition and salinity. Woody debris of 2–3 different size categories (all less than 50 cm diameter) was moderately abundant at the site, including a lot of debris that appeared to have been deposited from upstream by winter flows. Biological substrate cover was moderately abundant and dense, including epiphytes, algae detritus and leaf packs. These features provide a diversity of habitat types and food for aquatic fauna.

Aquatic plants covered 10 per cent of the site and consisted of charophytes and Ruppia spp. (vascular macrophyte) that is common in the salinised Avon system but is usually found in estuaries. This macrophyte cover would provide habitat and shelter for small and juvenile aquatic fauna. Much of the Ruppia at the site was covered in epiphytes. Benthic mats of filamentous algae were also observed which, during the day, can float to the surface as the algae photosynthesises. This density of algae has the potential to significantly impact the dissolved oxygen concentration in the water, which was observed with a diurnal range in dissolved oxygen of almost 6 mg/L – refer to water quality section for a further description.

Habitat was also provided by roots of riparian and terrestrial vegetation draped in the water along about half the length of both banks, although on the right bank, much of this was from tree roots exposed by erosion.

The substrate that could be assessed was mostly sand, with some silt and clay. Some areas were soft and depositional in nature, although there was no widespread evidence of sedimentation at the site.

The surface hydrology of the Avon River is highly variable. Between 1996 and 2023 the department’s gauging station at Ballardong Street (GS# 615024), which is about 12 km downstream of the assessment site, has recorded an average annual discharge of 80 gigalitres (GL), but with a standard deviation of 87 GL. Annually, the flow cycle has three main phases, typical of many river systems in the south-west of Western Australia:

• bank flow – medium to high flows with occasional inundation of the floodplain
• wet connected – connected pools allow biota to recolonise the broader aquatic habitat from the refuge pools and
• wet isolated – pools provide refuge habitat but are disconnected (Wheatbelt NRM 2013).

The period of disconnection (wet isolated phase) over the dry and hot summer months is likely to increase across the south-west of the state because of the effects of climate change. This is likely to have impacts on water quality and the movement of aquatic species to support life history requirements.

The geomorphology and hydrogeology of the Avon River is complex and highly compartmentalised, with many localised aquifers and strong interaction between groundwater and surface water. It appears that water quality in the Avon River pools is locally influenced by these interactions, with horizontal seeping and vertical upwelling of either fresh and/or saline waters, particularly during summer months (Wheatbelt NRM 2013). Gwambygine pool is additionally influenced by relatively freshwater inputs from the Dale River.

Based on records from DWER gauging stations along the Avon (at Toodyay, Northam and York) the Avon River dries to pools (wet disconnected flow phase) from late spring to early summer through to autumn. During this period, the pools are a critical refuge for aquatic fauna but are often under the greatest stress from water quality issues revolving around saline groundwater intrusions and increased incidence of potentially harmful algal blooms (e.g. cyanobacteria) due to eutrophication. The additional impact of sedimentation and filling of these pool refuges increases the threat to the ecological health of the system. To address this problem sediment has been removed from various pools throughout the Avon River in recent decades, including Gwambygine Pool in 2010.

At the time of sampling Gwambygine Pool was in the wet connected flow phase. The site was connected upstream of the pool with flow visible through a shallow riffle-like area where the river flowed over an area of exposed rock (see centre of image below). As water levels fall during the warmer months, the pool will become disconnected and remain so until the onset of winter flows.

The riparian zone (containing species adapted to living at the interface between aquatic and terrestrial environments) was 40 m wide on the left bank, and 10 m wide on the right bank, because of landform, but also clearing for agriculture. The vegetation at the site consisted of a native tree canopy with an exotic understory, typical of remnant vegetation in a disturbed or modified system.

The streamside tree layer was dominated by swamp sheoak (Casuarina obesa), which is more tolerant of the salinised conditions in the Avon River. Flooded gum (Eucalyptus rudis) and paperbarks (Melaleuca spp.) were also present, though the shrub layer was absent, and the ground cover was reduced (by erosion) and mostly consisted of weeds, including pasture grasses and dock leaf (Rumex spp.). There was evidence of natural recruitment of trees, which were in healthy condition and of moderate extent throughout the site. This included a lot of new growth from fallen paperbarks.

Erosion was evident in the site, particularly on the right bank (see image below).

Most of the right bank was affected by relatively severe erosion, with undercutting, exposed tree roots and soil loss from the banks observed. Less of the length of the left bank was affected by erosion and, where observed, it was less severe than on the right bank. There were no signs of livestock at the site, and the fencing on the right bank was in good condition. Further downstream, livestock (sheep) have been observed, with pugging and manure demonstrating they have access to the river. If this is a regular occurrence it is likely to increase erosion and nutrients in the water and impact vegetation.

Below is a graph of the water quality recorded during the 24-hour assessment.

Water temperature was optimal over the 24 hours with a maximum recorded temperature of 23.06°C (below 25°C threshold)¹. The diurnal temperature range was also optimal, below 4°C (3.52°C). It is important to consider that this result is from November, and the water temperature is likely to increase over the summer months, highlighting the importance of riparian shading for waterways.

Dissolved oxygen was also optimal, remaining above the 4 mg/L threshold² for the entire 24 h assessment period. However, the diurnal range of 5.94 mg/L (5.26–11.2 mg/L) was suboptimal. The oxygen range combined with the elevated maximum concentration demonstrates significant influence of biological factors (i.e., diurnal respiration and photosynthesis phases of filamentous algae). This diurnal flux is likely to be even greater over summer, with biological productivity and processes increasing in warmer temperatures. Respiration through the night may reduce dissolved oxygen concentrations to levels below the tolerances of aquatic species.

Specific conductivity (which can be used as an indicator of salinity) was an average of 12.28 mS/cm over the 24-hour assessment period, which is in the mid-saline range. While this is suboptimal, it is less saline than sites further downstream, indicating the freshwater influence of the Dale River on this section of the Avon River.

pH was optimal (average of 8.48), based on expectations of natural ranges for freshwater rivers of south-west Western Australia. Turbidity was low, 2.6 nephelometric turbidity units (NTU), and true colour was 50 true colour units (TCU).

Nutrient concentrations were both moderate; total nitrogen was 1.08 mg/L and total phosphorus was 0.041 mg/L.

Footnote:

¹ An upper limit for temperature is nominally set at 25°C based on the range recorded in sites where south-west native fish species are typically found to occur (Beatty et al. 2013).

² A lower limit of 4 mg/L is taken from the SWIRC where it is used as a guideline limit for condition scoring. This level aligns with Beatty et al. (2013) where south-west native fish species were shown to typically reside in sites with levels above 4 mg/L.

The species captured at the Gwambygine Pool site during each of the assessments since 2010 are provided below in the table below. The species expected to occur here are based on species found at the site and elsewhere within the wider subcatchment. As differences in habitat within a reach naturally influence species distributions, and variability in methods between sampling programs can affect the species caught, this list is only indicative.

Five species of fish and two crayfish species were recorded during the 2023 sampling. This included four native fish species, one exotic species of fish, and two native crustacean species. This is a slight increase in the number of species detected since the previous assessment in 2010. This diversity of aquatic fauna is also greater than the other two sites on the Avon mainstem, further highlighting the ecological importance of Gwambygine pool.

Western pygmy perch (including juveniles) were recorded for the first time at this site. Western minnows were also recorded, but nightfish were not detected, although they were recorded in 2010. Hopefully, this reflects sampling effort rather than a loss of species. Three single-wing fyke nets were used in fish collection, which only cover a small cross-section of the pool (limited to edge habitat). The exotic eastern gambusia (including juveniles) was also present, which presents a risk to populations of native fish through competition and fin nipping.

Two of the four native fish species recorded were blue-spot goby and western hardyhead; usually estuarine opportunistic species, they have become established inland in the salinised Avon system. Juveniles were recorded for both species indicating well-established populations. The two native crustaceans (crayfish and shrimp) recorded were gilgies and south-west glass shrimp.

The presence of two freshwater dependant native fish species (western pygmy perch and western minnow) and the two freshwater crustacean species (particularly the gilgie) illustrates the relatively freshwater nature of Gwambygine Pool within the salinised Avon catchment.

Note: collection of fauna from inland aquatic ecosystems across Western Australia requires a licence from the Department of Primary Industries and Regional Development (DPIRD) and the Department of Biodiversity, Conservation and Attractions (DBCA). All species collected must be reported to these agencies as part of licence conditions.

Only freshwater fish and crayfish that typically inhabit river channels are targeted by the standard SWIRC sampling methods; however, other species are sometimes captured or observed.  The following species were detected at the site:

• south-western snake-necked turtle (Chelodina oblonga)
• rakali (Hydromys chrysogaster) activity has been observed by local members of the York River Conservation Society. This species is listed by DBCA as a Priority 4 species (rare, near-threatened and other species in need of monitoring).
• Fluviolanatus subortus, a very small species of bivalve, endemic to Australia. This species is usually found in the lower reaches of estuaries attached to the underside of logs and rocks. Naturally found the Swan-Canning estuaries it may now be established in the Avon (like the blue spot goby and western hardyhead) as it offers a similar habitat.
• diving beetles
• dragonfly larvae

For more information on these and other aquatic species, please see the River Science fauna page.