Dale River - Reserve Pool

River condition of the Reserve Pool site (site code: SA720DALE1, site reference: 6154196) on the Dale River 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)

All previous assessments at this site are listed below.

• 2010 (October): Assessment of the ecological impacts of sediment removal at Reserve Pool (Department of Water)
• 2019 (November): Healthy Rivers (Department of Water and Environmental Regulation)
• 2023 (November): Healthy Rivers (Department of Water and Environmental Regulation)

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 Gwambygine Pool.

Other departmental data: The Reserve Pool site is about 12 km upstream of the Department of Water and Environmental Regulation’s (the department) flow gauging station on the Dale River known as Waterhatch Bridge (site reference: 615027), 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 Dale River is a tributary of the Avon River, and it experiences a more temperate climate with higher rainfall than the rest of the Avon basin. It has steeper landform gradients, with more incised, faster-flowing rivers than the eastern section of the Avon basin (Henning and Kelsey 2015). The headwaters of the Dale River are largely uncleared, due to the unsuitable steepness of the land for agriculture, unlike most of the rest of the Avon catchment. The Dale River is relatively fresh water (Department of Water [DOW] 2006) and is an important source of fresher water to the Avon River, where it joins between Beverley and York. This is an important factor in the relatively healthy condition of Gwambygine Pool on the Avon River, a few kilometres downstream of the tributary of the Dale and Avon rivers. The Dale River is suffering from the effects of sedimentation though, mostly seen through the filling of pools largely due to eroding banks along most of the river (DOW 2006). This increases the threat to the ecological health of the system by reducing the physical capacity of the pools to provide refuge for aquatic fauna over summer and autumn, as well as by loss of aquatic habitat diversity (through smothering).

10,000 cubic metres of sediment was removed from the pool in 2010, and an ecological assessment was conducted in October. Some of that sediment from the 2010 excavations is still present in the adjacent zone about 50 m from the river. The October 2010 assessment confirmed the presence of nightfish and western pygmy perch in low numbers (DOW 2011).

The Reserve Pool assessment site is on the Dale River about 95 km east of Perth. The healthy rivers assessments (2019 and 2023) have focused on the upstream end of the pool where sediment is increasingly building up again (following the 2010 removal of sediment from the pool) – see centre of image above.

The pool is a few hundred meters long and about 30 m wide. At the time of sampling the average pool width and depth in the site was 30–40 m and 1–1.5 m, respectively, although the centre of the pool was more than 2 m deep. Bankfull width was about 60 m.

The banks were concaved with some wide lower benches, and a range of bank gradients from steep to low. This was difficult to assess at the upstream end of the site because of the large slug of coarse sand that has both filled the pool and covered the right bank.

Reserve Pool was too deep in the centre for a comprehensive assessment of aquatic habitat, as such, the following habitat description relate to the accessible areas closer to the banks.

The aquatic habitat was moderately diverse and healthy but also heavily impacted by sedimentation which poses a risk to the long-term health of the pool.

The site was entirely pool habitat. The margins of the pool were relatively well shaded with tree overhang providing stream shading along 75–80 per cent of both banks and extended for about 3 m from the banks. This means much of the centre of the pool was unshaded, but this is offset by the fact the centre of the pool was deep where cooler water would offer fauna refuge from warmer surface waters. Woody debris was sparse and of 2–3 sizes (with larger wood not observed). Some woody debris was partly buried in both fine depositional sediment and heavier coarse sand. Biological substrate cover including leaf packs (which provide habitat and shelter for fauna) covered around half of the bottom of the pool that could be assessed (noting the middle was too deep to assess).

Aquatic plants covered about 25 per cent of the site (see image below). A submerged macrophyte, possibly a species of water nymph or naiad (Najas spp.) was observed, along with Ruppia spp. The aquatic macrophytes provide habitat and food for invertebrates and fish, particularly important as nursery habitat.

The aquatic habitat was broadly comparable to the previous assessment in November 2019. One noticeable difference was the current size of the sediment slug at the upstream end of the pool which was much greater in 2023.

The surface hydrology of the Dale River is highly variable. Between 1996 and 2024 the department’s gauging station at Waterhatch Bridge (GS# 615027), which is about 12 km downstream of the assessment site, has recorded an average annual discharge of 32 gigalitres (GL), but with a standard deviation of almost 26 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 geomorphology and hydrogeology of the Dale River, like the broader Avon system, is complex and highly compartmentalised, with many localised aquifers and strong interaction between groundwater and surface water. Water quality in the Dale River pools is locally influenced by these interactions, with horizontal seeping and vertical upwelling of either fresh and/or saline waters, particularly during summer.

Based on records from the Waterhatch Bridge DWER gauging station 12 km downstream, the Dale River ceases to flow (wet disconnected flow phase) during summer through to early autumn during most years. 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 and Dale Rivers in recent decades, including Reserve Pool in 2010.

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.

At the time of sampling, the pool was connected to the channel upstream by a very narrow and shallow section of water flowing through the sandy sediment that has been deposited at the upstream end of the pool. Flow was visible, but very slow, and not measurable with a flow meter. It is reasonable to assume the pool will become disconnected from the rest of the river through summer and autumn as water levels drop. The river at the department’s flow gauging station, around 12 km downstream (Waterhatch Bridge), consistently ceases to flow over summer. While this is a natural feature of many south-west rivers, it is important that these pools possess sufficient habitat quality and diversity, as well as maintain sufficient water quality, to support resident fauna over summer and autumn.

The riparian zone (containing species adapted to living at the interface between aquatic and terrestrial environments) was limited to 10 m wide on both banks, but largely because of landform rather than any clearing. There was simply a rapid change to a more terrestrial based vegetation e.g., wandoo (Eucalyptus wandoo) at the site, which is illustrated in the image below. The vegetation at the site consisted of a largely native tree canopy with an exotic understory, typical of remnant vegetation in disturbed or modified systems.

The riparian tree layer consisted of mature flooded gum (Eucalyptus rudis) and paperbarks (Melaleuca spp.), with the paperbarks being at the end of their flowering. The ground cover layer was reduced and consisted mostly of exotic grass with some dock weed (Rumex spp.). Native ground cover at the site consisted of some dense stands of shore rush (Juncus kraussii), particularly on the left bank. This rush is common throughout the Avon and Dale Rivers. Some saltbush (Atriplex spp.) was also recorded through the site, indicating some impact of salinity on the site. While the Dale River is relatively fresh, the salinity of the site was classed as low saline, see water quality section. A native riparian shrub layer was absent. There was evidence of natural recruitment of trees, which were in healthy condition but limited in extent. There were also some areas of apparently planted rushes. The condition of the riparian vegetation was similar to that observed in 2019.

There was a moderate extent of erosion through the site, but this was of limited severity. Generally, the banks were structurally stable since the nature of the pool is more depositional than erosive. This is illustrated by the ongoing sedimentation issues impacting the pool, and this also highlights wider erosion issues upstream in the catchment.

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

Water temperature over the 24 hours was optimal as the 25°C threshold¹ was not exceeded and the diurnal range was less than 4°C (2.93°C). Dissolved oxygen was also optimal over the 24 hours, as it did not fall below the low threshold² value of 4 mg/L. Ideally, the diurnal range in dissolved oxygen concentration should be no more than 4 mg/L and at the site over the 24 hours it was 3.22 mg/L. While the diurnal range in dissolved oxygen was optimal it is reasonable to assume it will increase through the warmer summer months. This potential for greater diurnal flux in dissolved oxygen could place additional stress on aquatic fauna. It is expected there would be refuge from adverse conditions at depth, if the pool can maintain the depth and volume required, in the long term, with the sediment load it is receiving.

Specific conductivity (which can be used as an indicator of salinity) was an average of 8.98 mS/cm over the 24-hour assessment period, which is in the low saline range, but fresher than the other three sites assessed on the Avon River. pH was optimal (average of 7.71), based on expectations of natural ranges for freshwater rivers of south-west Western Australia. Turbidity was low, 2.9 nephelometric turbidity units (NTU), and true colour was 50 true colour units (TCU) which was consistent with the field observation of ‘clear’ relating to the degree of tannin staining.

Nutrients were optimal with a low total nitrogen concentration (0.61 mg/L) and a total phosphorus concentration (0.027 mg/L) at the lower end of the moderate range.

The water quality was generally similar to that observed in 2019, bearing in mind the observations are only over a 24-hour period and subject to a range of influencing factors such as current weather and air temperatures and past winter rain and resulting flow.

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 Reserve 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.

Six species of fish and two crustacean species were recorded during the 2023 sampling. This included five 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 2019.

Western minnow was recorded for the first time at this site. Also recorded were western pygmy perch and nightfish, but all in relatively low abundance. This is probably because single-wing fyke nets were used because of the depth of the pool, as a result only about 5–10 percent of the pool cross-section was covered. This is likely to result in fewer fish being caught than using the usual method of double-wing nets across a whole river channel, for example. However, the most abundant fish recorded was the exotic eastern gambusia (including juveniles), which suggests a well-established population in the pool. Eastern gambusia present a risk to populations of native fish through competition and fin nipping.

In addition, two of the five 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 western pygmy perch and western hardyhead (as well as eastern gambusia), which would suggest suitable habitat conditions within the pool to support breeding and recruitment for those species. The two native crustaceans (crayfish and shrimp) recorded were gilgies and south-west glass shrimp.

The presence of three freshwater dependant native fish species (nightfish, western pygmy perch and western minnow) and the two freshwater crustacean species (particularly the gilgie) illustrates the relatively freshwater nature of the Dale River within the salinised Avon catchment. While this represents an improvement in species richness from 2019, it could reflect sampling effort (i.e., that the pool has been assessed more times). In other words, the species were likely always present; it is just that previous assessments failed to detect them, which illustrates the importance of repeated, routine surveys to build a comprehensive picture of the species present.

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 was observed, with remains of prey (shells of crayfish), and paw prints found at the site. Rakali have also been observed by local members of the Friends of the Dale River community group. This species is listed by DBCA as a Priority 4 species (rare, near-threatened and other species in need of monitoring).
• diving beetles
• dragonfly larvae
• leeches

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