Preston River - Dardanup Road West

River condition of the Dardanup Road West site (site code: PR40PRES1, site reference: 6114021), on the Preston River, was assessed between 14 and 15 February 2023. This was compared to a previous assessment in 2020.

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 February 2020.

Water quality loggers were deployed for an extended period (see below). This enabled assessment of water quality responses to changes in climate, streamflow and factors such as the intactness of vegetation within the river corridor. Among other things, this allows detection of potential suboptimal water quality conditions which can be missed during the standard 24-hour assessment.

• 2019–2020 (Dec–Apr): Healthy Rivers
• 2022–2023 (Nov–Apr): Healthy Rivers

Other departmental data: The Dardanup Road West site is about 7 km downstream of the Department of Water and Environmental Regulation’s (the department) flow gauging station known as Boyanup Bridge (site reference: 611004), which has been in operation since 1980.

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 Dardanup Road West HRP site is on Preston River, about 10 km south-east of Bunbury, in the south-west of Western Australia.

The site is in a meandering plane-bed channel within a broad river valley, typical of the coastal plain. The average water width and depth at the time of sampling was 8 m and 0.5–0.99 m respectively. The bankfull extent of the floodway was particularly deep (about 8–12 m) and wide (up to 60 m), suggesting very high winter storm flows. The river was within a relatively low-gradient section, with stepped banks and some sections of wide lower bench, more so on the right bank. The left bank was dominated by very high and steep banks as it was largely on the outside of a bend.

Most of the site (98 per cent) was channel habitat with a small section of shallow run (2 per cent). The in-stream aquatic habitat was varied, providing a diverse range of potential niches and conditions for aquatic fauna (see woody debris, depth and substrate composition below). Widespread sedimentation presented a significant ongoing risk to habitat condition, with some areas of substrate habitat being smothered.

At the time of sampling, water depth was varied, from an average of 0.5–0.99 m and up to 2 m deep in the open channel and between 0.05–0.24 m in the small section of shallow run habitat.

Woody debris was present in a variety of sizes, as would be expected naturally, but the abundance was only moderate, less than would be expected. The reduced abundance of woody debris (especially considering the diversity of sizes present) could be because of sedimentation covering the wood and/or may be being pushed downstream with very high winter flows (with reduced replacement because of clearing through the upper catchment).

The substrate featured pebble, gravel, sand and silt, with sand and silt dominant. Biological substrate cover (algae, detritus and leaves) was present across most of the site (60–100 per cent) but in moderate density.

Overhanging roots, overhanging banks and bank vegetation draped in the water provided habitat along up to half of the bank lengths (10–49 per cent).

Stream shading was near optimal on the left bank where 70 per cent of the bank was shaded by the tree canopy, up to 8 m across the river from the bank, with an average channel width of 8 m. On the right bank the trees were generally set further back from the bank, with only 35 per cent of the channel shaded to an average of 5 m from the bank. Shrub shading was extensive on both banks (85 per cent), extending 1–2 m from the bank across the river. The presence of a shrub layer at the site (which is often reduced or absent in agriculture areas) was encouraging.

The Preston River is a perennially flowing system along much of its length because of water releases from Glen Mervyn Dam for the Preston Valley Irrigation Cooperative, as well as from a net input of groundwater from the superficial aquifer (Department of Water 2015, unpublished) on the coastal plain (west of the Darling Scarp).

At the time of the assessment the site was connected and flowing. The department’s flow gauging station at Boyanup Bridge (614004) – 15 km upstream of the site – measured a total daily discharge of 10.7 ML/day during the field assessment. The minimum daily discharge recorded at that station during the period the long-term water quality logger was deployed was 8 ML on 22 January 2023.

The relatively high dry-season flows are unusual for south-west rivers. The resulting flow varied between 0.3 m/s and <0.1 m/s through the site at the time of sampling. Connectivity was further demonstrated by large-bodied freshwater cobbler moving upstream through the site in February.

The riparian zone (containing species adapted to living at the interface between aquatic and terrestrial environments) was 30–35 m wide on both banks and mostly native (except for the ground cover layer).

A mature tree canopy of mostly flooded gum (Eucalyptus rudis), with some peppermint trees (Agonis flexuosa), was present. Some of the larger trees were also set back from the river by channel shape, on the right bank particularly. This resulted in less optimal shading on the right bank, though this was offset by the physical depth of the bankfull channel (8–12 m through the site). Shading was also reduced towards the downstream end of the site, where riparian vegetation has been cleared around a road bridge.

A native, woody shrub layer was present, which was notable as this is often reduced or absent in many south-west river systems, through agricultural areas. In the streamside zone (the first 10 m from the riverbank) the shrub layer – largely tea tree (Melaleuca spp.) – covered most of the right bank (>75 per cent). Natural recruitment of both trees and shrubs was observed in a moderate abundance and good health.

The ground cover layer was mostly exotic, consisting of pasture grass and Watsonia spp. (bugle lily). The bare ground near the road bridge provided good conditions for coloniser weed species, and cotton bush was observed here, but in previous years deadly nightshade and wild melons (Citrullus lanatus var. lanatus) have also been observed in the same area.

Channel stability was varied. The left bank was more affected by erosion, both in extent and severity. Many trees on the steeper left bank showed exposed roots (some about 5 m or more above current water level) and undercutting and slumping was observed in places (see image below); however, this is not unusual on the outside of bends with high winter flows creating strong eroding forces.

Below is a graph of the long-term water quality recorded at Dardanup Road West from November 2022 to April 2023.

Water temperatures were near optimal; the upper temperature threshold¹ of 25°C was only exceeded for 0.76 per cent of the deployed period. Any exceedances were minor and short-lived. Diurnal ranges in water temperature were similarly optimal with the daily range of 4°C not exceeded.

Water quality was optimal in terms of dissolved oxygen. The dissolved oxygen concentration never fell below the low threshold² of 4 mg/L. The dissolved oxygen diurnal range was also generally optimal (<4 mg/L).

This is a relatively unusual situation for south-west river systems over summer, especially in the current drying climate. The optimal water quality observed is likely supported by the strong flow releases and baseflow through summer months, when many systems would be experiencing much slower, or no, flows. Under these conditions the water is aerated as it flows through riffles and snags of woody debris, maintaining an optimal dissolved oxygen concentration.

Specific conductivity (which can be used as an indicator of salinity) was optimal. Specific conductivity was at the very lower end of the marginal range (on the cusp of fresh) with an average value of 0.81 mS/cm over the November 2022 to April 2023 logger deployment. Specific conductivity was stable over that time (see purple line on bottom graph – above).

pH was stable over the long-term logger deployment and within acceptable ranges (6.5–8.0), based on expectations of natural ranges for freshwater rivers of south-west Western Australia.

The long-term water quality recorded over this assessment was broadly consistent with the previous assessment (2019–2020), indicating optimal conditions are being maintained, although this is seemingly largely supported by strong baseflow from groundwater contributions to surface flow.

Nutrient concentrations (total nitrogen and total phosphorus) were both low and considered optimal. Turbidity was low (5.7 nephelometric turbidity units [NTU]), which matched field observations of ‘clear water’. The true colour of the water was 25 true colour units (TCU), which was consistent with the field observations of ‘light tea’ coloured water.

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 recorded at the Dardanup Road West site in 2023 are provided in the table below. The table includes a list of all species previously reported in the subcatchment, which provides an indication of species that may occur at the assessment site. As differences in habitat within a subcatchment 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 three species of crustacean were recorded during the 2023 sampling. This included three species of native freshwater fish (western minnow, freshwater cobbler and nightfish), one estuarine opportunistic species (south-western goby), and three native freshwater crustaceans (smooth marron, gilgie and south-west glass shrimp). The only exotic fish species recorded was the eastern gambusia. The presence of the nightfish in 2023 was the first time the species had been recorded in this subcatchment.

There was a slight increase in the number of species detected in 2023 compared with the 2020 assessment. This included a first recording of nightfish and western minnow, which were not detected in the 2020 assessment. Of the species recorded in 2020, only the western hardyhead was not detected in 2023; but as this species is an estuarine opportunist and highly mobile within the lower reaches of rivers, its absence is not concerning. The abundance of freshwater cobbler was lower in 2023. In 2020 more than 150 cobbler were caught in the downstream-facing fyke net while moving upstream, and in 2023 about 25 individuals were caught in the same way, moving upstream. This is most likely related to temporal variability in species movement.

Across all species, while the diversity was moderately healthy, the abundance was generally low. The abundance of juveniles was also low, and juveniles were only detected for cobbler. Overall, the slightly improved diversity and similar abundances of native fish indicate a stable aquatic biota value for the site in 2023, but the generally low abundances in 2020 and 2023 are potentially concerning, especially as a similar pattern was observed at the Sylvan Way site (about 8 km downstream). It is possible that abundances are impacted by natural movement patterns within the connected channel; however, this will be closely observed through future monitoring.

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:

• Carter’s freshwater mussel (Westralunio carteri) was visually observed. This mollusc is the sole endemic freshwater mussel species in Western Australia and currently is listed as vulnerable on the International Union for Conservation of Nature’s (IUCN) red list of threatened species, (because of declining range, which is largely attributed to the effects of salinity)
• Leeches were captured in fish traps.

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