Preston River - Fowler Street

River condition of the Fowler Street site (site code: PR163PRES1, site reference: 6110054), on the Preston River, was assessed between 14 and 15 February 2023.

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)

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.

• 2021–2022 (Nov–Mar): Healthy Estuaries WA
• 2022–2023 (Nov–Apr): Healthy Rivers

Other departmental data: The Fowler Street site is about 5 km upstream of the Department of Water and Environmental Regulation’s (the department) flow gauging station known as Lowden Road Bridge (site reference: 611009), which has been in operation since 1999.

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 Fowler Street HRP site is on Preston River, about 50 km south-east of Bunbury, in the south-west of Western Australia.

The site is in a meandering, low-gradient channel within a 5 km wide river valley, on the Darling Plateau. At the time of sampling the site was a channel-pool sequence with a water depth that was moderately varied. The average water width and depth was 5 m and 0.5–0.99 m respectively. Channel depth (to the top of the banks) was more than 2 m and the bankfull extent of the floodway was about 35 m wide, suggesting high winter storm flows. This was confirmed by the landholder who mentioned high floods encroach far into the paddock adjacent to the river. The banks were convex with vertical slopes in areas of greater erosion and stepped with steep slopes in other parts. The channel shape was U-shaped through the whole assessment site.

The aquatic habitat was optimal. Water depth was varied, offering a range of flow conditions. All sizes of woody debris were present in moderate abundance, which provides food and shelter for a range of aquatic fauna.

The physical substrate was sand, silt and clay with a small amount silt deposition that produced a moderate sized plume when disturbed. Biological substrate cover (algae, detritus and leaves) was limited to less than one-third of the site but moderately dense where it did occur. This is likely a natural feature of a site, with bed material moving through the site under higher flows and accumulating in eddies and areas of lower water velocity.

Some habitat was also provided by overhanging roots and bank vegetation draped in the water (up to half of the bank length)

Stream shading was moderate, impacted by historical clearing and lerps (psyllids) – obvious in many large trees in the area (see image below). Lerps are sap-sucking insects that can cause defoliation of trees in heavy infestations. See further information under the vegetation section below.

Stream shading was provided by tree canopy along about 70 per cent of the left bank, and 50 per cent of the right bank. Shading by shrubs was more limited, and was greater on the left bank than the right.

The Preston River has perennial flows along much of its length from water released from Glen Mervyn Dam for the Preston Valley Irrigation Cooperative, as well as from a net input of groundwater from the superficial aquifer downstream on the coastal plain (Department of Water 2015, unpublished). Water released from Glen Mervyn Dam flows down Lyall’s Mill Stream which joins the Preston River about 5 km upstream of the Fowler Street site. The effect of releases from Glen Mervyn Dam on the habitat and water quality conditions at the Fowler site can be seen in several parameters including flow, dissolved oxygen, and conductivity/salinity.

At the time of the assessment the site was connected and flowing. The department’s flow gauging station at Lowden Road Bridge (site ref 611009) is 5 km downstream of the Fowler Street site. Over the period of long-term water quality logger deployment (November 2022 to April 2023) the lowest recorded daily discharge (flow) was 2.1 ML/day on 14 December 2022. After this, the irrigation releases from Glen Mervyn Dam started and the daily discharge was 5–9 ML/day for the remainder of the deployment period (see graph in water quality section).

At the time of sampling, flow measurements through the site varied between 0.3 m/s and <0.1 m/s, and connectivity was further demonstrated by large-bodied freshwater cobbler moving upstream (recorded in the downstream-facing fyke net) through the site in February.

The riparian zone (containing species adapted to living at the interface between aquatic and terrestrial environments) was 20 m wide on the left bank, largely because of restoration works by the landholder, and was mostly native (except for the ground cover layer). The riparian vegetation on the right bank was 4 m wide on average.

All riparian layers of native vegetation (tree, shrub and ground cover) were reduced, mostly by clearing for agriculture but also by pests (e.g. lerps) and weeds (e.g. buffalo grass, penny royal mint, onion grass and blackberry). Note: the impact of lerps is linked to the loss of shrub habitat which is home to their natural predators (various insects, arachnids and birds).

A mature tree canopy of mostly flooded gum (Eucalyptus rudis) provided shading, but this was impacted by the effect of lerps, as shown in the aerial photo (see aquatic habitat section above). Some yarri or blackbutt (Eucalyptus patens) and swamp banksia (Banksia littoralis) had been planted on the left bank. The native shrub layer comprised tea tree (Melaleuca spp.), which also provided some stream shading. Ground cover was made up of some native rushes and sedges and a few kangaroo paws but was dominated by exotic pasture grass. Natural recruitment of woody native vegetation (both tree and shrub) was limited, but supplemented on the left bank by targeted planting, and was in moderate health.

The channel had good structural integrity though there was evidence of some erosion from high winter flows. The image below shows the importance of tree and shrub roots in stabilising banks. Although minor in this case, the slumping of the far bank into the channel illustrates that the largely exotic ground cover vegetation provides only limited bank protection.

Below is a graph of the continuous water quality recorded at Fowler Street from November 2022 to April 2023.

Water temperature was optimal; the upper temperature threshold¹ of 25°C was only exceeded for less than 1 per cent of the deployed period, and any exceedances were minor and short-lived. The diurnal range in water temperature was optimal (<4°C).

Water quality was also optimal in terms of dissolved oxygen. Dissolved oxygen concentrations only fell below the low threshold² of 4 mg for less than 0.5 per cent of the deployed period. The few exceedances were minor and short-lived. The diurnal range in dissolved oxygen was less ideal, with a range >4 mg/L early in the deployment which gradually improved to <4 mg/L by mid-December. This suggests there may have been some biological drivers of dissolved oxygen during late spring. This was possibly because of algae that may have been disrupted by increased flows because of irrigation water releases from Glen Mervyn Dam that began around that time. The gradual reduction in the diurnal range in dissolved oxygen continued until about halfway through the water quality logger deployment and then stayed within an optimal range until the logger was retrieved in April 2023.

Another parameter apparently influenced by flow was the specific conductivity. Specific conductivity (which can be used as an indicator of salinity) was optimal after a clear reduction from about 1.5 mS/cm (marginal) to about 0.7 mS/cm (fresh) which coincided with the increased flows recorded at Lowden Road Bridge gauging station (because of water releases for irrigation from Glen Mervyn Dam).

pH (average value of 7.18) was within acceptable ranges (6.5–8.0), based on expectations of natural ranges for freshwater rivers of south-west Western Australia. In the early period of logger deployment the daily range in pH was relatively greater than at any other time between November 2022 and April 2023. Together with the higher diurnal flux in dissolved oxygen during the same period, this greater variation in pH over a 24-hour cycle is likely to be because of elevated algal activity. This is because the chemical by-products of algal photosynthesis and respiration affect the pH.

The optimal water quality recorded at this site is consistent with previous long-term water quality recorded for the Healthy Estuaries WA project, including the reduction in salinity with the onset of water releases from Glen Mervyn Dam.

Nutrient concentrations (total nitrogen and total phosphorus) were both low and considered optimal. Turbidity was low (4.7 nephelometric turbidity units [NTU]). The true colour of the water was 28 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 Fowler Street 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.

Six species of fish and three species of crustacean were recorded during the 2023 sampling. This included four species of native freshwater fish (western minnow, freshwater cobbler, western pygmy perch, and nightfish), one estuarine opportunistic species (western hardyhead), 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 western hardyhead in 2023 was the first time the species had been recorded in this subcatchment.

As with other sites in the Preston River, both in this round of sampling and in 2019–20, the overall abundance of aquatic fauna was relatively low even though species diversity was quite rich. Of the nine species observed, juveniles were recorded for six of them. This indicates that habitat and water quality meets life stage requirements of the species. The relative low abundance could be related to the overall high connectivity caused by artificially maintained flows, which increases spread of animals through the wider area. Of all the aquatic fauna, only freshwater cobbler could be described as abundant (63 individuals), and 60 of those were recorded moving upstream (captured in the downstream-facing fyke net). The freshwater cobbler is a particularly mobile species that will migrate several kilometres within a ‘home range’.

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 during the 2023 assessments. This mollusc is the sole endemic freshwater mussel species in Western Australia and is currently 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).
• dragonfly larvae caught in fish nets

Motion-sensor cameras deployed between November 2022 and April 2023 also recorded several species:

• rakali (Hydromys chrysogaster) – recorded on multiple occasions. This species is listed by DBCA as a Priority 4 species(rare, near-threatened and other species in need of monitoring). Rakali have been recorded on motion-sensor camera previously at this site.
• Australian wood duck
• Pacific black duck
• swamphen
• ibis
• kangaroo
• fox (exotic)
• king skink

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