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Serpentine River - Lowlands

Basin : Murray River

The condition of the Serpentine River (Waangaamaap Bilya) at Lowlands (site code: MR127SERP1, site reference: 6144121) was assessed between 21 and 22 November 2022. Results are compared to 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 three previous assessments at this site are listed below. Note: aquatic fauna sampling methods have been specified given variations between surveys.

YearPeriodAuthorAssessment
2017NovemberHealthy Rivers (Department of Water and Environmental Regulation [the department])Full SWIRC assessment (SWIRC method)
2019NovemberHealthy Rivers (Murdoch University for the department)Fish and crayfish trapping only
2020SeptemberHealthy Rivers (Murdoch University for the department)Fish and crayfish trapping only

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.

  • 2017–2018 (Nov–May): Healthy Rivers
  • 2022–2023 (Nov–May): Healthy Rivers

Other departmental data: The Lowlands site is about 3 km upstream of the Department of Water and Environmental Regulation’s (the department) flow gauging station known as Lowlands (site reference: 614114), which has been in operation since 1998.

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.

Condition summary

The image below indicates the conditions at the Lowlands site in November 2022. November is within the Noongar season of Kambarang (second spring), which is traditionally seen as a transformational time of year with many plants flowering. The season is marked by longer periods of dry weather and rising temperatures. Further images are provided in the gallery at the bottom of the page to show general site conditions.

A summary of site condition over the latest monitoring period is provided in the sections below. For other site data, please contact the department’s River Science team (please reference the site code and sampling dates).

River setting

The Lowlands site is on the Serpentine River on the southern Swan Coastal Plain about 40 km south of Perth.

The Serpentine River and tributaries through the low-lying coastal plain have been modified to deeply incised drainage channels to support agriculture in the region. A major exception to this is in Lowlands Reserve which is a 1300 ha block of uncleared remnant vegetation. The area has a greater degree of naturalness than much of the rest of the Serpentine River system.

The assessment site is within a relatively low-gradient section of the river with a meandering active channel, typical of natural river form expected through the Swan Coastal Plain.

Aquatic habitat

At the time of sampling, aquatic habitat was characterised as a channel/pool sequence and was in near-optimal condition.

A diverse range of habitat offered a variety of niches for fauna. Channel depth was highly varied (0.25–2 m) and there was a moderate variety of substrate types (pebble, gravel and sand) with minor impact because of sedimentation from upstream sources. Woody debris was moderately abundant and varied (all sizes of wood were observed from small twigs to large tree trunks).

Overhanging banks and roots draped in water provided additional habitat along up to half of the bank lengths. Vegetation draped in the water was present over about half of the bank length. In-stream vegetation, dominated by water ribbon (Cycnogeton or Vallisneria spp.) covered 40 per cent of the channel. Rushes and sedges existed along most of the edge of the channel.

Tree canopy shading was optimal on both banks (extending >90 per cent along both banks), providing shading for 4–5 m over the channel from the banks. With an average channel width of 7 m, this amount of shading from the bank generally provided shade across the entire river channel.

Flow & connectivity

The Serpentine River is dammed at two locations in the upper catchment, on the Darling Scarp – at Serpentine Dam and Serpentine Pipehead. Flows are artificially controlled through dam releases at two locations downstream of the dams – above Serpentine Falls (at the downstream end of the Scarp) and about 5 km further downstream at Halls Road. In all but the driest years (e.g. 2016), releases keep the river connected to the Lowlands gauging station about 3 km downstream of the assessment site. However, maintaining this flow regime is increasingly difficult given reduced inflows to the dams and declining groundwater levels as a result of an increasingly dry climate. The Serpentine Falls are a natural permanent barrier to upstream movement of fish. The two dams are also a permanent barrier to fish movement.

At the time of sampling the site was connected in terms of fish passage. The daily discharge (flow) at the Lowlands gauging station was about 8.5 ML over the period of the 24-hour assessment. The image below illustrates flow at a riffle at a crossing immediately downstream of the site. Over the long-term logger deployment period, the flow decreased through the summer months to a minimum discharge of 0.15 ML/day (8 Feb 2023). Flows increased later in the year (see the top graph in the water quality section below).

Vegetation

The riparian zone (containing species adapted to living at the interface between aquatic and terrestrial environments) was intact, extensive (50–70 m wide) and almost entirely native on both banks.

As illustrated in the image above, the native riparian vegetation was healthy and extended along most of the bank length, providing both habitat and stream shading. Eucalyptus rudis (flooded gum) was the dominant tree species and provided shading along most (90–95 per cent) of the banks and across the river for 4–5 m from the banks. As the river width was 6–8 m, this meant the river was well shaded by the tree canopy. The shrub layer was largely made up of Melaleuca spp. (tea tree) and the ground cover was a variety of rushes and sedge including sword grass, as well as bracken. The extent and channel-overhang of the shrub layer was greater on the left bank because of the natural channel form and bank shape.

The width of the riparian zone varied along the reach, with species such as paperbark and flooded gum extending further from the bank in some places. In other parts of the site terrestrial species (e.g. Corymbia calophylla [marri]) were observed closer to the banks. This was a function of small differences in bank height and slope along the reach and related frequency of inundation. Abundant and healthy recruitment of native woody vegetation was observed for both trees and shrubs. A few exotic weeds were also observed including fig trees, cotton bush and arum lily.

The channel was generally stable, with erosion affecting less than half the bank length, though more of the right bank was affected than the left. However, where erosion was noted, it was usually of a high severity and the affected banks had poor structural integrity. This was observed most on the outside of bends where erosive forces, during high flows, are greatest. The image below illustrates this, showing a severely undercut tree where the soil has been washed away. This pattern of erosion is relatively natural and typical of low-gradient, meandering river reaches, although changes to the natural flow regimes could also be a contributing factor (i.e. longer periods of low water levels followed by flashier flows because of cleared catchment upstream).

Overall, the roots of abundant native vegetation supported the banks making them stable through most of the site.

Water quality

Below is a graph of the continuous water quality recorded at the Lowlands site from November 2022 to May 2023.

Water temperature was generally optimal. Maximum water temperature remained below 25°C for most (96 per cent) of the assessment period, with only minor exceedances of a short duration, between November 2022 and May 2023. The 25°C threshold1 is used as the upper range for optimal conditions. This demonstrates that the intact cover of vegetation through the river corridor can mitigate the high ambient temperatures recorded during the period (Jandakot airport maximum was 39.6°C on 21 January 2023). This contrasts with an assessment site further downstream (MR83SERP1) where vegetation through the corridor has been historically cleared and water temperatures through the same period exceeded the 25°C threshold for much longer (24 per cent of the deployment time).

Dissolved oxygen was below the 4 mg/L threshold2 for almost two-thirds (64 per cent) of the deployment period and was also below 2 mg/L for 15 per cent of the total deployment. The dissolved oxygen was consistently below the 4 mg/L threshold2 for an extended period between mid-January and mid-March. This period of suboptimal dissolved oxygen aligns with lowest flow recorded at the Lowlands gauging station (see table 2 above). This is partly a natural process caused by slow moving water in meandering, low-gradient rivers not mixing oxygen into the water. But it is important to note the flow regimes in this river are modified by human activity. The diurnal dissolved oxygen range was generally optimal (<4 mg/L), and without apparent biological drivers of oxygen (e.g. algae). This would at least not place additional stress on aquatic fauna during the prolonged period of suboptimal dissolved oxygen between January and March. This pattern of diurnal dissolved oxygen is much more natural than the greater diurnal range (because of greater biological drivers) recorded at the downstream site near Peel Main Drain (site code: MR83SERP1).

Specific conductivity (which can be used as an indicator of salinity) was within the marginal to marginal-brackish range throughout the deployment, which was still well within tolerances for south-west native fish species. pH was stable and within acceptable ranges, based on expectations of natural ranges for freshwater rivers of south-west Western Australia.

Nutrient concentrations were optimal, with total nitrogen classed as low (0.45 mg/L), and total phosphorus classed as moderate (0.074 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.

Species found at the site

Fish and crayfish

The species recorded at the Lowlands site in 2022 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.

Fish and freshwater crustaceans (crayfish and shrimp) observed at the Lowlands site (site code: MR127SERP1, site reference: 6144121) on the Serpentine River.

Two species of fish and two species of crustaceans were recorded during the 2022 sampling. This included one native freshwater fish species (western minnow) and two native freshwater crustaceans (gilgie and south-west glass shrimp). One exotic fish species was recorded (eastern gambusia). Juveniles were recorded for western minnow and gilgies, but also for the exotic eastern gambusia, indicating sufficient habitat quality and diversity to meet life cycle needs for these species.

Overall, species diversity has declined since 2017. In 2017 western pygmy perch and nightfish were recorded, including juveniles for both species. Both species were absent in 2022. Abundance was also reduced from that observed in 2017. Excluding south-west glass shrimp, almost 800 individual fish and crayfish were recorded in 2017, but in 2022 less than 100 were recorded. This may be partly because of the timing of sampling and fish movements, or because of variation in flow between different sampling events. This difference in flows would alter the timing of fish being able to navigate barriers downstream of the Lowlands site (e.g. flow-gauging station weirs). Alternatively, it may be an indication that the suboptimal conditions through the wider system have impacted the aquatic fauna. Future sampling will consider multiple assessments under a range of conditions and seasons to improve the representativeness of aquatic fauna data. Considering that the Lowlands site is one part of a connected and complex aquatic habitat, it is possible that during high-stress summer periods the fauna are making use of refuges in other parts of the river within the reserve.

Two native species, freshwater cobbler and smooth marron, are expected at the site but have not been recorded in any of the sampling events. However, they have both been recorded as recently as 2019 and 2020 (respectively) at another site about 5 km upstream, which reduces concerns about bioconnectivity in the Serpentine River.

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.

Other aquatic fauna

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 observed during the 2022 and 2017 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).
  • Rakali (Hydromys chrysogaster) activity was observed, with remains of prey (shells of Carter’s freshwater mussel and crayfish) found at the site. This species is listed by DBCA as a Priority 4 species (rare, near-threatened and other species in need of monitoring). Rakali have also been recorded through fauna trapping and on a motion-sensor camera by DBCA further upstream in the Lowlands reserve.

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

Terrestrial fauna was recorded through the riparian habitat by long-term motion sensor cameras. Native species included common brushtail possum, kangaroo and Gould’s sand goanna (Varanus gouldii), commonly known as a racehorse goanna (see gallery). Exotic species were also recorded including black rat and red fox.

Several bird species were also recorded including kookaburra, cormorant, pacific black duck, wood duck and white-faced heron.
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