Serpentine River - Downstream Peel Main Drain

The condition of the Serpentine Downstream Peel Main Drain site (site code: MR83SERP1, site reference: 6144122) on the Serpentine River (Waangaamaap Bilya) was assessed between 21 and 22 November 2022. Results are compared to a previous assessment in 2017.

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 November 2017.

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 site is about 8 km downstream of the Department of Water and Environmental Regulation’s (the department) flow gauging station known as Dog Hill (site reference: 614030), which has been in operation since 1979.

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 Serpentine Downstream Peel Main Drain site is on the Serpentine River on the southern Swan Coastal Plain, about 50 km south of Perth.

Many tributaries of the Serpentine River are deeply incised drainage channels, which were modified to drain the low-lying lands to support agriculture in the region. This includes the Peel and Birriga main drains and Manjedal Brook (and its tributaries) upstream of the assessment site, and others that flow into the Serpentine lakes system south of the site.

The assessment site is within a low-gradient and highly modified section of the middle Serpentine River. The riparian zone has been cleared of trees and shrubs and the channel sits between two flood levees. In essence this section of the river is an open drain.

At the time of sampling in November, the system was open channel habitat. The site subsequently dried to two small pools in late summer. While this has happened in this section of the Serpentine River previously, it is not a regular occurrence.

The aquatic habitat was in poor condition. Both channel depth and substrate type (sand) were uniform and offered little habitat diversity in terms of depth, flow and structure. The only biological substrate recorded was algae and fine detritus, with woody debris and leaf litter absent. Riparian zone vegetation was mostly exotic grass, which was continuous through the assessment site and draped in water, providing some aquatic habitat structure. In-stream vegetation covered 5–10 per cent of the site and consisted of Azolla spp. (water fern) and Otellia ovaflora (swamp lily).

The image below shows swamp lily in a mass of filamentous algae, which is indicative of conditions throughout the site. High algal density is usually a symptom of nutrient enrichment, and this was supported by the water samples collected (see water quality section below). Because of the clearing of woody vegetation in the riparian zone there was no stream shading of the channel.

In 2017–18 the declared plant Pontederia crassipe, formerly known as Eichorina crassipes, (water hyacinth) was recorded on multiple site visits. Encouragingly, no observations of this noxious weed were made during any of the 2022–23 site visits. This suggests management of water hyacinth by the Department of Primary Industries and Regional Development has been successful.

The Serpentine River is dammed at two locations within the Darling Scarp in the upper catchment – Serpentine Dam and Serpentine Pipehead Dam. 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 (refer to site summary for site code: MR127SERP1).

Flow in the middle reaches (where the MR83SERP1 site is located) is also influenced by deep drainage, and flow in the lower reaches through the lakes system is also tidally influenced by the Bindjareb Djilba (Peel-Harvey estuary).

Flow at the time of sampling was low (about 0.1 m/s) and uniform through the width and depth of the channel, typical of drains. At this time the site was connected in terms of fish passage; however, within a few months the reach had dried to a series of disconnected pools (see below). The site was disconnected for several months.

This image illustrates the suboptimal aquatic habitat including uniform depth, absence of in-stream structural habitat, and limited shade. In addition to increased risk from predators, these conditions result in significant impacts to water quality, particularly temperature and dissolved oxygen – refer to the discussion in the water quality section below.

Trees and shrubs within the stream corridor were historically removed to enhance drainage to support agriculture. The only fringing vegetation present was ground cover, which consisted mostly of pasture grass and scattered weeds, with some native rushes.

Despite the lack of deep-rooted vegetation, only minor erosion was observed. This is largely because of the low erosive force of flows through this section of the river, with low summer flows and a wide channel floodplain to mitigate higher winter flows. Significant sedimentation was recorded through the site which is likely because of unimpeded runoff of sediment from the catchment and from erosion of bed and banks further upstream. This is illustrated by the obvious sand deposition that was noted through site and can be seen on the right side of the image above.

Below is a graph of the continuous water quality recorded at the Serpentine Downstream Peel Main Drain site from November 2022 to May 2023. 

Water quality was suboptimal through much of the monitoring period.

Water temperature exceeded the 25°C threshold¹ for 24 per cent of the deployment period, and during January and February the temperature occasionally exceeded 30°C. Above 30°C it is expected that south-west native aquatic fauna would experience thermal stress. The diurnal range in water temperature also exceeded the desired maximum of 4°C for most of summer and autumn, placing further stress on aquatic fauna. The lack of shading of the channel allowed maximum exposure to sunlight, contributing to these suboptimal conditions. Note: at the Lowlands assessment site further upstream (site code: MR127SERP1), which retains remnant trees and shrubs, water temperature only exceeded the lower 25°C threshold for 4 per cent of the deployment period.

Dissolved oxygen was also suboptimal. Levels fell below the 4 mg/L threshold² for 49 per cent of the deployment period and below 2 mg/L for 32 per cent of the time, which represents significant stress to aquatic species. The diurnal range in dissolved oxygen was also high. which indicates a strong influence from biological drivers (i.e. algal blooms producing oxygen during the day [photosynthesis] and consuming oxygen during the night [respiration]). During the period between late January to April, as temperatures increased and the water levels declined and disconnected, the effect of algal growth and temperature on dissolved oxygen increased.

Specific conductivity (which can be used as an indicator of salinity) was classed as high brackish. Many south-west fish are reported to tolerate short periods of elevated salinity; however, their capacity to withstand these conditions (resilience) declines with additional stress. For example, they may not be able to tolerate these conditions when also responding to elevated temperatures or low dissolved oxygen. Specific conductivity increased steadily as the water level fell and the system transitioned from an open flowing channel to small, disconnected pools.

pH was generally within acceptable ranges, based on expectations of natural ranges for freshwater rivers of south-west Western Australia. The diurnal range increased towards the end of the period when the site was disconnected (late March to early April) and this, like the increased diurnal range in dissolved oxygen, is likely to be because of algal activity in the small pool as the chemical by-products of photosynthesis and respiration affect the pH. The pH data is incomplete because the probe drifted away from reference calibration between maintenance checks, which is not surprising given the algal growth that was observed on and around the monitoring equipment.

Nutrient concentrations were elevated and are consistent with the dense growth of filamentous algae observed at the site. Total nitrogen was moderate (1.2 mg/L) and total phosphorus was very high (0.25 mg/L). Total nutrient concentrations are a combination of soluble and insoluble fractions, with soluble nutrient fractions often an indication of manmade sources such as inorganic fertilisers. About 92 per cent of the total nitrogen and 60 per cent of the total phosphorus were soluble forms, which suggests artificial fertiliser was as significant source of nutrients. This demonstrates excess use of fertiliser through the catchment (i.e. runoff of fertiliser not used by crops and pastures) which not only impacts waterway health but also results in a financial loss to farmers.

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 Serpentine Downstream Peel Main Drain site in 2022 are provided in Table 1 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.

Eight species of fish and four species of crustaceans were recorded during the 2022 sampling. This included three native freshwater fish species (western minnow, western pygmy perch, and nightfish) two native freshwater crustaceans (gilgie and south-west glass shrimp) and two native estuarine-freshwater species (blue-spot goby and western hardyhead). The goby and hardyhead can complete life cycles in both freshwater and estuarine environments but are most common in the lower reaches of rivers where they can move into estuaries. Two exotic fish (eastern gambusia and goldfish) and two exotic crustaceans were recorded (yabby and indistinct river shrimp). The indistinct river shrimp is a recently discovered pest species in Western Australia – for more information see our species page.

The diversity of species recorded in 2022 was identical to 2017. The aquatic fauna was moderately diverse and met expectations based on previous assessments, but the catch was dominated by the estuarine opportunist blue-spot goby and western hardyhead, in terms of abundance. These two species accounted for 80 per cent of the total catch. This was similar to 2017, but overall abundance was reduced by about one third in 2022. Juveniles were recorded for western pygmy perch which indicates conditions suitable for all life cycle stages for this species at, or near, the site. Juveniles were also recorded for blue-spot goby, western hardyhead, and eastern gambusia. The presence of an established population of eastern gambusia presents a direct risk to native fauna through predation (on fish larvae and frog eggs), and through competition and aggressive behaviours like fin nipping.

Considering the suboptimal water quality conditions and the fact the site largely dried over late summer, it is likely aquatic fauna are being supported by local refuges, most likely the lakes downstream of the site (as conditions observed at the site are not expected to support native aquatic communities). In a drying climate these refuge locations will become increasingly important.  The riverine habitat through the middle-lower Serpentine River requires urgent action to protect ecosystem health in a drying climate. In addition to addressing nutrient sources, priority actions to improve ecosystem resilience should include revegetation of the riparian corridor and installation of aquatic habitat (large wood) to provide immediate protection for species.

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 turtles (Chelodina oblonga)
• Motorbike frogs (Litoria moorei)
• Moaning frogs (Heleioporous eyrei)
• Dragonfly larvae and diving beetles
• Rakali (Hydromys chrysogaster) – evidence of activity based on discarded crayfish (see image below). This species is listed by DBCA as a Priority 4 species(rare, near-threatened and other species in need of monitoring).

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