Hotham-Williams Sub-Catchment River Health Assessment

Seeking to build on its ecological knowledge of the Hotham and Williams rivers sub-catchment (southwest Western Australia), the Peel-Harvey Catchment Council (PHCC) partnered with WRM to conduct spring and autumn river health assessments at six sites across the two systems.

The Hotham and Williams rivers are two major tributaries of the Murray River, one of south west Western Australia’s largest river systems. Each have their origins in the southern Wheatbelt region, an area with relatively low annual rainfall (~500 mm) that has been extensively cleared for agriculture. This has lead to increased salinity levels in both the Hotham and Williams rivers – a result of secondary salinisation, where the removal of deep-rooted native vegetation has caused the watertable to rise, pushing stored salts that have built-up over thousands of years to the surface and into the rivers. The major detrimental effects of salinisation on stream health include the death of riparian zone trees (with flow-on effects of stream bank destabilisation and erosion) and reductions in stream biodiversity, with “sensitive” native fish, invertebrate and plant species being replaced with invasive and/or salt-tolerant taxa. Other environmental challenges faced by the Hotham and Williams rivers include the damming of sections for recreational and agricultural use, and physical damage to river banks and vegetation where livestock access has not been restricted.

Through the Hotham-Williams Rivers and Tributaries’ Natural Resource Management and Conservation Project, one of the PHCC’s main goals is to work with landowners and the community to address existing natural, conservation and cultural resource management knowledge gaps of the Hotham and Williams rivers, build community capacity for future land management, and undertake research and restoration projects to protect and enhance catchment health, biodiversity and agricultural sustainability. In order to gather baseline information on ecosystem health for this project, WRM were approached by PHCC to lead the technical river health assessment (RHA) surveys, with in-kind and financial support to be provided to PHCC by Newmont Goldcorp Boddington (NGB) and the Western Australian Department of Water and Environmental Regulation (DWER). With over a decade of experience conducting biological assessments on the Hotham River, WRM were thrilled to be given the opportunity to take on the assessments.

RHAs use the Southwest Index of River Condition (SWIRC) methodology, recently developed by DWER to provide a standardised methodology to assess the health of rivers in the south west of Western Australia by collecting data on variety of river and catchment health indicators, including fringing zone (riparian) vegetation, aquatic habitat, aquatic biota (including fish and macroinvertebrates) and water quality. Employing the SWIRC methodology, WRM visited six sites across the Hotham-Williams sub-catchment – four sites on the Hotham River, and two on the Williams – in the spring of 2019. At first glance there was a stark difference in the ecological condition of river sites depending on location within the catchment – sites located in the headwater reaches of the rivers (where pastoral clearing is most extensive and rainfall lowest) tended to be highly saline, with heavily reduced riparian zones and a limited number of native fish species (however, native fish were recorded at all sites). Conversely, sites further to the west (downstream) tended to be 50 – 75% less saline, with intact riparian zones and relatively diverse of native fish and crayfish assemblages. Overall, four native fish species (western minnow, western pygmy perch, nightfish and blue-spot goby) and one native crayfish species (gilgie) were recorded during the survey, alongside the highly invasive and widespread eastern mosquitofish (gambusia).

Another round of surveys are planned for autumn 2020, which will provide information on seasonal variations in water quality and aquatic biota in the two rivers. The data and information collected will contribute not only to PHCC’s Hotham-Williams Rivers and Tributaries’ Natural Resource Management and Conservation Project, but also DWER’s Healthy Rivers initiative.

For more information on south west Western Australia’s unique river systems, and the river health assessment process, visit DWER’s Healthy Rivers website.