Thomson River

Carran Carran (Thomson River) flows from the slopes of the Baw Baw Plateau to join Durt-Yowan (Latrobe River) south of Sale (Figure 2.3.1). The major tributaries of the Thomson River are the Aberfeldy and Jordan rivers in the upper reaches and Wirn wirndook Yeerung (Macalister River) in the lowest reach. Two major structures regulate flow on the Thomson River: Thomson Reservoir — the largest water supply storage for metropolitan Melbourne — and Cowwarr Weir — a regulating structure that supplies irrigation water to parts of the Macalister Irrigation District.

Thomson Reservoir harvests most of the flow from the upper catchment of the Thomson River and significantly affects the flow in all downstream reaches. The Aberfeldy River now provides most of the natural flow to the Thomson River below Thomson Reservoir and is essential for providing natural freshes and a high flow.

Water for the environment is held in the Thomson Reservoir and released into the river as required. Reach 3 of the Thomson River (from the Aberfeldy River confluence to Cowwarr Weir) is the highest priority for delivery of water for the environment due to its heritage river status, high-value native streamside vegetation, high-quality in-stream habitat and low abundance of exotic fish species.

At Cowwarr Weir, the Thomson River splits into the old Thomson River course (reach 4a) and Rainbow Creek (reach 4b) (see Figure 2.3.1). Passing flow throughout the year is split two-thirds down reach 4a and one-third down reach 4b to avoid impacts to irrigators located on Rainbow Creek. Water for the environment is primarily delivered to the old Thomson River course (reach 4a) to support fish migration as Cowwarr Weir impedes fish movement through Rainbow Creek.

The Heyfield wetlands is a cluster of pools located between the Thomson River and the township of Heyfield. The construction of levees and weirs along the Thomson River means that river water rarely enters the wetlands. While the largest pool receives stormwater from the Heyfield township, smaller ponds rely on rainfall or pumped water for the environment to maintain environmental values. These values include wetland plant communities planted in recent years as part of a comprehensive revegetation program.

System map

Thomson System — Grey river reaches have been included for context. The numbered reaches indicate where relevant environmental flow studies have been undertaken. Coloured reaches can receive environmental water.

Environmental watering objectives in the Thomson River

Restore populations of native fish, specifically Australian grayling 
Enhance the structure of native fish communities 

Maintain the existing frog population and provide suitable habitat

Increase the abundance of platypus

Maintain the natural invertebrate community

Provide freshwater habitat for migratory and non-migratory wetland birds within the Gippsland Plains landscape

Improve water quality in the Thomson River estuary

Maintain or enhance the physical form of the channel to provide a variety of channel features and habitats for aquatic animals

Enhance river function by maintaining substrate condition and enabling carbon cycling

Maintain the structural diversity and appropriate distribution (zonation) of streamside vegetation along the riverbank and reduce terrestrial encroachment/invasion (Thomson River)

Increase the recruitment and growth of native in-stream, fringing and streamside vegetation (Thomson River)

Maintain the existing vegetation and promote the growth and establishment of semi-aquatic species (Heyfield wetlands)

Enhance the resilience of semi-aquatic species (Heyfield wetlands)

Environmental values

The Thomson River supports native species of migratory fish that need to move between the sea and freshwater environments to complete their life cycles, including Australian grayling, tupong, short- and long-finned eel, Australian bass and pouched and short-headed lamprey. A focus for environmental flow management is the Australian grayling, which is a threatened species
in Victoria. Australian grayling spawn in response to autumn freshes, and the larvae and juveniles spend time at sea before returning to the freshwater sections of coastal rivers.

The composition and condition of streamside vegetation vary throughout the Thomson River catchment. The vegetation is intact and in near-natural condition above Thomson Reservoir in the Baw Baw National Park. Streamside vegetation between Thomson Reservoir and Cowwarr Weir is mostly in good condition but is affected by exotic weeds, including blackberry and gorse. Below the Cowwarr Weir, the vegetation is degraded due to stock access and widespread weed invasion.

The Heyfield wetlands are among the few remaining freshwater wetland sites in the Gippsland Plains landscape. They provide habitat for aquatic and terrestrial animals, including threatened migratory birds that prefer shallow, slow-moving waterbodies.

Traditional Owner cultural values and uses

The Gunaikurnai have had a continued connection to Gunaikurnai Country for many thousands of years, including with
the waterways in the Latrobe system, into which Carran Carran (Thomson River) feeds. For the Gunaikurnai as Traditional Owners, there are immense challenges to heal, protect and manage Country which has been drastically altered since colonisation. “As Gunaikurnai, we see our land (Wurruk), waters (Yarnda), air (Watpootjan) and every living thing as one. All things come from Wurruk, Yarnda and Watpootjan and they are the spiritual life-giving resources, providing us with resources and forming the basis of our cultural practices. We have a cultural responsibility to ensure that all of it is looked after” (Water is Life: Traditional Owner Access to Water Roadmap 2022 - Gunaikurnai Nation Statement). This cultural landscape is dependent on culture and Aboriginal management.

The Gunaikurnai Land and Waters Aboriginal Corporation (GLaWAC) is working with the West Gippsland CMA to determine how to express Gunaikurnai objectives for water in a way that contributes to seasonal watering proposals from the perspective of traditional custodians with traditional knowledge. GLaWAC was a member of the Steering Committee and Project Advisory Group for the 2020 review of the Carran Carran (Thomson River) FLOWS study and has membership on the Thomson Environmental Water Advisory Group (EWAG).

GLaWAC Cultural Water Officers have completed Aboriginal Waterways Assessments on Carran Carran (Thomson River) and are assessing how to protect and further the river’s cultural values and uses. Traditionally, Carran Carran (Thomson River) was an important meeting place and a place to camp. Today, most of Carran Carran is inaccessible to the Gunaikurnai, making it difficult to meet and yarn along the river.

Assessments for watering requirements of Carran Carran (Thomson River) for the Gunaikurnai have been based on cultural indicators, including:

the condition of the lower Latrobe wetlands (which Carran Carran helps supply)
the condition and prevalence of plants and animals with cultural values and uses
species known to be indicators of water quality, water regimes and healthy Country.

GLaWAC is working with the West Gippsland CMA to share traditional knowledge of plant and animal species of cultural significance in and around the waterways of the Latrobe Valley and the importance of specific watering decisions to support them. Watering requirements to support cultural values and uses include:

timing of deliveries of water for the environment planned in partnership with GLaWAC to support a seasonal flow regime and wet and dry periods that contribute to healthy Country
maintaining freshwater supply to the Durt-Yowan (Latrobe River) estuary, Dowd Morass, Sale Common and Heart Morass, and associated freshwater habitats; the lower Latrobe wetlands are an important resource for the Gunaikurnai
providing connectivity between reaches and onto floodplains to support dependent plants and animals with cultural values and uses of significance to the Gunaikurnai
maintaining water quality to support the health of native plants and animals with cultural values and uses of significance to the Gunaikurnai.

West Gippsland CMA engaged with GLaWAC on Thomson watering priorities for 2023-24, with engagement planned to continue in the 2023-24 water year.

Social, recreational and economic values and uses

In planning the potential environmental watering actions in Table 2.3.1, the West Gippsland CMA considered how environmental flows could support values and uses, including:

water-based recreation (such as kayaking, canoeing, fishing and swimming)
riverside recreation and amenity (such as birdwatching, camping, hiking and duck hunting)
community events and tourism (such as community education, events at the Heyfield wetlands and visitation by locals and non-locals)
socioeconomic benefits (such as maintaining bankside vegetation and preventing erosion and the potential loss of private and public land). icons.

If the timing or management of planned environmental flows may be modified to align with a community benefit, this is acknowledged in Table 2.3.1 with the following

	Watering planned to support water sports activities (canoeing and kayaking)
	Watering planned to support peaks in visitation

Autumn, winter and spring freshes in the Thomson River create ideal conditions for white water rafting, kayaking and canoeing. The timing of environmental flows may be adjusted to optimise opportunities to support these recreation activities, where it does not compromise environmental outcomes. For example, a fresh that aims to cue the migration of Australian grayling and other native fish may be timed to coincide with recreation events or holiday periods when people take advantage of favourable rafting or kayaking conditions. Kayaking and rafting activities have inherent risks, and large environmental flows are ramped up and down over several days to avoid sudden changes in water levels that may affect river users. The West Gippsland CMA also provides notification of planned large releases of water for the environment to alert river users about potential increases in the water level and velocity.

Interested community members can register on the West Gippsland CMA website to receive notification of upcoming watering events.

Scope of environmental watering

The term ‘environmental watering’ refers to the active delivery of water for the environment to support particular environmental objectives by altering the flow in a river or the water level in a wetland. While other terms are also used to describe the delivery of water for the environment, ‘environmental watering’ is deliberately used here and in seasonal watering statements to ensure consistency in the legal instruments that authorise the use of water for the environment in Victoria.

Table 2.3.1 describes the potential environmental watering actions in 2023-24, their expected watering effect (that is, the intended physical or biological effects of the watering action) and the longer-term environmental objectives they support. Each environmental objective relies on one or more potential environmental watering actions and their associated physical or biological effects.

Table 2.3.1 Potential environmental watering actions, expected watering effects and associated environmental objectives for the Thomson system

Potential environmental watering action	Expected watering effects	Environmental objectives
Thomson River (targeting reach 3)
Winter/spring/autumn low flow (125-350 ML/ day during July to November and 230-350 ML/day during April to June 2024)	Maintain a minimum level of habitat and maintain water quality in pools and riffles for waterbugs and fish (when delivered at 125 ML/day); habitat availability and condition are increased when delivered at greater magnitudes Regulate the water temperature and wet large woody debris to provide food and shelter for waterbugs and fish Maintain sufficient water depth to facilitate platypus and fish movement between localised habitats and increase foraging opportunities (further enhanced when delivered at greater magnitudes) Wet low-lying benches (when delivered at greater magnitudes) to prevent encroachment by invasive plants and permit seed dispersal Additional benefits to the Thomson River estuary (reach 6) are expected when provided at 350 ML/day magnitude: partially flush the upper water column, helping to sustain waterbug communities and fish by maintaining oxygen levels prevent high salinity levels, helping to maintain emergent macrophyte vegetation provide freshwater to the Latrobe system
Spring fresh(es) (one to two freshes of 800-900 ML/day for five to seven days during September to November)	Trigger the migration of adult and juvenile native fish (in particular, the upstream migration of juvenile Australian grayling and Australian bass from marine/estuarine habitats) Improve and maintain streamside vegetation by inundating the benches and providing variable water levels for plant zonation Carry plant seeds from the upper catchment for deposition downstream Deposit fine particulate sediments on the benches and prevent pools from infilling Scour substrates to remove accumulated fine sediment and biofilms to improve habitat and food for waterbugs Additional benefits to the Thomson River and its estuary (reach 6) are expected when provided at 900 ML/day magnitude: wet vegetation on higher benches partially flush the upper water column in the Thomson River estuary, helping to sustain waterbug communities and fish by maintaining oxygen levels prevent high salinity levels, helping to maintain emergent macrophyte vegetation provide freshwater to the Latrobe system
Summer/autumn low flow (125 ML/day during December to March)	Maintain habitat and water quality in pools and riffles for waterbugs and fish Facilitate localised movement between habitat types for small-bodied native fish and platypus Prevent encroachment into the in-stream channel by invasive plants
Summer/autumn fresh(es) (one to two freshes of 230-350 ML/day for seven days during December to March)	Wet aquatic and fringing vegetation to maintain its condition and support its growth Wet low-lying benches to prevent encroachment by invasive plants and enable vegetation zonation Provide velocity and depth diversity and prevent sediment smothering by fine sediments When delivered in February-March (at 230 ML/day), the fresh also aligns with and supports native fish movement: trigger downstream migration of adult short- and long-finned eel and upstream movement of juvenile Australian bass increase the water depth over riffles to facilitate local movement between habitats for large-bodied native fish
Autumn freshes (two freshes of 800 ML/day for five to seven days during April to May)	Trigger the migration of adult and juvenile native fish, in particular: the downstream migration and spawning of adult Australian grayling (April) the downstream migration of adult tupong and upstream migration of adult and juvenile Australian bass (May) Carry plant seeds and propagules from the upper catchment for deposition downstream and help maintain zonation of vegetation Prevent infilling of pools by mobilising fine sediments and depositing them on existing bars and benches, to provide substrate for vegetation Scour substrates to remove accumulated fine sediment
Heyfield wetlands
Fill (during August to September)	Wet ponds to capacity, to stabilise the banks and support the spring growth of semi-aquatic vegetation Provide freshwater habitat for waterbirds and frogs (such as growling grass frogs and golden bell frogs)
Top-ups as required to maintain water level (during October to May)	Top up ponds before summer to maintain vegetation and enhance recruitment by triggering the release of seeds Top up ponds in late summer to ensure the survival of newly planted wetland vegetation Maintain habitat for waterbirds and frogs (such as growling grass frogs and golden bell frogs) When delivered in April to May, top-ups provide drought refuge habitat for waterbirds and frogs following prolonged dry conditions
Partial drawdown (during April to May)	Oxygenate surface soils, break down accumulated organic matter and cycle nutrients Enhance waterbird food availability by exposing the mudflats and provide access to burrowing invertebrates

Page last updated: 01/12/22