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 Carran Carran (Thomson River) are the Aberfeldy and Jordan rivers in the upper reaches and Wirn wirndook Yeerung (Macalister River) in the lowest reach. Most natural flow originates from the Aberfeldy River. Two major structures regulate flow on Carran Carran (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 Carran Carran (Thomson River) and has a significant effect on the flow in all downstream reaches. The natural flow from the Aberfeldy River, which meets Carran Carran (Thomson River) below Thomson Reservoir, is essential for providing natural freshes and high flows in Carran Carran (Thomson River).

Water for the environment is held in the Thomson Reservoir and released into the river as required. Reach 3 of Carran Carran (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, Carran Carran (Thomson River) splits into the old Carran Carran (Thomson River) course (reach 4 a) and Rainbow Creek (reach 4b) (see Figure 2.3.1). Passing flows throughout the year are split two-thirds down reach 4 a and one-third down 4b to avoid impacts to irrigators located on Rainbow Creek. Water for the environment is primarily delivered to the old Carran Carran (Thomson River) course (reach 4 a) to support fish migration because Cowwarr Weir impedes fish movement through Rainbow Creek.

The Heyfield wetlands is a cluster of pools located between Carran Carran (Thomson River) and the township of Heyfield. Due to the construction of levees and weirs along Carran Carran (Thomson River), river water rarely enters the wetlands; and 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 that have been planted as part of a comprehensive revegetation program in recent years.

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 

Maintain/enhance the structure of native fish communities

Maintain the existing frog population and provide suitable habitat

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

Maintain or enhance river function by maintaining substrate condition and enabling carbon cycling

Increase the abundance of platypus

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

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

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

Environmental values

Carran Carran (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 flows management is the Australian grayling, which

is listed as 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 one of the few remaining freshwater wetland sites in the Gippsland Plains landscape area. 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 over 27,000 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. The Gunaikurnai see all of Country as interconnected with only separation between clans, not cultural landscapes of land, waterways, coasts, oceans and natural and cultural resources. The 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. This has included GLaWAC membership on the Steering Committee and Project Advisory Group for the 2020 review of the Carran Carran (Thomson River) FLOWS study and GLaWAC membership of the newly formed Thomson and Latrobe Environmental Water Advisory Groups (EWAGs).

GLaWAC cultural water officers have recently completed an Aboriginal Waterways Assessment on Carran Carran, and they are assessing how to document, protect and further the river’s cultural values and uses. Traditionally, Carran Carran 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 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 sharing with the West Gippsland CMA its 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 embody healthy Country
maintaining freshwater supply to the 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 the GLaWAC Cultural Water Team on watering priorities for 2022-23.

Social, recreational and economic values and uses

In planning the potential watering actions in Table 2.3.1, 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).

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 icons.

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

Autumn, winter and spring freshes in Carran Carran (Thomson River) create ideal white water rafting conditions for kayakers and canoers. 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 the white water rafting 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.

Recent conditions

The Thomson River catchment had average to above-average rainfall and above-average temperatures throughout much of 2021-22. A significant rainfall event in June 2021 caused major flooding, with flow peaking at 74,000 ML/d upstream of

Cowwarr Weir. Two other separate natural bankfull flows in spring also helped to achieve environmental objectives that cannot be achieved with managed releases of water for the environment. High rainfall throughout winter and spring boosted inflows to

Thomson Reservoir and provided significant increases to allocations. Most water for the environment for the Thomson system is allocated up-front at the start of the water year, with additional allocation throughout the year based on inflows to Thomson Reservoir.

Delivery of water for the environment in the Thomson system was managed according to a wet scenario in 2021-22, and all the high priority (tier 1) planned environmental flows were met. Natural flows from the Aberfeldy River helped to achieve or exceed environmental flow recommendations in reach 3 of the Thomson River throughout most of the year. Water for the environment was used during 2021-22 to deliver a spring fresh of 800 ML per day for seven days in late October to encourage the recruitment of juvenile migratory fish species, a summer fresh of 350 ML per day for seven days to maintain and support the growth of aquatic and fringing vegetation, and an autumn fresh of 800 ML per day for seven days to trigger the migration of adult and juvenile native fish. Significant rainfall across the catchment filled the Heyfield wetlands in winter, eliminating the need for planned environmental water deliveries. Water levels in the wetlands were maintained throughout spring, providing habitat for waterbirds, frogs and turtles.

Water for the environment that was not used in 2021-22 will be carried over to support watering actions in 2022-23 or beyond. However, Thomson Dam was 90 percent full in May 2022, and if high rainfall continues over winter and spring 2022, some of the carried-over water may be forfeited (in line with entitlement rules) if the dam operator needs to make spill releases.

The Thomson River catchment has had wetter-than-average conditions for the past two years, which have delivered many large flow events. Fish surveys conducted in the middle and lower reaches of the Thomson River have detected Australian grayling, river blackfish and the strong recruitment of tupong. These results highlight the importance of spring and autumn freshes, which support the spawning and recruitment of migratory native fish species. Tupong were also detected upstream of the newly constructed Horseshoe Bend fishway in 2021 and 2022, indicating fish are migrating upstream and using the fishway to access habitat in the upper reaches of the Thomson and Aberfeldy rivers.

Scope of environmental watering

Table 2.3.1 describes the potential environmental watering actions in 2022-23, 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
Carran Carran (Thomson River) (targeting reach 3)
Winter/spring/autumn low flow (125-350 ML/ day during July to November 2021 and April to June 2022)	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 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 (in August)	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 September to December)	Top up ponds before summer to maintain vegetation and enhance recruitment by triggering seed release Maintain habitat for waterbirds and frogs (such as growling grass frogs and golden bell frogs)
Partial drawdown (during December to February)	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

Scenario planning

Table 2.3.2 outlines potential environmental watering and expected water use under a range of planning scenarios.

It is important to deliver a mix of low flows and freshes throughout the year in Carran Carran (Thomson River), but the magnitude, duration and frequency of these events will generally be lower under drought and dry climate scenarios than under average and wet scenarios. However, a large carryover from 2021-22 and forecast high allocations at the start of 2022-23 mean the available supply of water for the environment will likely be high under all climate scenarios, which may allow flows

to be delivered at greater-than-normal rates under the dry and drought scenarios. The estimated water demands for planned watering actions presented in Table 2.3.2 do not account for potential unregulated flows. As seen in recent years, natural tributary inflows are likely to achieve many of the planned watering actions under wetter climate scenarios, and therefore, most or all of the tier 1a and tier 1b actions proposed for the Thomson River under wet and possibly average scenarios should be achievable with available supply.

Under all climate scenarios, the highest-priority watering actions for Carran Carran (Thomson River) are 800 ML per day freshes in autumn and spring (in October/November), which target migratory fish movement into or out of the system. These events are essential to cue the spawning and recruitment of the threatened Australian grayling population and other native migratory fish species, which have shown positive signs of recruitment over the last two years. These events are necessary every year under average and wet climate scenarios to ensure regular recruitment and to align with environmental cues in the broader landscape. They are generally less important in dry or drought scenarios, but they are considered important to deliver even under drier conditions in 2022-23 to consolidate recent population growth and to potentially supplement populations in east Gippsland that were affected by the 2019-20 bushfires. Where possible, the spring and autumn freshes may be timed to coincide with long weekends to provide additional recreational benefits for river users. Two autumn freshes will likely be delivered under all climate scenarios, but under drought and dry scenarios, the duration may be reduced from seven to five days to conserve water. Freshes that last for five days are expected to trigger some fish migration, although total fish movement is likely to be less than for a seven-day fresh. Providing an additional 800-900 ML per day fresh in September is important under all scenarios to support vegetation outcomes, but there is unlikely to be enough water for the environment to actively deliver these events in drought or dry climate scenarios. It will be important to deliver two summer/autumn freshes under all climate scenarios to clear fine silt and biofilms from in-stream habitat and facilitate the movement of native fish and platypus.

Delivery of low flows throughout the year is expected to change, depending on the climate scenario. A flow of 125 ML per day in reach 3 is the target magnitude from December to March, and it is the minimum recommended flow between May and November. This flow magnitude is expected to be delivered with operational passing flows under all climate scenarios.

Increasing the low-flow magnitude up to 350 ML per day between July and November and April to June is preferred under all climate scenarios, to improve outcomes for fringing and streamside vegetation. If water for the environment is limited under drier scenarios, the low flow may likely only be raised in July (to 300 ML per day) and May to June (to 230 ML per day) to help fish and platypus move throughout the reach at critical breeding and dispersal times.

The recommended water regime for the Heyfield wetlands is the same under all climate scenarios to help recently planted semi-aquatic and terrestrial fringing plants to establish and promote natural recruitment. Water for the environment will likely be needed to fill and top up the wetlands under drought and dry climate scenarios. Natural run-off is likely to meet some or all of the recommended watering actions at the Heyfield wetlands under average and wet climate scenarios.

Under all climate scenarios, a minimum of 2,600 ML is prioritised for carryover into 2022-23 to meet critical early-season, low- flow requirements in Carran Carran (Thomson River).

Planning scenario table

Table 2.3.2 Potential environmental watering for the Thomson system under a range of planning scenarios

Planning scenario	Drought	Dry	Average	Wet
Expected river conditions	Passing flow and limited natural flow from Aberfeldy River and other tributaries contribute to low flow A large volume of consumptive water is released from storage	Passing flow and natural flow from Aberfeldy River and other tributaries contribute to low flow and some freshes Amoderate volume of consumptive water is released from storage	Passing flow and natural flow from Aberfeldy River and other tributaries contribute to low flow and periods of high flow and freshes A small volume of consumptive water is released from storage	Natural flow from Aberfeldy River and other tributaries is expected to meet most low-flow requirements, provide large freshes and sustained high flow Minimal volume of consumptive water released from storage
Expected availability of water for the environment	25,000-28,000 ML	28,000-31,000 ML	31,000-34,000 ML	34,000-37,000 ML
Carran Carran (Thomson River) (targeting reach 3)
Planning scenario	Drought	Dry	Average	Wet
Potential environmental watering – tier 1 (high priorities)	Tier 1a (can be achieved with predicted supply)
	Winter/spring/ autumn low flow (partially delivered: 300 ML/day in July 2022, 230 ML/day in June 2023 and 125 ML/day at other times) Spring fresh (one fresh, of lower duration and magnitude) Summer/autumn low flow Summer/autumn freshes (two freshes, one at upper, one at lower magnitude) Autumn freshes (two freshes, one of lower duration [in May])	Winter/spring/ autumn low flow (partially delivered: 300 ML/day in July 2022 and 230 ML/ day in May to June 2023 and 125 ML/ day at other times) Spring fresh (one fresh, of longer duration but lower magnitude) Summer/autumn low flow Summer/autumn freshes (two freshes, one at upper, one at lower magnitude) Autumn freshes (two freshes)	Winter/spring/ autumn low flow (partially delivered: 300 ML/day in July 2022 and 350 ML/d in May to June 2023 and 125 ML/day at other times) Spring fresh (one fresh, of longer duration but lower magnitude) Summer/autumn low flow Summer/autumn freshes (two freshes, one at upper, one at lower magnitude) Autumn freshes (two freshes)	Winter/spring/ autumn low flow (partially delivered: at 350 ML/day in July 2022 and April to June 2023 and 125 ML/day at other times) Spring freshes (two freshes, of longer duration but lower magnitude) Summer/autumn low flow Summer/autumn freshes (two freshes, of upper magnitude and duration) Autumn freshes (two freshes)
	Tier 1b (supply deficit)
	Winter/spring/ autumn low flow (at 350 ML/day during July to November 2022) Spring freshes (two freshes [one replacing tier 1a fresh],delivered of longer duration and one additional spring fresh of longer duration and lower magnitude) Autumn fresh (tier 1a fresh delivered of longer duration [in May])	Winter/spring/ autumn low flow (at upper magnitude continuously) Spring fresh (one additional fresh, of longer duration and lower magnitude)	Winter/spring/ autumn low flow (at upper magnitude continuously) Spring fresh (one additional fresh, of longerduration and lower magnitude) Summer/autumn freshes (deliver both tier 1a freshes at upper magnitude)	Winter/spring/ autumn low flow (at upper magnitude continuously) Spring fresh (one tier 1a fresh at upper magnitude)
Potential environmental watering – tier 2 (additional priorities)	N/A
Heyfield wetlands
Potential environmental watering – tier 1 (high priorities)	Tier 1a (can be achieved with predicted supply)
	Fill (in August) Top-ups (two, in September-December) Partial drawdown (during December to February)
	Tier 1b (supply deficit)
	N/A
Potential environmental watering – tier 2 (additional priorities)	N/A
Possible volume of water for the environment required to achieve objectives	21,000 ML (tier 1a) 22,500 (tier 1b)	26,000 ML (tier 1a) 25,700 ML (tier 1b)	33,400 ML (tier 1a) 21,200 ML (tier 1b)	43,500 ML¹ (tier 1a) 14,600 ML (tier 1b)
Priority carryover requirements for 2023-24	2,600 ML

¹ While the demand is in excess of available supply, it is expected that some of the events will be at least partially met with natural inflows under a wet scenario.

Engagement

Table 2 shows the partners with which West Gippsland CMA engaged when preparing the Thomson system seasonal watering proposal.

Seasonal watering proposals are informed by longer-term regional catchment strategies, regional waterway strategies, environmental flow studies, water management plans and other studies. These incorporate a range of environmental, cultural, social and economic perspectives and longerterm integrated catchment and waterway management objectives. For further details, refer to the West Gippsland Regional Catchment Strategy and West Gippsland Waterway Strategy.

Table 2 Partners and stakeholders engaged in developing the Latrobe system seasonal watering proposal

Partner and stakeholder engagement
Cowwarr Landcare Group Heyfield Wetlands Committee of Management Waterwatch volunteers Birdlife Australia Melbourne Water Southern Rural Water Gippsland Water Department of Environment, Land, Water and Planning Individual landholders Tourism operators VRFish Arthur Rylah Institute (Department of Environment, Land, Water and Planning) Gunaikurnai Land and Waters Aboriginal Corporation

Page last updated: 01/12/22