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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 environmental watering 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 4a) and Rainbow Creek (reach 4b) (see system map below). Passing flows throughout the year are split two-thirds down reach 4a 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 4a) to support fish migration because Cowwarr Weir impedes fish movement through Rainbow Creek.

The Heyfield wetlands is a cluster of several 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.

Traditional Owners
Environmental water holder

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

icon-objectives-fish
Restore populations of native fish, specifically Australian grayling

Maintain/enhance the structure of native fish communities
Frog icon
Maintain the existing frog population and provide suitable habitat
Landscape icon
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
Platypus icon
Increase the abundance of platypus
Plant icon
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, promote the growth and establishment of semi-aquatic species (Heyfield wetlands)

Enhance the resilience of semi-aquatic species (Heyfield wetlands)
Insect icon
Restore and maintain the natural invertebrate community
bird icon
Provide freshwater habitat for migratory and non- migratory wetland birds within the Gippsland Plains landscape
Water icon
Maintain and 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 Thomson River catchment. The vegetation is intact and 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 thousands of years, including with the waterways in the Latrobe system, into which Carran Carran (Thomson River) feeds. For the Gunaikurnai as traditional custodians, there are immense challenges to heal, protect and manage Country which has been drastically altered since colonisation. Gunaikurnai see all of Country as connected with no separation between landscapes, waterways, coasts and oceans and natural and cultural resources – the cultural landscape is interdependent.

The Gunaikurnai Land and Waters Aboriginal Corporation (GLaWAC) are 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. FLOWS studies provide guidance about the timing, watering duration and amount of water needed by native plants and animals and are therefore a critical input to the annual water for the environment planning process.

GLaWAC cultural water officers have also 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, the majority 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 environmental watering 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.

Social, recreational and economic values and uses

In planning the potential watering actions in Table 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 and events at the Heyfield wetlands, and visitation by locals and non-locals)
  • socio-economic benefits (such as outdoor education businesses and helping to maintain bankside vegetation, preventing erosion and potential land loss.).

If the timing or management of planned environmental flows may be modified to align with a community benefit, this is acknowledged in Table 1 with the following icons.

Kayak icons

Watering planned to support water sports activities (e.g. canoeing, kayaking, rowing, swimming, water skiing)

Camping icon

Watering planned to support peaks in visitation (e.g. camping or other public activities on long weekends or school holidays)

Autumn, winter and spring freshes in Carran Carran (Thomson River) create ideal whitewater 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, the spring fresh, which aims to cue the migration of Australian grayling and other native fish, may be delivered over the Melbourne Cup racing carnival weekend in November when many people take advantage of the Tuesday public holiday to spend a long weekend kayaking on the Carran Carran (Thomson River).

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 Carran Carran (Thomson River) catchment had average to above-average rainfall throughout much of 2020-21. The majority of 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. Consistent inflows occurred throughout winter and spring, boosting water availability and resulting in further allocations, which by the end of spring were 17 percent greater than at the same time in 2019-20. Water was released from Thomson Reservoir throughout the year to meet minimum passing flow requirements, irrigation demand and some environmental flow demands. Environmental flows in Carran Carran (Thomson River) were managed in line with average and wet climate scenarios — note that the potential watering actions for 2020-21 were the same under both scenarios — and most planned environmental flows in winter and early spring 2020 were met by natural flows.

Carran Carran (Thomson River) had several natural freshes in July and August including one event that peaked above 6,000 ML per day at Coopers Creek gauge. Water for the environment was used to deliver a spring fresh of 800 ML per day at Coopers Creek in late September/early October 2020 to support vegetation outcomes. A natural fresh that peaked at about 3,000 ML per day in late October/early November provided a natural trigger for fish to migrate upstream from marine/estuarine habitats. Over summer, operational and natural flows met low-flow requirements and partially met one summer/autumn fresh. Water for the environment was used to deliver a fresh in March 2021 to support vegetation growth and flush sediments, and to deliver two autumn freshes of 800 ML per day to trigger downstream fish migration. It was also used to maintain low flows through autumn 2021. No water for the environment was delivered to the Heyfield wetlands in 2020-21: significant rainfall across the catchment filled the wetlands in winter and water levels were maintained throughout spring, providing habitat for waterbirds, frogs and turtles.

All of the high priority (tier 1) planned environmental watering actions for Carran Carran (Thomson River) were met in 2020-21. Some tier 2 watering actions were also partially met, which contributed to environmental outcomes in the Thomson River estuary.

Environmental monitoring indicates improved environmental outcomes in the Thomson system and some recovery from drier conditions between 2017 and 2019. Fish surveys in February 2021 detected successful recruitment following spring freshes in 2020. Specific findings included the catch of 19 Australian grayling in the middle to lower reaches and the highest catch rate of tupong in the 17 years of surveying. Some tupong were also caught upstream of Horseshoe Bend, which indicates fish are using the recently constructed fishway to access habitat in the upper reaches of Carran Carran (Thomson River) and the Aberfeldy River. On a landscape scale, west Gippsland catchments may play an important role in providing habitat for coastal migratory fish populations, given many catchments in east Gippsland were affected by the 2019-20 bushfires. Environmental watering in 2021-22 will aim to maintain and where possible build on the environmental outcomes achieved in 2020-21. Scientists at the Arthur Rylah Institute have recommended maintaining low flows in 2021-22 to promote the upstream dispersal and survival of new tupong recruits.

Scope of environmental watering

Table 1 Potential environmental watering actions, expected watering effects and associated environmental objectives for the Thomson system

Potential environmental watering action

Functional watering objectives

Environmental objectives

Carran Carran (Thomson River) (targeting reach 3)

Winter/spring/autumn low flow (125-350 ML/day during July to November 2021 and May 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 is increased when delivered at higher 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 higher magnitudes)
  • Wet low-lying benches (when delivered at higher 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

Fish iconPlatypus iconPlant iconInsect iconWater drop icon

Spring fresh(es) (one to two freshes of 800-900 ML/day for five to seven days during September to November)

Camping iconKayak icons

  • Watering planned to support water sports activities
  • Watering planned to support peaks in visitation
  • 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

Fish iconMountain iconsPlant iconInsect iconWater drop icon

Summer/autumn low flow (125 ML/day during December to April)

  • 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
  • Wet low-lying benches to prevent encroachment by invasive plants and enable vegetation zonation

Fish iconPlatypus iconPlant iconInsect icon

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
  • 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

Fish iconMountain iconsPlant icon

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

Fish iconMountain iconsPlant icon

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)

Frog iconPlant iconHeron icon

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)

Frog iconPlant iconHeron icon

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

Frog iconPlant iconHeron icon

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: 22/01/21