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The Thomson River flows from the slopes of Mt Whitelaw on the Baw Baw Plateau to join the Latrobe River south of Sale. The major tributaries of the Thomson River are the Aberfeldy, Jordan and Macalister rivers, with most unregulated flows originating from the Aberfeldy River.

Environmental water in the Thomson system is held in Thomson Reservoir. Reach 3 of the Thomson River (from the Aberfeldy River confluence to Cowwarr Weir) is the priority for environmental watering due to its heritage river status, relatively intact native riparian vegetation, high-quality in-stream habitat and low abundance of exotic fish species.

At Cowwarr Weir, the Thomson River splits in two and water can move down the old Thomson River course (reach 4a) and Rainbow Creek (reach 4b). The preference is to pass environmental water down the old Thomson course to enable fish migration, as Cowwarr Weir prevents migration through Rainbow Creek.

Storage manager
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 values

Australian grayling are a threatened fish species and the focus of environmental watering in the Thomson River. Australian grayling are known to spawn in response to autumn high flows, and the eggs and juveniles spend time at sea before returning to the freshwater sections of coastal rivers. In addition to Australian grayling a further six migratory fish species are found in the Thomson including the common galaxias, tupong and short-finned eel.

Vegetation is intact and near-natural upstream of Thomson Reservoir in the Baw Baw National Park. Riparian vegetation upstream of Cowwarr Weir is mostly in good condition, but infestations of blackberry and gorse are widespread. Downstream of the Cowwarr Weir, the vegetation is degraded due to stock access and widespread weed invasion.

Social and economic values

Thomson Reservoir is very important because it is the largest storage in Melbourne's water supply system. The system also supplies water to irrigators and towns in Gippsland.

The Thomson River downstream of Thomson Reservoir is popular for canoeing and kayaking. West Gippsland CMA provides flow information before releases so that people can safely take advantage of the improved paddling conditions provided by environmental water.

The waterways in the Thomson system (including the Thomson River) continue to hold significance for Traditional Owners. Waterways and wetlands in the region are important ceremonial places and for thousands of years the Thomson River has provided resources such as food and materials to the Gunaikurnai people.

Environmental watering objectives in the Thomson River

Fish icon
Protect and boost populations of native fish, specifically Australian grayling, by providing pool habitat and flows for fish to move upstream and downstream, and by encouraging fish to spawn
Landscape icon
Scour silt build-up within the river bed to provide increased and diverse habitat for plants and animals to colonise
Plant icon
Provide water for plant life to germinate, establish and grow on the river bank

Conditions mid-2016

In 2015–16 summer rainfall was near-average, but in the important winter and spring inflow periods, rainfall and river flows were well-below-average. Rainfall in autumn was also below-average. Despite the dry conditions there was sufficient environmental water available to deliver water to give Australian grayling and other fish species an opportunity to migrate and spawn. This was supported by a transfer of water from the Yarra system.

In 2015 the Australian grayling spawning response was the best ever recorded in this system, corresponding to the release of an autumn fresh in the Thomson River. Spawning responses were also observed in the previous three years and there is high certainty that flows of a specific magnitude and duration will elicit a spawning response.

Bankfull flows and winter freshes have not occurred often in recent years because the Thomson Reservoir substantially reduces the duration and frequency of high-volume flows. While these flows are important, there is insufficient water held in the Thomson River entitlement to provide them in addition to the highest-priority flows in spring and autumn.

Scope of environmental watering

Table 1 Potential environmental watering actions and objectives for the Thomson system

Potential environmental watering

Environmental objectives

Spring freshes (1–2 freshes of 800 ML/day for 4 days each in September–October)

  • Provide a migration cue for juvenile Australian grayling
  • Inundate and regenerate riparian vegetation
  • Scour sediment exposing fresh habitat areas

Spring baseflows (230 ML/day from October–November)

Autumn freshes (1–2 freshes of 600–800 ML/day for 4 days each in April–May)

  • Provide a migration and spawning cue for Australian grayling and other aquatic species
  • Inundate and regenerate riparian vegetation
  • Scour sediment exposing fresh habitat areas

Autumn/winter baseflows (230 ML/day from May–June)

Winter freshes (up to 4 freshes of 800 ML/day for 4 days in June–August)

  • Maintain/enhance the native fish community structure by providing opportunities for localised fish movement between habitats

Summer/autumn freshes (up to 7 freshes of 230 ML/day for 4 days in December–April)

  • Maintain/enhance the native fish community structure by providing habitat availability, large woody debris inundation
  • Inundate and regenerate riparian vegetation
  • Scour sediment exposing fresh habitat areas

Scenario planning

The highest priority in the Thomson River is to provide environmental water to promote Australian grayling breeding in autumn and recruitment (when fish survive to settlement or maturity) in spring. Australian grayling live for about three years, so it is important to provide optimal breeding conditions at least every two out of three years to maintain their long-term viability. Similarly, it is important to provide spring recruitment flows two out of every three years to attract juvenile Australian grayling into the Thomson River to sustain the local population.

Environmental water for breeding has been delivered successfully for the last seven years in succession with great results. Recruitment flows have been provided for the last two years. While the delivery of water for breeding and recruitment in consecutive years reduces the urgency to provide environmental water in 2016–17, water for recruitment in spring is prioritised first because they were infrequent before the last two years.

In drought conditions, water releases will focus on delivering baseflows (low flows) in winter and spring to maintain suitable aquatic and riparian habitat. Water for breeding and recruitment isn't delivered in a drought scenario.

In dry conditions and subject to water availability, spring freshes (small pulses of water) will be released along with baseflows to provide for juvenile Australian grayling. Savings of water under drought and dry conditions will be carried over into 2017–18, by which time breeding and recruitment will be important to maintain the Australian grayling population.

If unregulated river flows are naturally high (such as under the average and wet scenarios), all available water may be used to maximise opportunities for Australian grayling. Matching releases with natural high-flow conditions will also improve habitat by scouring sediment and regenerating riparian vegetation.

Table 2 Potential environmental watering for the Thomson system under a range of planning scenarios

Planning scenario

Drought

Dry

Average

Wet

Expected river conditions

  • Unregulated flows very limited
  • Large volumes of consumptive water released from storage
  • Unregulated flows from the Aberfeldy River and other tributaries contribute to baseflows and freshes
  • Moderate volumes of consumptive water released from storage
  • Unregulated flows from the Aberfeldy River and other tributaries contribute to baseflows, freshes and high flows
  • Some consumptive water released from storage
  • Unregulated flows from the Aberfeldy River and other tributaries contribute to baseflows, freshes and sustained high flows
  • Possible spills from Thomson Reservoir and minimal consumptive water released from storage

Expected availability of environmental water1

  • 10,000–12,000 ML
  • 10,000–14,000 ML
  • 10,000–18,000 ML
  • 18,000–23,000 ML

Potential environmental watering – tier 1 (high priorities)

  • Spring and autumn baseflows
  • One spring fresh
  • Spring and autumn baseflows
  • Two spring freshes
  • Spring and autumn baseflows
  • One autumn fresh
  • Two spring freshes
  • Spring and autumn baseflows
  • One autumn fresh
  • One winter fresh

Potential environmental watering – tier 2 (additional priorities)

  • One spring fresh
  • One autumn fresh
  • One summer fresh
  • One autumn fresh
  • One summer fresh
  • One winter fresh
  • One autumn fresh
  • One summer fresh
  • One winter fresh
  • One summer fresh

Possible volume of environmental water required to achieve objectives2

  • 5,600 ML (tier 1)
  • 8,900 ML (tier 2)
  • 9,400 ML (tier 1)
  • 5,100 ML (tier 2)
  • 18,000 ML (tier 1)
  • 10,000 ML (tier 2)
  • 23,000 ML (tier 1)
  • 5,300 ML (tier 2)

Priority carryover requirements

  • 4,600 to 6,400 ML
  • 0 ML

1 The first 10,000 ML of Thomson Reservoir inflows is allocated to the environment at the beginning of the water year. An additional 8,000 ML may be available in the system, subject to an entitlement amendment being completed to facilitate its use.

2 Environmental water requirements for tier 2 actions are additional to tier 1 requirements.

Risk management

In preparing its seasonal watering proposal, West Gippsland CMA considered and assessed risks, and identified mitigating strategies, relating to implementing environmental watering. Risks and mitigating actions are continually reassessed by program partners throughout the water year.

Engagement

Waterway managers meet communities on environmental watering regionally, although other program partners also play a role.

In each region of Victoria, community engagement on environmental watering happens when environmental watering objectives and priorities are scoped (long term and annually), when delivering environmental water, and when reporting on environmental watering results.

In the West Gippsland region communities are involved in decisions about the Thomson and Macalister river systems and the Lower Latrobe wetlands. The community is engaged informally and through the Macalister river Project Advisory Group.

Who is engaged and how

Recreational users

A representative of VRFish sits on the Macalister river Project Advisory Group.

West Gippsland Catchment Management Authority engages with Field & Game Australia when developing the Lower Latrobe wetlands seasonal watering proposal (annual scoping). Field & Game Australia shares wetland bird count data with the Catchment Management Authority which helps with reporting on environmental watering outcomes.

The Catchment Management Authority notifies many recreational user groups about environmental water deliveries. These groups include canoe clubs, outdoor education companies and school groups (many of whom come from outside of the region). These groups are interested in managing safety and also achieving opportunities for shared benefits from environmental watering (e.g. kayaking on higher flows in the Thomson river).

Environment groups

Representatives from environment groups (Environment Victoria, Landcare and Native Fish Australia) sit on the Macalister river Project Advisory Group.

Irrigators/farmers

Irrigation representatives sit on the Macalister river Macalister river Project Advisory Group.

Southern Rural Water sends environmental water delivery notifications to their irrigation customers on behalf of the Catchment Management Authority. These irrigation customers are often primarily interested in environmental watering from a safety and infrastructure protection perspective.

Traditional owners

Through the Catchment Management Authority Indigenous facilitator, the Gunaikurnai Land and Waters Aboriginal Corporation has been invited to join the Macalister river Project Advisory Group.

The Catchment Management Authority and Gunaikurnai Land and Waters Aboriginal Corporation have a Memorandum of Understanding. The Catchment Management Authority recognises the recently developed Gunaikurnai Land and Waters Aboriginal Corporation draft Whole Of Country Plan which contains aspirations for rivers and wetlands in Gippsland.

Councils

The Wellington Shire Council is a member of the Macalister river Macalister river Project Advisory Group.

The Wellington Shire Council, the Latrobe City Council and the Baw Baw Shire Council are all on the Southern Rural Water environmental water delivery notification list.

General public

Public signage informing the general public about upcoming environmental water deliveries is set up a few days before a planned delivery along key recreational sites in the upper Thomson River. This is for primarily for public safety purposes.