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Thomson Reservoir harvests most of the flow from the Thomson River upper catchment and has a significant effect on flow in all downstream reaches. Unregulated flows from the Aberfeldy River, which meets the Thomson River downstream of Thomson Reservoir, are essential for providing natural freshes and high flows in the Thomson River.

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 environmental watering due to its heritage river status, high-value native riparian 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 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 Thomson River course (reach 4a) to support fish migration, because Cowwarr Weir impedes fish movement 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

The Thomson River supports six native species of migratory fish that need to move between the sea and freshwater environments to complete their life cycles. 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 high flows, and the larvae and juveniles spend time at sea before returning to the freshwater sections of coastal rivers.

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

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. Reduce competition from exotic fish.
Fish icon
Restore populations of native fish, specifically Australian grayling, by providing pool habitat and flows for fish to move and to cue spawning
Landscape icon
Maintain channel form diversity including pools, to provide a variety of habitats for aquatic animals
Plant icon
Maintain and restore the structural diversity and zonation of riparian vegetation. Increase the recruitment and growth of native riparian vegetation.
Insect icon
Restore and maintain the natural invertebrate community

Environmental watering objectives in the Heyfield wetlands

Frog icon
Maintain the existing frog populations and provide suitable habitat
Plant icon
Maintain the existing vegetation, promote the growth and establishment of semi-aquatic species. Enhance the resilience of semi-aquatic and riparian woodland species.
bird icon
Provide freshwater refuge habitat for migratory and non-migratory wetland birds within the Gippsland Plains landscape. Continue to support observed terrestrial woodland and grassland birds by maintaining their riparian woodland habitat.

Recent Conditions

In recent years, water for the environment has primarily been used to deliver autumn and spring freshes to create spawning and recruitment opportunities for native fish including Australian grayling, Australian bass and tupong. Low flows have also been provided to enable fish to move between habitats along the river.

The Thomson system has experienced mostly dry conditions over the past two years. 2018–19 saw belowaverage rainfall through winter and spring and some of the highest ever recorded temperatures were observed in summer.

Passing flows in the Thomson River were modified during July 2018 to allow some water for the environment to be saved for use later in the year. The modification was agreed by the VEWH, West Gippsland CMA, Southern Rural Water and Melbourne Water, and it saved 1,270 ML of water for the environment. Those savings enabled a winter fresh to be delivered in early August 2018. This and a second partial fresh that was delivered late in August were timed to assist the migration and spawning of Australian bass.

In November 2018, a spring fresh was delivered to trigger the upstream movement of juvenile native fish from the sea and estuary. This fresh was timed to coincide with environmental and unregulated flows from the Macalister and Latrobe Rivers, to optimise outcomes in the lower Latrobe system.

In response to low rainfall throughout summer, consumptive water demands were high. Releases to meet these consumptive orders, combined with limited unregulated flows, met the targets for summer freshes from December 2018 to February 2019.

Construction works for the Thomson River fishway at Horseshoe Bend prevented the delivery of an autumn fresh in 2019. Autumn freshes are needed to trigger migration and spawning of native fish and are generally provided at least two out of every three years. Once completed, the fishway is expected to enhance the response to environmental flows, as it allows fish to move upstream of the Horseshoe Bend Tunnel (a known barrier to fish movement) to access about 85 km of high-quality habitat in the upper Thomson and Aberfeldy rivers.

A combination of water for the environment and consumptive water were used to maintain low flows in the Thomson River over autumn.

Scope of environmental watering

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

Potential environmental watering action

Functional watering objectives

Environmental objectives

Thomson River

Autumn fresh (one to two freshes of 800 ML/day for four days each during April
to May)

  • rigger the downstream migration (and spawning) of Australian grayling
  • Carry plant seeds from the upper catchment for deposition
    downstream
  • Deposit sediments on benches, to provide substrate for vegetation
  • Wet the bank/bench to deliver dissolved and/or fine particulate organic matter
  • Scour substrates to remove accumulated fine sediment

Fish iconMountain iconsPlant iconInsect icon

Autumn/winter low flows (up
to 230 ML/day during April
to June)

  • Increase the available habitat for waterbugs
  • Wet large woody debris to provide food and shelter for waterbugs and fish
  • Increase water depth to provide habitat for fish and provide
    opportunities for localised fish movement between habitats

Fish iconMountain iconsInsect icon

Spring low flows (230 ML/day
in November)

  • Provide connectivity along the river to enable fish passage between habitats and supporting juvenile recruitment of native species
  • Increase available habitat for waterbugs
  • Wet large woody debris to provide food and shelter for waterbugs and fish
Fish icon Insect icon

Spring freshes (one to two freshes of 800 ML/day for four days each during October to November)

  • Trigger upstream fish migration from marine/estuarine habitats, encouraging tupong spawning and the recruitment of juvenile native
    species including Australian grayling and Australian bass
  • Improve and maintain riparian vegetation by inundating the benches and providing variable water levels for plant zonation
  • Carry plant seeds from the upper catchment for deposition downstream
  • Deposit dissolved and/or fine particulate sediments on the benches
  • Scour substrates to remove accumulated fine sediment

Fish iconMountain iconsPlant icon

Winter fresh (one to two freshes of 800 ML/day for four days each during June
to August)

  • Trigger native fish (such as tupong and Australian bass) to migrate towards estuary for spawning (Australian bass) or breeding (tupong)
  • Carry plant seeds from the upper catchment for deposition downstream
  • Deposit dissolved and/or fine particulate sediments on the benches, to help regenerate semi-aquatic vegetation communities
  • Scour substrates to remove accumulated fine sediment

Fish iconMountain iconsPlant icon

Summer freshes (up to seven freshes of 230 ML/day for four days each during
December to April)

  • Increase the water depth to provide habitat for native fish
  • Wet aquatic and fringing vegetation to maintain its condition and support its growth
  • Scour sediment to expose fresh habitat area

Fish iconMountain iconsPlant iconInsect icon

Heyfield wetlands

Fill (August)

  • Wet ponds to capacity, to stabilise the banks and support the spring growth of semi-aquatic vegetation
  • Provide freshwater refuge habitat for waterbirds and frogs (such as growling grass frogs and golden bell frogs)

Frog iconPlant iconHeron icon

Partial fill (during October to
November)

  • Top up ponds to maintain the existing vegetation and enhance its recruitment by triggering seed dispersal
  • Provide freshwater refuge 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

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 indeveloping the Latrobe system seasonal watering proposal

Partner and stakeholder engagement
  • Heyfield Wetlands Committee
  • BirdLife Australia
  • Landcare groups
  • Waterwatch volunteers and coordinators
  • Arthur Rylah Institute
  • Individual landholders
  • Baw Baw Shire Council
  • Wellington Shire Council
  • Melbourne Water
  • Southern Rural Water
  • Victorian Environmental Water Holder
  • Kayakers
  • Canoe clubs
  • VRFish
  • Angling clubs
  • Local birdwatchers
  • Tourism information centres
  • Regional tourism associations
  • Gunaikurnai Land and Waters Aboriginal Corporation