VEWH - Thomson River

The Thomson River flows from the slopes of the mountains of the Baw Baw Plateau to join the Latrobe River south of Sale. The major tributaries of the Thomson River are the Aberfeldy and Jordan rivers in the upper reaches and the Macalister River in the lowest reach. Most unregulated flows originate from the Aberfeldy River. 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 which supplies irrigation water to parts of the Macalister Irrigation District.

The Thomson River and its tributaries continue to be an important place for Traditional Owners and their Nations. The Registered Aboriginal Party in the region is the Gunaikurnai Land and Waters Aboriginal Corporation.

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 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 has national conservation importance. 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 upstream of 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.

Social and economic values

Thomson Reservoir is the largest storage in Melbourne's water supply system, and it supplies water for irrigation, industry and towns in Gippsland. The Macalister Irrigation District is the largest irrigation area in southern Victoria and a major economic driver for the region.

The upper reaches of the Thomson River — from Thomson Reservoir to Cowwarr Weir — and its tributary the Aberfeldy River — in the Baw Baw National Park — are classified as heritage river areas under the Heritage Rivers Act 1992, based on their ecological, historical and recreational values. The Horseshoe Bend tunnel is an important European heritage site in this area; it is a legacy of early gold mining in the area. The tunnel is a significant barrier to fish movement and therefore limits the ability to meet all environmental flow objectives in the upper reaches of the system.

The Thomson River is highly valued for recreation downstream of the Thomson Reservoir to Cowwarr Weir. The area is popular for camping, kayaking and canoeing. Avid kayakers, canoeists and outdoor recreational operators often take advantage of the whitewater conditions provided with releases of water for the environment in the upper reaches of the Thomson River. The local communities have a strong connection with the waterways and the visual amenity and recreational opportunities they provide.

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Environmental watering objectives in the Thomson River

Restore populations of native fish, specifically Australian grayling, by providing pool habitat and flows for fish to move and to cue spawning 

Scour silt build-up within the river bed to improve the quality of in-stream habitat for aquatic plants and animals Prevent encroachment of terrestrial vegetation into the stream channel

Increase recruitment and growth of native riparian vegetation Prevent encroachment of terrestrial vegetation into the stream channel

Conditions 2018

Releases of water for the environment in recent years have focused on providing autumn and spring freshes for spawning and recruitment opportunities for native fish species including Australian grayling, tupong and Australian bass. Low flows have also been provided to enable fish to move between habitats along the river.

The Thomson system has experienced variable climate conditions over the past two years. There was averageto-above-average rainfall in winter and spring 2016–17 resulting in several unregulated flow peaks and minor flooding, while dry conditions followed in summer, autumn and winter. Dry conditions persisted into 2017–18 with below-average rainfall throughout most of the year. Environmental flows in 2017–18 aimed to deliver freshes to provide spawning and recruitment opportunities for Australian grayling and low flows in winter and spring to maintain the quality of fish habitat.

Passing flows in the Thomson River were modified for July 2017 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, Gippsland Water and Melbourne Water, and it saved 2,500 ML of water for the environment. Those savings, and additional allocations associated with the new environmental entitlement for the Thomson River which was gazetted in June 2017, enabled a range of environmental flows to be delivered through spring, summer and autumn.

From October to November 2017, spring low flows were delivered to allow fish to move between habitats in individual river reaches. An unregulated event provided a short fresh in mid-September 2017, and water for the environment was used to extend its duration to attract Australian grayling into the upper reaches of the river. Low flows in May to June 2018 were met through a combination of managed releases for environmental and consumptive use. In April 2018, an autumn fresh was provided to prompt migration and spawning of Australian grayling.

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 October–November)	Encourage recruitment of juvenile migratory fish species from the estuary and ocean
Autumn/winter low flows (up to 230 ML/day from May–July)¹	Provide improved passage along the river to enable localised fish movement between habitats
Spring low flows (230 ML/day November)	Provide improved passage along the river to enable fish to move between habitats
Winter freshes (up to 4 freshes of 800 ML/day for 4 days in June–August)	Provide a migration and spawning cue for migratory fish species including tupong and Australian bass Maintain diversity and increase riparian vegetation
Autumn freshes (1–2 freshes of 800 ML/day for 4 days each in April–May)	Provide a migration and spawning cue for migratory fish species including tupong and Australian bass Maintain diversity and increase riparian vegetation
Summer/autumn freshes (up to 7 freshes of 230 ML/day for 4 days in December–April)	Provide habitat for native fish Maintain diversity and increase growth of submerged aquatic vegetation Scour sediment exposing fresh habitat areas Provide habitat for waterbugs

¹ Passing flows may be flexibly managed at rates less than 230 ML per day in July.

Risk management

In preparing its seasonal watering proposal, West Gippsland CMA considered and assessed the risks of environmental watering and identified mitigation strategies. Program partners continually reassess risks and mitigation actions throughout the water year.

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 engaged in developing the Thomson system seasonal watering proposal

Partner engagement
Melbourne Water Southern Rural Water Victorian Environmental Water Holder