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The Werribee River flows south-east from the Wombat State Forest near Ballan before dropping through the Werribee Gorge to Bacchus Marsh and then flowing into Port Phillip Bay at Werribee. The Lerderderg River is a major tributary that joins the river at Bacchus Marsh.

The priority river reaches for environmental flow delivery in the Werribee system are the reach downstream of Lake Merrimu (reach 6), the reach within Werribee (reach 9) and the estuary: these support a diverse range of native fish species, waterbugs and platypus. Flows targeting the estuary are expected to provide some benefits to reach 8 and water may also be delivered for environmental objectives in this reach under suitable conditions. Environmental water released from Lake Merrimu can be re-harvested in Melton Reservoir, minus en route losses. It can then be held and re-released from Melton at a later date for specific lower Werribee River outcomes. Flows are measured downstream of Lake Merrimu (reach 6), downstream of Melton Reservoir (reach 8) and at the Werribee Diversion Weir for reach 9 and the estuary (see system map below).

Waterway manager
Storage manager
Environmental water holder

System map

Werribee 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 Werribee River

Habitat icon
Maintain habitat for frogs, waterbugs and platypus
Landscape icon
Move built-up silt from riffles (in the shallower parts of the river)
Water icon
Maintain pool water quality for fish and platypus and inundate estuary salt marsh with brackish water
Plant icon
Maintain diverse macrophytes (large water plants) and shrubs to provide shade and food for organisms further up the food chain
Fish icon
Protect and boost native fish populations (including black bream and galaxiids) by providing pool habitat and flows for fish to move up and downstream and encouraging fish to spawn

Environmental values

The Werribee system supports a range of native fish including large populations of black bream and other species (such as the river blackfish, flathead gudgeon, short-finned eel, tupong and Australian smelt and several species of galaxiids). A diverse community of frogs and waterbugs inhabit the upper reaches and platypus are present in the lower reaches. The freshwater-saltwater interface of the Werribee River estuary is a regionally significant ecosystem due to the many aquatic plants and animals it supports, providing juvenile habitat and for the successful recruitment of fish such as black bream.

Social and economic values

The Werribee River provides the opportunity for recreational activities including fishing, bird watching, passive boating (canoeing, kayaking) and bushwalking. The system also provides irrigation water for agricultural industries throughout the Bacchus Marsh and Werribee areas and domestic water for Melton and Bacchus Marsh. Significant Aboriginal cultural heritage sites have been found along the riverbank and escarpments including fish traps, artefacts and burial sites. The Werribee River continues to be a place of significance for Traditional Owners and their Nations in the region.

Conditions mid-2016

Rainfall into the Werribee system has been below-average for the past four years. Melton Reservoir has not spilled since October 2012 and consequently there has been minimal natural outflow to Port Phillip Bay. Environmental watering has been required to provide a large portion of flows in reach 9 and the estuary. Dry conditions persisted in 2015–16 with environmental watering focusing on the most critical objectives to help protect the health of the system under dry conditions.

Baseflows and two freshes were provided to Pyrites Creek (reach 6) using environmental water. Maintaining baseflows is important to provide suitable frog habitat in winter/spring, while the freshes targeted outcomes for pygmy perch and waterbugs. The dry conditions affected deliveries, with significant losses resulting in the freshes not meeting the target flows and noticeably less of the releases from Lake Merrimu reaching Melton Reservoir downstream. A persistent trickle flow occurred throughout summer in the upper parts of Pyrites Creek as a result of leakage from Lake Merrimu while the lower section of the reach ceased to flow for extended periods.

With continuing dry conditions and low environmental water availability, it is becoming increasingly difficult each year to meet the flow objectives for the lower Werribee River. Issues such as blue-green algae and floating aquatic weeds were again evident in reach 9 through the Werribee township. These issues highlight the low-flow and high-nutrient loads in the river and affect fishing, boating and the general enjoyment people have from being near the river. Two freshes to the lower Werribee River were delivered in January and March 2016; while primarily aimed at maintaining water quality and supporting fish passage, they also flushed the blue-green algae. These freshes were too small to have a significant or long-lasting effect on reducing the aquatic weed build-up.

The Werribee River will benefit from an additional 1,100 ML of water made available from Lake Merrimu in 2015–16. At the time of writing, this water was planned to be delivered in winter 2016, primarily to provide a large fresh event to the lower Werribee River, an event that has not occurred since 2012. This larger fresh is aimed at providing a significant flush to the river, improving habitat for fish and platypus and removing the aquatic weed accumulation. This will improve water quality in the lower Werribee River and is expected to result in less weed, algae and other issues in 2016–17, although follow-up flows are likely to be important to achieving this.

Scope of environmental watering

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

Potential environmental watering

Environmental objectives

Pyrites Creek (reach 6)

Spring/summer freshes (up to 3 freshes of 30 ML/day for 2 days in September–December)

  • Improve waterbug habitat by scouring silt and sand from riffles
  • Promote vegetation growth

Spring/summer high flows (130 ML/day for 2 days in September–December)

  • Flush organic matter from benches
  • Promote recruitment and growth of riparian vegetation

Winter/spring/summer baseflows (2 ML/day [or natural] in June–December)

  • Provide waterbug and frog habitat

Lower Werribee River (reaches 8, 9 and the estuary)

Spring/summer freshes (up to 2 freshes of 50–80 ML/day for 2 days in November–December)

  • Promote juvenile black bream recruitment
  • Promote longer-distance movement of fish through reach 9

Winter/spring/summer baseflows (10 ML/day in June–December)

  • Provide black bream habitat for spawning
  • Provide habitat for waterbugs and fish and support vegetation growth in reach 9

Autumn baseflows 10 ML/day during March–May

  • Promote downstream migration of diadromous fish (fish that move between freshwater and saltwater to complete their life cycle) to the estuary
  • Provide habitat for waterbugs and fish and to support vegetation growth in reach 9

Summer/autumn freshes (up to 3 freshes of 80 ML/day1 for 2 days during January–April)

  • Maintain pool water quality for fish and platypus in reach 9
  • Promote recruitment of juvenile black bream in the estuary
  • Scour silt and algae from riffles in reach 8

Winter/spring/summer freshes (up to 4 freshes of 350 ML/day for 3 days during June–December)2

  • Promote diversity of riparian vegetation in reaches 8 and 9
  • Provide fish movement cues (all)
  • Inundate saltmarsh vegetation with brackish water in the estuary

1 Recommendation is for 137 ML delivered in one day. The recommendation has been revised due to operational constraints to be 160 ML delivered over 2 days. Monitoring has shown that this achieves the hydraulic and water quality objective.

2 If this watering action is not delivered in 2015–16, it may be delivered in 2016–17 using the 1,100 ML of additional water provided in 2015–16. This is not shown in the scenario planning table below as it is intended to be delivered in 2015–16 at the time of writing.

Scenario planning

As seasonal conditions improve across the planning scenarios from drought to wet, additional actions become a priority for environmental watering. The critical flows planned to be delivered under the drought and dry scenarios focus on deliveries to Pyrites Creek (reach 6) and freshes to protect the lower Werribee River, by maintaining water quality. However, the amount of water available may not be sufficient to meet all these demands, particularly under drought conditions. The expected volume of environmental water required to achieve the desired objectives increases as conditions become wetter, as re-harvesting Lake Merrimu releases in Melton Reservoir cannot occur when Melton Reservoir is spilling.

When possible, winter releases from Lake Merrimu to Pyrites Creek (reach 6) will be captured in Melton Reservoir, making the volume that reaches the reservoir available for releases downstream later in the water year. This is an essential management option to enable the best use of very limited environmental water under drought and dry conditions. Under average or wet conditions Melton Reservoir is likely to be spilling, meaning releases from upstream will spill through the reservoir and provide a small increase in unregulated flow downstream.

Carrying over some water into 2017–18 is essential to help protect the health of Pyrites Creek (reach 6) in the following year under dry conditions.

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

Planning scenario

Drought

Dry

Average

Wet

Expected river conditions

  • No unregulated flows
  • Minimal consumptive releases out of storage into reach 8 in summer/autumn
  • No unregulated flows below Melton Reservoir, minimal passing flows to reach 6
  • Consumptive releases out of storage into reach 8 in summer/autumn
  • Unregulated spills in winter/spring from Melton into reaches 8 and 9 and the estuary; most reach 6 baseflows met by passing flows
  • Consumptive releases out of storage into reach 8 in summer/autumn
  • Unregulated spills in winter/spring from Melton into reaches 8 and 9 and the estuary; all reach 6 baseflows provided
  • Consumptive releases out of storage into reach 8 in summer/autumn

Expected availability of environmental water

  • 350 ML carryover
  • 0 ML allocation
  • 50 ML inflows
  • 400 ML total
  • 350 ML carryover
  • 500 ML allocation
  • 200 ML inflows
  • 1,050 ML total
  • 350 ML carryover
  • 700 ML allocation
  • 400 ML inflows
  • 1,450 ML total
  • 350 ML carryover
  • >800 ML allocation
  • >900 ML inflows
  • >2,050 ML total

Potential environmental watering –

tier 1 (high priorities)

  •  Winter/spring/summer baseflows  (reach 6)
  • Two spring/ summer freshes (reach 6)
  • Two summer/autumn freshes (lower reaches)
  • Winter/spring/summer baseflows (reach 6)
  • Three spring/ summer freshes (reach 6)
  • Two summer/autumn freshes (lower reaches)
  • Autumn baseflows (lower reaches)
  • Spring/summer freshes (lower reaches)
  • Three spring/summer freshes (reach 6)
  • Two summer/autumn freshes (lower reaches)
  • Autumn baseflows (lower reaches)
  • Two spring/summer freshes (lower reaches)
  • Winter/spring/summer baseflows (lower reaches)
  • Three spring/summer freshes (reach 6)
  • Spring/summer high flows (reach 6)
  • Two summer/autumn freshes (lower reaches)
  • Autumn baseflows (lower reaches)
  • Two spring/summer freshes (lower reaches)
  • Winter/spring/summer baseflows (lower reaches)

Potential environmental watering –

 tier 2 (additional priorities)

  • Winter/spring/summer freshes (lower reaches)
  • Autumn baseflows (lower reaches)
  • Winter/spring/
    summer freshes (lower reaches)
  • Additional winter/spring/summer freshes (lower reaches)
  • Additional winter/spring/summer freshes (lower reaches)

Possible volume of environmental water required to achieve objectives1

  • 350 ML (tier 1)
  • 1,500 ML (tier 2)
  • 700 ML (tier 1)
  • 1,300 ML (tier 2)
  • 900 ML (tier 1)
  • 1,300 ML (tier 2)
  • 1,200  ML (tier 1)
  • 1,300 ML (tier 2)

Priority carryover requirements

  • 200 ML

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

Risk management

In preparing its seasonal watering proposal, Melbourne Water 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.

Further Information on Scenario Planning

Engagement

Table 3.5.3 shows the partners, stakeholder organisations and individuals with which Melbourne Water engaged when preparing the Werribee system seasonal watering proposal.

Seasonal watering proposals are informed by longer-term regional waterway strategies, environmental water management plans and environmental flow studies, which incorporate environmental, cultural, social and economic considerations.

Table 3.5.3 Partners and stakeholders engaged in developing the Werribee system seasonal watering proposal

Partner and stakeholder engagement

  • Werribee River Community Advisory Group including representatives of Melton, Wyndham and Moorabool councils, Waterwatch, Werribee Riverkeeper, Western Melbourne Catchment Network, Friends of Werribee Gorge and Longforest Mallee, Pinkerton Landcare, Friends of Toolern Creek, Werribee South Fishing Club, Werribee Anglers Club and Port Phillip and Westernport CMA
  • Southern Rural Water and licensed diverters
  • Victorian Environmental Water Holder

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.

Communities in the Melbourne region are involved in decisions about the Tarago, Yarra and Werribee river systems through each system's Environmental Water Advisory Group.

Who is engaged and how

Recreational users

Through formal advisory groups coordinated by Melbourne Water, representatives of recreational user groups are engaged. Advisory group members receive notifications about planned environmental water deliveries via the community bulletin.

For recreational users, safety related to higher water levels are a key concern.

On occasion, Melbourne Water informs recreational users directly about environmental watering (such as the Werribee fishing club or Canoes Victoria).

Environment groups

Representatives of environment groups are engaged through formal advisory groups coordinated by Melbourne Water. Advisory group members receive notifications about planned environmental water deliveries via the community bulletin.

Melbourne Water has informal relationships with various environment groups (such as Landcare, Birdlife Australia, Yarra Riverkeepers, Environment Victoria and Waterwatch) and meet with these groups on an as-needs basis.

Some groups, such as Waterwatch, share monitoring information with Melbourne Water.

Landholders/farmers

Through the Yarra Diversions newsletter, diversions licence holders receive information and updates on environmental watering.

Landholders and farmers are engaged through formal advisory groups coordinated by Melbourne Water. Advisory group members receive notifications about planned environmental water deliveries via the community bulletin.

Traditional Owners

Melbourne Water is doing a collaborative project with the Wurundjeri Tribe Land and Compensation Heritage Council and the Victorian Environmental Water Holder to document the Wurundjeri cultural values in the Yarra river system. The aim of this project is to increase understanding of values that can be supported with environmental water to achieve shared benefits for Aboriginal people from environmental watering (Aboriginal environmental outcomes).

Councils

Through formal advisory groups coordinated by Melbourne Water, Council representatives (from various local councils across region) are engaged. Advisory group members receive notifications about planned environmental water deliveries via the community bulletin.

Melbourne Water publishes an annual two-pager for each Council about activities undertaken in their catchment, including delivery of environmental flows.

General public

Melbourne Water communicates and engages with the general public through their website, media releases and Facebook and Twitter. Melbourne Water community bulletins are posted on their webpage, Facebook and Twitter, and issued to media contacts.