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Rosslynne Reservoir is the only major storage in the Maribyrnong catchment, and it is located in the upper reaches of Jacksons Creek near Gisborne. The release capacity of 20 ML per day from the reservoir is a significant constraint on the outcomes that can be achieved by environmental flow deliveries in the Maribyrnong system. Water for the environment is primarily used to support outcomes in Jacksons Creek between Rosslynne Reservoir and the confluence with Deep Creek (that is, environmental flow reaches 6 and 7 shown in Figure 3.4.1). These two reaches are described as upper and lower Jacksons Creek respectively.

The VEWH does not hold an environmental entitlement in the Maribyrnong system, and it relies on opportunistic, temporary trade to meet demands. Each year for the last six years, Melbourne Water and the VEWH have worked with local diversion licence holders to purchase unused water that can be delivered to support environmental outcomes. This arrangement is negotiated each year and will only occur with the agreement of all parties involved.

System map

Maribyrnong system

Environmental watering objectives in the Maribyrnong River

Fish icon
Protect and increase populations of native small-bodied fish
Landscape icon
Maintain channel morphology
Platypus icon
Maintain the platypus population
Plant icon
Maintain and improve the condition, abundance, diversity and structure of in-stream and riparian vegetation.

Prevent terrestrial vegetation encroachment into the channel.
Insect icon
Support a wide range and high biomass of waterbugs, to break down dead organic matter and support the river’s food chain
Water icon
Maintain water quality, particularly dissolved oxygen concentrations

Environmental values

The upper Maribyrnong catchment contains areas of intact streamside vegetation, which provide important habitat for native fish including migratory short-finned eels, common and ornate galaxias, flathead gudgeon, tupong and Australian smelt. A large population of waterbugs provides abundant food for a significant platypus population in several reaches in the Maribyrnong system.

Recent conditions

The Maribyrnong catchment experienced lower-thanaverage rainfall between summer 2016–17 and summer 2018–19, and inflows to Rosslynne Reservoir during that period were well-below average. The storage level in April 2019 was 17 percent (4,400 ML), which is the lowest volume since mid-2016. The dry conditions meant that winter/spring low-flow requirements in reaches 6 and 7 were either not met or only partially met by natural catchment run-off over the last two years.

In 2018–19, water for the environment was used to deliver autumn freshes to reaches 6 and 7, to prevent adverse water quality conditions and to flush fine sediments from hard surfaces. These functions are critical during very low flow periods to maintain suitable habitat and food resources for small-bodied native fish, waterbugs and platypus. The autumn freshes also provided opportunities for fish to disperse throughout each reach and helped support in-stream and riparian vegetation.

Scope of environmental planning

Table 1 Potential environmental watering actions and objectives for the Maribyrnong River

Potential environmental watering action

Functional watering objective

Environmental objective

Summer/autumn freshes (up to five freshes of 20–40 ML/day for up to seven days) during December to May

  • Flush pools to maintain water quality
  • Scour substrates to remove fine sediment
  • Inundate the in-stream vegetation and riparian benches to support the growth of native riparian plants and to limit encroachment by terrestrial plant species
  • Provide passage for small-bodied native fish and platypus between habitats
Fish iconMountain iconsPlatypus iconPlant iconInsect iconWater drop icon 

Winter/spring high flows (20–40 ML/day) during June to November

  • Inundate the in-stream vegetation and riparian benches to support the growth of native riparian plants and to limit encroachment by
    terrestrial plant species
  • Scour substrates to remove fine sediment
  • Provide passage for small-bodied native fish and platypus between habitats

Fish iconMountain iconsPlatypus iconPlant iconInsect icon

Summer/autumn low flows (4–6 ML/day) during December to May

  • Maintain waterbug habitat by providing suitable depth over riffles, maintaining pools and inundating large woody debris
  • Inundate the in-stream vegetation
  • Provide passage for small-bodied native fish and platypus between habitats

Fish iconPlatypus iconPlant iconInsect icon

Engagement

Table 2 shows the partners, stakeholder organisations and individuals Melbourne Water consulted when preparing the Maribyrnong 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 Port Phillip and Western Port Regional Catchment Strategy and Melbourne Water's Healthy Waterways Strategy.

Table 2 Partners and stakeholders engaged in developing the Maribyrnong system seasonal watering proposal

Partner and stakeholder engagement
  • Friends of the Maribyrnong Valley
  • Jacksons Creek Econetwork
  • Landcare groups
  • Licenced diverters from the Maribyrnong River at Keilor
  • Department of Environment, Land, Water and Planning
  • Western Water
  • Southern Rural Water
  • Victorian Environmental Water Holder
  • Wurundjeri Woi Wurrung Cultural Heritage Aboriginal Corporation