Maribyrnong

The Maribyrnong catchment is located to the north-west of Melbourne. The main waterways in the catchment are Jacksons Creek, which flows south-east from Mount Macedon, and Deep Creek, which flows south from Lancefield (Figure 3.4.1). These two tributaries join at Keilor North to form Mirrangbamurn (Maribyrnong River), which flows south to join Birrarung (Yarra River) at Yarraville before flowing into Port Phillip Bay.

Rosslynne Reservoir is in the upper reaches of Jacksons Creek near Gisborne and is the only major storage in the Maribyrnong catchment. The reservoir has a maximum release capacity of 20 ML per day under ideal conditions, which significantly constrains the environmental outcomes that can be achieved in the Maribyrnong system. Water for the environment is primarily used to support environmental outcomes in Jacksons Creek between Rosslynne Reservoir and the confluence with Deep Creek (that is, delivery of water for the environment to reaches 6 and 7 shown in Figure 3.4.1). Jacksons Creek is a known groundwater-dependent ecosystem on the national Groundwater Dependent Ecosystems Atlas. This means ecological components in the system rely on groundwater for at least some period of time.

The VEWH does not hold an environmental entitlement in the Maribyrnong system, and it relies on opportunistic, temporary trade to meet demands. Melbourne Water (as diversion manager) and the VEWH work with local diversion licence holders to purchase unused water when it is available to support environmental outcomes. This arrangement is negotiated each year, is subject to water availability in the bulk entitlement and storage capacity, and only occurs with the agreement of all parties involved.

Maribyrnong pie chart

Proportion of water entitlements in the Maribyrnong system held by private users, water corporations and environmental water holders on 30 June 2020

System map

Environmental watering objectives in the Maribyrnong River

Protect and increase populations of native smallbodied fish

Maintain or increase platypus populations

Maintain and improve the condition, abundance, diversity and structure of in-stream and streamside vegetation

Support a wide range and high biomass of waterbugs to break down dead organic matter and support the river’s food chain

Maintain water quality, particularly 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 of the Maribyrnong system.

Recent conditions

The Maribyrnong catchment had above-average rainfall during 2021-22, leading to high soil moisture and the largest tributary inflows to reaches downstream of Rosslynne Reservoir for more than five years. Inflows to Rosslynne Reservoir were also above average during 2021-22, and the storage reached 66 percent capacity in early February 2022. Tributary inflows provided good flow conditions in lower Jacksons Creek (reach 7) and the Maribyrnong River, but Rosslynne Reservoir harvested inflows from higher in the catchment and restricted flow through reach 6.

The VEWH purchased 314.5 ML of unused allocation from licence holders in the Maribyrnong system to provide environmental flows to Jacksons Creek in summer/autumn 2021-22.

Water for the environment was managed under a wet climate scenario during 2021-22 in the Maribyrnong system. Planned watering actions were met by natural flows in lower Jacksons Creek (reach 7), and managed releases from Rosslynne Reservoir partially met planned watering actions in upper Jacksons Creek (reach 6). Target low-flow magnitudes were met in upper Jacksons Creek for most of the critical summer/autumn period, and five freshes were also delivered. However, the water delivery infrastructure at Rosslynne Reservoir limits releases to a maximum of 15 ML per day, which makes it impossible to fully deliver the recommended summer/autumn fresh.

The target magnitude for winter/spring low flows and summer/autumn freshes has been reduced from 40 ML per day to 15 ML per day in this year’s plan to reflect the flows that can be realistically delivered. These flows are smaller than the system requires for good environmental health, and the expected effects associated with those watering actions have been modified accordingly. The smaller flows are not expected to improve the condition of streamside vegetation or grow native fish and

platypus populations. However, they will help to maintain habitat for existing populations and reduce the risk of poor water- quality incidents and are therefore still necessary. Melbourne Water will continue to work with the reservoir operator and relevant government agencies to improve water delivery capacity from Rosslynne Reservoir.

Traditional Owner cultural values and uses

Melbourne Water is working with the Registered Aboriginal Parties within the Maribyrnong system — the Wurundjeri Woi Wurrung Cultural Heritage Aboriginal Corporation and the Bunurong Land Council Aboriginal Corporation — to develop overarching partnership agreements to frame relations and obligations between them to strengthen relationships and increase Traditional Owners’ involvement in the planning and delivery of water for the environment. The intent is for Traditional Owners to be active partners in the planning, delivery and monitoring of all works and deliveries of water for the environment associated with Mirrangbamurn (Maribyrnong River), including the environmental watering program.

There are more opportunities for Melbourne Water and the VEWH to work with the Traditional Owner groups to identify and better integrate cultural values and their flow requirements into the environmental watering program on an ongoing basis.

Social, recreational and economic values and uses

In planning the potential watering actions in Table 3.4.1, Melbourne Water considered how environmental flows could support social values such as community connection and amenity by planning flows that will maintain healthy habitat and improve water quality.

Scope of environmental planning

Table 1 Potential environmental watering actions, expected watering effects and associated environmental objectives for the Maribyrnong system

Potential environmental watering action	Expected watering effects	Environmental objectives
Winter/spring low flow (15 ML/day during June to November)	Maintain depth in pools and riffles to provide habitat for small-bodied native fish, platypus and waterbugs Prevent terrestrial vegetation encroachment
Summer/autumn low flow (4-6 ML/day during December to May)	Maintain pool habitat availability for small-bodied fish and platypus during low-flow periods Maintain a > 0.1 m median depth over riffles to provide macroinvertebrate habitat and inundate in-stream vegetation. Maintain continuous flow to limit pool stratification and maintain water quality
Summer/autumn freshes (five freshes of 15 ML/ day for four days every 4-6 weeks during December to May)	Increase depth over riffle to provide local movement of small-bodied native fish and platypus during the low flow period Maintain habitat and food resources for waterbugs Flush pools to maintain water quality

Scenario planning

Table 3.4.2 outlines potential environmental watering and expected water use under a range of planning scenarios.

There is no permanent environmental entitlement in the Maribyrnong system, so water for the environment can only be delivered in 2022-23 if other entitlement holders are willing to sell some of their annual allocations to the VEWH.

Adequate low flows throughout the year and summer/autumn freshes are a high priority under all climate scenarios to maintain habitat for native fish and platypus and to prevent incidences of poor water quality. Local catchment run-off, tributary inflows and groundwater contributions are likely to meet and exceed these flow requirements in lower Jacksons Creek (reach 7) under average and wet climate scenarios. However, mandated passing flows and water for the environment will be needed to achieve these watering actions in upper Jacksons Creek (reach 6) under all climate scenarios and in lower Jacksons Creek (reach 7) under a dry climate scenario.

The VEWH is unable to carry over water in the Maribyrnong system to support multi-year planning.

Planning scenario table

Table 3.4.2 Potential environmental watering for the Maribyrnong system under a range of planning scenarios

Planning scenario	Dry	Average	Wet
Expected river conditions	Low volumes of unregulated flows Passing flows may meet some low-flow objectives Some baseflow from groundwater contributions in Jacksons Creek	Unregulated flows meet some objectives Passing flows may meet several low-flow objectives Groundwater contributions provide baseflow in Jacksons Creek	Unregulated flows meet most objectives Passing flows may meet most low-flow objectives Groundwater contributions provide baseflow in Jacksons Creek
Expected availability of water for the environment	There is no environmental entitlement in the Maribyrnong system. Water will need to be traded with willing irrigators to support tier 1b watering actions
Maribyrnong system (targeting reach 6)
Potential environmental watering – tier 1 (high priorities)	Tier 1a (can be achieved with predicted supply)
	N/A
	Tier 1b (supply deficit)
	Winter/spring low flow Summer/autumn low flow Summer/autumn freshes (three freshes)
Possible volume of water for the environment required to achieve objectives	2,400 ML
Priority carryover requirements for 2023-24	N/A: the VEWH is unable to carry over water in the Maribyrnong system to support multi- year planning

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
Environment Victoria Friends of Holden Flora Reserve Friends of the Maribyrnong Valley Inc. Independent community members Jacksons Creek EcoNetwork Native Fish Australia Waterwatch Coordinators Department of Environment, Land, Water and Planning Melbourne Water (Service Delivery) Southern Rural Water Western Water Aboriginal Victoria Commissioner for Environmental Sustainability Victoria Environment Protection Authority Victoria Hume City Council Maribyrnong City Council Moonee Valley City Council Parks Victoria Port Phillip and Westernport CMA Victorian Fisheries Authority Victoria Police Licensed diverters from the Maribyrnong River at Keilor Atlas Ecology Pty Ltd Blackbird Cruises

Partner and stakeholder engagement

Environment Victoria
Friends of Holden Flora Reserve
Friends of the Maribyrnong Valley Inc.
Independent community members
Jacksons Creek EcoNetwork
Native Fish Australia
Waterwatch Coordinators
Department of Environment, Land, Water and Planning
Melbourne Water (Service Delivery)
Southern Rural Water
Western Water
Aboriginal Victoria
Commissioner for Environmental Sustainability Victoria
Environment Protection Authority Victoria
Hume City Council
Maribyrnong City Council
Moonee Valley City Council
Parks Victoria
Port Phillip and Westernport CMA
Victorian Fisheries Authority
Victoria Police
Licensed diverters from the Maribyrnong River at Keilor
Atlas Ecology Pty Ltd
Blackbird Cruises

Page last updated: 01/07/22