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 2.4 Macalister system
Scenario planning
Table 2.4.2 outlines the potential environmental watering and expected water use under a range of planning scenarios.
The assumed supply of environmental water for 2020–21 enables planning for a suite of freshes and low flows in winter, spring and autumn under all climate scenarios. An autumn fresh to cue the downstream migration of Australian grayling is the highest-priority fresh, to achieve the minimum two required spawning cues for this species over a three- year period; autumn high flows were achieved in 2019–20 but not delivered in the Macalister or Thomson rivers during 2018–19 due to construction of the Horseshoe Bend fishway.
A winter fresh to cue the downstream movement of tupong and Australian bass towards the Latrobe estuary for breeding and spawning, and a spring fresh to cue
the upstream migration of adult fish from estuarine environments and promote the recruitment of juveniles are also planned. Low flows may be delivered either side of freshes or when operational flows supplying irrigators are insufficient to maintain water quality and connectivity through the river system, particularly below Maffra Weir and during autumn and winter when there is little delivery of water to supply irrigation customers. Depending on the magnitude of the fresh and whether or not it follows a storage spill, additional objectives for watering floodplain vegetation and flushing pools may be achieved.
In a dry, average or wet scenario, a spill at Lake Glenmaggie is expected in spring. Water for the environment may be used following the peak of the spill to extend the event
and ensure the water level does not drop off too suddenly. Freshes in spring aim to cue the upstream migration of adult migratory fish from estuarine environments and promote the recruitment of juveniles. Under a drought scenario, Lake Glenmaggie is not expected to spill, and a smaller fresh will likely be delivered in late spring instead
(still meeting flow objectives for migratory fish), followed by low flows to maintain water depth and hydraulic habitat for native fish. During summer and autumn, small-magnitude freshes (to improve water quality and maintain connectivity between habitats for aquatic animals) are planned under all scenarios, but the duration and frequency of the freshes is variable. For example, under drought and dry scenarios, it may be necessary to increase the duration of summer autumn freshes from three to 10 days to prevent pools stagnating. If passing flows provided below Maffra Weir reduce or cease under a drought scenario, water for the environment may be used to supplement reduced passing flows to maintain connectivity and oxygen levels below Maffra Weir.
If additional water for the environment is available under any scenario, it will potentially be used to extend the duration and magnitude of winter/spring low flows or to deliver an additional winter/spring fresh to increase flow variability and provide more opportunities for fish and platypus to move throughout the system.
A critical carryover volume of at least 1,500 ML is identified into 2021–22 under all scenarios, to ensure there is enough water to deliver winter low flows in July and August 2021.
          72 | Victorian Environmental Water Holder | Seasonal Watering Plan 2020–21

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