Page 65 - VEWH Seasonal Watering Plan 2020-21
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Table 2.3.1 Potential environmental watering actions and objectives for the Thomson River (continued)
2.3 Thomson system
  Potential environmental watering action
  Functional watering objectives
   Environmental objectives
   Summer/autumn low flow (340 ML/day at reach 6 during December to May)
  • Partially flush the upper water column in the Thomson estuary, helping to sustain waterbug communities and fish by maintaining oxygen levels
• Prevent high salinity levels, helping to maintain emergent macrophyte vegetation
• Provide freshwater to the Latrobe River system
             Heyfield wetlands
         Fill (in August)
  • Wet ponds to capacity, to stabilise the banks and support the spring growth of semi-aquatic vegetation
• Provide freshwater habitat for waterbirds and frogs (such as growling grass frogs and golden bell frogs)
                Top-ups to maintain water level, as required (during October to November)
 • Top up ponds before summer to maintain the existing vegetation and enhance its recruitment by triggering seed dispersal
• Provide freshwater habitat for waterbirds and frogs (such as growling grass frogs and golden bell frogs)
 Partial drying (during December to February)
  • Oxygenate surface soils, break down accumulated organic matter and cycle nutrients
• Enhance waterbird food availability by exposing the mudflats and provide access to burrowing invertebrates
   Scenario planning
Table 2.3.2 outlines the potential environmental watering and expected water use under a range of planning scenarios.
The highest-priority potential watering actions in the Thomson River under all scenarios in 2020–21 are freshes in autumn, for which the primary objective is to trigger the migration and spawning of Australian grayling. It is critical to provide autumn freshes under all scenarios in 2020–21 to achieve the minimum two required spawning cues for Australian grayling over a three-year period: autumn freshes were achieved in 2019–20 but not delivered in 2018–19 due to construction of the Horseshoe Bend fishway. Spring freshes, smaller-magnitude summer/autumn freshes and winter/spring low flows are also planned to be delivered under all scenarios. The only difference between scenarios is the intention to deliver an extra fresh in early spring under average and wet conditions, to improve outcomes for streamside vegetation.
If possible, spring and autumn freshes in the Thomson system will be timed to coincide with releases in the Macalister and Latrobe systems, to optimise fish movement and optimise outcomes in the Latrobe estuary and lower Latrobe wetlands. Outcomes for the Latrobe estuary rely on flow contributions from all three systems.
The second of a two-year watering trial at the Heyfield wetlands is planned to continue in 2020–21 under all scenarios. The trial involves filling the wetlands in winter and providing top-ups as needed throughout spring, to maintain water levels for aquatic vegetation and to provide habitat for waterbirds and frogs. It is then intended the wetlands will be allowed to draw down over summer and autumn, to oxygenate the soil and allow nutrient cycling.
There are no critical watering actions that cannot be delivered with expected environmental water holdings (that is, there are no tier 1b priorities). Additional environmental watering priorities (tier 2) have been identified for the Thomson River under drought and dry scenarios. Additional water is required to deliver summer/autumn low flows in the Thomson River estuary (reach 6) to freshen water quality and support estuarine fish.
The tier 2 potential environmental watering actions identified are critical to achieving environmental objectives in the
long term. They could be delivered in the current year if circumstances allow, but they can likely be deferred without significant environmental harm. The summer/autumn low flow targeting reach 6 is currently a tier 2 action, as further research is required to establish the specific salinity level tolerances and management triggers for determining
when critical flushing of the Thomson River estuary is required, including whether the salt wedge pushing too high upstream could affect breeding and egg drift. In 2020–21 this watering action is not prioritised over freshes and winter/spring low flows, as it requires a significant volume of water and it can be partially met by passing flows from both the Thomson and Macalister rivers. In addition, freshes in both rivers may be timed to coincide, potentially achieving some of the functional objectives.
Under all scenarios, 5,500 ML of water for the environment is prioritised for carryover at the end of 2020–21, to meet critical early season demands in the Thomson River — spring low flows and freshes — under a drought scenario in 2021–22.
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