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 2.3 Thomson system
Recent conditions
The start of the 2019–20 water year was warmer and
drier than average in the Thomson River catchment, but above-average rainfall over summer increased inflows to the Thomson Reservoir and boosted water availability. Thomson Reservoir did not spill, so moderate releases were made throughout the year to supply minimum passing flow requirements, to meet irrigation demand and for environmental watering. Environmental watering actions were delivered in line with dry conditions during the first half of the year and average conditions from late summer.
Other than bankfull and overbank flows (which cannot be managed with water for the environment), all recommended environmental flows for the Thomson River were achieved through natural flows, managed environmental flows, operational deliveries or a combination of these.
Water for the environment was used to meet high-priority freshes in spring and autumn and to maintain target low flows as needed. The spring and autumn freshes are particularly important to cue native fish to move between habitats, supporting their breeding and recruitment.
Water for the environment was delivered to Heyfield wetlands in August and October 2019 to enhance the growth of recently planted aquatic and streamside vegetation and to increase feeding habitat for waterbirds.
Scope of environmental watering
Table 2.3.1 describes the potential environmental watering actions in 2020–21, their functional watering objectives (that is, the intended physical or biological effect of the watering action) and the longer-term environmental objectives they support. Each environmental objective relies on one or more potential environmental watering actions and their associated physical or biological functions.
Table 2.3.1 Potential environmental watering actions and objectives for the Thomson River
  Potential environmental watering action
  Functional watering objectives
   Environmental objectives
   Autumn fresh (two freshes of 800 ML/day for seven days during April to May)
    • Trigger the downstream migration (and spawning) of Australian grayling (April)
• Trigger the downstream migration of tupong and Australian bass (May)
• Carry plant seeds from the upper catchment for deposition downstream
• Deposit sediments on benches, to provide substrate for vegetation
• Wet the bank/bench to deliver dissolved and/or fine particulate organic matter
• Scour substrates to remove accumulated fine sediment
               Spring fresh (one to two freshes of 800 ML/day for seven days during September to November)
     • Trigger upstream fish migration from marine/estuarine habitats and the recruitment of juvenile native species including Australian grayling and Australian bass (October to November)
• Improve and maintain streamside vegetation by inundating the benches and providing variable water levels for plant zonation
• Carry plant seeds from the upper catchment for deposition downstream
• Deposit fine particulate sediments on the benches
• Scour substrates to remove accumulated fine sediment
        Summer/autumn fresh (two freshes1 of 230–350 ML/ day for seven days during December to March)
 • Increase the water depth to provide habitat for native fish
• Wet aquatic and fringing vegetation to maintain its condition and support its growth
• Maintain the physical form and functioning of the channel through mobilisation of fine sediments
         Autumn/winter/spring low flow (125–350 ML/day during May to November)2
• Increase the available habitat for waterbugs
• Regulate the water temperature and wet large woody debris to provide food and shelter for waterbugs and fish
• Increase the water depth to facilitate platypus and fish movement between localised habitats and increase foraging opportunities
• Wet low-lying benches to prevent encroachment by invasive plants and permit seed dispersal
            1 Additional summer freshes are likely to be met with operational water releases.
2 Passing flows may be flexibly managed at rates less than 230 ML per day in July.
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