Page 61 - VEWH Seasonal Watering Plan 2020-21
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System overview
The Thomson River flows from the slopes of the Baw Baw Plateau to join the Latrobe River south of Sale. The major tributaries of the Thomson River are the Aberfeldy and Jordan rivers in
the upper reaches and the Macalister River in the lowest reach. Most natural flow originates from the Aberfeldy River. Two major structures regulate flow on the Thomson River: Thomson Reservoir — the largest water supply storage for metropolitan Melbourne — and Cowwarr Weir — a regulating structure which supplies irrigation water to parts of the Macalister Irrigation District.
Thomson Reservoir harvests most of the flow from the Thomson River upper catchment and has a significant effect on flow in all downstream reaches. Natural flow from the Aberfeldy River, which meets the Thomson River below Thomson Reservoir, is essential for providing natural freshes and high flows in the Thomson River.
Water for the environment is held in the Thomson Reservoir and released into the river as required. Reach 3 of the Thomson River (from the Aberfeldy River confluence to Cowwarr Weir) is the highest priority for environmental watering due to its heritage river status, high-value native streamside vegetation, high-quality in-stream habitat and low abundance of exotic fish species.
At Cowwarr Weir, the Thomson River splits into the old Thomson River course (reach 4a) and Rainbow Creek (reach 4b) (see Figure 2.3.1). Passing flows throughout the year are split two-thirds down reach 4a and one-third down 4b to avoid impacts to irrigators located on Rainbow Creek. Water for the environment is primarily delivered to the old Thomson River course (reach 4a) to support fish migration, because Cowwarr Weir impedes fish movement through Rainbow Creek.
The Heyfield wetlands is a cluster of several pools located between the Thomson River and the township of Heyfield. Due to the construction of levees and weirs along the Thomson River, natural wetting of river waters to the wetland rarely occurs; and while the largest pool receives stormwater from the Heyfield township, smaller ponds rely on rainfall or pumped water for the environment to maintain environmental values. These values include significant revegetation that has been done in recent years.
Environmental values
The Thomson River supports six native species of migratory fish that need to move between the sea and freshwater environments to complete their life cycles. A focus for environmental flows management is the Australian grayling, which is listed as a threatened species in Victoria. Australian grayling spawn in response to autumn freshes, and the larvae and juveniles spend time at sea before returning to the freshwater sections of coastal rivers.
The composition and condition of streamside vegetation varies throughout the Thomson River catchment. The vegetation is intact and near-natural condition above Thomson Reservoir in the Baw Baw National Park. Streamside vegetation between Thomson Reservoir and Cowwarr Weir is mostly in good condition but is affected by exotic weeds including blackberry and gorse. Below the Cowwarr Weir, the vegetation is degraded due to stock access and widespread weed invasion.
The Heyfield wetlands are one of the few remaining freshwater wetland sites in the Gippsland Plains landscape area, and they are a source of habitat for aquatic and terrestrial animals that prefer shallow, slow-moving waterbodies, including threatened migratory birds.
Environmental watering objectives in the Thomson system
2.3 Thomson system
    Restore populations of native fish, specifically Australian grayling
Maintain/enhance the structure of native fish communities
Reduce competition from exotic fish
     Maintain the existing frog population and enhance opportunities for breeding
      Maintain channel form diversity including pools, to provide a variety of habitats for aquatic animals
   Increase the abundance of platypus
    Maintain and restore the structural diversity and zonation of streamside vegetation and reduce terrestrial encroachment/invasion (Thomson River)
Increase the recruitment and growth of native in-stream, fringing and streamside vegetation (Thomson River)
Maintain the existing vegetation, promote the growth and establishment of semi-aquatic species (Heyfield wetlands)
Enhance the resilience of semi-aquatic and streamside woodland species (Heyfield wetlands)
     Restore and maintain the natural invertebrate community
   Provide freshwater habitat for migratory and non-migratory wetland birds within the Gippsland Plains landscape
Continue to support observed terrestrial woodland and grassland birds by maintaining their streamside woodland habitat
Victorian Environmental Water Holder | Seasonal Watering Plan 2020–21 | 59

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