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Lachlan River - low dissolved oxygen levels

Information for community and land managers

Up to 15 000 ML of the water quality allowance has been allocated for use in the Lachlan River system to mitigate low levels of dissolved oxygen in the aftermath of recent flooding.

River users may notice a darker water colour in parts of the Lachlan River and its tributaries over the coming weeks as natural flooding has caused a reduction in dissolved oxygen (DO) levels in water that is flowing from inundated floodplain areas into the river.

These current overbank flood flows are unregulated and are a natural occurrence.

Low dissolved oxygen after natural flooding is caused by the decomposition of inundated leaves, grass and other organic material – the process that breaks down the material consumes oxygen in the water and also causes it to become darkly discoloured (sometimes called ‘hypoxic blackwater’) and can often have a strong unpleasant odour.

Low dissolved oxygen levels in creeks, rivers and wetlands can cause stress (and even death) to fish and other aquatic animals which rely on oxygen in the water to breathe.

The Water Sharing Plan for the Lachlan Regulated River Water Source 2016 provides for a water quality allowance (WQA) of 20 000 ML to be maintained for the management of water quality.

On 3 November, up to 15 000 ML of the water quality allowance has been allocated for use in the Lachlan River system to maintain a steady base flow as flooding subsides, and deliver better quality water to affected areas downstream of Forbes. The flow level will be wholly in-channel and will allow water on the floodplain to continue returning to the river.

This use of the WQA may not be able to prevent some fish deaths but it is expected to reduce the severity and duration of any such events by creating local refuges with increased oxygen levels, by enabling fish to seek and move to areas of better water quality, and will also assist in the recovery of stressed native fish.

The decision had been made in consultation with the Lachlan Environmental Water Advisory Group (EWAG) and a Technical Advisory Group (TAG) established for this purpose. These groups include representatives from Central Tablelands Local Land Services who Chair the Lachlan EWAG, DPI Water, DPI Fisheries, WaterNSW, Lachlan Valley Water, Commonwealth Environmental Water Office and NSW Office of Environment and Heritage.

Daily flow rates will be managed taking into account the contribution of tributary inflows upstream of Forbes in conjunction with releases of WQA from Wyangala Dam. The Lachlan EWAG and TAG will continue to monitor and reassess the situation, which will inform the duration of the water release.

Members of the public are requested to notify NSW DPI Fisheries if they observe any stressed or dead fish in the region.

  • Fishers Watch Hotline -1800 043 536
  • Senior Fisheries Manager, Western - 0408 663 338

Information on reporting fish kills.

Media enquiries: Kylie Krause | 0439608370