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Blackwater risk rises with river levels

The likelihood of water quality issues has increased after the Murray–Darling Basin experienced its wettest November on record.
Published: 17 December 2021

As temperatures start to climb in the weeks ahead, the possibility of blackwater will increase, if conditions stay wet under a La Niña weather pattern.

Warmer temperatures will also favour the growth of blue-green algae, and following any floods, government agencies will also be carefully monitoring salinity levels.

There is a greater risk of blackwater events as leaf litter and other carbon debris are swept into waterways. Dark or tannin-coloured water is natural and indicates nutrients present in the water, an important part of feeding river productivity. It can be a good food source for water bugs, fish, frogs, birds, platypus and turtles.

It becomes a problem when carbon levels are so high that the process of decomposition sucks up the oxygen that fish and other creatures rely on, creating hypoxic (low oxygen) blackwater and making it difficult for fish and other aquatic animals to survive.

In the past 2 decades, large-scale hypoxic blackwater events have been associated with late spring or summer floods following extended drought in the Murray and Murrumbidgee rivers, and this season the risk will affect the northern Basin as well.

River managers, scientists and environmental water holders are monitoring rivers for water quality risks and will work to reduce the any impacts where possible.

To report fish deaths and blackwater events

  • New South Wales – Fishers Watch hotline: 1800 043 536
  • Victoria – Environmental Protection Authority pollution hotline: 1300 372 842
  • Queensland – Department of Environment and Science: 1300 130 372
  • South Australia – Fishwatch Hotline: 1800 065 522
  • Australian Capital Territory – Access Canberra: 13 22 81

Blackwater at the junction of Broken Creek and River Murray
Blackwater can be seen emerging from the left of this image taken at the junction of Broken Creek and the Murray River. Photo credit: WaterNSW.

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