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Blackwater events happen when too much carbon is in the water. This occurs when there are large amounts of decaying organic matter in the river. These events affect water quality in the Murray–Darling Basin, harm fish and other aquatic life, and make it harder to treat water so it is fit for humans to drink.

To find out whether there are current blackwater events in your area, check the Murray–Darling Basin Authority's (MDBA) water quality risk map.

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Why blackwater is a problem in the Murray–Darling Basin

Blackwater events have caused several severe fish deaths in the Murray–Darling Basin. These events often occur when a long drought is followed by flooding. This happened in late 2010. After a long drought in the previous decade, flooding rains caused a long lasting blackwater event that flowed through the lower parts of almost all rivers in the Basin and caused many fish deaths.

What causes blackwater events

Between floods and during droughts, dead leaves and other plant matter build up on the ground instead of being washed into the river. Bacteria begin to break the plant matter down. When significant rain finally comes, the build-up of dead and decaying plant matter is carried by rain and floodwater into the river.

Once in the water, bacteria continue to break down the leaves and other plant matter. This process uses up a lot of the oxygen in the water, so there is less oxygen for fish and other aquatic organisms to breathe. The decaying matter releases carbon that makes the water look black, giving these events the name ‘blackwater’.

Various factors affect the severity of blackwater events, including the type and amount of plant material, air and water temperatures, and the length of time since it last rained.

Bushfires can also influence water quality and cause conditions similar to blackwater events. After a bushfire, rain can carry ash and burnt material into rivers. Plants and trees have been burned, which means the soil is more easily eroded and can also be washed into rivers. This can cause unpredictable changes to the water chemistry, including devastating blackwater events.

How blackwater affects rivers

Courtesy of Goulburn Broken Central Catchment Management Authority​ and North Central Catchment Management Authority​.

Severe blackwater events decrease water quality.

When too much oxygen is removed from the water, fish and other organisms struggle to breathe and may suffocate and die. Large fish like the Murray cod need more oxygen, so they tend to die first.

Blackwater events also affect communities that use the river. While drinking water is always treated to remove bacteria and sediments, it may need additional treatment after a blackwater event before it is fit to drink.

Blackwater events are a natural part of the Basin ecosystem, as are the floods that cause them. Both have long-term benefits for the health of the river. When the organic matter washed into the river is broken down, carbon and nutrients are released into the water. This boost supports fish, birds and other wildlife.

What the MDBA is doing about blackwater

The Basin Plan sets the amount of water that can be taken from the Basin each year, while leaving enough for rivers, lakes and wetlands and the plants and animals that depend on them.

Water that is ‘allocated’ to the environment is managed by environmental water holders, including the Commonwealth Environmental Water Holder (CEWH). The officers of the CEWH work with state and local water managers to develop strategies to reduce the risk of blackwater events. For example, smaller and regular watering of floodplains will reduce the amount of decaying leaves that could flow into the river during bigger floods. This reduces the severity of blackwater events during floods. Water that is set aside for the environment can help with regular inundation of floodplains.

Under the plan, Basin state governments must also take blackwater risks into account in their water resource plans. These plans include water quality management plans that identify:

  • causes of decreased water quality
  • risks to water quality
  • water quality and salinity targets
  • cost-effective ways to achieve water quality objectives.

When a blackwater event occurs

It can be difficult to prevent the immediate effects of blackwater events. When a flood leads to a severe blackwater event, better quality water may be able to be released from dams or lakes, flushing the area with oxygen-rich water. This helps aquatic life to survive. However, planning the release of the right amount of water at exactly the right time can be difficult, and sometimes there is no stored water available. The Commonwealth Environmental Water Holder works with Basin state governments to develop strategies to support this.

Advice for recreational river users

If you are planning activities on or around rivers that have recently been flooded, remember to:

  • check the water quality in your area on the alert map
  • be aware there may be hazards underwater that aren’t visible
  • be especially careful of fast-flowing water
  • treat water before drinking it by boiling it (boil for at least 3 minutes) or using a carbon water filter.

Report fish deaths

  • 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

Related links

Updated: 11 Nov 2021