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Blackwater occurs naturally when floods wash leaves, grass and organic  material off riverbanks and floodplains into waterways.

The affected water appears darker, often similar to the colour of black tea. Blackwater can be a good food source for fish and other aquatic life. High levels of organic matter in waterways, combined with warm weather, can cause oxygen levels in the water to drop, harming or killing fish and other creatures in the river. This is known as hypoxic (low oxygen) blackwater. Hypoxic blackwater events affect water quality in the Murray–Darling Basin, harm fish and other aquatic life.

To find out if there is a risk of blackwater events in your area, check the Murray–Darling Basin Authority's water quality risk map.

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Why severe blackwater is a problem in the Murray–Darling Basin

Hypoxic (low oxygen) blackwater events have caused several mass fish deaths in the Murray–Darling Basin. These events often occur when a long drought is followed by flooding.

What causes severe blackwater events

Between floods and during droughts, dead leaves and other organic matter builds up on the ground instead of being washed into the river. When significant rain finally comes, material is carried by rain and floodwaters.

Once in the water, bacteria 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 blackwater events.

How blackwater affects rivers

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

Severe (hypoxic) 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, early floodplain watering when its cooler in winter and spring, which helps to breakdown organic material before it gets warmer. 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 severe 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 delivered to provide localised areas 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 during times of flood, 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 water quality issues or fish deaths

Find out who to contact if you need to:

  • report a suspected water quality issue (including blue-green algae or blackwater)
  • get more information about water quality in your area
  • use water after a water quality incident
  • report dead or dying fish in your area.
Updated: 23 Dec 2022