During warm conditions, this organic material uses much of the available oxygen as it breaks down, leaving less for aquatic life to survive. The affected water appears darker, often similar to the colour of black tea.
Water quality in the Murray–Darling Basin is monitored by individual Basin state and territory governments and specific water quality alerts are issued by respective state agencies.
Cause of blackwater
Between floods and during droughts, dead leaves and other organic materials build up on the ground instead of being washed into waterways. When significant rain finally comes, the built-up plant matter is carried by rain and floodwater into the waterways. Once in the water, bacteria break down this organic material.
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 between high rainfalls induced flooding.
Bushfires can also influence water quality and cause conditions similar to blackwater events. After a bushfire, rain can carry ash and burnt material into waterways. The cover offered by plants and trees is lost, meaning the soil is more easily eroded and can be washed into rivers. This can contribute to unpredictable changes to the water chemistry, including to blackwater events.
How blackwater affects rivers
Severe (hypoxic) blackwater events decrease water quality.
Courtesy of Goulburn Broken Central Catchment Management Authority and North Central Catchment Management Authority.
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.
High levels of organic matter in waterways, combined with warm weather, can cause oxygen levels in the water to drop. 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.
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. Hypoxic blackwater events have caused several mass fish deaths in the Murray–Darling Basin.
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.
Though the impacts from hypoxic blackwater are intense, the effects are usually relatively short-lived.
Reducing the impact of blackwater
Water that is ‘allocated’ to the environment is managed by environmental water holders, including the Commonwealth Environmental Water Holder (CEWH).
The CEWH works with state and local water managers to develop strategies to reduce the risk of blackwater events. For example, floodplain watering in cooler months allows slower breakdown of 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
If you are planning activities on or around rivers that have recently been flooded, remember to:
- check the water quality in your area from the local authorities and on our water quality threats 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.
Find out who to contact if you need to report water quality issues or fish deaths
Last updated: 29 April 2025