Issues for river health

Blackwater

As a result of flooding across several catchments in New South Wales and Victoria washing large amounts of organic matter from the floodplain into tributaries and the River Murray, there is an increased risk of water quality issues like low-oxygen blackwater.

Blackwater is starting to affect the river upstream. South Australia is already experiencing low dissolved oxygen levels and a blackwater event is expected in South Australia.

Governments and water authorities across the basin are working together to monitor the unfolding conditions. Given the scope of natural flooding, prevention is not possible and mitigation is limited.

There may be increased water treatment to help mitigate the impacts of blackwater.

DEW is working with the Murray-Darling Basin Authority, SA Water, upstream states and fish scientists to plan operations at Lake Victoria for the expected blackwater event to provide a haven of better-quality water for native fish in and around Lake Victoria, utilising lessons from the 2016 blackwater event.

What is blackwater?

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

High levels of organic matter washed into waterways are then consumed and broken down by bacteria. Particularly when combined with warm weather, this can cause oxygen levels in the water to drop. This is known as hypoxic (low oxygen) blackwater, which can have a dark or blackish colour and a strong, unpleasant smell.

It is important to note that black coloured water does not necessarily mean hypoxic blackwater.

Dissolved oxygen levels in the Murray are typically around 7 mg/L to 11 mg/L. Fish and other aquatic animals may become stressed when oxygen levels drop below 4 mg/L and have difficulty surviving when oxygen levels drop below 2 mg/L. Large-bodied native fish, such as Murray cod, are particularly vulnerable.

Although hypoxic blackwater events may result in the loss of fish and other aquatic life, the impacts of these events on the environment are usually short-term and oxygen levels increase again as the flooding subsides.

Naturally occurring events such as these underpin the broad health of rivers. They provide nutrients that drive the overall production of our river and wetland systems. In the longer-term, native fish, water birds and other organisms benefit from the increased production in the river, boosting food supplies and supporting breeding cycles.

Is blackwater safe?

Blackwater in the River Murray poses no direct public health risk. In a small number of people, blackwater may cause a skin irritation due to sensitivity to natural organic matter in the water.

Swimming in rapidly flowing and highly turbid water should be avoided. Find out more about River safety.

Water supplies and quality

It is recommended not to drink water from the River Murray at any time unless it is treated.

SA Water’s water treatment plants are designed to manage a range of source water quality challenges, including blackwater in the River Murray. Regular monitoring and sampling of South Australian sections of the river, as well as strategic locations upstream, is part of SA Water’s proactive approach to maintaining safe, clean drinking water for its customers.

Blackwater is acceptable for livestock and animals to drink, but they prefer cleaner water.

Impacts on fish and crustaceans

Large-bodied native fish, such as Murray cod are particularly vulnerable to hypoxic blackwater and can become distressed when dissolved oxygen levels fall.

Report large numbers of dead fish or other aquatic animals to the 24-hour FISHWATCH hotline on 1800 065 522. Reports of fish kills can also be made through the SA Fishing app. Further information about fish kills and how to report is also available on the PIRSA website.