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Blackwater event expected in Murray River

Areas not flooded in more than 20 years may contribute to a Murray River blackwater event in coming weeks, authorities have warned.

The Murray Darling Basin Authority (MDBA) said heavy rainfalls had mobilised large amounts of organic matter, such as leaves and wood, from the forest floor and floodplain.

That matter is set to decay in coming weeks, resulting in low dissolved oxygen levels, which could cause fish to die.

"The high level of carbon and low levels of oxygen in the water due to the load of decaying organic matter is not unexpected," MDBA Head of River Management David Dreverman said.

"River users may also notice dead or distressed fish as a result of the sudden drop in oxygen levels."

Some higher parts of the floodplain, particularly in the Edward and Wakool systems, are expected to add to high carbon load as they may not have been flooded for more than 20 years, Dreverman said.

“With high carbon loads and floods over this extent of floodplain, it is possible that impacts will be widespread, extending downstream, potentially into South Australia,” he said.

“In most cases, it is not possible to dilute the blackwater, but we are working closely with state agencies and environmental water holders to monitor the situation and will identify opportunities as they arise.

“That includes looking at whether better quality water can be delivered to affected areas to create local refuges with increased oxygen levels.”

Similar blackwater responses could be expected in tributaries such as the Goulburn and Murrumbidgee valleys.

MDBA provides live river data, warnings and alerts here.