A NSW Government website

Drought, floods and extreme events

Hypoxic blackwater

Find information about blackwater including water quality stages, dissolved oxygen water quality updates and details on historic blackwater events.

Dead fish in the river at Menindee

Water quality stages for Blackwater

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What do the stages mean?


Stage 1

Water quality monitoring shows indicators within normal range

Stage 1 - Normal management

  • All water quality and climatic indicators within normal/tolerable ranges.
  • Dissolved oxygen above 4 mg/L at all times. Low risk to aquatic ecosystems.

Stage 2

Water quality monitoring has detected conditions which indicate a potential threat to the aquatic ecosystem.

Stage 2 - Emerging drought

Any or all of:

  • Daily dissolved oxygen levels dropping below 4 mg/L at night/early morning but increasing to above 4 mg/L during the day. Can impact on fish health, but may not result in deaths.
  • Forecast rainfall, existing inflows and storage levels indicate increased likelihood of unregulated, overbank flows that will inundate dry floodplain, which is likely to have significant build-up of organic material.

Stage 3

Water quality presents an immediate threat to aquatic ecosystems. Urgent management response is required to avoid fish death or similar event of high ecological implications.

Stage 3 - Severe drought

Further deterioration of water quality conditions indicated by any, or all of:

  • Local reports of fish gasping at the water surface or deaths. Reports of crayfish leaving the water.
  • Dissolved oxygen dropping below 2 mg/L at night/early morning. Water temperature remaining below 25 °C. High risk to aquatic ecosystems. Fish deaths may occur.
  • Storage levels at near full capacity. Existing high flows already in the system. Forecast for heavy rain which will result in the inundation of previously dry floodplain.

Stage 4

Water quality is causing significant impact on aquatic ecosystems with potentially catastrophic outcomes – action is required to minimise or mitigate against further mass fish death.

Stage 4 - Critical drought

  • Confirmed reports of widespread fish deaths.
  • Dissolved oxygen level remaining below 2 mg/L and water temperature above 25°C. Very high risk to aquatic ecosystems, and fish deaths occurring.
  • Weather forecasts indicate poor water quality is likely to deteriorate further

Fish kills in Menindee

Fish kills in Menindee

Find information and updates on the ongoing monitoring and management of fish deaths in the Lower Darling-Baaka River at Menindee and downstream.

Dissolved oxygen water quality updates -
Murray Darling Basin

2024 updates

2023 updates

2022 updates 

2021 updates

Dissolved oxygen water quality updates -
Southern Basin

Historic Blackwater events

Hypoxic blackwater events are a natural occurrence and they do not always lead to fish deaths. Fish deaths that are likely linked to hypoxic blackwater have been reported in the Barwon and Darling Rivers, as well as the southern Basin since the late 1800s.

The newspaper accounts, dating from 1884, are focussed on the Barwon and Darling Rivers.

The links below are reports from the Murray River from as far back as 1892.

Menindee lake, Kinchega National Park - Image credit: Natasha Webb/DCCEEW

Learn the basics about hypoxic water, what it is and why it matters.

Further information

The Department of Climate Change, Energy, Environment and Water (DCCEEW) has information on hypoxic blackwater events and water quality.

You can find maps of water quality threats and a short video on blackwater at the Murray–Darling Basin Authority website.

Real-time water data by WaterNSW includes DO values for some river gauges in NSW.