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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

What is 'Blackwater'?

‘Blackwater’ is a term used when high levels of organic material and tannins in a river discolour the water making it appear black. The water can then become hypoxic (low oxygen) when the organic material decomposes reducing the oxygen in the water. Blackwater events occur during flooding when organic material is washed off the river bank and floodplain and into the river system.

Low dissolved oxygen levels in inland rivers and creeks can lead to stress and death in native fish and other aquatic animals.

Hypoxic blackwater events are a natural occurrence and 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.

Oxygen in water is critical for aquatic life. When oxygen levels in water get very low, we call this hypoxia. This video explores the causes of hypoxic blackwater water and what we can do to manage it.

What causes blackwater?

Most hypoxic blackwater events happen after prolonged dry periods, when temperatures are warm and there has been an extensive build-up of organic material, such as leaf litter, which is then washed into the river from runoff and high flows.

While blackwater has always occurred, less frequent freshes and flooding allows larger quantities of organic material to accumulate on river banks and floodplains. This increases the incidence of hypoxic events. When this material is washed into waterways in times of flood, increased bacterial activity can result in deoxygenation of the floodwater. This process is more pronounced during summer flooding.

The severity of blackwater events is determined by the amount, age and type of organic matter in the path of the flood and whether it has been previously submerged in water.

What are the effects of blackwater?

Hypoxic blackwater usually has short-term harmful impacts on the environment. Low levels of dissolved oxygen, combined with the toxic components of some organic matter, can lead to the death of aquatic organisms.

Some native fish and crustacea are especially vulnerable to oxygen deprivation. Fish are sometimes able to escape the most badly affected areas by swimming upstream or downstream. The chemicals released from organic material can also make water bodies more alkaline or acidic, potentially resulting in toxic effects on some aquatic organisms.

Despite short-term effects on aquatic organisms, the floods which lead to blackwater are an essential and valuable part of the long-term health of river, floodplain and wetland ecosystems, particularly after prolonged drought. These events help break down organic material which supply additional nutrients to drive the overall production of river and wetland systems. In the long-term, native fish, waterbirds and other organisms benefit from the increased production that boosts food supplies and supports breeding cycles.

What are the risks to humans? 

Risks to human health are low if direct contact with hypoxic blackwater is avoided. Thorough cleansing is advised after any contact with affected water and discoloured or dead fish should not be eaten because of possible health risks.

Hypoxic blackwater may have social and economic impacts related to the higher costs of treating water for consumption and short-term loss of amenity and recreation opportunities. If suitable town water treatment is not available, boil water alerts may need to be issued for some towns.

Watch the video

Oxygen in water is critical for aquatic life. When oxygen levels in water get very low, we call this hypoxic water. This video explores the causes of hypoxic water and what we can do to manage it.

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.

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.