Hypoxic 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 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. Prolonged periods at or below four milligrams per litre typically result in physiological stress to fish and crustaceans. Fish may be seen ‘gasping’ for air near the surface or floating on their sides, and crayfish may leave the water. Fish and other aquatic animals have difficulty surviving when oxygen levels drop below two milligrams per litre.

Fish kills can be quite localised around areas where oxygen levels have been depleted. For more information on fish kills, see the NSW Department of Primary Industries Fish kills in NSW.

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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

Historic hypoxic 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.

NSW Murray-Darling Basin dissolved oxygen water quality updates

NSW Murray-Darling Basin dissolved oxygen - water quality updates 2023

NSW Murray-Darling Basin dissolved oxygen - water quality updates 2022

What are the causes of hypoxic 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.

What is the likelihood of a hypoxic blackwater events occurring in the summer of 2021-2022?

The Bureau of Meteorology is predicting above average rainfall across eastern Australia until late summer or early autumn 2022.  The increased chance of flooding combined with warm water temperatures, a high load of leaf litter in floodplain forests and full storages, all increase the risk of hypoxic blackwater events. Most of these conditions will already be in place across the southern valleys during the spring and summer of 2021/22.

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.

Further information

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

The Murray–Darling Basin Authority has prepared a water management 101 fact sheet on blackwater and a map of threats to water quality in the Murray–Darling Basin. A short video on blackwater is also available.

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