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.

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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 2020?

The severity and longevity of the current drought will most likely result in ‘severe’ blackwater events in many valleys following major flows. Experts agree it is not a matter of if – but when– particularly in the southern valleys in spring and summer where there has been a very high build-up of leaf litter in floodplain forests, Bureau of Meteorology predictions are for wetter conditions, and temperatures will increase during the last quarter of 2020.

Further information

The Department of Environment and Energy 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.