Blackwater events and water quality

​​​​​​​Blackwater events occur when flooding washes organic material into waterways, where it is consumed by bacteria, leading to a rise in dissolved carbon in the water.

Water appears black due to the release of dissolved carbon compounds, including tannins, as the organic matter decays, similar to the process of adding water to tea leaves.

Rising levels of dissolved carbon causes a sudden depletion of dissolved oxygen in water, which is essential for aquatic organisms that need to breathe underwater.

Use this guidance to help you understand and manage the effects of blackwater events on water quality in Australia.

Conditions that lead to blackwater events

Blackwater can be a natural feature of lowland river systems.

Prolonged dry periods lead to:

• high air and water temperatures, contributing to increased bacterial activity and lower concentration of dissolved oxygen in water
• extensive build-up of organic material such as leaf litter.

Less frequent flooding leads to the accumulation of larger quantities of organic material on river banks and floodplains. When this debris is washed into the waterway during a flood, bacteria consumes the organic matter, releasing carbon and depleting the levels of dissolved oxygen in the water. This process is more pronounced during summer flooding.

Severity of blackwater events is determined by amount, age and type of leaf litter and whether it has been previously submerged in water. Debris from river red gums is a significant contributor as it contains large amounts of dissolved organic carbon.

Although blackwater occurs naturally, in managed river systems the magnitude and timing of floods have been altered by the collection and delivery of water for agriculture and other consumptive purposes. This can potentially result in more frequent blackwater events.

Primary drivers of blackwater events

Effects of blackwater events

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 localised death of aquatic organisms.

Native fish and crustaceans are especially vulnerable to oxygen deprivation, although fish are sometimes able to escape the most badly affected areas by swimming upstream or downstream.

Chemicals released from organic material can also make water bodies more alkaline or acidic, potentially disrupting normal pH balances and resulting in toxic effects on some aquatic organisms.

Human health risks are low if you:​​

• avoid direct contact with blackwater
• do not eat discoloured or dead fish.

If you do come into contact with blackwater, you are advised to thoroughly clean all affected areas.

Negative social and economic effects are possible, due to higher costs of treating water for consumption and short-term loss of amenity and recreation opportunities.

Benefits of blackwater

Despite short-term negative effects on aquatic organisms, the floods which lead to blackwater are an essential and valuable requirement for the long-term health of river, floodplain and wetland ecosystems, particularly after prolonged drought.

These events help break down organic material which supply additional carbon and nutrients to drive overall production in river and wetland systems.

Native fish, waterbirds and other organisms benefit from increased production, because it:

• boosts food supplies
• supports breeding cycles.

During blackwater events, downstream systems also benefit from the organic inputs once the water has re-oxygenated.

Managing blackwater events

Flooding that leads to blackwater events is a natural feature of Australian river systems, limiting the capacity to prevent and manage negative impacts.

Frequency or severity of such events can be reduced by managing water systems to:

• reinstate natural wetting and drying cycles
• ensure adequate flows to re-oxygenate water.

In temperate areas, cyclical flooding in winter and spring removes excessive organic material from floodplains when there are more moderate temperatures, and bacterial activity is low. This watering also helps reduce the amount of leaf litter falling by alleviating water stress on vegetation.

In managed river systems, releasing more water from storages into the system may help flush blackwater and improve water quality, after floods have receded​.

State and territory agencies have guidance and provisions for managing blackwater.

Murray-Darling Basin

The Murray-Darling Basin Authority (MDBA), the statutory agency responsible for managing the Basin’s water resources, manages the River Murray water quality monitoring program. The program maintains a uniform system for measuring, analysing and presenting data to create a picture of current and long-term river health.

Information generated by the program helps to assess the impact of human development in the river system, detect sources of pollution and respond to changes in condition such as algal blooms and blackwater events.

The Murray-Darling Basin Plan sets out a series of water quality targets which aim to:

• protect and enhance water quality in the Basin
• return more natural flow regimes to the system
• acquire water entitlements with the objective of returning more water to the environment.

These entitlements become part of the Commonwealth’s environmental water holdings and are managed to restore the health of rivers, floodplains and wetlands.

Environmental watering helps to achieve more natural wetting and drying cycles, flushing out toxicants, and improving overall water quality. Restoring natural flow regimes can shorten the time between floods, reducing the build-up of leaf litter and the severity of blackwater events.

Glossary

Carbon: element in all living beings and consumed by plant matter as carbon dioxide to produce energy, new organic matter and release oxygen.

Dissolved oxygen: form of oxygen in water which aquatic organisms are able to breathe through specialised respiratory systems.

Ecosystem: specific composition of animals, plants and micro-organisms which interact with one another and their environment.

Environmental water: water released from a reservoir to maintain downstream water levels, or water retained in a system to satisfy ecological requirements.

Floodplain: portion of a valley next to a river channel which is periodically covered with water during flooding.

pH: measurement of acidity and alkalinity, denoting the concentration of hydrogen ions in a solution. A pH of 7 is neutral.