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Blackwater events occur when returning floodwater contains elevated levels of dissolved organic carbon. Normally the export of carbon from floodplains to a river channel is a beneficial process, providing sustenance to lowland river ecosystems. However if the rate of oxygen consumption during decomposition of the organic carbon is faster than it can be replenished from the atmosphere, this can cause oxygen depletion in the water column, with catastrophic short-term consequences for fish and crustaceans.

The black appearance of the water is due to the release of carbon compounds (including tannins) as the organic matter decays – similar to the process of adding water to tea leaves.

Blackwater with low level of dissolved oxygen may cause stress to fish, crayfish and other aquatic animals. When the dissolved oxygen reaches a very low level it can result in fish deaths. For example, Large bodied native fish (e.g. Murray Cod) require at least 2 milligrams dissolved oxygen (DO) per litre in the water to survive, but may begin to suffer at levels below 4-5 mg DO per litre.

Blackwater can occur as a result of floods and is a natural phenomenon due to the convergence of a number of conditions. The amount of organic matter will depend on factors such as the type, quantity and age of the leaf litter and whether the litter has previously been flooded.

Increasing air and water temperatures also have the potential to lower dissolved oxygen levels in affected rivers. The decade long drought preceding the 2010-11 floods in the southern Murray–Darling Basin allowed the accumulation of large amounts of organic material on the floodplains.

A Murray cod showing signs of oxygen deprivation due to blackwater
A Murray cod shows signs of oxygen deprivation in blackwater.

Managing blackwater

Environmental watering can sometimes assist in the management of a blackwater event. The addition of oxygenated water provides an immediate boost to oxygen concentrations in water with extremely low levels of dissolved oxygen and can also provide localised relief for aquatic life.

The logistics of delivering environmental water to an event are complex and take into account travel times, the amount of oxygen in the water and the amount of water required. Consideration is given to how the organic matter will react with the incoming water. Care must be taken to ensure that flooding is not exacerbated by intervention. It is important to remember that environmental water is only one aspect of managing blackwater event, and only in some cases can it be used with success.

Benefits of blackwater

Blackwater can actually contribute to major improvements in the long term health of the Murray–Darling Basin. For example, the 2010–11 floods were very beneficial for floodplain forests, wetlands and rivers. While fish deaths occurred as a result of blackwater, benefits to native fish resulted from the large amount of carbon entering the system. Carbon enters the food web, increasing the zooplankton and macroinvertebrate communities, which in turn act as food sources for fish.

In Australia, flood pulses are the trigger for many species to breed (boom and bust cycles). Floods generally inundate many wetlands, prompting thousands of water birds to breed, nest and fledge the next generation. This includes ibis, spoonbills, egrets, cormorants and nankeen night herons, with breeding persisting well after the flooding.

Advice for recreational river users

If you are planning activities on or around rivers that have recently been flooded remember to:

  • check water quality
  • be aware there may be submerged hazards
  • be especially careful of fast flowing water
  • treat water before consumption - treatment techniques include boiling (at a rolling boil for at least 3 minutes) or using a carbon water filter

If you are concerned or notice dead fish please contact state authorities: