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Droughts can cause organic matter to build up on the floodplain and banks of rivers. When substantial rain finally occurs and water flows over river banks onto the floodplain, it collects leaves and other organic matter on the water’s return to the river.

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, 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. 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.

Managing blackwater

Adding water from dams and lakes 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 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.

Long-term 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 beneficial for some floodplain forests, wetlands and rivers. While some fish deaths occurred as a result of blackwater, long-term 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.

Flood pulses are the trigger for many species to breed (boom and bust cycles). Floods can inundate 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: