The large numbers of bloated and smelly fish running up along the Ward River appears to be a Blackwater event.
SWNRM advises that they are sending away some of the dead carcasses and water samples to confirm the event.
Queensland Health says while risks to human health are low-direct contact with Blackwater should be avoided. They advise thorough cleansing after any contact with affected water and discolored or dead fish should not be eaten because of possible health risks.
the Charleville Fishing and Restocking Club members have been flat out clearing away dead fish from the River and surrounds and we should expect to see ore float to the surface over the next few days.
What is a Blackwater Event?
A Blackwater event occur when returning floodwater contains large amounts of organic matter, such as leaves and wood which in turn becomes dissolved organic carbon, as the waters push that matter into river systems from the bush floor.
As it enters the river it creates low dissolved oxygen levels and that rate of oxygen consumption during decomposition is faster than it can be replenished from the atmosphere.
This causes oxygen depletion in the water, with, as we are seeing, catastrophic short-term consequences for fish.
The black appearance of the water is due to the release of the dissolved carbon compounds, including tannins, as the organic matter decays.
A Blackwater Event however is good news in the long-term. The Murray Darling River Basin Authority says it is a naturally occurring event.
The MDBA’s head of River Management, David Dreverman, says the high level of carbon and low levels of oxygen in the water due to the load of decaying organic matter is not unexpected and can contribute to major improvements in the long-term health of the river system.
Blackwater In the Murray–Darling Basin. For example, the 2010–11 floods were very beneficial for floodplain forests, wetlands and rivers.
While fish deaths occurred because 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.
Mr Dreverman says in most cases it is not possible to dilute the Blackwater.