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Blackwater: Questions and Answers

The unprecedented December inflows are resulting in blackwater with low dissolved oxygen in many locations across the Murray region, and there is a risk of this moving downstream to South Australia in coming weeks.

Blackwater: Questions and Answers

Crayfish at Nyah.

While blackwater is a natural occurrence in low-lying river systems, the severity and extent of this event is due to the prolonged drought and subsequent widespread flooding.

Please find below some of the most common questions and answers relating to the blackwater event.

What is Blackwater?
Blackwater is a natural part of the ecology of lowland river systems.   It can provide a valuable source of carbon to rivers, but it can also create low levels of dissolved oxygen in water which can be harmful to fish and other aquatic species. 

Blackwater usually develops in late spring or summer on flooded wetlands or floodplains which have large accumulations of organic material.  It can also occur in rivers if large amounts of leafy or woody material are washed in from storms.  

The two most important factors influencing the development of blackwater are temperature of the water and the amount of carbon (ie leafy litter and woody debris) present.    The risk of blackwater developing increases with increasing amounts of organic material and increasing  temperatures, which is why blackwater is most often seen after November or December.   

Where is blackwater currently occurring?
Blackwater is currently affecting some sections of the River Murray, Edward and Wakool river systems and most recently in the lower Goulburn River and Broken Creek, Lower Darling Anabranch, Murrumbidgee and Loddon rivers.

Is blackwater harmful to humans?
The inundation of flood plains during peak flood events may lead to the formation of blackwater via the leaching of tannins from the decomposition of organic vegetation. These blackwater events have been shown to lead to stress and mortality in fish due to reduced oxygen levels in the water. 

As with any flood waters event it is best to avoid contact with blackwater due to safety hazards submerged in the water. However, if you do come in contact with blackwater ensure you wash with soap and water.  Do not drink blackwater.  Take care to protect any cuts or wounds to your skin from coming into contact with blackwater. 

Are fish in systems affected by blackwater safe to eat?
In assessing whether a fish that you have caught is safe to eat it is important to use commonsense approach. Do not eat fish that are dead when you find them or that do not look healthy when caught.

Can stock drink the blackwater?
Blackwater may contain micro organisms and chemicals that may be harmful to humans.  It is best to prevent stock from drinking blackwater

Is it still safe to swim, water ski in affected systems?  
Blackwater contains a lot of decaying debris.  The floodwaters that help create blackwater may contain micro organisms and chemicals that may be harmful to humans.  It is best to avoid recreational activities where contact with blackwater is likely to occur.  If you do come in contact with blackwater ensure you wash with soap and water.  Do not drink blackwater.  Take care to protect any cuts or wounds to your skin from coming into contact with blackwater. 

Who should I report fish deaths to?
Fish deaths can be reported to the EPA's Pollution Watch Line on 1800 444 004.

Where to go for more information:

Click here for the media release relating to the blackwater event, which was issued by Parks Victoria on December 21, 2010.

Please regularly check the Murray Darling Basin Authority's website for updates on the blackwater event in the Murray River.

 

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