THE blackwater events this season have resulted in significant stress and fish deaths in parts of the Murray system due to low oxygen levels in the water.
It has also resulted in a fair bit of misinformation.
We’ve seen relatively widespread blackwater this season because the floods in parts of the Murray and its tributaries reached so far across the floodplain they mobilised up to 25 years’ worth of organic matter.
An immense quantity of leaves, wood, bark and other high-carbon material builds up over many years. In the not too distant past, these higher floodplains had more regular flushing than the current river regulation and land practices allow.
These days flushing away the build-up of matter is limited mainly to the lower floodplains through a complex process of environmental watering (it’s never a case of “simply adding water”).
One thing that’s clear is that environmental watering and works programs on the Murray did not give rise to the blackwater we’re now seeing.
In fact, environmental watering goes a long way towards healthier ecosystems by flushing the lower floodplains.
The Murray Darling Basin Authority, local land and water managers, environmental water holders and scientists continue to devote a lot of effort to exploring the causes and impacts of blackwater.
The investigation into the 2010-11 and other events found actions such as regularly flushing carbon from the floodplains helps but that it is very difficult to prevent and manage large-scale blackwater, particularly in river systems regulated to support development.
Right now, these same people are working to mitigate the impact of this latest event. Environmental water is being used to create pockets of refuge and dilution.
For example, a release in the Murrumbidgee recently has produced some great results by raising oxygen levels in parts of the river.
If we want to limit blackwater events in the future, more frequent environmental watering of the lower floodplain is part of the answer.
MDBA environmental management executive director