Risk of Blackwater this Summer

By the Victorian Environmental Water Holder

Blackwater can be a natural feature of lowland river systems and can occur during flooding when organic material – sticks, leaves, bark and grass – is washed off the floodplain and into the river systems.  This can lead to a sudden decrease in the oxygen available to fish and other animals in our rivers, which can sadly lead to their death.

River regulation (supplying water to towns, industry and farms), and a warming climate characterised by longer dry periods, has reduced the long-term frequency of floodplain flooding. After three hot and dry years, floodplains across our river systems have a high load of leaf litter – much of which hasn’t moved since 2016. This build-up of organic material on the floodplain has increased the risk of blackwater events when floods do occur. The current weather pattern as well as the high chance of La Niña conditions this spring and summer means that the risk of floods is high, and consequently, so it the risk of blackwater.

However, not all blackwater is bad and it can be difficult to know the difference.  Good blackwater often looks like ‘gum-leaf tea’ with a pale brown appearance and naturally occurs due to the breakdown of leaf litter and other vegetation. This type of blackwater has many benefits including a boost to the food web in the forest, creek and river. The number of bugs in the water increases, which in turn act as a food source for fish, waterbirds and other aquatic animals.

Over the past 3 years, targeted environmental watering has reduced the build-up of organic matter in some areas, lessening the potential impacts of blackwater events. This is similar to how planned burns can lessen the impact of bushfires.

Just as land managers carry out frequent cool burns to avoid one big hot, damaging bushfire, environmental water managers try to reduce excessive leaf litter build up on floodplains with regular flooding.  Aside from boosting vegetation and providing connectivity across a floodplain, this flooding also aims to reduce the negative impacts of blackwater events.

There is not enough environmental water available to dilute blackwater events in large systems, however, environmental water can help in smaller, isolated waterways.

In many cases mitigation measures to resolve blackwater events are limited but water agencies will work together to determine if and when any management options are available.

This short video explains the causes and effects of Blackwater.

For more information, visit the Victorian Environmental Water Holder’s website.

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