Blackwater

Blackwater occurs naturally when floods wash leaves, grass and organic cropping material from riverbanks and floodplains into waterways.

Blackwater causes problems when the bacteria population in the water increases. This can happen when there’s a lot of food around for the bacteria – for example, carbon-rich leaves and sticks from plants. We can’t see this bacteria.

The increase in bacteria population means there are a lot more living creatures in the water consuming oxygen. This sudden population explosion of bacteria – together with warm weather – can cause the bacteria to suck most of the oxygen from the water. The water becomes ‘hypoxic’, meaning low in oxygen. Unfortunately, this can cause fish deaths.

The dark colour associated with blackwater comes from a range of carbon molecules that dissolve into the water, the most common being tannins. Tannins are a naturally occurring substance found in native plants. When floodwaters wash over fallen leaves and branches, tannin dissolves into the water.

The tannins are not harmful to humans or animals. In fact, they are found in a variety of the foods we eat – including chocolate, nuts, tea and red wine.

Blackwater events

Not all blackwater events lead to low oxygen resulting in fish deaths. The only way we can tell if the water is low is oxygen is by measuring the dissolved oxygen. A dissolved oxygen reading:

• over 6.5mg/L is a healthy waterway with enough oxygen for the fish
• below 2mg/L is a hypoxic waterway (not enough oxygen) and could result in fish deaths.

This means there are 2 types of blackwater events:

• a blackwater event with enough oxygen
• a hypoxic blackwater event – one where there is not enough oxygen.

Neither event is long-term. Eventually, healthy oxygen levels return and tannins levels drop.

For more information see:

• North Central Catchment Management Authority – What is toxic blackwater?
• Murray–Darling Basin Authority – Blackwater.

Blackwater monitoring

During a blackwater event, we closely monitor and continuously fine-tune the treatment processes at our water treatment plants. This improves the clarity and taste of the water.

Our urban water supply is safe to drink during a blackwater event; however, the water may be discoloured. Blackwater may cause filtration problems for irrigators. Rural water is untreated and not suitable for drinking at any time.

Get blackwater updates

To find out if there is a risk of a blackwater event in your area, check the Murray–Darling Basin Authority’s water quality threats map and follow us on Facebook.