FACT Sheet: Blackwater

What is blackwater?

Blackwater events are a natural part of the ecology of lowland river systems during flooding. When accumulations of organic matter such as eucalypt leaves and twigs, decay in wetlands or waterways, the decay process darkens the water turning it black. As the organic matter decays oxygen in the water is consumed, sometimes at a rate faster than oxygen can be replenished. This can result in low levels of dissolved oxygen during some blackwater events. The low levels of dissolved oxygen may cause stress to fish, crayfish and other aquatic animals. When the dissolved oxygen reaches very low levels it can result in fish deaths.

What causes blackwater events?

Blackwater events are a natural phenomenon due to the convergence of a number of conditions. The amount of organic matter will depend on a number of factors such as the type and age of the leaf litter, the amount of litter that has accumulated on the floodplain or dry river channel, and whether or not the litter has been previously flooded. The long period of drought has lead to the accumulation of large amounts of organic material on the floodplains. The increasing air and water temperatures and the late season flooding have the potential to lower dissolved oxygen levels in affected rivers.

Is the current blackwater event caused by environmental watering?

No. The current blackwater event is the result of natural floods. Prior to the most recent floods, some environmental water releases were made early in the season when the weather was cooler to maintain low levels of water on the floodplain between successive flood peaks. However, these releases were much lower and much shorter than the floods, and did not inundate any new areas of floodplain. In the Edward–Wakool system, river operations have been releasing water with high dissolved oxygen from the irrigation escape network.  This is having a positive impact at some locations.

Benefits of blackwater

Blackwater contributes to major improvements in the long term health of the system — the recent floods have been very beneficial for the floodplain forests, wetlands and rivers. While there may be some fish deaths as a result of blackwater events, longer term benefits to native fish due to the large amount of carbon entering the system can be seen. Carbon enters the food web, increasing the zooplankton and macroinvertebrate communities, which in turn act as food sources for fish.  

This flooding event has attracted many thousands of waterbirds such as ibis, spoonbills, egrets, cormorants and nankeen night herons, these species and others have been observed nesting. With the current flooding event it is expected these birds will continue to breed over the next few months.

Advice to recreational river users

Recreational river users are advised to take the water quality into account when planning their activities around rivers that are or have recently flooded. 

• With any flood event it is best to be careful in and around with the water due to possible hazards submerged in the water.

• Water from recently flooded rivers should be treated before consumption; techniques such as boiling water at a rolling boil for 3 minutes or using a carbon water filter are advised. 

Fish deaths should be reported to the relevant state authorities:
Victoria – Environmental Protection Agency Pollution Watch Line 1800 444 004
New South Wales – Department of Industry and Investment, Fisheries contact number 1300 550 474

Latest Water Quality Bulletin

• Latest water quality bulletin
• Latest water quality bulletin

Dissolved oxygen status Map

Dissolved oxygen status Map

Archived water quality bulletin

Archived Dissolved oxygen status Maps