What is a blackwater event?
Blackwater events can be a natural feature of lowland river systems. Blackwater can occur when flooding washes organic material into waterways, where it is consumed by bacteria, leading to a rise in dissolved carbon in the water.
Carbon is a source of food for bacteria, fish and macroinvertebrates. However, the when the organic material releases dissolved carbon it causes bacteria and microbes to multiply and deplete the dissolved oxygen. This process is more pronounced during summer flooding.
Hypoxia is the term used when waterways have very low or depleted oxygen. Low levels of dissolved oxygen, combined with the toxic components of some organic matter, can lead to the localised death of aquatic organisms. Native fish and crustaceans are especially vulnerable to oxygen deprivation, although fish are sometimes able to escape the most badly affected areas by swimming upstream or downstream.
Water appears black due to the release of dissolved carbon compounds, including tannins, as the organic matter decays, similar to the process of adding water to tea leaves.
Blackwater may be seen following prolonged inundation of dry channels containing leaf litter, or by remobilisation of existing blackwater. Normally, the movement of organic material from the floodplain to a river channel produces good results by nourishing lowland river ecosystems with carbon.
Less frequent flooding can lead to the accumulation of larger quantities of organic material on river banks and floodplains. Severity of a blackwater event is determined by amount, age and type of leaf litter and whether it has been previously submerged in water.
Responding to blackwater events
Blackwater events that lead to hypoxic conditions are managed by many agencies. This includes government, Environment Protection Authority (EPA) Victoria, Catchment Management Authorities (CMAs), water corporations, and local councils. CMAs take the lead in preparedness, monitoring and planning for events and working with the community and recovery. EPA Victoria and or Department of Health lead response during an active event.
The monitoring to prepare and respond to these emergencies is provided in part, through our Water Measurement Information System (WMIS).
Why monitor an event in real time?
- Events can be fast and have a major impact on a range of uses
- Aquatic organisms
- Amenity and Recreational opportunities including fishing and swimming
- Irrigation, stock and domestic
- Town water supply may be affected, and water treatment plant operations benefit from early warning.
What can be done in real time?
- Water treatment plants operators can
- stop taking affected water & use an alternative supply.
- adjust the treatment processes.
- Notifications can be issued to:
- drinking water customers where quality cannot be guaranteed e.g. issue boil water notices
- rural water users to make informed decisions re suitability of water for irrigation, domestic and stock use.
- Fish/aquatic animal species may be removed/relocated until conditions change
- Water corporations may be able to release water from storages to address the poor water quality.
Reviewed 10 September 2024