Home » Flood mitigation » Floodplain management » Understanding blackwater: Technical note
This technical note summarises current scientific knowledge of blackwater and results from the recent collaborative Australian Research Council project ‘Episodic estuarine hypoxia: resolving the geochemistry of coastal floodplain blackwaters’, which focussed on the Richmond River floodplain and was led by Professor Leigh Sullivan.
The intention of this technical note is to provide a common, plain-English understanding of blackwater to guide management at a local government, state agency, industry and landowner level. It was compiled by Chrisy Clay (Rous County Council) and Suzanne Acret (Ballina Shire Council) with the input and endorsement of Professor Leigh Sullivan (University of Canberra), Professor Scott Johnston (Southern Cross University) and Dr Vanessa Wong (Monash University).
Rous County Council and Ballina Shire Council have an active interest in water quality issues associated with flood mitigation infrastructure. In developing this technical note, both councils seek to raise awareness and interest of blackwater and provide an up to date understanding of the issue and the complexity in addressing it.
Clay C., Acret S., Sullivan L. (2020) Blackwater: what is it, what causes it, what can be done about it? Technical Note. Rous County Council and Ballina Shire Council. Click here to download a copy (2.4mb).
|Blackwater mixing with brown floodwater in Bungawalbyn Creek.|
What is blackwater?
How is blackwater formed?
On the Richmond floodplain, blackwater forms from the rapid biological breakdown of plant material in water. This is part of the natural carbon cycling process.
However, drainage of the floodplain has affected the formation of blackwater in several ways. Floodplain drainage includes the construction of artificial channels and levees, straightening of natural waterways and installation of floodgates.
|Graph: General water quality processes after flooding. (Johnson, S. et al 2003.)|
Is it acidic? Or influenced by acid sulfate soils?
Blackwater may form in acid sulfate soil environments (i.e. low-lying areas on the floodplain), but the chemistry of acidic discharge and blackwater is usually different. Both issues may impact a particular waterbody (such as a creek or a drain), but they form from different processes.
What are the impacts of blackwater?
|Dead fish at Ballina after flooding in 2008.|
How can the formation of blackwater be minimised?
|Deep inundation of the floodplain during 2008 major flood.|
Where are the priority areas for management in the Richmond?
Can we stop blackwater events from ever happening again?
The geomorphic features of the Richmond River catchment make it particularly vulnerable to deoxygenation or blackwater events. In terms of catchment to floodplain ratio, it has the largest coastal floodplain in NSW relative to its catchment size. The run off from such a large floodplain, particularly following summer flooding, into a relatively small receiving water of the Richmond River estuary can deoxygenate extensive stretches for prolonged periods of time. In comparison, the Clarence River is the reverse, with an extensive catchment and a large river, and a less mature and smaller floodplain (but has large sources of potential blackwater generation).
This Australian Research Council project ‘Episodic estuarine hypoxia: resolving the geochemistry of coastal floodplain blackwaters’ aimed to improve our understanding of blackwater events and identify the key factors that produce severe blackwater events. As part of the work, it looked at common plant species including native wetland species and pasture species found in low-lying floodplain areas and analysed their potential to deoxygenate floodplain waters during flood events. This work will support changed floodplain land management recommendations in the future.
It also provided the following information:
|L: Blackwater discharging into the Tuckean Broadwater. R: A typical, low lying floodplain backswamp.|
Vithana C.L., Sullivan L.A., Shepherd T. (2019) Role of temperature on the development of hypoxia in blackwater from grass. Science of The Total Environment 667: 152-159.
Vithana C.L., Sullivan L.A., Shepherd T. (2017) Effect of schwertmannite and jarosite on the formation of hypoxic blackwater during inundation of grass material. Water Research 124: 1-10.
Eyre B.D., Kerr G., Sullivan L.A. (2006) Deoxygenation potential of the Richmond River Estuary floodplain, northern NSW, Australia. River Research Applied 22: 981–92.
Johnston S.G., Slavich P.G., Sullivan L.A. and Hirst P. (2003) Artificial drainage of floodwaters from sulfidic backswamps: effects on deoxygenation in an Australian estuary. Marine and Freshwater Research 54, 781-795.
Johnston S., Kroon F., Slavich P., Cibilic A. and Bruce A. (2003) Restoring the balance: Guidelines for managing floodgates and drainage systems on coastal floodplains. (NSW Agriculture: Wollongbar, Australia).
Southern Cross GeoScience (2019) Episodic estuarine hypoxia: resolving the geochemistry of coastal floodplain blackwaters – Summary of project findings. Southern Cross GeoScience Technical Report No. 119. Prepared for Rous County Council. Southern Cross University, Lismore NSW 2480.
Wong V.N.L., Johnston S.G., Burton E.D., Bush R.T., Sullivan L.A., Slavich P.G. (2011) Anthropogenic forcing of estuarine hypoxic events in sub-tropical catchments: Landscape drivers and biogeochemical processes. Science of the Total Environment. 409, 5368-5375.
Wong V.N.L., Johnston S.G., Bush R.T., Sullivan L.A., Clay C., Burton E.D. and Slavich P.G. (2010) Spatial and temporal changes in estuarine water quality during a post-flood hypoxic event. Estuarine, Coastal and Shelf Science. 87, 73-82.
Wong V.N.L, Walsh S., Morris S. (2018) Climate affects fish-kill events in subtropical estuaries of eastern Australia. Marine and Freshwater Research. (69) 1641-1648.