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Flooding in the Murray–Darling Basin

Floods are a natural part of Australia's water cycle and crucial to providing water to floodplains. Floods can also be damaging, presenting challenges for community and the environment.

Floods occur when there is lots of rain that causes flows in the river to increase to levels that result in water moving onto floodplains. Flooding can occur suddenly or slowly—sudden and heavy rainfall can cause floods to quickly rise in minutes or hours causing flash flooding and are usually associated with relatively small catchment areas. In large catchment areas floods can occur slowly from a build-up of rain that causes runoff, resulting in significant floods that inundate large areas of land for days, weeks or months.

Floods can vary in size and frequency depending the on rainfall, catchment conditions, the river and if there is any flood infrastructure in place to mitigate flooding.

In the northern part of the Basin, floods typically occur in winter/spring as a result of winter/spring rains. Large floods also occur in summer as a result of tropical cycles and tropical lows. In the southern part of the Basin, the majority of the floods occur in winter and spring, although there has been some large floods in summer and autumn.

Key contacts during flood

During times of flood and high releases people should stay up to date with the latest information from the Bureau of Meteorology. The Bureau provides:

  • flood watches and flood warnings
  • rainfall and river level forecasts and information.
The Bureau of Meteorology is responsible for issuing flood warnings to the general public. Check www.bom.gov.au/australia/warnings for up-to-date flood warnings in your area.

Other key contacts include:

The Murray-Darling Basin Authority does not provide advice to the general public on flooding or evacuations.

Key facts

Flooding is a natural process and is critical to the health of our rivers.

During times of flood anyone that may be affected should follow the advice of the Bureau of Meteorology and local authorities to ensure their safety.

Floods and the impact of flooding can vary greatly

Floods provide nutrients and energy for floodplains, including wetlands and forests. Floods are critical for providing water to billabongs and flushing sediment out of the river bed.
Flooding can create blackwater, which can impact water quality and cause fish deaths.

The flooding of rivers following heavy rainfall is the most common form of flooding in Australia.

Some rivers only carry water during times of flood—during average or dry years, they may be dry riverbeds or creeks.

 

Expected water quality issues

Potential water quality issues caused by flooding include:

  • High turbidity, which is created by issues such as sediment flushed into rivers by erosion, following bushfires, or stirred up by carp. Turbidity reduces the penetration of sunlight, affecting plants and aquatic life.
  • Blackwater, which is a naturally occurring process caused by nutrient-rich organic material such as leaves and bark washing into rivers during a flood. As this organic matter decays, tannins are released, giving the water a blackish appearance. The process can also release chemicals that make water more alkaline or acidic and reduce dissolved oxygen levels. However, this natural process is not always a threat and is important to the productivity of the river, providing nutrients to supply the food chain that aquatic life depends on.
  • Low dissolved oxygen levels, which can occur in drought and flood conditions. Low dissolved oxygen levels suffocate and kill aquatic life (such as fish and shrimps) in large numbers. In drought, oxygen levels throughout the water column can quickly reduce when stratified water bodies with low-oxygen deeper layers rapidly mix with oxygenated surface layers due to sudden changes in climate conditions. In floods with large inputs of organic matter, the process responsible for blackwater can rapidly consume the oxygen in a river or lake.

Management of dams and infrastructure during floods

Dams exist in many catchments across the Basin, both in the northern and southern Basin. In some instances, dams provide an opportunity to mitigate floods, depending on how much water can be captured.

Some dams have designated space that is used to capture water during times of flood, while others do not.

Related links

 

Updated: 02 Apr 2020