How bushfires affect water in the Murray–Darling Basin

As well as devastating communities, bushfires can change the quality and amount of water in the Basin's rivers, streams and dams.

This page explains

• How bushfires reduce water quality
• How bushfires can affect water stored in dams
• How bushfires change the amount of water in the system
• How bushfires change the way water flows over land and into rivers
• Report water quality issues or fish deaths

How bushfires reduce water quality

After a bushfire, ash, burnt material, soil, sediment and organic matter can enter waterways and reduce water quality. Poor quality water may need additional treatment to make it fit to drink.

During and after a fire, ash and burnt material fall from the sky, landing on the waterways and on the ground. When they land on the ground they cover the layer of dead plant matter that usually covers the soil. When it rains, the plant matter, ash and burnt material are all washed into rivers and streams.

The plants that usually cover the soil have also been destroyed, which means soil and clay are more easily washed into the water when it rains.

The presence of these materials reduces water quality.

• The additional nutrients in the water from ash and burnt plant matter are consumed by bacteria that use up the oxygen in the water.
• The extra sediment means there are more solid particles in the water. This can damage the health of aquatic animals, as well as covering holes and other spaces where they live or lay their eggs.

The results of these changes to water quality can't always be predicted, but may include blackwater events that reduce the oxygen available for aquatic organisms to breathe, and can contribute to fish deaths.

The impact a bushfire has on water quality depends on each catchment's characteristics, such as the amount and intensity of rain that fell after the fire and the type of forest that was burnt.

You should know

• Bushfires can reduce water quality straight away and for many years after.
• The effects of a bushfire on water quality and quantity can be different in different areas.
• Ash, soil, sediment and other materials that wash into water change the water chemistry, which can harm animals and plants.
• Town water supplies affected by bushfires may need extra treatment.
• Check with your local water supplier or state government about water quality in your area after a bushfire.

Water quality in dams after a bushfire

The quality of the water stored in dams can also be reduced by bushfires, depending on how much bushfire-affected water flows into the dam and how it mixes with the water already in storage.

If too much bushfire-affected water flows in, the sediment and ash mix with the stored water. This can lower the amount of oxygen dissolved in the water and increase the risk of algal blooms.

How bushfires change the amount of water in the system

Bushfires can also affect the amount of water that ends up flowing over the land and into waterways. This water, which mainly comes from rain, is called surface water runoff.

If it rains before plants begin to grow back, most of the surface water runoff will flow over the soil and into waterways. However when plants start to regrow, surface runoff drops to levels lower than before the fires took place, because the growing plants take extra water from the soil.

Over the next couple of decades, the plants mature and water runoff returns to normal.

The amount of surface runoff depends on the severity of the fire, each catchment's characteristics, and the intensity, timing, and amount of rain that falls after a bushfire, as well as where it falls.

How the rivers are managed after a bushfire

After serious bushfires, the Murray–Darling Basin Authority (MDBA) and state governments are responsible for different parts of the recovery process.

Water quality

The MDBA works with Basin state governments to see how the bushfires affect water quality and availability in the River Murray system during and after a fire, and to predict what will happen in the future. However, it is not the MDBA's responsibility to manage changes to water quality caused by bushfires. Water managers in each state are responsible for managing water quality. Water may need additional treatment before it is suitable for drinking. Contact your local water authority to find out more.

Fish and aquatic wildlife

Basin state governments work to relocate fish in areas likely to experience blackwater events as a result of fires. They can also release water from dams to help improve oxygen levels in the water.

However, these efforts don't always make a significant difference to the survival of fish and other wildlife.

Revegetation

After plants have been destroyed by fire, newly exposed soil and clay washes into rivers. As plants regenerate in the burnt areas, the amount of soil being washed away decreases. Generally, it is best to allow native plants to regenerate naturally after a fire, though this can take many years. In extreme cases, some replanting may be done to prevent erosion in areas where water quality may be badly impacted.