The impacts of low-oxygen water

Low-oxygen water, also known as hypoxic water, poses a great risk to aquatic ecosystems. When the oxygen levels in water are too low, it can threaten native fish and other aquatic life.

In some places we can use water for the environment to create refuge for native fish when low-oxygen water is present. This provides a small area for fish to obtain oxygen to increase their chance of survival.

What is low-oxygen water?

Low-oxygen water is water that is low in dissolved oxygen. This happens when bacteria break down large amounts of organic matter in rivers. This could be sticks, grass and crops that get washed in during heavy rain. This often happens after or during flood events, when water moves between rivers and floodplains. Or it can happen if there has been a long time between flood events, where organic matter has built up on the floodplain and is flushed into the river in one large deposit.

These bacteria grow and use up oxygen in water. When it's warm, bacteria grow faster and increase the risk of low-oxygen water. Warm water also has less oxygen than cool water.

When the amount of oxygen in the water gets too low, fish and other aquatic life can die. Native fish and crustaceans are at a higher risk of dying.

Low-oxygen water appears black. This is because of the high levels of tannin in the water (similar to putting a tea bag in a cup of water).

Low-oxygen water isn't permanent. Once the bacteria use up all the organic matter, they die off. Oxygen levels in the water will return to normal, but this can take days to weeks.

Using water for the environment in low-oxygen conditions

We have limited options to reduce the impact of low-oxygen water on native fish and aquatic life.

When it floods, the water levels are often too high to dilute using water for the environment. Delivering oxygenated water depends on having access to good quality water. It also depends on how quick we can get it to the parts of the river that need it. This can be hard during floods.

In some cases, we can deliver oxygenated water to parts of a river. This creates refuge areas of better-quality water. As long as species move to these areas, it can increase their chance of survival. We work with scientists, river managers, and communities to do this.

Tackling low-oxygen water in the Basin

Scientists have recruited locals from Deniliquin to measure water quality in the Edward / Kolety-Wakool river.