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The Basin has 23 major rivers. Almost 4 million people rely on water from these rivers and catchments for the survival of their families, communities and industries. Animals and plants have evolved and adapted to the seasonal patterns of water flow through the Basin’s rivers.

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The Basin has 23 major rivers. Almost 4 million people rely on water from these rivers and catchments for the survival of their families, communities and industries. Animals and plants have evolved and adapted to the seasonal patterns of water flow through the Basin's rivers.

There is so much life and prosperity dependent on the water of the Basin that it is quite extraordinary that less than 15% of the area of the Basin contributes to runoff in the river systems - except in times of flood.

Most of the water in the Basin comes from the southern and western sides of the Great Dividing Range, flowing into the 23 major rivers, including Australia's 3 longest, the Murray, Murrumbidgee and Darling rivers. High evaporation rates, expansive floodplains and significant water diversions for towns and agriculture mean that much of the surface water in the Basin does not reach the ocean.

Groundwater is also an important water source for Basin communities and industry. Groundwater is water contained in soil or rock below the ground surface. It comes from surface water that flows downwards through sediment or rock, forming an aquifer. In turn, the aquifer may reconnect with the soil surface, or with rivers and streams somewhere else in the landscape.

The water of the Basin's rivers and the aquifers beneath its plains quickly attracted European settlers, and the Basin's land and water resources were put to work. Dams were built to capture water in the upper catchments, and river flow was regulated to ensure water supply met water demand - which was often contrary to natural flow patterns.

Basin residents and governments have long recognised the need to share the resources of the Basin equitably and sustainably. For more than 100 years there have been agreements, policies and plans addressing the use and management of the waters of the Basin. In the 21 st century the pressure on the river system is significant. Population growth, thriving agricultural industries and ongoing cycles of drought and flood must be balanced with the need to carefully manage the natural environment - which underpins the traditional culture, modern social fabric and economy of the Basin.

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The Basin has 23 major rivers. Almost 4 million people rely on water from these rivers and catchments for the survival of their families, communities and industries. Animals and plants have evolved and adapted to the seasonal patterns of water flow through the Basin's rivers.

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A land of sweeping plains, Of ragged mountain ranges, Of droughts and flooding rains..."
These few well-known lines of Dorothea Mackellar's poem 'My Country' perfectly describe the Murray-Darling Basin.

The Murray-Darling Basin is an area of over 1 million square kilometres in south-eastern Australia. It is made up of 23 river catchments and is bounded by the Great Dividing Range in the south and east, the central Queensland sandstone belt in the north, and the arid interior in the west.

Named for the way that water flows in its vast area, all the streams, creeks and rivers of the Basin flow towards the Murray or Darling rivers, and ultimately to the Southern Ocean. The watercourses mostly start as fast flowing streams in the Great Dividing Range. As the rivers flow inland across wide riverine plains, the river channels widen and meander through the countryside, slowing down the pace of water flow through the system. A large amount of water evaporates along the way, a considerable amount is soaked up by wetlands, and in the last 200 years, much water is used to supply towns and cities, industry and irrigation.

The Basin is a landscape that is over 60 million years old. While its basic shape and foundations largely have remained unchanged during this time, earthquakes, volcanic activity and the coming and going of ice ages have shaped the Basin to the 23 major rivers, over 30,000 wetlands and expansive riverine plains that we see today.

The sheer size of the Basin means that it encompasses several climate zones: semi-arid, subtropical, temperate and cool temperate. In the Great Dividing Range, where most rivers rise, average annual rainfall is high, often over 1,000 mm. As the rivers work their way inland, annual rainfall is increasingly lower and evaporation rates increase.

Throughout the Basin, variability is a feature of the weather experienced from one season to the next. Drought and flood have long been part of the Basin climate, and climate change predictions are that seasonal variability will be greater than experienced in the last 200 years.

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A land of sweeping plains, Of ragged mountain ranges, Of droughts and flooding rains..." These few well-known lines of Dorothea Mackellar's poem 'My Country' perfectly describe the Murray-Darling Basin. The Murray-Darling Basin is an area of over 1 million square kilometres in south-eastern Australia.

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Why is the Murray Darling River important to Australia 

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Why is the Murray Darling River important to Australia 

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-The Murray-Darling Basin is the largest and most complex river system in Australia. it runs from Queensland, through New South Wales and the Australian Capital Territory, Victoria and South Australia it spans 77,000 kilometres.

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More than 2.6 million Australians call the Murray–Darling Basin home.

Over 3 million people have access to fresh, clean drinking water from the Basin.

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A mass fish death is a sudden and unexpected mass mortality of wild or cultured fish. There are many causes of fish kills and often they are caused by natural events.

Fish death event can occur in both freshwater systems and estuarine systems and are more likely to occur in summer (January and February). Events in summer are more likely due to:

higher water temperatures (and consequently lower dissolved oxygen levels)

frequent severe and sudden storm/ flood events

lower water levels in freshwater river systems during these months.

There are a range of causes of fish deaths, including water quality changes, pollution, infection, or a combination of these. In almost half of all reported fish kill events the cause is unknown.

Some causes include:

dissolved oxygen levels (e.g. caused by bushfires, flood events, decaying vegetation matter)

high or low temperatures

drought or dry conditions

algae and algal blooms

acidic runoff

pesticide/chemical pollution

dumping of waste fish or bycatch

disease/infections

dam releases.

Often, a large percentage of fish kills are caused by several factors acting together. For example drought conditions, blue-green algae and a change in temperature, resulting in reduced dissolved oxygen, collectively are thought to have led to the fish death event in the Lower Darling in January 2019.

Fish deaths are usually the result of sudden events or changes in the local environment. As far as possible, fish will avoid adverse environmental conditions and swim to another area to avoid harm. However, if the entire or a large proportion of the waterway is affected, there are barriers to movement, or the adverse conditions appear very rapidly, then fish are unable to relocate and a fish death event often results.

Areas of risk

A number of locations across the Basin have been identified as risk areas for fish death events. Basin governments are already taking action on the ground to mitigate these risks and respond to recent events.

Actions underway include:

adding water can protect key refuges for native fish by improving water quality. During drought, there is limited water available and water storages are low, meaning this option is not always available.

aerating water can increase the amount of oxygen in the water in targeted areas, improving water quality. Aerators can only improve oxygen levels for a small area (about the size of a tennis court).

moving fish to key waterholes can protect these animals in times of drought. This method is not an option when fish are too stressed and are unlikely to survive the journey.

new technologies are being considered to increase turbulence in the water, which improves water quality. Many technologies are in trial stage and a full evaluation of these methods is yet to occur.

A recommended action plan has also been provided to the Australian Government Minister responsible for water.

If you want to report on fish deaths please use the following contact numbers:

New South Wales Fishers Watch hotline: 1800 043 536

Queensland Department of Environment and Science: 1300 130 372

Victoria Environmental Protection Authority pollution hotline: 1300 372 842

South Australia Fishwatch Hotline: 1800 065 522

Australian Capital Territory Access Canberra: 13 22 81

Reduced dissolved oxygen and its effects on fish

Low levels of dissolved oxygen can cause stress (and even death) to fish and other aquatic animals which rely on oxygen in the water to breathe.

Dissolved oxygen in water comes from both the atmosphere-by exchange across the air-water interface, mostly from wind and wave action, or agitation during flows in streams and rivers-and is released by aquatic plants, via photosynthesis.

The normal range of dissolved oxygen for water is between 6-8 milligrams per litre (mg/L). However, this varies between coastal and inland rivers and estuarine and marine waters.

Most aquatic animals, including fish, extract the oxygen they need from the water through their gills. Very low levels of dissolved oxygen will cause suffocation and death of aquatic animals.

It is not necessary for the water to become completely deoxygenated for a fish kill to occur. The critical minimum level varies with different species and different physical conditions, but as a general guide, few fish species will tolerate prolonged exposure to levels below 3 mg/L. Larger fish species, such as Murray cod, tend to become stressed and/or die first due to their greater oxygen requirements.

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A mass fish death is a sudden and unexpected mass mortality of wild or cultured fish. There are many causes of fish kills and often they are caused by natural events. Fish death event can occur in both freshwater systems and estuarine systems and are more likely to occur in summer (January and February).

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and rocks.