A NSW Government website

Surface water science

Water quality of surface water environments

Find out more about water quality and why we monitor water quality in NSW.

Samples taken for water quality testing.

The quality of water in our rivers and aquifers is dynamic, varying with time and location. Natural processes and human activities can affect water quality. Natural factors include the local geology which influences salt and nutrient inputs as well as climate, riparian vegetation and stream flow. There are a wide range of human practices that may change water quality. This includes sewage discharge, mine waste, land clearing and urban and agricultural development.

The National Water Quality Management Strategy establishes guideline values for various water quality measures. Following consultation with the community, interim water quality objectives are in place in NSW. They help decision makers consider water quality in both state-wide strategic planning and at the local level in assessing impacts of proposed development.

Water licences authorise the management of contaminated irrigation or mining water, sewage discharge and other point source pollution.

The department has developed water quality management plans for each water resource plan area in NSW.

What is bad water quality?

At times, both natural events and human activities can cause poor water quality in both surface and groundwater systems. This video explores three types of poor water quality.

What is hypoxic water?

Oxygen in water is critical for aquatic life. When oxygen levels in water get very low, we call this hypoxic water. This video explores the causes of hypoxic water and what we can do to manage it.


Salt is a natural part of the landscape of NSW and some rivers and aquifers have naturally high salinity levels. For example, salinity levels in groundwater aquifers can range from that of rainwater to more than ten times the concentration of sea water. Tree clearing, intensive irrigation and discharge of saline wastewaters can speed up the release of salt into soils and water sources. Mines, industry, sewage treatment plants and climate fluctuations can also contribute. Over-extraction of water from an aquifer can also cause salt-water intrusion in coastal regions.

Salinity is the presence of soluble salts. It is measured in water by its ability to transmit an electric current (electrical conductivity or EC). But this method is not foolproof. It needs to be supplemented with chemical analysis. Water for drinking should ideally have a salt concentration less than 800 μS/cm (microsiemens/centimetre). Levels greater than 1000 μS/cm can cause problems for some irrigated crops and can damage aquatic ecosystems.

Salinity is influenced by climate and geography. Salt may be dissolved into surface waters, mobilised and redeposited many times over decades. Steep catchments and high rainfall areas are less likely to have salinity problems than flat catchments in low rainfall areas.

A salinity audit of the Murray Darling Basin published in 1999 predicted that salinity levels were rising and could cause serious problems for water use and the environment within 20 to 50 years. As a result the NSW Government developed the NSW Salinity Strategy. This strategy recognised that to slow down the increase in salinity, we need to:

  • protect and manage our native vegetation
  • use our land so that less water goes into the groundwater table
  • use water more effectively and efficiently
  • use engineering solutions
  • make better use of land affected by salt
  • focus our efforts on priority salinity hazard landscapes.

The Basin Salinity Management Strategy (BSM2030) sets the direction for salinity management across the Murray Darling Basin for the next 15 years.

The department has a role in the operation of salt interception schemes in south western NSW.


Turbidity is an indicator of water quality. Turbidity refers to how clear the water is. The greater the amount of total suspended solids in the water, the murkier or muddier it appears and the higher the measured turbidity. Increased turbidity can be an issue, particularly in inland areas.

In most rivers, turbidity increases after rainfall and flooding. This is because of soil erosion and the increased amount of suspended sediments in the water. As the sediment settles out of the water, it can  cause sedimentation of rivers and dams, and can smother aquatic plants. The suspended sediments can also absorb and transport microorganisms, nutrients, heavy metals, pesticides and other chemicals.

Turbid water is a problem for country town water supplies. It is difficult and costly to remedy and may lead to health problems.


Temperature is also an indicator of water quality. Temperature is measured spatially and temporally. Aquatic plants and animals have adapted to the natural seasonal variation in temperatures. This makes them susceptible to rapid changes of temperature or significantly different seasonal temperatures (e.g. cold summer water temperatures). Water temperature is important for triggering breeding events, growth, movement within rivers, food resources and survival. This makes it an important indicator for surface water environments.

What is cold water pollution?

Cold water pollution is an artificial decrease in the natural temperature of water in rivers. A layer of cold water forms deep within large dams bound by walls higher than 15 metres. Water from this cooler, bottom layer is typically released through the existing deep outlets in the older, major dams of New South Wales.

The water released can be up to 12°C colder than the water in the river upstream and downstream of the dam. Large volumes of cold water flowing from dams lowers the natural temperature of the downstream river. This can be damaging to aquatic ecology over long distances downstream. The effect is greatest during the warmer months from spring to autumn when large volumes of water from the cooler, bottom layers are released for irrigation.

Which dams cause cold water pollution?

Most small dams and weirs (walls less than 15 metres) in NSW do not cause cold water pollution because the water is not deep enough to form layers. Instead, the water is regularly mixed by incoming stream flow and the wind, so cold water pollution is less likely to occur. Modern dams have outlets that release water from a warmer surface level.

An Environmental Trust study in 2002-03 assessed 3,000 dams and weirs in NSW finding eight that caused severe cold water pollution. Moderate impacts were evident in 14 and four had less severe impacts.

In NSW large volumes of unseasonably cold water flows from major dams. Cold water can persist sometimes for hundreds of kilometres downstream. At least 2,000 kilometres of the State's rivers are affected by cold water pollution.

Controlling cold water impacts can lead to major improvements in river health, recovery of populations of native fish and aquatic biodiversity.

How does cold water pollution damage the environment?

In the fragile ecosystems of Australian rivers and wetlands the effects of cold water can be deadly for species of native fish. Some of them are threatened species. Cold water pollution can suppress the breeding and growth of native fish, kill juvenile fish and affect other aquatic life.

The life-cycles of fish and other aquatic creatures are finely tuned to the natural daily and seasonal variations in temperature. Large volumes of cold water disturb the delicate ecological balance. It creates an unseasonal environment that ecologists have compared to an 'eternal winter'.

In spring and summer the rising temperature of the water becomes an important environmental cue, triggering spawning of native fish. A release of cold water from a major dam can suppress spawning for up to 300 km downstream. The ability of native fish to reproduce, grow and maintain sustainable numbers is reduced. Introduced species such as carp flourish as they compete with native fish for food and habitat. Some species of native fish can disappear from large sections of the river.

Some examples of the effects of cold water releases on native fish in NSW include:

  • elimination of trout cod, Macquarie perch and freshwater blackfish from large sections of the Murrumbidgee River downstream from Blowering Dam
  • loss of trout cod, Macquarie perch and freshwater catfish from the Murray River downstream from Hume Dam
  • loss of silver perch, Murray cod, rainbowfish and bony herring from the Macquarie River for up to 300 km downstream from Burrendong Dam
  • suppressed breeding of native fish, particularly silver perch, in the Namoi River as far as 100 km downstream of Keepit Dam
  • 50 per cent of juvenile silver perch killed after only 30 days of exposure to cold water in a study conducted by NSW Fisheries.

Cold water pollution can also affect river health by reducing or altering the food sources available for animals within the aquatic food chain including: micro-organisms, insects, water birds, frogs and platypus.

What is being done in NSW?

The NSW Government is working with dam owners, community groups and environmental scientists to identify those areas most affected. We are finding methods to prevent or mitigate cold water pollution. The department, in partnership with other key agencies, is implementing a strategy to control cold water pollution from dams identified for priority action in NSW.

Outcomes include:

  • 26 dams identified as a priority for action, including nine high priority dams, after assessing 3,000 dams and weirs in NSW
  • operating protocols reviewed for major dams with multi–level offtake towers and guidelines implemented to minimise release of cold water while risk managing algal blooms
  • structural modifications considered for multi–level offtake towers to make them safer and easier to operate
  • trialling an innovative thermal curtain at Burrendong Dam
  • increased coverage of temperature monitoring around major NSW dams
  • feasibility evaluation of using surface mounted impellors or submerged curtains as low–cost alternatives to multi–level offtake towers, to allow warmer water to be released
  • interagency group established to develop a cold water pollution strategy for NSW.

The following publications were developed by the Cold Water Pollution Interagency Group:

Dissolved oxygen

Dissolved oxygen is the amount of oxygen present in the water. Waterbodies receive oxygen from the atmosphere and from aquatic plants. Running water absorbs more oxygen from the atmosphere than still water.

Fish and other aquatic animals need oxygen from the water they live in to survive. If oxygen levels in water drop, animals can become stressed, or even die. When oxygen levels get very low, we call this hypoxic water. Hypoxia develops when aquatic organisms consume oxygen faster than it can be replaced. High, low, and no flow conditions in our rivers can lead to hypoxic water.

In dry periods, with low or no flows and high air temperatures, rivers can disconnect into pools. The quality of the water in these remnant pools can become poor. The risk of fish deaths generally increases under these conditions as aquatic organisms compete for what can become a limited oxygen supply. Sometimes low oxygen can persist throughout the waterbody, especially in shallow waters. In deeper pools, water at the bottom of the pool can become very low in oxygen. A change in weather conditions can cause this to mix with the rest of the pool resulting in stress to aquatic organisms.

During high flows or floods, organic material, like dead leaves and other plant matter, are washed from the floodplain into the river. Similarly, after an extended dry period or bushfire, rainfall can wash high volumes of organic material or ash into rivers as flows resume. Bacteria begin feeding on this organic material, consuming oxygen in the process - sometimes more oxygen than can be replenished. These are essential natural processes which provide food for the river ecosystem, however, a sudden, rapid drop in oxygen may have adverse impacts, such as fish deaths.

Aquatic plants and algae increase oxygen levels during the day through photosynthesis, however oxygen levels reduce overnight through respiration which consumes oxygen. This can lead to hypoxic water in the morning before oxygen levels increase again from photosynthesis.

All these conditions can cause fish deaths and loss of aquatic life. Unfortunately, due to the large areas where these conditions occur, we have limited tools to manage hypoxic water:

  • aerators can increase oxygen within a small area, such as a weir pool;
  • flows carrying high levels of organic material can be diverted away from high risk areas, and
  • when water is available, we can release fresh water from upstream storages to dilute the hypoxic water.

We’re working hard to minimise the risk of hypoxic water in our rivers by trying to maintain adequate flow so pools don’t become disconnected, inputs of organic material are dispersed and algal blooms are monitored. Refer to the dissolved oxygen water quality updates for the latest hypoxic blackwater information.

Simple things you can do

  • Be on the lookout for any changes in water quality - i.e. changes to the colour or odour of your local waterway and report them via the Fishers Watch Phoneline on 1800 043 536.
  • Contact the phoneline if you observe any dead fish or fish starting to gasp at the water surface.


Algae are a natural part of aquatic ecosystems. But some algae can produce toxins. Toxins can be damaging to humans, domestic animals, livestock and aquatic organisms. Exposure through drinking or contact with the water may be harmful.  Toxin producing algae can be found in freshwater, as well as brackish and marine waters.

Blue-green algae (also known as cyanobacteria) are the only group of toxin producing algae found in freshwater. Blue-green algae are microscopic bacteria that live in water and can photosynthesise. This is why they are often called ‘algae’.

Although microscopic, when blue-green algae form colonies and accumulate rapidly, they can become visible to the naked eye as an algal bloom. Some species of blue–green algae can produce potent liver and neurotoxins. Some can also be skin irritants. But not all blue–green algae species are toxic. And potentially toxic blue-green algae species do not produce toxins all the time.

Toxic marine and estuarine algae can also affect recreational water use. Some of these algae produce toxins that can cause illness if ingested. Others can cause skin irritations. These algae are often microscopic but some cause red coloration to water. This is a phenomenon known as red tides.

The Impact of river flows on algal bloom formation (PDF, 250.19 KB) in the Darling, Murray and Murrumbidgee Rivers has been researched.

For further information on identifying algal blooms, dangers and problems, and prevention and control, visit WaterNSW.

Algal alerts and contacts

Current algal alerts are published by WaterNSW.

Contact the relevant regional Co-ordinator at WaterNSW to report an algal bloom.

State strategy for algal management

Our role of the department in algal management is to:

  • co-ordinate and support the State Algal Advisory Group
  • provide advice on algal issues and the government strategic direction of algal management and planning in NSW.

The State Algal Advisory Group provides the over-arching policy advice and framework for the management of fresh water and marine algal blooms. Membership of the State Algal Advisory Group is made up of the relevant NSW State Agencies, NSW local government and the Murray Darling Basin Authority.

Information for water utilities

A framework for raw waters used as a source for potable supply is available at Water Quality Research Australia.