Cold water pollution is an artificial decrease in the temperature of river water in a natural ecosystem.

What causes cold water pollution?

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 stored water is stratified and when large volumes of water are released for irrigation.

Cold water pollution strategy in NSW

It is easy to think of cold water as harmless but 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 threatened species.

In NSW large volumes of unseasonably cold water flowing from major dams have suppressed the breeding and growth of native fish, killed juvenile fish and affected other aquatic life. This can persist sometimes for hundreds of kilometres downstream. At least 2,000 kilometres of the State's rivers are thought to be seriously affected by cold water pollution.

The NSW Government is working with dam owners, community groups and environmental scientists to identify the areas most seriously 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.

Which dams cause cold water pollution?

Most small dams and weirs in NSW do not cause cold water pollution because they have walls lower than 15 metres. Other dams have outlets that release water from a warmer surface level and some do not need to release water in large volumes.

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.

Controlling cold water impacts would create major improvements in river health, recovery of populations of native fish and aquatic biodiversity.

How does cold water pollution damage the environment?

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 by creating 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?

NSW Government agencies and corporations are collaborating to investigate the causes and effects of cold water pollution and how to manage its impacts. Some outcomes include:

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

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