Temperature - cold water pollution
Cold water pollution is an artificial decrease in the temperature of river water in a natural ecosystem.
A layer of cold water forms deep within large dams bound by walls higher than 15 metres. Water from this bottom layer is typically released through the 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 of the dam. Large volumes of cold water flowing from dams lowers the natural temperature of the river downstream, damaging aquatic ecology over long distances. The effect is greatest during the warmer months from spring to autumn when large volumes of water are released for irrigation.
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 threatened species of native fish.
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, 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, and to find methods to mitigate or prevent cold water pollution. The NSW Office of Water, in partnership with other key agencies, is implementing a strategy to control cold water pollution from dams identified for priority action in NSW.
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, 14 that caused moderate impacts, and four that caused less severe impacts. The following map of NSW shows the dams identified as high, medium and low priority for addressing cold water pollution.
Controlling cold water impacts from the high and moderate priority dams would improve temperature conditions downstream or up to 2,000 km affected in NSW. Major improvements would be gained for river health, recovery of populations of native fish and aquatic biodiversity.
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, lowering the overall temperature of water downstream from a dam, 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, competing 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.
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
In April 2011 guidelines for managing cold water releases, developed by the Cold Water Pollution Interagency Group was released:
- NSW Cold Water Pollution Strategy: Guidelines for managing cold water releases from high priority dams (PDF 183 KB).
In July 2012, the Cold Water Pollution Interagency Group released their report on Stage 1 of the Cold Water Pollution Strategy:
Astles, K.L., Winstanley, R.K., Harris, J.H. and Gehrke, P.C., 2003. Experimental study of the effects of cold water pollution on native fish. A final report for the Regulated Rivers and Fisheries Restoration Project. NSW Fisheries Final Report Series No. 44. ISSN 1440-3544. 55pp.
Murray–Darling Basin Commission, Native Fish Strategy.
Preece RM and Jones HA 2002, 'The effect of Keepit Dam on the temperature regime of the Namoi River, Australia', River Research and Applications, 18:397–414.
Preece RM 2003, Cold water pollution below dams in NSW: A desktop assessment (PDF 944 KB), map (PDF 1.55 MB) NSW Department of Infrastructure, Planning and Natural Resources.
Rish S, Preece R and Dudgeon S 2000, Status of temperature management capabilities at water storages operated by DLWC, NSW Department of Land and Water Conservation, CNR 2000.033.
Ryan TJ and Preece RM 2003, Potential for thermal shock in the Murray-Darling Basin, a report to the Murray-Darling Basin Commission (Natural Resources Management Strategy), Victorian Department of Sustainability and Environment, Heidelberg, and NSW Department of Sustainable Natural Resources, Parramatta.
Sherman, B 2000, Scoping options for mitigating cold water discharges from dams , Consultancy Report 00/21, CSIRO Land and Water, Canberra.
Sherman B, Todd CR, Koehn JD, Ryan T 2007, Modelling the impact and potential mitigation of cold water pollution on murray cod populations downstream of Hume Dam, Australia. River Research and Applications 23:377-389.