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Impact of thermal pollution on upland streams

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Taking a dip in a cool mountain stream is highly invigorating. The water is clear and bracing and the surroundings are Australian bushland at its best...

...But when the temperature of the water drops even further – because of water as cold as 3°C released from the base of an upstream reservoir – it's not just people that might find the temperature drop uncomfortable. Many plants and animals living in the stream find the cold surge to be beyond their tolerable limits and their growth rate slows or they even disappear.

Similarly, in winter, the dam’s water will be warmer than normal, upsetting the natural temperature cycle and the breeding cues it creates.

Releasing water from Bendora dam. Cold water emerging from the dam can have ecological effects in the Cotter River for 10 km or more until it warms up.Releasing water from Bendora dam. Cold water emerging from the dam can have ecological effects in the Cotter River for 10 km or more until it warms up.

In work funded by eWater CRC, Dr Kit Rutherford and his colleagues have been measuring temperature drops in the Cotter River (near Canberra) brought about by periodic releases of cold water from Bendora Dam. They have used these measurements, and meteorology and flow records, to build a heat-flow model that can predict how far downstream the effects will persist. Water managers could use the model to control releases in a way that avoids damaging environmental effects.

Rutherford has a long-term interest in the subject. He is a physicist and ecologist from the National Institute of Water and Atmospheric Research in New Zealand, and in 2006 spent time with CSIRO Land and Water in Canberra working with Australian colleagues on the project. The team – from the University of Canberra, Central Queensland University, and the ACT Government – have recently released their findings, and they make interesting reading.

The researchers found that water released from Bendora could be many degrees cooler than the natural stream. Temperature differences between the surface and 3 m depth could be 11°C in summer, and even cooler at greater depth. The effect is most pronounced immediately downstream – and then gradually dissipates – but it can persist for an appreciable distance, depending on flow rate, weather, and the level of the offtake from which the water is released. At a modest flow of one cubic metre per second, the team found that an effect could be detected as far as 20 km downstream, right down to the next reservoir on the river, the Cotter dam.

Claire Sellens, thigh-deep in the icy cold water of the Cotter River, taking vegetation canopy measurements for feeding data into the thermal model.Claire Sellens, thigh-deep in the icy cold water of the Cotter River, taking vegetation canopy measurements for feeding data into the thermal model.

The macroinvertebrate specialist in the team was Dr Claire Sellens from Central Queensland University, and her field work often involved wading thigh-deep into icy cold water. To lessen effects on fish, Sellens suggests that dam managers should make efforts to keep spring and summer temperatures above 17°C. This could be done by using variable depth offtakes or by ensuring that the dam's temperature-stratified layers are turbulently mixed.

“This temperature would greatly improve spawning and would improve the growth rate of juveniles,” she says, “since we now know that cooling below 15°C can harm the survival of eggs and larvae.

“Slower growth means that small fish are vulnerable to predators for longer, so this also affects survival rates.”

Some alien fish species, like trout, are more tolerant of low temperatures than native ones, so higher temperatures would help sustain native populations.

“Cold temperatures are also likely to affect other life in the stream – macroinvertebrates, plants, and the entire food web,” she says, “but at this stage it isn’t possible to be clear on how big the impact will be.

“Fish are probably the tip of the iceberg, metaphorically speaking, and we need more research in this whole area, which is becoming more important as environmental flows receive greater attention.”

The intensive monitoring that Rutherford, Sellens and the team did on the Cotter River enabled them to develop a more accurate model of how the temperature of water released from a dam increases as it flows downstream. The team used temperature sensors, radiation sensors, and meteorological data to calibrate the model, which, for any given flow rate, relates temperature to distance downstream. The measurements showed the model could predict water temperature to within one or two degrees, enough to allow preliminary assessments of downstream ecological effects.

Besides setting out the details of the model, part of the report is also devoted to a review of current knowledge of ecological impact of dam releases.

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