Water temperature and thermal

Generating electricity creates a lot of wasted energy in the form of heat, which then needs to be cooled before it can be discharged into the environment.

This water is usually much warmer than river water, although prior to discharging the temperature is usually reduced using cooling towers, which circulate water to remove the heat. The water can then be either returned to the water body from where it was extracted, or cooled using air contact and evaporation in a cooling tower.

Most animals and plants that live in waterways prefer a certain temperature range for optimum growth and reproduction. When temperatures change outside a species' preferred range, mahinga kai can be significantly impacted.

Potential impacts of changing temperatures on water quality and mahinga kai

• Decreases in available oxygen (DO) with increasing temperatures - reduces oxygen available for mahinga kai.
• Increases in fish metabolic rates - sudden changes, like those found at a discharge points, are more likely to cause stress and possibly death.
• Increases in algal abundance and changes in the dominant species present - as the water gets warmer algal growth increases, often resulting in algae blooms.
• Changes in the amount and type of animals present - some animals cannot tolerate extreme changes in temperature and will avoid these areas (habitats) of a waterway if unfavourable.
• Changes in migration patterns - water temperature triggers the time of migration for breeding as mahinga kai travel to and from the sea. Changes in water temperature and flow throughout streams or rivers or at localised points can have significant impacts on movement of fish through the water column (deep and shallow).

Learn more about the potential environmental impacts of temperature changes in waterways