Temperature is the degree of heat or cold within a substance or liquid, in this instance water in a waterway.

Guideline values for the temperature of water in a waterway are:

Excellent  < 16º C

Fair              16 to 20º C

Poor         > 20º C

Water temperature has a substantial effect on aquatic life. If the overall temperature of a waterway is altered, a change in the aquatic plants and animals can be expected.

An increase in water temperature reduces the amount of oxygen in the water, making it difficult for fish and invertebrates to breathe. As their physical need for oxygen increases, simultaneously there is a decrease in the amount of oxygen available in the water.

Cold-water fish, such as trout and salmon, are very sensitive to temperature change and if temperature increases above 20º C their bodies suffer stress. New Zealand’s native fish are generally tolerant of temperatures up to about 25º C while some native invertebrates can become stressed when temperatures exceed 18º C.

Water temperature depends largely on the time of year and on weather conditions although stream type can also contribute. For example, a spring-fed lowland stream, such as the Styx River, experiences reasonably stable temperatures throughout the year compared to streams fed from rainfall and melting snow. Other factors that can affect water temperature include changes in the shape of the stream channel, reduction in overhanging vegetation, diminished water flow, or, alternatively, a discharge into the waterway.

The temperature of the river is a surface reading only. It is calculated using a long mercury thermometer supplied as part of a SHMAK kit. To be able to compare temperature readings over a period of time each reading needs to be taken at approximately the same time of the day.

To obtain a water temperature reading:

  • Place the thermometer in a reasonable depth of water at an undisturbed part of the site, at the edge of the river and leave it in the water until a stable temperature is reached (usually around 1-2 minutes).
  • Take the thermometer out of the river, immediately read the temperature scale, and record the result on the datasheet.

To obtain an air temperature reading:

  • Leave thermometer to reach a stable temperature (about 1 minute) in a shady spot, if possible, off the ground.