Clarity is the term used to describe the clearness or transparency of water in a waterway.

Water clarity varies depending on the amount of materials that have become mixed and suspended in the water. Suspended material, such as sediment and algae, restricts light passing through the water column.

Clarity is measured by the distance in centimetres at which an object becomes invisible to the naked eye. This is influenced by the amount of mixed or suspended sediment and/or algae present in the water sample.

Guideline values for water clarity measurements are:

Why is clarity measured?

Generally people perceive clear water as clean and healthy and more attractive in waterways. Clear water is also generally more desirable ecologically as a habitat for aquatic plants and animals. Reduced light passing through the water can limit plant growth which in turn, affects aquatic life relying on the plants for food. Clear water helps optimise water quality conditions and allows sunlight to penetrate into the water.

Obviously there will be times, such as during flooding, when water clarity declines. However, if poor water quality persists in times of stable flow, plant growth can be limited and the feeding patterns of fish and invertebrates can be affected.

Murky waters may result in excessive amounts of sediment being deposited on the streambed. This deposited sediment settles into the spaces between the stream stones where fish and many invertebrates like to shelter. For this reason streams with sedimented beds are generally home to less fish and invertebrates.

How is clarity measured?

Water clarity is measured using a clarity tube. The tube, open at one end, usually comes as part of a SHMAK kit. The clarity measuring part of the kit includes 2 magnets; one for use outside the tube, the other magnet with a black disc attached is used inside the tube. Also included is a black rubber cap that is fitted over the open end of the tube.

To obtain a water clarity reading:

• Using a container, collect undisturbed water from the stream and pour it into the clarity tube, filling it to the brim. Ensure that there are no air bubbles in the tube.
• Insert the magnet with the black disc inside the tube, with the disc closest to the open end of the tube. Hold this in place by attaching the other magnet to the outside of the tube. Fit the rubber cap over the open end of the tube.
• Take the tube out of direct sunlight, and while holding it parallel with the ground, look into the clear end of the tube.
• Slide the magnets along the tube until the black disc is no longer visible. Read this position on the outside of the tube, and note down this reading, which is recorded in centimetres.
• Repeat the above 2 steps, then swap with your survey partner to allow your partner to also take a reading so that a total 3 readings are recorded.
• Add the 3 readings taken, and then divide the total readings by 3 to get an average distance in ‘cms’. The resulting figure is the water clarity measurement to be recorded.