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What you need

  • White plastic disc approximately 10 cm diameter

  • Length of thick wire approximately 50 cm long

  • Permanent waterproof coloured markers

What you do

Colour sections of the plastic disc in the pattern shown in the diagram below. Insert a length of wire through the centre of the disc and secure. Bend over and tape cut ends of wire to ensure their safety. This home made Secchi disc is used to measure the levels of suspended sediment in a river. Drop the disc into the water and lower until the colours are no longer visible. Pinch the wire on the surface of the water and remove. Measure the depth at which the colours vanished and compare with other sites.

When you use it

This technique can be used to investigate changes in the river channel along its length, particularly with variations in the land use of the surrounding area, for example agriculture. It can also be used to compare rivers in different settings.