Turbidity is a measure of water clarity, specifically how much suspended matter reduces the capacity of light to penetrate the water. Materials that become mixed and suspended, reduce water clarity. During periods of heavy runoff after spring melting of snow and ice, or heavy down pours of rain, silt-laden water can be seen running into rivers and estuaries. One tool used to measure water clarity is the secchi disk. This activity is used to introduce students to the idea of turbidity and show them a simple way to measure how turbid water is. 

Teacher Note:  All engage activities may be conducted in one class period or presented as the content is covered in the web resource. The water clarity activity should take approximately 10 minutes if using 5 gallon buckets with three turbid conditions. If you use an alternative activity, plan accordingly.

Objectivessecchi disk



Secchi disk in the water.

National Science Education Standards

9-12 A: Science as Inquiry

9-12 B: Physical Science

9-12 C: Life Science

9-12 E: Science and Technologysecchi disk

9-12 F: Science in Personal and Social Perspectives

Lesson Resources

Info on secchi disks:

Secchi disks can be ordered from scientific supply stores and can be found for less than $30.  If you choose to make your own, below are some basic instructions.

Obtain a white Frisbee, plastic lid, white plastic plate or laminated white piece of paper (circle). Cut out a circle with a diameter of 20cm. Punch a hole in the center.The color of the secchi disk will be split up into alternating black and white sections (2 white, 2 black, see photograph above). You can use a black permanent marker or black acrylic paint to color in the black portion of the disk. Tie a knot at one end of about 5 ft of rope and place string through the hole.  Attach some weights to the bottom of the disk so it easily sinks to the bottom of the bucket. Metal washers work well for this purpose. You may wish to mark the rope every cm for ease of taking measurements. Your new, homemade secchi disk is ready for use!