A Secchi disk is a simple, circular plate divided into alternating black and white quadrants(or plain white), attached to a marked, weighted line. Lowered into a body of water, the depth at which the pattern vanishes and reappears measures the water’s transparency and turbidity.

Using a Secchi Disk

To take a highly accurate transparency reading, the measurement process follows specific, standardized steps:

• Prepare: Remove any sunglasses to avoid impaired vision or glare. Position yourself on the shady side of a boat or dock, ideally between 10:00 a.m. and 2:00 p.m.
• Lower: Slowly lower the disk straight down into the water column until the distinction between the black and white quadrants is no longer visible. 
• Record and Average: Note the depth of disappearance, then pull the disk up slightly until you can just barely see it again. The average of these two depths is your final Secchi depth