Secchi Disk Depth
One of the most commonly used measurements for water clarity is the Secchi disk depth (pronounced she SEHK-ki) . The Secchi disk is a simple and inexpensive device used by both citizens and scientists for measuring water clarity.
The device generally consists of a 20-centimeter (8-inch) disk made of wood or plastic. Some are painted with alternating black and white quadrants, or they can be solid white. A non-stretching piece of rope or light chain is attached through the center; the rope is marked in increments of feet or meters. A small weight is attached beneath the disk so that it will sink quickly and the line will remain taught while measurements are being made.
The disk is lowered below the surface until it just disappears from view; that depth is referred to as the Secchi disk depth.