The Secchi disk is a tool used to measure the clarity of water. It is a circular plate, usually divided into 4 alternating quadrants of black and white. The diameter is typically 20 centimeters (roughly 8 inches).
The Secchi disk is lowered by a rope into the water until it is no longer visible. The rope is marked at the surface of the water with a clothespin. The Secchi disk is then lowered a couple of feet deeper into the lake and slowly brought back up until the disk is visible. The rope is again marked with a clothespin at the surface of the water. The distance from the Secchi to each clothespin is measured and the average of the two is the Secchi transparency readings.