Measurement

Secchi Depth

A measurement of water clarity determined by lowering a black-and-white disk into the water until it disappears from view.

What It Means for Your Lake

Secchi depth is the most widely used field measurement for lake water clarity. Named after Italian astronomer Angelo Secchi, who developed the technique in 1865, it involves lowering a 20-centimeter (8-inch) black-and-white disk into the water on a calibrated line. The depth at which the disk disappears from sight is the Secchi depth. In Minnesota and Wisconsin lakes, Secchi depth typically ranges from less than 1 foot in highly eutrophic lakes to over 25 feet in pristine oligotrophic lakes. Secchi readings are most meaningful when taken during summer months (June through September) under consistent lighting conditions, ideally on the shady side of the boat between 10 AM and 2 PM. Because it requires no lab equipment, Secchi depth is the metric most commonly collected by volunteer lake monitors. LakeQuality weights Secchi depth at 35% of the overall grade because it directly reflects what swimmers and boaters experience. A Secchi depth above 15 feet earns an A grade, while readings below 3 feet earn an F. Long-term Secchi depth trends are the best indicator of whether a lake is improving or declining in overall health.

Frequently Asked Questions