Measurement
Secchi Depth
A measurement of water clarity determined by lowering a black-and-white disk into the water until it disappears from view.
Secchi depth is the simplest, oldest measurement of lake water clarity — invented in the 1860s and still the gold standard. A black-and-white disk is lowered into the water on a calibrated rope; the depth at which it disappears from view is the Secchi depth. On the LakeGrade rubric, Secchi depth is one of three direct inputs. The cutoffs are: above 4.5 m = A, above 3.0 m = B, above 2.0 m = C, above 1.0 m = D, below 1.0 m = F.
A practical note for anglers and homeowners: Secchi depth is the easiest water-quality measurement to make yourself. A homemade Secchi disk and a calibrated rope are sufficient; the readings will be comparable to professional measurements within about 10%.
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
What is secchi depth?
A measurement of water clarity determined by lowering a black-and-white disk into the water until it disappears from view.
Why does secchi depth matter for lake health?
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 disapp...
Related Terms
Water Clarity
The transparency of lake water, typically measured by Secchi depth, a primary indicator of water quality and recreational suitability.
Trophic State Index
The Carlson Trophic State Index (TSI) is a numerical scale from 0 to 100 that classifies lakes by their nutrient and productivity levels.
Lake Grade
An A-through-F letter grade assigned by LakeQuality to summarize overall lake water quality based on clarity, phosphorus, and chlorophyll-a measurements.
Source: EPA National Aquatic Resource Surveys, 2026.