Secchi Depth
A measurement of water clarity determined by lowering a black-and-white disk into the water until it disappears from view.
Measurement
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. LakeQuality shows Secchi depth on every lake that has a reading, as measured. EPA's National Lakes Assessment does not assign a condition class to clarity, so the disk reading stands on its own beside the phosphorus and chlorophyll-a classes. Our methodology page explains how each reading is shown.
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 the lakes LakeQuality covers, 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. Secchi depth is measured and shown on every lake page. It carries no EPA condition class: EPA’s National Lakes Assessment does not assign one to Secchi, and dark water can come from algae, clay, or natural colour. Readings above 15 feet are exceptionally clear; readings below 3 feet are opaque. Long-term Secchi depth trends remain the best field indicator of whether a lake is improving or declining.
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
The transparency of lake water, typically measured by Secchi depth, a primary indicator of water quality and recreational suitability.
The Carlson Trophic State Index (TSI) is a numerical scale from 0 to 100 that classifies lakes by their nutrient and productivity levels.
The measurements a state or federal agency published for a lake, shown on its LakeQuality page: Secchi clarity, total phosphorus, total nitrogen, chlorophyll-a and turbidity, with the EPA National Lakes Assessment condition class for the lake’s ecoregion beside each nutrient and algae reading.
Source: EPA National Aquatic Resource Surveys, 2026.