Water Clarity Explained: What a Secchi Depth Reading Really Tells You

What a Secchi reading actually measures

A Secchi depth reading measures water transparency: the depth at which a standardized black-and-white disk, lowered on a marked line, disappears from sight. Named after the 19th-century astronomer Angelo Secchi, who first used the method in 1865, the disk is still the workhorse of lake monitoring because it is cheap, repeatable, and needs no power. A volunteer or technician lowers the disk until it vanishes, notes that depth, then raises it until it reappears, and averages the two.

The number captures how much light penetrates the water column. Anything that blocks or scatters light — suspended algae, fine sediment, or dissolved organic color — shortens the reading. Because clarity responds to the same nutrient and algae dynamics that define lake health, Secchi depth is a reliable, low-cost proxy for overall condition. Across the 5,469 lakes we track, 5,159 carry at least one Secchi-based clarity measurement drawn from the EPA Water Quality Portal and state monitoring programs.

What counts as a "good" Secchi number?

A good Secchi reading is generally above roughly 15 feet for crystal-clear water, 10 to 15 feet for good clarity, and 6.5 to 10 feet for moderate clarity; below about 3 feet is very murky. The clearest lakes in our data sit well into the crystal-clear range. Our current clarity leader is Adams Lake in Vilas, where crystal clear, you can see 100 ft down.

For the full list, see the clearest lakes ranking. Keep in mind that the "right" number is lake-specific: a shallow prairie lake that reads 8 feet may be in excellent shape for its type, while a deep northern lake that drops from 25 feet to 12 feet is showing real decline.

Clear is not the same as clean

Clarity measures light, not safety — and the two can diverge in both directions. The most common surprise is the tannin-stained lake. Bog and wetland-fed lakes carry dissolved organic carbon leached from peat and decaying leaves, which tints the water tea-brown and can hold a Secchi reading to a few feet even when the lake is biologically clean and low in nutrients. That color is natural staining, not pollution.

The reverse matters more for swimmers: water can look reasonably clear and still be unsafe. A Secchi disk cannot detect the bacteria behind a swim advisory, and it does not distinguish between the two things that cloud water. Algae-driven turbidity is greenish and signals a nutrient problem — often the early stage of a harmful algae bloom. Sediment-driven turbidity is brownish and usually temporary, spiking after storms, wind events, or spring runoff. Both lower the Secchi number, but they mean very different things: one is a chronic health signal, the other a passing weather effect. This is why we pair clarity with phosphorus and chlorophyll-a rather than reading it alone.

How clarity feeds a lake's grade

Secchi depth is one of the three pillars of our grading methodology, alongside total phosphorus (the nutrient that fuels algae) and chlorophyll-a (a direct measure of algae concentration). Together those metrics also place a lake on the trophic scale — from nutrient-poor oligotrophic lakes with deep clarity to nutrient-rich eutrophic lakes where clarity collapses. Because the three metrics move together, a lake with consistently deep Secchi readings almost always earns a strong grade, and a lake losing clarity year over year is usually losing ground on nutrients too.

To go deeper on the field method itself, read our Secchi depth guide, or look up any term in the water-quality glossary. Where lakes are gauged, live water temperature and level from USGS NWIS stations add same-day context on top of the long-term clarity record.