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 14,370 lakes we track, 11,661 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.

Why the letter does not score Secchi

EPA's National Lakes Assessment 2022 Technical Support Document grades phosphorus, nitrogen, chlorophyll-a, and turbidity against ecoregion condition classes. It does not grade Secchi. Dark water can mean algae (already captured by chlorophyll-a), natural tannin stain, or suspended clay. Scoring clarity as if it were pollution failed bog lakes and clay reservoirs — which is why LakeQuality reports the disk reading on every lake page and keeps it out of the letter. The full engine is in Lake Water Quality Grades Explained and on the methodology page.

The two still move together often enough to be useful. A lake with consistently deep Secchi readings usually has low algae; a lake losing clarity year over year is often gaining nutrients. That is a correlation, not an identity. Carlson's trophic index can still be computed from Secchi alone, which is why TSI appears on some unrated lakes — and why this site only states a trophic class on rated ones. See Trophic State Explained and the field method in the Secchi depth guide. Where lakes are gauged, live temperature and level from USGS NWIS add same-day context on top of the long-term clarity record.