Secchi Depth and Water Clarity, Explained
Secchi depth is the oldest and simplest water quality measurement still in daily use: how deep you can see into a lake. It is easy to grasp, cheap to collect, and remarkably informative — clarity integrates the effects of algae, sediment, and dissolved color into a single number. Across the 7,139 LakeQuality lakes with clarity data, the average Secchi reading is 6.8 feet. This guide explains what the number captures, how it becomes a grade, and what pushes it up or down.
How the measurement works
The method dates to 1865, when astronomer Pietro Angelo Secchi lowered a white disk into the Mediterranean to gauge transparency. Today a monitor lowers a standard black-and-white Secchi disk on a measured line and records the depth where it disappears, then averages that with the depth where it reappears on the way up. The reading is taken on the shady side of the boat around midday. Because the protocol is so consistent, Secchi data collected by state agencies and volunteers decades apart remain directly comparable — a big reason it anchors the EPA Water Quality Portal record we build on.
How clarity maps to a grade
LakeQuality grades clarity on Metropolitan Council thresholds. A reading deeper than 14.8 feet (4.5 m) earns an A — you can usually see the bottom across much of the lake. Roughly 9.8 to 14.8 feet is a B, 6.6 to 9.8 a C, and 3.3 to 6.6 a D. Below 3.3 feet the water is effectively opaque and earns an F, with underwater hazards hidden and algae blooms likely. Clarity is one of three equally weighted inputs to the overall letter grade, and it is the one most visible to anyone standing on a dock.
What drives clarity up and down
Three factors dominate. Algae is usually the biggest lever — more phosphorus feeds more algae, which scatters light and shrinks the Secchi depth, which is why clarity, phosphorus, and chlorophyll-a tend to move together. Sediment clouds shallow, wind-swept, or runoff-fed lakes even when nutrients are moderate. Dissolved organic color — tannins leached from wetlands and forests — stains some lakes tea-brown and lowers clarity naturally, without indicating pollution. That last factor is exactly why subtropical, often-tannic Florida lakes are graded on a separate scale rather than penalized by northern cutoffs.
The clearest lakes in the dataset
The deepest Secchi readings we record belong to cold, low-nutrient lakes — the kind where the disk vanishes only far below the surface:
| Lake | County | State | Grade | Secchi |
|---|---|---|---|---|
| Adams Lake | Vilas | WI | A | 100 ft |
| St James Pit Lake | St. Louis | MN | A | 47.6 ft |
| Clear Lake | Rock | WI | A | 37.5 ft |
| Bear Lake Southeast Basin | Kalkaska | MI | A | 36.5 ft |
| Higgins Lake East Basin | Roscommon | MI | A | 35 ft |
| Higgins Lake Northwest Basin | Roscommon | MI | A | 34.5 ft |
| Whitefish Bay Deep Lake | Chippewa | MI | A | 34.4 ft |
| Higgins Lake | Roscommon | MI | A | 34 ft |
Frequently Asked Questions
What is a Secchi disk?
A Secchi disk is a plain black-and-white disk lowered into the water on a marked line. The depth at which it just disappears from view is the Secchi depth, a simple, standardized measure of water transparency used by lake monitors worldwide since the 1800s.
What is a good Secchi depth?
On the LakeQuality clarity scale, a reading over 14.8 feet earns an A and roughly 9.8 to 14.8 feet a B. Below about 3.3 feet the water is opaque and earns an F. The clearest lakes we track exceed 25 feet of visibility.
What makes water cloudy?
Three things reduce clarity: algae (driven by phosphorus), suspended sediment (from runoff, wind, or shallow stirred bottoms), and dissolved organic color (tannins that stain some lakes tea-brown). Algae and sediment lower quality; natural tannic color can lower clarity without meaning the lake is polluted, which is why Florida lakes are graded on their own scale.
How does Secchi depth relate to the overall grade?
Secchi clarity is one of the three equally weighted metrics behind the letter grade, alongside phosphorus and chlorophyll-a. It is the most directly observable of the three — the one you can essentially read with your own eyes from a dock or boat.
Sources: North American Lake Management Society (Secchi disk method); EPA Water Quality Portal (measurements); Metropolitan Council clarity thresholds. Clarity is a classification of measured transparency, not a safety certification. Data last refreshed 2026-07-05.
More Lake Guides
Learn how to read Secchi depth, phosphorus, chlorophyll-a, and trophic state measurements, and what they tell you about your lake.
Practical steps every lakefront property owner can take to reduce nutrient runoff, maintain shoreline buffers, and protect water quality.
Use LakeQuality data to evaluate lake properties before buying, what the grade, depth, fish species, and trends mean for your investment.
The exact method behind every LakeQuality grade: how EPA Water Quality Portal measurements become an A-through-F letter using Metropolitan Council thresholds, medians, and the Carlson Trophic State Index.
How to read a water quality grade before a swim, why a grade is a classification of past measurements rather than a same-day clearance, and the visual signs of a harmful algal bloom.
How water clarity, temperature, and algae shift across the seasons, from ice-out through fall turnover, and what that means for the clearest swim, the best bite, and the sharpest Secchi reading.
How to pick a lake for kids and first-timers: forgiving panfish, public shore access, and clean, safe water. What to look for in a grade, which species to target, and how to plan the trip.
How the fishing changes through the year as lakes warm, stratify, and turn over — where fish hold in each season, why water quality and temperature drive the bite, and how to time a trip.