Updated August 2026
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 12,064 LakeQuality lakes with clarity data, the average Secchi reading is 8.8 feet. This guide explains what the number captures, why it is shown but not scored, 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 relates to the nutrient and algae readings
Secchi depth carries no EPA condition class. EPA's National Lakes Assessment assigns condition classes to phosphorus, nitrogen, chlorophyll-a, and turbidity, none to Secchi, because low clarity is either redundant with chlorophyll-a (dark because of algae) or misleading (dark because of clay or natural colour). A reading deeper than 15 feet is exceptionally clear; below about 3 feet the water is effectively opaque, with underwater hazards hidden. Those bands describe the swimming experience. How the nutrient and algae readings are classified is on the methodology page.
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 read against their own reference conditions rather than 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 | Secchi |
|---|---|---|---|
| Adams Lake | Vilas | WI | 100 ft |
| Gun Lake | Seton Lake | BC | 58.4 ft |
| Cadden Lake | Parry Sound District | ON | 57.4 ft |
| Slocan Lake | Slocan River | BC | 53.5 ft |
| Trout Lake | Duncan Lake | BC | 53.3 ft |
| Meade Lake | Meade | KS | 53 ft |
| Greenwater Lake | Thunder Bay District | ON | 47.9 ft |
| St James Pit Lake | St. Louis | MN | 47.6 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?
Ten feet or more is good recreational clarity. Readings above 15 feet are exceptionally clear; the clearest lakes we track exceed 25 feet. Below about 3 feet the water is effectively opaque. Those bands describe what you can see; they carry no EPA condition class.
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 tannic Florida lakes are read against their own reference conditions.
How does Secchi depth relate to the nutrient and algae readings?
Secchi clarity is measured and shown on every lake page with no condition class beside it. EPA does not assign a Secchi condition class, because dark water can be algae (already captured by chlorophyll-a), suspended clay, or natural dissolved colour. It is still the measurement 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); EPA National Lakes Assessment 2022 (why Secchi is not scored). Clarity is a measurement of transparency, not a safety certification. Data last refreshed 2026-08-26T11:20:36.830Z.
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 water-quality readings, depth, fish species, and trends mean for your investment.
How EPA Water Quality Portal measurements are read against EPA National Lakes Assessment ecoregion condition classes on every LakeQuality lake page. Secchi clarity is shown as a measurement with no class. No reading is a swimming clearance.
How to read a lake’s nutrient and algae readings before a swim, why a reading is a record 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, low-algae water. What to look for in the readings, 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.