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 11,661 LakeQuality lakes with clarity data, the average Secchi reading is 8.7 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 letter
LakeQuality does not grade Secchi depth. 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. They are not why a lake received its letter. The letter lives 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 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 | NR | 100 ft |
| Lake Maumelle | Pulaski | AR | NR | 99 ft |
| Cadden Lake | Parry Sound District | ON | A | 57.4 ft |
| Meade Lake | Meade | KS | NR | 53 ft |
| Crescent Lake | Klamath | OR | A | 51.7 ft |
| Greenwater Lake | Thunder Bay District | ON | A | 47.9 ft |
| St James Pit Lake | St. Louis | MN | NR | 47.6 ft |
| Fremont Lake | Sublette | WY | B | 42.8 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 are not the A–F letter.
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 measured and shown on every lake page. It is not scored into the letter. EPA does not assign a Secchi condition class, because dark water can be algae (already graded via 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 2026-08-02T04:54:46.732Z.
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