What Is Secchi Depth? Water Clarity Explained
Secchi depth is the simplest and most intuitive measure of lake water quality: how far can you see down into the water? It is measured in feet using a Secchi disk, a simple tool invented in 1865 by Angelo Secchi that is still the standard method today.
How It Works
A Secchi disk is an 8-inch diameter disk painted in alternating black and white quadrants. A volunteer or scientist lowers it on a calibrated rope from the shaded side of a boat. They record two depths: the depth where the disk disappears from view, and the depth where it reappears as it is raised. The Secchi depth is the average of those two numbers.
What the Numbers Mean
| Secchi Depth | Grade | What You See |
|---|---|---|
| Over 15 ft | A | Crystal clear, you can see the bottom in shallow areas, deep blue color |
| 10 - 15 ft | B | Good clarity, visible well below the surface, blue-green color |
| 6.5 - 10 ft | C | Moderate, some haze, green tint from algae |
| 3.3 - 6.5 ft | D | Murky, cannot see feet when standing waist deep, distinct green color |
| Under 3.3 ft | F | Very murky, almost opaque, pea-soup green or brown |
What Affects Water Clarity
- Algae, the biggest factor. More algae = murkier water. Chlorophyll-a levels directly correlate with Secchi depth.
- Suspended sediment, soil particles from runoff, especially after heavy rain or in shallow wind-mixed lakes.
- Dissolved organic matter, "tea-colored" water from bog drainage. Common in northern Minnesota and Wisconsin. These lakes may have low algae but still reduced Secchi depth.
- Lake depth, deep lakes resist mixing and tend to maintain better clarity than shallow lakes.
Clearest Lakes We Track
These lakes have the deepest Secchi depth readings, the clearest water in our dataset:
| Lake | County | State | Clarity | Grade |
|---|---|---|---|---|
| Adams Lake | Vilas | WI | 100 ft | A |
| St James Pit Lake | St. Louis | MN | 47.6 ft | A |
| Clear Lake | Rock | WI | 37.5 ft | A |
| Bear Lake Southeast Basin | Kalkaska | MI | 36.5 ft | A |
| Higgins Lake East Basin | Roscommon | MI | 35 ft | A |
| Higgins Lake Northwest Basin | Roscommon | MI | 34.5 ft | A |
| Whitefish Bay Deep Lake | Chippewa | MI | 34.4 ft | A |
| Higgins Lake | Roscommon | MI | 34 ft | A |
| Bear Lake | Kalkaska | MI | 33 ft | A |
| Crystal Lake | Benzie | MI | 32 ft | A |
Frequently Asked Questions
What is Secchi depth?
Secchi depth measures water clarity, how deep you can see into a lake. It is measured in feet by lowering a black-and-white disk (called a Secchi disk) into the water until it disappears from view. The depth where it vanishes is the Secchi depth. Deeper readings mean clearer water.
What is a good Secchi depth for a lake?
A Secchi depth of 15 feet or more (grade A) indicates crystal clear water. 10-15 feet (grade B) is good clarity. 6-10 feet (grade C) is moderate. Below 3 feet (grade F) means the water is very murky. For swimming, you generally want at least 4-5 feet of visibility.
How is Secchi depth measured?
A standard 8-inch black-and-white disk is lowered on a calibrated rope from the shaded side of a boat. The observer notes the depth where the disk disappears, then lowers it further and records the depth where it reappears. The Secchi depth is the average of those two readings.
Why does Secchi depth change throughout the year?
Secchi depth is lowest (water is murkiest) in summer when algae growth peaks, especially in July and August. Spring runoff can temporarily reduce clarity. Fall turnover mixes the water and can reduce clarity briefly. Winter often has the best clarity under ice, though it is rarely measured.