What Is a Good Secchi Depth for a Lake?
A good Secchi depth is anything above 6.6 feet (2 meters). Exceptional is above 14.8 feet (4.5 m) — those are the gin-clear "boundary waters" lakes. Anything under 3.3 feet (1 m) is murky. Secchi depth is the single best at-a-glance measure of water clarity, and clarity tracks closely with algae, phosphorus, and overall cleanliness.
The Secchi disk method
A Secchi disk is a 20-cm black-and-white disk that has been used to measure water clarity since 1865, when Father Pietro Angelo Secchi developed the technique on a Vatican research vessel. You lower it on a marked rope until you can no longer see it, then raise until it is visible again. The average of those two depths is the Secchi depth. Cheap, repeatable, and remarkably accurate.
The full scale (in feet)
- Above 14.8 ft (4.5 m) — Exceptional. Crystal clear, oligotrophic. Boundary Waters character.
- 9.8-14.8 ft (3-4.5 m) — Excellent. Clear, low algae, rich submerged plant life.
- 6.6-9.8 ft (2-3 m) — Good. Most "clean" lakes. Mesotrophic.
- 3.3-6.6 ft (1-2 m) — Moderate. Some nutrient enrichment, visible algae in summer.
- Under 3.3 ft (1 m) — Murky. Heavy algae or sediment. Eutrophic to hypereutrophic.
Why Secchi depth matters
Secchi depth predicts almost everything else about water quality. Clarity drops when algae populations grow (chlorophyll-a goes up), when phosphorus enrichment fuels that algae growth, or when sediment from runoff or shoreline erosion stays suspended. A long-term decline in Secchi depth is the earliest visible signal of lake degradation.
It also predicts what plants and fish a lake supports. Lakes with Secchi above 10 feet support submerged aquatic vegetation deep into the water column, which in turn supports walleye, smallmouth bass, and other clear-water species. Lakes with Secchi under 3 feet typically support carp, bullhead, and other turbidity-tolerant species.
Seasonal variation
Secchi depth changes throughout the year. It is typically deepest in early spring (after ice-out, before plankton populations recover) and shallowest in late summer (peak algae). The values reported on LakeQuality are growing-season averages — typically May through September — which captures the relevant range for swimming and recreation.