How to Tell If a Lake Is Clean (5 Things to Check)
A clean lake has Secchi depth above 6 feet, phosphorus under 30 µg/L, and chlorophyll-a (algae) under 10 µg/L. You can also tell visually: clean lakes have firm sandy or rocky shorelines, no green or blue-green scum, water that looks blue rather than green or brown, and minimal weed mats near shore. The five signals below — three from the data, two from the shore — let you judge cleanliness with confidence.
Signal 1: Secchi depth (water clarity)
A Secchi disk is a black-and-white disk lowered into the water on a marked rope. The depth at which it disappears is the Secchi depth, the simplest measure of water clarity. Above 14.8 feet (4.5 m) is exceptional. Above 6.6 feet (2 m) is good. Under 3.3 feet (1 m) means heavy algae or sediment.
Signal 2: Phosphorus
Phosphorus is the limiting nutrient for algae growth. Below 20 µg/L is excellent. 20-30 µg/L is good. 30-60 µg/L flags moderate enrichment. Above 60 µg/L means the lake is at chronic bloom risk every summer.
Signal 3: Chlorophyll-a (algae)
Chlorophyll-a directly measures algae concentration. Under 5 µg/L is excellent. 5-10 µg/L is good. 10-20 µg/L is moderate. Above 20 µg/L flags bloom-prone lakes. This is the single best predictor of swimming safety.
Signal 4: Shoreline condition
A clean lake has a firm sandy or rocky shoreline, not muck. The water near shore is clear enough to see the bottom. Submerged plants (pondweed, eelgrass) are present but not choking the water — submerged plants indicate good water clarity, while floating mats of duckweed or filamentous algae indicate enrichment.
Signal 5: Color and smell
Clean lake water looks blue, blue-green, or tea-colored (from natural tannins in tamarack and bog drainage — this is fine). Murky brown, milky green, or bright pea-soup green means trouble. Clean lakes smell faintly earthy, sometimes like wet rocks or pine; sewage, chemical, or strong musty odors are warning signs.