Are Lakes Safe to Swim In? A Data-Driven Guide
Most lakes in Minnesota and Wisconsin are safe to swim in most of the time. The risk depends on three things: water clarity (Secchi depth), algae levels (chlorophyll-a), and recent advisories from the local health department. A lake with Secchi depth above 6 feet, chlorophyll-a under 10 µg/L, and no posted advisory is generally safe. The hard part is checking before you swim.
The three numbers that matter
For lake-water swimming risk, only three measurements matter much:
- Secchi depth (water clarity). Above 6 feet is clear; under 3 feet is murky enough that you cannot see your feet.
- Chlorophyll-a (algae concentration). Under 10 µg/L is safe; above 20 µg/L flags bloom-prone water.
- Phosphorus — predicts whether algae is likely to spike in the next few weeks. Above 30 µg/L means the fuel is loaded.
What "unsafe" actually means
When a lake is described as unsafe for swimming, the risks are usually one of: (1) blue-green algae toxins (the most serious — can cause skin rash, GI illness, or worse), (2) E. coli or other fecal bacteria (typically from stormwater runoff after heavy rain or from agricultural drainage), or (3) heavy sediment or particulate matter that obscures hazards underwater.
In the Upper Midwest, the dominant summer risk is blue-green algae. Bacterial advisories spike for 24-72 hours after big rainstorms.
Before you swim, check
- The lake's LakeQuality grade (A or B = generally safe; C = swim with awareness; D or F = check advisories first)
- Local health department advisories — Minnesota: MDH; Wisconsin: DNR
- Visual signs at the beach: green or blue-green scum, dead fish, an unusual odor
- Whether it has rained heavily in the last 48-72 hours (bacterial spike window)