Safest Lakes to Swim in Wyoming

The lakes ranked above are the swim-safest lakes in Wyoming for 2026 — all have grade A or B water and chlorophyll-a below 10 µg/L.

Three signals: (1) water clarity — Secchi depth above 6 feet is a strong sign of low suspended sediment and pathogens; (2) chlorophyll-a below 10 µg/L means low risk of harmful algae bloom; (3) a current grade of A or B from year-round sampling. Wyoming DEQ and county health departments post seasonal advisories — always check before you swim, especially in late summer when blue-green algae bloom risk peaks.

Most monitored Wyoming lakes are safe to swim in under normal conditions, but water quality varies widely. The lakes ranked above are filtered to grade A and B with chlorophyll-a below 10 µg/L — meaning they consistently test cleaner and have lower algae bloom risk than the state average. Always check posted advisories before swimming and avoid water after heavy rain (bacterial contamination spikes from runoff).

Algae blooms (especially blue-green / cyanobacteria) are most common in shallow, warm, nutrient-rich (eutrophic) lakes during late summer. Lakes with high phosphorus and chlorophyll-a measurements are at higher risk. The lakes on this page are filtered to exclude high-algae lakes. For the opposite list, see our most-polluted-lakes trend page.

Wyoming lakes are typically safest to swim in late spring through early summer (May–July), when water is cool and algae bloom risk is lowest. Risk rises in late summer and early fall (August–September) when warm water and accumulated nutrients drive cyanobacteria blooms. Always avoid swimming after heavy rainfall (bacterial spikes from runoff) and check current state DNR advisories the day you go.

Too few Wyoming lakes carry an A–F grade to rank the state on cleanliness, so there is no Wyoming cleanest-lakes list. The lakes on this page cleared the swimming-safety filter: an A or B grade plus a chlorophyll-a reading below 10 µg/L. Each lake page carries its own grade and the sampling behind it.