Is Lake Wilhelm safe to swim?

As of July 20, 2026, Lake Wilhelm in Mercer County, Pennsylvania is not recommended for swimming. Satellite imagery measured 1,236,468 cells/mL of blue-green algae on July 19, 2026, above the 100,000 cells/mL advisory threshold. Rainfall over the past week totaled 0.0 in, which can wash bacteria and nutrients into the water.

How this read is built

Lake Wilhelm is a lake in Mercer County, Pennsylvania. Satellite imagery measured 1,236,468 cells/mL of blue-green algae on July 19, 2026, above the 100,000 cells/mL advisory threshold.

Blue-green algae (cyanobacteria) blooms are most common from midsummer into early fall, when the water is warm, calm, and rich in nutrients from runoff. They can produce toxins that make people sick and can be rapidly fatal to dogs, so skip water that looks green, scummy, or like spilled paint, and keep children and pets out of it.

Over 7 summers of satellite monitoring since 2020, Lake Wilhelm has reached blue-green algae advisory levels in 7 of them, most recently in July 2026. This is a lake that blooms most summers. Its highest recorded reading was 1,236,468 cells/mL in August 2024.

Is it safe to swim in Lake Wilhelm right now?

Lake Wilhelm is not recommended for swimming today. Satellite imagery measured 1,236,468 cells/mL of blue-green algae on July 19, 2026, above the 100,000 cells/mL advisory threshold. Conditions can change quickly with wind, heat, and rain, so check the posted signs and look at the water before you get in.

Does Lake Wilhelm have blue-green algae (cyanobacteria)?

Yes. Satellite monitoring measured 1,236,468 cells/mL of blue-green algae (cyanobacteria) at Lake Wilhelm on July 19, 2026, above the 100,000 cells/mL advisory level. Avoid swimming and keep children and pets out of the water.

Can dogs swim in Lake Wilhelm?

No. Blue-green algae at Lake Wilhelm is above the safe-contact threshold (1,236,468 cells/mL on July 19, 2026). Cyanobacteria toxins can be rapidly fatal to dogs, which swallow it while swimming and lick it from their fur. Keep dogs out of the water and away from any scum along the shore.

How often does Lake Wilhelm have blue-green algae blooms?

Over 7 summers of satellite monitoring since 2020, Lake Wilhelm has reached blue-green algae advisory levels in 7 of them, most recently in July 2026. This is a lake that blooms most summers. Its highest recorded reading was 1,236,468 cells/mL in August 2024.

How many Pennsylvania lakes and beaches are unsafe for swimming today?

Today, 10 Pennsylvania waters are above a safe-contact threshold and 9 more warrant caution, out of 41 monitored waters SwimZone tracks in the state.

How is Lake Wilhelm's water quality measured?

SwimZone builds this read from EPA CyAN satellite imagery for blue-green algae, official beach and fecal-bacteria advisories where a program monitors Lake Wilhelm, and a recent-rainfall overlay, all refreshed daily. It was last updated July 20, 2026.