Is Lake Thunderhead safe to swim?

As of July 20, 2026, Lake Thunderhead in Putnam County, Missouri is not recommended for swimming. Satellite imagery measured 608,391 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 Thunderhead is a lake in Putnam County, Missouri covering about 1.3 sq mi, the 12th-largest of the 32 lakes SwimZone monitors in Missouri. Satellite imagery measured 608,391 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 Thunderhead has reached blue-green algae advisory levels in 6 of them, most recently in July 2026. This is a lake that blooms most summers. Its highest recorded reading was 1,689,285 cells/mL in September 2021.

Is it safe to swim in Lake Thunderhead right now?

Lake Thunderhead is not recommended for swimming today. Satellite imagery measured 608,391 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 Thunderhead have blue-green algae (cyanobacteria)?

Yes. Satellite monitoring measured 608,391 cells/mL of blue-green algae (cyanobacteria) at Lake Thunderhead 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 Thunderhead?

No. Blue-green algae at Lake Thunderhead is above the safe-contact threshold (608,391 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 Thunderhead have blue-green algae blooms?

Over 7 summers of satellite monitoring since 2020, Lake Thunderhead has reached blue-green algae advisory levels in 6 of them, most recently in July 2026. This is a lake that blooms most summers. Its highest recorded reading was 1,689,285 cells/mL in September 2021.

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

Today, 14 Missouri waters are above a safe-contact threshold and 1 more warrant caution, out of 34 monitored waters SwimZone tracks in the state.

How is Lake Thunderhead'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 Thunderhead, and a recent-rainfall overlay, all refreshed daily. It was last updated July 20, 2026.