Is Tappan Lake safe to swim?

As of July 20, 2026, Tappan Lake in Harrison County, Ohio is not recommended for swimming. Satellite imagery measured 1,103,832 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

Tappan Lake is a lake in Harrison County, Ohio covering about 3.3 sq mi, the 10th-largest of the 42 lakes SwimZone monitors in Ohio. Satellite imagery measured 1,103,832 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, Tappan Lake 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 1,346,304 cells/mL in July 2023.

Is it safe to swim in Tappan Lake right now?

Tappan Lake is not recommended for swimming today. Satellite imagery measured 1,103,832 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 Tappan Lake have blue-green algae (cyanobacteria)?

Yes. Satellite monitoring measured 1,103,832 cells/mL of blue-green algae (cyanobacteria) at Tappan Lake 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 Tappan Lake?

No. Blue-green algae at Tappan Lake is above the safe-contact threshold (1,103,832 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 Tappan Lake have blue-green algae blooms?

Over 7 summers of satellite monitoring since 2020, Tappan Lake 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 1,346,304 cells/mL in July 2023.

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

Today, 26 Ohio waters are above a safe-contact threshold and 12 more warrant caution, out of 100 monitored waters SwimZone tracks in the state.

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