Is Delaware Lake safe to swim?

As of July 20, 2026, Delaware Lake in Delaware County, Ohio is not recommended for swimming. Satellite imagery measured 558,757 cells/mL of blue-green algae on July 18, 2026, above the 100,000 cells/mL advisory threshold. Rainfall over the past week totaled 0.9 in, which can wash bacteria and nutrients into the water.

How this read is built

Delaware Lake is a lake in Delaware County, Ohio covering about 1.8 sq mi, the 21st-largest of the 42 lakes SwimZone monitors in Ohio. Satellite imagery measured 558,757 cells/mL of blue-green algae on July 18, 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, Delaware Lake has reached blue-green algae advisory levels in 6 of them, most recently in August 2025. This is a lake that blooms most summers. Its highest recorded reading was 1,201,885 cells/mL in July 2024.

Is it safe to swim in Delaware Lake right now?

Delaware Lake is not recommended for swimming today. Satellite imagery measured 558,757 cells/mL of blue-green algae on July 18, 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 Delaware Lake have blue-green algae (cyanobacteria)?

Yes. Satellite monitoring measured 558,757 cells/mL of blue-green algae (cyanobacteria) at Delaware Lake on July 18, 2026, above the 100,000 cells/mL advisory level. Avoid swimming and keep children and pets out of the water.

Can dogs swim in Delaware Lake?

No. Blue-green algae at Delaware Lake is above the safe-contact threshold (558,757 cells/mL on July 18, 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 Delaware Lake have blue-green algae blooms?

Over 7 summers of satellite monitoring since 2020, Delaware Lake has reached blue-green algae advisory levels in 6 of them, most recently in August 2025. This is a lake that blooms most summers. Its highest recorded reading was 1,201,885 cells/mL in July 2024.

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 Delaware 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 Delaware Lake, and a recent-rainfall overlay, all refreshed daily. It was last updated July 20, 2026.