Is Flat Lake safe to swim?

As of July 20, 2026, Flat Lake in McHenry County, North Dakota is not recommended for swimming. Satellite imagery measured 365,109 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

Flat Lake is a lake in McHenry County, North Dakota covering about 0.4 sq mi, the 205th-largest of the 216 lakes SwimZone monitors in North Dakota. Satellite imagery measured 365,109 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, Flat 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 2,979,223 cells/mL in August 2021.

Is it safe to swim in Flat Lake right now?

Flat Lake is not recommended for swimming today. Satellite imagery measured 365,109 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 Flat Lake have blue-green algae (cyanobacteria)?

Yes. Satellite monitoring measured 365,109 cells/mL of blue-green algae (cyanobacteria) at Flat 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 Flat Lake?

No. Blue-green algae at Flat Lake is above the safe-contact threshold (365,109 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 Flat Lake have blue-green algae blooms?

Over 7 summers of satellite monitoring since 2020, Flat 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 2,979,223 cells/mL in August 2021.

How many North Dakota lakes and beaches are unsafe for swimming today?

Today, 167 North Dakota waters are above a safe-contact threshold and 4 more warrant caution, out of 241 monitored waters SwimZone tracks in the state.

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