Is South Waubay Lake safe to swim?

As of July 20, 2026, South Waubay Lake in Day County, South Dakota carries a swim caution. Satellite imagery measured 50,121 cells/mL of blue-green algae on July 19, 2026, elevated but below 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

South Waubay Lake is a lake in Day County, South Dakota covering about 3.7 sq mi, the 19th-largest of the 127 lakes SwimZone monitors in South Dakota. Satellite imagery measured 50,121 cells/mL of blue-green algae on July 19, 2026, elevated but below 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, South Waubay 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 2,814,900 cells/mL in July 2023.

Is it safe to swim in South Waubay Lake right now?

South Waubay Lake carries a swim caution today. Satellite imagery measured 50,121 cells/mL of blue-green algae on July 19, 2026, elevated but below 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 South Waubay Lake have blue-green algae (cyanobacteria)?

Low levels. Satellite monitoring measured 50,121 cells/mL at South Waubay Lake on July 19, 2026, below the 100,000 cells/mL advisory threshold. Keep an eye out for surface scum, which can concentrate quickly.

Can dogs swim in South Waubay Lake?

Use caution. Blue-green algae at South Waubay Lake are elevated (50,121 cells/mL on July 19, 2026). Dogs are far more sensitive to cyanobacteria than people, so keep them out of the water and away from surface scum.

How often does South Waubay Lake have blue-green algae blooms?

Over 7 summers of satellite monitoring since 2020, South Waubay 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 2,814,900 cells/mL in July 2023.

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

Today, 89 South Dakota waters are above a safe-contact threshold and 3 more warrant caution, out of 142 monitored waters SwimZone tracks in the state.

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