Is Opitz Lake safe to swim?

As of July 20, 2026, Opitz Lake in Marshall County, South Dakota is not recommended for swimming. Satellite imagery measured 1,946,718 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

Opitz Lake is a lake in Marshall County, South Dakota covering about 2.6 sq mi, the 28th-largest of the 127 lakes SwimZone monitors in South Dakota. Satellite imagery measured 1,946,718 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, Opitz 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 3,433,231 cells/mL in August 2024.

Is it safe to swim in Opitz Lake right now?

Opitz Lake is not recommended for swimming today. Satellite imagery measured 1,946,718 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 Opitz Lake have blue-green algae (cyanobacteria)?

Yes. Satellite monitoring measured 1,946,718 cells/mL of blue-green algae (cyanobacteria) at Opitz 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 Opitz Lake?

No. Blue-green algae at Opitz Lake is above the safe-contact threshold (1,946,718 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 Opitz Lake have blue-green algae blooms?

Over 7 summers of satellite monitoring since 2020, Opitz 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 3,433,231 cells/mL in August 2024.

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