Is Olsen Slough safe to swim?

As of July 20, 2026, Olsen Slough in Nelson County, North Dakota is clear to swim. Satellite imagery detected no blue-green algae bloom as of July 19, 2026. Rainfall over the past week totaled 0.0 in, which can wash bacteria and nutrients into the water.

How this read is built

Olsen Slough is a lake in Nelson County, North Dakota covering about 0.6 sq mi, the 173rd-largest of the 216 lakes SwimZone monitors in North Dakota. Satellite imagery detected no blue-green algae bloom as of July 19, 2026.

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, Olsen Slough 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 2,307,932 cells/mL in August 2025.

Is it safe to swim in Olsen Slough right now?

Olsen Slough is clear to swim today. Satellite imagery detected no blue-green algae bloom as of July 19, 2026. Conditions can change quickly with wind, heat, and rain, so check the posted signs and look at the water before you get in.

Does Olsen Slough have blue-green algae (cyanobacteria)?

No blue-green algae bloom is detectable at Olsen Slough as of July 19, 2026 (below the satellite's roughly 10,000 cells/mL detection limit). Cyanobacteria can still appear within hours in warm, calm, nutrient-rich water, so look before you swim.

Can dogs swim in Olsen Slough?

No bloom is detected at Olsen Slough today, but dogs are much more vulnerable to blue-green algae than people, and blooms can form within hours. Always keep dogs away from water that looks green, scummy, or discolored, and rinse them off after swimming.

How often does Olsen Slough have blue-green algae blooms?

Over 7 summers of satellite monitoring since 2020, Olsen Slough 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 2,307,932 cells/mL in August 2025.

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 Olsen Slough'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 Olsen Slough, and a recent-rainfall overlay, all refreshed daily. It was last updated July 20, 2026.