Is Nelson Reservoir safe to swim?

As of July 20, 2026, Nelson Reservoir in Phillips County, Montana is not recommended for swimming. Satellite imagery measured 299,353 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

Nelson Reservoir is a reservoir in Phillips County, Montana covering about 6.4 sq mi, the 14th-largest of the 124 lakes SwimZone monitors in Montana. Satellite imagery measured 299,353 cells/mL of blue-green algae on July 19, 2026, above the 100,000 cells/mL advisory threshold. Larger, warmer waters like this can hold a bloom for weeks once nutrients and calm weather line up.

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, Nelson Reservoir 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 2,180,635 cells/mL in August 2025.

Is it safe to swim in Nelson Reservoir right now?

Nelson Reservoir is not recommended for swimming today. Satellite imagery measured 299,353 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 Nelson Reservoir have blue-green algae (cyanobacteria)?

Yes. Satellite monitoring measured 299,353 cells/mL of blue-green algae (cyanobacteria) at Nelson Reservoir 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 Nelson Reservoir?

No. Blue-green algae at Nelson Reservoir is above the safe-contact threshold (299,353 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 Nelson Reservoir have blue-green algae blooms?

Over 7 summers of satellite monitoring since 2020, Nelson Reservoir 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 2,180,635 cells/mL in August 2025.

How many Montana lakes and beaches are unsafe for swimming today?

Today, 19 Montana waters are above a safe-contact threshold and 2 more warrant caution, out of 135 monitored waters SwimZone tracks in the state.

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