Is Cooney Reservoir safe to swim?

As of July 20, 2026, Cooney Reservoir in Carbon County, Montana is clear to swim. Satellite imagery measured 14,800 cells/mL of blue-green algae on July 19, 2026, well below the 100,000 cells/mL advisory threshold. Rainfall over the past week totaled 0.6 in, which can wash bacteria and nutrients into the water.

How this read is built

Cooney Reservoir is a reservoir in Carbon County, Montana covering about 1.2 sq mi, the 59th-largest of the 124 lakes SwimZone monitors in Montana. Satellite imagery measured 14,800 cells/mL of blue-green algae on July 19, 2026, well 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, Cooney Reservoir has reached blue-green algae advisory levels in 3 of them, most recently in July 2026. Its highest recorded reading was 662,435 cells/mL in September 2021.

Is it safe to swim in Cooney Reservoir right now?

Cooney Reservoir is clear to swim today. Satellite imagery measured 14,800 cells/mL of blue-green algae on July 19, 2026, well 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 Cooney Reservoir have blue-green algae (cyanobacteria)?

Low levels. Satellite monitoring measured 14,800 cells/mL at Cooney Reservoir 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 Cooney Reservoir?

No bloom is detected at Cooney Reservoir 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 Cooney Reservoir have blue-green algae blooms?

Over 7 summers of satellite monitoring since 2020, Cooney Reservoir has reached blue-green algae advisory levels in 3 of them, most recently in July 2026. Its highest recorded reading was 662,435 cells/mL in September 2021.

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