Is Katy Lake safe to swim?

As of July 20, 2026, Katy Lake in Sheridan County, Montana is not recommended for swimming. Satellite imagery measured 5,885,495 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

Katy Lake is a lake in Sheridan County, Montana covering about 0.5 sq mi, the 107th-largest of the 124 lakes SwimZone monitors in Montana. Satellite imagery measured 5,885,495 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, Katy Lake has reached blue-green algae advisory levels in 6 of them, most recently in July 2025. This is a lake that blooms most summers. Its highest recorded reading was 7,178,323 cells/mL in August 2023.

Is it safe to swim in Katy Lake right now?

Katy Lake is not recommended for swimming today. Satellite imagery measured 5,885,495 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 Katy Lake have blue-green algae (cyanobacteria)?

Yes. Satellite monitoring measured 5,885,495 cells/mL of blue-green algae (cyanobacteria) at Katy 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 Katy Lake?

No. Blue-green algae at Katy Lake is above the safe-contact threshold (5,885,495 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 Katy Lake have blue-green algae blooms?

Over 7 summers of satellite monitoring since 2020, Katy Lake has reached blue-green algae advisory levels in 6 of them, most recently in July 2025. This is a lake that blooms most summers. Its highest recorded reading was 7,178,323 cells/mL in August 2023.

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