Is Thompson Lake safe to swim?

As of July 20, 2026, Thompson Lake in Kootenai County, Idaho is clear to swim. Satellite imagery measured 5,804 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 0.0 in, which can wash bacteria and nutrients into the water.

How this read is built

Thompson Lake is a lake in Kootenai County, Idaho covering about 0.5 sq mi, the 62nd-largest of the 63 lakes SwimZone monitors in Idaho. Satellite imagery measured 5,804 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, Thompson Lake has reached blue-green algae advisory levels in 6 of them, most recently in September 2025. This is a lake that blooms most summers. Its highest recorded reading was 1,272,047 cells/mL in July 2022.

Is it safe to swim in Thompson Lake right now?

Thompson Lake is clear to swim today. Satellite imagery measured 5,804 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 Thompson Lake have blue-green algae (cyanobacteria)?

Low levels. Satellite monitoring measured 5,804 cells/mL at Thompson Lake 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 Thompson Lake?

No bloom is detected at Thompson Lake 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 Thompson Lake have blue-green algae blooms?

Over 7 summers of satellite monitoring since 2020, Thompson Lake has reached blue-green algae advisory levels in 6 of them, most recently in September 2025. This is a lake that blooms most summers. Its highest recorded reading was 1,272,047 cells/mL in July 2022.

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

Today, 9 Idaho waters are above a safe-contact threshold and 1 more warrant caution, out of 66 monitored waters SwimZone tracks in the state.

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