Is Big Salmon Lake safe to swim?

As of July 20, 2026, Big Salmon Lake in Flathead County, Montana carries a swim caution. Satellite imagery measured 26,852 cells/mL of blue-green algae on July 19, 2026, elevated but below the 100,000 cells/mL advisory threshold. Rainfall over the past week totaled 0.4 in, which can wash bacteria and nutrients into the water.

How this read is built

Big Salmon Lake is a lake in Flathead County, Montana covering about 1.5 sq mi, the 54th-largest of the 124 lakes SwimZone monitors in Montana. Satellite imagery measured 26,852 cells/mL of blue-green algae on July 19, 2026, elevated but 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, Big Salmon Lake has reached blue-green algae advisory levels in 1 of them, most recently in September 2025. Blooms here have been occasional rather than annual. Its highest recorded reading was 299,353 cells/mL in September 2025.

Is it safe to swim in Big Salmon Lake right now?

Big Salmon Lake carries a swim caution today. Satellite imagery measured 26,852 cells/mL of blue-green algae on July 19, 2026, elevated but 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 Big Salmon Lake have blue-green algae (cyanobacteria)?

Low levels. Satellite monitoring measured 26,852 cells/mL at Big Salmon 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 Big Salmon Lake?

Use caution. Blue-green algae at Big Salmon Lake are elevated (26,852 cells/mL on July 19, 2026). Dogs are far more sensitive to cyanobacteria than people, so keep them out of the water and away from surface scum.

How often does Big Salmon Lake have blue-green algae blooms?

Over 7 summers of satellite monitoring since 2020, Big Salmon Lake has reached blue-green algae advisory levels in 1 of them, most recently in September 2025. Blooms here have been occasional rather than annual. Its highest recorded reading was 299,353 cells/mL in September 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 Big Salmon 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 Big Salmon Lake, and a recent-rainfall overlay, all refreshed daily. It was last updated July 20, 2026.