Is American Falls Reservoir safe to swim?

As of July 20, 2026, American Falls Reservoir in Bannock County, Idaho is clear to swim. Satellite imagery detected no blue-green algae bloom as of July 19, 2026. Rainfall over the past week totaled 0.0 in, which can wash bacteria and nutrients into the water.

How this read is built

American Falls Reservoir is a reservoir in Bannock County, Idaho covering about 56 sq mi, the 3rd-largest of the 63 lakes SwimZone monitors in Idaho. Satellite imagery detected no blue-green algae bloom as of July 19, 2026. 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, American Falls Reservoir has reached blue-green algae advisory levels in 5 of them, most recently in August 2025. This is a lake that blooms most summers. Its highest recorded reading was 1,689,285 cells/mL in September 2024.

Is it safe to swim in American Falls Reservoir right now?

American Falls Reservoir is clear to swim today. Satellite imagery detected no blue-green algae bloom as of July 19, 2026. Conditions can change quickly with wind, heat, and rain, so check the posted signs and look at the water before you get in.

Does American Falls Reservoir have blue-green algae (cyanobacteria)?

No blue-green algae bloom is detectable at American Falls Reservoir as of July 19, 2026 (below the satellite's roughly 10,000 cells/mL detection limit). Cyanobacteria can still appear within hours in warm, calm, nutrient-rich water, so look before you swim.

Can dogs swim in American Falls Reservoir?

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

Over 7 summers of satellite monitoring since 2020, American Falls Reservoir has reached blue-green algae advisory levels in 5 of them, most recently in August 2025. This is a lake that blooms most summers. Its highest recorded reading was 1,689,285 cells/mL in September 2024.

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