Is Island Park Reservoir safe to swim?

As of July 20, 2026, Island Park Reservoir in Fremont County, Idaho carries a swim caution. Satellite imagery measured 20,220 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 0.2 in, which can wash bacteria and nutrients into the water.

How this read is built

Island Park Reservoir is a reservoir in Fremont County, Idaho covering about 11.8 sq mi, the 8th-largest of the 63 lakes SwimZone monitors in Idaho. Satellite imagery measured 20,220 cells/mL of blue-green algae on July 19, 2026, elevated but below the 100,000 cells/mL advisory threshold. 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, Island Park Reservoir has reached blue-green algae advisory levels in 6 of them, most recently in August 2025. This is a lake that blooms most summers. Its highest recorded reading was 1,839,344 cells/mL in August 2023.

Is it safe to swim in Island Park Reservoir right now?

Island Park Reservoir carries a swim caution today. Satellite imagery measured 20,220 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 Island Park Reservoir have blue-green algae (cyanobacteria)?

Low levels. Satellite monitoring measured 20,220 cells/mL at Island Park 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 Island Park Reservoir?

Use caution. Blue-green algae at Island Park Reservoir are elevated (20,220 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 Island Park Reservoir have blue-green algae blooms?

Over 7 summers of satellite monitoring since 2020, Island Park Reservoir has reached blue-green algae advisory levels in 6 of them, most recently in August 2025. This is a lake that blooms most summers. Its highest recorded reading was 1,839,344 cells/mL in August 2023.

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