Is Hells Canyon Reservoir safe to swim?

As of July 20, 2026, Hells Canyon Reservoir in Idaho is not recommended for swimming. Satellite imagery measured 831,195 cells/mL of blue-green algae on July 19, 2026, above the 100,000 cells/mL advisory threshold. Rainfall over the past week totaled 0.1 in, which can wash bacteria and nutrients into the water.

How this read is built

Hells Canyon Reservoir is a reservoir in Idaho covering about 3.6 sq mi, the 22nd-largest of the 63 lakes SwimZone monitors in Idaho. Satellite imagery measured 831,195 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 6 summers of satellite monitoring since 2020, Hells Canyon Reservoir has reached blue-green algae advisory levels in 6 of them, most recently in July 2026. This is a lake that blooms most summers. Its highest recorded reading was 1,272,047 cells/mL in August 2022.

Is it safe to swim in Hells Canyon Reservoir right now?

Hells Canyon Reservoir is not recommended for swimming today. Satellite imagery measured 831,195 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 Hells Canyon Reservoir have blue-green algae (cyanobacteria)?

Yes. Satellite monitoring measured 831,195 cells/mL of blue-green algae (cyanobacteria) at Hells Canyon Reservoir 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 Hells Canyon Reservoir?

No. Blue-green algae at Hells Canyon Reservoir is above the safe-contact threshold (831,195 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 Hells Canyon Reservoir have blue-green algae blooms?

Over 6 summers of satellite monitoring since 2020, Hells Canyon Reservoir has reached blue-green algae advisory levels in 6 of them, most recently in July 2026. This is a lake that blooms most summers. Its highest recorded reading was 1,272,047 cells/mL in August 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 Hells Canyon 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 Hells Canyon Reservoir, and a recent-rainfall overlay, all refreshed daily. It was last updated July 20, 2026.