Is Horsethief Reservoir safe to swim?

As of July 20, 2026, Horsethief Reservoir in Valley County, Idaho is not recommended for swimming. Satellite imagery measured 1,946,718 cells/mL of blue-green algae on July 19, 2026, above the 100,000 cells/mL advisory threshold. Rainfall over the past week totaled 0.0 in, which can wash bacteria and nutrients into the water.

How this read is built

Horsethief Reservoir is a reservoir in Valley County, Idaho covering about 0.4 sq mi, the 63rd-largest of the 63 lakes SwimZone monitors in Idaho. Satellite imagery measured 1,946,718 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 7 summers of satellite monitoring since 2020, Horsethief Reservoir has reached blue-green algae advisory levels in 3 of them, most recently in July 2026. Its highest recorded reading was 1,946,718 cells/mL in July 2026.

Is it safe to swim in Horsethief Reservoir right now?

Horsethief Reservoir is not recommended for swimming today. Satellite imagery measured 1,946,718 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 Horsethief Reservoir have blue-green algae (cyanobacteria)?

Yes. Satellite monitoring measured 1,946,718 cells/mL of blue-green algae (cyanobacteria) at Horsethief 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 Horsethief Reservoir?

No. Blue-green algae at Horsethief Reservoir is above the safe-contact threshold (1,946,718 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 Horsethief Reservoir have blue-green algae blooms?

Over 7 summers of satellite monitoring since 2020, Horsethief Reservoir has reached blue-green algae advisory levels in 3 of them, most recently in July 2026. Its highest recorded reading was 1,946,718 cells/mL in July 2026.

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