Is Gunlock Reservoir safe to swim?

As of July 20, 2026, Gunlock Reservoir in Washington County, Utah is not recommended for swimming. Satellite imagery measured 354,898 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

Gunlock Reservoir is a reservoir in Washington County, Utah covering about 0.4 sq mi, the 48th-largest of the 48 lakes SwimZone monitors in Utah. Satellite imagery measured 354,898 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, Gunlock Reservoir has reached blue-green algae advisory levels in 2 of them, most recently in June 2023. Blooms here have been occasional rather than annual. Its highest recorded reading was 420,749 cells/mL in June 2023.

Is it safe to swim in Gunlock Reservoir right now?

Gunlock Reservoir is not recommended for swimming today. Satellite imagery measured 354,898 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 Gunlock Reservoir have blue-green algae (cyanobacteria)?

Yes. Satellite monitoring measured 354,898 cells/mL of blue-green algae (cyanobacteria) at Gunlock 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 Gunlock Reservoir?

No. Blue-green algae at Gunlock Reservoir is above the safe-contact threshold (354,898 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 Gunlock Reservoir have blue-green algae blooms?

Over 7 summers of satellite monitoring since 2020, Gunlock Reservoir has reached blue-green algae advisory levels in 2 of them, most recently in June 2023. Blooms here have been occasional rather than annual. Its highest recorded reading was 420,749 cells/mL in June 2023.

How many Utah lakes and beaches are unsafe for swimming today?

Today, 7 Utah waters are above a safe-contact threshold and 0 more warrant caution, out of 53 monitored waters SwimZone tracks in the state.

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