Is Johnson Valley Reservoir safe to swim?

As of July 20, 2026, Johnson Valley Reservoir in Sevier County, Utah is clear to swim. Satellite imagery detected no blue-green algae bloom as of July 19, 2026. Rainfall over the past week totaled 0.7 in, which can wash bacteria and nutrients into the water.

How this read is built

Johnson Valley Reservoir is a reservoir in Sevier County, Utah covering about 1.1 sq mi, the 23rd-largest of the 48 lakes SwimZone monitors in Utah. Satellite imagery detected no blue-green algae bloom as of July 19, 2026.

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, Johnson Valley Reservoir has reached blue-green algae advisory levels in 4 of them, most recently in June 2025. Its highest recorded reading was 1,465,895 cells/mL in July 2024.

Is it safe to swim in Johnson Valley Reservoir right now?

Johnson Valley 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 Johnson Valley Reservoir have blue-green algae (cyanobacteria)?

No blue-green algae bloom is detectable at Johnson Valley 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 Johnson Valley Reservoir?

No bloom is detected at Johnson Valley 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 Johnson Valley Reservoir have blue-green algae blooms?

Over 7 summers of satellite monitoring since 2020, Johnson Valley Reservoir has reached blue-green algae advisory levels in 4 of them, most recently in June 2025. Its highest recorded reading was 1,465,895 cells/mL in July 2024.

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