Is Big Johnson Reservoir safe to swim?

As of July 20, 2026, Big Johnson Reservoir in El Paso County, Colorado is not recommended for swimming. Satellite imagery measured 701,105 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

Big Johnson Reservoir is a reservoir in El Paso County, Colorado covering about 0.4 sq mi, the 100th-largest of the 107 lakes SwimZone monitors in Colorado. Satellite imagery measured 701,105 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, Big Johnson Reservoir has reached blue-green algae advisory levels in 6 of them, most recently in June 2026. This is a lake that blooms most summers. Its highest recorded reading was 4,559,351 cells/mL in September 2024.

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

Big Johnson Reservoir is not recommended for swimming today. Satellite imagery measured 701,105 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 Big Johnson Reservoir have blue-green algae (cyanobacteria)?

Yes. Satellite monitoring measured 701,105 cells/mL of blue-green algae (cyanobacteria) at Big Johnson 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 Big Johnson Reservoir?

No. Blue-green algae at Big Johnson Reservoir is above the safe-contact threshold (701,105 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 Big Johnson Reservoir have blue-green algae blooms?

Over 7 summers of satellite monitoring since 2020, Big Johnson Reservoir has reached blue-green algae advisory levels in 6 of them, most recently in June 2026. This is a lake that blooms most summers. Its highest recorded reading was 4,559,351 cells/mL in September 2024.

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

Today, 26 Colorado waters are above a safe-contact threshold and 3 more warrant caution, out of 113 monitored waters SwimZone tracks in the state.

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