Is Sweetwater Reservoir safe to swim?

As of July 20, 2026, Sweetwater Reservoir in San Diego County, California is not recommended for swimming. Satellite imagery measured 299,353 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

Sweetwater Reservoir is a reservoir in San Diego County, California covering about 1.4 sq mi, the 98th-largest of the 193 lakes SwimZone monitors in California. Satellite imagery measured 299,353 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, Sweetwater Reservoir has reached blue-green algae advisory levels in 5 of them, most recently in July 2025. This is a lake that blooms most summers. Its highest recorded reading was 1,103,832 cells/mL in September 2022.

Is it safe to swim in Sweetwater Reservoir right now?

Sweetwater Reservoir is not recommended for swimming today. Satellite imagery measured 299,353 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 Sweetwater Reservoir have blue-green algae (cyanobacteria)?

Yes. Satellite monitoring measured 299,353 cells/mL of blue-green algae (cyanobacteria) at Sweetwater 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 Sweetwater Reservoir?

No. Blue-green algae at Sweetwater Reservoir is above the safe-contact threshold (299,353 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 Sweetwater Reservoir have blue-green algae blooms?

Over 7 summers of satellite monitoring since 2020, Sweetwater Reservoir has reached blue-green algae advisory levels in 5 of them, most recently in July 2025. This is a lake that blooms most summers. Its highest recorded reading was 1,103,832 cells/mL in September 2022.

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

Today, 28 California waters are above a safe-contact threshold and 4 more warrant caution, out of 452 monitored waters SwimZone tracks in the state.

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