Is Hetch Hetchy Reservoir safe to swim?

As of August 20, 2026, Hetch Hetchy Reservoir in Tuolumne County, California has no advisory posted. EPA's CyAN satellite detected no blue-green algae bloom in the latest clear image (August 19, 2026), which is an estimate, not a guarantee. Modeled rainfall over the past week was about 0.0 in (Open-Meteo, a gridded estimate that can differ from a local gauge), which can wash bacteria and nutrients into the water.

How this read is built

Hetch Hetchy Reservoir is a reservoir in Tuolumne County, California covering about 3.1 sq mi, the 43rd-largest of the 186 lakes SwimZone monitors in California. EPA's CyAN satellite detected no blue-green algae bloom in the latest clear image (August 19, 2026), which is an estimate, not a guarantee.

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 EPA CyAN satellite imagery since 2020, Hetch Hetchy Reservoir has not shown a satellite-estimated bloom-level signal. This is an estimate, not a guarantee, and conditions can change quickly, so check the current read and official advisories before you swim.

It is currently peak season for blue-green algae, when warm, calm water lets blooms form fastest. This water has no recorded advisory-level bloom in its satellite history, which makes it a comparatively steady summer swim spot; conditions still change between samples, so check the current read before you go.

Is it safe to swim in Hetch Hetchy Reservoir right now?

Hetch Hetchy Reservoir has no advisory posted today. EPA's CyAN satellite detected no blue-green algae bloom in the latest clear image (August 19, 2026), which is an estimate, not a guarantee. Conditions can change quickly with wind, heat, and rain, so check the posted signs and look at the water before you get in.

Does Hetch Hetchy Reservoir have blue-green algae (cyanobacteria)?

EPA's CyAN satellite detected no blue-green algae bloom at Hetch Hetchy Reservoir in the latest clear image (August 19, 2026). This is a satellite estimate below the sensor's detection limit, not a guarantee: cyanobacteria can appear within hours in warm, calm, nutrient-rich water, so look before you swim.

Can dogs swim in Hetch Hetchy Reservoir?

No bloom is detected at Hetch Hetchy 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 Hetch Hetchy Reservoir have blue-green algae blooms?

Over 7 summers of EPA CyAN satellite imagery since 2020, Hetch Hetchy Reservoir has not shown a satellite-estimated bloom-level signal. This is an estimate, not a guarantee, and conditions can change quickly, so check the current read and official advisories before you swim.

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

Today, 0 California waters are above a safe-contact threshold and 21 more warrant caution, out of 445 monitored waters SwimZone tracks in the state.

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

• Blue-green algae and dogs→
• What does blue-green algae look like?→
• When is algae season?→