Is Nicasio Reservoir safe to swim?

As of August 20, 2026, Nicasio Reservoir in Marin County, California has no advisory posted. EPA's CyAN satellite detected no blue-green algae bloom in the latest clear image (July 21, 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

Nicasio Reservoir is a reservoir in Marin County, California covering about 1.4 sq mi, the 98th-largest of the 186 lakes SwimZone monitors in California. EPA's CyAN satellite detected no blue-green algae bloom in the latest clear image (July 21, 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, Nicasio Reservoir has shown a satellite-estimated bloom-level signal in 6 of them, most recently in June 2026. It shows a bloom-level signal most summers. Its highest satellite estimate was about 480,000 cells/mL in September 2022. These are satellite estimates, not water samples or posted advisories; check your state or county health department for official status.

It is currently peak season for blue-green algae, when warm, calm water lets blooms form fastest. This lake has bloomed in most monitored summers (6 of 7), historically peaking around September. Early summer, before the water fully warms, has generally been the safer window here; check the current read before you go either way.

Is it safe to swim in Nicasio Reservoir right now?

Nicasio Reservoir has no advisory posted today. EPA's CyAN satellite detected no blue-green algae bloom in the latest clear image (July 21, 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 Nicasio Reservoir have blue-green algae (cyanobacteria)?

EPA's CyAN satellite detected no blue-green algae bloom at Nicasio Reservoir in the latest clear image (July 21, 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 Nicasio Reservoir?

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

Over 7 summers of EPA CyAN satellite imagery since 2020, Nicasio Reservoir has shown a satellite-estimated bloom-level signal in 6 of them, most recently in June 2026. It shows a bloom-level signal most summers. Its highest satellite estimate was about 480,000 cells/mL in September 2022. These are satellite estimates, not water samples or posted advisories; check your state or county health department for official status.

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 Nicasio 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 Nicasio 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?→