San Antonio Reservoir algae bloom status

San Antonio Reservoir algae bloom history

California's Water Boards hold 7 bloom reports for San Antonio Reservoir going back to 2017. Reports came in every year since then, with the busiest year being 2018 (3 reports).

State staff recommended an advisory 5 times: 1 Danger and 4 Caution. Danger is the highest level and means people and pets were told to stay out of the water and off the shore.

Blooms have been recorded every year for the last 5 years in the data.

That puts San Antonio Reservoir in the top 20% of the 322 California lakes on this site for the number of years with blooms.

When do algae blooms happen at San Antonio Reservoir?

At San Antonio Reservoir, reports are most often logged in January, May and June.

About 71% of reports fall between May and September, when warm, calm, sunny weather lets blue-green algae multiply.

Blooms can appear, move with the wind and fade within hours or days, so the time of year is only a guide.

Recent San Antonio Reservoir algae reports

What the California results mean

California recommends one of three advisory levels when a bloom is checked, matched to the signs posted at the shore.

Caution

Toxins may be present. Stay away from scum and keep pets and livestock from drinking the water.

Warning

Toxins are at higher levels. No swimming, and keep children and pets away from the water.

Danger

High toxin levels. Stay out of the water and off the shore until further notice.

Bloom reported

A bloom was reported, often by a member of the public, without a formal advisory level.

Is San Antonio Reservoir safe for dogs?

Dogs are at the greatest risk from toxic algae. They swallow water while swimming, eat scum on the shore and lick algae off their fur afterwards, and a small amount can kill a dog within hours. With blooms recorded in 5 different years here, take particular care at San Antonio Reservoir in January, May and June.

• Keep your dog on a lead near water that looks green, scummy, foamy or discoloured, and away from algae washed up on the shore.
• If your dog gets in, rinse it straight away with clean water and don't let it lick its coat.
• Call a vet immediately if your dog vomits, drools, has diarrhoea, staggers, shakes or seems weak after being near the water.

Read the full guide to toxic algae and dogs: symptoms, what to do in the first minutes and how vets treat it.

Health effects of toxic algae

Blue-green algae (cyanobacteria) can produce toxins that affect the liver, nervous system and skin. Swallowing water during a bloom can cause stomach pain, vomiting and diarrhoea. Skin contact can cause rashes, and breathing spray from the water can irritate the eyes, nose and throat. Children are at higher risk because they are smaller and more likely to swallow water.

How to spot a bloom

Toxic blooms can look like pea soup, spilled paint or green streaks on the surface, and they can also be blue, brown, white or red. Scum often collects along the shore downwind. If you can't see your feet in knee-deep water, or you aren't sure, stay out.

Don't drink untreated lake water, and don't boil it to make it safe: boiling doesn't destroy algal toxins. If you feel unwell after contact with the water, contact a doctor or Poison Control on 1-800-222-1222.

Nearby lakes with algae reports

• Lake San Antonio1 km away, 6 reports, last in 2026
• Lake Nacimiento6 km away, 3 reports, last in 2022
• Atascadero Lake45 km away, 3 reports, last in 2026

All Monterey County lakes with algae reports

Questions about San Antonio Reservoir

Is there algae in San Antonio Reservoir right now?

There are no bloom reports for San Antonio Reservoir in the last 30 days of California's data. The most recent was a bloom report on 1 February 2021. California's data is checked every day, so for today's conditions check the California HABs Portal map and the signs at the shore.

When was the last algae bloom at San Antonio Reservoir?

The most recent entry in California's data is from 1 February 2021: a bloom report.

Has San Antonio Reservoir had toxic algae before?

Yes. Blooms are recorded every year from 2017 to 2021, most often in January, May and June.

Can dogs swim in San Antonio Reservoir?

Keep dogs out of the water whenever a bloom is reported or the water looks green, scummy or discoloured. Dogs are the animals most often killed by algal toxins because they swallow water and lick their fur. If your dog swims in water that might have algae, rinse it with clean water and call a vet if it vomits, drools, staggers or seems weak.

Can you eat fish caught in San Antonio Reservoir during a bloom?

If you keep fish caught during a bloom, health agencies advise removing the guts and liver and rinsing the fillets in clean water before cooking, because toxins collect most in the organs. Some agencies advise not eating fish from an active bloom at all, so follow local guidance.