Arastradero Lake algae bloom status

Arastradero Lake algae bloom history

California's Water Boards hold 2 bloom reports for Arastradero Lake going back to 2024. Reports came in every year since then.

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

When do algae blooms happen at Arastradero Lake?

Reports are spread across the year, with 50% between May and September.

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

Recent Arastradero Lake 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 Arastradero Lake 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.

• 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.

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

• Alpine Pond8 km away, 3 reports, last in 2025
• Shoreline Lake10 km away, 3 reports, last in 2026
• Seal Slough22 km away, 3 reports, last in 2023
• Alameda Creek Quarry Ponds26 km away, 11 reports, last in 2021
• Quarry Lakes26 km away, 8 reports, last in 2026

All Santa Clara County lakes with algae reports

Questions about Arastradero Lake

Is there algae in Arastradero Lake right now?

There are no bloom reports for Arastradero Lake in the last 30 days of California's data. The most recent was a bloom report on 24 March 2025. 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 Arastradero Lake?

The most recent entry in California's data is from 24 March 2025: a bloom report.

Has Arastradero Lake had toxic algae before?

Yes. Blooms are recorded every year from 2024 to 2025.

Can dogs swim in Arastradero Lake?

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 Arastradero Lake 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.