San Diego River algae bloom status

San Diego River algae bloom history

California's Water Boards hold 16 bloom reports for San Diego River going back to 2018. Reports came in during 7 of the 9 years since then, with the busiest year being 2018 (3 reports).

State staff recommended an advisory 1 time: 1 Warning. Warning is the middle level and means toxins were high enough to advise against swimming.

Blooms have been recorded every year for the last 6 years in the data.

That puts San Diego River in the top 10% of the 318 California lakes on this site for the number of years with blooms.

When do algae blooms happen at San Diego River?

At San Diego River, reports are most often logged in May, June and August.

About 88% 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 Diego River algae reports

Recent reports came from 11 different locations around San Diego River, including San Diego River, Sefton Park, Upstream of San Diego Mission Road and Upstream end of Walker Preserve. A bloom in one cove or beach doesn't mean the whole lake is affected, and conditions can differ a short distance away.

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 Diego River 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 7 different years here, take particular care at San Diego River in May, June and August.

• 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

• Kumeyaay Lake8 km away, 2 reports, last in 2025
• Chollas Creek9 km away, 2 reports, last in 2025
• Mission Bay13 km away, 6 reports, last in 2026
• Sweetwater River15 km away, 3 reports, last in 2025
• Lindo Lake19 km away, 5 reports, last in 2024

All San Diego County lakes with algae reports

Questions about San Diego River

Is there algae in San Diego River right now?

There are no bloom reports for San Diego River in the last 30 days of California's data. The most recent was a bloom report on 19 June 2026 at San Diego River, Sefton Park. 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 Diego River?

The most recent entry in California's data is from 19 June 2026 at San Diego River, Sefton Park: a bloom report.

Has San Diego River had toxic algae before?

Yes. Blooms are recorded in 7 of the 9 years from 2018 to 2026, most often in May, June and August.

Can dogs swim in San Diego River?

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 Diego River 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.