Tuolumne River algae bloom status

Tuolumne River algae bloom history

California's Water Boards hold 5 bloom reports for Tuolumne River going back to 2019. Reports came in during 4 of the 8 years since then, with the busiest year being 2022 (2 reports).

State staff recommended an advisory 1 time: 1 Caution. Caution is the lowest level and means people and pets should stay away from scum and discoloured water.

Reports have fallen in the last three years compared with the three before.

When do algae blooms happen at Tuolumne River?

At Tuolumne River, reports are most often logged in January, August and September.

About 80% 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 Tuolumne River algae reports

Recent reports came from 5 different locations around Tuolumne River, including NF ~0.6mi below Basin Cr. (536TMR003), Highway 120 Bridge and Pate valley. 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 Tuolumne 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 4 different years here, take particular care at Tuolumne River in January, August and September.

• 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

• Merced River24 km away, 11 reports, last in 2022
• Pine Mountain Lake26 km away, 6 reports, last in 2025
• Stanislaus River50 km away, 2 reports, last in 2020
• New Melones Reservoir52 km away, 4 reports, last in 2021
• West Walker River57 km away, 3 reports, last in 2025

All Tuolumne County lakes with algae reports

Questions about Tuolumne River

Is there algae in Tuolumne River right now?

There are no bloom reports for Tuolumne River in the last 30 days of California's data. The most recent was an algal mat alert sign on 12 August 2026 at NF ~0.6mi below Basin Cr. (536TMR003). 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 Tuolumne River?

The most recent entry in California's data is from 12 August 2026 at NF ~0.6mi below Basin Cr. (536TMR003): an algal mat alert sign.

Has Tuolumne River had toxic algae before?

Yes. Blooms are recorded in 4 of the 8 years from 2019 to 2026, most often in January, August and September.

Can dogs swim in Tuolumne 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 Tuolumne 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.