Merced River algae bloom history
California's Water Boards hold 11 bloom reports for Merced River going back to 2020. Reports came in every year since then, with the busiest year being 2021 (6 reports).
State staff recommended an advisory 3 times: 3 Caution. Caution is the lowest level and means people and pets should stay away from scum and discoloured water.
Blooms have been recorded every year for the last 3 years in the data.
- 2020 1 report
- 2021 6 reports
- 2022 4 reports
When do algae blooms happen at Merced River?
At Merced River, reports are most often logged in June, August and September.
About 91% 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 Merced River algae reports
Recent reports came from 10 different locations around Merced River, including El Portal, Superintendents bridge and Pohono Bridge. A bloom in one cove or beach doesn't mean the whole lake is affected, and conditions can differ a short distance away.
| Date | Location | Result |
|---|---|---|
| 30 August 2022 | El Portal | Algal mat alert sign |
| 29 August 2022 | Superintendents bridge | Algal mat alert sign |
| 29 August 2022 | Pohono Bridge | Algal mat general awareness sign |
| 29 August 2022 | Happy Isles Bridge | Algal mat general awareness sign |
| 21 October 2021 | at top of Nevada Falls | Bloom reported |
| 5 September 2021 | Highway 140 and Briceburg Rd | Bloom reported |
| 30 August 2021 | at Saxon Creek | Bloom reported |
| 16 August 2021 | SF ~2.6mi above main stem | Caution |
| 23 June 2021 | NF below Bean Cr. | Caution |
| 22 June 2021 | SF ~2.6mi above main stem | Caution |
| 6 August 2020 | Merced River, at Briceburg | Bloom reported |
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 Merced 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 3 different years here, take particular care at Merced River in June, 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
- Tuolumne River24 km away, 5 reports, last in 2026
- Pine Mountain Lake38 km away, 6 reports, last in 2025
- Chowchilla River47 km away, 3 reports, last in 2023
- H.V. Eastman Lake51 km away, 13 reports, last in 2026
- Indian Lakes53 km away, 2 reports, last in 2020
Questions about Merced River
Is there algae in Merced River right now?
There are no bloom reports for Merced River in the last 30 days of California's data. The most recent was an algal mat alert sign on 30 August 2022 at El Portal. 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 Merced River?
The most recent entry in California's data is from 30 August 2022 at El Portal: an algal mat alert sign.
Has Merced River had toxic algae before?
Yes. Blooms are recorded every year from 2020 to 2022, most often in June, August and September.
Can dogs swim in Merced 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 Merced 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.
About this data
This page uses official records from the California State Water Resources Control Board, which updated daily. Reports are grouped by waterbody. The site was last rebuilt on 24 October 2026.
States only test some lakes, some of the time, so a lake without a recent report hasn't necessarily been checked. This page is independent, isn't affiliated with the State of California, and isn't health advice. View the source data.