New Melones Reservoir algae bloom history
California's Water Boards hold 4 bloom reports for New Melones Reservoir going back to 2019. Reports came in every year since then, with the busiest year being 2020 (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.
Blooms have been recorded every year for the last 3 years in the data.
- 2019 1 report
- 2020 2 reports
- 2021 1 report
When do algae blooms happen at New Melones Reservoir?
At New Melones Reservoir, reports are most often logged in July and August.
About 100% 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 New Melones Reservoir algae reports
| Date | Location | Result |
|---|---|---|
| 21 July 2021 | below Camp 9 bridge | Caution |
| 25 August 2020 | New Melones Reservoir, Tuttle Town Recreation area | Bloom reported |
| 25 August 2020 | New Melones Reservoir, Glory Hole Recreation area | Bloom reported |
| 24 July 2019 | Not given | 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 New Melones 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 3 different years here, take particular care at New Melones Reservoir in July 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
- Stanislaus River10 km away, 2 reports, last in 2020
- Lake Tulloch20 km away, 8 reports, last in 2026
- Salt Spring Valley Reservoir27 km away, 3 reports, last in 2022
- Pine Mountain Lake32 km away, 6 reports, last in 2025
- Tuolumne River52 km away, 5 reports, last in 2026
Questions about New Melones Reservoir
Is there algae in New Melones Reservoir right now?
There are no bloom reports for New Melones Reservoir in the last 30 days of California's data. The most recent was a Caution advisory on 21 July 2021 at below Camp 9 bridge. 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 New Melones Reservoir?
The most recent entry in California's data is from 21 July 2021 at below Camp 9 bridge: a Caution advisory.
Has New Melones Reservoir had toxic algae before?
Yes. Blooms are recorded every year from 2019 to 2021, most often in July and August.
Can dogs swim in New Melones 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 New Melones 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.
About this data
This page uses official records from the California State Water Resources Control Board, which updated daily. Reports are grouped by waterbody, and New Melones Reservoir combines reports from Calaveras County and Tuolumne County. 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.