Is San Luis Reservoir safe? Toxic algae status
Merced County, California. Harmful algal bloom (blue-green algae) reports from the California State Water Resources Control Board.
Data checked 24 October 2026. California data is updated daily.
17
bloom reports since 2016
9 of 11
years with a bloom reported (2016–2026)
9
advisories recommended: 6 Caution, 2 Warning, 1 Danger
Year by year
Each box shows bloom reports logged that year, coloured by the highest advisory recommended.
Reports at San Luis Reservoir are most often logged in January, March and May.
Recent reports
| Date | Location | Result |
|---|---|---|
| 21 May 2026 | Pacheco pass WB side | Bloom reported |
| 3 January 2023 | Dinosaur Point Boat Launch | Danger |
| 1 January 2023 | Basalt Boat Launch (lake-wide) | Warning |
| 1 January 2023 | Pacheco Pumping Plant | Warning |
| 6 June 2022 | Dinosaur Point Boat Launch | Caution |
| 28 March 2022 | Pacheco Pumping Plant | Caution |
| 28 March 2022 | Basalt Boat Launch | Caution |
| 20 December 2021 | Basalt Boat Launch | Bloom reported |
| 13 June 2021 | Waterbody name submitted:Agua Hedionda Lagoon;Landmark submitted:None | Caution |
| 4 May 2021 | Basalt Boat Launch | Caution |
| 11 May 2020 | San Luis Reservoir, Pacheco Pumping Plant | Bloom reported |
| 11 May 2020 | San Luis Reservoir, Basalt Boat Launch | Bloom reported |
What the results mean
Algae may be present. Stay away from scum and discoloured water, and keep pets and livestock from drinking it.
Toxins at higher levels. No swimming, and keep pets and children well away from the water.
High toxin levels. Stay out of the water and off the shoreline.
Dogs and toxic algae
Dogs face the greatest risk because they swallow water and lick algae off their fur. If a bloom has been reported, keep dogs out of the water and away from scum on the shore. If your dog swims in water that might have algae, rinse it with clean water straight away and call a vet if it seems unwell.
Nearby lakes with bloom records
Common questions
Is San Luis Reservoir safe to swim in?
The most recent entry in California's published data is from 21 May 2026: bloom reported (Pacheco pass WB side). Blooms have been recorded at San Luis Reservoir in 9 different years, so treat green, scummy or discoloured water with caution. For current conditions, check the California HABs Portal map and posted signs.
When does San Luis Reservoir get algae blooms?
In the state's records, reports are most often logged in January, March and May. Blooms are most likely in warm, calm weather, but they can appear at other times of year.
Source: California State Water Resources Control Board, official data. This page is independent and not affiliated with the State of California.