Is Lake Temescal safe? Toxic algae status
Alameda 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.
10
bloom reports since 2016
9 of 11
years with a bloom reported (2016–2026)
6
advisories recommended: 4 Caution, 0 Warning, 2 Danger
Year by year
Each box shows bloom reports logged that year, coloured by the highest advisory recommended.
Reports at Lake Temescal are most often logged in January, May and June.
Recent reports
| Date | Location | Result |
|---|---|---|
| 7 September 2026 | Rio Dell (more specific location pending follow up with RP) | Bloom reported |
| 1 January 2023 | Lake Temescal | Danger |
| 2 May 2022 | East Bay Regional Parks | Caution |
| 27 December 2021 | — | Bloom reported |
| 4 January 2021 | — | Bloom reported |
| 6 January 2020 | Lake Temescal | Caution |
| 17 June 2019 | Lake Temescal | Caution |
| 2 January 2018 | Lake Temescal | Bloom reported |
| 6 June 2017 | Lake Temescal | Caution |
| 28 June 2016 | Lake Temescal | Danger |
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.
Common questions
Is Lake Temescal safe to swim in?
The most recent entry in California's published data is from 7 September 2026: bloom reported (Rio Dell (more specific location pending follow up with RP)). Blooms have been recorded at Lake Temescal 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 Lake Temescal get algae blooms?
In the state's records, reports are most often logged in January, May and June. 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.