California
Toxic algae in Alameda County lakes
23 lakes and waterways in Alameda County have harmful algal bloom reports in California's official data. The most recent entry is for Lake Temescal: There are no bloom reports for Lake Temescal in the last 30 days of California's data. The most recent was a bloom report on 7 September 2026. Lake Temescal, San Francisco Bay and Quarry Lakes and 7 more have had blooms in three or more years.
Lakes with bloom history
Newest report first. Each lake has its own page with the full history and what the results mean.
- Lake Temescal Bloom reported
- San Francisco Bay Bloom reported
- Quarry Lakes Danger
- Lake Merritt Bloom reported
- San Francisco Bay Central Bloom reported
- Lake Del Valle Caution
- Lake Chabot Danger
- Redwood Creek Bloom reported
- Shadow Cliffs Lake Caution
- Del Valle Reservoir Caution
- Alameda Creek Quarry Ponds Danger
- Shadow Cliffs Reservoir Bloom reported
Other waterbodies with a report
These have one or two reports in a single year, which isn't enough history for a page of their own yet.
- Bethany Reservoir 2024
- Codornices Creek 2021
- Dyer Reservoir 2022
- High Valley Pond 2021
- Lakeshore Lake 2021
- Oakland Estuary 2022
- Oakland/Alameda Estuary 2022
- Ruby Hill Community Retention Pond 2026
- San Francisco Bay Lower 2023
- Welch Creek 2021
- Welch Creek Pond 2021
Source: California State Water Resources Control Board, official data, checked 24 October 2026. For today's conditions see the California HABs Portal map. No report doesn't mean a lake is safe: most water is only tested occasionally.