California
Toxic algae in San Mateo County lakes
15 lakes and waterways in San Mateo County have harmful algal bloom reports in California's official data. The most recent entry is for Alpine Pond: There are no bloom reports for Alpine Pond in the last 30 days of California's data. The most recent was a bloom report on 27 August 2025 at Midpeninsula Regional Open Space District. Alpine Pond and Seal Slough 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.
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.
- Butano Creek 2024
- Central Lake 2022
- Foster City Marina Lagoon 2024
- Leslie Creek 2024
- O'Neill Slough 2017
- Pescadero Creek 2022
- Pilarcitos Creek 2018
- Quarry Pond 2025
- San Gregorio Creek 2022
- San Mateo Marina Lagoon 2025
- San Pedro Creek 2022
- Sausal Pond 2025
- Upper Emerald Lake 2020
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.