Users of San Gregorio State Beach should avoid the creek, as the algae presents serious health risks
July 1, 2022
  • Note: As of Aug. 31, 2022, recent testing confirms the presence of toxic algae in San Gregorio Beach lagoon and creek. The warnings below about the risk in bodies of water on the Coastside remain in effect.

    Redwood City – Water sampling at San Gregorio State Beach near Half Moon Bay has revealed the presence of toxic algae in San Gregorio Creek lagoon. In coordination with the San Francisco Bay Quality Control Board, San Mateo County’s Environmental Health Services has posted notices in publicly accessible areas of the state beach, warning users not to enter the creek or allow children or pets to do so.

    Harmful algal blooms (HABs) produce toxins that are potentially fatal to people, pets, and livestock. The compounds produced by bacteria in the algae can cause numerous health effects in humans and animals that range from stomach pain to nerve damage and, in some circumstances, death.

    Testing in San Gregorio Creek lagoon revealed the presence of algal mats, which are a type of bloom that grows on bottom surfaces. These mats can be attached to the bottom or become detached and float in the water or on the surface and wash up on shore.

    The waters and banks of San Gregorio State Beach lagoon and the creek upstream for a quarter mile should be avoided due to risks from the algae. While testing occurs regularly on beaches and adjacent bodies of water, with warnings posted, any standing or slow moving fresh or brackish water with visible signs of algae growth should be avoided this summer.

    Adults and children should not touch, eat, or swallow any algal mats. Dogs should not eat algal mats or drink from the water (dogs are not permitted on San Gregorio State Beach). Call a doctor or veterinarian immediately if a person or pet becomes sick after coming into contact with or ingesting algae or water from San Gregorio Creek or elsewhere where algae is visible.

    More information about harmful algal blooms is available here:

    “Harmful algal blooms can be very dangerous,” said Heather Forshey, director of Environmental Health Services. “As families enjoy the water this summer, we urge everyone to be especially mindful of children and pets near any body of water where there is visible algae.”

    The next water sampling event for San Gregorio Creek lagoon is scheduled for mid-August, so users of the state beach should avoid the lagoon, creek, and nearby areas until the water is declared free of toxic algae and the posted notices are removed.

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    Media Contact

    Preston Merchant
    Communications Officer
    San Mateo County Health
    (650) 867-1661