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Harmful Algal Blooms


A harmful algal bloom (HAB) is a buildup of blue-green algae that creates a green, blue-green, white or brown coloring on the surface of ponds, lakes and other slow-moving waterways, sometimes occurring as mats or scum.

Blooms are most common between June and September. They grow in warm, stagnant and nutrient-rich water, and are becoming increasingly common both in California and nationwide.

Human exposure to water containing toxic harmful algal blooms, for example by direct body contact or ingestion, can result in a number of symptoms including the following:

  • Rashes
  • Eye, nose, mouth or throat irritation
  • Allergic reactions
  • Headache
  • Gastrointestinal upset, such as abdominal pain, nausea, vomiting and diarrhea


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  • Discovery Bay Information

    May 23, 2023

    Pre-holiday assessments have been organized by the State Waterboards which involve sampling at popular recreational waterbodies prior to major holiday weekends, including Memorial Day, Independence Day, and Labor Day, and working with waterbody managers and the county to determine most appropriate signage to inform visitors of potential freshwater harmful algal blooms (FHABs). You can find the results from sampling on the FHAB Incident Report Map. Results are distributed the Friday before the noted major holiday weekends and provide recommendations for posting signs at popular public access locations around the waterbody.

    For More Information Please Visit:



  • Other sites in Contra Costa County have experienced harmful algal blooms




  • FAQs

    Is it safe for me to go in the water?

    It depends where you are and if there are advisories posted in the area.

    A CAUTION sign means:

    • Do not swim or wade near algae or scum
    • Keep your children away from algae in the water or on the shore
    • Do not drink the water or use it for cooking
    • Do not let pets or livestock go into or drink the water or eat scum on the shoreline
    • Do not eat shellfish from the water

    A WARNING sign means:

    • No swimming
    • Stay away from scum and cloudy or discolored water
    • Do not use the water for drinking or cooking
    • Do not let pets or livestock go into or drink the water, or go near the scum on the shoreline
    • Do not eat shellfish from the water
    • For fish caught here, throw away guts and clean fillets with tap water or bottled water before cooking

    A DANGER sign means:

    • Stay out of the water until further notice. Do not touch scum in the water or on shore
    • Do not let pets or livestock drink or go into the water, or go near the scum on shore
    • Do not eat fish or shellfish from the water
    • Do not use the water for drinking or cooking. Boiling or filtering will not make the water safe

    Exposure to HAB can cause rashes, skin and eye irritation, allergic reactions, gastrointestinal upset and other effects. At high concentrations, exposure can result in serious illness or death. For more information, see California Water Quality Monitoring Council HAB Portal.

    What about my pets?

    Keep pets out of water with harmful algal blooms. Dogs are especially vulnerable to getting sick and there have been reports of dogs dying following exposure associated with drinking the water or licking algae from their fur after wading or swimming in water with blooms.

    Who should I contact if I swam in the water and now have symptoms?

    If you think you or someone else is displaying symptoms of HAB poisoning, contact your healthcare provider or the California Poison Center Help Hotline at 1-800-222-1222.

    What can I do to get rid of harmful algae?

    We do not recommend attempting to treat or remove the algae. Harmful algae blooms are part of the environment's food chain and eventually dissipate on their own. Chemical treatment is often ineffective and can result in more toxins being released into the water when the algae cells die, and can have unintended effects on other species.

    Algal blooms result from changes in water conditions that encourage sudden growth of the species. The best way to reduce and prevent blooms is to reduce water pollution, particularly from runoff containing fertilizers or pesticides. Pick up trash dumped in waterways, and make sure all household sewer systems are working properly. Minimize the use of fertilizers whenever possible.

    Do I neeed to wash my boat after it has been in HAB infested waters?

    There are no specific actions regarding boats that have been in bloom-infested waters, but the Division of Boating & Waterways has guidance on preventing invasive species: Clean, Drain, and Dry Boat Cleaning Procedures


  • Pets & Vets

    Keep pets out of water with harmful algae blooms. Dogs are especially vulnerable to getting sick and there have been reports of dogs dying following exposure associated with drinking the water or licking algae from their fur.


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  • How to Avoid Toxic Exposure

    • Always assume that a algal bloom is toxic
    • Do not swim and avoid jet-skiing, wind surfing or water skiing in water with scum layers or blooms
    • Do not allow children or pets to swim in water with scum layers or blooms
    • Do not drink or use water containing algae. Boiling will not remove algal toxins
    • Do not eat fish or shellfish caught or harvested in a bloom area
    • Respect any water-body closures by public health authorities

    What to Do When Exposed?

    • If you come into contact with a bloom, wash your skin and hair thoroughly. If wearing a swimsuit, pay careful attention to skin under the swimsuit
    • If your pet comes into contact with a bloom, wash it thoroughly with clean water to prevent algae ingestion while your pet cleans itself. If you believe your pet may have ingested dry or liquid algae contact a licensed veterinarian right away
    • If you ingest algae or suspect that you may have and begin to experience symptoms, contact your healthcare provider or California Poison Control 1-800-222-1222


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