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Beach and Creek Mouth Monitoring


Water Sampling

Water samples from natural recreational waters in San Mateo County are sampled each week for concentrations of indicator bacteria.

Subscribe to beach and creek sampling email updates here. 

If concentrations of indicator bacteria exceed State or County standards, the area is posted to warn users that they may become ill if they engage in water contact activities in the posted area.

As a reminder, we advise against swimming in ocean waters during and 3 days after a storm. When swimming in ocean waters, always stay at least 300 feet from a flowing storm drain or creek mouth.

Join the Monday morning San Mateo County Volunteer Sampling Team and help protect beach users.  For more information, see the Volunteer Opportunities in the FAQ section below.

To view current data and posted sites, please view the map here: San Mateo County Interactive Beach Map

The State Water Resources Control Board has statewide recreational water sampling locations and historical data available to the public: Safe to Swim Map


The following list was last updated Wednesday, July 17, 2024, based on samples collected Monday, July 15, 2024. 

Subscribe to beach and creek sampling email updates here. 

Certain beaches and creeks may not have been sampled this week due to closures or inaccessibility. Sites that were not sampled are shown in gray on the San Mateo County Interactive Beach Map (view in desktop mode if map is unresponsive).

The following locations have elevated levels of indicator bacteria and are posted with warning and/or closure signs. 

Ocean Beaches
  • Linda Mar Beach at San Pedro Creek
  • Fitzgerald Marine Reserve
  • Pillar Point #5 (Capistrano Beach)
  • Pillar Point #9 (Outer Harbor)
Creeks (Where they meet or cross the beach)
  • Pilarcitos Creek
Bay Beaches  
  • Lakeshore Park
  • Erckenbrack Park
  • Gull Park

Signs limiting the recreational use of these waters have been posted and will remain in place until the level of indicator bacteria drop below State thresholds.  You can also check for closures and postings on San Mateo County’s Beach/Creek Posting Hotline announcement at (650) 599-1266.  Read on for more information on the County’s Recreational Water Quality Monitoring Program, including:

  • Description of the County’s Monitoring Program
  • Posting Criteria
  • Explanation of Signs
  • What Waters to Avoid
  • Description of Exposure Symptoms
  • Volunteer Opportunities

Recreational Water Quality Monitoring in San Mateo County

Thousands of people use San Mateo County swimming areas annually. On any one day, people can be found swimming, surfing, body-surfing, diving, or just walking through the water’s edge at any of the Ocean or Bay water beaches in San Mateo County.

Generally, these waters are clean and safe for swimming. However, to ensure public health and safety, San Mateo County Environmental Health Services, with the help of volunteer samplers, regularly test these waters for contamination. If problems are found the public is notified and steps are taken to control the source of contamination.


How often does the County test the water?

San Mateo County routinely tests 43 sites each week, including ocean beaches, creek mouths (where the creek meets or crosses the beach), and Bay/lagoon beaches. Sites are sampled every Monday morning by a combination of County staff and volunteers, primarily associated with the local chapter of the Surfrider Foundation.  The County evaluates the potential presence of disease-causing organisms by testing for “indicator” bacteria. When high indicator bacteria levels are found, the site is posted with a warning sign and is re-tested to confirm the results.

What criteria are used to evaluate the sample results?

Samples are analyzed by the San Mateo County Public Health Laboratory for three bacteria indicator parameters specified by State regulations. These indicator bacteria include total Coliform, Escherichia coli (E. coli), and Enterococcus. Sample analytical results for these indicator bacteria are evaluated against both a single-sample maximum and against the geometric mean of at least 5 weekly samples (collected within a 30 day period) for each parameter. The State-specified trigger levels for each parameter are, for the single-sample maximum and geometric mean respectively, total Coliform (10,000 or 1,000), E. coli (400 or 200), and Enterococcus (104 or 35), all in units of most-probable-number of colony-forming-units per 100 milliliters (mpn/100ml).

What do the posted signs mean?

If a site exceeds state standards, the site is posted with Water Quality Advisory/Warning signs. Advisory or Warning signs have red and black lettering on a white background and state: “These waters are contaminated and are not suitable for swimming or other water contact activities”. Warning signs are posted when bacterial standards have exceeded state swimming standards but there is no confirmed source of contamination.

Persons using recreational water posted by County Health Officials do so at their own risk. These signs are posted to alert the public that there may be a risk of illness associated with swimming in contaminated waters and are an advisory rather than a quarantine or beach closure.

When there is a confirmed sewage spill that directly affects a beach, the beach is posted as CLOSED. The beach is not re-opened until it has been tested for indicator bacteria as described above and meets minimum standards for human contact.

What waters should I avoid?

Avoid waters which are posted with warning signs or which look unhealthy.

Obvious signs of unhealthy water include stagnant, frothy black or brown water, or a cloudy white or off-white colored water, or water with an offensive odor. An oily slick or sheen may indicate gasoline or oil products in the water.

When swimming at ocean beaches with creeks that cross the beach, it is best practice to avoid recreational activity in the creek, as well as in the ocean water in the immediate area where the creek meets the ocean. While the creek water may look calmer and safer for you or your child to play in, it typically has higher bacteria counts than the ocean water.

Avoid going in the water during rainstorms. Swimming in areas close to urban streams and storm drain outlets should be avoided during and for up to three days after the end of a rainfall event due to the increased levels of bacteria caused by rainfall runoff.

Can I get sick from swimming in the ocean or streams?

Swimming in water that is contaminated by sewage, animal waste, chemicals, or other types of contamination can result in minor illnesses such as; skin rashes, eye and ear infections, and upset stomachs, or more severe and potentially life-threatening diseases such as cholera, hepatitis, and meningitis.

Sources of contamination include sewage and industrial waste discharges and contributions from residential sources such as storm drain runoff and domestic animal wastes. Sewage can come from failing septic systems, discharges to storm drains, or cracked or blocked sewer lines in sewered areas. Large concentrations of birds or marine mammals may also be sources of contamination which result in illness.

In spite of the heavy recreational water use in San Mateo County, there have been very few reports of illnesses linked to water contact. Water-borne illnesses such as cholera and meningitis that have life-threatening impacts in other countries have not been shown to be a problem at local swimming areas.

Typical symptoms of the illness due to the presence of disease-causing organisms are: skin rashes, ear and eye infections, gastrointestinal upsets (diarrhea, nausea, stomach aches), open wound infections, continuing headaches.

What should I do if I think I have gotten sick from the water?

See your doctor based on the symptoms you experience. If you think your illness was due to water contact within San Mateo County, please contact County Environmental Health Services at (650) 372-6200 to make a report. The information you should provide is: name, dates and areas you were swimming, symptoms, reasons you believe you got sick from swimming, and a telephone number that you or your health care provider can be reached for any other information we may need.

Volunteer Opportunities

Join the Monday morning San Mateo County Volunteer Water Sampling Team and help protect beach users by sampling beach, creek, and bay waters. 

  • San Mateo County Environmental Health Services is looking for volunteers that we will train to collect beach, creek, and lagoon water samples Monday mornings to be tested for indicator bacteria.
  • County staff will provide training, needed equipment, and supplies for the sampling.
  • Samples are collected every Monday morning, and volunteers drop samples off at convenient locations in either Half Moon Bay or Pacifica for transport by County staff to the Public Health Laboratory for analysis.
  • Sampling occurs year-round, and the frequency, duration, and extent of volunteering is flexible.

Note: You must provide your own transportation in order to collect and drop off water samples.

If you are interested in gaining valuable experience helping to protect the public at San Mateo County recreational waters, please contact San Mateo County Environmental Health staff at

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This page was last updated 5/8/24

IMPORTANT NOTICE: ANNUAL MUSSEL QUARANTINE IS IN EFFECT FROM MAY 1st – OCTOBER 31st. For more information, read California Department of Public Health’s press release

Approximately every two weeks Environmental Health Services staff collects mussel and plankton samples along San Mateo County’s coastline as part of the State’s Mussel Sampling Program.