Body of News Release
Marin County, CA – Water quality at Marin County beaches remained strong during the 2025 monitoring season, according to the 36th annual report from Heal the Bay. The annual report rates water quality at beaches along the U.S. West Coast using bacterial monitoring data collected by local agencies.
The majority of Marin beaches continued to receive strong A and B grades during the summer dry weather. In fact, 78% of Marin beaches received A’s and B’s. However, these results were far below the historical average of 97%.
This year’s report also showed that 91% of California beaches received A and B grades in summer dry conditions; 81% in winter dry conditions; and 61% during wet weather. This year, Northern California in general saw a lot less rain about 30% below the usual 10-year average. While the drier conditions appeared to benefit wet weather grades regionally, Marin saw the opposite trend during summer dry weather: fewer beaches earned safe-to-swim grades compared to recent years. That unusual pattern since lower rainfall is typically associated with better water quality warrants further investigation by beach managers.
Marin’s water quality monitoring program, overseen by the Marin County Community Development Agency’s (CDA) Environmental Health Services Division (EHS), gathered data from 31 bayside and oceanside monitoring locations over 31 weeks from April 2025 through October 2025 to come up with the grades. Twenty-six of the sites are marine beaches and five are fresh water recreational sites. The beaches are managed by a variety of agencies: 12 federal beaches, five state beaches, eight County beaches, and six local or privately managed beaches. Two intermittent creeks -- Chicken Ranch Ditch at Chicken Ranch Beach, and Dillon Beach Ditch at Dillon Beach -- are measured when there is enough water for sampling.
When looking at the summer dry average grades of Marin sites in a span of five years, 97% of beach sites received A and B grades. During the wet weather average events from April through March, 19 sites received A and B grades, and only three sites received C and F grades. The five-year wet weather beaches grade average is 84%, with 90% receiving either an A or a B, likely as a result of lower rainfall.
In terms of rainfall, California saw 16% less rain (332 inches) during this reporting period compared with the 10-year average of 397 inches. Heal the Bay reports Marin received 21 inches of rain between April 1, 2025, and March 30, 2026, 8% below the historical 10-year average of 23 inches. The full impact of the decreased rainfall in winter months is difficult to assess, because most of the local beaches are not monitored during winter.
Rain alleviates drought conditions, but it also results in more pollutants, including bacteria, being flushed into streams, the bay, and the ocean. Pushed by rainwater, contaminants flow from streets in the form of trash, fertilizer, pet waste, metals, and automotive fluids.
Marin experienced multiple sewage spills during the 2025-26 observation period, resulting in approximately 55,000 gallons of wastewater reaching surface waters across 10 incidents, according to the California Environmental Protection Agency website.
The spills in question, falling under the jurisdiction of various sanitation districts, have been diligently addressed by the responsible authorities. The spills were promptly reported, and appropriate measures were taken to mitigate any potential impact on the environment and public health.
Marin EHS has monitored ocean, bay, and freshwater sites since 2003 and posts sample results weekly. The samples are processed by the Napa-Solano-Yolo-Marin County Public Health Lab. The tests quantify the most probable number of total coliforms, E. coli, and enterococcus bacteria present in each water sample.
An advisory sign is posted to alert the public if lab results indicate water samples exceed the State of California standards for recreational waters. The California Department of Public Health advises beach users to avoid contact with recreational waters where advisory signs are posted. People in contact with elevated bacteria levels in recreational waters may become ill.
Heal the Bay recommends beach users never swim within 100 yards on either side of a flowing storm drain, creek, or river in any coastal waters during a rainstorm and to stay out of the water for at least three days after a storm has ended.
Learn more about Environmental Health Services Division’s testing program on the webpage.