Across 280 state-monitored California beaches, bacteria samples fail 2.0× as often after rain. In Santa Cruz County the jump is 3.2×.
Hannah · · 4 min read
The old advice is to stay out of the ocean for about three days after it rains, because storm drains flush bacteria from streets, pets and leaky sewers to the beach. We wanted to see how strong that effect is in the state’s own sampling data, beach by beach.
We matched 538,835 bacteria sample results from California’s Beach Watch program (January 2020 to October 2026) to the nearest NOAA rain gauge, and compared days after rain with dry days.
The beaches whose water quality changes most after rainShare of sampled days over a state bacteria standard: dry days vs days after 0.1 in. of rain
Across 280 beaches and 95,850 sampled beach-days, a beach exceeded at least one state single-sample bacteria standard on 20.8% of dry days and 42.3% of days after rain, 2.0× as often.
The pattern is broad. Of the 161 beaches with enough wet and dry samples to compare, 142 failed more often after rain. The median beach’s failure rate rose by 17.4 percentage points, and 76 beaches rose by 20 points or more.
Many of the biggest jumps are at lagoons, harbors and channels, where water mixes slowly with the open ocean. At Colorado Lagoon in Long Beach, samples failed on 18% of dry days and 86% of days after rain. At Manhattan Beach, which is usually clean (3.6% of dry days), the after-rain failure rate was 60%.
After rain, more samples fail in almost every countyShare of beach-days over a state standard, dry vs after rain, by county
One county stands out the other way: Monterey’s 15 beaches had no exceedances on the 30 after-rain beach-days we could classify. That is a small sample: only 30 of Monterey’s 2,194 sampled beach-days followed rain.
Santa Cruz County
In Santa Cruz County, the after-rain effect is larger than the statewide average. Its 25 monitored beaches failed on 10.9% of dry days and 34.9% after rain, 3.2× as often.
Santa Cruz County beaches, dry vs after rainBeaches with at least 15 after-rain and 30 dry sampled days, 2020–2026
Main Beach and Cowell Beach, on either side of the Santa Cruz Wharf, fail most often overall: 25% and 25% of dry days, rising to 57% and 54% after rain. Seabright Beach shows the sharpest contrast: it failed on only 2.5% of dry days but 31% of days after rain.
Read this before comparing beaches
More stations, more chances to fail. We count a beach-day as failing if any monitoring station at that beach exceeded a standard. Main Beach has 7 stations and Cowell Beach 6, while most beaches (62% statewide) have one; the most-sampled beach has 31. A multi-station beach will look worse than a one-station beach with the same water. Compare a beach with itself (dry vs after rain) rather than ranking beaches against each other.
Sampling is not random. Agencies sample more at busy beaches, more in summer and sometimes more after a known problem. Only 12 of Santa Cruz County’s 25 beaches had enough after-rain samples (15) to include in the comparison.
Methods
Samples. State Water Resources Control Board fecal indicator bacteria results, 2020 to present, rows from the Beach Watch program (2020-01-01 to 2026-10-07). Replicates are reduced to the highest value per station, analyte and day. 862 unusable results were skipped.
Standards. California AB 411 single-sample standards per 100 mL: total coliform 10,000, fecal coliform or E. coli 400, enterococcus 104 (culture) or 1,413 (DNA method), plus the total-to-fecal ratio rule. A "less than" result at or below the standard passes.
Rain. A beach-day is after rain if the nearest NOAA GHCN-Daily gauge within 20 km (with at least 80% of days reported since 2020) recorded at least 0.1 in. (2.54 mm) in the roughly 72 hours before sampling. It is dry if every one of those days was reported and below that. Otherwise the weather is unknown and the day is left out of the comparison (7,933 beach-days statewide).
Jump = after-rain failure rate minus dry failure rate, in percentage points, only for beaches with at least 15 after-rain and 30 dry sampled days.
Limits
A sample is a snapshot of one spot at one moment, and results arrive a day or more after sampling, so this history describes risk, not today’s water. Rain gauges can be up to 20 km from the beach. Official advisories and closures, not this analysis, decide whether a beach is open; check the county health department or Can I swim today? for the latest samples.
Data and code
Every number and chart in this post is generated from these files: