Rain is the single biggest reason beaches fail
When a storm moves through, it washes whatever is on the ground — pet and wildlife waste, fertilizer, oil, and the contents of overwhelmed storm drains — straight into the surf. That runoff carries a spike of the fecal-indicator bacteria that beach programs test for, which is why so many advisories land in the day or two after heavy rain. Many coastal counties issue a precautionary "rain advisory" at certain beaches whenever rainfall passes a set threshold, before a single sample even comes back.
The effect is concentrated where the built environment drains to the shore: beaches near storm-drain outfalls, river and creek mouths, marinas, and aging sewer infrastructure. A beach on an open, well-flushed coast recovers quickly as clean ocean water dilutes the plume; an enclosed bay, harbor, or Great Lakes beach with weak circulation can stay elevated for days. That geography is a large part of why beaches with similar visitors can earn very different grades.
Sewage and infrastructure failures
Beyond ordinary runoff, the sharpest spikes come from sanitary sewer overflows and combined sewer overflows — events where a system carrying sewage is overwhelmed (often, again, by rain) and discharges partly treated or untreated waste. Broken pipes, failing septic systems near the shore, and boat sewage discharge add to the load. These are the events most likely to escalate from an advisory to a full closure, because the contamination is directly from human waste rather than diffuse runoff.
This is also why the same handful of beaches show up on "worst" lists year after year. A beach's bacteria problem is usually a watershed problem: fix the outfall, separate the sewers, or restore the wetland that once filtered the runoff, and the samples improve. Until then, the pattern repeats — which is exactly what a multi-year grade captures and a single day's advisory cannot.
How the monitoring system actually works
The framework comes from the federal BEACH Act of 2000, which directs coastal and Great Lakes states to monitor recreational beaches for bacteria and to notify the public when water quality is unsafe. States and their county health departments run the sampling on their own schedules — typically weekly during the swim season, more often at high-use or problem beaches — and report results into the EPA BEACON system, which BeachGrade draws on for advisory status.
When a sample exceeds the applicable single-sample threshold (enterococci 104 CFU/100 mL in marine water, E. coli 235 CFU/100 mL in fresh water), the agency posts an advisory and usually resamples. The beach reopens once follow-up samples fall back below the limit. An "advisory" is a public-health recommendation against swimming; a "closure" is an enforced prohibition, reserved for the highest-risk situations such as a known sewage spill.
What the data can and cannot tell you
BeachGrade summarizes this monitoring across 8,447 beaches in 35 states and territories, but coverage is uneven. Some states sample intensively and report promptly; others test fewer beaches or report on a lag. A beach with no advisory on record may genuinely be clean, or it may simply be lightly monitored — which is why a raw "no advisory" flag on an unmonitored beach means "no data," not "never contaminated." Our grades lean on sample counts precisely so this distinction stays visible.
The practical takeaway is simple: if it has rained hard in the last 48 hours, assume urban and suburban beaches are riskier than usual, favor open coastline over enclosed water, and check the official advisory for the specific beach and day. BeachGrade tells you which beaches have a history of trouble; the day-of advisory from your state or county tells you whether trouble is present right now.