SwimZone

Methodology

How the swim-safety read works

Every SwimZone water gets a single plain verdict for the day, refreshed as new monitoring comes in. Here is exactly what goes into it, how the pieces combine, and where the read stops.

Clear to swimUse cautionAdvisory

The inputs

Two very different hazards close a swimming spot, and they are monitored in different ways, so the read is built from a few layers: what official bacteria sampling says, what cyanobacteria monitoring says, and how much recent rain has likely moved things since the last sample. The worse of the two hazards sets the day's verdict.

  1. 1

    Fecal bacteria (E. coli)EPA BEACON + state beach programs

    Runoff after rain, failing septic systems, and stormwater carry fecal bacteria into the water. State and federal programs sample designated beaches for E. coli (fresh water) or enterococcus (marine), and post an advisory when a sample is over the state limit. We read those official results and advisories as the first hazard, because a posted bacteria advisory is the most common reason a beach is closed to swimming.

  2. 2

    Cyanobacteria (blue-green algae)EPA CyAN + state HAB programs

    Warm, calm, nutrient-rich water can bloom with cyanobacteria, which sometimes produce toxins that are dangerous to people and can be deadly to dogs. On larger lakes, EPA's CyAN satellite estimates bloom intensity; smaller waters rely on state harmful-algal-bloom programs and reported sightings. We read the most recent bloom status or advisory as the second hazard.

  3. 3

    Recent rain and runoffOpen-Meteo

    Sampling lags. A lab result can be a day or more old, and a heavy rain since then can spike bacteria before the next sample is taken. We read each water's recent rainfall, so a fresh soaking on top of an otherwise clear beach is flagged as a reason to use caution rather than assume yesterday's all-clear still holds.

How they combine

The verdict is the worse of the two hazards, never an average. If either bacteria or cyanobacteria is under an official advisory, the water reads Advisory. If either is elevated, or heavy recent rain makes the last clear sample unreliable, it reads Use caution. Only when both hazards are clear and no fresh runoff is flagged does a water read Clear to swim.

Every water page shows the two hazards side by side, with the source and date behind each, so you can see exactly why it reads the way it does.

Limits and caveats

This is a summary of public monitoring, not a safety guarantee and not a lab test of the exact spot you plan to swim. Monitoring lags real time, blooms and bacteria move with wind and rain within a single water body, and a "clear" read reflects the last official sample, not this minute.

What we cover. We publish reads for waters with official monitoring: designated public beaches and larger lakes with a state or federal program. Many small ponds and undeveloped shorelines are not monitored by anyone, so an absence of an advisory is not proof a water is safe.

Look before you get in. Avoid water that is scummy, discolored, or smells bad, keep children and dogs away from visible algae, and follow posted signs and local health guidance. For more on recreational water safety, see EPA beach and swimming guidance.