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About these forecasts

For each spot, SwimSignal traces the river upstream, finds every monitored storm overflow whose water reaches it, and combines what those overflows are doing now with how likely each is to spill over the next five days, allowing for travel time, bacterial die-off and dilution. It is a forecast, not a water test: "low" is not a guarantee of clean water, and inclusion of a spot is not a statement that it is safe to swim there. How accurate is it?

What the levels mean

A day's level is the highest of these. A spot's page shows each one.

Sewage spills: The exposure index (0–100): the modelled chance that a spill from an overflow upstream reaches the spot. Low under 15, moderate 15 to 40, high 40 to 70, very high 70 or more.
Water quality: On rivers with overflows upstream: the chance a water sample would show E. coli over 900 per 100 ml, from any source, runoff after rain included. Low under 10%, moderate 10 to 25%, high 25 to 50%, very high over 50%. The minimum bathing-water standard lets about one sample in ten be over 900.
Environment Agency rating: At a designated bathing water, the EA's yearly rating from up to four seasons of lab samples. When it is poor, advice against bathing applies all season, so the level is at least high on every day. Where there is no daily forecast, the rating sets the level: excellent or good low, sufficient moderate, poor high.
Algae: At a designated bathing water, the EA sampler's last look, if under two weeks old: enough algae to be objectionable makes the level at least high, some at intervals at least moderate.
Not included: The EA's short-term advice against bathing after a pollution incident or an algal bloom. A bathing water's page links to the EA's page, which has it.

Low: Nothing in the forecasts or the EA's rating flags the day. That is not a guarantee of clean water, and it says nothing about currents, cold water or algae.
Moderate: Some chance that sewage reaches the spot, or that a water sample would fail.
High: Sewage reaching the spot, or a failing sample, is a real prospect; or the EA rates the water poor.
Very high: Sewage reaching the spot, or a failing sample, is likely.
No level: No monitored overflow upstream, or no river connection the model can trace, and no EA rating. There is nothing to flag, which is not the same as clean water.

What each part of a spot's page means

Right now: Which upstream storm overflows are discharging now or finished within 48 hours, from the water companies' live feeds.
Exposure index (0–100): The modelled chance that at least one upstream overflow spills and its discharge reaches the spot after travel time, die-off and dilution. The spill part is verified against what the overflows did; the die-off and dilution weights are physical estimates, not calibrated against water samples, so it is an index, not a probability. The colour levels (low to very high) come from it.
E. coli > 900: The estimated probability that a midday water sample would exceed 900 E. coli cfu/100 ml, from rainfall at the spot, the exposure index, water-body type and season, fitted to Environment Agency samples at river and lake bathing waters and tested on years and sites it never saw. It counts bacteria from any source, so rain alone can raise it when the exposure index is low. 900 is the threshold used in the inland "sufficient" classification, which is a 90th-percentile calculation over four seasons, not a pass/fail test of one sample. It is validated on rivers in the May–September sampling season only; on lakes it has no demonstrated skill and is not shown. The EA takes no samples from October to April, so then nothing tests it: the figure still counts towards the level, so that a wet autumn day does not read as clean water, but it is marked †, and the date's own effect is held at its value at the nearer end of the season (30 September or 1 May) rather than carried on with nothing to check it.
Algae: Designated bathing waters only: the Environment Agency sampler's latest look at the water, an observation, not a forecast. It does not tell blue-green algae from harmless kinds.
What it does not know: Anything about a specific farm, road, outfall or algal bloom: only monitored storm overflows are modelled.

Saved spots, offline use and your data

Saved spots are kept in this browser, and sent nowhere unless you turn on alerts on the Saved page. After a first visit the page and the latest forecast are stored on your device, so the site opens without signal at the water's edge; it then says how old the forecast is. Privacy notice.

Add SwimSignal to your Home Screen

It then opens from its own icon, without the browser's bars. No account is needed.

iPhone or iPad: tap Share, then Add to Home Screen.
Android: open the browser's menu and look for Install app or Add to Home screen.

On iPhone and iPad the Home Screen app keeps its own saved spots: spots saved in Safari do not carry over, so save them again in the app.