Saved spots

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About these forecasts · How accurate is it?

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 risk" 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 spill risk: A 0–100 score for sewage from upstream reaching the spot. Low under 15, moderate 15 to 40, high 40 to 70, very high 70 or more.
Sewage spills right now: Today only: the overflows upstream discharging now, or stopped lately, from the water companies' live feeds. Today's level is at least right now's.
E. coli estimate: 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 risk 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. It is tested only for May to September, so it sets the level only then: from October to April, when the EA takes no samples, it does not set the level, and a spot's page says "Out of season" in its place, with the figures under "Experimental estimate". A spot's page says how it has done on live samples so far.
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 through the bathing season, 15 May to 30 September, and the level is at least high risk on every day. Where there is no daily forecast, a rating of sufficient makes the level moderate risk and poor makes it high risk; excellent or good leave the plain level below.
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 risk, some at intervals at least moderate risk.
Lab sample: At a designated bathing water, the EA's latest sample, if under two weeks old and over 900 E. coli per 100 ml: the level is at least moderate risk.
Not included: The EA's short-term advice against bathing after a pollution incident or an algal bloom: SwimSignal cannot read it, so a bathing water's page says it could not be checked. Opening "Today's EA advice" there loads the EA's own panel, which has it. Nor the overflows in Wales, which are not in the data: where a spot's river runs from Wales, its page says how much of it is in Wales.

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 be over 900 E. coli per 100 ml.
High: Sewage reaching the spot, or a sample over 900, is a real prospect; or the EA rates the water poor.
Very high: Sewage reaching the spot, or a sample over 900, is likely.
No monitored overflow found upstream: The trace found no monitored storm overflow within 60 km upstream, or the river network draws no river into the lake, so there is no sewage spill risk and no E. coli estimate. That is not the same as no risk. Other risks apply: algae, wildlife, runoff and bathers. Check the signs at the water. Where a rating of sufficient or poor, algae at the last check or a recent sample raises the level, that sets it instead.
No level: No forecast in this update, or no river or lake on the network near the spot. There is nothing to go by, which is not the same as clean water.

Under its level, a spot's page says in one line what to do, from published health advice for swimmers. Each line and its sources.

What each part of a spot's page means

Right now: Which upstream storm overflows are discharging now or stopped lately, from the water companies' live feeds. A spill counts in full from its start until its water has passed the spot, then for less over the next 48 hours. Today's level is at least right now's. A spill reported running for over two days is named as possibly a monitor fault, and still counts.
Sewage spill risk (0–100): A 0–100 score for sewage from upstream reaching the spot. It is built from the chance that at least one overflow upstream spills and its discharge arrives, 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 the score is not a probability. The colour levels (low to very high) come from it.
E. coli estimate: The estimated chance that a midday water sample would show over 900 E. coli per 100 ml, from rainfall at the spot, the sewage spill risk, 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 sewage spill risk 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: it does not set the level, a spot's page gives the figure only under "Experimental estimate", 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. A spot's page says how it has done on the samples taken since the forecast began, and, where the river runs from Wales, what a test on Welsh river samples found.
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.
Water temperature: Rivers only, where an Environment Agency sensor on the same river, within 15 km in a straight line and on the same side of the tidal limit, has reported in the last day: its latest reading, with how far up or down the river it is and how old the reading is. Where there is no such sensor nothing is shown; the temperature is never estimated. It is not part of the level.
What it does not know: The treated sewage that sewage works let out every day, and anything about a specific farm, road, outfall or algal bloom: only monitored storm overflows are modelled. Nor does it use the wind, which decides where river water goes in a lake. What the forecast does not count, and what you can do.

Saved spots, offline use and your data

Saved spots, the names of your lists and your swim journal are kept in this browser, and sent nowhere unless you turn on alerts on the Saved page (alerts send which spots are saved, not the lists' names or the journal). 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.