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

Sewage spills

The exposure index, a 0–100 score of how likely sewage from upstream is to reach 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 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. In the May to September season only: from October to April, when the EA takes no samples, the estimate is shown marked † but does not set the level.

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 on every day. Where there is no daily forecast, a rating of sufficient makes the level moderate and poor makes it high; 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, 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 sewage risk from monitored overflows

The trace found no monitored storm overflow upstream, so there is no spill forecast and no E. coli estimate. Other risks apply: algae, wildlife, runoff and bathers. Check the signs at the water. Where a rating of sufficient or poor, or algae at the last check, raises the level, that sets it instead.

No river connection: overflows cannot reach this lake

A lake with no river flowing into it in the network, so sewage from an overflow cannot reach it by water. The same other risks apply.

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.

Right now

Which upstream storm overflows are discharging now or finished within 48 hours, from the water companies' live feeds.

Exposure index (0–100)

A 0–100 score of how likely sewage from upstream is to reach 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 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 is still shown, marked †, but it does not set the level, 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.

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

Anything about a specific farm, road, outfall or algal bloom: only monitored storm overflows are modelled.

Saved spots and your swim log are kept in this browser, and sent nowhere unless you turn on alerts on the Saved page (alerts send the list of saved spots, not the log). 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.

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• 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.