Explaining the readings

If a site has a designated bathing water status, it means that bacteria harmful to human health (such as E. coli and intestinal enterococci) are monitored weekly during the bathing season by environmental agencies and information on the water quality is displayed publicly. This allows river users to make informed decisions about getting in the water and potential risks. The sites that we are testing do not have bathing water status but are used frequently by swimmers, paddlers, rowers and others.

The tests by environmental agencies classify water quality from excellent to poor, the latter being advice against bathing. The numbers reported are in CFU per 100ml, where CFU is "colony forming units", a measure of the amount of bacteria.

For inland waters such as the Thames, the Defra classifications are: E. coli 500 or below is excellent; up to 1,000 is good; above 1,000 is poor. Intestinal enterococci 200 or below is excellent; up to 400 is good; above 400 is poor. We show readings at or below these "good" limits in green and above them in red.

This is a simplification: Defra classifications are based on data for a whole 20-week season, using the 90th and/or 95th percentile, not individual samples. See Defra under "Links" for a fuller description.