What does he mean by his statement “Sometimes, our measurements don’t tell the real story”, with regards to water quality in rivers?

Answer

Rivers may be graded as unsuitable for swimming are not necessarily unsuitable at everygiven point in time. These grades are based on a five-year period of monthly E. coli sampling, that is used to calculate the likelihood that a swimmer would acquire a bacterial infection if they swam once on an average day during that period. However, the actual risk can vary greatly from day to day, particularly because high E. coli concentrations generally occur at times of high flow. This means ‘unsuitable’ sites can be safe for swimming during some days or times of the year, generally coinciding with the summer and low-flow conditions most people prefer for swimming. This is why councils sample designated swimming sites for E. coli more frequently during summer, and this is the best information for determining if it is safe to swim.