On a journey along the Thames – where fury at pollution has spawned a wave of local activism – it is clear that the decline of rivers is among this government’s worst legacies

Red kites swoop above Fawley Meadows as Dave Wallace dips a sampling beaker into the deep green water of the River Thames on a late spring day. A sharp wind blows droplets upstream towards the arches of Henley Bridge, while the might of the river, its path here straight and wide, pulls downstream towards Windsor, on its 215-mile odyssey to the North Sea.

Today, the water meadows along its banks host blue and white striped marquees, lined up in uniform rows for the Henley regatta. After the rowers depart, the river bears the swimmers who follow. They dip, jump and dive its depths at an annual festival of open water races, echoing the galas that took place in Victorian days.