To stop streets and houses from flooding, storm overflows act as relief valves and allow extra wastewater from drains and sewers to flow untreated into rivers. A new storm overflow interactive map has been produced by Thames Water. The map can be viewed at https://www.thameswater.co.uk/edm-map, and this evening at 21:00, the map showed that storm overflow was happening upstream and downstream from Abingdon.
I think the site monitored nearest to Abingdon is the one pictured near the Abingdon treatment works and was not discharging.
Thanks to Newcomer for letting me know about this from an article he read in the Guardian at https://www.theguardian.com/environment/2023/jan/05/interactive-map-shows-thames-water-raw-sewage-discharges-in-england-rivers.