Thames Water’s first annual report on London’s new sewer tunnel estimates that storm-sewage overflow volume into the tidal Thames fell about 86% in 2025 against the 2020–2023 baseline. The October 13 report combines calculated and modelled flows, rather than counting individual spills.
The tunnel intercepts storm sewage and sends it onward for treatment. The operator says rainfall differences alone cannot explain the reduction, but 2025 was relatively dry and wetter-year evidence is still needed. These first-year findings do not establish that the river is safe for swimming.