London's "Super Sewer" Cut Storm Overflows to the Thames by About 86% in 2025

London's Thames Tideway Tunnel is making a visible difference to the river. On 13 October 2026, Thames Water published the first annual Tideway Benefits Report, which found storm overflows into the tidal Thames fell by about 86 percent compared with the period before the tunnel.

What the report found

• Annual overflow volume in 2025 was just under 3 million cubic metres, against a pre-Tideway average of more than 21 million.
• Days of elevated health risk linked to overflows fell from 75.7 percent to 10.4 percent.
• About 84 tonnes of sewage-derived litter, such as wet wipes, were kept out of the river in 2025.
• Monitoring by the charity Thames21 found sewage-related items fell from 30 percent of foreshore litter to 18 percent.
• Only one low-oxygen event in 2025 was linked to overflow activity.

The tunnel's operation is also moving from Tideway to Thames Water, with Tideway retaining ownership.

Why it matters

Fewer overflows mean cleaner water for wildlife and for people who swim, row and paddle on the river. A Zoological Society of London conservation lead said investment in water treatment infrastructure "delivers tangible improvements in water quality."

Caveats

This comes from a Thames Water release, and the 86 percent figure is based on modelled volumes rather than measured discharges. The first year was a commissioning phase, and the release notes that 2025 was unusually dry. Tideway's own Henrietta Gibb said the system "will fully prove itself over the longer term." Earlier design targets aimed for a larger reduction of around 95 percent in a typical year.