The first annual Tideway Benefits Report shows storm overflows to the tidal Thames reduced by 86% ( based on modelled volumes) after its first year of operation compared to the pre-Tideway period.

The tunnel has prevented approximately 84 tonnes of sewage-derived litter, items such as wet wipes and sanitary products, from entering the river in 2025, helping improve conditions for wildlife and river users.

The report has revealed significant improvement in river health following the opening of the super sewer in 2025.

Henrietta Gibb, Director of Impact and Engagement at Tideway, said:

“These first-year results show the profound difference the Thames Tideway Tunnel is already making to the river. The dramatic reduction in sewage discharges, litter and days of elevated health risk means cleaner water, better conditions for wildlife and a healthier Thames for everyone who uses and enjoys it.

“Protecting the Thames for generations to come has always been at the heart of this project and, while the system will fully prove itself over the longer term, it is hugely rewarding to see the progress already being made.”

Storm overflows to the tidal Thames have been successfully reduced by 86%, compared to pre-Tideway levels, as the operation of this significant asset is transferred to Thames Water from Tideway, the company set up to deliver the project.

The findings mark a new chapter for the River Thames and signals that London’s principal waterway, which was once declared biologically dead in the late 1950s, is on the road to recovery.

The report, which provides the first full-year assessment of the environmental benefits delivered by the tunnel during its commissioning phase, shows concrete improvements across all four of its core objectives: 

• reducing sewage discharges
• improving water quality
• lowering sewage-derived litter 
• minimising days associated to elevated health risks for river users.

Annual storm overflow volumes fell by approximately 86% to just under three million cubic metres in 2025, compared to a pre-Tideway average of more than 21 million cubic metres.

The report re-emphasises that while weather can influence discharge volumes, with last year being unusually dry, the reduction that was seen in 2025 substantially exceeds what would be expected from rainfall patterns alone and demonstrates the impact of the tunnel.

The benefits are also being experienced first hand by millions of people who use the Thames each year. According to the report, days associated with elevated health risks linked to storm overflows have reduced drastically from an average of 75.7% of days to just 10.4% in 2025, compared to a time when the Thames Tideway Tunnel didn’t exist.

The tunnel system has also helped tackle one of the most visible forms of river pollution. During 2025, an estimated 84 tonnes of sewage-derived litter, including wet wipes, sanitary products and plastics, were prevented from entering the Thames. Across the wider London Tideway Tunnel system, around 130 tonnes of sewage-derived litter were intercepted before reaching the river.

To further re-enforce this outlook, monitoring by environmental charity Thames21 recorded a significant reduction in sewage-derived litter on the foreshore, with sewage-related items falling from 30% of litter recorded before the tunnel became operational to 18% in 2025.

The findings complement the Zoological Society of London's (ZSL) recent State of the Thames report, which highlighted encouraging signs of recovery across the river and demonstrated how decades of investment and action have helped improve conditions for wildlife and people. However, the ZSL report also warned that pollution and climate change remain significant challenges, underlining the importance of continued action to improve the health and resilience of the Thames.

Alison Debney, Conservation Lead UK Nature Recovery at ZSL said:

“The publication of this report, alongside our latest State of the Thames report published earlier this year, provides a clear demonstration that investment in water treatment infrastructure delivers tangible improvements in water quality, benefiting both public health and wildlife.

Tess Fayers, Director of Waste and Bioresources at Thames Water, commented:

"This report shows that the Thames Tideway Tunnel is already delivering real benefits for the River Thames.

“As responsibility for operating the Thames Tideway Tunnel passes to Thames Water, we are seeing dramatically fewer sewage discharges entering the river, less sewage-derived litter reaching our foreshores and significantly improved river water quality.”

"The River Thames is one of the world's great urban rivers and a vital habitat for wildlife. This is an important milestone for the river and a significant step towards a cleaner, healthier and more resilient Thames for future generations."

The report also found that only a single dissolved oxygen depletion event in 2025 linked to sewage overflow activity, as the tunnel creates better conditions for fish and other aquatic wildlife in the tidal Thames.

Thames Water and Tideway will continue to publish annual reports tracking the environmental benefits delivered by the tunnel as they move into long-term operation.

Click here to read the Tideway Benefits Report in full.