A new report from the Zoological Society of London (ZSL) says that the River Thames – a river that was once declared ‘biologically dead’ - is ringing in 2026 with a new lease of life - although experts warn that rising water temperatures and pollution are threatening the hard-won resurrection.

The updated The State of the Thames report by the ZSL team highlights how decades of dedicated action have bettered the Thames for people and nature, with improvements including lower concentrations of toxic metals and more ‘breathable’ water for the river’s underwater wildlife.

The report also shows how rapidly regulation can bring benefits; plastic cotton bud sticks, once one of the most commonly found litter items in the Thames, have all but disappeared from its shores following the UK’s sale-ban in 2022. The Society is hoping the same will be achieved for plastic wet wipes, another form of pollution found along the Thames, which are due to be banned in the UK later this year.

However, the report also highlights the need for cross-government action to protect the Thames and the essential services that it provides to the nine million people living along it – which include providing drinking water, food, livelihoods, protection from coastal flooding, and offsetting the damaging impacts of carbon emissions.

Marking the start of ZSL’s 200th anniversary year, the detailed 100 page report is an update to the first The State of the Thames report, published in 2021. Bringing together the expertise of 21 organisations working across the Thames, the report collates the most up-to-date monitoring data available from 21 different indicators used to assess the health of the Thames - providing conservationists, policy makers and industry leaders with an essential overview of the progress made so far to recover the iconic river.

Other wins for public health and the environment include the drop in concentrations of toxic copper and zinc in the Thames to below levels at which they damage the aquatic ecosystem. Zinc concentrations in 2024 were half the concentrations of 1990, while copper concentrations are only a quarter of that in 1990.

Additionally, while nitrate levels have steadily increased over the last 35 years, the concentrations of another monitored nutrient, phosphorous, have decreased since 1970. As excess levels of these nutrients can lead to the overgrowth of algae that deplete the river of essential oxygen, this finding marks another win for the Thames -which the report attributes in part to the upgrade of five London sewage treatment works and investment in the Lee Tunnel.

Both nitrate and phosphorous are found naturally in rivers, but are also associated with sewage, industrial waste and agricultural run-off - underscoring the importance and impact of investment and changes in land management in addressing these issues.

The upgrades were followed by a £4.6 billion investment in The Thames Tideway Tunnel, which was fully connected to the Thames Water network in 2025. The 'super-sewer', along with the Lee Tunnel and treatment works upgrades, are expected to help capture 95% of the volume of untreated sewage currently entering the tidal Thames in a typical year. Since autumn 2024, the tunnels have captured almost 13 million tonnes of sewage that would have been otherwise discharged into the Thames.

Climate change and pollution putting the estuary in a fragile position

However, the report is warning that from toxic ‘forever chemicals’ to rising temperatures and sea-levels, the data shows that climate change and pollution - from sources including road run-off, sewage and pharmaceuticals - are putting the estuary in a fragile position.

The report flags up that the effects of chemicals of emerging concern, another new indicator, are just beginning to be understood and whilst current data is insufficient to provide trends, it is important to highlight the urgent need for monitoring.

The summer temperature of the capital's waterway has increased on average by 0.13⁰C each year since 2007, equivalent to a 1⁰ C temperature rise every eight years. In the heart of the city, water levels by Tower Bridge and the Tower of London have also been rising - by 5mm each year on average since 1993. As water levels continue to rise, so does the risk of upstream waters becoming increasingly more saline - leading to the loss of rare freshwater tidal habitat along the Thames and the interwoven communities of wildlife that it supports.

Concentrations of nitrates, which are harmful in high amounts, have also gradually increased in the Thames and two of its monitored tributaries – the Rivers Lee and Darent – all of which risks further unbalancing the delicate ecosystem and threatening the important services it provides.

Thames Water funded the preparation of the report – which highlights that the content remains independent and that the water company had no editorial influence over the content.

Writing in the foreword to the report, Dr Andrew Terry, Director of Conservation and Policy, ZSL says:

“However, our work is far from over. We must accelerate large-scale habitat restoration for both habitats and species that will boost resilience and provide access opportunities for local communities. The Thames faces persistent pressures from climate change and increasing levels of nutrients, plastics and forever chemicals – pressures that are set to intensify as London’s population grows. And, while progress has been made on wastewater management, we must address the sources of pollution, such as runoff from roads and land management of areas adjacent to the river.

“Collaboration remains our most powerful tool. The combination of civil society, public sector, private sector and knowledge institutions working together is essential for success. New legislation, such as the Environmental Improvement Plan, Biodiversity Net Gain and Local Nature Recovery Strategies, coupled with the Mayor’s Clean and Healthy Waterway Strategy, provides cautious optimism for the future. We hope these frameworks and emerging highintegrity financing mechanisms, such as blue carbon and biodiversity credits, will drive necessary investment into nature recovery.”

Click here to download the report in full