The State of the Thames 2021

by ZSL on

The tidal stretch of the River Thames is arguably London’s greatest asset.

As the point at which the River Thames meets the sea, the Tidal Thames is an estuary with immense societal, economic, and environmental importance. Millions of people every year walk along its riverside paths, explore its foreshore, and enjoy boat rides on its waters. However, there is a lot that people don’t know about the tidal stretch of the River Thames. For instance, there are over a hundred fish species that can be found below its murky waters, including sensitive species such as seahorses and European smelt – a fish that smells of cucumber. Porpoises can often be spotted in the Thames Estuary, and seals have been seen as far upstream as Teddington!

Short-snouted seahorse
Short-snouted seahorse

Once declared ‘biologically dead’

While the presence of these and other species in the Tidal Thames indicate a healthy ecosystem, this has not always been the case. For years, tonnes of household and industrial sewage were regularly deposited into its waters, causing significant damage to the environmental health of the river. In 1957, stretches of the Tidal Thames were declared biologically dead by the Natural History Museum. Since then, the river’s water quality and ecological health have both significantly improved, due largely to advances at sewage treatment works.

Faulty drains threaten Thames water quality

What’s changed since 1957?

These improvements in environmental health inspired The State of the Thames 2021. This report uses a set of indicators and associated time series data to celebrate the environmental successes of the Tidal Thames since 1957. For example, dissolved oxygen, which is critical for aquatic life, has remained at consistently healthy concentrations in recent years. Phosphorus, which at high concentrations can be dangerous to aquatic life, has been declining in the last 15-20 years since phosphate stripping was introduced to major sewage treatment works. After years of habitat decline, saltmarsh area has started to increase in recent years. Seal populations have been rising steadily since the early 2000’s, as have wading birds. In addition, efforts have been made to open connectivity between the Tidal Thames and its tributaries for eels and other fish species.


At the same time as celebrating these successes, the report also highlights the current challenges facing the Tidal Thames. In doing so, it emphasises the areas in need of our attention if we want to protect and continue to improve the Thames environment. One of the most worrying challenges to the estuary is climate change, seen by rising water temperature and sea level, which can both have damaging effects on habitats and wildlife. Another concern is chemical contaminants, which have many different sources and can often go undetected, sometimes causing irreparable harm to aquatic life. Furthermore, both larger macroplastics and the barely visible microplastics, are found in abundance within the water column and on the foreshore. Finally, while improvement to sewage treatment has helped prevent high concentrations of phosphorus, nitrate concentrations have been gradually rising over time.

Changes for the future

Encouragingly, much is being done to address these threats. The Environment Agency’s Thames Estuary 2100 Plan aims to use nature-based solutions to address sea level rise and the associated flood risk. Thames21’s Thames River Watch Programme, the Port of London Authority’s Thames Litter Strategy, and ZSL’s #OneLess campaign are all promising initiatives to confront plastic pollution. To address the remaining water quality issues caused by an outdated sewage system, Tideway is constructing a new ‘super sewer’, which will capture and store most of the millions of tonnes of raw sewage that currently overflow into the estuary when it is completed in 2025.

seal on the thames

One of the most important goals of The State of the Thames 2021 is to help instil a sense of pride in our river and how far it has come in the past 60 years. When people recognise the importance of the Tidal Thames and all the intangible benefits it provides (mental, physical, spiritual, and cognitive benefits), they will be willing to fight to protect and improve this valuable ecosystem!

Read the State of the Thames 2021 report in full

Select a blog

Careers at ZSL

Our people are our greatest asset and we realise our vision for a world where wildlife thrives through their ideas, skills and passion. An inspired, informed and empowered community of people work, study and volunteer together at ZSL.

Nature at the heart of global decision making

At ZSL, a key area of our work is the employment of Nature-based Solutions – an approach which both adapt to and mitigates the impacts of climate change. These Solutions, which include habitat protection and restoration, are low-cost yet high-impact, and provide multiple benefits to people and wildlife. We ensure that biodiversity recovery is at the heart of nature-based solutions. 

ZSL London Zoo

A blog for lovers of ZSL London Zoo, bringing you extraordinary animal facts and exclusive access to the world's oldest scientific zoo.

ZSL Whipsnade Zoo

Do you love wildlife? Discover more about our amazing animals at the UK's biggest zoo!


We're working around the world to conserve animals and their habitats, find out more about our latest achievements.


From the field to the lab, catch up with the scientists on the cutting edge of conservation biology at ZSL’s Institute of Zoology.


A day in Discovery and Learning at ZSL is never dull! The team tell us all about the exciting sessions for school children, as well as work further afield.

Artefact of the month

Every month, one of the pieces held in ZSL’s Library and at ZSL Whipsnade Zoo will feature here as Artefact of the Month.

Wild About

Read testimonials from our Members and extracts from ZSL's award winning members' magazine, Wild About.

Asia Conservation Programme

ZSL works across Asia, from the famous national parks of Nepal to marine protected areas in the Philippines. Read the latest updates on our conservation.

Remote Sensing in Ecology and Conservation

An Open Access journal for research at the interface of remote sensing, ecology and conservation.