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Why is the River Thames Brown?

The River Thames is a muddy river with a silt bed, which gives it a brown appearance. The murky brown colour of the water has led many to believe that the Thames is dirty and polluted, but it’s actually a lot cleaner than it looks.

The River Thames has made a remarkable recovery since the industrial revolution, and is today clean enough to support over one hundred species of fish.

This post is about why the River Thames looks brown, as well as how polluted it really is.

Why is the River Thames Brown?

Why is the River Thames Brown?

The River Thames appears brown because there is silt on the riverbed. This silt is made up of fine particles which disperse in the water and make it look muddy.

There are six different types of sediment that can be found on a riverbed:

  1. Silt (mud/clay)
  2. Sand
  3. Cobbles
  4. Boulders
  5. Gravel
  6. Bedrock

The material found at the bottom of the river will affect the appearance of the river, as well as the kind of wildlife it will attract. For example, trout require silt free gravel for laying their eggs. Chalk streams like the River Wandle, tend to have gravel or flint beds, which are ideal for trout.

Carp on the other hand like silt riverbeds as the silt contains natural food sources for them such as bloodworm. Since the River Thames has a silt bed it is therefore a magnet for carp.

The photograph below shows the River Thames flowing past Battersea Park in Wandsworth. You can clearly see the brown colour of the water.

River Thames near Battersea Park in Wandsworth
River Thames near Battersea Park in Wandsworth

Has the Thames Always Been Brown?

The Thames has always been brown because it’s always had a silt riverbed. The brown water of the river Thames may have even been the original source of the river’s name.

In Sanskrit Tamas तमस् means “darkness”, and the Celtic word “tame” means “dark one”. The river Thames may have been named the “dark one” due to its dark brown, muddy water.

Due to the mud on the riverbed, the Thames would have looked just as brown to the Romans in 43AD (when they made Londinium their capital), as it does today.

Is the Thames the Cleanest River in the World?

Believe it or not the River Thames is actually one of the cleanest rivers in the world.

Many have mistakenly believed the Thames to be dirty due to its brown colour, and its historical reputation for being polluted, but this is not the case.

Back in the 1950s the River Thames was filled with sewage and industrial waste, and was declared biologically dead by the London’s Natural History museum.

Since then however considerable work has been done to clean up the river Thames.

Today it is a haven for over 125 species of fish, as well as other wildlife such as eels, seahorses, sharks, grey seals and oysters.

The photograph below shows a short snouted seahorse, which is one of the species found in the outer tidal Thames and Thames estuary.

short snouted seahorse

River Thames Pollution History

Nineteenth Century

In Victorian times the River Thames was London’s main source of drinking water. The river was, at the time, infamously dirty and thick with human sewage. Diseases such as typhoid and scarlet fever were common.

In 1831 cholera broke out which led to investigations into the cause of the disease. No one realised the water from the Thames was causing the disease, but instead blamed it on miasma. This was a theory that diseases such as cholera were caused by a noxious form of “bad air”.

In 1858 a heatwave in London caused foul smells from the River Thames. The Great Stink was one of the main turning points for the river. The smell reached the Houses of Parliament, and this, along with the cholera outbreaks led to laws being passed to clean up the Thames, and to stop waste being dumped into the river.

A plan was finally agreed in 1865 to divert sewage away from the River Thames. The new sewer system was created by Sir Joseph William Bazalgette, and was instrumental in relieving London from more cholera epidemics.

Twentieth Century

Despite the creation of a new sewage system, the Thames river was still heavily polluted during the first half of the twentieth century.

During the second world war, bombing destroyed some of the old Victorian sewers causing tonnes of raw sewage to once again spill into the Thames.

Post war Britain did not have the resources to fix the sewers, and it wasn’t until the 1960s that the problem was resolved. During the 1960s industrial waste was removed from the river and oxygen levels increased. Biodegradable detergents also came into use at this time.

Despite this however, water quality standards were still not good enough to support fish populations or aquatic life.

In the 1970s there was increasing concern for the environment, and more regulations were put on pesticides and fertilisers that could end up in the river.

In 1989, under Prime Minister Margaret Thatcher, the Water Act and the privatisation of water companies led to the National Rivers Authority being established.

The National Rivers Authority was one of the forerunners of the Environment Agency, existing between 1989 and 1996, and was responsible for water quality management, pollution prevention, and many aspects of the management of aquatic ecology.

It was during the 1980s that the river saw the return of salmon. From this time until the end of the twentieth century, considerable effort was made to clean up the River Thames.

Since the start of the twenty first century, the amount of toxic metals, and dangerous contaminants in the river has reduced. In 2010 the Thames won the Theiss River Prize for the most outstanding river restoration. Today the river Thames is one of the world’s cleanest rivers.

River Thames in Hammersmith
River Thames in Hammersmith: Photograph by Olivia Herlihy

Is the Thames Clean Enough to Swim in?

The Port of London Authority allows swimming in the river Thames from Putney Bridge through to Teddington, however swimming in the Thames is certainly not encouraged.

Despite efforts to clean up the river, there are still unresolved issues and questions about whether the river is clean enough to swim in.

Over the past few years over 2 billion litres of raw sewage has been discharged into the Thames, and a range of heavy metals have been found in the river bed. These metals are potentially toxic to humans.

The river Thames was also found to have some of the highest recorded levels of microplastics than any river in the world. Microplastics have been shown to cause damage to human cells.

Parts of the Thames are also tidal which can be dangerous for even a strong swimmer. Eddies and undertows are also a major threat for swimmers as they can suck people under the water in seconds.

Swimming in the Thames in central London, or any parts of the river in greater London outside of the stretch from Putney Bridge to Teddington, is prohibited.

Egyptian Goose in the River Thames
Egyptian Goose in the River Thames: Photograph by Olivia Herlihy

Other Posts About the River Thames

River by Hammersmith Bridge
River Thames by Hammersmith Bridge: Photograph by Olivia Herlihy

Why is the River Thames Brown?

Thank you for reading my post about why the river Thames is brown. The river Thames runs 229 miles from from Gloucestershire, though the centre of London, to Essex, where it meets the North Sea.

The entire length of the river Thames is brown, but the river Wandle, one of its tributaries, is a chalk stream, and is clear. Today the river is home to many marine species, water beetles, aquatic invertebrate species and other wildlife.


Friday 19th of April 2024

Fascinating post about the origins of the name Thames . Thoroughly enjoyed another interesting and informative posts. So enjoyable and encouraging to go and visit the various sites you have mentioned and appreciate our wonderful capital city. Once again thank you Olivia, an avid follower of your work.

Y O Keame

Saturday 19th of August 2023

About 20 years ago, I stayed for a month in London not asking thames brown. Today I got the answer. Your detailed information gives me a perfect understanding. I still remember crossing the tower bridge and swimming in the community center. Thanks

Mary Kate Mulligan

Monday 9th of January 2023

Very interesting!