Discover the tidal Thames River

Words and photos: Wanda Bodnar and Paul Hyman

Active360 has been operating on the tidal Thames since 2011. We provide private and group lessons and specialised training on learning the Tideway code and trips abroad. Since 2017, we have also been running ‘Thames Natural History Tours’. These sessions allow novice and returning paddlers to discover more about this beautiful river’s historical significance and natural history. The idea for the tours was developed when Wanda, a marine scientist, started coaching with us four years ago, and we started thinking about different ways we could engage more people with the aquatic environment.

The 215-mile-long River Thames is the United Kingdom’s second-longest river. It meanders through Oxford, Reading, Henley-On-Thames, Windsor and London before joining the North Sea via the Thames Estuary. Because of its connection with the North Sea, the Thames is under the influence of the tides up to Teddington. Hence, we call this stretch of the river the tidal Thames or the Tideway. 

The tidal Thames has two high tides and two low tides each day, with a tidal range of approximately seven metres. It is also subject to monthly tidal cycles. When the Earth, Sun and Moon are lined up in a row during spring tides, the average tidal range is slightly larger. When the sun and the moon pull the water in two different directions during neap tides, the high tides are a little lower, and the low tides are a little higher than average. 

Thanks to this non-stop movement, the sediment is continuously suspended in the water column, giving it its trademark brown colour. This brown colour is often seen as a sign of pollution (‘dirty old river’), but this is the sign of well-mixed nutrients in the water, making the tidal Thames a rich feeding ground for aquatic species. 

The tidal Thames has been at the centre of human history and development. Since the arrival of the first settlers during the Mesolithic times, it has been the source of food and drinking water, worship, an important transportation and trade route… and an open sewer. By the mid-1800s, the population of London was about two million people, and due to the lack of a sewer system, waste discharge into leaky cesspits and the river was common practice. 

Following the great stink of 1858, when the smell in London became so bad that the curtains of the riverside buildings had to be soaked in lime chloride, members of the Parliament rapidly passed a new law that allowed the construction of a new sewer system. Joseph Bazalgette, a civil engineer and Chief Engineer of the Metropolitan Board of Works, was given responsibility for the work, and the project was completed in 1875. 

Heated water
However, the newly built sewer system brought only temporary relief as the population continued to grow, and the tributaries of the Thames were not part of the sewer network. Also, the Blitz significantly damaged London’s sewer system, and following the war, electricity-generating power stations started to appear along the river. The latter had an adverse effect: the heavily heated water used to cool these power stations would be discharged into the river, depleting the water of its precious dissolved oxygen. 

Consequently, fish populations significantly declined, and in 1957 the river was declared biologically dead by the Natural History Museum. However, thanks to environmental regulations put in place since the 1970s and the profuse work of charities and non-profit organisations in recent decades, the health of the river has been improving – and will improve further once the Tideway Tunnel (known as London’s Super Sewer) is completed in a few years. Nevertheless, the tidal Thames is already one of the central places of recreation for Londoners. 

Our Thames Natural History tours run through an area called the Arcadian Thames – a multi-layered stretch of the river rich with wildlife and history. The Arcadian Thames, overseen by the Thames Landscape Strategy, is the most significant connected open space in London. Paddling upriver from Kew Bridge, you can discover places like Syon Park, the last remaining tidal meadow on the tidal Thames, Isleworth Island, a nature reserve home to two species of rare molluscs, and Corporation Island, a rich heronry. 

We also discuss the new and still growing threat of plastic pollution, mainly coming from the discarded litter near the riverside. We actively encourage people to think about their consumption habits to protect aquatic wildlife and influence manufacturers, supermarkets, takeaways, local authorities, and others to do the same. These discussions are critical since, in recent years, we tend to encounter more and more seals on the river.  

Seals in the tidal Thames
The two species of seals found in the tidal Thames are the grey and harbour seals. Grey seals are frequent visitors, whilst harbour seals are residents with a breeding population in the outer Thames Estuary. Grey seals (Halichoerus grypus) are the larger of the two species, with a longer, ‘roman nose’ profile and parallel nostrils. Harbour seals (Phoca vitulina) are the smaller ones with a cat-like face and V-shaped nostrils. They can be spotted in the river chasing fish and hauled out on the Thames foreshore.

Both the grey and harbour seals’ body is well insulated with a thick layer of blubber or fat, except for their head and flippers. During their annual moult, they gradually lose their coat, subsequently increasing the blood flow below the skin’s surface to encourage fur growth, making them lose more heat. Their flippers have a dense network of blood vessels and regulate their body temperature when out of the water. To avoid getting too cold, they can shut off the blood circulation close to their skin and lift their head and flippers, taking up their signature banana-shaped pose. To avoid getting too warm, they dangle their head and flippers into the water and utilise blood circulation to cool their body. 

In recent years, the population of these beautiful animals has been improving, and there have been many more seal sightings. In most cases, when hauled out, they are simply getting warm or resting while digesting their food, so it is best to leave them undisturbed.

Magical moments
Nevertheless, close seal encounters can also happen on the water whilst paddling. These are quite magical moments, especially in that they always approach the paddleboard on their terms. If they climb on, we always advise people to remain calm, keep their distance and not touch or push them off. They always roll back into the water after a few minutes. 

Most recently, Active360 has helped form a local group called ‘Seal Watch’, which will aim to gather more data on seal haul-out sites along the river and raise awareness of the presence and well-being of these important marine mammals. Seal Watch also hopes to persuade the Government to give seals more protection in law and secure signage on riverside over the next few months to ask dog owners to keep their dogs under control in places where seals regularly haul out.

To find out more about Active360’s led Thames Natural History tours, visit

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