The River Thames and its tributaries


Some of the River Thames tributaries in London.

The River Thames plays a vital role in our landscape, culture, and economy. It’s the longest river in England and the second longest in the UK after the River Severn. Its 215 miles flow from Kemble in Gloucestershire to the Thames Estuary in Southend-on-Sea, where it meets the North Sea. Along the way, it’s fed by many tributaries.

A tributary is a body of water that feeds into a larger stream, river – also called a parent river or mainstem –, or lake. Tributaries, also known as affluents, help drain drainage basins and don’t flow directly into the ocean. Most large rivers, such as the Thames, are formed from many tributaries. The River Thames itself is believed to have been a tributary of the ancestral River Rhine around 30 million years ago.

It is thought by some that there are more than 30 tributaries that flow in the Thames. The main ones are the Coln, Churn, Evenlode, Kennet, Lea, Leach, Loddon, Ock, and Windrush. The Coln and the Churn rivers join the Thames in Wiltshire; the Evenlode, Ock, and Windrush in Oxfordshire; the Kennet and Loddon in Berkshire; the Lea in East London; and the Leach in Gloucestershire.

The River Kennet is the largest one at 45 miles long and is also one of England’s most important chalk streams. It’s a biodiversity sanctuary, home to many plant and animal species. The stretch of river between Marlborough and Woolhampton is designated a Site of Specific Scientific Interest (SSSI) for its unique species and high conservation value.

There are many protected sites within the Thames and its tributaries, both aquatic and terrestrial. Unfortunately, just like most rivers in the UK, these waterways are affected by different pollution sources and other pressures such as invasive species, water abstraction and urbanisation.

At Thames21 we aim to improve the River Thames and its tributaries by taking a multi-stakeholder approach. We empower people through our education programme; we use nature-based solutions to enhance rivers’ health; we conduct pioneering research to advocate for better policies; and we have a dedicated and engaged network of volunteers who support our projects.

There are many volunteer opportunities to get involved in our projects. Check our events page or contact us to find out more.