In the UK we have fundamentally changed the way our rivers work, straightening and deepening their channels to get water off the land as quickly as possible. This has effectively turned them into water chutes, which benefited our agricultural and industrial industries by providing drainage and water transport corridors. We have also built weirs and locks to control the water flow.

But this fragmentation has led to some major problems. A river is meant to meander across the landscape and have areas either side that can flood safely, called floodplains. Floodplains allow for natural drainage, create wetlands that combat pollution and create habitats that are very important for migratory fish. The lack of floodplains as well as the presence of man-made barriers have contributed to the decline of some of the most imprtant migratory fish species, as they are unable to reach their breeding, nursery and feeding grounds.

Various EU directives and national legislation require improvements to our rivers and, in turn, fish migration. Some great work has already been done to ‘re-meander’ rivers, restore wetlands and reconnect floodplains. And technical solutions to man-made barriers – collectively known as “fish passes” – are helping to recreate fishes’ natural migration routes. Although they vary in form and complexity, many enable fish to navigate barriers by swimming and leaping up a series of low steps leading into the water on the other side.

Thames Estuary Partnership has been working on an exciting project called the Greater Thames Estuary Fish Migration Roadmap and, to make our in-house barrier dataset more robust, we need your help to gather more data.

With the data collected will be able to visualise entire migratory routes and create a roadmap identifying ‘Highways’, ‘A-roads’ and ‘B-roads’ that migratory fish would use if there were no man-made barriers preventing them. As well as helping to prioritise where fish passes are needed, the completed roadmap will help to identify opportunities for river restoration and habitat creation.



Fish pass

The animated illustration below highlights the complex lifecycle of the European Eel. During their larval stage they make their transatlantic journey from the Sargasso Sea into European rivers (including the River Thames). Here, they start their metamorphosis first as glass eels, then as elvers growing and maturing into yellow then into silver eels. Silver eels then make their transatlantic journey back onto the Sargasso Sea where they mate and spawn. Interestingly, throughout their lifecycle the from and coloration of their eye also changes adapting to the different environment.
Image source: Awaroo

The example image below shows the result of river restoration efforts carried out on the Hogsmill in recent years. The Hogsmill is classifed as 'A-road' and it is directly connected to the River Thames which is classified as 'Highway'. With either pass (fish and/or eel) installation and barrier removal/modification works, it is visible how the river has been opened up (red/closed waterways turning into green/open waterways) allowing migratory fish to move upstream and downstream in the Hogsmill as well as to and from the River Thames. You can read more about the excellent work that has been carried out by the South East Rivers Trust here.


The interactive map below focuses on man-made barriers within Greater London. By clicking/tapping on the top right corner of the map the layer control will collapse allowing you to tick and untick the different map layers to show and hide. The 'Barriers' layer will show all the exisiting barriers in London with each marker showing more information after clicking/tapping on it.

Using the Barrier Information Form, you can help us confirm the type and exact location of barriers and whether there is a fish pass installed with photo evidence.

Barrier Information Form

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