Project Overview:
Welcome to Denham Eel Pass Project page.
Groundwork South, Affinity Water and private landowners are working in partnership to resolve barriers to the migration of European Eels in the Colne River Catchment in Denham. This is an exciting project to improve eel passage in Colne Valley Regional Park and deliver environmental benefits for an endangered species and is also part of the Landscape Partnership Scheme.
Project Background:
The Colne Valley Park has over 200km of rivers and canals with over 60 lakes which host a wide variety of 16 native fish species including Barbel, Chub, Dace and European Eel.
European eels (Anguilla anguilla) once thrived in London’s rivers but the number of young joining the adult populations has dropped so dramatically since the 1980s that they are now classified as ‘Critically Endangered’. They migrate over 3000 miles all the way from the Sargasso Sea to European freshwaters, where they live most of their adult life. However, the migration of eels is often prohibited due to physical barriers in the river channel, such as weirs, preventing access to their living habitats which affects the resilience of eel stocks within the river catchment. However, there is a solution as with the installation of eel passes to allow free movement.
Three barriers have been identified: one on the River Colne and River Frays at Denham Lodge in New Denham and River Frays and Colne at Denham Country Park.
What is currently happening?
Three barriers have been identified: one on the River Colne at Denham Lodge in New Denham and River Frays and Colne at Denham Country Park.
The River Colne is culverted and flows under Willow Ave where the weir is located. We are investigating and designing the installation of a modular eel pass which are tiles mounted onto the edge of the weir enabling eel to transverse the structure. This is a simple but effective solution to eel passage with minimal impact and disturbance.
The weir structure at Denham Country Park is an impressive structure controlling the flow between the Colne and Frays rivers as well as hosting the Environment Agency’s telemetry stations. These factors mean the weir cannot be altered in any way that may impact its primary purpose and changes to the water level must be minimal. Therefore, a low impact solution has been identified – modular larinier pass. The pass would span both steps of the weir, providing a suitable climbing substrate for eels to transverse the structure a simple but effective solution. This design is a low impact solution with the low volumes of water required flow through the pass passage with calculations to ensure minimal impact on telemetry, dynamics of the weirs and disturbance to the local environment. This is great news as it brings a simple solution with no landscape changes as well as great wins for the critically endangered European eels, which will be enabled to traverse to the next section of the river.
The construction phase will be occurring in September 2022.
Find out what happened next here: Denham Eel Passes Installed.
This pictures shows how Glass Elvers negotiate a brush Eel Pass, many thanks to Cliff Carson for allowing us to use.
Contact Us
If you have a question about the project or would like further detail please contact us.
Questions and Answers and Slippery Eel Facts
What are the structures being bypassed?
Wier and other in channel features act as massive walls which the young eels can’t bypass preventing them reaching vital habitats for their development. By inputting eel passes /eel ladder that will allow the young eels to reach the next stretch of river.
How do eels migrate when they are so tiny?
Eels rely on transatlantic currents such as the Gulf Stream to carry them across the Atlantic Ocean to our shores. Once they reach our rivers, they will use tides to carry them upstream. Once they reach non tidal sections it’s up to them to swim, but by this stage they have changed in shape, appearance and strength and are better suited to swimming.
How do we know they spend 20 years here? Have they been tagged?
Fish can be aged via analysing a scale sample or otolith (ear bone) under a microscope. They contain age rings, much like the trunk of a tree, and when counted will tell you how old a fish is and how long it has spent in sea or freshwater.
Do the eel passes work both ways or is it easier to get downstream? Presumably the pass is designed for the elvers?
Eel passes are designed for upstream migration. Providing a barrier does not cause damage to fish passing downstream, eels are generally able to plop over the crest of a weir and into the pool downstream. No pass required unless there are features that may damage the eel as it moves over the weir.
What is it about the Colne Valley Park that makes it ideal habitat for an eel?
Bountiful wetland habitat with abundant sources of food (freshwater invertebrates and fish). Eels will make use of the rivers in the Colne Valley Regional Park and all the waterbodies connected to them (canals, gravel pit wetlands, ditches, ponds). Plus eels have been migrating to the Colne for thousands of years and it is this inherent instinct that directs them here.
How long is a mature eel?
60-100cm. Bigger in some exceptional cases.
How will this benefit me?
The project aims to provide sustainable solutions to resolve barriers to fish migration, issues affecting the river corridor and associated species. It will enable free movement for fish for them to reach areas previously inaccessible so increasing the amount of fish seen upstream and encouraging high species diversity. Helping a ecosystem thrive resulting in a more dynamic and diverse populations which benefits the health and enjoyment of the local blue and green spaces.
Will access to the area be restricted during the works?
Walking routes and throughfare will still be accessible as normal however no access will be allowed into the construction area.
What trees will be removed?
One tree will be removed to enable access to the works. This will be replaced with another.
How will we be notified about the works?
Via this website and posters on site.
