Erbistock Weir, Wales

Erbistock Weir removal boosts biodiversity of the River Dee

The River Dee

The River Dee is located in north Wales, UK. It is the largest river in North Wales with a catchment area of more than 1800 km². It is one of the most highly regulated rivers in Europe, and along with Llyn Tegid it has been designated as a Special Area of Conservation (SAC). From its headwaters in the uplands of Eryri (Snowdonia), the Dee descends via Llyn Tegid, the largest natural lake in Wales. After flowing through a broad valley to Corwen, it turns eastwards through the Vale of Llangollen, under the famous Pontcysyllte Aqueduct World Heritage Site, before breaching the Welsh foothills near Bangor-on-Dee, and meandering northwards through the Cheshire plain to its tidal limit just below Chester.

The River Dee has significant importance for both economic and ecological reasons. The main uses of the river are farming, predominantly cattle and sheep grazing, and water abstraction to supply for 2.5 m people. It also supports tourism through recreational activities like angling, canoeing and navigation. The River Dee is inhabited by several species included in the Annex II of EU Habitats Directive, like sea lamprey (Petromyzon marinus), brook lamprey (Lampetra planeri), river lamprey (Lampetra fluviatilis), bullhead (Cottus gobio) and otter (Lutra lutra). This river cathment is particularly important though due to the presence of Atlantic salmon (Salmo salar) and the floating water-plantain (Luronium natans).

The Erbistock Weir

Erbistock Weir (Figure 1) was located on the River Dee near Erbistock in Wrexham (Figure 2). It was built approximately between 1820 and 1840 to supply water to a mill and was privately owned. Records of a structure at the site though date back to the 11^th century.

Figure 1. Erbistock Weir, Dee River, Wales, UK. The Baulk fish pass visible on the right-hand bank (looking downstream) © LIFEDeeRiver

Figure 2. Location of Erbistock Weir, Dee River, Wales, UK © Crown copyright and database right 2024

The weir was a significant barrier to the migration/passage of a range of fish species as evidenced by acoustic tracking work carried out during the LIFEDeeRiver project. More specifically, upstream migration of Atlantic salmon was systematically delayed or blocked by the presence the Erbistock Weir (Figure 3). The impact of the weir on sea lamprey and river lamprey was more severe as any upstream movement was prevented completely (Figure 3).

Figure 3. Acoustic tracking work carried out during the LIFEDeeRiver project demonstrating the impact of Erbistock Weir on fish migration. Note: the solid horizontal line represents Erbistock Weir, the yellow triangles on the Y axis represent the location of acoustic receivers in the river and the red dots indicate a detection by the receivers of an acoustic tag attached to a fish. © LIFEDeeRiver

Barrier removal

The removal of Erbistock Weir was delivered under Notice, which was served under the following legislation: Section 172 and Schedule 20 of the Water Resources Act 1991, Section 10 of the Salmon and Freshwater Fisheries Act 1975 Act 1995, Article 9 of the Natural Resources Body for Wales (Establishment) Order 2012. The planning for the removal took around three years, including discussions around heritage value of the structure and Detailed Design of the demolition works (Figure 4).  The latter included – amongst others – assessments of the flood risk and the impact on water depth changes and morphology.

Figure 4. Part of the design of the demolition works for the Erbistock Weir, Dee River, Wales, UK © LIFEDeeRiver

The demolition work was put out to mini-competition and was awarded to S.E. Metcalfe a‘i Gwmni. The removal works (Figure 5) took a total of 11 weeks to complete on the ground. Prior to the demolition (in 2023) Heneb archaeologists carried out a level 2 survey, covering the landscape and setting of the features (mill building and weir). During the removal of the weir the archaeologists were overseeing the works, which allowed them to record the construction materials, artifacts and fully document the weir. This ensured that all this information would be recorded and made available to the public, thus protecting its historical significance.

Figure 5. Removal works of the Erbistock Weir, Dee River, Wales, UK © LIFEDeeRiver

The removal of the Erbistock Weir (Figure 6) was delivered as part of the LIFEDeeRiver project, a £6.8 million, cross-border (Wales and England) project that will bring multiple benefits to the environment, particularly improving the numbers of salmon, lamprey and freshwater pearl mussels, helping them become more sustainable in the future. The project is funded through the LIFE programme, with additional funding from Welsh Government, Environment Agency, Dŵr Cymru/Welsh Water, Eryri National Park Authority, and the River Restoration Centre. Total cost of the removal was ~£510000.

Figure 6. The Erbistock Weir site, Dee River, Wales, UK, following the completion of the demolition works © LIFEDeeRiver

The LIFEDeeRiver team spent a lot of time prior to the works talking to local anglers, landowners, and interested parties. Historically, below the weir has always been one of the more productive fishing beats on the Dee, due to the fact that salmon were held up there. Anglers were generally supportive of the work, as they understood through discussions with staff that the removal would be better for the river as a whole.

There were some mixed feelings about the removal of the weir as many local people remembered it as somewhere they played when they were children, or went to watch salmon jumping.  During delivery, we ensured there was a member of the LIFEDeeRiver team on site every day to talk to people, and many of these people appreciated why we were doing the work, even if they maybe would prefer for it not to happen.

Challenges

Safety on site:

The safety of staff working on site during the removal of the Erbistock Weir was of paramount importance during this project. As a result, a specialist organization (outdoor expertise) was contracted to provide (a) pre-works site specific training and (b) on-site monitoring during the works.  The pre-works training involved generic working in water training, but also a site-specific day with the rescue team and the contractors to understand and plan for site specific issues. During the works a rescue team was in position onsite the whole time staff were working in the water.  They also carried out ad-hoc rescue drills during the works.

Access across land:

The access for the site was approximately 580 metres across an arable field.  For much of the time the work was being carried out, the weather was warm and dry, however there were periods of intense rainfall which made the ground very wet. To mitigate for this, the contractor installed a metal trackway across the field, which allowed deliveries and staff to safely access the site. If this had not been installed, deliveries of concrete and materials would have been delayed, and the rescue team may not have been able to make it to site. Although this was a considerable expense, it was worth it to ensure the project remained on schedule.

Staff on site – Comms:

Comms was central to effectively delivering this project, and ensuring the correct messages were disseminated to the public. Prior to the works, an article was placed in the local community newsletter, and during the work interpretation boards with QR codes were installed around the site. We also ensured that staff were present every day to talk to any members of the public that came to the site (it was by a public footpath).

Ecological and community benefits

The benefits of the removal of Erbistock Weir on fish communities have already been visible. Following the weir removal, 25 sea lamprey redds (nests) have been discovered upstream of the former barrier – a clear and tangible sign that environmental interventions on the River Dee are gradually transforming the ecosystem for the better (Video 1). In July 2025, one of these redds was captured by drone just 40 metres downstream of Manley Hall gauging weir. This observation confirms that the removal of the Erbistock Weir has opened up an additional 4 kilometres of critical habitat, enabling sea lamprey to return to spawning grounds long restricted by man-made obstacles.

https://www.facebook.com/reel/1437896090960795

Video 1. Sea lamprey redds (nests) been discovered upstream of the former Erbistock Weir site © LIFEDeeRiver

The redd surveys will be carried out every year during the river and sea lamprey spawning periods (conditions allowing). Drone surveys will also continue allowing the production of ortho-rectified images of the stretch upstream of the weir site (Figure 7), so that changes to the river can be assessed.

Figure 7. Ortho-rectified images of the stretch upstream of the Erbistock Weir site created by drone footage (above) before and (below) after the removal of the structure © LIFEDeeRiver

The weir removal has ensured that passage for all fish species, not just sea lamprey, is now unhindered for the first time in nearly 200 years. While the project is yielding significant results above the former weir site, the concentration of sea lamprey redds immediately below Manley Hall gauging weir highlights that this structure remains a significant bottleneck to further upstream migration. Many fish are still forced to spawn just downstream of this structure, restricting their natural instinct to migrate further upstream to key spawning areas.

Watch a live interview from the former Erbistock Weir site:

More information is available on the following sites: 

https://naturalresources.wales/LIFEDeeRiver?lang=en

https://naturalresources.wales/about-us/news-and-blogs/news/biodiversity-boost-for-the-river-dee-after-erbistock-weir-removal/?lang=en

https://www.ecrr.org/Portals/27/ECRRTechnicalNewsletter%201-2025-part%20River%20Dee.pdf

Before & After photo pair

The restored site at Erbistock Weir (left) before and (right) after the removal works © LIFEDeeRiver

With many thanks to Joel Rees-Jones from Natural Resources Wales/Cyfoeth Naturiol Cymru for providing the information and the visuals presented herein.

Prepared by Foivos A. Mouchlianitis