Dorchester Weirs Fish Pass

Rivers in the UK have gone through hundreds of years of human modification, often impacting habitats of plants, fish and mammals living in our watercourses.

The weirs at Bucks Pool and at Overy Mill, which control the flows to Overy Mill and down the Thame act as barriers to fish moving up and down the river, impeding fish migration between the River Thame and the River Thames.

The weir at Bucks Pool has deteriorated and is vulnerable to failure. Its failure would likely change flows down both the River Thame and the Mill Stream through Overy Mill in an uncontrolled and unpredictable way, potentially resulting in adverse outcomes for river ecology and flood risk.

The River Thame Conservation Trust, working with the Hurst Water Meadow Trust and the Environment Agency have partnered to find a solution, which enables fish passage, addresses the deterioration of the weir and maintains the existing flow splits down the Mill stream and River Thame. All fish species will benefit from the improved fish passage. This includes highly migratory species native to the River Thame such as European Eel (historically present) and Brown Trout (historically more widespread), plus numerous coarse fish species (which have declined in abundance), and “minor” fish species such as bullhead, minnow and stone loach.

Key Project Partners

River Thame Conservation Trust

  • Lead organisation responsible for delivering the project.
  • Expertise in river conservation, project management, hydrology and ecological restoration.
  • Contract management of designers, modellers and baseline data gathering.

Environment Agency

  • Steering committee member on the project, advising on regulatory guidance and providing fisheries expertise.
  • Technical oversight of fish passage design.
  • Funding through the development phase of the project.

Landowners including Hurst Water Meadow Trust

  • Hurst Water Meadow Trust is a local volunteer-led charity that manages the meadow adjacent to the proposed work.

Current Project Timeline

A project of this complexity requires consideration for public amenity and safety, flood risk, biodiversity and other environmental and social constraints and often takes time to deliver.

Development and design optimisation
– 2020 to 2023
1st community engagement event – June 2023
Complete Concept Design - January 2024
Environmental Baseline - Largely complete, to be completed by Spring 2024
Design drawings and costings - March 2024
Stakeholder Engagement – February to May 2024
2nd community engagement event – April 2024
Planning Application target date - May 2024
Additional fundraising – March 2024 to Spring 2025
Detailed Design – June 2025Construction - Late summer 2025


How will the river change?

Our current preferred design option will involve installation of a rock ramp directly downstream of Bucks Pool where it narrows. At the upstream end of the rock ramp there will be a partially submerged weir which has been hydraulically designed to ensure existing flows down the Mill Stream and the River Thame are maintained, while allowing passage to all fish species.  The new weir will be sensitively integrated into the riverbed / banks and faced with natural materials.  

Directly downstream of the new weir and for approximately 20m there will be a gradually sloping rock ramp, comprising stones, boulders and naturalised bank protection, creating an attractive cascade with flow characteristics to enable fish passage.  

The sloping sides into Bucks pool with be landscaped to have a shallower gradient and there will planting in keeping with the existing environment around the pool and the rock ramp. The barrier will have the effect of increasing the water depth in Bucks Pool by up to 75 cm.


How will risks of flooding change and what about the risk of the Mill Stream drying out?

The fish passage solution has been hydraulically designed to maintain the existing low to high flows and not affect the current levels of flood risk in Dorchester upon Thames. Extensive hydraulic analysis has been conducted to evaluate the design. Topographical survey has been completed of the entire section of the channel, hydrometric data has been gathered and a 2D hydrodynamic model has been built to simulate flooding both pre and post rock ramp. The approach has been verified with the EA. The design has been developed to ensure no impact to existing flooding mechanisms.    

Additionally, the replacement of the currently failing, and highly vulnerable weir structure with a new structurally sound and purposely designed fish pass will remove the risk of an uncontrolled and unpredictable failure of the existing weir with unknown consequences for flood risk.

For low flows, hydraulic analysis has been completed to ensure that the low flow notches in the new weir are designed to maintain the existing flow splits and ensure the Mill Stream receives the same low flows as it does currently. In addition, the design will include flexibility so that the flow down the Mill Stream can be adjusted if required into the future.  


Will the bridge and existing deteriorated weir structure remain?

The current preferred design option will leave the existing weir fully submerged and move the control of flows downstream of the Bucks pool to the new weir. So the presence of the existing weir and bridge will not impact the efficacy of the design and the associated hydraulics.  

The proposed design ensures that the high velocities currently present at the existing weir during non-flood flows will be reduced significantly. Reduced velocities in turn lower the risk of obstruction by fallen trees (and the associated increase in flood risk from the Mill Stream), plus reduces risk associated to maintenance/ obstruction removal.  Nevertheless, we are conscious that aesthetically this may be of concern to the local community and therefore any changes to the existing weir and bridge will be subject to stakeholder input and availability of funding.  


Who will be responsible for ongoing maintenance?

Following an initial verification period, in which RTCT will gather data to confirm functionality. Maintenance responsibilities will be with the riparian landowner. RTCT will develop a maintenance plan along with the riparian owners as part of the project delivery. 


How will this improve the River when water quality is an ongoing concern?

RTCT acknowledges that poor water quality is a major concern across the entire Thame catchment.  We continue to engage with Thames Water, river users and landowners and have ongoing projects conceived to improve the overall water quality in the Thame. Facilitating fish passage is one of the most effective means of improving fish populations and their resilience.  RTCT will continue to pursue the longer-term objective of improved water quality, whilst also pursuing river habitat restoration such as fish passage.  These objectives are not mutually exclusive.


What about people who use the area for recreation and swimming?

Consideration for recreation and access has been part of the design process. In response to concerns raised in community consultation, the banks of Bucks Pool will be landscaped with a shallower gradient and clear signage will be installed. Ongoing collaboration with the Hurst Water Meadow Trust and engagement with local communities will inform implementation of these considerations.


What about angling?

The hydraulics in Bucks Pool will change and as such the characteristics of the Bucks pool will change from an angling perspective.  Bucks Pool will remain fishable and the project will have an overall positive impact on fish communities in the River Thame.  Access for angling is not anticipated to change.

Funding and Resources

To date, this project has been funded and is continuing to be funded by the Environment Agency through their Water Environment Investment Fund and a grant provided by the Garfield Weston Foundation.

This support has allowed us to commission specialist services from:

Environment Agency:

Catchment Coordinator - Kay Lidgard

Fish Passage Advisor - Darryl Clifton Dey

Fisheries Technical Specialist - Stuart Manwaring


Five Rivers Consultancy Ltd,

Aquamaintain Ltd.

Hydraulic and hydrological Modelling:

Horritt Consulting

Tree Surveying:

Lofthouse Wildlife Surveys Conservation Arboriculture

Archaeological Review:

Oxford Archaeology Ltd

Get Involved

We encourage active participation from the community and stakeholders to contribute to the success of the Dorchester Weirs Fish Pass project. If you are interested in getting involved, here are some ways you can engage:

Stay Informed: Keep up-to-date with project developments by following our official communication channels, including our website, social media, and newsletters.

Sign up to our newsletter

Attend Public Meetings: Join us at public meetings and consultations to learn more about the project, ask questions, and provide your valuable input.

Volunteer Opportunities: Look out for opportunities to volunteer for activities related to the project, such as habitat restoration or community engagement events.

Sign up as a volunteer

Get Involved

We are reliant on the support of individuals, organisations, and businesses to make this project a reality. Your financial contributions can play a crucial role in achieving our goals. Here's how you can help:

Individual Donations: Consider making a personal donation to support the ecological restoration of the River Thame. Every contribution, no matter the size, makes a difference.


Sponsorship: Businesses and organisations can contribute to the project's success by providing sponsorship. Your support will be acknowledged, and you'll be part of a vital initiative for environmental conservation.

Community Fundraising: Organise or participate in community fundraising events to gather support for the project. Your collective efforts can have a significant impact.

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