What does Thames Water propose?
Thames Water proposes an above-ground reservoir with walls (‘bunds’) up to 25 metres high, comparable to an eight-storey building. It would hold an extraordinary volume of water—150 billion litres, equivalent to 150 million tonnes in weight (and 1 billion baths), and cover 4.0 square miles (roughly the size of Gatwick Airport). The proposed reservoir is the largest of its kind in Europe — three times bigger than the maximum dimensions of 50 billion litres considered suitable for the site during the public inquiry held in 2010.
Where will it be?
The proposed reservoir would be situated between Steventon, Drayton, Marcham, East Hanney and Abingdon, and would be built across farmland on a flood plain which is essential to the surrounding area. This presents major construction challenges and threats to the area. Access for construction vehicles would be via the A34 and Marcham, a known congestion hotspot with a high accident rate. Millions of tons of rock and gravel will be brought to site via the congested Great Western train line, requiring a large new marshalling yard to be built between Grove and Steventon.
How will it work?
The proposal would be to pump untreated water from the River Thames to fill a reservoir, with water pumped from Sutton Courtney / Culham, although this is likely to be restricted during high rainfall due to quality concerns. A £1 billion pipeline would then transfer water from the reservoir outside the Thames Valley area to two other water companies, Affinity (15%) and Southern Water (30%). An emergency drain tunnel, currently in early planning stages, would need to be tunnelled under the A34. It would release billions of litres of water into the River Thames at Sutton Courtney / Culham in the event of an emergency.
When would it be built?
If the reservoir receives a Development Consent Order in 2026, the construction would be started in 2027, with completion planned at the end of 2037. It would then take until the end of 2039 to fill the Reservoir. The reservoir would then open in 2040.
What would not be built?
Unlike the nearby Farmoor Reservoir, the proposed Abingdon reservoir is unsuitable for watersports or recreational facilities, with access strictly controlled. Amenities such as walks, trees, and picnic spots are impractical for a reservoir of this size and strategic importance. The reservoir will not effectively aid flood management due to its 25 metre high banks and limited pumping during floods. It cannot fill with rainwater, requiring 70 years to do so, nor would it reduce river pollution, as claimed.
A Timeline of Events
Thames Water has been focused on building the reservoir since the 1990s.
1996
The reservoir idea first appears.
2006 – 2009
In early planning documents to provide water for Oxford and Swindon, Thames Water includes the idea of a major reservoir (100 million cubic metres) in Oxfordshire.
2010
A public inquiry rejects the Abingdon reservoir plans:
- too big (maximum size that should be considered: 50 million cubic metres).
- neither fit for purpose nor compliant
- not justified by facts, Thames Water had overestimated demand
- important alternatives to the proposed reservoir had not been properly investigated — in particular, options to transfer water from the River Severn to the River Thames to supply London’s reservoirs.
2019
In the draft Water Resource Management Plan, Thames Water proposes the reservoir again — this time, at 150 million cubic metres. This plan is vigorously opposed.
2022
Thames Water, now in partnership with water companies, Affinity Water and Southern Water, proposes a reservoir (100 million cubic metres) as part of the Water Resources South East Regional Plan:
- now referred to as water for London, but almost half to be transferred out of the water-stressed Thames Water area to sell to Southern Water and Affinity.
2023
The draft and regional plans are revised, with no separate public consultation:
- no justification of how this represents ‘best value’.
- proposed reservoir capacity increased from 100 to 150 million cubic metres increasing safety and flooding risks.
December
GARD calls for a public inquiry, requesting that:
- inclusion of the reservoir in Thames Water’s plan not be allowed to proceed for further development without a proper and transparent assessment of the costs, environmental impact, drought resilience compared to alternatives, and safety and flood risk potential, with the inquiry process and findings made available to the public.
This request is supported unanimously by local councils, MPs and environmental group Campaign for Rural England (CPRE).
2024
Thames Water, in its 2024 Water Resources Management Plan, sets out how it intends to supply water from 2025 to 2030. The reservoir is chosen in the only scenario to 2040 that Water Resources South East has considered.
In August, the new Secretary of State waves the plans for the proposed reservoir through to the next stage (starting the application for a Development Consent Order), ignoring the unaddressed problems and GARD’s call for a public inquiry.
It has since emerged that the Environment Agency, not in favour of the plan, wrote to Thames Water in April 2024 raising 14 issues. Thames Water made changes to the plan in July 2024. These changes and the response were not published until October 2024, after Steve Reed approved it.
Within this some major alterations were made without any opportunity for public scrutiny. The amendments demonstrate a reduced water deficit after 2050 and further call into question the need for a mega reservoir of this scale and it’s selection as the ‘best value option’.
Thames Water insist to DEFRA that ‘the level of estimating detail, relative design maturity and significant
allowances for uncertainty in the SESRO cost estimate give confidence that it is a reliable basis for comparison with other WRMP schemes.’2025
GARD’s sister company ‘SAFERWaterS’ and CPRE launch a claim challenging the Secretary of State’s decision to advance Thames Water’s plans for the proposed reservoir, with the aim to reverse this decision and call for a public inquiry with full scrutiny.
GARD and CPRE deliver a petition with over 6600 signatures asking for a public inquiry.
A Judicial Review in the High Court granted took place on 25–26th June but the claim was rejected and public inquiry denied.
7 Weeks later Thames Water admitted the costs have escalated from £2.2 to £6.6 Billion
2026
Application for Development Consent Order to be made.
2027 – 2028
Development Consent Order examination and decision on the proposed reservoir to be made.