Why does GARD object to the proposed reservoir?
What initially began as opposition to an ill-conceived reservoir proposal and lack of transparency from Thames Water has evolved into a deeper understanding of a broken system by which future water resources are selected and justified.
GARD believes the current water resource planning process is fundamentally broken. Large-scale infrastructure projects, such as the proposed reservoir, are being advanced without proper scrutiny, accountability, or evidence to support their necessity or feasibility. Critical, technical and environmental flaws in the plan for the proposed reservoir have been ignored or deflected, despite warnings from GARD, CPRE, local councils, and MPs. This must be addressed before the public is burdened with decades of unnecessary costs and environmental harm.
“Private water companies have an incentive to back the creation of supply-side projects like reservoirs above demand-reducing measures like education programmes, said Kevin Grecksch a departmental lecturer in the University of Oxford’s school of geography and the environment.”
(Big Issue, January 2025)
GARD’s objections:
Flood risk
The proposed reservoir would be constructed in a flood-prone area. This raises concerns about two types of flooding: fluvial (river) flooding and groundwater flooding. Thames Water has yet to understand local ground conditions adequately. In February 2024, its experts warned that the reservoir could raise groundwater levels by up to 1 metre without extensive mitigation measures. Such an increase would exacerbate flooding, given that the water table is already less than half a metre below ground level given that the water table is less than half a metre below ground in some of the areas surrounding the reservoir. Mitigation measures are yet to be designed or costed. The resulting increase in cost is likely to be large,
Additionally, a 2018 report highlighted that there was insufficient space on-site to compensate for the loss of the existing flood zone. Proposed plans include excavation of large flood lakes immediately adjacent to a main road and new housing.
Despite over 20 years of planning, there are still no reliable models to predict the proposed reservoir’s effects on flood risks, raising concerns about the adequacy of preparation and mitigation efforts. All proposed plans include diverting more water into the River Ock catchment, which flooded multiple times in 2024.
Financial concerns
Water companies in England and Wales have been able to exploit a flawed system to prioritise profit over sustainable solutions. Original estimates for the build cost were £2.7bn (2022 prices) and with a lifetime cost of £7.5bn (2022 prices); the proposed reservoir is among the most expensive of the nationally significant schemes.
At the Judicial Review (25–26 June 2025), TW maintained the £2.2bn figure. Yet mid-August, just seven weeks later, TW publicly revised the cost to two to three times that amount. The latest published by Thames Water places the initial capital expenditure (Capex) at £6.6 billion in 2022–23 prices. Adjusting for construction price inflation of 7.6% from 2022/23 to 2024/25, this figure rises to approximately £7.1 billion in 2024/25 prices. However, this figure excludes the costs of the Thames to Affinity Transfer (T2AT) and Thames to Southern Transfer (T2ST), both of which are essential components of the SESRO scheme. Based on previous estimates (from Ofwat, at 2022/23 prices), T2AT is expected to cost around £400 million, and T2ST approximately £1.1 billion, bringing the total initial Capex to £8.7 billion.
In only October 2024 TW assured regulators their estimates were ‘robust’ and spelt out that an increase of costs of between £669m and £803m would be the tipping point when the Severn Thames Transfer scheme would represent better value.
Now, despite a >£4 billion increase, without a re-evaluation, they continue to assert that this reservoir offers customers ‘best value’ over fixing leaks and transferring water and press ahead.
Customers will start paying for the reservoir 12 years before water even starts to be delivered and continue to pay during its 250-year lifetime. Not only will all Thames Water, Affinity Water and Southern Water customers pay for construction and operational costs, but also the £3 billion under Ofwat’s return on regulated capital value (RCV) mechanism for every £1 billion spent. The reservoir is an asset that makes Thames Water more valuable, only to its investors, and is therefore more attractive than water management alternatives.
Major UK infrastructure projects are notorious for overrunning on budget and time. Design revisions, cost escalations, and regulatory delays seriously undermine the credibility of approving schemes such as this reservoir on the basis that they represent best value for bill payers.
Learn more:
In this interview GARD’s finance lead discusses how the reservoir costs are still widely inaccurate and the scale of cost rises yet to come. He explains how Thames water bill payers would be faced with a 25% increase in bills to pay for construction, long-term financing and shareholder returns.
About the economic motivations behind the push for reservoirs in the 2025 Cunliffe Water Enquiry.
Lack of drought resilience
The empty Cocksburn Reservoir
GARD’s modelling demonstrates that the reservoir could run dry during prolonged drought, leaving it significantly depleted at the start of summer droughts and especially vulnerable to multi-year droughts.
While the proposed reservoir would increase water storage by pumping water from the River Thames, it would not provide “new water” to fill the reservoir, and relying on rainfall is not the answer, as it would take 70 years to fill it naturally. Records show that, if the proposed reservoir had been there in the 1975-76 drought, it could not have been filled from the Thames River for 17 consecutive months.
Poor drought resilience is a concern echoed by the Environment Agency, which advised against advancing the scheme. Other countries are adopting more sustainable, drought-resilient solutions than outdated reservoirs.
Read more:
Oxford University’s Kevin Grecksh argues why more reservoirs aren’t the answer to the UK’s water shortage issues.
Damage to the environment
Habitat loss and biodiversity decline
Analysis by environmental expert, Dr Julian Parfitt, shows that the proposed reservoir would destroy productive farmland, hundreds of ancient trees, and miles of hedgerows.These are all habitats for protected species such as bats, birds, otters, voles and polecats. Ecosystems are created over many hundreds of years, hence the loss of biodiversity from the proposed site cannot be fully mitigated, and Thames Water downplays the true environmental impact, while exaggerating mitigation by any wetlands it promises to create.
Carbon footprint
Construction and operation of the reservoir would generate a substantial carbon footprint—the largest of all the proposed water resource projects—from materials, excavation, transport, and energy use—undermining net zero goals.
Irreversible impact on the landscape
The visual and physical transformation of a large swathe of Oxfordshire’s rural landscape would be permanent, altering the character and heritage of the area for generations, and also altering the local climate, with low-lying fog.
Flawed justification
Government data indicates that total water demand in the UK is expected to decline by approximately 15% by 2050. This predicted decline is driven by long-overdue pressure to reduce leakage and encourage greater water efficiency. Yet many large-scale water infrastructure projects, including reservoirs, are being justified on the basis of increasing demand. This fundamental contradiction undermines the case for large new reservoirs.
Unexamined “environmental destination” targets are driving Water Resource Management Plans, including Thames Water’s putting forward the proposed reservoir, without proper cost–benefit analysis. For example, the £3 billion lower River Lea water abstraction project is deemed unnecessary for river ecology. However, experts suggest only 1/3 of proposed schemes are vital for protecting chalk streams, crucial to the UK water ecosystem, while nearly 70% (with replacement infrastructure costing over £35bn)—are unnecessary for good ecological river health and could cause environmental harm. This lack of scrutiny risks misusing billions of consumer funds on projects that could damage both the environment and public finances. The Environment Agency acknowledges this:
“There has been no assessment of the actual ecological benefits that flow compliance [aka abstraction reductions] would deliver, or of their total cost, best value timing or affordability.”
Environment Agency,
National Water Framework 2025, Appendix C.
Social disruption
The extensive scale of the reservoir project would bring 10 years of heavy vehicle movements, intensifying traffic, noise, and air pollution. The Marcham Interchange on the A34 would become a pivotal access point for construction traffic to the proposed reservoir site, exacerbating existing congestion and safety issues at this accident-prone junction.
Tunnelling plans under the A34 (for the inlet and outlet pipes between the reservoir and the Thames) would lead to disruption more widely across Oxfordshire, affecting not only the interchange but also the surrounding region. Engineering works may result in partial road closures, traffic diversions and delays, causing a ripple effect of disruptions.
The cumulative impact would compromise one of Oxfordshire’s most vital transport corridors for an extended period, negatively affecting commuters, businesses, and emergency services, and placing immense strain on the area’s infrastructure and economy.
Embankment failure
A breach in one of the reservoir walls (embankment failure) is extremely rare, but with 150 billion litres of water (70% retained above ground), the impact could be catastrophic. Detailed analysis and robust planning for emergency scenarios should have been fully developed and transparently assessed from the outset. For example, if a fault were detected, draining the reservoir to the River Thames would take 15 days and require local villages and parts of South Abingdon to be evacuated. The risk of undetected breaches, with potentially fatal consequences for communities in East Hanney, Steventon, and South Drayton, highlights that this shouldn’t have been ignored.
Criticism has arisen over Thames Water’s delay in completing a mandatory Dam Breach Assessment, which evaluates property damage and potential fatalities. Professor Christopher Binnie, adviser to GARD and expert water engineer, has highlighted the lack of credible emergency drawdown plans, with safety and flood risks for downstream towns and villages including Wallingford, Goring, and Henley.
Read more:
Professor Chris Binnie (MA, DIC, Hon DEng, FREng, FICE, FCIWEM) comments on the engineering design of the proposed reservoir in his report: