London is desperate for more rain. After months of unusually dry weather, the capital is living through drought conditions and its depleted rivers, groundwater and reservoirs need sustained rainfall to recover. Yet there is an awkward complication. When rain comes, London also needs it not to come too quickly.
The contradiction is more familiar than it might sound. In July 2021, two intense storms flooded more than 2,000 properties across 24 London boroughs, including homes, schools, hospitals and Underground stations. One of those storms produced around five times as much water as parts of London’s sewer network could accommodate. Five years later, almost 320,000 London homes and businesses are estimated to be at high risk of surface-water flooding.
London therefore finds itself contemplating flooding in the middle of a drought. We need enough rain, sustained over time, to replenish the water resources on which the city depends, but intense rainfall can overwhelm drains, sewers and the ability of the ground to absorb it. London’s acres of roads, roofs and pavements compound the problem by giving rain fewer places to go. A summer downpour can cause considerable damage without necessarily putting enough water back where we need it.
It points to a more interesting way of thinking about London’s relationship with water. The challenge is not simply whether we have too much or too little. It is whether water of the right quality is in the right place, in the right quantity, at the right time, and whether the infrastructure exists to move it when it isn’t.
London has been working on that problem for almost two thousand years.
The Thames is one of the fundamental reasons London exists where it does. For the Romans, a navigable tidal river connected their new settlement to the sea while a bridge connected the roads on either side. For the city that followed, the river became a source of water, transport, industry and trade.
As London grew, however, exploiting that geographical advantage required increasingly ambitious engineering. Local wells and watercourses could not indefinitely support an expanding population. The New River, completed in 1613, carried drinking water for around 40 miles from Hertfordshire to Clerkenwell. More than two centuries later, London’s growth created the opposite problem: disposing safely of the waste produced by millions of people. Joseph Bazalgette’s great Victorian sewer system responded by intercepting sewage that had previously entered the central Thames and carrying it eastwards.
The pattern is more important than the individual projects. London has never finished building its water infrastructure because the problem itself keeps changing. A larger population requires more water. More buildings and paved surfaces change the way rainfall moves through the city. Rising living standards alter consumption. A changing climate shifts the assumptions about rainfall, drought and sea levels on which previous generations based their engineering.
The result is not one London water system, but layers of infrastructure built centuries apart and still working together.
Perhaps the most visible example stands across the Thames at Woolwich.
The Thames Barrier became operational in 1982 after decades of work to reduce London’s exposure to catastrophic tidal flooding. Its purpose is easy to understand: when a dangerous combination of high tide and storm surge threatens the capital from the North Sea, its gates can be raised to stop that water travelling further upstream.
Yet the Barrier’s busiest ever winter demonstrated how much more complicated London’s water problem can be.
During the exceptionally wet winter of 2013/14, enormous volumes of rain fell across the Thames catchment and travelled downstream towards London. The Barrier closed a record 50 times during that flood season, but on 41 occasions the principal reason was high river flow rather than water coming in from the sea. By closing around low tide, the Barrier could prevent the incoming tide from occupying space in the river, creating capacity for the freshwater coming downstream when the gates reopened.
The Thames Barrier was conceived primarily to stop the sea coming into London. During its busiest winter, most of its work involved helping London deal with rainwater trying to get out.
The distinction matters because flooding is not a single problem. The Thames can rise, intense rain can overwhelm a neighbourhood far from the river, and rain falling many miles upstream can eventually reach London through the wider catchment. The Environment Agency’s current strategy explicitly describes the Barrier as protecting London from both tidal surges and significant river flooding.
London has also recently completed another enormous piece of engineering to address what happens when rain reaches the city’s sewers.
Much of central London’s Victorian network combines wastewater and rainwater in the same pipes. Bazalgette’s system included overflow points so that, when heavy rain overwhelmed its capacity, diluted sewage could discharge into the Thames rather than back up into streets and buildings. A safety mechanism that made sense for the nineteenth-century city became a serious source of river pollution as London grew.
Rather than attempting to replace thousands of miles of existing sewer, the solution has been to add another layer. The 25-kilometre Thames Tideway Tunnel became fully connected to London’s existing sewer network in February 2025, intercepting storm sewage that would previously have overflowed into the river. Tideway says the new system is now stopping around 95 per cent of the sewage spills that previously entered the tidal Thames and, by June this year, had captured more than 20 million tonnes of sewage.
Neither project means London has “solved” flooding. Tideway primarily addresses sewage pollution from overloaded combined sewers, while the Barrier manages flood risk in the tidal Thames. Surface-water flooding remains a major risk in its own right. What they demonstrate is London’s repeated ability to identify a water problem and build extraordinary infrastructure in response.
This week brought a reminder that even those solutions eventually need successors. Under the Environment Agency’s current Thames Estuary 2100 strategy, an option for the Barrier’s long-term future is due to be selected by 2040 and operational by 2070. But new assessments reported this week suggest that increased use and the resulting pressure on maintenance could mean replacement is required between 2050 and 2060. With a new barrier potentially taking decades to plan and build, a decision that once appeared comfortably in the future may be becoming considerably more immediate.
In other words, London may need to begin planning the successor to one of its most famous pieces of infrastructure while the existing one is still doing exactly what it was designed to do.
The present drought poses the opposite question. If London needs more water, where can we get it?
It is an obvious question in Britain, an island famous for rain, and particularly in London, where an enormous river flows through the middle of the city. Why, when reservoirs are depleted, can’t we simply use more of the water surrounding us?
The answer is embodied in one of London’s less familiar pieces of infrastructure at Beckton.
At Beckton, water from the tidal Thames is brackish, containing a mixture of freshwater and seawater, and cannot simply be pumped into London’s drinking-water network. London’s desalination plant there was completed in 2011 as a strategic drought resource, capable of treating estuarine water when conventional supplies are under pressure.
Its treatment process gives some idea of the engineering involved. Water is abstracted from the estuary when the tide is low and salinity is lower, before passing through clarification, conventional filters and ultrafiltration membranes. Reverse osmosis then forces the water through extremely fine membranes to remove dissolved salts. The treated water has minerals restored, is disinfected and can then be blended with other treated drinking water before entering London’s supply network. The concentrated brine left behind also has to be managed before discharge.
Reverse osmosis is energy-intensive, which is one reason desalination has been conceived as a strategic drought resource rather than an everyday source of London’s drinking water. Beckton is infrastructure built in the hope that it will not routinely be required.
That makes its current position especially interesting. After years in which it has rarely operated, work is now under way to bring Beckton back into service by the end of 2026, not primarily to rescue London from this summer’s drought, but to help water resources recover over winter and strengthen supplies for next year.
There are legitimate questions about why such expensive resilience infrastructure has not been available during the current drought. Those sit within a much wider debate about the management and financing of London’s water system which is beyond the scope of this article. The more fundamental infrastructure question is fascinating enough: how much should a city invest in systems that may sit largely unused for years precisely because they exist for exceptional circumstances?
Beckton also makes visible another feature of London’s water geography. Supplying the capital has never meant relying solely on water falling within London itself. The New River reached into Hertfordshire four centuries ago; today’s system depends upon rivers, reservoirs and groundwater across a much larger region. Future resilience is likely to extend that geography further through new storage and water transfers.
Even then, having enough water in the wider system does not guarantee that it reaches the person who needs it. In July, a burst main on Old Ford Road in Bow flooded the surrounding area to a depth of up to a metre. At the same time, some of the 650 affected households lost water, power or gas. It was a strikingly literal demonstration of the central problem: too much water in the street while people living beside it could not necessarily get water from their taps.
London’s water security therefore depends not only upon finding enough water, but upon the condition and capacity of the infrastructure that stores, treats and moves it.
Put these projects beside one another and something striking emerges. A Victorian sewer system still operates beneath London, now augmented by a super sewer completed in the 2020s. A desalination plant built in the 2010s is being prepared for renewed use to help London recover from the drought of 2026. A flood barrier opened in the 1980s is still protecting the capital, while decisions about what replaces or augments it have to begin decades before the existing structure reaches the end of its useful life.
Infrastructure operates on a very different timescale from most decisions about the city. Engineers planning London’s water system today have to make assumptions about the city of 2050, 2070 and beyond: how many people will live here, where their homes will be, how much water they will use, how high sea levels will rise, and how patterns of rainfall will change.
That is particularly relevant when London is simultaneously deciding how it should grow. The Draft London Plan published in July plans for 558,000 additional homes between 2027 and 2036 and looks towards the city’s development to 2050. As The London Society’s own analysis has highlighted, infrastructure is one of the central constraints on whether London’s planned growth can actually be delivered.
Water has to be part of that conversation from the beginning. New homes require additional supply and sewer capacity, while development changes how rain moves across the landscape. The Draft London Plan continues an established shift in how London thinks about drainage, putting the use of rainwater as a resource at the top of its drainage hierarchy, followed by infiltration and green infrastructure before controlled discharge into drains and sewers.
This represents a different kind of infrastructure from Bazalgette’s sewers or the Thames Barrier. Instead of one enormous structure attempting to move water away, thousands of smaller interventions can help make space for it within neighbourhoods. London’s future water resilience is likely to require both approaches: major engineering planned generations ahead and more modest changes incorporated into streets, parks and new developments as the city evolves.
There is something particularly revealing about discussing all this during a drought. London needs enough sustained rainfall to help restore rivers, groundwater and reservoirs after a long dry period, yet experience also tells us that the city has to be ready for the possibility that some of that rain will arrive with an intensity its streets and drainage systems struggle to accommodate.
That is not evidence that London’s infrastructure has failed. In many respects, the history is more encouraging. London has repeatedly encountered limits in the way it manages water and responded by changing the city: bringing water from further away, constructing sewers, building flood defences, intercepting sewage before it reaches the Thames and, more recently, finding ways to turn brackish estuarine water into drinking water. None of those solutions has been final, because London itself has never stopped changing.
The old children’s round offers an appealingly simple instruction: London’s burning, pour on water. In the middle of a drought, it sounds particularly tempting. Two thousand years of living beside the Thames suggest that the real challenge is rather more complicated. London’s future depends not simply on having enough water, but on our ability to store it when it is scarce, make room for it when there is too much, keep it clean enough to use, and build the infrastructure to do all three long before we know exactly what future London will require.
