Ask most folks to name a river that flows through London and they'll all name the exact same one. The one that starts with a "T."

Yet unbeknownst to many — presumably including a decent number of Britons — there are dozens of smaller rivers and canals flowing through the heart of the British capital city aside from the Thames: the Effra, the Tyburn, the Walbrook, the Westbourne and the once-mighty Fleet to name a few.

So why then do these waterways go largely unmentioned and unnoticed?

The answer is a straightforward one. It's because you can't actually see many of them as they were buried deep beneath the city streets eons ago.

Discharge of River Westbourne, London The point where the River Westbourne, a small tributary of the Thames that's been largely buried beneath the city, discharges into the Inner London Tideway in Chelsea. (Photo: Dun.can/flickr)

Thanks to environmental nonprofit 10:10 Climate Action, the so-called "lost rivers" of London are back in the news. While it might come as a surprise that these mysterious sunken waterways are being touted by the group as a promising green heat source, it's likely an even bigger surprise — or at least to those who aren't familiar with London's vast, complicated and filth-riddled history — that they even exist at all.

London isn't the only city to be built atop subterranean watercourses that have gradually been paved over and buried, some in their entirety, to make way for new development. Toronto, Brussels, Vienna, Moscow, Sydney, New York City and Philadelphia all are home to rivers, creeks, brooks and streams that have been forced underground. It is however, the first major city where there's a movement underway to tap into its lost waterways on a city-wide basis as a means of curbing greenhouse gas emissions.

Illustration of London's 'lost rivers' An illustration depicting London's numerous 'lost' rivers. Once vital and filled with life, these waterways were subsequently used as sewage canals and later buried in the 19th century. (Photo: Wikimedia Commons)

Seeking heat, underground

London's underground waterways — most of them tributaries of the Thames or the River Lea — are now predominately now part of the city's contemporary sewer system.

This isn't too dramatic of a departure from hundreds of years ago during London's unfortunate pre-sewer era when the tributaries were used as open-air conduits for dumping raw sewage, industrial refuse and all means of unsavory detritus. Everything went in — and it all flowed into the Thames (which has recovered quite nicely after decades of work.)

As Tom Bolton, author of "London's Lost Rivers: A Walkers Guide," writes for the Telegraph, the waste-clogged, disease-carrying rivers that crisscrossed the city were a "big problem" and didn't exactly help things along as London experienced massive growth spurts at the end of the 18th century and into the 19th century.

Floods, filth, stench and disease put off Georgian and Victorian house-buyers. In Mayfair, the Tyburn was tucked away under mews. In West Norwood, the Effra was buried deep under grids of new Victorian villas.
The Fleet was legendarily filthy. Redesigned as a Venetian-style canal by Christopher Wren after the Fire of London, it was quickly overtaken by grim reality. Jonathan Swift, in 1710, wrote about the Fleet filled with ‘the sweepings from butchers' stalls, dung, guts and blood.' A few years later Alexander Pope described how ‘Fleet-ditch with disemboguing streams / Rolls the large tribute of dead dogs to the Thames.' It is no surprise then that the lower Fleet was culverted in huge storm sewer tunnels where it has remained ever since.

Like Bolton, who so eloquently describes these waterways as "invisible threads, binding London together under the surface while the city roars above," 10:10 Climate Action has buried rivers on the brain.

Illustration of River Fleet An 18th century painting depicting the mouth of the River Fleet, a major tributary of the Thames. Fleet Street, an iconic London thoroughfare, takes it name from this now-buried waterway. (Photo: Wikimedia Commons)
The London-based environmental charity, which, among other things, has campaigned for solar-powered schools and the extension of Daylight Saving in Britain by one hour as a way of slash electricity-related emissions, has launched a new report that explores how the Rivers Westbourne, Tyburn, Fleet et al. could play a vital role in fostering a cleaner, greener London.

The idea is a simple yet ingenious one and has been executed on a smaller scale in other locales such as the campus of Borders College in Scotland and a governmental building in Stuttgart, Germany.

Steamy underground sewer systems, whether they were once natural watercourses or not, are primo places to extract natural heat using water-source heat pumps, which the Guardian describes as working like "reverse refrigerators." Once extracted, this captured heat can be transported to neighboring office buildings and residential complexes, negating the need for burning fossil fuels for heat, which is the norm in London. In turn, a significant amount of air-polluting carbon emissions are avoided.

Describing London's unlikely potential heat sources as previously being a "cracking place to throw your sewage into," 10:10 goes on to explain the basics behind heat pump technology:

Heat pumps use electricity to harvest low-grade heat (as in not-very-hot-heat) from the surrounding area and use it to heat space and water in buildings. It's essentially moving heat from one place to another, using some simple physics to amplify its effects. Putting just one unit of electrical energy in will produce three to five units of heat energy.
What about low carbon cooling? Heat pumps look pretty good here too — the same device can be used to provide heating in winter and cooling in summer. Not bad.

Of course, this is all a carbon-curbing concept for now. A sizable amount of infrastructural work would need to be performed before London, where 75 percent of all household energy is used for heat and water heating, starts tapping into underground rivers-turned-sewerage culverts. Existing buildings also need to be retrofitted for improved efficiency, which 10:10 calls the "biggest first step."

What's more, it's unclear if deploying heat pumps to lost rivers across the city is even financially viable. But as Leo Murray, the director of campaigns for 10:10, tells the Guardian: "The mayor has set ambitious targets for phasing out gas burning in London over the coming years and we are going to need every bit of low-carbon heat we can get our hands on to meet these goals."

Exterior of Buckingham Palace Buckingham Palace is due to get greener, cleaner and more efficient in the coming years. Environmental campaigners think drawing heat from a subterranean river should be part of the scheme. (Photo: Jimmy Harris/flickr)

Royal sewer heat on the horizon?

In presenting the case for a city heated by buried rivers instead of gas boilers, 10:10 partnered with Edinburgh-headquartered community energy organization Scene to detail a few specific sites that would benefit greatly from such an undertaking.

The most notable candidate for sewer-supplied heating is none other than Buckingham Palace.

As 10:10 details, the monarchy, to its credit, has already embarked on an ambitious 10-year retrofit of Buckingham Palace with the goal to slash the historic royal residence's carbon emissions by 40 percent. Part of this entails installing a biogas boiler. But as 10:10 points out, this will only supply the palace with roughly 5 percent of its heat and hot water and that "Her Majesty could do with some more low carbon heat."

This is where the River Tyburn, sections of which flow through nearby Regents Park and directly beneath the palace itself, comes in. After poring over maps and river flow data, it was concluded that the potential for substantial heat harvesting just north of the palace is excellent due to the river's "heavy flow." A single heat pump could provide the palace's wintertime heating needs and then some provided that the proper energy-saving upgrades are made.

Historic photo of Brockwell Lido The sometimes frigid water temps at Brockwell Lido, a public swimming pool built in 1937, could be kept nice and toasty throughout the year thanks to a nearby buried river. (Photo: Wikimedia Commons)

Similar to a sewer-heated public piscine in Paris, there's also the potential for keeping London's outdoor public swimming pools at a comfortable temperature using captured sewer heat. Brockwell Lido, a large pool complex in Herne Hill, south London, could benefit from its proximity to the River Effra, a large meandering stream that was buried in the mid-19th century. The pool, which is open year-round, is currently not heated.

As for the River Fleet, one of the largest — and, as mentioned, foulest — of the Thames' former tributaries, it could help dramatically minimize the energy bills at Acland Burghley School, a secondary school in Tufnell Park that's home to a decent-sized rooftop solar array. By installing a water-source heat pump near the school along a subterranean tributary stream, all of the building's annual heating needs could be met. What's more, the solar array could partially power the pump, making it a "truly renewable heating" system per 10:10.

Peering down into London's River Fleet Taking in the 21st century views of London's once-powerful (and wildly polluted) River Fleet. (Photo: Andrea Vail/flickr)

The group goes on to detail two other sites — the Hammersmith Town Hall redevelopment project and the Somers Town Heat Network — that would benefit from harnessing the city's extensive network of underground rivers.

"With the threat of climate change looming ever larger, we need to get moving on decarbonising our heating and heat pumps offer a smart solution," writes 10:10. "It's time to take the plunge."

You can peruse the group's full, 23-page report, "Lost Rivers: A Pathfinder Project Exploring the Potential to Harvest Heat Energy From London's Hidden Underground Rivers," here.

Matt Hickman ( @mattyhick ) writes about design, architecture and the intersection between the natural world and the built environment.