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The Trevi Fountain is one of Rome’s most 
spectacular sights. Across a pale green pool,

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between two tritons wrestling with 
winged horses, from beneath the feet

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of the mighty sea-god Oceanus, a silver 
cascade rushes over steps of stone.

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Impressive though all this is, the most 
remarkable part lies behind the riot of statues,

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where the water that feeds the fountain flows, 
as it has for more than two millennia, through

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the concrete channel of a Roman aqueduct.
Greek engineers began building aqueducts

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as early as the sixth century BC. A stone-lined 
channel carried spring water to Archaic Athens,

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and Samos was served by an aqueduct that plunged 
through a tunnel two-thirds of a mile (1 km) long.

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Even more impressive systems appeared during 
the Hellenistic era, when the acropolis of

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Pergamon was supplied with water flowing 
under pressure through huge lead pipes.

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The Roman aqueducts differed from their 
Greek predecessors in their use of arches

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and hydraulic concrete. But it was sheer number 
and scale that truly set them apart. Hundreds of

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aqueducts were constructed across the Roman world, 
some well over 50 miles (80 km) long and capable

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of delivering millions of gallons each day.
Contrary to what you might assume, the majority of

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Roman aqueducts were not built to supply drinking 
water. Most Roman cities existed for centuries

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before they constructed their first aqueduct, and 
had established networks of wells and cisterns.

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More often than not, aqueducts were luxuries, 
designed to supply bath complexes, ornate

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fountains, and the houses of the elite.
The time-consuming and extremely expensive

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process of building an aqueduct began 
with locating a usable water source.

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Lakes were almost never chosen – stagnant water 
was regarded as unhealthy – and rivers were only

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tapped in exceptional cases, since they carried 
sediment and fluctuated seasonally. Usually,

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the source was a hillside spring.
A Roman aqueduct was an artificial river,

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flowing downhill from source to city. The channel 
gradient had to be both gentle and consistent. If

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it was too steep, the mortar lining would 
begin to erode; if it was too gentle,

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water would stagnate. Most Roman aqueducts descend 
only five or ten feet every mile (1.5 – 3 m / km),

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and some have slopes as gradual as 1 in 
20,000 – that is, a few inches per mile.

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To maintain such miniscule gradients, 
Roman engineers relied on the dioptra

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and the chorobates. The dioptra – an ancestor of 
the modern theodolite – was a sighting platform

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used to measure the relative position and height 
of distant points. The chorobates, a long table

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with a central channel, was a water level. 
With competent use of these instruments and

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adequate stocks of manpower and money, an 
aqueduct could be built almost anywhere.

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For most of their length, Roman 
aqueducts ran underground,

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following the contours of the landscape as 
they slowly descended from their sources.

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Although the water flowing down them was rarely 
more than knee-deep, their channels were made

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tall enough for maintenance workers to walk along 
without stooping. To minimize leakage, the masonry

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walls were coated with waterproof cement.
When an aqueduct had to cross a valley,

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its gradient was maintained by elevating 
the channel on rows of masonry arches.

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Until the reign of Augustus, these arcades 
were normally built with blocks of local stone.

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Later, they tended to be brick-faced concrete. 
Exceptionally deep depressions might be bridged

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with two or even three tiers of arches.
The most spectacular example is undoubtedly

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the Pont du Gard, just outside Nimes. No less 
than 160 feet (50 m) high, it consists of huge

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blocks of limestone laid without mortar, which 
support a channel so carefully graded that its

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level descends less than an inch from end to end.
When an aqueduct crossed a valley too deep for a

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bridge, Roman engineers built an inverted siphon 
– a pipe running at ground level from a header

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tank on one side of a valley to a receiving tank 
on the other side. As long as the receiving tank

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was at least slightly lower than the header, the 
water in the pipe would rise to its own level,

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flowing up the slope and out of the valley.
The counterparts of the siphons and bridges

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that allowed aqueducts to traverse valleys were 
the tunnels that carried them through hills.

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Unless the cutting was exceptionally deep, the 
usual construction method involved excavating

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a series of shafts and boring in both directions 
from the bottom. This didn’t always go as planned:

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an inscription from North Africa records how 
two work gangs, tunneling from either side

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of a mountain, became disoriented and 
began digging in opposite directions.

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When – having hewn through hills, vaulted 
valleys, and marched majestically o’er

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the plains – an aqueduct finally 
reached the city it was to supply,

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its terminus was often marked by a spectacular 
fountain. Most of its water, however,

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was channeled into the distribution tanks that 
the Romans called castella. These fed batteries of

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pipes, which in turn led to smaller distribution 
tanks. Pompeii had 12 of these; Rome had 247.

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In the northwestern provinces, water pipes were 
often made of tree trunks joined with iron bands.

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In the eastern Mediterranean, they might consist 
of long lines of hollowed stone blocks. Most

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Roman pipes, however, were made of terracotta or 
lead. Although they knew that lead caused health

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problems, the Romans persisted in making pipes 
from it, simply because lead was cheap, easy to

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work, and didn’t rust. The Romans were only saved 
from lead poisoning by the swiftness with which

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water flowed through the pipes, and by the calcium 
deposits that tended to coat their insides.

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Although most cities with aqueducts drew some 
of their drinking water from wells or cisterns,

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aqueduct water was – at least in the city 
of Rome itself – regarded as healthier

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and better-tasting. Most households 
accessed aqueduct water by drawing it

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from a public fountain or basin (or by paying a 
water-carrier to fetch it for them). By one count,

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Rome had 1,352 fountains. In Pompeii, there 
was one for about every 160 inhabitants.

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Baths were almost equally common. In 
the city of Rome alone, aside from

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the colossal imperial thermae, there were more 
than 850 neighborhood baths by late antiquity.

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The largest complexes used so much water 
that they needed dedicated aqueducts.

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The Baths of Caracalla, for example, 
were fed by a spur of the Aqua Marcia,

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and featured a reservoir with 32 chambers and a 
capacity of more than 2,000,000 gallons (8,000,000

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L). The outflow of wastewater from these baths 
was copious that it was used to power watermills.

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Private connections to aqueduct 
water were relatively rare.

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In Rome, the process for installing a 
tap involved appealing to the emperor,

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bringing the emperor’s authorization to the 
water commissioner, and finally receiving

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a calix, a bronze nozzle stamped with the 
owner’s name. The grant was not permanent:

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as soon as the recipient died or 
sold his home, his calix was removed.

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Although some private connections were granted 
to the owners of industrial facilities,

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most belonged to members of the elite, who used 
the water to supply the gardens, fountains,

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and private baths of their mansions. The situation 
seems to have been broadly similar in Pompeii,

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where only 10% of households had access to piped 
water, but that 10% used it so extravagantly

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that one house had no fewer than 33 faucets.
Maintaining the aqueducts was a constant struggle.

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In the city of Rome, a permanent staff of 700 
installed new pipes, braced collapsed arches,

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and kept the channels clear. Mud and 
stones had to be removed from the

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settling tanks – depressions in the 
channel designed to catch suspended

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sediment and debris – and mineral deposits 
were periodically scraped from the walls.

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Although Rome’s aqueducts seem to have been 
fairly well-maintained until late antiquity,

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not all cities were so scrupulous, and some 
aqueducts became completely clogged with debris.

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When they weren’t clogged, the aqueducts were 
awesome manifestations of the Roman knack

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for practical engineering on a monumental scale. 
The aqueduct that served Carthage ran 55 miles

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(90 km) from a sacred spring to the 
cavernous cisterns of the city’s great baths.

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The aqueduct that Augustus built along 
the Bay of Naples was even longer,

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and featured at least a dozen branches 
supplying the naval station at Misenum,

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the elaborate seaside villas at Baiae, and 
the doomed cities of Pompeii and Herculaneum.

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The aqueduct of Constantinople, whose channels 
had a combined length of over 300 miles

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(500 km), filled a vast series of 
artificial lakes and covered reservoirs.

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Most impressive of all were 
the eleven aqueducts of Rome,

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which may have collectively carried as much 
as a million cubic meters of water each day.

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Tapping springs and streams in the surrounding 
hills, and carried over suburban villas

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and market gardens on miles-long arcades, Rome’s 
aqueducts entered the city proper in a spectacular

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web of pipes, conduits, and distribution tanks.
Since only a few of the aqueducts were high enough

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to supply all 14 of the city’s regions, most 
had a fairly localized distribution network.

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The whole system, however, was interconnected, so 
that if one aqueduct were shut down for repairs,

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another could be diverted to replace it. This 
feature was not always appreciated, since Rome’s

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aqueducts ranged in quality from the Aqua Marcia 
– fed by the emerald pools of a delicious mountain

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spring – to the Aqua Alsietina, whose water was 
so muddy that it was considered undrinkable.

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But thanks to hundreds of millions of sestertii in 
funding, endless maintenance work, and the basic

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quality of their construction, the aqueducts 
continued to flow long after the emperors were

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gone. A few, as we’ve seen, are still flowing 
today, another living legacy of ancient Rome.

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If you enjoyed this video, please consider 
supporting toldinstone on Patreon. You might also

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enjoy my book, Naked Statues, Fat Gladiators, 
and War Elephants. Thanks for watching.