Roman Aqueducts and Water Systems: Engineering for a Million People

The city of Rome, at its height in the early third century CE, was the largest city in the world. The population of about a million people lived in an area of roughly thirteen square kilometers, with a population density higher than modern Manhattan. The city’s millions of inhabitants needed a million cubic meters of water a day — for drinking, for cooking, for cleaning, for the great imperial bath complexes, for the public fountains, and for the industrial uses that kept the city supplied. To meet this demand, the Romans built the largest and most sophisticated water-supply system of the ancient world: a network of eleven great aqueducts, hundreds of kilometers of underground channels, and thousands of kilometers of distribution pipes. For the broader engineering context, see Roman Engineering and Architecture. For the materials that made the system possible, see What Was Roman Concrete?.

The Roman water system was not just a marvel of engineering. It was a public service, paid for by the state, supplying free water to the citizens of Rome, and it was managed by a single official, the curator aquarum, the commissioner of the water supply, who was one of the most important administrators of the imperial government. The water system is the clearest example we have of the Roman state at work: a long-term, well-funded, technically sophisticated program of public works, governed by a permanent bureaucracy, designed to serve a vast population at a moderate cost.

The Aqueducts of Rome

The first aqueduct of Rome was the Aqua Appia, built in 312 BCE by the censor Appius Claudius Caecus, the same man who built the Appian Way. The Aqua Appia was almost entirely underground, about 16 kilometers long, and supplied the city with about 75,000 cubic meters of water a day from springs to the east of Rome. The Romans learned quickly that an underground aqueduct was more reliable than an elevated one: the water was protected from the sun, which encouraged the growth of algae, and from enemies, who might poison or divert it.

By the time of the emperor Frontinus, who wrote a detailed treatise on the water system of Rome around 97 CE, the city had nine aqueducts in service, with a total length of about 430 kilometers, of which only about 50 kilometers were on arches above the ground. By the time of the emperor Trajan and his successor Hadrian, an eleventh aqueduct, the Aqua Traiana and the Aqua Alexandrina, had been added. The eleven aqueducts of imperial Rome, in order of construction, were:

Aqua Appia, 312 BCE, 16 km
Anio Vetus, 272 BCE, 64 km
Aqua Marcia, 144 BCE, 91 km — the longest, and famous for the coldness and purity of its water
Aqua Tepula, 125 BCE, 18 km
Aqua Julia, 33 BCE, 22 km
Aqua Virgo, 19 BCE, 21 km — the only ancient aqueduct still in use today, supplying the Trevi Fountain
Alsietina, 2 BCE, 33 km — the lowest, supplying the naumachia of Augustus
Aqua Claudia, 38 – 52 CE, 69 km
Anio Novus, 38 – 52 CE, 87 km
Aqua Traiana, 109 CE, 33 km
Aqua Alexandrina, 226 CE, 22 km

The total capacity of the system, according to Frontinus, was about 1 million cubic meters a day, although the actual amount delivered was probably less because of leakage and illegal tapping.

The water reached the city in a single pipe, the castellum aquae, the water castle, at the highest point of the urban area. From there, it was distributed through a network of lead pipes, the fistulae, to the public fountains, the public baths, the imperial palaces, and the privileged private users. About 40 percent of the water went to public uses, 30 percent to the imperial household and the public buildings, and the remaining 30 percent to private users who paid an annual tax for the privilege.

The Channel: The Specus

The water of an aqueduct flowed in a covered channel, the specus, the Latin word for “channel” or “conduit.” The specus was typically a stone or concrete channel, about 60 to 90 centimeters wide and 1.5 to 2 meters high, with a smooth floor that allowed the water to flow freely. The channel was covered by a stone slab, with inspection and access shafts at regular intervals.

The Romans built the specus to a careful gradient, with a typical fall of about 1:200 to 1:1000. A steeper fall would erode the channel; a gentler fall would allow sediment to build up. The gradient was maintained with great care. Modern measurements of the Pont du Gard in southern France have shown that the gradient is 1:3000 over the bridge’s 25 kilometers — a precision of about 25 centimeters over the whole length, achieved without modern surveying instruments.

The Romans were not the first to build aqueducts. The Greeks had built a few short ones in their cities, and the Etruscans had built an underground system in some Italian cities. But the Romans were the first to apply the technology on a continental scale, and the specus system they developed was the basis of all later water engineering.

The Pont du Gard

The most spectacular surviving Roman aqueduct is the Pont du Gard, the three-tiered stone bridge across the Gardon River near Nîmes in southern France. The Pont du Gard was part of the Aqua Nemausus, the aqueduct that supplied the city of Nîmes, and it was built in the mid-first century CE, probably under the emperor Claudius.

The Pont du Gard is 49 meters high and 275 meters long, with three tiers of arches. The lowest tier has six arches, each 20 meters wide. The middle tier has eleven arches. The top tier, which carried the specus, has thirty-five smaller arches. The stones of the bridge are fitted together without mortar, and the joints are so precise that a knife blade cannot be inserted between them. The bridge has stood for two thousand years, surviving the fall of Rome, the medieval period, the early modern period, and the construction of the modern highway that runs past its base. It is a UNESCO World Heritage Site and is one of the most visited tourist sites in France. For the role of Nîmes in the empire, see Roman Gaul.

The Aqueduct of Segovia

The other great surviving Roman aqueduct is the Aqueduct of Segovia in central Spain, a 28-meter-high bridge that carries the Aqua Segoviensis into the old city of Segovia. The aqueduct is about 17 kilometers long in total, with the most spectacular section being the bridge across the Plaza del Azoguejo in the center of the city. The bridge, built in the late first or early second century CE, has 167 arches, two stories, and is built of granite blocks fitted together without mortar. It was in continuous use until the mid-nineteenth century, and it is the symbol of the city of Segovia. For the Iberian context, see the history of the Punic Wars, which brought Spain into the Roman orbit.

The Aqua Claudia

The Aqua Claudia was the most impressive of the aqueducts of Rome itself. Built under the emperor Caligula in 38 CE and completed under Claudius in 52 CE, the Aqua Claudia ran 69 kilometers from springs in the Anio valley east of Rome, partly in a covered specus and partly on arches. The surviving remains of the Aqua Claudia outside Rome are some of the most dramatic pieces of Roman aqueduct construction. The emperor Claudius celebrated the completion of the aqueduct with a great triumphal procession across the Roman Forum, and the surviving inscription on the aqueduct is one of the longest and most boastful of Roman building inscriptions.

The Calix and the Settling Tanks

The water in an aqueduct was full of sediment, leaf litter, and the occasional small animal. Before the water was distributed to the city, it had to be cleaned. The Romans did this in a series of settling tanks, the piscinae, that were built at intervals along the aqueduct. The piscinae were large concrete basins, with weirs and baffles that slowed the water and allowed the sediment to fall to the bottom. The settled sediment was periodically cleaned out, and the cleaned water was passed on to the next stage.

The final cleaning of the water was done at the calix, the head of the distribution system. The calix was a special tank with a series of calibrated bronze nozzles, the calices, that distributed the water to the various pipes. The sizes of the calices were precisely specified, and the calibration of the system was a science in itself. Frontinus, the water commissioner, wrote a detailed chapter on the geometry of the calices in his treatise on the water system.

The Distribution Castella

From the calix, the water was distributed through a network of castella, the distribution tanks, to the various parts of the city. The castella were typically built of concrete and faced with brick or stone, with a series of inflow and outflow pipes. The castella had two functions: to maintain the pressure in the distribution system, and to allow the curator aquarum to regulate the flow.

The castella were connected to the public fountains, the public baths, the imperial palaces, and the privileged private users by a network of lead pipes, the fistulae. The fistulae were made of lead sheets rolled into a cylinder and welded at the seam. The standard widths were the quinaria (about 2.3 cm), the denaria (about 4.5 cm), and so on, with each larger size carrying about 2.5 times the flow of the next smaller. The fistulae were stamped with the name of the owner of the property, a useful record for the curator aquarum when investigating illegal tapping.

The lead pipes are the basis of the modern myth that the Romans were poisoned by lead. The modern view is that the lead content of the Roman water was probably not high enough to cause widespread lead poisoning, and the lead pipe network itself was much less common than the terra-cotta pipes that were used for the smaller distribution systems. For the social context of the water system, see Roman Society and Daily Life.

Maintenance Access

A Roman aqueduct was built to be inspected and cleaned. The specus had access shafts, the puticuli, at intervals of about 75 meters, with stone covers that could be lifted to allow access. The shafts were also used as ventilation shafts, allowing the air to circulate in the covered channel and reducing the growth of algae.

The aqueducts were also built with a series of bypass channels at the major points. The bypass channels allowed water to be diverted from one section of the aqueduct while the section was being cleaned or repaired. The Aqua Marcia, the longest of the Rome aqueducts, had at least three major bypass channels. For the military context, see Roman Forts and Frontiers, which depended on the same engineering principles of supply and maintenance.

The Cloaca Maxima

The Romans were the first civilization to build a comprehensive sewage system, and the Cloaca Maxima of Rome is the most famous piece of Roman sanitary engineering. The Cloaca was originally a covered drainage channel built in the sixth century BCE to drain the marshy valley between the Capitoline and Palatine hills into the Tiber. The channel was progressively extended, and by the imperial period it was a network of stone and concrete tunnels running under the entire city. The Cloaca Maxima was kept in good repair throughout the imperial period, and the quaestor of the Roman people, the quaestor urbanus, was responsible for its upkeep. The Cloaca Maxima is still in use today, two thousand five hundred years after it was built, draining the modern city of Rome into the Tiber.

The Curator Aquarum

The most important official in the Roman water system was the curator aquarum, the “commissioner of the waters.” The office was created by Augustus in 11 BCE and was held by a senator of consular rank. The curator aquarum had overall responsibility for the eleven aqueducts of Rome, the distribution system, the public fountains, the public baths, and the public sewers.

The curator aquarum was assisted by a substantial staff, the familia aquarum, the “household of the waters.” The familia included architects, accountants, supervisors, plumbers, and a permanent force of public slaves. The curator had the power to fine illegal users of the water, to revoke the licenses of abusive plumbers, and to investigate any tampering with the system. The curator was also a judge, and he heard disputes between water users in his own court. The office was, in the imperial period, one of the most coveted in the senatorial career.

The most famous of the curatores was Sextus Julius Frontinus, the general, governor, and writer who held the office from 97 to 100 CE. Frontinus wrote the De Aquaeductu, the treatise on the water system of Rome, which is one of the most important sources for the technology of the Roman water system and the only surviving technical treatise on Roman engineering. For the broader context, see Roman Engineering and Architecture.

The Legacy of the Roman Water System

The Roman water system was the most important piece of public infrastructure of the ancient world. It allowed a million people to live in a city that would have been impossible without a continuous supply of clean water. It was a triumph of engineering, of organization, and of public administration. The principles that the Romans developed — the use of gravity flow, the precision of the gradient, the use of settling tanks, the careful calibration of the distribution, the organized maintenance — are still the basis of modern water engineering.

The water system was also a monument to the Roman state’s commitment to public welfare. The system was paid for by the state, the water was free, and the most basic public service — clean drinking water — was available to every citizen. The great baths of Rome — the Baths of Caracalla, the Baths of Diocletian, the Baths of Trajan — would have been impossible without the aqueducts, and they are among the most impressive monuments of the ancient world. The Roman water system is the ancestor of the modern municipal water supply, and the engineers of the nineteenth century, when they designed the first modern water systems, looked back to the Romans as their masters.