How Did Roman Aqueducts Work?

The Trevi Fountain is one of Rome's most spectacular sights. Across a pale green pool, between two tritons wrestling with winged horses, from beneath the feet of the mighty sea god Oceanus, a silver cascade rushes over steps of stone. Impressive though all this is, the most remarkable part lies behind the riot of statues, where the water that feeds the fountain flows, as it has for more than two millennia, through the concrete channel of a Roman aqueduct. Greek engineers began building aqueducts as early as the 6th century BC. A stone-lined channel carried spring water to archaic Athens, and Samos was served by an aqueduct that plunged through a tunnel more than one kilometer long. Even more impressive systems appeared during the Hellenistic era, when the Acropolis of Pergamum was supplied with water flowing under pressure through huge lead pipes. The Roman aqueducts differed from their Greek predecessors in their use of arches and hydraulic concrete, but above all it was sheer number and scale that set them apart. Hundreds of aqueducts were constructed across the Roman world, some well over 50 miles, that is 80 kilometers, long. Contrary to what you might assume, the majority of Roman aqueducts were not built to supply drinking water. Most Roman cities existed for centuries before they constructed their first aqueduct, and had established networks of wells and cisterns. More often than not, aqueducts were luxuries, designed to supply baths, ornate fountains, and the houses of the elite. The time-consuming and extremely expensive process of building an aqueduct began with locating a usable water source. Lakes were almost never chosen, stagnant water was regarded as unhealthy, and rivers were only tapped in exceptional cases, since they carried sediment and fluctuated seasonally. Usually, the source was a hillside spring. A Roman aqueduct was an artificial river, flowing downhill from source to city. The channel gradient had to be both gentle and consistent. If it was too steep, the mortar lining would begin to erode. If it was too gentle, water would stagnate. Most Roman aqueducts descend only five or ten feet every mile, that is one and a half to three meters every kilometer, and some have slopes as gradual as one in twenty thousand, that is a few inches per mile. To maintain such minuscule gradients, Roman engineers relied on the dioptera and chorobates. The dioptera, an ancestor of the modern theodolite, was a sliding platform used to measure the relative position and height of two distant points. The chorobates, a long table with a central channel, was a large water level. With competent use of these instruments, and adequate stocks of manpower and money, an aqueduct could be built almost anywhere. For most of their length, Roman aqueducts ran underground, following the contours of the landscape as they slowly descended from their sources. Although the water flowing down them was rarely more than knee-deep, their channels were made tall enough for maintenance workers to walk along without stooping. To minimize leakage, the masonry walls were coated with waterproof cement. When an aqueduct hit across a valley, its gradient was maintained by elevating the channel on rows of masonry arches. Until the reign of Augustus, these arcades were normally built with blocks of local stone. Later, they tended to be brick-faced concrete. Exceptionally deep depressions might be bridged with two or even three rows of arches. The most spectacular example is undoubtedly the Pont du Gard, located just outside Nîmes. No less than 160 feet, 50 meters, high, it consists of huge blocks of limestone laid without mortar, which support a channel so carefully graded that its level descends less than an inch from end to end. When an aqueduct crossed a valley too deep for masonry arches, Roman engineers built an inverted siphon, that is, a pipe running at ground level from a header tank on one side of the valley to a receiving tank on the other side. As long as the receiving tank was at least slightly lower than the header, the water in the pipe would rise to its own level, flowing up the slope and out of the valley. The counterparts of the siphons and bridges that allowed aqueducts to traverse valleys were the tunnels that carried them through hills. Unless the cutting was exceptionally deep, the usual construction method involved excavating a series of shafts and boring in both directions from the bottom. This didn't always go as planned. An inscription from North Africa records how two work gangs, tunneling from either side of a mountain, became disoriented and began digging in opposite directions. When, having hewn through hills, vaulted to valleys, and marched majestically over the plains, an aqueduct finally reached the city it was designed to supply, its terminus was often marked by a spectacular fountain. Most of its water, however, was channeled into the distribution tanks that the Romans called castella. These fed batteries of pipes, which in turn led to smaller distribution tanks. Pompeii had 12 of these. Rome had 247. In the northwestern provinces, water pipes were often made of tree trunks joined with iron bands. In the eastern Mediterranean, they might consist of long lines of hollowed stone blocks. Most Roman pipes, however, were made of terracotta or lead. Although they knew that lead caused health problems, the Romans persisted in making pipes from it, simply because lead was cheap, easy to work, and didn't rust. The Romans were only saved from lead poisoning by the swiftness with which water flowed through their pipes, and by the calcium deposits that tended to coat their interiors. Although most cities with aqueducts drew some of their drinking water from wells or cisterns, aqueduct water was, at least in Rome itself, regarded as healthier and better tasting. Most households accessed aqueduct water by drawing it from a public fountain. By one count, Rome had 1,352 fountains. In Pompeii, there was a fountain for every 160 inhabitants. Baths were almost equally common. In the city of Rome alone, aside from the colossal imperial Thermae, there were more than 850 neighborhood baths by late antiquity. The largest complexes used so much water that they required dedicated aqueducts. The Baths of Caracalla, for example, were fed by a spur of the Aqua Martia, and featured a reservoir with 32 chambers, and a capacity of more than 2 million gallons, 8 million liters. The outflow of wastewater from the baths was so copious that it was used to power water mills. Private connections to aqueduct water were relatively rare. In Rome, the process for installing a private tap involved appealing to the emperor himself, bringing the emperor's authorization to the water commissioner, and finally receiving a calyx, a bronze nozzle stamped with the owner's name. The grant was not permanent. As soon as the recipient died or sold his house, his calyx was removed. Although some private connections were granted to the owners of industrial facilities, most belonged to members of the elite, who used the water to supply the gardens, fountains, and private baths of their mansions. The situation seems to have been broadly similar in Pompeii, where only 10% of households had access to piped water, but that 10% used it so extravagantly that one house had no fewer than 33 faucets. Maintaining the aqueducts was a constant struggle. In the city of Rome, a permanent staff of 700 installed new pipes, braced collapsing arches, and kept the channels clear. Mud and stones had to be removed from the settling tanks, depressions in the channel designed to catch suspended sediment and debris, and mineral deposits were periodically scraped from the walls. Although Rome's aqueducts seem to have been fairly well maintained until late antiquity, not all cities were so scrupulous, and some aqueducts became completely clogged with debris. Speaking of clogs, this video is sponsored by the Woosh Pipe and Drain Company of Astoria, New York City. If you live in Queens and want to keep your plumbing flowing free, call Steve at Woosh and make it drain. When they were functioning as intended, the aqueducts were awesome manifestations of the Roman knack for practical engineering on a monumental scale. The aqueduct that served Carthage ran 55 miles, 90 kilometers, from a cavernous cisterns beneath the city's great baths. The aqueduct that Augustus built along the Bay of Naples was even longer, and featured at least a dozen branches, which supplied the naval station at Mycenaeum, the elaborate seaside villas of Baiae, and the doomed cities of Pompeii and Herculaneum. The aqueduct of Constantinople, whose channels had a combined length of over 300 miles, 500 kilometers, carried water into a series of vast artificial lakes and reservoirs. Most impressive of all were the 11 aqueducts of Rome, which may have collectively carried as much as a million cubic meters of water each day. Tapping springs and streams in the surrounding hills, and carried over suburban villas and gardens on miles-long arcades, Rome's aqueducts entered the city proper in a spectacular web of pipes, conduits, and distribution tanks. Since only a few of the aqueducts were high enough to supply all 14 of the city's regions, most had a fairly localized distribution network. The whole system, however, was interconnected, so that, if one aqueduct were shut down for repairs, another could be diverted to replace it. This feature was not always appreciated, since Rome's aqueducts ranged in quality from the aqua martia, fed by the emerald pools of a delicious mountain spring, to the aqua alciatina, whose water was so muddy that it was considered undrinkable. But thanks to hundreds of millions of sestertii in funding, endless maintenance work, and the basic quality of their construction, the aqueducts continued to flow long after the emperors were gone. A few, as we've seen, are still flowing today, a living legacy of ancient Rome. If you enjoyed this video, please consider supporting Toland Stone on Patreon. You might also enjoy my book, Naked Statues, Thack Gladiators, and Royal Offense. Thanks for watching!