The first Roman aqueduct was built to transport water into cities to supply community baths and inaugurated an advance in hydraulic engineering and architecture that continued to operate in Europe for many centuries.

The engineering of aqueduct construction was complicated and based on a piecemeal evolution of techniques over five hundred years. The water, tapped from springs, streams, or reservoirs, was allowed to descend down a gently sloping channel until it reached a point of distribution. The main conduit was generally rather tall and narrow, approximately 5 feet by 2 feet (1.5 meters by 0.6 meter). Because of the gravitational flow of the water, aqueducts had to be laid out carefully. If their slope was too slight, the water became sluggish; if it was too steep, pressures would build up to cause overflowing and ruptures. Vitruvius (fl. first century b.c.e.), the celebrated architect of Augustus’s time, recommended a fall of one in two hundred, an excellent gradient which could not, however, always be maintained. Because the velocity of the water needed to be controlled and hills and valleys had to be circumvented, an aqueduct was usually much longer than the straight distance between inlet and outlet. The aqueducts operated on the principle of constant offtake, so the outlets within the city were always open. Once an aqueduct was finished, it required constant maintenance to clean out deposits left by the hard Roman water. Frontinus, Sextus Julius

A Roman aqueduct in Segovia, Spain.

(PhotoDisc)

An aqueduct could be constructed in three different ways. It could be led through a tunnel cut into the ground, laid in a ditch and covered over, or raised above the surface of the ground on a wall or series of arches. This last procedure generally recommended itself because its building materials were cheap while lead pipe for laying underground was expensive. Still, a long aqueduct often made use of all three methods. The first Roman aqueduct, the Aqua Appia of 312 b.c.e., ran its 10-mile (16-kilometer) course mostly underground, as did the Aqua Anio Vetus of 272 b.c.e. The third Roman aqueduct, however, the Aqua Marcia, built in 144-140 b.c.e., drew its waters from the Anio River behind Tivoli only 16 miles (26 kilometers) northeast of Rome and traveled 58 miles (93 kilometers) before it reached the city. It was a high-level type aqueduct that crossed the Roman campagna on a series of arches nearly 6 miles (10 kilometers) long and entered the city at a level of 195 feet (60 meters) above the Tiber River, high enough to supply the Capitoline, Caelian, and Aventine hills. When the Aqua Tepula of 125 b.c.e. and the Aqua Julia of 40 b.c.e. were built, their channels were superimposed upon those of the Marcian aqueduct near its approach to the city, making Tepula and Julia high-level types as well. The Aqua Virgo of 19 b.c.e., terminating in the Campus Martius, and the Aqua Alsietina of 2 b.c.e., supplying the Trastevere, were again underground. The famous Aqua Claudia was a very lofty structure reaching the highest hills of the city including the Palatine. The Aqua Traiana of 109 c.e., with its outlet on the Janiculum, had underground conduits, while the Aqua Alexandrina of 226 c.e. was carried on arches for most of its course.

The same methods of construction were used in the provinces. Occasionally refinements were made, as in the aqueducts of Lyons, where siphons were used to carry water down and then up the other side of a valley or ravine. Siphons were probably used more often than most scholars have yet recognized. Bridges of arches were utilized by Roman engineers only in valleys that were up to 164 feet (50 meters) deep. Deeper valleys were crossed by means of siphons, and the number of siphons also depended on the number of valleys traversed by the aqueduct. Among the more famous remains of provincial aqueducts is the magnificent Pont du Gard, the bridge of the aqueduct for the city of Nîmes in southern France. This aqueduct, 31 miles (50 kilometers) long, crossed the Gard on a triple row of arches 900 feet (274 meters) long and 162 feet (49 meters) above the river. Built during the reign of Augustus, it remains one of the finest extant examples of Roman engineering. Hardly less spectacular is the gray granite bridge of the aqueduct of Segovia in Spain. Still in use, it crosses the valley next to the city on a series of arches 2,530 feet (771 meters) in length and 94 feet (29 meters) at their greatest height. In this category belongs also the magnificent span of the Roman aqueduct at Cherchel in Algeria, which served ancient Caesarea. One of the most impressive sights in north Africa is the ruins of the aqueduct built by Hadrian to bring water from inland mountains to Carthage. It was 82 miles (132 kilometers) in length, and some of its arches, 341 of which are still standing, reached a height of 130 feet (40 meters).

Although it is impossible to say when Roman aqueduct construction reached its apogee, one might well hold that the building of the famous Aqua Claudia best represents that point in history. When this aqueduct was dedicated on August 1, 52 c.e., the birthday of the emperor whose name it bears, the famous Sextus Julius Frontinus (c. 35-c. 103 c.e.) was already seventeen. Frontinus was the man who, as curator aquarum from 97 c.e. until some time before 103, was to be the authority on Roman aqueducts through his celebrated work De aquis urbis Romae (n.d.; Aqueducts, 1925).

The Claudia, begun in 38 c.e. under Emperor Caligula, is often regarded as the highest sustained achievement in Roman aqueduct construction. As the last and in many ways the most impressive stone aqueduct, Frontinus considered it especially magnificent. Conceived on a greater scale than any before it, it stretched virtually across the campagna, drawing water from a spring near Tivoli to enter Rome by the now-famous Porta Maggiore, the eastern entrance into the city. Of its far-reaching line of arches, one section of 154 still stands intact together with many smaller groups of arches, one of which preserves the highest point of the aqueduct at more than 88 feet (27 meters). The enormous size of the undressed stones, laid dry, makes the work remarkable. Together with its associated aqueduct, the Aqua Anio Novus, it cost the Roman government fifty-five million sesterces.

Beyond Frontinus, primary accounts of the aqueducts are limited. Strabo (64/63 b.c.e.-after 23 c.e.) praised the skills of the engineers in his geographical works, while Vitruvius included one chapter on aqueducts in his famous ten books on architecture. Frontinus himself appears to have been an administrative manager with no hydraulic experience, composing De aquis urbis Romae to show that he had mastered knowledge of the aqueducts. Yet enough has been learned of the aqueducts to know that Romans perceived them as the mark of civilized living, well worth the constant maintenance and expense they required. Successfully supplying millions of gallons of water to Rome’s major cities, aqueducts were made to look beautiful to display them as the focus of civic pride that they were.

• Aicher, Peter J. Guide to the Aqueducts of Ancient Rome. Wauconda, Ill.: Bolchazy-Carducci, 1995. Provides a history of the construction of the aqueducts, an in-depth discussion of each of the eleven aqueducts of Rome, and a field guide to the ruins. Appendices include an information chart on the ancient aqueducts, information on modern Roman aqueducts, and a summary of relevant inscriptions. Maps, figures, bibliography, and index.
• Evans, Harry B. Water Distribution in Ancient Rome: The Evidence of Frontinus. Ann Arbor: University of Michigan Press, 1994. Evans, a historian of ancient engineering, includes a translation of De Aquis and analysis of Frontinus’ descriptions of Roman aqueducts.
• Hodge, A. Trevor. Roman Aqueducts and Water Supply. 2d ed. Newburyport, Mass.: Focus: 2002. A comprehensive study that covers all aspects of aqueduct construction and function. An unusually practical study of ancient water supply methods.
• Hodge, A. Trevor. “Siphons in Roman Aqueducts.” Scientific American 252 (June, 1985): 114-119. Well-explained account of the technology involved in Roman aqueducts.
• Landels, J. G. Engineering in the Ancient World. Rev. ed. Berkeley: University of California Press, 2000. Includes a substantial chapter on water engineering, as well as chapters on Vitruvius and Frontinus. The author is a rare combination of classicist and engineer.
• O’Connor, Colin. Roman Bridges. New York: Cambridge University Press, 1993. O’Connor’s explanation of Roman engineering in this area treats the aqueduct as a special bridge.