Water has always been one of the most important resources for a city, and ancient Rome was famous for its great wealth of public fountains, baths, artificial pools and reservoirs, naval arenas (naumachiae), irrigation channels, and other similar facilities. During a time length of over 500 years, eleven main aqueducts, plus a considerable number of branches, were built for Rome's urban needs. It has been assessed that, all together, the total output of these aqueducts was considerably more than the daily amount of water the modern city can rely upon today.
This plenty, which no other place in the world ever had, was worth Rome the name of regina aquarum, "queen of the waters".
Interestingly, the Romans did not give a name to the aqueducts themselves, but to the water they carried, so that most of them were referred to as aqua (Aqua Appia, Aqua Marcia, Aqua Iulia, etc.), often named after the ruler or government officer who sponsored or presided over their making.

Since the time Rome was founded, the inhabitants could rely on the water of the Tiber, which ran along the western urban boundary (today it cuts the modern city into two halves), and on its main tributary, the Aniene, which meets the larger river about 4 km (2 ½ mi.) north of the earliest city walls, on a spot now surrounded by modern districts. During the age of kings, and for some time of the republican age, the population satisfied its needs by drawing water directly from these rivers, from minor channels, and from a number of smaller sources such as wells and cisterns for rain water. In the 4th century BC the size of the city and the growth of the population, including the many immigrants, the foreign merchants and the slaves, required a larger supply.

the course of the Tiber and of the Aniene, compared to Rome's extension in different ages

In fact, in 312 BC censor Appius Claudius had the first aqueduct built; it drew water from a spring located between 7 and 8 miles east of the city, although the total length of its course measured no less than 11 miles (the reasons of such crooked direction will be explained in part II). The making of the following aqueducts came at an average rate of one every 60 years or so, but in AD 52 two of them were built almost at the same time.

The length of the aqueducts was expressed in passus ("steps"), a measure corresponding to 1.482 m or 4 ft 10¼ in). More approximately, they were measured in milia passus, i.e. roman miles, actually meaning "thousands of steps", equals to 1.482 km, or 0.92 mi.

The following is a list of the main ancient aqueducts, according to a chronological ordering, which also shows which parts of the city they served. The location of their springs refers to the distance and position from the city center, expressed in roman miles.

AQUEDUCT	NAME REFERENCE	YEAR OF CONSTRUCTION	LOCATION OF THE SPRINGS	LOCATION OF THE MAIN OUTPUT
AQUA APPIA	censor Appius Claudius Caecus	312 BC	7-8 miles east	Circus Maximus (south west) branches for most districts
ANIO VETUS  [1]	"old Aniene"	269 BC	29 miles east	Porta Esquilina (south-east)
AQUA MARCIA	praetor Quintus Marcius	144 BC	36 miles east	Quirinal Hill (north-east)
AQUA TEPULA	"warm water", due to its temperature	125 BC	10 miles south-east	Porta Collina (north-east)
AQUA IULIA	emperor Octavianus' family name	33 BC	12 miles south-east	Porta Viminalis (north-east) branches for many districts
AQUA VIRGO	"maiden water", from a legend	19 BC	8 miles east	Campus Martis (north-west)
AQUA ALSIETINA	Lake Alsietinus (now Martignano)	2 BC	14 miles north-west	Trans Tiberim (west)
AQUA CLAUDIA	emperor Claudius	AD 52	38 miles east	Porta Praenestina (south-east) branches for most districts
ANIO NOVUS	"new Aniene"	AD 52	38 miles east	it shared the same output of the Aqua Claudia
AQUA TRAIANA	emperor Trajan	AD 109	13 miles north-west	Janiculum Hill (west)
AQUA ALEXANDRINA	emperor Alexander Severus	AD 226	14 miles east	Pantheon, Campus Martis (north-west)

[1] - The original name was Anio;  Vetus ("old") was added over 200 years later, when the Anio Novus was built

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The rate of flow of each aqueduct was calculated in quinariae. Scholars assessed that 1 quinaria was equals to 0.48 litres/second. The most powerful of the eleven aqueducts, the Anio Novus, drew 4,738 quinariae, which meant a supply of almost 200 million litres per day!

Rome's waterworks were supervised by a high officer whose title was curator aquae, i.e. "water curator". It is thanks to one of these curators, Sextus Iulius Frontinus (late 1st century AD), who wrote a detailed essay on this subject, that most data concerning the administration, the features and the course of the roman aqueducts are known today. The earliest graphic record of these waterworks appears in the Forma Urbis Romae, an ancient and very detailed city plan dating to the early 3rd century AD, carved in stone in scale 1:240, some fragments of which still exist. One of them in particular features an output at the end of a duct, with the word AQVEDVCTIVM.

above: a 1590 map, showing a no longer existing part of the Aqua Virgo (highlighted in yellow), on the the Pincio hill; on the right: a map dated 1472 features a part of the Arcus Caelemontani (a branch, see part III), behind the Colosseum

Several maps of Rome from the Renaissance and Baroque ages, instead, show three-dimensional aerial views of the many parts of the viaducts still standing by the 15th-17th centuries. Thanks to these sources and to archaeological excavations, it has been possible to trace the course of most ancient roman aqueducts, although due the city's development in time very little of these majestic structures has survived.

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