Aqueducts are man-made conduits constructed to carry water. The term aqueduct comes from words meaning "to lead water" in Latin, the language of the Romans who were the first builders of large aqueducts. Aqueducts carry water from natural sources, such as springs, into cities and towns for public use.

The first aqueducts

Wells, rivers, lakes, and streams are the oldest sources of water. In the ancient world however, rivers and lakes were also sometimes used as places to dispose of sewage and trash. Water from rivers that flowed though several villages often carried disease-causing organisms. Aqueducts provided a way for a plentiful supply of clean water to be piped into cities.

The earliest aqueducts were also used to transport water for irrigation (watering crops). Aqueducts were used in ancient India, Persia, Assyria, and Egypt as early as 700 b.c.e. The Romans, however, are regarded as the most famous ancient aqueduct builders.

Between 312 b.c.e. and 230 c.e., the most complex and efficient ancient system of aqueducts was built to supply the city of Rome with water. Outside of the capital city of Rome, the Romans built aqueducts throughout their large empire. Ruins of ancient aqueducts can still be seen in Italy, Greece, North Africa, Spain, and France.

How ancient aqueducts functioned

Ancient aqueducts used tunnels and channels (passages for water to flow) to transport water. The earliest irrigation aqueducts were simple canals and ditches dug into the ground. In order to keep water for use by people clean, aqueducts that supplied people with water featured covered channels or pipes.

The first aqueduct made of stone-covered waterways was built by the Assyrians around 690 b.c.e. Centuries later, Roman aqueduct builders perfected the closed channel design, building thousands of miles (kilometers) of stone aqueducts throughout the Roman Empire.

Ancient aqueducts were carefully planned before they were constructed. Water flowed through the channels by the force of gravity alone. The rate of flow (how many gallons could flow through the conduit in a day) was determined by the force of the spring that fed the aqueduct.

Aqueduct channels were constructed with a gradual slope (angle) so that water from the source could flow downhill to its destination. There were no pumps that could move water up a hill or slope. Thus, when crossing hilly terrain, aqueducts were built on stone bridges and in tunnels.

Pipes made of stone or a type of baked clay called terra cotta carried water through carved out tunnels. Aqueduct bridges (or elevated spans) were required to withstand the heavy weight of water. Spectacular Roman aqueduct bridges featuring several stories (or tiers) of strong arches can still be seen today. Some are still in use!

After the aqueducts entered the city, water flowed into public cisterns (large pools or wells that store water) or flowed from public fountains. In Rome, some citizens had water from the aqueducts piped directly to their homes. Wastewater was carried by sewer systems that emptied into outlying streams that normally did not feed into the aqueduct.

Like modern water supply systems, ancient aqueducts required constant maintenance. Where aqueducts ran underground, shafts (tunnels) were built to provide access to the aqueduct for repairs.

Chalk and other minerals built up in the conduits and required regular cleaning. Wars, earthquakes, storms, and floods sometimes damaged whole sections of aqueducts. Fixing aqueducts was an expensive undertaking and required the work of strong laborers and skilled engineers.

Innovations in aqueduct technology

After the fall of the Roman Empire in the fifth century, aqueduct building ceased in Europe. For centuries, the scientific knowledge necessary to build aqueducts, aqueduct bridges, and sewers was lost. Rome and some other cities continued to use their ancient aqueducts.

However, during the Middle Ages (500-1500 c.e.), people mostly used wells and rivers as a source of water. During the Renaissance (1300s-1600s), a renewed interest in classical architecture and engineering led scholars of the day to rediscover how ancient water systems worked and how aqueducts were constructed.

In the 1600s, aqueducts were once again included in public water systems. In France, a system of pumps moved water from a river to an aqueduct system that began on the crest (high point) of a nearby hill. An aqueduct spanning 38 miles (61 kilometers) carried water into the city of London, England. The Chadwell River to London aqueduct flowed over 200 small bridges.

In the eighteenth and nineteenth centuries, innovations such the steam pump permitted water to be pressurized. Pressurized water is water that is mixed with air or steam that, with the help of a pump, can be moved forcefully through pipes and conduits.

This allowed water systems to move water over any terrain. Aqueducts and water pipe systems carried water over greater distances with the aid of pressurized water. Pressurization also created a need for stronger pipes. Instead of terra-cotta, pipes were made of metals or concrete.

Between the 1830s and 1900, the growing city of New York constructed several aqueducts to bring spring and river water into the city from sources over 120 miles (193 kilometers) away. These aqueducts incorporated new and old aqueduct technology.

They employed pumps and deep underground pipe systems, but the Old Croton aqueduct, in use until 1955, also featured a Roman-like aqueduct bridge. Today, the three major aqueduct systems that serve New York City deliver nearly 1.8 billion gallons (approximately 6.8 billion liters) of water per day.

Aqueducts today

Aqueducts remain an important and efficient means of delivering clean water to cities. Today’s aqueducts are longer and able to carry more water than ancient aqueducts. Pumps and pressurized water flow permit aqueducts to flow up a slope. Improved pipe materials allows today’s aqueducts to be completely hidden deep underground.

The largest modern aqueduct system in the world has been under construction since the 1960s. When finished, the aqueduct will carry water 600 miles (966 kilometers) through the state of California, from the northern part of the state south to the Mexican border.