Construction of the aqueduct began during the reign of the Roman emperor Constantius II (r. 337–361) and was completed in 373 by the emperor Valens (r. 364–378). The aqueduct remained in use for many centuries. It was extended and maintained by the Byzantines and the Ottomans.
Initially, the Aqueduct of Valens carried water from springs at Danımandere and Pınarca; the channels from each spring met at Dağyenice. This 4th-century first phase of the system was 268 kilometres (167 miles) long. A second, 5th-century phase added a further 451 kilometres (280 miles) of conduits that took water from Vize, 120 kilometres (75 miles) away from Constantinople.
The final and most visible aqueduct bridge in the system survives in the Fatih district of Istanbul, Turkey. Named in Turkish: Bozdoğan Kemeri, lit. ’Aqueduct of the Grey Falcon’, it is an important landmark in the city, with its arches passing over Atatürk Boulevard (Atatürk Bulvarı).
The Bozdoğan Kemeri spans the valley between the hills that are today occupied by the Istanbul University and the Fatih Mosque, formerly the site of the Church of the Holy Apostles. The surviving section is 921 metres long, about 50 metres less than the original length.
The construction of a water supply system for the city of Byzantium began under the emperor Hadrian. Constantine the Great re-founded the city and greatly expanded it which meant that demand for fresh water greatly increased.
The Valens aqueduct, which originally got its water from the slopes of the hills between Kağıthane and the Sea of Marmara, was merely one of the terminal points of this new wide system of aqueducts and canals—which eventually reached over 250 kilometres (160 mi) in total length, the longest such system of Antiquity—that stretched throughout the hill-country of Thrace and provided the capital with water.
Once in the city, the water was stored in three open reservoirs and over a hundred underground cisterns, such as the Basilica Cistern, with a combined capacity of over one million cubic metres.
The water comes from two lines from the north-east and one coming from the north-west, which join together outside the walls, near the Adrianople Gate (Edirne Kapı).
Near the east end of the aqueduct there is a distribution plant, and another lies near Hagia Sophia. The water feeds the zone of the imperial palace.
During Byzantine times, two roads important for the topography of medieval Constantinople crossed under the eastern section of the aqueduct.
