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.

[5] The Valens aqueduct, which originally sourced its water from the slopes of the hills between Kağıthane and the Sea of Marmara,[6] 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.

[8] The exact date that construction on the aqueduct began is uncertain, but it was completed in 368 AD during the reign of Valens, whose name it bears.

The spectacular Bozdoğan Kemeri section lay along the valley between the third and fourth hills of Constantinople, occupied respectively at that time by the Capitolium and the Church of the Holy Apostles.

[9] The structure was inaugurated in 373 by the urban prefect Clearchus, who commissioned a Nymphaeum Maius in the Forum of Theodosius, that was supplied with water from the aqueduct.

[9][a] After a severe drought in 382, the Emperor Theodosius I built a new line (the Aquaeductus Theodosiacus), which took water from the north-eastern region known today as the Belgrade Forest.

[5] In the 4th century, Gregory of Nazianzus described the Aqueduct of Valens as an "underground and aerial river" (Patristic Greek: ὁ ὑποχθόνιος καὶ τὸ ἀέριος ποταμός, romanized: ho hypokhthónios kai tò aérios potamós).

[1] This was thought to be an exaggeration until archaeological survey revealed the 227-kilometre (141-mile) course of the channel from Danımandere to Constantinople, with another 41-kilometre (25-mile) line to Pınarca making the total system inaugurated by Valens 268 kilometres (167 miles) long.

[1] Likewise, the claim of Hesychius of Miletus in the Patria of Constantinople that the aqueduct extended to Vize (Medieval Greek: Βιζύη, romanized: Bizyē) was correct.

[2]: 13  The second, 5th-century phase of the Aqueduct of Valens – 451 kilometres (280 mi) long – carried water from springs at Pazarlı, at Ergene, and near Binkılıç.

[1] Another two open-air cisterns inside the Constantinian Walls were both fed by the Aqueduct of Valens: one at Saraçhane and the other on Bab-ı Ali Caddesi.

[11]: 124 Under Emperor Theodosius II (r. 402–450), the aqueduct's water was directed exclusively to the Nymphaeum, the Baths of Zeuxippus and the Great Palace of Constantinople.

[6][13] In 1075, the official in charge of the aqueduct's maintenance was Basil Maleses, the protovestes and former judge recorded by the Greek historian Michael Attaleiates as having been captured by Alp Arslan's Seljuks at the Battle of Manzikert.

'logothete of the waters' and attested only from Attaleiates's history, was descended from the official in charge of aqueducts mentioned by Frontinus one millennium earlier in the Roman imperial period.

[1][14] After the First Crusade's passage through Constantinople, both William of Malmesbury and Odo of Deuil mentioned the working Aqueduct of Valens in their histories.

[6] Nevertheless, according to Ruy Gonzáles de Clavijo, a Castilian diplomat who traveled to Constantinople en route to an embassy to Timur in 1403, the aqueduct was still functioning.

[16] ^ a: The Nymphaeum, which was called "abundant water" (δαψιλές ὔδωρ, dapsilés hýdor in Greek), was one of the four Nymphaea existing in the city in the first third of the 4th century.