The Great Nymphaeum - or Great Fountain - was located in Region X near the site were Emperor Theodosius was to build his forum, Forum Taurus (now the courtyard of the main building of Istanbul University). It was most likely built by the city prefect Clearchus during his first tenure (372-3). According to Crow 2022, p. 76, the Great Nymphaeum distributed the water from the Valens line to the higher parts of the new city, icnluding the Binbirdirek Cistern (which containted a castellum aquae).
Literary Evidence
Multiple Byzantine historians (Jerome, Socrates, Cedrenus, Zonaras) attest that Clearchus built the Nymphaeum during his first tenure (372-3). Cedrenus and Zonaras use the term "Nymphaeum Maius".
This Nymphaeum is attested alongside three other nymphaea: one in Region IV and V, and another outside the city in Region XIV (Rhegion?).
André Thévet (1553) still recorded "se dégorge dans un grand vaisseau de marbre pour l'usage public des citoiens" (Cosmographie du Levant, XIX, 3, Lyon, 1554, 53) (Janin, 1964, p. 200-201).
Archaeological Evidence
Excavations have been done near the Forum of Theodosius (now Beyazıt) Bauer discovered vaulted conduits from the north, leaving him to conclude that the remains east of the arch [of Theodosius] in the Forum Tauri as the Nymphaeum of Region X (Crow et al., 2008, 127). See Bayülgen & Saner, 2024, p. 19-22 for a discussion of the remains excavated at the Forum of Theodosius, especially near the arch.
Most major cisterns (such as Aetius, Aspar, Forum of Theodosius I, Binbirdirek) lay alongside the route of the Valens line. Crow et al., 2008, p. 123 speculate whether the Great Fountain served as end point after the completion of the Valens line before the later cisterns were added.