The Kırkçeşme water supply system was an Ottoman aqueduct system that brought water from the Belgrade Forest, north of Constantinople. It probably followed the Hadrian’s aqueduct system that was later developed and restored during the Byzantine era. While part of this system was restored by Mehmet II, the main features of the Kırkçeşme system were built by Mimar Sinan between 1554 and 1563. In terms of both cost and scale, the Kırkçeşme water supply system - the most extensive Ottoman water supply system - can be considered Sinan's greatest work.

The Kırkçeşme water supply system originated in the Belgrade Forest, where one of the main aquiferous areas in the vicinity of Constantinople was located. It is likely that the aqueduct system built by Hadrian (117-138) to supply Byzantium corresponds to Kırkçeşme supply system. Valens (364-378), Theodosius (379-395) and other emperors developed the water supply system of Constantinople, by redeveloping or building new aqueducts. Hadrian’s line continued to be used and developed in Late Antiquity, probably entering the city just north of Kaligaria Gate (Eğri Kapı), passing near the cistern near the Pantokrator Monastery and ending at the Basilica Cistern. Constantinople probably heavily relied on the water sources from the Belgrade Forest by the 12th century, as the long-distance water supply coming from Thrace ceased to function. It seems that this aqueduct system was not repaired following the Latin conquest of Constantinople in 1204, after which cisterns became the main source of water in the city while the remains of the aqueduct system decayed.

Evidence suggests that an earlier system also originated from the waters of the Belgrade Forest, though it is difficult to determine the surviving Roman and Byzantine features of the water supply system as the Ottomans extensively redeveloped it. There are, for example, remains of the Roman or Byzantine channels around the Cebecikoy Stream. Recent research has also confirmed that the lowest tier of Eğri Kemer has substantial Roman or Byzantine remains. In addition, it is possible that Uzunkemer also has trace of Roman or Byzantine foundations, while Ottoman dams, like the Büyük Bent, also were probably built on the site of Byzantine era dams. Mehmet II ordered work on the aqueduct system to supply shortly after the conquest of Constantinople in 1453. It seems that the ruins of Byzantine aqueducts were still prominent when he began to rebuild the water systems, making it unclear how much was restored or rebuilt. One of his main water projects involved redeveloping the water system in Halkalı to supply the Aqueduct of Valens (Bozdoğan Kemeri). However Mehmet II also redeveloped part of the water system in the Belgrade Forest, using the water sources on the southern bank of the Cebecikoy Stream. This system was known as Kırkçeşme (Turkish “forty fountains”) after a public fountain built by Mehmet near the Aqueduct of Valens. The fountain, which was demolished to widen Atatürk Boulevard in the 1940s, was decorated with a Byzantine relief of peacocks.

The Kırkçeşme water supply system was restored and expanded by Mimar Sinan between 1554 and 1563. This system, which brought large quantities of water from the Belgrade Forest on the north side of the city, was the biggest to be built during the Ottoman period. It seems that Suleiman the Magnificent (1520-1566) commissioned this water system after noticing water leaking from an old aqueduct when hunting around the Kağıthane Stream. This project was essential for his capital as increases in Constantinople’s population were causing water shortages in the city. Mimar Sinan’s system collected water from the branches of the Kağıthane Stream in the Belgrade Forest, partly using the old Roman water channels as a guide as Mehmet II had done before. This huge water supply system had 55 kilometers of channels and 33 aqueducts, five of which were monumental with several tiers of arches. It also included water distribution basins and a city distribution network with around 300 public fountains. This system was perhaps the most advanced system of its era. At the same time, many of its features, such as the aqueducts, settling basins, covered channels, distribution basins, and fountains, are related by the water systems of Rome and Constantinople built centuries earlier. This water system was built around the same time as Süleymaniye Mosque, another project of Mimar Sinan.

The Kırkçeşme system was divided into two main branches - a northern branch and an eastern branch. The monumental Eğri Kemer (Kovukkemer) and Paşa Kemer aqueducts were on the eastern branch, while Uzun Kemer was on the northern branch. These branches came together at a distribution pool known as Başhavuz, after which the main line passed over the Alibey Stream by means of the Mağlova Kemer Aqueduct, then over the Cebeciköy Stream by means of the Güzelcekemer Aqueduct, after which the line united with the Cebeciköy branch. After a distance of 22 kilometers it reached the Eğirkapı Distribution Chamber outside the city walls. The main line leaving Eğirkapı continued as far as the Tezgahçılar Distribution Basin, before separating into two lines, one terminating in the Hagia Sophia Distribution Chamber Chamber and the other in the Tahtakale Distribution Chamber. Another line passed through the city walls at Sulukule and divided into two branches, one which went to Haseki and the other to Yedikule.

The Kırkçeşme supply system began to supply the city with water after it was completed in 1563. However, heavy flooding in 1563 destroyed the Mağlova and Kurt Kemer aqueducts and extensively damaged Uzunkemer, Eğri Kemer, and Güzelcekemer. It functioned again in 1564 after Sinan rebuilt and restored the damaged sections of the system. It has been estimated that it supplied the city with 12,000-17,000 cubic meters of water day, depending in part on the season. Staring in the 17th century, dams were built to increase the water supply. These dams included the Karanlık Bent (1620), Büyük Bent (1724), Avyad Bendi (1765) and Kirazlı Bent (1818). It is possible that there was a Byzantine dam built in the 4th century around the site of the Büyük Bent and perhaps the Avyad Dam as well.