Christian Egypt kept the river's rhythm and changed its meaning. The deserts on either side of the valley became the birthplace of monasticism: Antony withdrew into the eastern desert in the late third century, Pachomius organised the first communities in the Thebaid, and the monasteries of Scetis in Wadi Natrun, west of the delta, have been inhabited continuously for some sixteen centuries. The Coptic Orthodox Church still uses a calendar whose year begins in September with the flood season and still prays, in a set petition, for the waters of the river to rise to their proper measure.
Under Islam the river remained the country's central fact and its ceremonial one. The cutting of the dam at the Khalij canal in Cairo, when the Nilometer on Rhoda Island registered a sufficient flood, was the great annual festival of medieval Cairo, and the custom persisted in altered form into the modern period. Upstream, the Blue Nile rises at Lake Tana in the Ethiopian highlands, whose island monasteries hold some of the oldest manuscripts of the Ethiopian church, so that the river links two of the oldest Christian traditions along its own length.
The Aswan High Dam, completed in 1970, ended the inundation. Lake Nasser drowned Lower Nubia, its people were resettled, and the temples of Abu Simbel and the island of Philae were cut apart and rebuilt on higher ground in an international salvage campaign.