The Amhara Highlands receive 80%of the total rainfall of Ethiopia and is the most fertile and hospitable regions of Ethiopia. The Amhara Region is the location of Lake Tana, the source of the Blue Nile, at Bahir Dar. The flow of the Blue Nile reaches maximum volume in the rainy season (from June to September) when it supplies about two-thirds of the water of the Nile proper. The Blue Nile, along with the Atbara River to the north, which also flows out of the Ethiopian Highlands, caused the annual Nile floods that contributed to the fertility of the Nile Valley. This supported the rise of ancient Egyptian civilization and Egyptian mythology. With the completion in 1970 of the Aswan High Dam in Egypt, the Nile floods ended. Lake Tana has a number of islands, whose numbers vary depending on the level of the lake; it has fallen about 2 metres (6.6 ft) in the last 400 years. According to Manoel de Almeida (a Portuguese missionary in the early 17th century), there were 21 islands, seven to eight of which had monasteries on them "formerly large, but now much diminished."When James Bruce visited the area in the later 18th century, he noted that the locals counted 45 inhabited islands, but wrote he believed that "the number may be about eleven." A mid-twentieth century account identified 37 islands, of which 19 were said to have or have had monasteries or churches on them.
The lake islands were the home of ancient Ethiopian emperors. Treasures of the Ethiopian Church are kept in the isolated island monasteries (including Kebran Gabriel, Ura Kidane Mehret, Narga Selassie, Daga Estifanos, Medhane Alem of Rema, Kota Maryam and Mertola Maryam). The body of Yekuno Amlak is interred in the monastery of St. Stephen on Daga Island; other Emperors whose tombs are on Daga include Dawit I, Zara Yaqob, Za Dengel and Fasilides. Other important islands in Lake Tana include Dek, Mitraha, Gelila Zakarias, Halimun, and Briguida.
In the late 20th century, the scholar Paul B. Henze reported being shown a rock on the island of Tana Qirqos and being told it was where the Virgin Mary had rested during her journey back from Egypt. He was also told that Saint Frumentius, the bishop known for introducing Christianity to Ethiopia, was "allegedly buried on Tana Cherqos