Lake Tana is the water source of the Blue Nile and is the largest lake in Ethiopia. Lake Tana is located in the north-western Ethiopian highlands, and according to the Statistical Abstract of Ethiopia for 1967/68 (which admittedly is kind of old information), the lake is approximately 84 kilometers long and 66 kilometers wide, with a maximum depth of 15 meters, and an elevation of 1,840 meters. Lake Tana is “fed” by the Little Abay, Reb and Gumara Rivers and its surface area ranges from 3,000 to 3,500 km² depending on season and rainfall that year. The lake level has been regulated since the construction of the control weir where the lake discharges into the Blue Nile, which regulates the flow to the Tis Abay falls and the hydro-power station.

Lake Tana has a number of islands, whose numbers vary depending on the level of the lake. The level of the lake has fallen about 6 feet in the last 400 years. According to Manoel de Almeida (who was a Portuguese missionary in the early 17th century), there were 21 islands; seven to eight of which had monasteries on them whose numbers were "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 stated he believed that "the number may be about eleven." A more modern geographer named 37 islands. Of those thirty seven islands, the modern geographer believed 19 have or had at one time monasteries or churches on them.

Lake Tana also supports a notable fishing industry; according to the Ethiopian Department of Fisheries and Aquaculture, 1,454 tons of fish are landed each year at Bahir Dar, which the department estimates is fifteen per cent of its sustainable amount.

Lake Tana has more than fish and islands. The remains of ancient Ethiopian emperors and treasures of the Ethiopian Church are kept in the isolated island monasteries. On the island of Tana Qirqos is a rock shown that was shown to Paul B. Henze, on which he was told the Virgin Mary had rested on her journey back from Egypt. Henze was also told that Frumentius, who introduced Christianity to Ethiopia, is "allegedly buried on Tana Cherqos." The body of Yekuno Amlak is interred in the monastery of St. Stephen on Daga Island; other Emperors whose tombs are on Daga include Emperor Dawit I, Emperor Zara Yaqob, Emperor Za Dengel and Emperor Fasilides. Other important islands in Lake Tana include Dek Island and Meshralia.

The monasteries on the islands are believed by many of the Ethiopians to rest on earlier religious sites and some of those sites include the fourteenth century Debre Maryam, the eighteenth century Narga Selassie, Tana Qirqos (which is said to have housed the Ark of the Covenant before it was moved to Axum), and Ura Kidane Mecet, known for its regalia. A ferry service links Bahir Dar with Gorgora via Dek Island and various lakeshore villages.