Lake Tana (; previously transcribed Tsana) is the largest lake in Ethiopia and a source of the Blue Nile. Located in Amhara Region in the north-western Ethiopian Highlands, the lake is approximately long and wide, with a maximum depth of , and an elevation of . Lake Tana is fed by the Gilgel Abay, Reb and Gumara rivers. Its surface area ranges from , depending on season and rainfall. The lake level has been regulated since the construction of the control weir where the lake discharges into the Blue Nile. This controls the flow to the Blue Nile Falls (Tis Abbai) and hydro-power station.
In 2015, the Lake Tana region was nominated as a UNESCO Biosphere Reserve recognizing its national and international natural and cultural importance.
Overview
thumb|left|Views over Lake Tana
thumb|left|The Island Church on Lake Tana
thumb|left|A local tour guide demonstrates how a stone is struck to signal meal times at a monastery on Zege Peninsula
thumb|left|Beginning of the [[Blue Nile river by its outlet from Lake Tana]]
thumb|left|A resort hotel on Lake Tana in [[Bahir Dar]]
Lake Tana was formed by volcanic activity, which blocked the flow of inflowing rivers in the early Pleistocene, about 5 million years ago.
The lake was originally much larger than it is today. Seven large permanent rivers feed the lake as well as 40 small seasonal rivers. The main tributaries to the lake are Gilgel Abbay (Little Nile River), and the Megech, Gumara, and Rib rivers. A 20th-century geographer named 37 islands, of which he believed 19 have or had monasteries or churches on them.
Remains of ancient Ethiopian emperors and treasures of Ethiopic Christianity are kept in the isolated island monasteries (including Kebran Gabriel, Ura Kidane Mehret, Narga Selassie, Daga Estifanos, Medhane Alem of Rema Island, Kota Maryam, and Mertola Maryam). On the island of Tana Qirqos is a rock shown to Paul B. Henze, on which he was told Mary, mother of Jesus had rested on her journey back from Egypt; he 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. Emperors whose tombs are also 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.
The monasteries are believed to have been built during the Middle Ages over earlier religious sites. They include the fourteenth-century Debre Maryam, and 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 Mehret, known for its regalia. A ferry service links Bahir Dar with Gorgora via Dek and various lakeshore villages.
There is also Zege Peninsula on the southwest portion of the lake. Zege is the site of the Azwa Maryam monastery.
Lake Tana was also a central location of the Beta Israel, complete with the only Jewish monasteries in the world, before their immigration to Israel.
Water characteristics and floods
Compared to other tropical lakes, the waters in Lake Tana are relatively cold, typically ranging from about . The water has a pH that is neutral to somewhat alkaline and its transparency is quite low.
Because of the large seasonal variations in the inflow of its tributaries, rain and evaporation, the water levels of Lake Tana typically vary by in a year, peaking in September–October just after the main wet season. When the water levels are high, the plains around the lake often are flooded and other permanent swamps in the region become connected to the lake. The lake's nutrient levels are low. Among these, L. acutirostris, L. longissimus, L. megastoma and L. truttiformis are strictly piscivorous, and L. dainellii, L. gorguari, L. macrophtalmus and L. platydorsus are mostly piscivorous. The remaining Labeobarbus in Lake Tana have other specialized feeding habits: L. beso (non-endemic and not closely related to the others) feeds on algae, L. surkis mostly on macrophytes, L. gorgorensis on macrophytes and molluscs, L. brevicephalus on zooplankton (however, juveniles of all members of the species flock feed on zooplankton), L. osseensis on macrophytes and adults insects, and L. crassibarbis, L. intermedius (non-endemic but closely related to the others), L. nedgia and L. tsanensis on benthic invertebrates like chironomid larvae. Among the endemic Labeobarbus, eight species spawn in the lake's wetlands and the remaining move seasonally into its tributaries where they spawn. Nevertheless, in a review that compared catches in 2001 to those ten years earlier, it was found that typical sizes of both the tilapia and the catfish had significantly decreased, and populations of the Labeobarbus barbs that breed in the tributaries had significantly declined. In the early 2000s, the local government for the first time introduced a fisheries legislation and it is hoped this will have a positive effect on the fish populations. Hippos are present, mostly near the Blue Nile outflow.