Lake Tana (Amharic:ጣና ሐይቅ,romanized: T’ana ḥāyik’i; previously transcribedTsana[1]) is the largestlake inEthiopia and a source of theBlue Nile. Located inAmhara Region in the north-westernEthiopian Highlands, the lake is approximately 84 kilometres (52 miles) long and 66 kilometres (41 miles) wide, with a maximum depth of 15 metres (49 feet),[2] and an elevation of 1,788 metres (5,866 feet).[3] Lake Tana is fed by theGilgel Abay,Reb andGumara rivers. Its surface area ranges from 3,000 to 3,500 square kilometres (1,200 to 1,400 square miles), depending on season and rainfall. The lake level has been regulated since the construction of the controlweir where the lake discharges into the Blue Nile. This controls the flow to theBlue Nile Falls (Tis Abbai) and hydro-power station.
In 2015, the Lake Tana region was nominated as aUNESCO Biosphere Reserve recognizing its national and international natural and cultural importance.[4]
Views over Lake TanaThe Island Church on Lake TanaA local tour guide demonstrates how a stone is struck to signal meal times at a monastery on Zege PeninsulaBeginning of theBlue Nile river by its outlet from Lake TanaA resort hotel on Lake Tana inBahir Dar
Lake Tana was formed by volcanic activity, blocking the course of inflowing rivers in the earlyPleistocene epoch, about 5 million years ago.[5]
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.[5]
Lake Tana has a number of islands, whose number varies depending on the level of the lake. It has fallen about 6 feet (1.8 m) in the last 400 years. According toManoel de Almeida (aPortuguese 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."[6] WhenJames Bruce visited the area in 1771, he noted that the locals counted 45 inhabited islands, but stated he believed that "the number may be about eleven."[6] Anton Stecker, in 1881, made a detailed examination of the lake, enabling substantially accurate maps,[1] and counted 44 islands.[7] A 20th-century geographer named 37 islands, of which he believed 19 have or had monasteries or churches on them.[6]
The monasteries are believed to have been built during the Middle Ages over earlier religious sites. They include the fourteenth-centuryDebre Maryam, and the eighteenth-century Narga Selassie, Tana Qirqos (said to have housed theArk of the Covenant before it was moved toAxum), and Ura Kidane Mehret, known for its regalia. A ferry service linksBahir Dar withGorgora via Dek Island and various lakeshore villages.
There is alsoZege Peninsula on the southwest portion of the lake. Zege is the site of theAzwa Maryam monastery.
Compared to other tropical lakes, the waters in Lake Tana are relatively cold, typically ranging from about 20 to 27 °C (68–81 °F). The water has apH that is neutral to somewhatalkaline and its transparency is quite low.[9]
Because of the large seasonal variations in the inflow of its tributaries, rain and evaporation, the water levels of Lake Tana typically vary by 2–2.5 m (6.6–8.2 ft) 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.[9]
Since there are no inflows that link the lake to other large waterways and the main outflow, theBlue Nile, is obstructed by theBlue Nile Falls, the lake supports a highly distinctive aquatic fauna, which generally is related to species from theNile Basin.[10] The lake's nutrient levels are low.[9]
Lake Tana supports a largefishing industry, mainly based on theLabeobarbus barbs, Nile tilapia and sharptooth catfish. According to the Ethiopian Department of Fisheries and Aquaculture, 1,454 tons of fish were landed in 2011 atBahir Dar, which the department estimated was 15% of its sustainable amount.[13] 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 theLabeobarbus barbs that breed in the tributaries had significantly declined.[9] Among the endemic fish, most are considered threatened (endangered orvulnerable) ordata deficient (available data insufficient for evaluating a status) by theIUCN.[14] 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.[9]
Other serious threats are habitat destruction and pollution.Bahir Dar has become a large city and it is rapidly growing; itswastewater is generally released directly into the lake.[9] The vegetation in the lake's wetlands, which are an important nursery for theLabeobarbus and other fish, are being cleared at a fast pace. A potentially serious threat to the unique ecosystem would be anintroduction of a large and efficient predatory species like theNile perch, which has been implicated in numerous extinctions inLake Victoria. The piscivorousLabeobarbus of Lake Tana are relatively inefficient predators that can only take fish up to about 15% of the length of the predator itself.[9]
Among other fauna, the lake supports relatively few invertebrates: There are fifteen species ofmollusks, including one endemic, and also an endemic freshwatersponge.[10]
^abVijverberg, Jacobus; Sibbing, Ferdinand A.; Dejen, Eshete (2009). "Lake Tana: Source of the Blue Nile".The Nile. Monographiae Biologicae. Vol. 89. pp. 163–192.doi:10.1007/978-1-4020-9726-3_9.ISBN978-1-4020-9725-6.
^abcC.F. Beckham and G.W.B. Huntingford,Some Records of Ethiopia, 1593-1646, (series 2, no. 107; London:Hakluyt Society, 1954), p. 35 and note.
^abcdefghijklmnVijverberg, J.; F.A. Sibbing; E. Dejen (2009). "Lake Tana: Source of the Blue Nile". In H.J. Dumont (ed.).The Nile. Monographiae Biologicae. Vol. 89. Springer Science + Business Media B.V. pp. 163–193.ISBN978-1-4020-9725-6.
^abde Graaf, Dejen, Sibbing and Osse (2000).Barbus tanapelagius, A New Species from Lake Tana (Ethiopia): its Morphology and Ecology. Environmental Biology of Fishes 59 (1): 1-9.
^de Graaf, Megens, Samallo, Sibbing (2007).Evolutionary origin of Lake Tana's (Ethiopia) small Barbus species: indications of rapid ecological divergence and speciation. Animal Biology 57(1): 39–48.
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