Monographiae Biologicae Volume 89, 2009, pp 163-192

Lake Tana: Source of the Blue Nile

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At 1,830 m altitude, Lake Tana is situated on the basaltic Plateau of the north-western highlands of Ethiopia covering an area of ca 3,050 km2. It is poor in nutrients and the source of the Blue Nile River (Great Abbay), with a catchment area of ca 16,500 km2. The Lake has been formed by volcanic activity, blocking the course of inflowing rivers in the early Pleistocene times ca 5 million years ago. The lava also separated the Lake and its headwaters from the lower Blue Nile basin by 40 m high falls at Tissisat, 30 km downstream from the Blue Nile outflow. Terraces suggest that the Lake was originally much larger than it is today. Seven large permanent rivers feed the lake as well as ca 40 small seasonal rivers. The main tributaries to the lake are Gilgel Abbay (Little Nile River), Megech River, Gumara River and the Rib River. Together they contribute more than 95% of the total annual inflow. The Blue Nile is the only outflowing river. The shallow lake (average depth 8 m, max. depth 14 m) is Ethiopia's largest lake, containing half the country's freshwater resources, and the third largest in the Nile Basin. In the main rainy season (July—August) the inflowing rivers carry heavy load of suspended silt into the lake, thereby increasing the turbidity of the lake water. The suspended sediments reduce the under water light intensity and as such the primary production, the basis of the food web. The fish community of the Lake is dominated by cyprinid fishes, 20 of the 27 fish species (e.g. Labeobarbus spp., Barbus spp., Garra spp.) are endemics to the Lake catchment. This speciation was possible because the incipient Lake offered new habitats for adaptive radiation and maintained its isolation for millions of years from the lower Blue Nile. Wetlands are located all around the lake, with the exception of the Northeast. Together they are the largest in the country and integral parts of the complex Tana-ecosystem. They consist of permanent swamps, seasonal swamps, and areas subjected to regular inundation. During the raining period these wetlands are connected with the lake. They act as nurseries for most of the fish populations in the lake, and serve as breeding ground for water fowl and mammals. Around the lake and its catchment, including the town of Bahir Dar, live about 2 million people. This lake and adjacent wetlands provide directly and indirectly a livelihood for more than 500,000 people. The Blue Nile drains the NE Ethiopian Plateau (total catchment: 324,000 km2). Already in ancient Egypt civilization this river was of key importance to early agriculture and today the river is still of critical importance for the economies of Sudan and Egypt.