Birds of the Nile

Abstract

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