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lake, water, nile, abbai and blue

TANA, a lake of North-East Africa, chief reservoir of the Abbai or Blue Nile. Tana lies between I I° 36' and 12° 16' N. and 37° 2' and 40' E., 5,690 ft. above the sea, on the northern portion of the Abyssinian plateau. Its greatest length is 47 m., its greatest breadth 44 m., and it covers approximately 1,100 sq.m., having a drainage area, including the lake surface, of some 5,400 sq.m. The shores are well defined, generally flat, but at places the mountains descend somewhat abruptly into the water. Elsewhere the land rises in gentle undulations, except at the mouths of the larger tributary streams, where are alluvial plains of considerable size. At the south-east end the lake forms a bay about I 1 miles long, and from three to eight miles across. From this bay the Abbai issues. The whole coast-line is con siderably indented and many narrow promontories jut into the lake. The island of Dek (eight m. long by four broad) is in the south-western part of the lake. Near it is the smaller island of Dega. Numerous islets fringe the shores.

Lake Tana is fed by three large rivers and by many petty streams. The chief tributary is the Little Abbai, which enters the lake at its south-west corner. This river, and the Abbai itself or Blue Nile which issues from the lake, are regarded as one and the same stream and a current is observable from the inlet to the out let. Next in importance of the affluents are the Reb and Gumara, which run in parallel courses and enter the lake on its eastern side. The outlet of the lake is marked by openings in a rocky ledge, through which the water pours by two or three channels, in a succession of rapids, uniting within a couple of miles into one river—the Abbai with a width of 65o ft.

The average annual rainfall in the Tana catchment area is estimated at 31 ft., and the volume of water received by the

lake yearly from this source and from affluents at about 6,572, 000,000 of cubic metres. The average seasonal alteration of the lake level is not more than about II metres.

Three technical missions have been sent by the British and Egyptian Governments conducted respectively by Mr. Dupuis (1903), Mr. Buckley (1916) and Mr. Grabham (192o–I ) to re port on the feasibility of utilizing the lake as a reservoir with the object of regulating the supply of water (3,500 million cu.m. in a normal year) discharged from the lake into the Blue Nile for irrigation needs in the Sudan and Egypt. As a result a scheme of works has been proposed, and it is estimated that these works, in addition to securing a more even distribution of water during the year, would by storing the surplus water in years of heavy rainfall form a reserve of 8,000 million cu.m. to tide over the deficiency in years of bad supply. The Abyssinian Government, however, has consistently opposed the project.

Tana has been identified with the Coloe Palus of the ancients, which was described by Ptolemy as a chief reservoir of the Egyp tian Nile and the source of the Astapos, which was certainly the Blue Nile. In 1625 it was visited by the Portuguese priest Jeronimo Lobo, and in 1771 by James Bruce. It was formerly known by the name of Dembea.

See NILE and ABYSSINIA, and the authorities there cited. The British Blue Book, Egypt, No. 2, 1904, which contains a special report (with maps) upon Lake Tana. Also the most valuable Report of the Mission to Lake Tana (1920-21) issued by the Egyptian Ministry of Public Works in 1925. (C. F. R.)