TSANA  , a

lake of North-East Africa, chief reservoir of the Abai or Blue Nile . Tsana lies between 11° 36' and 12° 16' N. and 37° 2' and 37° 40' E., filling a central depression in the Abyssinian highlands . It is about 5690 ft. above the sea, but from 2500 to 3000 ft. below the mountain plateau which encircles it . Its greatest length is 47 m., its greatest breadth 44 m., and it covers, approximately xtoo sq. m., having a drain-age area, including the lake surface, of some 5400 sq. m . In shape it may be compared to a pear, the stem being represented by the escaping waters of the Abai . The shores of the lake are well defined, generally flat, and bordered by reeds, 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 eleven miles long, and from three to eight miles across, and from this bay the Abai issues . The whole of the coast-line is considerably indented and, many narrow promontories jut into the lake . The island of Dek (8 m. long by 4 broad) is in the south-western part of the lake . Near it is the smaller island of Dega, whilst numerous islets fringe the shores . Lake Tsana is fed by three large rivers and by many petty streams .

The chief tributary is the Abai, which enters the lake at its south-

west corner through a large papyrus swamp . This river, and the Abai or Blue Niie which issues from the lake, are regarded as one and the same stream and a current is observable from the inlet to the outlet . 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 into a lagoon-like expanse . Thence it issues by two or three channels, with a fall of about 5 ft. in a succession of rapids . These channels unite within a couple of miles into one river—the Abai with a width of 65o ft . After passing a large number of rapids in the first sixteen miles of its course the Abai enters a deep gorge by a magnificent fall—the Fall of Tis Esat—the water being confined in a channel not more than 20 ft. across and falling 15o ft. in a single leap . The gorge is spanned by a stone bridge built in the 17th century . From this point the Abai makes its way through the mountains to the plains of Sennar, as described in the article NILE . The average annual rainfall in the Tsana catchment area is estimated at 3; ft., and•the volume of water received by the lake yearly at 6,572,000,000 of cubic metres . More than half of this amount is lost by evaporation, the amount discharged into the river being placed at 2,924,000,000 cubic metres . The seasonal alteration of the lake level is not more than 5 ft .

The

rainy season lasts from the beginning of June to the end of September . During this period the discharge from the lake is, it appears, little greater than in the dry season, the additional water received going to raise the lake level . Thus the rise in the Blue Nile, in its lower course, would seem to be independent of the supply it derives from its source . Tsana has been identified with the Coloe Palus of the ancients, which although placed x 2° too far south by Ptolemy was described by him as a chief reservoir of the Egyptian 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 . Dr . Anton Stecker, in 1881, made a detailed examination of the lake, enabling the cartographers to delineate it with substantial accuracy . By the Portuguese of the 77th century the lake was styled Dambia, 1 Sven Hedin, Scientific Results of a Journey in Central Asia, 1899-5902, iii . 344 (Stockholm, 1905-1907) . and this name in the slightly altered form of Dembea was in use until towards the close of the 19th century . By many Abyssinians the lake is called Tana, but the correct Amharic form is Tsana . See NILE and ABYSSINIA, and the authorities there cited .

The

British Blue Book, Egypt, No . 2, 1904, contains a special report (with maps) upon Lake Tsana by Mr C . Dupuis, of the Egyptian Irrigation Service . In the Boll. soc. geog. italiana for December 1908 Captain A . M . Tancredi gives the results (also with maps) of an Italian expedition to the lake . (W . E . G.; F . R .