1911 Encyclopædia Britannica/Tsana

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 1100 sq. m., having a drainage 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 Nile 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 650 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 150 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|Rain in the Tsana catchment area is estimated at 33 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 12° 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 17th century the lake was styled Dambia, 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. C.)