Page:Africa by Élisée Reclus, Volume 1.djvu/79

 in fish, chiefly cyprides of a different species from those of the Nile. A kind of bivalve also occurs, resembling the oyster in appearance and flavour.

Issuing from the lake at an altitude of 6,200 feet, the Abai flows at first towards the south-east, forming a first fall near Woreb, 5 miles below the outlet. Expanding lower down to a width of about 650 feet, it winds along through shady fields to the Tis-Esat, or Alata Falls, where it is suddenly precipitated from a height of over HO feet into a yawning chasm shrouded in vapour. In the centre of the cascade stands a pyramidal rock surmounted by a solitary tree constantly agitated by the breeze. Immediately below this spot the Abai plunges into a winding gorge, at one point scarcely 8 or 10 feet wide, crossed by a bridge of Portuguese construction. Some 30 miles farther on it is crossed by another bridge, the central arch of which has given way, its broken fragments forming a reef amid the tumultuous waters underneath. The whole distance between these two bridges is little more than a succession of falls and rapids, with a total descent of at least 2,000 feet. Alpine masses tower to the right and left above the gorge, which seems to have no outlet. But after describing a complete semicircle round the Abyssinian plateau, the Abai emerges on the plain in a north-westerly direction. The fall in this vast circuit is altogether over 4,000 feet, while throughout its lower course, terminating at the Khartum confluence, the incline is scarcely perceptible. Here it winds in gentle meanders between its alluvial banks, which are constantly yielding to the erosive action of the stream.

During the dry season the Bahr-el-Azraq diminishes in volume downwards, and in many places may be easily forded. For more than half the year the Yabus and Tumat, its chief tributaries from the south, are apparently merely dried-up wadies, although the water still percolates beneath the sands. The Rahad, or Abu-Atraz, also one of its large eastern affluents, which rises on the west slope of the Abyssinian border range, is completely exhausted for a long way above the confluence before the wet season. But from June to the middle of September, when the rain falls in torrents on the mountain slopes, its vast bed overflows its banks, supplying abundant water to the cultivated riverain tracts. The Dender, however, another river rising in Abyssinia, appears to be perennial. Nowhere else would it be more useful or more easy to construct reservoirs and control the discharge than in this hydrographic basin, which at the confluence of the two great arteries at Khartum stands at an altitude variously estimated at from l,250 to 1,400 feet above sea-level.

The northern as well as the southern section of the Abyssinian plateau is also comprised in the Nile basin. But here the affluents of the great river rise, not on the western slope, but in the very heart of the highlands, close to the range forming the water-parting between the Mediterranean and the Red Sea. The Takkaze, main headstream of the whole Atbara hydrographic system, has its source at an elevation of nearly 7,000 feet, and flows at first westwards, as if to fall into Lake Tsana. But the gorge through which it descends between its crystalline schist walls rapidly attains a level fur lower than that of the Ethiopian uplands. At the point where the river trends northwards it has already fallen to an altitude of con-