606 THE FRESH-WATER LOCHS OF SCOTLAND 
to lateral pressure, which has also given rise to the great central 
ridge running from the mountains of Abyssinia and those flanking 
the Red Sea in the north to the continuation of these same ridges in 
the shape of the Drakensberg Mountains in the extreme south. 
The lakes on the line west of the Victoria Nyanza — Tanganyika, 
Kivu, Edward, and Albert — drain to the Congo or the Nile, but 
those on the east and those in the depression north of Lake Rudolf 
have no outlet to the sea. Properly speaking, the latter should have 
been referred to along with the lakes of the inland drainage areas of 
Northern Africa ; but as they lie in one of the branches of this 
gigantic valley svstem, they are described after the lakes of the Nile, 
the Congo, and the Zambesi (see p. 618). These lakes were explored 
in 1893 by J. S. Gregory.^ 
Apart from the seasonal variations in level, most of the lakes 
of East Africa show periodic fluctuations, while some have supposed 
that a progressive desiccation of the whole region is traceable, 
tending to the ultimate disappearance of the lakes. Such a 
drying-up has no doubt been in progress during long geological 
ages, but is probably of no practical importance at the present time. 
The periodic fluctuations in the level of Lake Tanganyika are such 
that its outflow appears to be intermittent. After rising steadily 
for some years after 1871, a fall seems to have set in about 1879, 
which before the end of the century had carried the lake back within 
its natural bed. Within the same time the neighbouring Lake 
Rukwa has in great part dried up. Others of the East African lakes 
have on the contrary risen in level, Nyasa having been unusually high 
in 1896, and Rudolf in 1896-98; so that, if the fluctuations are due 
to variations of rainfall, these do not aff'ect the whole lake-region 
simultaneously in the same direction. In the case of Victoria Nyanza, 
a variation to the extent of 5 feet has been thought to recur in periods 
of eighteen to twenty-five years. Since 1896 records of the seasonal 
variations have been kept at stations north of the lake, the maximum 
in the year having been so far about 15 inches. 
River Nile. The Nile is a good example of an old river system (see fig. 73), 
the basin of which has been subjected to various earth-movements, 
and now, partly as a result of these, partly in consequence of the 
geological structure of the country through which it flows, presents 
the somewhat unusual spectacle of a river with two plain tracts at 
two very distant points and levels in its course. The valleys of the 
Bahr-el-Jebel and White Nile form the upper plain tract, and the valley 
of Egypt the lower. The latter is simply a cleft in the desert plateau, 
and is regarded as having been determined in the first instance by 
1 See The Great Rift Valley, London, 1896. 
