9. Lake Victoria Nyanza.

—Lake Victoria, the true source of the Nile, lies on the Equator, and fed by abundant rains and numerous streams, discharges its surplus waters over the Ripon Falls, and gives birth to the Victoria Nile. Its most important feeder, the Kagera, whose southernmost tributary rises in the Kangosi hills 2000 metres above sea level in south latitude 4°, has a length of some 600 kilometres. The direct line across the lake from the mouth of the Kagera to the Ripon Falls is 220 kilometres, so that in academical language the length of the Nile at the Ripon Falls is already 820 kilometres. Lake Victoria lies 1129 metres above sea level, and has an area of 60,000 square kilometres; though until the parallels of longitudes are definitely settled, the lake may be credited with an area of between 60,000 and 65,000 square kilometres, constituting it the largest fresh water lake in the old world. Its waters are beautifully clear and perfectly sweet. The depth of the lake is not great and it is covered with many islands. The greatest depth found by Commander Whitehouse in the northern half of the lake has been 73 metres, while the bays are shallow. The northern, southern and eastern shores of the lake, as described by Sir William Garstin, are generally clear, while the western shore, especially at the mouth of the Kagera, is flat, marshy and covered with papyrus. The country surrounding the lake is undulating or hilly and rises to a height of 700 metres above the lake at the south-east corner. The rocks are generally granites, chrystalline schists and quartzites, etc. The hills are covered with red clay and marl on the higher lands, while the valleys consist of a rich black loam.

The catchment basin of the lake is 244,000 square kilometres of which 60,000 are water. Most of the important streams feeding the lake traverse extensive marshes and swamps and must lose a great part of their waters by evaporation. According to Capt Lyons (Appendix III of Sir William Garstin’s report), the climate of the lake basin is typically that which is known as equatorial; two rainy seasons and two dry seasons make up the year, the rains coinciding more or less with the equinoxes and the dry seasons with the solstices, except that the second minor rains are delayed about 1 to 2 months after the autumn equinox. As Capt. Lyons hopes soon to publish a monograph on the meteorology of the Nile valley, I shall say little about the details of rainfall of the different catchment basins, contenting myself with broad principles and main features. March, April and May form the greater rainy season, and October, November and December the lesser. The rainfall of the former season may be considered twice as heavy as that of the latter, but it is the latter which practically decides the height of the lake in the following year. This, according to Capt. Lyons, is due to the fact that in the summer months, when the rainbelt lies to the north of the lake, the dry south winds must blow across the lake basin even though the diurnal reversal of winds on the lake is not mastered by them. These dry winds greatly increase the evaporation, and there is a marked diminution of the water between July and November, which must be primarily due to the increased evaporation.

The rainfall in the catchment basin may be taken as 1250 millimetres per annum on the average. As the evaporation off the lake is probably the same, the area of the lake may be left out of the catchment altogether. The balance of the catchment basin amounts to 184,000 square kilometres, on which there is a mean annual rainfall of 230 cubic kilometres. The mean discharge of the Victoria Nile over the Ripon Falls appears to the approximately 580 cubic metres per second or 18 cubic kilometres per annum. This represents about ¹⁄₁₂ the mean rainfall. The greatest discharge of the lake seems to be about 850 cubic metres per second and the lowest 450. As the lake has risen in a single year 80 centimetres, which represents an increase of water of 48 cubic kilometres, and has fallen 60 centimetres which represents a decrease of water of 36 cubic kilometres, it will be seen that the discharges from the lake are factors of less importance in determining the level of the lake than the heavier rainfall and diminished evaporation in a year of good rain, and the lighter rainfall and increased evaporation in a year of poor rain. The great function of Lake Victoria in the economy of the Nile supply is the insuring of a nearly constant discharge of water into the Victoria Nile, and providing much of the evaporation which comes down in the catchment basin itself in the shape of rain.

The principal feeders of Lake Victoria are the following streams:—

The northern and western feeders are generally perennial streams, while many of the southern and eastern are torrents.