AFRICA/Rift Valley 29 Figure Geyser activity in the volcanically active area around Lake Bogoria, Kenya limited, occurring only to the north and south of Lake Tanganyika This contributes to the different characters of the lakes in the two branches as not only can the volcanic rocks fill the basins, leaving less space for large lakes, but also many of the rock types are rich in salts, which contribute to the salinity of the lakes once they are released by weathering Large and active volcanoes that sit outside the rift structure are a striking feature of the landscape Mounts Kilimanjaro and Kenya, for example, are favourite targets for climbers, and both sit on the flanks of the rift (Figure 7) Hydrology and Climate The East African Rift system sits astride the equator, extending from 12 N to 15 S, and this dictates the overall character of the climate Superimposed on this are the effects of the rift topography, with its uplifted domes, faulted flanks, and depressed central valleys Rainfall is lowest in the northern parts of Ethiopia and increases southwards into northern Kenya The region is generally desert or semi-desert with vegetation limited to sparse grasses and scrub South of where the rift branches the rainfall is higher, with the western branch being wetter than the eastern one The uplifted mountains that make up the margins of the rift are wetter and cooler than the valley bottom; for example, an annual figure of over 2000 mm of rainfall has been recorded in the Ruwenzori Mountains near Lake Mobutu The doming that accompanied the rifting in East Africa has had a major impact on the present river systems The development of the rift disrupted a pre-existing continental drainage system in which a few large rivers with vast integrated drainage basins dominated the landscape As the area was domed and faulted and the new valley formed, the rivers adjusted to the new landscape: some lost their headwaters, others were created, some gained new areas to drain The overall effect was to divert much of the drainage north into the Nile system and west into the Congo drainage, with only a few small rivers now reaching the Indian Ocean Inside the valley, the rivers are generally short and small, ending in a lake not far from the river source, but a few rivers run along the rift, often caught between faulted hills, and discharge into lakes far from their original sources, e.g the Kerio River in Kenya has its source near Lake Baringo but discharges into Lake Turkana more than 200 km to the north (Figure 8) The segregation of the underlying structure into topographically distinct sections exerts an overriding control on the character and distribution of lakes throughout the rift It provides the framework within which the balance between movement of water into the basin, from rainfall and rivers, and evaporation from the surface will work The largest and deepest lake, Lake Tanganyika, is in the wetter western branch of the rift in a particularly deep section It covers an area of over 40 000 km2 and is more than 1400 m deep at its deepest point Lakes in the eastern branch are smaller and shallower; for example Lake Bogoria is an average of less than 10 m deep, and if the climate changes and rainfall decreases they soon become ephemeral, drying out completely during periods of drought