Mounts Kilimanjaro and Kenya, for example, are fa-vourite targets for climbers, and both sit on the flanks of the rift Figure 7.. South of where the rift branches the rainfall is higher,
Trang 1limited, occurring only to the north and south of Lake
Tanganyika This contributes to the different
charac-ters 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
fa-vourite 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 12N to 15S, 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
ex-ample, 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 km2and 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
Figure 6 Geyser activity in the volcanically active area around Lake Bogoria, Kenya.
AFRICA/Rift Valley 29