Explain the influence of earth movements on the formation of lakes in East Africa. 

Candidates should define a lake as a hollow in the Earth’s surface filled by water
• Lakes in East Africa differ in permanency, salinity, shape, size and depth.
• Earth movements are differential movements of the Earth’s crust which are caused by internal/endogenic/diastrophic forces. They may be slow or rapid, lateral or vertical. Result from convective currents in the mantle that cause tension, compression, Earth quakes, isostatic re-adjustment etc.

Candidates should identify earth movements which cause lake formation e.g. coastal warping, faulting and folding. These movements cause hollows which are then filled with water to form lakes.

Coastal warping
The extensive downward and upward movement of the crust led to formation of depressions and uplands. Down warping occurred in central and South Eastern Uganda to form depressions occupied by lakes Victoria, Kyoga, Wamala and Nakivale.
• Before warping took place, land in central Uganda was sloping to the West and rivers like Katonga, Kagera, Rwizi, Kafu were flowing to the west.
• During warping, Eastern and Western Uganda were up warped while central Uganda down warped forming basins.
• After warping, rivers reversed their flow eastwards; emptying their waters into the basins to form lakes Victoria and Kyoga.
Diagram(s) to illustrate river flows

• The formed lakes are shallow in depth, having fresh water, irregular outlines/shore lines and extensive swamps around them bays/headland.
Faulting
The breaking and subsequent displacement of crustal rocks by forces of tension or compression led to the formation of following types of lakes:
• Grabens are hollow lakes/Rift valley lakes found in rift valley floor occupying graben hollow. Grabens result from forces of tension, compression forces. E.g. Albert, Tanganyika, Edward, George, Natron, Eyasi etc.

According to tensional forces
– Radioactive and convective currents from the mantle caused lines of weakness.
– Normal faults were created in the crust.
– Displacement occurred in the crust forming the rift valley.
– Secondary faulting took place in the rift valley to form grabens.
– Formed grabens were later occupied by water to from rivers, rain to form lakes.

According to compressional forces
– Radioactive and convective currents from the crust produced lines of weaknesses.
– Reversed faults were formed in the crust
– Crustal displacement occurred to form the rift valleys
– Secondary faulting took place in the rift valley forming grabens.
– The formed grabens were occupied by water from rivers, rain to form lakes.

– The formed lakes are characterized by being narrow, elongated and deep with steep bank , taking shape of the grabens.
– Examples include lakes; Albert, Tanganyika, Turkana, Edward etc.

Tilt block lakes occupy depressions between tilted uplands/ridges.
– Tension and compression forces led to formation of several parallel faults dividing up the crust into several blocks.
– Faulted landscape was then subjected to uplift or sinking at different rates and then tilting in one direction forming angular ridges and depressions.
– Water from rain /rivers fill the depressions to form a lake(s) e.g. Lake olbolossat in Aberdares in Kenya.

Diagrammatic illustration