Why is end-to-end navigation impossible on most rivers in Africa?

African rivers have a certain specificity of flow. Most of them originate in the highlands of the central part of the continent and on their way to the oceans does not flow through the plains, but along mountain canyons and gorges, which makes the rivers very turbulent and winding. Also, waterfalls are often formed on them. Some rivers, such as the Nile, receive a lot of water, but since they flow through the savannah and desert, they lose volume due to evaporation and from this the upper reaches are shallow and not suitable for navigation.