They prevented travel to the interior of Africa. _ Airiana
Rapids
cataract
V-shaped valleys typically include rapids and waterfalls. These valleys are formed by fast-flowing rivers eroding downwards into the landscape, resulting in steep sides and features like rapids and waterfalls due to the force of the moving water.
Waterfalls or river rapids
You could say καταρράκτες [=waterfalls]
A lock is the mechanism for raising and lowering ships in a river that has rapids or waterfalls. Locks are used to make navigation an easier task.
Portage.
CAtaract
A mechanism for raising or lowering ships in a river that has rapids or waterfalls is called dam and lock system. It is a special kind of dam system.
you can see waterfalls on the internet but also in the rain forest and any place where there are many large rapids in a large rivers
West