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They prevented travel to the interior of Africa. _ Airiana
cataract
Rapids
You could say καταρράκτες [=waterfalls]
Waterfalls or river rapids
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.
CAtaract
Portage.
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
"Waterfalls and rapids"