Fall Line
fall line
Rapids
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.
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.
A fall line is a place where the elevation of the land drops sharplycausing rivers to form waterfalls or rapids.
cataract
You could say καταρράκτες [=waterfalls]
Waterfalls or river rapids
Portage.
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
upper course also called youthful stage by W.M. Davis