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.
The feature is called a "fall line." It is the natural boundary where rivers descend from higher elevations to lower coastal plains, resulting in waterfalls and rapids. This geological formation often marks the transition between different types of terrain and can significantly influence the development of cities and transportation routes.
A fall line is a place where the elevation of the land drops sharplycausing rivers to form waterfalls or 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.
You could say καταρράκτες [=waterfalls]
Waterfalls or river rapids
Portage.
A group of rapids and small waterfalls is commonly referred to as a "cataract." This term is often used to describe sections of a river where the water flows rapidly over rocks, creating turbulence and drops. Cataracts can vary in size and intensity, contributing to the river's overall landscape.