This is from the Greek “kataraktes,” which means “downrush.”
Clearly, most people know “cataract” from the disorder of the mammalian eye. So, how did that get named for a waterfall? Wikipedia has some theories:
As rapidly running water turns white, so the term may have been used metaphorically to describe the similar appearance of mature ocular opacities. In Latin, cataracta had the alternative meaning “portcullis” and the name possibly passed through French to form the English meaning “eye disease” (early 15th century), on the notion of “obstruction”. Early Persian physicians called the term nazul-i-ah, or “descent of the water” – vulgarised into waterfall disease or cataract – believing such blindness to be caused by an outpouring of corrupt humour into the eye.
The most famous cataracts are those on the Nile. They have a Wikipedia page all their own, which says:
The Cataracts of the Nile are shallow lengths (or whitewater rapids) of the Nile river, between Khartoum and Aswan, where the surface of the water is broken by many small boulders and stones jutting out of the river bed, as well as many rocky islets. In some places, these stretches are punctuated by whitewater, while at others the water flow is smoother but still shallow.