Over population, is the answer
Answer:
I think its never but sorry if im wrong
The Cataracts of the Nile are shallow lengths (or white water rapids) of the Nile River, between Aswan and Khartoum, 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
Hope this helped
