I was about to send similar information to wbarmy. Northern rulers periodically sailed/rowed south to keep the southerners in their place with warfare. However, with weak leaders or chaotic conditions the south sometimes took over. I guess floating north was easier.
The Ethiopians or Nubians might have had the upper-hand for awhile, but they were not Egyptians. Egyptians were and are Semitic Arabs.
If you do not believe me, go tell one of them they are black. Just like the Sicilians!
https://www.youtube.com/watch?v=PcZCGMCDzbU