According to the web site Pete just posted, the population of Egypt was about 2 million at the end of the Old Kingdom. From what I have been able to determine, Egypt became essentially uninhabitable due to the catastrophic high floods at the end of the 12th Dynasty. However, once the annual flood levels returned to normal, the population would have very quickly recovered.
The Exodus of the Amarna Period was caused more by drought and plague than flood. There is an excellent article in the Fall 2006 issue of KMT Journal on plague in Egypt during the reigns of Amenhotep III and Akhenaten. I don't know how available KMT is in Australia, but in the USA most Borders and Barnes & Noble bookstores carry it.
In the 19th Dynasty, pharaoh Merenptah was made to be a kind of Moses-Akhenaten figure. He was "humbled" in the 5th year of his reign. Only six months after the death Ramses II, Merenptah was assassinated, so he did not get the chance to lead an Exodus ala Akhenaten. The role of Moses was subsequently forced upon a number on other Egyptian royals, namely Bocchoris, Ramses III/Psusennes, and Taharqa/Psamtik. See Part III of the on-line book for the details of their typecasting.
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