The history of the Nile encompasses several episodes, the last of these being the present river flowing south to north. Before that, Egypt witnessed several rivers flowing from east to west, following the upheaval of the Red Sea hills. This uplift with associated northerly trending fissures and fractures realigned the drainage such as the Qena River to flow from north to south in what is now Wadi Qena. Qena became a watershed for rivers flowing from north to south; the southern Allaqi River and a large river flowed south-westward to feed the recently discovered "radar imaged" rivers from southwest Egypt. Though these "radar imaged" rivers are old features - predating man- the discovery of an Acheulian hand axe on the bank of one of these streams proves that these rivers were active during Acheulian times. After the retreat of the Pliocene water from the Gulf occupying most of the present Nile valley, rejuvenation of old rivers flowing east-west and north-south took place. It was only during the Middle Pleistocene that the trapped Ethiopian and equatorial African waters breached the cataracts in northern Sudan, capturing many side rivers as it flowed through Egypt to become the real Nile. It took man several hundred thousand years to migrate from the relatively safe deserts on both sides of the Nile to its banks. These migrations were associated with drastic climatic changes from tropical savannah to arid and dry conditions. Modern desertification started approximately 7800 years ago.