The First (“Siennian”) cataract of the Nile, as an ancient conterminal area at the junction of Egypt and Lower Nubia, has never been an insuperable barrier between the two countries whose inhabitants, if necessary, passed it not only by land but straight by the River as well. The greatest difficulties in passing the cataract arose during the Egyptian season of “summer” with its lowest Nile levels, when the royal fleets on their way to Kush and backwards were, according to some pharaohs’ rock inscriptions on the island of Sehel, stopped by local reefs and shoals, which necessitated cleaning (or “digging”) the cataract channels regularly. It was the First cataract, as one of the shallowest stretches of the Main Nile that was chosen by British engineers (W. Willcocks et al.) for the large reservoir (Old Aswan) dam whose construction resulted in heavy inundation of the upstream island of Philae with its architectural monuments of the Late Egyptian and Graeco-Roman times. In the Predynastic period, the Nile discharge was much higher than today and had presumably a similar inundating effect along the whole cataract. Archaeologists argue that the southern side of Elephantine at the bottom of the cataract was then divided by the River into western and eastern islands, while today’s island of Elephantine was shaped apparently by the so-called “Neolithic drop” of the Nile in late fourth millennium BC. According to my hypothesis, this evolution of Elephantine was just a part of the complex hydrological and geomorphological metamorphosis of the Nubian and Egyptian Nile with the emergence, as its major result, of the famous “Siennian cataract” on the eve of pharaonic civilisation.