Until the Aswan High Dam was built, Egypt received a yearly inundation - an annual flood - of the Nile. The ancient Egyptians did not realise this, but the flood came due to the heavy summer rains in the Ethiopian highlands, swelling the different tributaries and other rivers that joined and became the Nile. This happened yearly, between June and September, in a season the Egyptians called akhet - the inundation. This was seen by the Egyptians as a yearly coming of the god Hapi, bringing fertility to the land. The goddess of the flood was the goddess Mehet-Weret, 'The Great Flood'.
The first signs of the inundation were seen at Swentet (Aswan) by the end of June, reaching its swelling to its fullest at Cairo by September. The flood would then decrease in size around two weeks later, leaving behind a deposit of rich, black silt. The amount of silt left behind due to the height of the Nile determined the amount of crops that the Egyptians could grow - if the inundation was too low, it would be a year of famine.
Original article: http://www.thekeep.org/~kunoichi/kunoichi/themestream/inundation.html#.WGQH6bXfWaO#ixzz4U9yCVkCP