What is the riddle of the Nile?

The Nile River is in Egypt. Each year, the river begins to rise in July and continues until October. Then its level is 7.5 m above the level in May. During high-water season, the river spills over the banks and deposits fertile silt on the fields. But there is almost no rainfall in Egypt and so the rise in water level was puzzling. Actually it is caused by heavy rains and melting snows in the Ethiopian mountains which sends water to the Blue Nile, a source of the Nile, which rises in Ethiopia.