Ancient Egypt was hostage to the Nile. The entire community depended for its existence on the abundant waters of the stately river. Its level rose and fell with rhythmic regularity, flooding large areas and feeding the land with the rich nutrients that made the valley fertile.
Over the centuries, these seasonal floods have been the lifeblood of Egyptian civilisation, and their failure could - and sometimes did - bring hardship, famine, pestilence and death.
The White Nile, as noted yesterday, springs south of the equator from the highlands of Burundi, before flowing through a series of lakes, and across a desert, to join the Blue Nile at Khartoum.
But it was what happened down-river from Khartoum that was the Nile's enigma down the centuries.
Since the 1960s the river's flow has been controlled to a large extent by the High Dam at Aswan, but before that a surge, causing a dramatic rise in the level of the water, moved northwards every year to reach the vicinity of Aswan by the end of June; it affected the ancient region of Thebes and the Valley of the Kings by late July, and finally arrived at Memphis and the delta region by September.
Then the waters gradually subsided, until the river reached its lowest level in the month of April.
Why this should happen was something of a mystery for centuries. Herodotus of Halicarnassus, for example, confessed himself nonplussed.
"Concerning the nature of this river," he wrote, "I was not able to learn anything either from the priests or from anyone besides, though I questioned them very pressingly.
"For the Nile is flooded for a hundred days, beginning with the summer solstice and after this time it diminishes and is during the whole winter very small. And on this head I was unable to obtain anything satisfactory from any one of the Egyptians, when I asked what is the power by which the Nile is in its nature the reverse of other rivers".
The wily Egyptians, naturally enough, had their own theories about the causes of this annual cycle, but saw no reason, so it seems, to share their knowledge with this nosy foreigner.
They attributed the rise and fall of the Nile to the Dog Star, Sirius, which is a prominent feature of the sky at the height of summer.
But nowadays we have a much more plausible explanation. We assign the seasonal flooding of the Nile to the tropical rains of the equatorial south where the White Nile begins its journey, and even more importantly to the melting snows of the Abyssinian uplands where the Blue Nile starts.