The river Nile was the source of life and prosperity in Egypt. The Ancient Egyptians relied on the annual floods of the Nile to irrigate their crops, but studies have shown that the way in which the river floods varies considerably. Working together, historians and climatologists have found links between years of low flooding and periods of instability in Egyptian society. Records show that the famines that followed low floods led to disease and civil unrest — possibly causing the collapse of the Old Kingdom.
The flooding of the Nile is the result of the yearly monsoon between May and August causing enormous precipitations on the Ethiopian Highlands whose summits reach heights of up to 4550 m (14,928 ft). Most of this rainwater is taken by the Blue Nile and by the Atbarah River into the Nile, while a less important amount flows through the Sobat and the White Nile into the Nile. During this short period, those rivers contribute up to ninety percent of the water of the Nile and most of the sedimentation carried by it, but after the rainy season, dwindle to minor rivers.
These facts were unknown to the ancient Egyptians who could only observe the rise and fall of the Nile waters. The flooding as such was foreseeable, though its exact dates and levels could only be forecast on a short term basis by transmitting the gauge readings at Aswan to the lower parts of the kingdom where the data had to be converted to the local circumstances. What was not foreseeable, of course, was the extent of flooding and its total discharge.
The Egyptian year was divided into the three seasons of Akhet (Inundation), Peret (Growth), and Shemu (Harvest). Akhet covered the Egyptian flood cycle. This cycle was so consistent that the Egyptians timed its onset using the heliacal rising of Sirius, the key event used to set their calendar.
The first indications of the rise of the river could be seen at the first of the cataracts of the Nile (at Aswan) as early as the beginning of June, and a steady increase went on until the middle of July, when the increase of water became very great. The Nile continued to rise until the beginning of September, when the level remained stationary for a period of about three weeks, sometimes a little less. In October it often rose again, and reached its highest level. From this period it began to subside, and usually sank steadily until the month of June when it reached its lowest level, again. Flooding reached Aswan about a week earlier than Cairo, and Luxor 5 – 6 days earlier than Cairo. Typical heights of flood were 45 feet (13.7 metres) at Aswan, 38 feet (11.6 metres) at Luxor (and Thebes) and 25 feet (7.6 metres) at Cairo.
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