The flooding of the Nile (Arabic: عيد وفاء النيل) has been an important natural cycle in Egypt since ancient times. It is celebrated by Egyptians as an annual holiday for two weeks starting August 15, known as Wafaa El-Nil. It is also celebrated in the Coptic Church by ceremonially throwing a martyr's relic into the river, hence the name, Esba` al-shahīd ('The Martyr's Finger'). Ancient Egyptians believed that the Nile flooded every year because of Isis's tears of sorrow for her dead husband, Osiris.
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 Atbara into the Nile, a less important amount is flowing 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, 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. Which was not foreseeable, of course, was the size 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.
If it were not for the Nile River, Egyptian civilization could not have developed, as it is the only significant source of water in this desert region.
Its other importance was its function as their gateway to the unknown world. The Nile flows from south to north, to its delta on the Mediterranean Sea. The floods were seen as the annual coming of the god. Possibly Egyptian mythology was based on this understanding, creating stories of gods or nature to give added importance to the processes and cycles that sustained Egypt.
Whilst the earliest Egyptians simply laboured those areas which were inundated by the floods, some 7000 years ago, they started to develop the basin irrigation method. Agricultural land was divided into large fields surrounded by dams and dykes and equipped with intake and exit canals. The basins were flooded and then closed for about 45 days to saturate the soil with moisture and allow the silt to deposit. Then the water was discharged to lower fields or back into the Nile. Immediately thereafter, sowing started, and harvesting followed some three of four months later. In the dry season thereafter, farming was not possible. Thus, all crops had to fit into this tight scheme of irrigation and timing.
In case of a small flood, the upper basins could not be filled with water which would mean famine. If a flood was too large, it would damage villages, dykes and canals.
The basin irrigation method did not exact too much of the soils, and their fertility was sustained by the annual silt deposit. Salinisation did not occur, since in summer, the ground water level was well below the surface, and any salinity which might have accrued was washed away by the next flood.
It is estimated that by this method, in ancient Egypt, some 2 million up to a maximum of 12 million inhabitants could be nourished. By the end of Late Antiquity, the methods and infrastructure slowly decayed, and the population diminished accordingly.
By 1800, Egypt had a population of some 2.5 million inhabitants.
Muhammad Ali Pasha, (1805–1848 Khedive of Egypt) modernized various aspects of Egypt. He endeavoured to extend arable land and achieve additional revenue by introducing cotton cultivation, a crop with a longer growing season and requiring sufficient water at all times. To this end, the Delta Barrages and wide systems of new canals were built, changing the irrigation system from the traditional basin irrigation to perennial irrigation whereby farmland could by irrigated throughout the year. Thereby, many crops could be harvested twice or even three times a year and agricultural output was increased dramatically.
Isma'il Pasha built the Ibrahimiya Canal, thereby greatly extending perennial irrigation. Although the British, during their first period in Egypt, improved and extended this system, it was not capable to store large amounts of water and to fully retain the annual flooding. In order to further improve irrigation, Sir William Willcocks in particular, in his role as director general of reservoirs for Egypt, planned and supervised the construction of the Aswan Low Dam, the first true storage reservoir, and the Assiut Barrage, both completed in 1902. However, they were still not capable to retain sufficient amounts of water to cope with dry summers, and although the Aswan Low Dam had been raised twice, between 1907–1912 and again 1929–1933, it could not fully meet the demands. At about the same time, the Sennar Dam was constructed on the Blue Nile as a reservoir in order to supply water to the huge Gezira Scheme on a regular basis. It was the first dam on the Nile to retain large amounts of sedimentation (and to divert a large quantity of it into the irrgation canals) and in spite of opening the sluice gates during the flooding in order to flush the sediments, the reservoir is assumed to have lost about a third of its storage capacity. In 1966, the Roseires Dam was added to help irrigating the Gezira Scheme. The Jebel Aulia Dam on the White Nile south of Khartoum was completed in 1937 in order to compensate for the Blue Nile's low waters in winter, but still, it was not possible to overcome a period of very low waters in the Nile and thus a drought some of which had been plagued Egypt since ancient times.
In order to overcome these droughts, Harold Edwin Hurst, a British hydrologist in the Egyptian Public Works from 1906 until many years after his retirement age, studied the fluctuations of the water levels in the Nile, and already in 1946 submitted an elaborate plan how a century storage could be achieved to cope with exceptional dry seasons occurring statistically once in hundred years. His ideas of further reservoirs using Lake Victoria, Lake Albert and Lake Tana and of reducing the evaporation in the Sudd by digging the Jonglei Canal were opposed by the states concerned. Eventually, Gamal Abdel Nasser, President of Egypt from 1956 to 1970, preferred the idea of the Aswan High Dam at Aswan in Egypt instead of having to deal with many foreign countries. The required size of the reservoir was calculated using Hurst's figures and mathematical methods. In 1970, with the completion of the Aswan High Dam which was able to store the highest floods, the annual flooding cycle in Egypt came to an end in Lake Nasser.
In the meantime, Egyptian population has risen to 92.5 million inhabitants (2016 estimate).