The Nile River floods from year to year during the summer period - the water overflows its banks and covers the surrounding land. It is this annual flood that makes the land fertile, thanks to which the Egyptian civilization exists. Since ancient times, the Egyptians have depended on the flooding of the Nile and its regular return to the river bed, growing bread in fertile fields. But the flood was unpredictable: it could be a moderate influx vital to the agricultural cycle, or there could be a severe flood with catastrophic consequences, washing away crops and people's homes.
If the river did not rise properly, drought and famine set in. The flood also played an important political and administrative role, as the quality of the crop was used to determine the amount of taxes paid by farmers. For this reason, the Egyptians began to measure the water level of the Nile using special structures called Nilometers.
At first, people simply put marks on the banks of the river, but later they began to build stairs, poles, wells and other structures. The royal priest monitored the daily level of the river and kept records. It was his responsibility to announce the expected arrival of the summer flood, or lack thereof. The ability to predict the volume of the coming influx of water became part of the mysticism of the Ancient Egyptian clergy.
The simplest design of a nilometer is a vertical column submerged in a river at marked depth intervals. Later, these columns began to be housed in an elaborate stone mine. One such nilometer can still be seen on Roda Island in central Cairo. It was built in 861 on the site of an earlier example.
Another historically important nilometer is found on Elephantine Island in Aswan. Its flight of stairs goes into the water, with depth markings along the walls. The southern border of the island was the first place where the annual flooding began.
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The most complex project involved a canal, or culvert, leading a long distance from the banks of the river to a special cistern. These wells were most often located within temples, where only priests and rulers were allowed. A particularly fine example of such a nilometer is found in the Temple of Kom Ombo in northern Aswan.
The ancient nilometers of Egypt continued to be used by later civilizations until the 20th century, when the construction of the Aswan dams ended the annual influx of the Nile.