Every year the river Nile begins to rise in the summer, the water overflows its banks and deposits slit on the surrounding floodplain. It is this annual flood that makes the land fertile allowing it to be cultivated and civilization to exist. Since the ancient times, the Egyptians depended on the Nile’s flood and its regular return for their sustenance. But the flood was unpredictable. While a moderate inundation was a vital part of the agricultural cycle, too much flood water was disastrous as it washed away crops and much of the infrastructure built on the flood plain. If the river failed to rise, it caused drought and famine. The flood also played an important political and administrative role, since the quality of the year's harvest was used to determine the amount of tax to be paid. The Egyptians therefore began measuring the Nile’s water level in order to predict the harvest.

Nilometer on Elephantine Island. The steps lead down to the Nile, while the short horizontal marks on the walls (to the left of the steps) recorded the heights of previous inundations. Photo credit

At first these records were little more than marks on the river bank, but later marked stairs, pillars, wells and other structures called nilometers were built. The royal priest monitored the day-to-day level of the river and kept records. It was his duty to announce the awaited arrival of the summer flood, or lack of. The ability to predict the volume of the coming inundation became part of the mystique of the Ancient Egyptian priesthood.

The simplest nilometer design is a vertical column submerged in the waters of the river, with marked intervals indicating the depth of the water. Later these columns began to be housed inside elaborate and ornate stone structure. One such nilometer can still be seen on the island of Rhoda in central Cairo. Although this nilometer was constructed in 861AD, it was built on a site of an earlier specimen.

Another historically important nilometer lies on the island of Elephantine in Aswan, which consist of a flight of stairs leading down into the water, with depth markings along the walls. Elephantine marked Egypt's southern border and was therefore the first place where the onset of the annual flood was detected.

The most elaborate design involved a channel or culvert that led from the riverbank, often running for a considerable distance, and then fed a well, tank, or cistern. These nilometer wells were most frequently located within the confines of temples, where only the priests and rulers were allowed access. A particularly fine example of such a nilometer can be seen at the Temple of Kom Ombo to the north of Aswan.

Egypt’s ancient nilometers continued to be used by later civilizations until the 20th century when the construction of the Aswan dams put an end to the Nile's annual inundation, rendering the nilometers obsolete.

Nilometer on Elephantine Island. View from the top of the steps. Photo credit

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The Nile near the bottom of the nilometer steps. The previous water level can be seen in the walls. Photo credit

The dome above the nilometer in Rhoda Island in Cairo. Photo credit

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The nilometer at Rhoda island comprises of a single column with markings on its body. Photo credit

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The nilometer at the Temple of Kom Ombo. Photo credit

The nilometer at the Temple of Kom Ombo. Photo credit

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Sources: Wikipedia / www.reshafim.org.il