Nilometer is the name given to one of several devices that are different in design but that all serve the same function: measuring water levels in the River Nile and thus allowing the keeping of comparative historic records.
Every summer, torrential rains in the highlands of Ethiopia cause a drastic increase in the volume of water flowing into the Nile from its tributaries. Between June and September, the reaches of the Nile running through Egypt would burst their banks and cover the adjacent flood plain. When the waters receded, around September or October, they left behind a rich alluvial deposit of exceptionally fertile black silt over the croplands. The inundation – akhet in the Egyptian language – was one of the three seasons into which the Ancient Egyptians divided their years. (See Season of the Inundation.)
It would be difficult to overstate the importance of the annual flood to Egyptian civilization. A moderate inundation was a vital part of the agricultural cycle; however, a lighter monsoon than normal would cause famine, and too much flood water would be equally disastrous, washing away much of the infrastructure built on the flood plain. Records from Pharaonic times indicate that on average, one year of out every five saw an inundation that was either over-abundant or fell short of expectations.
The ability to predict the volume of the coming inundation was part of the mystique of the Ancient Egyptian priesthood. The same skill also played a political and administrative role, since the quality of the year's flood was used to determine the levels of tax to be paid, in kind, by the peasantry to their rulers. This is where the nilometer came into play, with priests monitoring the day-to-day level of the river and announcing the awaited arrival of the summer flood.
The simplest nilometer design is a vertical column submerged in the waters of the river, with marked intervals indicating the depth of the water. One that follows this simple design, albeit housed in an elaborate and ornate stone structure, can still be seen on the island of Rodah in central Cairo. While this nilometer dates only as far back as 861 AD, when the Abbasid caliph al-Mutawakkil ordered its construction, it was built on a site occupied by an earlier specimen.
The second nilometer design comprises a flight of stairs leading down into the water, with depth markings along the walls. The best known example of this kind can be seen on the island of Elephantine in Aswan. This location was also particularly important, since for much of Egyptian history 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, with a deep, cylindrical well and a culvert opening in the surrounding wall, can be seen at the Temple of Kom Ombo to the north of Aswan.
While nilometers originated in Pharaonic times, they continued to be used by the later civilizations that held sway in Egypt. In the 20th century, the Nile's annual inundation was first greatly checked, and then eliminated entirely, with the construction of the Aswan dams. While the Aswan High Dam's impact on Egypt and its agriculture has been controversial for other, more complex reasons, it has also had the additional effect of rendering the nilometer obsolete.