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