Before the construction of the Aswan High Dam in the 1960s, the Nile used to flood annually, submerging the fields on either side of it, and depositing the rich nutrients that it had brought with it. If the flood was too high, the water would cause destruction, and would also eventually recede late in the year, disrupting the agricultural cycle. If it was too low, the soil would not be resupplied with nutrients, and agriculture would be impossible. Both outcomes could result in food shortages, or even famine.
Nilometers, as their name suggests, were used to measure flood levels so that dykes, levees, and canals could be prepared accordingly. Flood levels affected agricultural productivity, so another major purpose of Nilometers was to set taxation levels.
Built on the order of the Abbasid Caliph al-Mutawakkil in 247 AH/861 AD, the Nilometer on Rawda Island is one of the oldest in Egypt. It is also the oldest structure built after the Arab conquest (20 AH/641 AD) that survives in its original form. The annual seven‑day celebration of the Nile flood, which took place from the medieval period down to the end of the nineteenth century, was attended by the sultan himself along with his senior officials.
The Rawda Island Nilometer consists of a marble octagonal column 19 cubits in length in the middle of a stone-lined well that is rectangular at the top, and circular at the bottom. Quranic texts relating to water, vegetation, and prosperity are carved on the walls of the well, as are measuring marks, which were used to determine the height of the flood as the water rose through the well. A stairway spirals down to the bottom.