Although there is evidence that a nilometer exited in this location since the Pharaonic Period,  the Umayyads under Suayman Abd al-Malek had built a simple Nilomter similar to those of the earlier period here in about 715 AD, which was restored in 815 by Caliph al-Marmoun. It was destroyed by an exceptionally high flood in 850. The one on Rawda Island today was built by order of the Abbasid Caliph al-Mutawakkil (847-861) under the direction of Ahmad ibn Muhammad al-Hasib at the end of his reign in 861. It was devised by Abu'l 'Abbas Ahmad ibn Muhammad ibn Kathir al-Farqhani, a native of Farghana, West Turkestan who was known in the West as the astronomer Alfraganus. Ibn Tulun had the Nilometer restored between 872 and 873 AD, and it was again restored in 1092 by the Fatimid Caliph, al- Mustansir. It remains mostly original, except for the wooden painted conical dome roof which is a modern restoration. This cupola in the shape we know it today was modeled from an earlier example built after the Nilometer was bombarded by French troops during their occupation of Egypt. When this earlier dome was destroyed by a nearby factory explosion in 1825, it was refitted by using an 18th century painting by the Danish traveler, Fredrik Ludvig Norden as a reference. 

A look up into the dome covering the Nilometer

This Nilometer is a somewhat more sophisticated instrument then earlier examples such as the one on Elephantine Islandin the Nile at Aswan. It consists of a pit that extends well below the level of the Nile that in turns connects with the Nile through tunnels dug on three levels on its eastern side. These tunnels are now blocked off from the Nile, so that the Nilometer no longer functions. The pit, which is lined with stone, is circular at the bottom and rectangular at the top, is accessed by a staircase on the interior walls. Its walls have four recesses with pointed arches, and small, relatively thin columns to either side adorned with two types of zigzag framing decorations carved on its stone voussoirs. Though these arches, known as "tiers-point" are the same type as those used in Gothic architecture, they proceeded the Gothic arch by some four hundred years. 

In the center of the pit a marble, octagonal column with a Corinthian capital that rises from its depths surmounting a millstone. At the top there is a wooden beam spanning the Nilometer. To measure the Nile flood, this column is graded and divided into 19 cubits (a cubit is slightly more than half a meter, and hence, it was capable of measuring floods up to about 9.2 meters). The flood that this Nileometer measured was both important to the rulers of Egypt as well as the whole population. An ideal flood filed the Nilometer up to the sixteenth mark and less then this could mean drought and famine. On the other hand, if the measurement exceed the 19 cubits, a catastrophic flood was at hand. In the days prior to the expected flood, this column would be anointed with saffron and musk in order to help induce a good water level. 

Plain Kufic inscriptions adorn the walls of the Nilometer and are the earliest surviving examples of architectural epigraphy (inscriptions considered as a group) in Egypt. They are taken from Quranic texts that refers to water, vegetation and prosperity, and therefore have a talismanic meaning, but there is also secular text as well. These inscriptions were executed in white marble originally on a blue background, though the letters themselves were left in the natural stone color.  However, the inscription recording the establishment of the Nileomter by al-Mutawakkil has been removed. Creswell, a well known historian of this period, believes this was done by Ibn Tulun, who replaced it with additional verse, as part of a campaign to assert his independence from the foreign Caliphate. Part of the original inscriptions read:

"We send down rain as a blessing from heaven, whereby we cause gardens to spring forth and the grain to harvest." (50:9) "Hast thou not seen how that God has sent down out of heaven water, and in the morning the earth becomes green?" (22:62)

Left: A cross section of the Nilometer from Norden's Views; 
Right: An early lithograph of the Nilometer on Rawda (Roda) Island

Because of its importance in determining the prosperity Egypt would experience during the following year, this Nilometer was the departure point of the greatest of Cairo's celebrations throughout the medieval period. This was the Fath al-Khalij, the festival of the Opening of the Canal, which ceased in 1899 when the Khalij (Khalig) was filled in (this Nilometer itself probably continued to be used up until the last flood in 1970). The Khalij Canal started opposite Rawda Island, bordered the medieval city to the west, and irrigated its outlying gardens and fields. The Khalij canal was blocked with an earth dam and was cleaned before the flood. It would then be opened when the water level reached the sixteenth cubit level, when the caliph and later sultans and pashas would inaugurate the celebrations that lasted for several days. The summer flood from the Nile would then fill this canal, together with many ponds that would have winter beds green with vegetation. During the celebrations, decorated boats would crowed the waters, and among these, the most splendid would be that of the ruler. Those who witnessed this event refer to it as Cairo's most spectacular celebration. During the hot summer months, the Khalij and the ponds remained filled with pleasure boats and its shores were lined with entertainment.  Near the Nilometer was a mosque for prayers during the flood celebration, and a palace for banquets held by the various rulers. 

However, the grand celebration was not guaranteed as an annual event. When the Nile flood waters failed to reach the sixteenth cubic mark, the celebrations were canceled and prayers and fasting were held instead in order to ward off the expected drought and famine. 

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