Every summer the rivers flood the Nile, the water overflows its borders and creates in some parts the so called floodplains. In ancient Egypt the Nilometer was the structure used for measuring the Nile River water level as well as predicting how much the cultivation might have produced, in a similar way to modern income estimates. Different versions of Nilometers were built, the two most famous types are the one from Elephantine Island in Aswan and the one from Rhoda Island in central Cairo. The structure could be simple stairs or entire buildings with stairs inside.
Below: the Nilometre in the Elephantine Island. Picture via Wikipedia – licence CC BY-SA 3.0
Sometimes the water checkers were kept inside the temples and the access to the structure was reserved to the priests. In ancient Egypt it was extremely important to organise the stock for the year, so the use of the instrument was essential.
Below: a conic structure covers the Nilometer on the Southern spot of the Rhoda Island, on the Nile, in Cairo. The structure is modern but the nilometer dates back to 715 AD. Picture by Prong hunter shared via Wikipedia – licence CC BY-SA 3.0
If the lever of the waters on Nile appeared to be low, the harvest would have been insufficient to the maintenance of the Egyptian population, hence the politics had to organise itself in time with international trades. In case the Nile would have appeared as too loaded with water, this meant that it would have flooded the fields and therefore destroyed the cultivation, and the politic should have faced the famine.
To get a secure harvest, the Nile was supposed to remain within a certain range, more or less, in order to irrigate the fields in a balanced way
Below: the axle of measurement of the nilometer in Rhoda Island, Cairo. Picture by Baldiri shared via Wikipedia – licence CC BY-SA 3.0
The instrument revealed itself to be so useful that it was used up until the 20th century, when the Egyptian government, helped by the Soviet Union and by the money coming from the Suez canal, built up the Aswan High Dam in 1970, which mitigated extensively the level of the Nile.
Below: picture by Michal Huniewicz shared via Wikipedia – licence CC BY-SA 4.0
Source: Encyclopaedia Britannica