nilometre

thumb|Measuring shaft of the nilometer on Roda Island, Cairo built in 861 AD on the orders of the Abbasid Caliph [[al-Mutawakkil]] A nilometer is a structure for measuring the Nile River's clarity and water level during the annual flood season in Egypt. There were three main types of nilometers, calibrated in Egyptian cubits: (1) a vertical column, (2) a corridor stairway of steps leading down to the Nile, and (3) a deep well with a culvert. If the water level was low, the fertility of the floodplain would suffer. If it was too high, the flooding would be destructive. There was a specific mark