Nilometers were structures that measured the clarity and water level of the Nile River during flood seasons long before the construction of the Aswan Dam. There are three kinds of nilometers, the simplest comprising a tall column erected inside a stone structure called a stilling well such as the one on Rhoda Island in Cairo. Built in 861, the Rhoda nilometer contains an octagonal marble column supported by a wooden beam. A staircase spirals down the stilling well, and a finely ornamented conic dome covers the structure.

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