Nilometer

Every year the river Nile begins to rise in the summer, the water overflows its banks and deposits slit on the surrounding floodplain. It is this annual flood that makes the land fertile allowing it to be cultivated and civilization to exist.
But the flood was unpredictable. If the river failed to rise, it caused drought and famine. The Egyptians therefore began measuring the Nile's water level in order to predict the harvest.