A nilometer measures the rise and fall of what

Rivers
A Nilometer was a structure for measuring the Nile clarity and water level during the annual flood season.

It was a step-like structure that measured how high the flood would be. If it was low, there would be famine.

Between July and November, the reaches of the Nile running through Egypt would burst their banks and cover the adjacent flood plain.

When the waters receded, around September or October, they left behind a rich alluvial deposit of exceptionally fertile black silt over the croplands.

The inundation – akhet in the Egyptian language – was one of the three seasons into which the Ancient Egyptians divided their years. ( Season of the Inundation.)

The season was called "Akhet", and it was the season of flooding.

Read more...https://en.wikipedia.org/wiki/Nilometer