In the time of river valley societys in egypt what was left behind after the nile flooded?

The Nile left behind enriched soil which was perfect for farming

In Egypt, the River Nile creates a fertile green valley across the desert. It was by the banks of theriver that one of the oldest civilizations in the world ... it left thick rich mud (black silt) which was excellent soil to plant seeds in after it ... The ancient Egyptians could grow crops only in the mud left behind when the Nile flooded. [PPT]