What yearly event turned the Egyptian desert into good farmland?

As the territory is a desert, they could cultivate just a narrow strip along the Nile by usin an irrigation system= that is they dug canals and built gates so that they could water the fields with the water of the Nile after the flooding (it occured in June). They also used the shaduf, with which they were able to water the higher places as well. They grew wheat and barley.