Because the nile river flooded mostly than any river in Egypt and so after 7 or more weeks there will be silt left over and the water would dry up but the silt would get stuck in the dam. (Caused less damage as well).
The same as building a dam on any river. Dams are built to control flooding downstream during the wet season, generate power year round, and provide irrigation in the dry season.
When Egypt decided to build the dam their choice was made for them as the Nile is the only river they have.
They built dams.
By building dams - for example, the construction of the Aswan Dam in the 1960's meant that from 1970 the annual flood was controlled.
The Aswan Dams
The Aswan Dam was built in 1956 to control the Nile River floods and produce electricity. Before the dams were built, the Nile River flooded every year during late summer, when water flowed down the valley from its East African drainage basin.
Aswan dams.
Dams - yes, locks - no
To control the river and flooding and produce electricity.
Aswan High Dam
They built dams
to stop the water from flooding
On or by the Nile - The pyramids, the Sphinx, the Aswan dams, Cairo, Nasser Lake, etc
It is one of the major dams on the river Nile, located on the Blue Nile in Sudan