The remains of the oldest known large scale dam in the world -its two sides actually- still stand in Egypt. 

It is called Sadd Al Kafara ( = of the Pagans) and is described as a new case in hydria webpage. 

There is a mystery about why this dam was constructed in 2700 – 2600 BC in the valley eastern from Nile, some 30-40 km far from its banks.
Archeologists are not sure what purpose it served but speculate it was for flood control. 
Anyway after a decade of construction works, and probably before its completion the dam was taken away by a devastating flood. 

Read the full text on the Sadd al Kafara dam here:  

http://hydriaproject.info/en/case-studies/sadd-al-kafara-the-oldest-dam-in-the-world/relevance