Sumer thrived for many centuries, largely because of their advanced irrigation system. However, this irrigation system may have led to the downfall of Sumer. The water diverted from the rivers to irrigate the soil also carried harmful salts and other mineral matter. The soil became more acidic and the crops started to die. Sumer lost its economic strength, which in turn led to a loss of political and military strength. Sumer was soon too weak to fight off invaders and was conquered.

Below is a recent photo of the area where the Tigris and Euphrates merge. It no longer looks so fertile. It never quite recovered its former fertility though the reasons for this are complex. 

Photo from USA Today - Travel https://img-aws.ehowcdn.com/560x560p/photos.demandstudios.com/getty/article/81/26/84705772.jpg