Complex sentence: It is likely that most of the inhabitants had to give up and return to nomadism, or they might have moved to Mureybet, just 50 km upstream on the other side of the Euphrates, which expanded dramatically at this time.
Split into simpler sentences: It is likely that most of the inhabitants had to give up and return to nomadism. Many of them might have gone to Mureybet, just 50 km upstream on the other side of the Euphrates, which expanded dramatically at this time.