I've also seen arguments (unsourcable at the moment) that the Tigris and Euphrates used to drain separately, and that the adjustment when they merged could have

I've also seen arguments (unsourcable at the moment) that the Tigris and Euphrates used to drain separately, and that the adjustment when they merged could have come at approximately the same point in pre-history. Given the proximity to the origin points of both stories, (and seen from within their flood plains where people were already living) that would have looked like a huge flood, after which the local geography would have looked very different.