Earlier quoted context omitted.
Any idea what the small circular lake with the island in the middle, at bottom right might be? That's an interestingly precise shape. (It's surrounded by labels for desert, but I can't make out the labels on the island itself.)
If you look at the map, you can see Samosata up north on the Euphrates, Ressaina between Euphrates and Tigris and Zeugma to the west. Tharrana is next to the circular lake with the island. https://en.wikipedia.org/wiki/Samosata is the modern city of Samsat https://en.wikipedia.org/wiki/Rhesaina is Ressaina (see http://www.perseus.tufts.edu/hopper/text?doc=Perseus:text:19... ) https://en.wikipedia.org/wiki/Zeugma,_Com…
Here's an alternate proposal: Raqqah. Check it out on the map:
https://goo.gl/maps/ubW2hp1Wh6N2
It obviously isn't in a lake, but it looks like the the tributary coming down from Harran goes both west and east of Raqqah, making it an island of sorts. The shape made by the this "island" is reasonably close to circular. Finally, per the map, it's southeast of Zuegma and south of Harran.
Obviously hypothesising wildly, but this map is just way too much fun.